MATERIALS and GEOENVIRONMENT MATERIALI in GEOOKOLJE RMZ - M&G, Vol. 67, No. 3 pp. 91-159 (2020) Ljubljana, September 2020 Table of Contents Kazalo Original scientific papers Izvirni znanstveni članki Characterisation of the Wear of the Roller Cone Drill Bit 91 Caused by Improperly Chosen Drilling Regime Karakterizacija obrabe kotalnega dleta povzročene z nepravilno izbiro režima vrtanja J. Šporin Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils in Geotechnology 103 and Mining Applications Laboratorijske preiskave abrazivnosti kamnin in zemljin na področju geotehnologije in rudarstva B. Janc, V. Jovičič, Ž. Vukelič Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria 119 Retrogresija gnajsa z ortopiroksenom iz območja Iboropa Akoko, jugozahodna Nigerija E.J. Oziegbe, V.O. Olarewaju, O.O. Ocan, G.Costin High-resolution Biostratigraphic Studies of Step-1 Well, Offshore, Western Niger Delta 135 M. E. Nton, S. O. Famori Fractional Current Flow in the Subsurface Using Electrical Resistivity Method: 149 A Laboratory Approach Podpovršinsko gibanje električnega toka z uporabo električne uporovne metode: laboratorijski pristop James Adeyemo Adegoke, Gideon Oluyinka Layade, Temitayo Kadir 91 Original scientific article Received: Dec 07, 2020 Accepted: Dec 11, 2020 DOI: 10.2478/rmzmag-2020-0013 Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime Karakterizacija obrabe kotalnega dleta povzročene z nepravilno izbiro režima vrtanja Jurij Šporin* Faculty of Natural Sciences and Engineering, Department of Geotechnology, Mining and Environment, University of Ljubljana, Aškerčeva 12, Ljubljana, Slovenia * juNj.sporin@ogr.ntf.uni-lj.si Abstract The rapid drop in the penetration rate or failure of the drill bit during the drilling process delays the drilling process. In our investigation, the 'in situ' drilling parameters were monitored during the drilling process along with the roller cone drill bit, which is suitable for drilling in soft rock formations (IADC 136). The drill bit was thoroughly examined to determine its damage and wear occurred during drilling along with decreasing penetration rate. The modern and standardised investigation methods were used to analyse the rock materials and the micro- and macro-structure of the materials of the roller cone bit. The analyses were performed by means of optical and electron microscopes, simultaneous thermal analysis of the steel materials, analysis of the chemical composition of the materials of the drill bit and determination of the geomechanical parameters of the drilled rock. The resulting wear, localised fractures and cracks were quantitatively and qualitatively defined and the parameters were correlated to the drilling regime and the rock material. The results of our investigation of the material of the roller cone bit can serve as a good basis for the development of new steel alloys that can withstand higher temperatures and allow effective drilling without structural changes of the steel material. Keywords: drilling regime, roller cone bit, wear, reduced penetration rate, change of the material properties. Izvleček Upad napredka vrtanja ali odpoved dleta med vrtanjem imata za posledico daljši čas izvajanja vrtalnih del. V toku raziskave obrabe kotalnega dleta IADC 136, ki je namenjen uporabi pri vrtanju skozi mehkejše kamnine, so se izvajale opazovanja in meritve vrtalnih parametrov. Po končanem odseku vrtanja, ko je hitrost napredovanja drastično padla, se je kotalno dleto natančno pregledalo za ugotovitev nastalih poškodb med vrtanjem. Za ugotavljanje in določanje načina obrabe so se uporabile moderne in standardne metode. Izvedla se je analiza hribinskega materiala skozi katerega se je vrtalo, ter analiza mikro in makro strukture materialov kotalnega dleta. Analiza materialov se je izvedla z uporabo optičnega in elektronskega mikroskopa, simultane termalne analize jekla kotalnega dleta, določila se je kemijska sestava jekla kotalnega dleta in geomehanske karakteristike hribinskega materiala skozi katerega se je vrtalo. Izražena obraba kotalnega dleta se je kvantitativno in kvalitativno ovrednotila v povezavi z režimom vrtanja in karakteristikami hribine skozi katere se je vrtalo. Rezultati naše raziskave materialov kotalnega dleta lahko služijo kot dobra osnova za razvoj novih jeklenih zlitin, ki bodo prenašale visoke temperature, ki nastajajo med vrtanjem, brez strukturnih sprememb jeklenih materialov. Ključne besede: režim vrtanja, kotalno dleto, obraba, zmanjšani napredek vrtanja, sprememba lastnosti materiala. 3 Open Access. © 2020 Sporin J., published by Sciendo. |fee)r.^igTMl This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. 92 Introduction The time of effective drilling operations depends on the properties of the materials from which the roller cone bit components are manufactured. The roller bit will wear out due to the consequences of effects of the rock material it is drilling through and the drilling regime. The penetration rate decreases due to the wear of the bit. The following factors among all have a significant influence on the effective operation of the roller cone bit are the steel material from which the rollers and teeth of the bit are made, the drilling regime (load on the bit during drilling, torque, number of bit revolutions and the quantity and properties of the drilling fluid] and the properties of rock drilled. We analysed the mechanical properties of the steel material of the roller cone bit IADC 136, 155.57 mm (6 1/8"), which was drilling through carbonatic siltstone where sandstone plates and rare thin layers of clay and limestone exist in a total length of 87.89 m. Our research focused on the resistance of the steel material of the roller cone bit to the rock material using a selected drilling regime. To accomplish this, it was necessary to use scientific approaches to determine the causes that lead to wear of the bit and identify the weak points on the bit that need to be technologically modified in order to enhance the effective drilling time. Drilling Principle of the Roller Cone Drill Bit The roller cone bit contains cutting elements, i.e. teeth or inserts, which are mounted on the rollers. The rollers, which are inserted into the bearings of the bit, rotate around their axis. They are driven by rotating drilling rods that drives the body of the drill bit on which the roller is mounted. Nozzles (outlet] for the drilling fluid are attached to the bit body to effectively remove the rock particles from the borehole bottom of the borehole and to cool the rollers and teeth of the bit. The drilling fluid flows through the nozzles from the inside of the drill pipes into the area of the bit. When the orifice is reduced, which is the ratio of the cross-section of the drill bit to the cross-section of the nozzles, the velocity of the drilling fluid flow is increased significantly. The increasing velocity of the drilling fluid has a positive effect on the flow of the drilling fluid around the rollers, which effectively removes the drilled rock particles from the bottom of the borehole and contributes to the fracture of the borehole, especially in the softer, poorly bonded rocks [1-20]. The roller cone bit with steel teeth, which is investigated here, is used in softer, poorly bonded rock formations. The teeth of the rollers are large and sharp so that they can penetrate deep into the soft rock structures where the rock is crushed and removed from the crushed area. The teeth are protected with a carbide coating to improve durability, which increases the effective operating time. The axes of the rollers do not intersect at the point of the vertical axis of the bit, but do have an offset corresponding to the point of the vertical axis of the bit. The offset of the roller axis is normally in the range of 2°-5°. The load on tooth F, which causes the tooth to penetrate the rock (h), is a linear combination of the force acting against the surface at the tip of the tooth and the force acting against the surface formed by the inclined surfaces of the tooth. When rollers and bits with teeth on rollers are made of the same material, we could state the following [21]: F = (Ci-w + C2-h]-l-op, (1] where F is the load on a single tooth of the bit; C1 is the coefficient of friction on the surface at the tooth tip (-]; w is the tooth width (m]; C2 is the coefficient of friction on the tooth surface in contact with the rock (-]; h is the penetration depth of the tooth (m); l is the tooth length (m) and o is the compressive strength of the rock (Pa). ' The result of the reaction force F on the tooth of the bit in contact with the rock is equal to the load generated on the bit [22] and is given by: WOB = n t • F, (2) RMZ - M&G | 2020 | Vol. 67 | pp. 091-102 Sporin J. 93 where WOB is the weight on the bit (N) and nt is the number of teeth in contact with the rock. The volume of the crater of the crushed rock Vcrat (m3) created by one tooth, assuming that the crater has a conical shape, is stated by the following equation [22]: Vcrat = ~ ' n ' r' erat h, (3) where rcrat is the crater radius (m) and h is the crater depth (m). By determining the volume of a single crater formed by a single tooth, the penetration rate can be evaluated, which is defined by the volume of the rock craters on the entire cutting surface of the roller cone bit in number of revolutions [22] and can be stated as follows. ROP = ■ V, erat nt ■ RPM (4) 1bit where ROP is the rate of penetration (m/s); V is the volume of the crater of the removed crat rock (m3); RPM is the number of rotations of the bit (s-1) and Abtt is the cutting area of the bit (m2). As the bit advances through the rock, the teeth of the roller cone bit wear out. Further, the parameters that influence the penetration rate (ROP) of the roller cone bit are focused here to determine the wear of the bit. Bourgoyne and Young [23, 24] defined these as influential parameters as stated in Eq. (5). ROP = fi-fi-frfrfrfcfrL (5) where f is the effect of formation strength or rock durability; f is the effect of formation depth; f is the effect of formation compaction or pore pressure; f is the effect of differential pressure; f is the effect of bit diameter and bit weight; f is the effect of rotary speed; f is the effect of tooth wear and f8 is the effect of bit hydraulics. We investigated the wear of the roller cone bit, the occurrence of damage and the change in the properties of the bit materials under the conditions of the properties of the material drilled through, the drilling regime (weight on the bit, speed, etc.) and the hydraulics of the drilling fluid in the area of the bit. Materials and Methods When determining the characterisation of the wear of the roller cone bit at the time of the investigations, detailed analyses were made against the drilling regime, the materials from which the bit was made and the rock through which the bit was drilled. To achieve this purpose, in the first phase, we carried out a survey of the bit, which included an overview of the condition of the bit after drilling according to the IADC bit dull standard. In this part, we investigated the condition of the teeth and rollers of the bit after drilling through known rock. While drilling through the known rock, the following drilling parameters of drilling regime were monitored: — The penetration rate; — The length of the drilled interval; — The load on the bit; — The bit RPM; and — The amount, pressure and properties of the drilling fluid. We carried out a complete analysis of the steel materials of the roller cone bit, which included the following investigations: — Analysing the micro- and macrostructure of roll cone bit materials with an reflected light microscope (Olympus BX61 and Olympus SZ61 stereo microscope using the Analysis 6.0 image analysis system); — The composition of the carbide coating of bit teeth using the XRF (X-ray fluorescence) method (Thermo NITON XL3t XRF analyser); — A cross-sectional view of bit teeth using a scanning electron microscope (SEM) and EDS analysis (energy dispersive X-ray spec-troscopy) (Jeol JSM 5610); — A dilatometric analysis of tooth steel and tooth carbide coating with the low temperature dilatometer (Baehr-Thermoanalyse GmbH DIL 801); — Chemical analysis of the components of the teeth and rollers with an I CP (inductively coupled plasma) analyser (ICP-OES Agil-lent 720); Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime 94 — DSC (Differential Scanning Calorimetry] of tooth carbide coating and tooth steel with a thermal analyser (NETZSCH STA 449 C Jupiter] and — Vickers hardness tests of the tooth steel with a 100 g load using microhardness tester (Shi-madzu type M). The properties of the rock obtained by sampling were estimated based on the analysis of Rock Lab 1.0. Results The results of our analysis show the way and the mechanisms that cause wear of the bit material at fixed or established rock properties in Table 1. Drilling regime parameters Parameters Units Value connection with the operating parameters of the drilling. Drilling work Drilling was carried out with a 1992 N-1000 drill rig. The interval in which we observed the performance of the IADC 136 roller cone drill bit included a length of drilling interval from 1535.15 m to 1623.04 m depth, corresponding to a length of 87.89 m. The penetration rate during the observed interval ranged from 0.2 m/h to 0.4 m/h. The remaining drilling regime parameters are shown in Table 1. The 40 mass.% bentonite drilling fluid was used during drilling. The properties of the drilling fluid are shown in Table 2. Value 00-1623.04 Depth m 1535.15-1600.00 1600. Rotary speed rpm 35 45 Load on drill bit kN 30 40 Pump pressure MPa 5.5 5.5 Pump capacity m3/min 0.845 0.845 Table 2. The properties of drilling fluid Parameters Units Value Density kg/m3 1150-1170 Viscosity (Fann Funnel] s 45-48 Plastic viscosity (Fann Viscosimeter] mPas 16-18 pH - 9.5 Filtration mL/30 min 9.8-10 Rock Material Properties The rock material we drilled through was regularly sampled from drilling fluid. Each rock sample was examined and compared to a predetermined lithological column based on the results of geophysical and laboratory measurements in the surrounding boreholes. As far as the lithology of the material of the drilled interval concerned, silty claystone is dominant and layers of carbonatic siltstone with sandstone plates and thin layers of clay RMZ - M&G | 2020 | Vol. 67 | pp. 091-102 and limestone occur in the lower and upper parts of the sequence. The geomechanical properties of the drilled borehole section were evaluated based on experience. The strength properties were evaluated using the Hoek-Brown's criterion using RockScience software, Rock Lab 1.0. The estimated rock properties values for the carbonate silt are shown in Table 3. The estimated rock properties values for limestone are shown in Table 4. Sporin J. 95 Table 3. Results of the rock properties - carbonatic siltstone Table 5. Chemical analysis of the tooth material - body Parameters Units Value Elements Mas. % Cohesion MPa 0.722 Mn 0.79 Elastic module MPa 9375 Ni 0.74 Angle of internal friction o 20.94 Cr 0.60 Compressive strength MPa 25 Mo 0.50 Cu 0.27 Table 4. Results of the rock properties - limestone Si 0.26 Parameters Units Value C 0.17 Cohesion MPa 5.50 S 0.016 Elastic module MPa 28125 P 0.011 Angle of internal friction o 38.42 Compressive strength MPa 75 The results of the chemical analysis (Table 5) Roller Cone Bit Investigation After drilling and cleaning, the drill bit was examined using the IADC dull grading system. We found that the teeth of the drill bit were evenly worn out and several individual teeth were broken. There are no obvious erosion effects due to the rock particles in the drilling fluid that would result in erosion of the steel material of the bit. The condition of roller cone bit after the length of drilled interval of 87.89 m is shown by Figure 1. show that the steel material of the tooth body in this case is chromium, molybdenum, nickel low alloy steel, which is often used for case-hardened parts. At SEM, we analysed metallographically the steel base of the tooth (chromium, molybdenum, nickel low alloy steel), the contact area between the tooth body and the carbide coating and the carbide coating itself. We have determined the chemical composition of the steel of each component of the tooth through the analysis of EDS. Figure 2 (SEM) shows the areas where metallographic tests are performed on the teeth. Tables 6-9 show the results of the EDS analyses for each component analysed. Figure 1. Characteristic dull of the roller cone bit after drilled interval. Investigations of Tooth Steel Material We examined the steel material and the carbide coating of the teeth. The investigation of the chemical composition of the teeth material was carried out with an optical emission spectrometer (ICP). The results of the chemical composition of the examined steel (body) are shown in Table 5. Figure 2. Scanning electron microscope (SEM) image of tip of the bit tooth. Position 1 - carbide material; Positions 2 and 3 - carbide coating matrix; Position 4 - tooth body and Position 5 - mixed zone. Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime 96 Table 6. The elemental composition of carbide coating material (Figure 2 - Position 1) Elements Concentration at.% wt.% W 55.415 79.539 Co 42.380 19.500 Fe 0.807 0.962 Table 7. The elemental composition of the carbide coating matrix (Figure 2 - Position 2) Elements Concentration at.% wt.% Table 8. The elemental composition of the carbide coating matrix (Figure 2 - Position 3) Fe 86.096 68.599 W 1.709 29.094 Ni 1.921 1.608 Mn 0.892 0.699 Elements Concentration at.% wt.% Table 9. The elemental composition of the tooth body (Figure 2 - Position 4) Fe 86.117 68.616 W 11.103 29.124 Ni 1.511 1.265 Mn 1.269 0.995 Elements Concentration at.% wt.% Fe 95.427 95.385 Ni 3.164 3.323 Mn 0.660 0.649 Mo 0.218 0.375 Si 0.531 0.267 The elemental composition of the carbide coating material (Figure 2 - Position 1), which is determined by EDS, is shown in Table 6. The elemental composition of the carbide coating matrix (Figure 2 - Position 2), which is determined by EDS, is shown in Table 7. The elemental composition of the carbide coating matrix (Figure 2 - Position 3), which is determined by EDS, is shown in Table 8. Figure 3. DSC heating curve of the steel of the tooth body. Figure 4. DSC heating curve of carbide coating. The elemental composition of the tooth body (Figure 2 - Position 4), which is determined by EDS, is shown in Table 9. We carried out a simultaneous thermal analysis (STA) (Figure 3) of the steel sample of the tooth body. The curve shows that eutectoid transformation in the solid state starts at a temperature of 693.9°C and also there is an endother-mic peak. The process ends at 800°C when the metal matrix (iron) is transformed into a gamma-phase iron structure - austenite (g-Fe). The temperature 1407.2°C is the low temperature at which the eutectic structure or alloy begins to melt. The remaining material of the metal matrix (austenite) begins to melt at 1474.5°C. The temperature of 1474.5°C is also the solidus temperature of the steel from which the tooth body is made. The DSC (Differential scanning calorimetry) heating curve of the carbide coating of the teeth in the roller cone bit is shown in Figure 4. The carbide coating contains a thin layer of the mixing zone. The mixing zone is the area formed during the welding of the carbide coating to the tooth. It contains the properties of the steel of the tooth body and the carbide coating. At the RMZ - M&G | 2020 | Vol. 67 | pp. 091-102 Sporin J. 97 Figure 5. Dilatometric heating curve of the steel of the carbide coating and the steel of the tooth: 1 - tooth steel material (blue), 2 - carbide coating (black). temperature 692.5°C, the starting of the eutec-toid transformation was detected. This transformation is related to the layer of the mixed zone. Therefore, this curve is not typical for carbide alloys. At the temperature of 1467.3°C, the starting of melting process was detected. Figure 5 shows the dilatometric curve of the steel of the carbide coating and the steel of the tooth body. It can be seen from the heating curve of the steel of the tooth body that the temperature of the eutectoid transformation increases linearly. The slope of the curve corresponds to the linear expansion coefficient for chromium, molybdenum and nickel low alloy steel, while the slope of the heating curve of the sample of the carbide coating containing a thin layer of the mixing zone is small. A lower curve angle of the carbide coating sample containing a thin layer of the mixing zone is the result of a lower coefficient of thermal expansion. In view of the fact that the temperatures can locally reach up to 500°C during drilling despite intensive cooling, the curves show that the difference between the expansion properties is 0.05%. Because of this difference, the mixing zone is the preferred area for the formation of internal stresses and hence suitable for the formation and propagation of micro-fractures. The diagram shows the beginning of the eutectoid transformation of the tooth body steel sample, which starts at 680°C (A ) and ends at a temperature of 800°C (A 3). For the carbide coat- Table 10. Results of the Vickers hardness test Diagonals (mm) d1 d2 Hardness - (HV0.5) Average Note 37.23 46.24 532.3198 36.22 37.55 681.5125 36.4 40.81 622.1374 37.99 41.33 589.4786 37.54 41.66 591.2662 Tip of the tooth 37.43 39.54 626.0232 594.5670 37.57 39.86 618.6071 36.33 42.41 598.1947 38.87 42.45 560.8396 38.48 42.45 566.2580 39.01 42.84 553.6000 44.99 47.95 429.3664 44.44 48.37 430.5701 Tooth body 44.61 47.78 434.4937 433.5184 44.09 47.10 446.0042 45.14 48.04 427.1575 33.28 37.13 748.1087 31.73 34.04 857.3887 Carbide 33.64 36.53 753.2349 784.1543 coating— 34.58 36.51 733.8654 matrix 30.41 36.51 828.1739 24.67 25.96 1446.830 Carbide 24.43 25.78 1471.137 1458.984 coating— base ing sample containing a thin layer of the mixed zone, eutectoid transformation starts at 750°C. On the curve of the carbide coating sample containing a thin layer of the mixing zone, a deviation is observed which can be associated with the eutectoid transformation of the thin layer of the mixing zone, but the temperature is slightly higher. The reason for this higher temperature is due to the chemical composition of the mixing zone. A Vickers hardness test was performed to determine the changes in the hardness properties of the tooth due to the effects of drilling. The measurement points are shown in Figure 6 and the results of the tests are shown in Table 10. The average hardness of the steel of the tooth body is 433.5184 HV. The average hardness of Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime 98 _ivgm toyftho lwlh ■ ccdLng ', Joeth brfy i coaflins - nr-ïtrix \ Figure 6. Display of measurement positions of Vickers hardness tests. the steel at the tip of the teeth is much higher and is 594.5670 HV. The hardness of the carbide coating is, as expected, much higher and reaches up to 1471,137 HV. Discussion Monitoring of the operating parameters during drilling revealed that the roller cone bit was not loaded in accordance with the manufacturer's recommendations, which recommends that the bit load should be in the range of 15-27 kN. The load on the bit during drilling was between 30 kN and 40 kN. In addition, the number of bit rotations was too low and they were between 35 rpm and 45 rpm. The recommended number of bit rotations for this type of tool is between 60 rpm and 100 rpm. It can be noted that the bit was overloaded with weight and the number of bit rotations was too low. Examination of the microscopic image showed that the tips of the teeth were exposed to high temperatures and stresses. The influence of high temperatures and loads is shown in Figure 7. Figure 7 shows the tip of the tooth that was in contact with the rock. We can see changes in the colour of the steel, which changes from blue tones (at the point of contact between the rock and the steel] to brown tones that change the light from top to bottom. Figure 7. Microscopic photo of the tip of the tooth. Figure 8. A view at the tip of tooth that was in contact with the rock. Because of the apparent colour change previously observed under the optical microscope, the sample was examined with a scanning electron microscope (SEM). A characteristic view of the microstructure of the area with a SEM is shown in Figure 8. Figure 8 shows the condition of the tooth tip that was in contact with the rock during drilling. A change in the microstructure of the steel on the surface was observed. The change in the steel structure can be observed in two layers. The first layer, where the change is noticeable, is up to a thickness of 36 ^m. The second layer, in which the change in microstructure is less pronounced but still noticeable, moves to a depth of up to 92 ^m. The change in the steel microstructure of the tooth was due to the high temperatures and RMZ - M&G | 2020 | Vol. 67 | pp. 091-102 Sporin J. 99 mechanical stresses that occurred during drilling when the tip of the tooth was in contact with the rock. When the roller cone bit is rotated around its axis, the teeth got cooled. The teeth that were not in contact with the rock were cooled by the drilling fluid flowing out from the nozzles of the bit. In this situation, a temperature change occurs on the surface of the teeth when their temperature is very high and they are washed with drilling fluid at a lower temperature. The other reason for microstructural changes in the steel is that the bit was overloaded during drilling and the number of revolutions was too low when compared to the load. The consequence of such a drilling regime is the increase in temperature of the material at the tooth tip, which in combination with the load results in the hardening of the material at the tooth tip. Because of this reason the hardness of the steel material increased. The measured hardness (Vickers hardness] at that time was on average about 160 HV higher than the hardness of the steel material in the middle of the tooth. Using differential scanning calorimetry (DSC], we found that the temperature of the eutectoid point is 699.2°C and its completion occurs at a temperature of 800°C. Due to the intensive cooling of the teeth under the influence of the drilling fluid, the microstructural change process occurred only in the steel layer down to a depth of 92 ^m. Microstructural changes are only noticeable on the upper side of the teeth, while no changes were observed on the sides. The changes in the steel structure occurred mainly due to excessive operating temperatures and loads on the selected steel. As demonstrated by DSC and dilatometric analysis, the temperature at the beginning of the transformation from ferrite to austenite is about 680°C for the selected steel. This means that the temperature of 680°C (Ac1] is a point at which recrystalli-sation of the steel is rapidly accelerated. The recrystallisation temperature for steel, when calculated for theoretical information, is somewhere around 0.4 x TL (TL - liquidus temperature, °C) [25]. Using the software Thermo-Calc for thermodynamic modelling of the equilibrium phase, we calculated the liquidus temperature, which was 1511°C for the selected steel. Therefore, the recrystallisation temperature Figure 9. Photography of tooth steel material. Position 1 -carbide coating; Position 2 - mixed zone and Position 3 -tooth steel (body). for the analysed steel is about 604°C. We also calculated the equilibrium eutectoid temperature Ae1 and the transition temperature Ae3 with the same software. The temperature of t6he former was 685°C and the latter was 811°C. The equilibrium eutectoid temperature A and the transition temperature Ae3 were also calculated. The calculated values are similar to the values obtained from the dilatometric analysis of the steel (680°C and 800°C), assuming that the temperatures obtained from the calculation are in equilibrium. This confirmed the results of the dilatometric analysis. Due to the influence of high temperatures and rapid cooling, the carbide coating on the upper surface of the tooth gets decayed. The difference between the temperature-expansibility coefficients of the steel material and the carbide form was found by a dilatometric test at low temperature. This difference is significant as it causes increase in internal stress in the mixed zone. The mixed zone is formed by welding the carbide coating onto the tooth steel and represents a mixture of dissolved steel material of the tooth and the carbide coating (Figure 9]. In the mixed zone, during the heating and cooling processes, internal stresses begin to increase due to the different temperature coefficients of the materials, which lead to initiation and propagation of cracks. Cracks were observed on the edges of the teeth, which progressed through the carbide coating to the steel of the tooth (body] (Figure 10]. The occurrence of such cracks can be attributed Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime 100 Figure 10. Photography of cracks through carbide coating. Figure 11. Erosion channels at the top of the tooth. to the excessive load on the bit during drilling and the stress accumulation due to the weight load of the bit. The carbide coating is tough and erosion resistant, but brittle, which causes it to crack under excessive load and lateral forces. Due to the formation of such cracks at the tooth edges, the carbide coating is sheared or eroded, which resulted in a chipped tooth. The erosion effect caused by aggressive particles in the drilling fluid could not be expressed comprehensively so far, as only a short length of 87.89 m was drilled with this drill bit. Microscopic images show smaller erosion microchannels, which are not largely pronounced (Figure 11). Erosion microchannels occur only in the area of the tooth tip. There are no erosion channels along the tooth edges that are not protected by carbide coatings, which is partly due to the geometry of the teeth. Here, the teeth sides are quite steep, which means that the drilling fluid with abrasive particles leaves the tooth area within a short time. Conclusions In this article, we have described the wear of the IADC 136 roller cone bit drilled into a known standard rock material. The bit wear was characterised by the analysis of the different metallurgical properties of the steel and the carbide coating of the teeth of the bit, which are related to the drilling regime and the properties of the rock material. During drilling, the teeth of the bit are heated when are in contact with the rock and then cooled by the influence of the drilling fluid. At this stage, there is a compressive load on both the steel of the tooth and its carbide coating. Internal stresses developed in the materials of the bit teeth due to the load on the bit can expand through the elastic zone and reach the plastic zone, which results in local dissolution of the carbide coating and a local increase in the hardness of the steel material. Using an IADC 136 roller cone bit, the length of interval 87.89 m was drilled into a carbonatic siltstone with sandstone plates and thin layers of clay and limestone. The wear of the teeth of the roller cone bit was mainly observed at the tooth tip. During the systematic investigation of the steel structure of the teeth and the carbide coating, it was found that the structure of the steel material at the tip of the teeth was partially changed by the influence of high deformations. This change, which was reflected in the increase in the hardness of the steel material, was higher at the tip of the tooth which is in contact with the rock by 160 HV than at the tooth body. The change in the hardness of the steel material, expressed as a layer parallel to the tooth tip, extends to a depth of about 36 mm. The formation of this layer with a higher degree of hardness can be explained by the implementation of the weight on the drill bit (WOB), which was higher than the recommended and relatively low rotation (RPM), and it was lower than the recommended. The increase in the temperature of the tooth material during drilling was due to excessive loads and inefficient cooling, resulting in micro-fatigue reaching the RMZ - M&G | 2020 | Vol. 67 | pp. 091-102 Sporin J. 101 plastic zone of the steel material at the tip of the tooth. The tip of the tooth was worn out by a modified microstructure due to friction along the rock, and underneath it, a new layer with a modified microstructure formed periodically. From this, it can be concluded that the correct choice of roller cone bit can be recommended for a specified type of rock and operating parameters (WOB, RPM, drilling fluid] since it has a significant influence on the wear of the roller cone bit and thus on the extension or reduction of the effective operating time of the roller cone bit. The results of our investigation of the materials of the roller cone bits can provide a base and guidelines for the development of new steel alloys that can withstand higher temperatures and allow effective drilling without structural changes in the steel material. References [1] Al-Sudani, J.A. (2017): Real-time monitoring of mechanical specific energy and bit wear using control engineering systems. Journal of Petroleum Science and Engineering, 149, pp. 171-182, DOI: 10.1016/j. petrol.2016.10.038. [2] Bingham, M.G. (1965): A New Approach to Interpreting Rock Drillability. Petroleum Publishing Co.: Tulsa, OK, USA, 93 p. [3] Cheatham, J.B. (1958): An analytical study of rock penetration by a single bit tooth. In; Proceedings of the 8th Drilling and Blasting Symposium, Minnesota, MN, USA. [4] Dagrain, F., Lamine, E., Delwiche, R., Golard, N. (2013): Characterization of the performances of small diameter drill bits for the optimization of the drilling parameters. In: Proceedings of the 2nd International Conference on Stone and Concrete Machining (ICSCM), Dortmund, Germany. [5] Dutta, P.K. (1972): A theory of percussive drill bit penetration. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 9, pp. 543-567, DOI: 10.1016/0148-9062(72)90044-7. [6] Eremin, E.N., Yurov, V.M., Guchenko, S.A., Laurynas, V.C., Kasymov, S.S. (2016): Antifriction Superhard Coatings for Drill Bits and Boring Cutters. Procedia Engineering, 152, pp. 608-612, DOI: 10.1016/j.pro-eng.2016.07.663. [7] Evans, I., Murrell, S. (1962): Wedge penetration into coal. Colliery Engineering, 39, 11. [8] He, W., Chen, Y., He, J., Xiong, W., Tang, T., OuYang, H. (2016): Spherical contact mechanical analysis of roller cone drill bits journal bearing. Petroleum, 2, pp. 208-214, DOI: 10.1016/j.petlm.2016.03.002. [9] Jones, H.G., Norgren, S.M., Kritikos, M., Mingard, K.P., Gee, M.G. (2017): Examination of wear damage to rock-mining hardmetal drill bits. International Journal of Refractory Metals and Hard Materials, 66, pp. 1-10, DOI: 10.1016/j.ijrmhm.2017.01.013. [10] Karasawa, H., Ohno, T., Miyazaki, K., Eko, A. (2016): Experimental results on the effect of Bit wear on torque response. International Journal of Rock Mechanics and Mining Sciences, 84, pp. 1-9, DOI: 10.1016/j.ijrmms.2016.01.013. [11] Ma, D.K., Yang, S.L. (1985): Kinematics of the Cone Bit. Society of Petroleum Engineers Journal, 25, pp. 321-329, DOI: 10.2118/10563-PA. [12] Maurer, W.C. (1965): Bit-Tooth Penetration Under Simulated Borehole Conditions. Journal of Petroleum Technology, 17, pp. 1433-1442, DOI: 10.2118/1260-PA. [13] Naganawa, S. (2005): Dynamics modeling of roller cone bit axial vibration. Journal of the Japanese Association for Petroleum Technology, 70, pp. 333-346, DOI: 10.3720/japt.70.333. [14] Naganawa, S. (2012): Feasibility study on roller-cone bit wear detection from axial bit vibration. Journal of Petroleum Science and Engineering, 82-83, pp. 140-150, DOI: 10.1016/j.petrol.2012.01.014. [15] Njobuenwu, D.O., Wobo, C.A. (2007): Effect of drilled solids on drilling rate and performance. Journal of Petroleum Science and Engineering, 55, pp. 271-276, DOI: 10.1016/j.petrol.2006.08.012. [16] Olsson, M., Yvell, K., Heinrichs, J., Bengtsson, M., Jacobson, S. (2017): Surface degradation mechanisms of cemented carbide drill buttons in iron ore rock drilling. Wear, 388-389, pp. 81-92, DOI: 10.1016/j.wear.2017.03.004. [17] Paul, B., Sikarskie, D.L. (1965): A preliminary model for wedge penetration in brittle materials. Transactions of the American Institute of Mining Engineers, 232, pp. 373-383. [18] Warren, T.M. (1981): Drilling Model for Soft-Formation Bits. Journal of Petroleum Technology, 33, pp. 963-970, DOI: 10.2118/8438-PA. [19] Warren, T.M. (1987): Penetration Rate Performance of Roller Cone Bits. Society of Petroleum Engineers, 2, pp. 9-18, DOI: 10.2118/13259-PA. Characterisation of the Wear of the Roller Cone Drill Bit Caused by Improperly Chosen Drilling Regime 102 [20] Zhao, J., Zhang, G., Xu, Y., Wang, R., Zhou, W., Yang, D. (2018): Experimental and theoretical evaluation of solid particle erosion in an internal flow passage within a drilling bit. Journal of Petroleum Science and Engineering, 160, pp. 582-596, DOI: 10.1016/j.pet-rol.2017.10.068. [21] Hareland, G., Wu, A., Rashidi, B.A. (2010): New Drilling Rate Model for Tricone Bit and Its Application to Predict Rock Compressive Strength. In: 44th US Rock Mechanic Symposium and 5th U.S.-Canada Rock Mechanics Symposium, Salt Lake City, UT, USA. [22] Rashidi, B., Hareland, G., Wu, Z. (2015): Performance, simulation and field application modeling of rollercone bits. Journal of Petroleum Science and Engineering, 133, pp. 507-517, DOI: 10.1016/j.pet-rol.2015.06.003. [23] Bourgoyne, A.T., Young, F.S. (1974): A Multiple Regression Approach to Optimal Drilling and Abnormal Pressure Detection. Society of Petroleum Engineers Journal, 14, pp. 371-384, DOI: 10.2118/4238-PA. [24] Bourgoyne, A.T., Millheim, K.K., Chenevert, M.E., Young, F.S. (1991): Rotary Drilling Bits. In: Applied Drilling Engineering. SPE: Richardson, TX, USA, pp. 190-245. [25] Totten, G.E. (2006): Steel Heat Treatment: Metallurgy and Technologies, 2nd ed. CRC Press: Boca Raton, Florida, USA, 848 p. RMZ - M&G | 2020 | Vol. 67 | pp. 091-102 Sporin J. 103 Review paper Received: Aug 17, 2020 Accepted: Sep 07, 2020 DOI: 10.2478/rmzmag-2020-0012 Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils in Geotechnology and Mining Applications Laboratorijske preiskave abrazivnosti kamnin in zemljin na področju geotehnologije in rudarstva B. Janc*, V. Jovičič, Ž. Vukelič University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geotechnology, Mining and Environment, Aškerčeva 12, Ljubljana, Slovenia * blaz.janc@ntf.uni-lj.si Abstract In geotechnology and mining, tools and equipment interact with aggressive geological material, causing the wear of these components. For this reason, it is important to determine the rate of abrasivity of individual geological materials, depending on the type of interaction with the tool. Various abrasivity tests have been developed in laboratories. Some of them are general, while others are special. What they all have in common is that they attempt to determine the abrasivity of rocks or soils in relation to the wear of the test specimens. This article gives an overview of the laboratory test methods for assessing the abrasivity of geological materials, which are useful in the field of geotechnology and mining engineering. General and special abrasivi-ty tests are presented in detail. The aim of the article is to present existing laboratory tests to assess the abrasivity of rocks and soils, based on which further investigations of wear can be considered as part of a comprehensive approach to this tribological problem. Understanding of the wear mechanisms is the basis for the development of wear-resistant tools and models for predicting the tool life. Key words: laboratory testing, abrasivity, wear, geote-chnology, mining. Povzetek V geotehnologiji in rudarstvu so orodja in oprema v interakciji z agresivnim geološkim materialom, ki povzroča obrabo omenjenih komponent. Zaradi tega obstaja težnja po določevanju stopnje abrazivnosti posameznih geoloških materialov glede na vrsto interakcije z orodjem. V laboratorijih so bile razvite različne preiskave abrazivnosti. Ene od njih so splošne, druge namenske. Vsem pa je skupno to, da skušajo določiti abrazivnost kamnin ali zemljin glede na obrabo preizkušanca. V članku je predstavljen pregled laboratorijskih preiskav abrazivnosti geoloških materialov, ki so uporabne na področju geotehnologije in rudarstva. Splošne in namenske preiskave abrazivnosti so podrobno predstavljene. Namen članka je predstaviti obstoječe laboratorijske preiskave abrazivnosti kamnin in zemljin, na podlagi katerih je mogoče nadaljnje preiskovanje obrabe kot del celovitega pristopa pri tem tribološkem problemu. Razumevanje mehanizmov obrabe je podlaga za razvijanje orodij, odpornih proti obrabi ter modelov za napovedovanje njihove življenjske dobe. Ključne besede: laboratorijsko preiskovanje, abraziv-nost, obraba, geotehnologija, rudarstvo 3 Open Access. © 2020 Janc B., Jovicic V., Vukelic Z., published by Sciendo. |feai >a.-[.o7FM| This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. 104 Introduction Geotechnology as an engineering discipline is confronted with many cases in which tools and equipment in dynamic contact with geological material get worn out. They are exposed to high compression, shear and impact loads. The result of these loads is material damage to the tools and equipment used in the technological process, which gradually reduces their functionality. The areas of geotechnology where the wear of tools and equipment are most pronounced (Figure 1] can be divided into the following four main groups: 1] Tunnelling 2] Drilling 3] Mining 4] Mineral processing The first group includes the tunnelling sector, which deals with the construction of road and rail tunnels, as well as tunnels for water transport and sewerage. Tunnelling can be carried out conventionally by drilling and blasting or mechanised with tunnel boring machines (TBMs] or excavation machines (roadheaders]. Both the excavation methods lead to tool wear. In the conventional method, the drill bits are worn out when drilling boreholes, whereas in the mechanised method the disc cutters on the TBM or the teeth on the head of the excavation machine (roadheader] are worn out. The second group includes the drilling sector, which deals with the drilling of production, exploratory or technical boreholes. Production wells are intended for the production of minerals and energy, such as oil, gas and geother-mal energy. Exploratory boreholes are used for geological-geomechanical investigations. Technical boreholes are used for rock blasting and anchoring geotechnical constructions. All these operations, during which percussion or rotary drilling is performed, cause wear on the teeth or inserts of the drill bits, which are the main tool for drilling boreholes. The third group includes the mining sector, which deals with the surface or underground extraction of mineral resources, including metals, non-metals and energy resources. In the mining sector, wear occurs mainly in two RMZ - M&G | 2020 | Vol. 67 | pp. 103-117 areas: the extraction and transport sectors. In the extraction sector, tools are worn on mechanised mining excavation machines such as the longwall shearer, continuous miner and bucket wheel excavator. In the transport sector, wear occurs on transport machines and equipment. The fourth group includes the mineral processing sector, which deals with the processing and preparation of mineral resources for use in various industrial processes. Most pronounced is the wear of equipment used to increase the specific surface area of the mineral resources. This includes crushing and grinding machines. Wear occurs on parts of the equipment that are in contact with the mineral. These are usually plates in various crushers and grinding media in mills. In all the four groups described above, it is possible to find the occurrence of material wear that forms a tool or equipment that is in dynamic contact with geological material. In most cases, the materials of the tools and equipment are steels which, due to their wide availability and economic acceptance, currently have the most suitable wear resistance. Abrasivity and Tribological System Abrasivity plays an important role in the process of wear. It is a property that reflects the ability of a rock (geological material] to cause wear on materials [1]. The tribological system combines all components necessary for the wear process. These components are as follows: 1] Geological material 2] Tool material 3] Surrounding medium 4] Type of load (dynamic interaction] A change in any of the above components also changes the mode and rate of wear. Figure 2 schematically shows a tribological system with all four components. There are several methods for estimating the abrasivity of rocks and soils. In general, three categories can be distinguished [2]: 1] Indirect methods - assessment of abrasivity by a combination of different geomechanical properties such as hardness, uniaxial compres-sive strength, tensile strength and other fundamental properties. Janc B., Jovicic V., Vukelic Z. 105 Figure 1: Wear of tools in geotechnology and mining engineering. The most typical devices for each field are shown (adapted from [3-16]). Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils 106 Figure 2: Tribologicalsystem (schematic). 2] Direct methods - assessment of abrasivity by laboratory test methods where there is relative movement between the geological sample and the wear tool in contact under the influence of controlled standard conditions. 3] Holistic methods - assessment of abrasivity by a combination of the fundamental geome-chanical properties of the geological material and other parameters of the process and environment in which the wear occurs. Indirect methods for assessing abrasivity have the advantage of using data that are already available or are relatively easy to obtain. However, they do not take into account the process variables for certain types of wear. Therefore, they are usually not used individually but in combination with direct or holistic methods. The holistic methods for assessing abrasivity are very much related to certain equipment and processes and, therefore, have less value for other applications. The direct methods for assessing abrasivity use the standard conditions to determine the abrasivity for a specific type of wear, either low or high stress or impact abrasion. The tests investigate the effects of relative movements between the geological sample and a wear tool in contact [2]. Laboratory Test Methods for Assessing the Abrasivity of Geological Material Researchers have developed various laboratory devices to test the abrasivity of individual rocks and soils directly in the laboratory. The test methods differ depending on the design of the device or apparatus, the duration of the test, the method of causing wear, the shape and size of the abrasive material and the wear tool, and the purpose of the test. Some abrasivity tests are general and standardised to some extent. However, most of the tests are special and were intentionally developed for certain technological processes described in the introductory part of this article. It is worth noting that by far the largest number of abrasivity tests have been developed for mechanised tunnelling. A general test method for assessing the abrasivity of geological material includes the CERCHAR abrasivity test for rocks and the LCPC abrasivity test for soils or grain materials. Both tests have been developed in France and are standardised. Special test methods for assessing the abrasiv-ity of geological material are those developed for a specific application, such as tunnelling, drilling and mineral processing. The group of tests for tunnelling includes NTNU/SINTEF (AV, AVS and SATTM) and RIAT (rolling indentation abrasion test] abrasivity tests, which were developed for the estimation of tool wear in mechanised tunnelling. These abrasivity tests are not yet standardised. Miller abrasive test is used to determine the abrasivity of slurries and is standardised (ASTM G75 standard]. The Gouging abrasion test can be used to estimate the wear of machine parts during crushing and grinding processes. The test is not standardised. It is also worth mentioning some other abrasivity tests that fall within the field of road construction. These are the (standardised] Los Angeles abrasion test, the Nordic ball mill test and the Dorry abrasion test, which all test the wear resistance of road aggregates. CERCHAR Abrasivity Test The CERCHAR abrasivity test was originally developed by a French laboratory (Laboratoire du Center d'Etudes et Recherches des Charbonnages de France] for mechanised coal mining. RMZ - M&G | 2020 | Vol. 67 | pp. 103-117 Janc B., Jovicic V., Vukelic Z. 107 This method is described by two standards, the French standard AFNOR NF P 94-430-1 and the International standard ASTM D7625-10 [17]. Apparatus There are two types of testing apparatus. The first is the original design as developed at the CERCHAR centre and the second is a modified design as reported by West [17]. In the CERCHAR apparatus (Figure 3), the loaded stylus moves across the surface of a stationary rock sample. In the case of the West design (Figure 4), however, the rock sample moves under the loaded stationary stylus. Different implementations of the apparatus design lead to a test duration [17]. Both apparatus have a rigid vice for firmly clamping the rock sample. Sufficient rigidity of the apparatus ensures that there is no lateral pin chuck Figure 3: CERCHAR-type testing apparatus (adapted from [17]). movement during the test. The stylus is loaded with a weight of 70 N [17]. Stylus The stylus is made of standard chrome-vanadium cold-work tool steel and is hardened to Rockwell hardness HRC 55 ± 1. The stylus diameter should be at least 6 mm. The length of the stylus should be such that the visible part of the stylus between the pin chuck and the surface of the rock sample is at least 15 mm. The pin of the stylus (Figure 5) has a conical shape with an angle of 90°. The worn pin should be resharpened and examined under a microscope before being used for further testing [17]. \_/ Figure 5: Steel pin. (a) Before the test. (b) After the test (d is the wear flat) (adapted from [18]). Test Sample The rock sample can be circular or irregularly shaped. A test on freshly broken rock surface is recommended. The rough surface can be obtained by a Brazilian tensile test (Figure 6) on the rock disc or by firm hammer blows on a rock core or rock sample. Alternatively, the sample can be prepared with a water-cooled diamond saw blade. The surface to be tested should be cleared of debris or loose grains of rock. The size of the rock surface should be suf- Figure 4: West-type testing apparatus (adapted from [17]). b) Figure 6: The two halves of the sample disc with test scratches (1-5) after Brazilian tensile test. (a) Rough surface. (b) Smooth surface (adapted from [20]). Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils 108 ficient to allow five test scratches at a minimum distance of 5 mm and 5 mm from the edge of the rock surface [17]. Test Procedure A microscopic inspection of the stylus should be performed before the test. The rock sample should be firmly clamped in the vice with the rock surface as horizontal as possible. The stylus should be carefully lowered onto the rock surface. The length of a test scratch in the rock sample must be exactly 10 mm. Depending on the apparatus design used, the test duration is 1 (CERCHAR version) or 10 s (West version). The stylus pin must be in constant contact with the rock surface during the test [17]. The main parameters of the CERCHAR abrasivity test are summarised in Table 1. Table 1: Main parameters of the CERCHAR abrasivity test. Parameters Value Load (N) 70 Stylus hardness (HRC) 55 ± 1 Test scratch length (mm) 10 Sample material Rock (freshly broken surface) CERCHAR WEST Test duration (s) 1 10 Test result CERCHAR abrasivity index (CAI) At least five test repetitions must be carried out on the rock surface, each time with a new or re-sharpened pin [17]. Stylus Wear Measurement The measurement of the length or diameter of the pin wear (d) is carried out with optical and digital methods (microscope). The measurement of the pin can be performed in the side or top view. However, it is recommended to measure in the side view [19]. Figure 7 shows the possible measurements of pin wear. When measuring in the top view, the mean value between d1 and d2 is taken for d. a) b) Figure 7: Measurements of pin wear. (a) Side view (d is the distance between the edges of the worn surface). (b) Top view (d1 and d2 are diameters of the worn pin, measured from two orthogonal directions) (adapted from [17]). Calculation of CERCHAR Abrasivity Index For each measurement of pin wear (d), the CERCHAR abrasivity index (CAI) is calculated as CAI = d x 10 (1) where d is the length of the wear pin surface measured with an accuracy of 0.01 mm [17]. The dimensionless CAI value is given as the arithmetic mean of five or more test repetitions together with the standard deviation [17]. Classification System The system for classifying abrasivity after ISRM [17] is given in Table 2. This system differs slightly from that proposed by the CERCHAR centre. Table 2: Classification of the CERCHAR abrasivity index (CAI) [17]. Mean value Classification/abrasivity of CAI description 0.1-0.4 Extremely low 0.5-0.9 Very low 1.0-1.9 Low 2.0-2.9 Medium 3.0-3.9 High 4.0-4.9 Very high >5 Extremely high RMZ - M&G | 2020 | Vol. 67 | pp. 103-117 Janc B., Jovicic V., Vukelic Z. 109 LCPC Abrasivity Test The LCPC abrasivity test was developed in the 1980s by a French laboratory (Laboratoire Central des Ponts et Chaussées] to test soil abrasivity. The method is described in French Standard P18-579 [21]. Test Device The main components of the test device for determining abrasivity are the frame, the motor, the funnel tube, the metal impeller and the sample container. The device is shown schematically in Figure 8. The 750 W motor ensures the rotation of the axle. A metal impeller is attached to the end of the axle and is immersed in a sample container, which is cylindrical in shape with a diameter of 93 mm and a height of 100 mm. The metal impeller (test specimen] has a rectangular shape with dimensions motor funnel tube metal impeller sample container Test Sample The LCPC testing device is designed to investigate the abrasivity of granular materials with a grain size of 4-6.3 mm. According to French Standard P18-579 a sample of 500 g ± 2 g of a dry material with a grain size of 4-6.3 mm is required. The required grain size of the sample is achieved by sieving. Fractions below 4 mm and above 6.3 mm must not be used in the test [21]. Test Procedure The sample container is filled with the required granular material through the funnel tube of the testing device. The rectangular metal impeller rotates for 5 min in a sample container at 4,500 rpm. To determine the abrasivity of the sample material, it is necessary to determine the mass of the metal impeller before and after the test. The mass loss of a metal impeller is a measure of the abrasivity of the sample. With the loss of mass, the metal plate also deforms. The more abrasive the material sample is the greater the deformation and mass loss of the impeller [21]. Figure 9 shows schematically the metal impeller before and after the LCPC test. The main parameters of the LCPC abrasivity test are summarised in Table 3. Table 3: Main parameters of the LCPC abrasivity test. Parameters Value Rotational speed (min-1) 4,500 Sample material Soil/granular material Grain size of the sample (mm) 4-6.3 Sample mass (g) 500 ± 2 Test duration (min) Figure 8: LCPC abrasivity testing device with main components (adapted from [18]). 50 mm x 25 mm x 5 mm. The impeller is made of standardised steel with a Rockwell hardness of HRB 60-75. The steel impeller must be replaced after each test [21]. Test result LCPC abrasivity coefficient (LAC) LCPC breakability coefficient (LBC) Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils 110 Figure 9: Schematic representation of metal impeller before and after the LCPC test: 1 new impeller, 2 not abrasive, 3 abrasive/very abrasive, 4 very abrasive and 5 extremely abrasive (adapted from [21]). Calculation of LCPC Abrasivity Coefficient The LCPC abrasivity coefficient (LAC] is calculated as the coefficient of mass loss of the metal impeller and the sample mass [21] as LAC = mn — m M (2) where LAC is the LCPC abrasivity coefficient (g/t), m0 is the mass of the steel impeller before the LCPC test (g), m is the mass of the steel impeller after the LCPC test (g) and M is the mass of the sample material (=0.0005 t). The LAC for natural rocks and soils varies between 0 and 2,000 g/t. The value range can be divided into five classes. As there is a close linear correlation (Figure 10) between the LAC and the CAI index, the CAI abrasivity classification can be used [21]. 2000- 1500- 3 1250 O S? < O CL ü 750 500- 2500 Abrasivity term CAI extremely abrasive 4-6 8 9 y O ( i/o ^ o o very abrasive 2-4 < / - 0 nC 0 medium abrasive 1-2 cO O slightly abrasive 0.5-1 2 3 4 5 6 CERCHAR Abrasivity Index (CAI) [ ] Figure 10: Correlation between LAC and CAI [21]. Classification System The abrasivity classification system in relation to the CAI is given in Table 4 [21]. Table 4: Classification of LCPC abrasivity coefficient (LAC) in relation to the CERCHAR abrasivity index (CAI) [21]. LAC (g/t) CAI (0.1 mm) Abrasivity term 0-50 0- 0.3 Not abrasive 50-100 0.3 -0.5 Not very abrasive 100-250 0.5- -1.0 Slightly abrasive 250-500 1.0 -2.0 (Medium) abrasive 500-1,250 2.0- -4.0 Very abrasive 1,250-2,000 4.0- -6.0 Extremely abrasive NTNU/SINTEF Abrasivity Tests NTNU/SINTEF abrasivity tests have been developed at the Norwegian University of Science and Technology in Trondheim (NTNU-Norges teknish-naturvitenskapelige universitet] in cooperation with the research organisation SIN-TEF (Stiftelsen for industriell og teknisk forsk-ning). The AV abrasivity test was developed in the 1960s to evaluate the wear of drill bits. It provides a measure of rock abrasion or the ability to induce wear on a tungsten carbide specimen [22]. The AVS abrasivity test was developed between 1980 and 1983 based on the AV test. The purpose of the test is to evaluate the tool life of mechanised rock TBM tunnelling. Compared to the AV test, the AVS test differs in the specimen material and the duration of the test. The AVS is a measure of rock abrasion or the ability to induce wear on cutter ring steel [22]. Soil abrasion test (SATTM] was introduced in 2005 to evaluate the tool life of mechanised TBM tunnelling in soft ground and in soil. The test is a further development of the NTNU/SIN-TEF abrasion tests for rock. The test device for the soil abrasion test is identical to the device for AV and AVS abrasivity tests. The difference is in the sample material [23]. The abrasivity tests are quite similar, except that the AV test measures the wear of a tungsten carbide specimen and the AVS and SATTM tests measure the wear of a specimen made of TBM cutter ring steel [22, 23]. RMZ - M&G | 2020 | Vol. 67 | pp. 103-117 Janc B., Jovicic V., Vukelic Z. Ill Device The device is identical for all three abrasivity tests. In the individual tests, the grain size of the rock or soil sample and the material of the test specimen are different. The device, which is shown schematically in Figure 11, consists of a drive, a rotating steel disc, a test specimen, a weight, a sample material feeder and a suction device. rotating steel disc Figure 11: NTNU/SINTEF abrasivity device (adapted from [22]). Test Specimen The material of the specimen in the AV test is tungsten carbide. The specimen in the AVS and SATTM tests is steel taken from the TBM cutter ring. The test specimen is cuboidal with a rounded surface in shape. The length of the test specimen for the AV and AVS tests is 30 mm, the width is 10 mm and the radius is 15 mm. The test specimen for the AV and AVS tests is shown in Figure 12. 10 [mm] crushed rock sample < 1 mm R15 test specimen rotating steel disc Figure 12: Test specimen for rock abrasivity test AV and AVS (adapted from [23]). The length of the test specimen for the SATTM test is 30 mm, the width 20 mm and the radius 15 mm. The test specimen for the SATTM test is shown in Figure 13. During the rotation of the steel plate the rock or soil sample passes under the rounded part of the stationary test specimen. Figure 13: Test specimen for soil abrasivity test SAT™ (adapted from [23]). Sample Material Crushed rock powder with a grain size of less than 1 mm is used for the AV and the AVS tests. Therefore the sample material has to be crushed and/or ground and sieved before the test. For the SATTM test, a dry and gently crushed soil sample with a grain size of less than 4 mm is used. The SATTM test is applicable for the investigation of clay, silt and sand fractions [23]. Test Procedure Abrasivity tests AV, AVS and SATTM represent the time-dependent abrasion of tungsten carbide (AV) or TBM cutter ring steel (AV, SATTM), caused by rock powder or soil grains. The same test device is used for all the three tests. A circular steel disc with a circumference of 1,000 mm (318.3 mm diameter) is set horizontally and rotates around its axis at a speed of 20 rpm. A rock or soil sample falls via the feeder onto the upper outer part of the steel disc and forms a material belt. The mass flow rate of the sample material is approximately 80 g/min. In front of the feeder (in the direction of rotation) a test specimen is mounted and loaded with a normal force of 100 N. The test specimen is firmly clamped and is stationary during the test. The contact between the test specimen and the sample material causes abrasive wear of the test specimen. A suction device is installed behind the feeder (in the direction of rotation) to remove the sample material from Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils 112 the steel disc. This allows the test specimen to remain in contact with new material during the entire test. The length of the path that the test specimen travels on the sample material is 100 m for the AV test and 20 m for the AVS and SATTM test [22, 23]. The duration of the AV test is 5 min or 100 disc rotations, that is, 100 m. The duration of the AVS and SATTM test is 1 min or 20 disc rotations, that is, 20 m. The tests AV, AVS and SATTM are normally carried out on 2-4 test specimens. The deviation is very small and should not exceed 5 mg of weight loss if the test is performed correctly. The reported AV, AVS and SATTM values are the main values of 2-4 parallel tests [22]. The main parameters of the NTNU/SINTEF abrasivity tests are listed in Table 5. Table 5: Main parameters of the NTNU/SINTEF abrasivity tests. Parameters Value Load (N) 100 Rotational 20 speed (min-1) Material mass 80 flow (g/min) AV AVS SAT™ Test duration (min) 1 Sample material Crushed rock Crushed rock soil powder powder Grain size of the sample <1 <1 <4 (mm) Test specimen Tungsten Cutter Cutter ring steel material carbide ring steel Test result AV value AVS value SAT™ value AV, AVS and SAT™ values To determine the abrasivity, it is necessary to measure the mass of the test specimen before and after the test. The loss of mass of the test specimen is a measure of the abrasivity of the rock or soil sample. RMZ - M&G | 2020 | Vol. 67 | pp. 103-117 Classification System The classification given in Table 6 is based on the distribution of the recorded results of the 2,621 samples used to determine AV, 1,590 samples tested to determine AVS and 254 abrasivity measurements on soil samples for the determination of SATTM. The reason for proposing only three classification categories for SATTM is the relatively small amount of data compared to AV and AVS measurements [23]. Table 6: Classification of rock and soil abrasion on tungsten carbide (AV) and cutter steel (AVS, SATTM) test specimen [22]. Abrasion AV (mg) AVS (mg) SAT™ (mg) Extremely high >58.0 >44.0 Very high 42.0-57.9 36.0-44.0 High 28.0-41.9 26.0-35.9 >22.0 Medium 11.0-27.9 13.0-25.9 7.0-22.0 Low 4.0-10.9 4.0-12.9 <7.0 Very low 1.1-3.9 1.1-3.9 Extremely low <1.0 <1.0 RIAT Abrasivity Test The RIAT was developed at the Norwegian University of Science and Technology in Trondheim (NTNU-Norges teknish-naturvitenskapelige universitet). The aim of the test is to imitate the wear behaviour of TBM cutter discs in mechanised tunnelling. The rock and soil abrasivity tests described previously (CERCHAR, LCPC, NTNU) use sliding and impact contacts to induce wear. In contrast to the tests described above, the RIAT is based on a rolling contact that is more realistic for assessing disc cutter wear. Another difference between some abrasivity tests and the RIAT is that these tests use crushed rock samples instead of intact samples [24]. Device The RIAT device, schematically shown in Figure 14, consists of two interchangeable minia- Janc B., Jovicic V., Vukelic Z. 113 ture rolling discs that roll over and penetrate the surface of an intact rock sample. Rotation, torque and vertical thrust are provided by a drive unit [24]. The rolling diameter or the distance between two miniature discs is 60 mm. The diameter of the miniature rolling disc is 30 mm and the tip discs have a constant tip width of 4 mm. The diameter of the discs is 30 mm. The Rockwell hardness of the steel is HRC 50 ± 1 [24]. Test Procedure The test is carried out in such a way that two miniature discs roll over a rock sample under the influence of the normal thrust of 1,250 N at a speed of 40 rpm and a center distance of 60 mm. The test parameters were determined in relation to real cutter parameters in mechanised TBM tunnelling in hard rock. The test duration is 30 min. Rock dust and debris formed during the test should be removed from the surface of the rock to be tested to ensure that the miniature rolling disc is in constant contact with the rock sample. A combination of compressed air and suction is used for this purpose [24]. The main parameters of the RIAT are listed in Table 7. Figure 14: RIAT device (adapted from [24]). width is 4 mm [24]. Table 7: Main parameters of the RIAT [24]. Parameters Value Thrust (N) 1,250 Rolling velocity (min-1) 40 Test duration (min) 30 Disc hardness (HRC) 50 ± 1 Sample material Rock (smooth surface) Test result RIAT abrasivity index (RIATa) RIAT indentation index (RIATi) Test Sample The test sample is intact rock that can have any shape. It is recommended that the surface of the rock sample is smooth and horizontal. Usually a circular rock core is used for the test. In such a case the minimum diameter of the rock sample is 100 mm [24]. Miniature Rolling Discs Miniature rolling discs are made of hot work tool steel AISI type H13, which is normally used for the actual TBM cutter rings. The miniature Determination of the RIAT Abrasivity Index and the RIAT Indentation Index The result of RIAT is the RIAT abrasivity index (RIATJ and the RIAT indentation index (RIAT.]. RIAT is defined as the mass loss of the miniaa ture rolling disc, measured in milligrams after the test. A representative mean value is determined by at least three tests [24]. RIAT, is defined as the mean value of 10 evenly distributed measurements of the penetration depth of the miniature rolling disc into the rock 1250 N 4 mm miniature disc cutter 100 mm Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils 114 surface in 1/100 mm. The RIAT value is an indication of the penetration resistance of the rock or the hardness of the rock surface [24]. In the study reported by Macias et al. [24], eight types of rocks were selected for the RIAT. At least three parallel tests were carried out for each rock type, so that a total of 29 were performed. The lowest and highest RIATa of the test performed is 3 for limestone and 104 for quartzite. The lowest and highest RIAT, of the test carried out is 5 for quartzite and 380 for limestone. As the study shows, RIATa and RIATi are inversely correlated. A higher RIATa means a lower RIAT. [24]. Due to a small number of tests carried out, the classification for the RIAT has not yet been determined. Gouging Abrasion Test The gouging abrasion test was developed to assess rock abrasivity in environments where high stress loads are expected at high speeds and at different angles of incidence. These conditions can be achieved in many mineral processing and mining equipment. Abrasion under such conditions causes considerable wear of the tool material [2]. Apparatus A gouging abrasion test simulates very high stress abrasion under high-energy impact conditions [2]. The rock sample with a rectangular shape and flat surface is clamped in the sample holder. A steel pin with a 90° angle is attached to the end of the pendulum arm. The length of the pendulum arm is such that the steel pin forcibly touches the surface of the rock sample as it swings past, creating a scratch along the entire length of the rock sample [2]. The apparatus is shown schematically in Figure 15. The rock sample is held securely in the sample holder, which can be moved vertically and horizontally by means of the transfer screw or spring mechanism. The wear tool is rigidly positioned in the tool holder of the pendulum arm, which is released from its working position. The steel pin scratches over the rock Figure 15: Gouging abrasion test apparatus. (a) Front view. (b) Side view (adapted from [2]). surface as it moves. The pendulum has 300 J available impact energy. The impact speed is 5.2 m/s [2]. Test Sample The rock sample has a rectangular shape with a length of 80 ± 0.5 mm. Due to irregularities in the samples, the width can vary and take on values between 25 and 50 mm. The thickness of the sample is between 10 and 16 mm. The examined surface of the rock sample is flat and smooth, as is the case with a diamond saw cut. This eliminates the influence of the different surface roughness of the rock sample on the test results [2]. Wear Tool According to Golovanevskiy and Bearman [2], a wear tool of Rockwell hardness HRC 40-42 is recommended for the gouging abrasion test. The wear tool is made of steel AS 1444/4340-Y. The tool is 25 mm long with a diameter of 9.375 ± 0.25 mm. The tool pin has a 90° angle. Test Procedure The prepared rock sample is placed in the sample holder. It is necessary to ensure the horizontal surface of the sample to be examined. The apparatus is equipped with a screw device for fine adjustment of the sample height and a spring system for rapid horizontal movement [2]. The wear tool is carefully inserted into the wear tool holder of the pendulum arm. When the pendulum arm is fully vertical, the pin of the wear tool is directed vertically into the test surface of the rock sample in the holder. At this point, there are a few millimetres of clearance RMZ - M&G | 2020 | Vol. 67 | pp. 103-117 Janc B., Jovicic V., Vukelic Z. 115 Figure 16: Wear contact between tool and rock sample (adapted from [2]). between the pin of the wear tool and the surface of the rock sample [2]. The sample holder is then lifted together with the sample via a screw assembly to achieve a slight contact between the surface of the rock sample and the wear tool pin [2]. The pendulum arm is lifted to its initial position. The sample holder with the rock sample is then raised by 1.0 mm. The accuracy is measured with a micrometre. This allows a 1.0-mm deep wear tool cut in the rock sample. The wear tool is allowed to make a 78-mm long arc-shaped cut on the surface of the rock sample, as shown in Figure 16 [2]. The pendulum arm is released. The pin of a wear tool produces a cut or scratch when it moves over a rock surface. The pendulum arm continues to move to the highest point of the path even as it passes the rock sample. When the pin of the wear tool is no longer in contact with the rock surface after the first swing, the sample holder moves quickly in a horizontal direction perpendicular to the plane of the pendulum motion via the spring mechanism. Due to this displacement of the rock sample, the wear tool and the rock surface no longer come into contact when the arm is moved back. The sample after several tests is schematically shown in Figure 17. When the pendulum stops, the wear tool is removed from the holder and the pin wear is measured. The rock sample holder is returned to its original working position and is moved about 5 mm to provide a fresh surface in the rock sample for further testing. The described procedure is valid for a single test run. For each further test run, a wear tool with an unused pin is inserted into the wear tool holder of the pendulum arm and the procedure described above is repeated [2]. The main parameters of the gouging abrasion test are listed in Table 8. Figure 17: Rock sample after testing (schematic). Numbers 1-6 represent individual scratches from the test (adapted from [2]). Table 8: Main parameters of the gouging abrasion test. Parameters Value Impact energy (J) 300 Wear tool hardness (HRC) 40-42 Test scratch length (mm) 78 Sample material Rock (smooth surface) Test result Gouging abrasion index (Gi) Gouging Abrasion Index The gouging abrasion index (Gi) is determined in the same way as the CAI, that is, the average distance of pin wear in millimetres multiplied by a factor of 10 [2]. For the representative value of the Gi, several tests should be carried out on a single rock sample. In practice, it is advisable to perform at least three test runs on a single rock sample, using a new wear tool and a fresh wear surface of the rock sample. This can be done with three wear tools and a 25-30-mm wide rock sample [2]. Conclusion The wear of tools and equipment during technological processes in geotechnology and mining is associated with high economic costs. On the one hand, there are the material costs associated with the replacement of worn tools or equipment, and on the other hand, there are the costs of downtime of the work process. The abrasivity is directly related to tool wear. Knowledge of the tribological system and wear mechanisms helps us to understand this com- Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils 116 plex problem. However, once the basics are known and the problem is understood, the next step can be taken, that is, the attempt to prevent it at least partially and the possibility to model or predict tool wear. It is not possible to prevent the abrasive wear of geotech-nology and mining application tools in such an aggressive environment as the geological one. However, with the knowledge of the problem it is possible to develop materials that build tools. Every small step to improve the wear resistance of a material is very valuable. The four main components of the tribological system are the geological material, surrounding medium, tools and load type. It should be considered that the first two components mentioned above cannot be modified. However, the tool (and the material it is made of] and the load type can be modified. Any change to the component of the tribological system is reflected in the wear. Therefore, a detailed study of the wear mechanisms for a specific technological process (be it mechanised tunnelling, drilling, crushing or grinding of mineral raw materials] should be carried out, and on this basis an optimisation of the geometry and material of the tool in dynamic contact with the geological material and the type of loading should be carried out. Two factors are of particular importance for a comprehensive understanding of tool wear. These are the wear rate and the mode of wear. In a comprehensive approach, the wear rate can be defined as the first stage and the wear mode as the second stage of wear investigation. Laboratory test methods for assessing the abrasivity of rocks and soils are important because they allow the wear rate of tools that interact dynamically with the abrasive geological material to be determined. The wear rate can be determined immediately after the test is performed, for example, by weighing the loss of mass of the test specimen or by microscopic examination of the pin wear, depending on the test set-up. However, the second stage of the comprehensive approach is to determine the mode of wear, which can be derived from the first stage. This article presents a review of the laboratory tests to assess the abrasivity of rocks and soils, which are useful in the field of geotechnology and mining. These abrasivity tests can help us to gain a comprehensive understanding of tool wear, especially in the first stage, for determination of the wear rate. However, it is recommended that starting from the first stage, the study of wear be continued in the second stage, which is concerned with determining the mode of wear. Compared to the first stage, the second stage is more demanding and time-consuming, as it requires interdisciplinary material knowledge and several individual geochemical and metallurgical investigations at the microscopic level. A comprehensive approach to the investigation of wear is the basis for the development of wear resistant tools and models for the prediction of tool life in individual areas of geo-technology and mining. References [1] Petrica, M., Painsi, M., Badisch, E., Peinsitt, T. (2014): Wear Mechanisms on Martensitic Steels Generated by Different Rock Types in Two-Body Conditions. TribologyLetters, 53(3), pp. 607-616, DOI: 10.1007/ s11249-014-0298-z. [2] Golovanevskiy, V.A., Bearman, R.A. (2008): Gouging abrasion test for rock abrasiveness testing. International Journal of Mineral Processing, 85(4), pp. 111-120, DOI: 10.1016/j.minpro.2007.08.004. [3] BBT Brenner Base Tunnel [online]. Galleria di Base del Brennero - Brenner Basistunnel BBT SE [cited 3/5/2020]. Available on: https://www.bbt-se.com. [4] Marti Group [online, cited 3/5/2020]. Available on: https://www.marti.com. [5] Robbins [online]. The Robbins company [cited 3/5/2020]. Available on: https://www. therobbinscompany.com. [6] Krauze, K., Boioz, t., Wydro, T. (2015): Parametric Factors for the Tangential-Rotary Picks Quality Assessment. Archives of Mining Sciences, 60(1), pp. 265-281, DOI: 10.1515/amsc-2015-0018. [7] Boart Longyear [online]. Boart Longyear company [cited 3/5/2020]. Available on: https://www. boartlongyear.com. [8] Varel [online]. Varel International Energy Services [cited 3/5/2020]. Available on: http://www. vareloilandgas.com. [9] Tkalich, D., Yastrebov, V.A., Cailletaud, G., Kane, A. (2017): Multiscale modeling of cemented tungsten carbide in hard rock drilling. International Journal of Solids and Structures, 128, pp. 282-295, DOI: 10.1016/j.ijsolstr.2017.08.034. RMZ - M&G | 2020 | Vol. 67 | pp. 103-117 Janc B., Jovicic V., Vukelic Z. 117 [10] Yahiaoui, M., Paris, J.Y., Denape, J., Dourfaye, A. (2015): Multiscale and Comparative Study of WC-Co Tribological Behavior. In: Procédés et Génie civil: 27ème Journées Internationales Francophones de Tribologie: JIFT, Nantes, France, Do, M.T., Cérézo, V., Kapsa, P. (eds.). Presses des Mines: Paris, pp. 219-226. [11] Cavpower [online, cited 3/5/2020]. Available on: https://www.cavpower.com. [12] RWE [online]. RWE Group [cited 3/5/2020]. Available on: https://www.group.rwe. [13] Blumaq [online, cited 3/5/2020]. Available on: https://www.blumaq.com. [14] Sandvik [online, cited 3/5/2020]. Available on: https://www.rocktechnology.sandvik. [15] Metso [online, cited 3/5/2020]. Available on: https://www. metso.com. [16] Energosteel [online, cited 3/5/2020]. Available on: https://energosteel.com. [17] Alber, M., Yarah, O., Dahl, F., Bruland, A., Käsling, H., Michalakopoulos, T.N., Cardu, M., Hagan, P., Aydin, H., Özarslan, A. (2013): ISRM Suggested Method for Determining the Abrasivity of Rock by the CERCHAR Abrasivity Test. Rock Mechanics and Rock Engineering, 47(1), pp. 261-266, DOI: 10.1007/s00603-013-0518-0. [18] Käsling, H., Thuro, K. (2010): Determining rock abrasivity in the laboratory. In: Rock mechanics in civil and environmental engineering: proceedings of the European Rock Mechanics Symposium (EURO-CK) 2010, Lausanne, Switzerland, Zhao, J. (ed.). CRC Press: Leiden. [19] Plinninger, R., Käsling, H., Thuro, K., Spaun, G. (2003): Testing conditions and geomechanical properties influencing the CERCHAR abrasiveness index (CAI) value. International Journal of Rock Mechanics & Mining Sciences, 40(2), pp. 259-263, DOI: 10.1016/S1365-1609(02)00140-5. [20] Aydin, H. (2019): Investigating the effects of various testing parameters on Cerchar abrasivity index and its repeatability. Wear, 418-419, pp. 61-74, DOI: 10.1016/j.wear.2018.11.001. [21] Thuro, K., Singer, J., Käsling, H., Bauer, M. (2006): Soil Abrasivity Assessment Using the LCPC Testing Device. Felsbau, 24(6), pp. 37-45, DOI: 10.1201/ NOE0415444019-c103. [22] Dahl, F., Bruland, A., Jakobsen, P.D., Nilsen, B., Gr0v, E. (2012): Classification of properties influencing the drillability of rocks, based on the NTNU/ SINTEF test method. Tunnelling and Underground Space Technology, 28, pp. 150-158, DOI: 10.1016/j. tust.2011.10.006. [23] Jakobsen, P.D., Bruland, A., Dahl, F. (2013): Review and assessment of the NTNU/SINTEF Soil Abrasion Test (SATTM) for determination of abrasiveness of soil and soft ground. Tunnelling and Underground Space Technology, 37, pp. 107-114, DOI: 10.1016/j. tust.2013.04.003. [24] Macias, F.J., Dahl, F., Bruland, A. (2016): New Rock Abrasivity Test Method for Tool Life Assessments on Hard Rock Tunnel Boring: The Rolling Indentation Abrasion Test (RIAT). Rock Mechanics and Rock Engineering, 49(5), pp. 1679-1693, DOI: 10.1007/ s00603-015-0854-3. Laboratory Test Methods for Assessing the Abrasivity of Rocks and Soils 119 Original scientific paper Received: Jun 04, 2020 Accepted: Jun 16, 2020 DOI: 10.2478/rmzmag-2020-0009 Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria Retrogresija gnajsa z ortopiroksenom iz območja Iboropa Akoko, jugozahodna Nigerija EJ. Oziegbe1*, V.O. Olarewaju2, O.O. Ocan3, G.Costin4 1 Department of Geosciences, Faculty of Science, University of Lagos, Nigeria 2 Department of Geology, Obafemi Awolowo University, Nigeria 3 Department of Geological Science, College of Science, Engineering and Technology, Osun State University, Nigeria 4 Earth, Environmental and Planetary Sciences, Rice University, Texas, USA * eoziegbe@unilag.edu.ng Abstract In this article, we report the mineral chemistry and pe-trographic features of charnockitic exposure of Iboropa within Precambrian Basement Complex of Nigeria. The mineral assemblages are pyroxene, plagioclase, biotite, hornblende, alkali feldspars, microperthite, quartz and ilmenite, with apatite occurring as accessory mineral. Apatite occurs in abundance as euhedral crystals. Orthopyroxene observed is strongly pleochroic and has numerous microfractures, and it is hypersthene (En45Fs54Wo1) with low TiO2 and MnO, having extremely low percentage of CaO. Hypersthene is mantled by a complex corona of amphibole, and the amphibole is hornblende with a chemical formula: (K,Na)(Ca,Fe)2 (Fe,Mg,Al,Ti)5(Al,Si)8O22(OH)2. Plagioclase occurs as inclusions in both pyroxene and biotite. Biotite has high concentration of TiO2 and extremely low CaO. The opaque mineral observed is ilmenite and it is concentrated around hypersthene and amphibole. Rare earth element (REE) displays negative Eu anomaly with enrichment of light REE over heavy REE. Amphiboles surrounding orthopyroxene are evidences of retrograde reactions and are formed at the expense of orthopyroxene reacting with plagioclase and quartz in the presence of fluid. The relationship between the mineral assemblages suggests the retrogression of the gneiss that might be as a result of rehydration process, and it is a transition from granulite facies to amphibolite facies during a retrogressive form of metamorphism. Keywords: Hypersthene, microfractures, retrograde, microperthite, rehydration. Povzetek V tem članku predstavljamo mineraloško-kemične in petrografske značilnosti charnokita, ki se pojavlja na območju Iboropa znotraj predkambrijske podlage Nigerije. Opazovani minerali so: piroksen, plagioklaz, bi-otit, rogovača, alkalijski glinenec, mikropertit, kremen in ilmenit, z apatitom ki se pojavlja kot akcesorni mineral. Apatit je akcesorni mineral in se pojavlja v obilju kot evhedralni kristal. Opazovan ortopiroksen je močno pleokroičen in ima številne mikrorazpoke. Je hi-persten (En45Fs54Wo1) z nizkim TiO2 in MnO in ima ek-stremno nizek delež CaO. Hipersten je oplaščen s kompleksno korono amfibola, ki je rogovača s kemijsko formulo (K,Na)(Ca,Fe)2(Fe,Mg,Al,Ti)5(Al,Si)8O22(OH)2. Plagioklaz se pojavlja kot inkluzije v piroksenu in bio-titu. Biotit ima visoko koncentracijo TiO2 in ekstremno nizko CaO. Opazovan nepreseven mineral je ilmenit in je koncentriran okoli hiperstena in amfibola. REE kaže na negativno Eu anomalijo z obogatitvijo LREE nad HREE. Amfiboli, ki obkrožajo ortopiroksene, so dokaz retrogradne reakcije in nastanejo pri reakciji ortopiro-ksena s plagioklazom in kremenom v prisotnosti fluida. Razmerje med mineralno sestavo predlaga retro-gresijo gnajsa, ki bi lahko bila rezultat rehidracijskega procesa in je prehod granulitnega faciesa v amfibolitni facies med retrogresivno obliko metamorfizma. Ključne besede: hipersten, mikro razpoke, retrogradno, mokropertit, rehidracija. 3 Open Access. © 2020 Oziegbe E.J., Olarewaju V.O., Ocan O.O., Costin G., published by Sciendo. K°D'^'""SUI This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. 120 Figure 1. Sketch geological map of the Hoggar-Air-Nigeria Province showing the Neoproterozoic Trans-Saharan belt, resulting from terrane amalgamation between the cratons of West Africa and Congo and the East Saharan block [26]. Introduction Orthopyroxene-bearing gneiss is usually referred to as charnockitic gneiss or granulite, most especially if the orthopyroxene is hyper-sthene. In this article, the use of the term charnockitic gneiss will be adhered to. Charnockite are hypersthene-bearing rocks [1]. There has been an extensive study on charnockites and charnockitic rocks in India [2-5]. Charnock-ites are orthopyroxene-bearing anhydrous granitoids [1, 6], and they are of magmatic or metamorphic origin [7]. Charnockites are restricted to high-grade belt and those that are of metamorphic origin take their source either from igneous or from sedimentary protoliths during high-grade metamorphism under anhydrous conditions [8]. In some parts of south India and Sri Lanka, 'patchy' charnockites have been termed as 'arrested growth', 'in situ' charnockites or charnockitisation of amphibolite facies [9-15]. For the patchy charnockites of east Gondwana, metamorphic transformation from amphibolite facies gneiss occurred by two mechanisms: CO2 ingress from deep level and a drop in the pressure of fluid [11, 16-18]. The process of charnockitisation is propagated by the influx of the CO2-H2O fluid, which results in the migration of most basic elements from the initial rock towards the transition zones [19]. Touret and Huizenga [20] studied charnockite microstructures from magmatic to metamorphic and realised that microstructures alone cannot always provide solution to the igneous or metamorphic origin of charnockites due to high temperature of recrystallisation. In their study, further evidence for the presence of brine fluids includes the high-temperature fluid-mineral reactions at inter-grain boundaries (K-feldspar microveins and myrmekites) observed in both igneous and metamorphic charnockites. Yang et al. [21] gave the first report of Paleoproterozoic incipient charnockite from the North China Cratons. They documented centimetre- to decimetre-scale anhydrous zones of incipient charnockite within tonalite-trondhjemite-granodiorite (TTG) rocks that are adjacent to an intrusive charnockite. From their study, they concluded that the incipient charnockite formed at ultrahigh-temperature conditions of 890°C-970°C. The study area for this research is part of the Neoproterozoic basement of Nigeria (Figure 1). Rocks of the Basement Complex of Nigeria are made up of predominantly migmatitic and granitic gneisses; quartzites; slightly migma-tised to unmigmatised meta-sedimentary schists and meta-igneous rocks; charnockitic, gabbroic and dioritic rocks and members of the Older Granite suite mainly granites, granodio-rites and syenites [22]. The migmatite gneisses of South-Western Nigeria are usually composed of three components, all of which sometimes may be present in a single outcrop [22]. These components are (a) early gneiss, (b) mafic-ul-tramafic bands and (c) granitic or felsic components. Iboropa Akoko is about 10 km from RMZ - M&G | 2020 | Vol. 67 | pp. 119-134 Oziegbe E.J., Olarewaju V.O., Ocan O.O., Costin G. 121 Figure 2. Location map of the study area. Figure 3. Field photograph of charnockitic gneiss with quartzo-feldspathic material at Iboropa Akoko. Ikare (Figure 2), and Ikare area is dominated by migmatite gneiss-quartzite complex, making up 90% of the rock units [23]. The charnockitic gneiss of Iboropa is dark grey in colour (Figure 3), and it is poorly foliated. Other associated rocks in this area are grey gneiss, granitic gneiss, pelitic gneiss and pegmatite. Almost all granulite facies rocks described so far from the Nigerian Basement Complex are of charnockitic affinity. Rocks of the granulite facies are found in all crystalline basement of the world [24]. Hubbard [25] suggested that these granulite facies rocks represent relicts of an earlier and more widespread granulite facies metamor-phism of unspecified age, predating the Pan-African Orogeny. However, Rahaman and Ocan [23] suggested that the charnockitic rocks were original igneous rocks that retained their anhydrous affinity during the Pan-African Orogeny. Most of the other rocks of the basement complex described so far range in metamorphic grades from green-schist to upper amphibolite facies. Metamor-phic rocks that have attained higher grades of metamorphism are probably the cordierite-sil-limanite gneisses at Bena village in northern Nigeria, which Sacchi [27] ascribed to the granulite facies. On the published 1:250,000 G.S.N. Sheets 61, Akure, some localities within the migmatite gneiss-quarzite complex were indicated by Dempster [28] as containing gran-ulite facies mineral paragenesis. Petrographic evidence presented strongly suggests that the Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria 122 granulite facies mineral paragenesis in Ikare area is a result of prograde metamorphism [29]. In this respect, rocks of the granulite facies mineralogy in Ikare area are different from others of similar mineralogy described so far [23], as original igneous rocks, because of composition, especially low water content, retained their original (igneous] granulite facies mineralogy. Data used for this work are from the PhD thesis of the first author. Materials and Methods Detailed petrography was carried out using Petrographic Microscopes, both at the Department of Geology, Obafemi Awolowo University and Department of Geology, Rhodes University, South Africa. Major elements and trace elements were determined at the Central Analytical Facility (CAF], Stellenbosch University, South Africa, using X-ray fluorescence (XRF] and laser ablation ICP-MS, respectively. The mineral chemistry of the minerals was done using electron microprobe analysis (EPMA] at the Department of Geology, Rhodes University South Africa by a JEOL JXA 8230 Superprobe, using 4 WD spectrometers. Equipment operating conditions employed for the EPMA were 15 kV acceleration voltage, 20 nA probe current, beam size of ~1 [im and counting time 10 s on peak and 5 s on each lower and upper background, respectively. Natural standards were used for measuring the characteristic X-rays, and the ZAF matrix correction method was employed for quantification. Results Petrography The charnockitic gneiss is granoblastic in texture. The minerals that are present in the char-nockitic gneiss include pyroxenes, amphiboles, biotite, plagioclase feldspar, K-feldspar quartz, opaque minerals, while apatite occur as accessory mineral. Orthopyroxene occur as por-phyroblasts, changing from colourless to grey, highly pleochroic and changing from grey to pinkish, a property indicative of hypersthene (Figure 4). Some of the porphyroblasts have Figure 4. Photomicrographs showing (a) porphyroblast of pyroxene (Px) having numerous fractures. Pale biotite (Bt) along cleavage planes (PPL). (b) Plagioclase (Pl) occurring as inclusions in pyroxene (XPL). PPL, plane polarised light; XPL, crossed polarised light. inclusions of plagioclase and biotite (Figure 4] which make them poikiloblastic in texture. There is rimming of pyroxene by amphibole (Figure 5a]. Back scattered electron (BSE] image and elemental maps in Figure 5 a are also presented (Figures 5b and 6]. There are small patches of pale-coloured biotite along the cleavage planes of pyroxenes (Figure 4a]. Amphibole occurs as large, xenoblastic, dark green crystals and it is strongly pleochroic with inclusions of pyroxene (Figure 7a and 7b]. Also, amphiboles have close association with opaque minerals. Biotite observed is of two varieties, namely the light brown and the deep brown, and both are strongly pleochroic. The longer axes of biotite are slightly aligned in preferred orientations RMZ - M&G | 2020 | Vol. 67 | pp. 119-134 Oziegbe E.J., Olarewaju V.O., Ocan O.O., Costin G. 123 Figure 5. (a) Photomicrograph showing pyroxene (Px) rimmed by amphibole (Am). The opaque mineral (Opq) (PPL). (b) Back-scattered electron image of minerals shown in (a). PPL, plane polarised light; XPL, crossed polarised light. Figure 6. The compositional maps of minerals shown in Figure 5, showing elemental levels of Si, Mg, Na, Ca, Fe, Al, K, Mn and Ti. Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria 124 Figure 7. Photomicrographs showing: (a) amphiboles (Am) with inclusions of pyroxene (Px) (PPL). (b) Amphiboles with inclusions of pyroxene, surrounded by plagioclase (Pl) (XPL). (c) Biotite having inclusions of plagioclase feldspar (XPL). (d) Biotite having inclusions of plagioclase feldspar (PPL). PPL, plane polarised light; XPL, crossed polarised light. Figure 8. Photomicrographs showing: (a) pyroxene in close contact with areas occupied by transparent minerals: plagioclase and K-feldspar. Altered biotite (brownish mineral) surrounded by transparent mineral (PPL). (b) Pyroxene (Px), perthitic K-feldspar (Kfs) and plagioclase feldspar (Pl). The straight lamellae in the perthite (XPL). (c) Biotite (Bt) and apatite. The smaller needle-shaped apatite crystals (PPL). (d) K-feldspar (Kfs), quartz (Qtz), apatite (Ap) and plagioclase (Pl) (XPL). PPL, plane polarised light; XPL, crossed polarised light. RMZ - M&G | 2020 | Vol. 67 | pp. 119-134 Oziegbe E.J., Olarewaju V.O., Ocan O.O., Costin G. 125 which thus defines the foliation. The biotite grains have inclusions of plagioclase and apatite (Figure 7c and 7d). Plagioclase occurs as porphyroblasts, and grains are almost equidi-mensional. Plagioclase crystals observed exhibit both ablite and carlsbard twinning. There is a close association between plagioclase and pyroxene. Plagioclase occurs as inclusions in both pyroxene and biotite, and plagioclase crystals occur at the margins of amphiboles. K-feldspar has perthitic texture and is in close contact with pyroxene (Figure 8a and 8b]. Apatite is needle-like and colourless, occurring as inclusions in feldspars (Figures 8c and 8d]. Mineral Chemistry Pyroxene: The mineral composition of pyroxene in charnockitic gneiss shows a slight variation from core to rim (Table 1]. The range in composition for both the core and rim is: SiO2: 49.68-49.82%, MgO: 15.20-15.07%, FeO: 31.91-32.19% and CaO: 0.495-0.618%. This result confirms the orthopyroxene as hy-persthene and the formula can be written as MgFeSi2O6 (Table 1]. In terms of the pyroxene end members, the formula can be written as En45Fs54Woa (Table 1]. The BSE image shows no form of zoning (Figure 5b], while the compositional maps show the levels of element in the minerals (Figure 6]. The compositional maps show high levels of magnesium (Mg] and iron (Fe], but a very low level of calcium (Ca] in the pyroxene (Px]. Amphibole: The composition of the various oxides is (Table 1]: SiO2: 42.45%, TiO2: 2.038%, FeO: 18.10%, Al2O3: 10.39%, MgO: 9.16%, CaO: 11.30%, Na2O: 1.53%, K2O: 1.41% and Cl: 0.071%. The compositional maps (Figure 6] show amphibole with an appreciable amount of Fe, Ca and Mg but very low Na content. The formula of the amphibole is (K,Na](Ca,Fe]2(Mg, Fe,Ti,Al]5(Al,Si]8O22(OH]2 that is named as hornblende (T5 able 18]. 22 2 Biotite: The chemical composition of bio-tite crystals is (Table 2]: SiO2: 34.71-35.95%, FeO: 19.97-20.97%, MgO: 9.64-10.15%, Al2O3: 13.35-14.14%, TiO2: 4.84-5.23%, CaO: 0-0.07% and K2O: 8.54-9.74%. The elemental composition of the biotite shows that it is an-nite. Plagioclase: The chemical composition of 14 crystals of plagioclase shows the following range of values: SiO2: 58.14-60.19%, Al2O3: 24.06-25.77%, CaO: 7.18-8.58%, Na2O: 5.999-7.35% and K2O: 0.34-0.49%, and in terms of anorthite content it can be written as Ab59An39Or2, that is known as andesine (Figure 9]. The chemical formula of the plagioclase based on cations is (Na,Ca]AlSi3O8 (Table 3]. Table 1. The chemical composition of pyroxene and amphibole Pyroxene Amphibole Sample 1c 1r Crystal 1 2 3 4 5 6 SiO2 49.680 49.830 SiO2 42.453 41.804 43.680 42.904 43.139 42.786 TiO2 0.037 0.114 TiO2 2.038 1.450 1.505 1.669 1.826 1.993 Al2O3 0.618 0.682 Al2O3 10.389 10.342 9.819 10.039 10.174 10.186 FeO 32.470 31.910 FeO 18.095 17.894 17.156 18.178 18.975 18.580 Cr2O3 0.000 0.010 Cr2O3 0.000 0.000 0.043 0.132 0.053 0.101 MnO 0.585 0.712 MnO 0.058 0.016 0.111 0.132 0.049 0.054 MgO 15.200 15.070 MgO 9.163 9.361 9.943 9.474 9.399 9.622 CaO 0.618 0.495 CaO 11.305 11.394 11.575 11.465 11.431 11.455 Na2O 0.014 0.055 Na2O 1.533 1.338 1.108 1.386 1.406 1.443 K2O 0.057 0.030 K2O 1.409 1.430 1.215 1.411 1.347 1.349 Total 99.290 98.900 Cl 0.071 0.085 0.063 0.077 0.043 0.140 TSi 1.958 1.970 Total 96.510 95.110 96.180 96.740 97.79 97.610 Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria 126 Table 1. The chemical composition of pyroxene and amphibole (continue) Pyroxene Amphibole Sample 1c 1r Crystal 1 2 3 4 5 6 TAl 0.029 0.030 O_Cl 0.020 0.020 0.010 0.020 0.010 0.030 TFe3+ 0.014 0.000 TSi 6.553 6.517 6.681 6.580 6.556 6.512 M1Al 0.000 0.002 TAl 1.447 1.483 1.319 1.420 1.444 1.488 M1Ti 0.001 0.003 Sum_T 8 8 8 8 8 8 M1Fe3+ 0.044 0.026 CAl 0.442 0.416 0.449 0.393 0.377 0.337 M1Fe2+ 0.062 0.080 CFe+3 0.000 0.038 0 0 0 0 M1Cr 0.000 0.000 CTi 0.237 0.170 0.173 0.193 0.209 0.228 M1Mg 0.893 0.888 CMg 2.109 2.176 2.267 2.166 2.129 2.183 M2Mg 0.000 0.000 CFe+2 2.213 2.201 2.106 2.232 2.279 2.240 M2Fe2 0.950 0.949 Sum_C 5 5 5 5 5 5 M2Mn 0.020 0.024 BFe+2 0.123 0.095 0.089 0.099 0.132 0.125 M2Ca 0.026 0.021 BMn 0.008 0.002 0.014 0.017 0.006 0.007 M2Na 0.001 0.004 BCa 1.870 1.903 1.897 1.884 1.861 1.868 M2K 0.003 0.002 Sum_B 2 2 2 2 2 2 Sum (cat) 3.997 3.998 ANa 0.459 0.404 0.329 0.412 0.414 0.426 Ca 1.337 1.069 AK 0.277 0.284 0.237 0.276 0.261 0.262 Mg 45.770 45.260 Sum_A 0.736 0.689 0.566 0.688 0.675 0.688 Fe2_Mn 52.890 53.670 Sum_cat 15.736 15.689 15.566 15.688 15.675 15.688 JD1 0.000 0.108 CCl 0.019 0.022 0.016 0.020 0.011 0.036 AE1 0.202 0.186 Sum_oxy 23.102 23.000 23.023 23.031 23.016 23.002 CFTS1 2.056 1.168 CTTS1 0.056 0.174 CATS1 0.000 0.000 WO1 0.000 0.000 EN1 45.780 45.560 FS1 51.902 52.800 Q 1.931 1.939 J 0.002 0.008 WO 1.299 1.055 EN 44.460 44.670 FS 54.240 54.280 WEF 99.890 99.570 JD 0.000 0.032 AE 0.100 0.396 M1, third octahedral site; M2, second octahedral site; M3, first octahedral site; T, tetrahedral site; JD, jadeite; AE, aegirine; WO, wollastonite; EN, enstatite; FS, ferrosilite; CAT, cation. RMZ - M&G | 2020 | Vol. 67 | pp. 119-134 Oziegbe E.J., Olarewaju V.O., Ocan O.O., Costin G. Table 2. Chemical composition of biotite crystals Crystal 1 2 3 4 5 6 7 8 SiO2 35.20 35.10 35.19 34.71 35.52 35.34 35.95 35.42 TiO2 5.23 5.15 4.97 4.84 4.84 4.97 5.09 5.00 AlA 13.63 13.79 14.14 13.75 13.92 13.35 13.82 13.83 Cr2O3 0.04 0.06 0.00 0.03 0.02 0.05 0.03 0.00 FeO 21.02 20.48 20.75 20.96 19.97 20.59 20.41 20.72 MnO 0.00 0.04 0.03 0.03 0.00 0.01 0.00 0.02 MgO 9.88 9.95 10.15 9.93 10.00 10.07 9.99 9.64 CaO 0.01 0.07 0.00 0.04 0.02 0.00 0.00 0.04 Na2O 0.13 0.11 0.09 0.14 0.09 0.08 0.14 0.11 K2O 9.74 9.71 9.57 9.62 9.56 8.54 9.63 9.74 Cl 0.15 0.11 0.12 0.15 0.09 0.09 0.10 0.10 H2O 1.80 1.81 1.81 1.78 1.81 1.80 1.83 1.81 Total 95.03 94.57 95.01 94.2 94.03 93.09 95.16 94.62 Si 5.739 5.735 5.717 5.714 5.803 5.823 5.812 5.782 AlIV 2.261 2.265 2.283 2.286 2.197 2.177 2.188 2.218 AlVI 0.356 0.388 0.423 0.380 0.481 0.414 0.443 0.441 Ti 0.641 0.633 0.607 0.599 0.595 0.616 0.619 0.614 Fe2+ 2.866 2.798 2.819 2.885 2.729 2.837 2.759 2.829 Cr 0.005 0.008 0.000 0.004 0.003 0.007 0.004 0.000 Mn 0.000 0.006 0.004 0.004 0.000 0.001 0.000 0.003 Mg 2.401 2.423 2.458 2.437 2.436 2.474 2.408 2.346 Ca 0.002 0.012 0.000 0.007 0.004 0.000 0.000 0.007 Na 0.041 0.035 0.028 0.045 0.029 0.026 0.044 0.035 K 2.026 2.024 1.984 2.020 1.993 1.795 1.986 2.028 Cations 16.338 16.327 16.323 16.381 16.270 16.170 16.263 16.303 CCl 0.083 0.061 0.066 0.084 0.050 0.050 0.055 0.055 OH 1.959 1.970 1.967 1.958 1.975 1.975 1.973 1.972 O 24 24 24 24 24 24 24 24 Fe/(Fe + Mg) 0.5400 0.54 0.530 0.540 0.530 0.530 0.530 0.550 Mg/(Fe + Mg) 0.460 0.460 0.470 0.460 0.470 0.470 0.470 0.450 AlIV, aluminium in tetrahedral site; AM, aluminium in octahedral site. Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria Table 3. The chemical composition ofplagioclase crystals Crystal 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Si02 58.96 59.05 59.39 59.47 58.84 58.19 58.45 59.22 59.23 58.7 59.76 60.19 58.14 58.57 Ti02 0.02 0 0 0 0.04 0 0.37 0.01 0 0 0.04 0 0.03 0 ai2o3 25.53 25.59 25.55 25.03 25.72 25.77 25.8 25.65 25.34 25.26 24.06 24.78 25.24 25.49 FeO 0.12 0.07 0.05 0.15 0.02 0.07 0.24 0.07 0.11 0.17 0 0.08 0.01 0.01 MnO 0.01 0.01 0.05 0.02 0.01 0.04 0 0.03 0 0.04 0.02 0 0 0 MgO 0 0.01 0 0 0.01 0 0 0 0.02 0 0 0.01 0 0 BaO 0.01 0.04 0.08 0.05 0.07 0 0.07 0.03 0.09 0 0.06 0.01 0.08 0.03 CaO 8.05 8.04 7.98 7.82 8.13 8.58 8.49 8.21 7.78 7.88 7.18 7.3 8.34 8.15 Na20 6.87 6.88 5.99 7.04 6.75 6.7 6.26 6.62 6.93 7.01 7.35 7.35 6.07 6.53 k2o 0.38 0.37 0.39 0.39 0.34 0.37 0.36 0.34 0.37 0.36 0.49 0.41 0.37 0.39 Total 99.95 100.06 99.48 99.97 99.93 99.72 100.04 100.18 99.87 99.42 98.96 100.13 98.28 99.17 Si 2.639 2.64 2.659 2.661 2.633 2.616 2.617 2.642 2.651 2.642 2.697 2.683 2.642 2.639 Al 1.346 1.347 1.347 1.319 1.356 1.364 1.36 1.347 1.336 1.339 1.279 1.301 1.351 1.353 Fe3+ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Ti 0.001 0 0 0 0.001 0 0.012 0 0 0 0.001 0 0.001 0 Fe2+ 0.004 0.003 0.002 0.006 0.001 0.003 0.009 0.003 0.004 0.006 0 0.003 0 0 Mn 0 0 0.002 0.001 0 0.002 0 0.001 0 0.002 0.001 0 0 0 Mg 0 0.001 0 0 0.001 0 0 0 0.001 0 0 0.001 0 0 Ba 0 0.001 0.001 0.001 0.001 0 0.001 0.001 0.002 0 0.001 0 0.001 0.001 Ca 0.386 0.385 0.383 0.375 0.39 0.413 0.407 0.392 0.373 0.38 0.347 0.349 0.406 0.393 Na 0.596 0.596 0.52 0.611 0.586 0.584 0.543 0.573 0.602 0.612 0.643 0.635 0.535 0.571 K 0.022 0.021 0.022 0.022 0.019 0.021 0.021 0.019 0.021 0.021 0.028 0.023 0.021 0.022 Cations 4.994 4.995 4.937 4.997 4.989 5.003 4.971 4.979 4.992 5.002 4.998 4.995 4.958 4.98 X 3.986 3.987 4.006 3.98 3.99 3.98 3.989 3.989 3.987 3.981 3.977 3.984 3.994 3.992 Z 1.008 1.007 0.93 1.016 0.998 1.023 0.981 0.989 1.003 1.021 1.02 1.011 0.963 0.987 Ab 59.4 59.5 56.2 60.6 58.9 57.4 55.9 58.2 60.4 60.4 63.2 63.1 55.6 57.9 An 38.4 38.4 41.4 37.2 39.2 40.6 41.9 39.8 37.4 37.5 34.1 34.7 42.2 39.9 Or 2.2 2.1 2.4 2.2 1.9 2.1 2.2 1.9 2.1 2.1 2.8 2.3 2.2 2.2 Ab, albite;An, anorthite; Or: orthoclase. 129 Or Figure 9. Ternary diagram showing feldspar solid solution. Ilmenite: The result of the chemical composition of ilmenite for charnockitic gneiss shows the following values (Table 4): TiO2: 50.79%, FeO: 46.37% and SiO2: 0.046%. Major Element Geochemistry The composition of the major oxides is as follows (Table 5): SiO2 (50.93 wt%), Al2O3 (15.51 wt%), CaO (7.79 wt%), Fe2O3 (122.3 wt%), MgO (5.14 wt%), K2O (1.01 wt%), MnO (0.15 wt%), Na2O (3.33 wt%), TiO2 (2.15 wt%), P2O5 (0.65 wt%), and Cr2O3 (0.02 wt%). Trace Element Geochemistry For the compatible elements Zn, Cr and V, the value ranges are as follows (Table 5): Zn (124.1 ppm), Cr (95.31 ppm) and V (188.28 ppm). The immobile transition elements Co and Ni have values of 107.09 and 52.36 ppm, respectively. The incompatible elements show a wide variation, and the large ion lithophile elements (LILE) have the values of Cs (0.31 ppm), Sr (583.8 ppm), Rb (18.36 ppm) and Ba (479.77 ppm). Rare Earth Elements Geochemistry The rare earth elemental (REE) composition of the charnockitic gneiss shows light REE (LREE) enrichment and depletion of heavy REE (HREE) (Table 5). The samples display a pattern of negative Eu anomaly. Discussion The orthopyroxene found in the charnockitic gneiss of Iboropa is hypersthene, that is Fe-Mg-rich (Table 1) and it is associated with amphibole (hornblende) and plagioclase. Pyroxene analysed is a solid solution between enstatite and ferrosilite (En45Fs54Wo1), and because of the high Fe content it can be referred to as fer-rohypersthene. Orthopyroxene is an essential mineral for all rocks in the charnockite suite. Table 4. The composition of ilmenite in a charnockitic gneiss _Sample_1_ SiO2 0.046 TiO2 50.787 Al2O3 0.02 FeO 46.372 Cr2O3 0.075 MnO 0.477 MgO 0.267 CaO 0 Na2O 0.061 Total 98.11 Si 0.001 Al 0.001 Ti 0.986 Fe2+ 1.001 Cr 0.002 Mn 0.01 Mg 0.01 Ca 0 Na 0.003 Cations 2.014 The percentage of CaO in the analysed pyroxene is very low (0.5-0.6%), a value which is less than 1.0 and is consistent with the fact that Ca atoms per formula does not exceed 1.0, which occurs only in the M2 site [30]. The aluminium oxide content of the pyroxene is also low. Or-thopyroxene can be found to be metamorphic because the weight percentage of MgO + FeOT is greater than 44.00% [31]. Orthopyroxene present could have formed during increase in grade of metamorphism from either hornblende or biotite. In Northern Nigeria, charnockite has been derived from the transformation of bio-tite by the addition of ferrous iron-rich juvenile Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria 130 Table 5. Bulk rock composition of charnockitic gneiss; major elements (%), trace elements (ppm), REE (ppm) Major element 1 Trace element 1 REE 1 Wt% ppm ppm SiO2 50.93 V 188.28 La 57.71 Al2O3 15.51 Cr 95.31 Ce 119.2 CaO 7.79 Co 107.09 Pr 14.13 Fe2O3 12.3 Ni 52.36 Nd 54.64 MgO 5.14 Cu 41.17 Sm 9.05 K2O 1.01 Zn 124.1 Eu 2.27 MnO 0.15 Rb 18.36 Gd 7.325 Na2O 3.33 Sr 583.8 Tb 0.856 TiO2 2.15 Zr 294.09 Dy 4.365 P2O5 0.65 Nb 20.61 Ho 0.8 0.02 Mo 1.54 Er 2.02 LOI 0.34 Cs 0.31 Tm 0.251 Total 99.32 Ba 479.77 Yb 1.63 Hf 6.49 Lu 0.22 Ta 1.08 Total 274.4 Pb 8.51 K/YbN 25.39 Th 2.89 GdN/YbN 3.72 U 0.45 EuN/EuN* 0.852 Ce/Ce* 1.153 solution [32]. Orthopyroxene could be formed by any of the following reactions: Hornblende + Quartz => Clinopyroxene + Orthopyroxene + (1] Plagioclase + K-feldspar + H2O Biotite + Quartz => (2] Orthopyroxene + K-feldspar + H2O The earlier reactions are dehydration reactions resulting in a decrease in the activity of water and an increase in alkali activity [19]. Mineral assemblage show altered form of biotite (first-generation biotite] in close association with K-feldspar (Figure 8a]. This process is a high-grade dehydration of rocks of the amphib-olite facies to granulite facies and it can involve partial melting and in some cases fluid solid-state dehydration [33-35]. Orthopyroxene mantled by amphibole is an indication of retrogressive metamorphic event (Figure 5]. The retrogression of the charnockite could be due to various dehydration processes [36]. Retrogression involving the breakdown of orthopyroxene is quite common in granulite complexes [37, 38]. Orthopyroxene replaced by amphibole has been described in the granulite facies of gneiss in the granite Falls-Montevideo [39]. The hypersthene observed has numerous microfractures that may serve as a pathway for the movement of fluids (Figure 4a]. The development of biotite along cleavage planes (Figure 4a] is also an indication of retrogression [40]. Amphibole could have been formed by a process in which orthopyroxene reacts with plagioclase (anorthite-rich) and quartz in the presence of water, and this can be represented by the following equation: Orthopyroxene + Plagioclase 1 + (3] Quartz + H2O => Amphibole + Plagioclase 2 RMZ - M&G | 2020 | Vol. 67 | pp. 119-134 Oziegbe E.J., Olarewaju V.O., Ocan O.O., Costin G. 131 In this process, amphibole replaces orthopy-roxene in a rehydration reaction. Biotite (second-generation biotite] that is in close contact with orthopyroxene (Figure 4a] could have been a product of a reaction in which K-feld-spar is consumed by a rehydration process, and thus it can be represented by the following equation: Orthopyroxene + K-feldspar + H2O => Biotite (4] The biotite is annite-rich, an iron end member of biotite as indicated by the mineral chemistry (Table 2]. Such retrogressive reactions have been found to result from fluid (CO2 and H2O] activity and oxygen fugacity in the later stages of dehydration zone [41]. Based on the BSE image and elemental maps (Figures 5b and 6], there is no evidence of zoning in the pyroxene crystals. Hornblende and biotite in charnockites have been found to be due to retrogressive metamorphism of pyroxene granu-lite facies rocks [42, 43], and retrograde form of metamorphism has been found to obliterate the mineralogy of the granulite facies [24]. Pla-gioclase occurring as inclusions in pyroxene and biotite is an indication that it was formed very early enough as primary minerals. Opaque minerals that are concentrated along the cleavage planes of amphiboles crystals are evidences of metamorphic alteration. The Al2O3 content of the amphibole is fairly high, and chornockit-ic rocks usually have Al2O3 higher than that of the normal igneous rocks. Hornblende has less magnesium than the coexisting orthopyroxene and a moderate composition of TiO2 (2.04%]. Hornblende has high TiO2 (>2] which some authors have linked to the increase in the grade of metamorphism, and this link between temperature and Ti content of amphibole has been reported [44, 45]. There is a high value of TiO2 (4.84-5.23%] in biotite, an indication that it is uniformly rich in Ti, and a higher value of XFe to XMg. Biotite could have reacted with quartz to give the Fe-Ti ilmenite. There has been suggestion that increasing Ti content leads to increase in the stability field of biotite [46]. The crystals of biotite have MgO of nearly the same range of composition as that of amphiboles but far less than that of orthopyroxene. Plagioclase is the dominant feldspar observed and it is al- bite-enriched (Ab55.9An4i.9Or22-Ab63.2An34.iOr2.8] that is andesine and is indicative of amphibolite zone. The K-feldspar has perthitic texture (Figure 8b]. Perthite is an intergrowth of one feldspar within another, albite in orthoclase [47], and it is common in high-grade metamorphic rocks [48] due to the high temperature involved. The perthite observed has straight lamellae (Figure 7b], an indication of granulite facies. Based on the association of orthopyrox-ene and plagioclase, the granulite can be said to be of intermediate pressure [24], and one of the feature of this facies is the depletion of Th and U. The values of Cs have also been found to be depleted in high-grade metamorphosed rocks [24]. There is a depletion in the values of K, Rb, Th and U, and pyroxene gneiss with values of this nature has been suggested as a residue of partial melting in which there has been the removal of these elements from the original rocks [49]. The elements Th, U, Rb and Cs are usually present in the lattices of micas of the amphibolite facies but there is breaking down of micas to K-feldspar in the granulite facies, and K-feldspar do not concentrate Rb and Cs [24]. The Sr value in this rock is high, which might compound the low value of Rb, thus giving rise to low Rb/Sr ratio. REE patterns show LREE enrichment, and HREE-depleted patterns with negative Eu anomaly (Table 5], suggesting that this charnockitic gneiss of Iboropa could have been formed by partial melting and crystallisation. The enrichment of LREE and negative Eu anomaly are similar to the charnockitic rocks of Ado-Ekiti area of Nigeria [50]. The negative Eu anomaly reflects an intracrustal form of differentiation of the original igneous parent rock [51]. Conclusion The detailed petrographic observation and the mineral assemblage suggest a retrograde met-amorphic reaction. There are two generations each for biotite and plagioclase feldspar. The first-generation biotite is altered and it is concentrated in and around plagioclase as well as K-feldpar, while the second-generation biotite surrounds orthopyroxene. The high-grade minerals such as hypersthene and plagioclase gave Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria 132 rise to hydrous minerals amphiboles and bio-tite. Fluids responsible for the hydration reaction could have been transported along the numerous microfractures within the hypersthene grains. The presence of straight lamellae in the perthite is an evidence of granulite facies. The charnockitic gneiss could have been resulted from a partial melting process. Acknowledgements The use of JEOL JXA 8230 Superprobe, instrument sponsored by NRF/NEP grant 40113 (UID 74464] at Rhodes University, Graham-stown South Africa, is kindly acknowledged. The financial support from Tertiary Education Trust Fund (TETFUND) is recognised. References [1] Holland, T.H. (1900): The charnockite series, a group of Archean hypersthenic rocks in Peninsular India. Memoir Geological Survey of India, 28, 192-249. [2] Anantha, I., T.R., N.K. (1978): Geochemical Comparison of Archaean Granulites in India with Proterozoic Granulites in Canada. [3] Dobmeier, C., Raith, M.M. (2000): On the origin of 'arrested' charnockitization in the Chilka Lake area, Eastern Ghats Belt, India: a reappraisal. Geological Magazine, 137(1), pp. 27-37. [4] Ghosh, J.G., de Wit, M.J., Zartman, R.E. 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[40] Allen, P., Condie, K.C., Narayana, B.L. (1985): The geochemistry of prograde and retrograde charnock-ite-gneiss reactions in southern India. Geochimica et Cosmochimica Acta, 49(2), pp. 323-336. [41] Oyawale, A.A., Ocan O.O. (2020): Migmatization process and the nature of transition from amphib-olite to granulite facies metamorphism in Ikare area Southwestern Nigeria. Journal of Geology and Mining Research, 12(2), pp. 45-64. [42] Cooray, P.G. (1961): The Geology of the area around Rangala: Ceylon Geol. Survey Mem., 2, 138 p. [43] Cooray, P.G. (1962): Charnockites and their associated gneisses in the Pre-Cambrian of Ceylon. The Quarterly journal of the Geological Society of London, 118, pp. 239-273. [44] Raase, P. (1974): Al and Ti contents of hornblende, indicators of pressure and temperature of regional metamorphism. Contributions to mineralogy and petrology, 45(3), pp. 231-236. [45] Spear, F.S. (1981): An experimental study of hornblende stability and compositional variability in amphibolite. American Journal of Science, 281(6), pp. 697-734. [46] Dymek, R.F. (1983): Titanium, aluminum and inter-layer cation substitutions in biotite from high-grade gneisses, West Greenland. American Mineralogist, 68(9-10), pp. 880-899. Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria 134 [47] Spry, A. (1969): Metamorphic Textures. Pergamon Press, Oxford: England, 358 p. [48] Cayzer, N. (2002): Feldspar microtextures and the cooling histories of high-grade terrains. Ph.D. Thesis. University of Edinburgh, U.K., 300 p. [49] Fyfe, W.S. (1973): The granulite facies, partial melting and the Archaean crust. Philosophical Transactions of the Royal Society of London. Series A, 273, pp. 457-461. [50] Olarewaju, V.O. (1987): Charnockite-granite association in SW Nigeria: rapakavi granite type and char-nockitic plutonism in Nigeria? Journal of African Earth Sciences, 6(1), pp. 67-77. [51] Bhattacharya, S. Chaudhary, A.K., Basei, M. (2012): Original nature and source of Khondalites in the Eastern Ghats Province, India. Geological Society, London, Special Publications, 365, 147-159. RMZ - M&G | 2020 | Vol. 67 | pp. 119-134 Oziegbe E.J., Olarewaju V.O., Ocan O.O., Costin G. 135 Original scientific article Received: Dec 02, 2020 Received: Dec 07, 2020 DOI: 10.2478/rmzmag-2020-0014 High-resolution Biostratigraphic Studies of Step-1 Well, Offshore, Western Niger Delta Visoko resolucijske biostratigrafske preiskave morske vrtine Step-1 na območju zahodne delte reke Niger M. E. Nton* and S. O. Famori* Department of Geology, University of Ibadan, Ibadan, Nigeria Corresponding author: E-mail: ntonme2009@gmail.com, sfamori@yahoo.com Abstract A high-resolution biostratigraphic study of the STEP-1 well, offshore Western Niger Delta Basin, Nigeria, was carried out using foraminifera, calcareous nannofossils and palynomorphs. The study was aimed at identifying the biostratigraphic zones, age deductions as well as palaeoenvironmental and palaeoclimatic reconstructions. From the studied well section of 609 m (1,829-2,438 m), 50 ditch cuttings were used for foraminifera and calcareous nannofossils, while 25 samples were used for palynological studies at 12 m and 24 m intervals, respectively. Standard laboratory preparation techniques were employed for the three microfossil groups. Due to the occurrence of some forms such as Globigerina praebulloides, Haplophragmoides spp, Bolivina scalprata miocenica, Valvulina flexilis and Cyclammina cf. minima, two planktonic and one benthonic foraminifera zones were identified as follows: Lower N18, Upper N17 zone (early Pliocene, late Pliocene) and Cyclammina minima zone (late Miocene), respectively. Two biozones were recognized for the nannofossils and include NN12 (Ceratolithus cristatus zone) and NN11 (Discoaster berggrenii zone). These zones were assigned to early Pliocene and late Miocene, respectively. Other forms include Discoaster pentaradiatus, Sphenolithus abies and Ceratolithus armatus. Echitricolporites spinosus/ P800zone has been assigned for the Palynomorph assemblages and was dated late Miocene due to the quantitative occurrence of Cyperaceaepollis spp. Four identified major condensed sections include intervals at 1,926, 1,987, 2,097 and 2,316 m, which have been dated 5.0, 5.8, 6.3 and 7.0 Ma, respectively. Based on the benthonic foraminiferal species and Palynological Marine Index, a shallow marine environment is deduced for the studied interval which was interpreted to be deposited under both wet and dry palaeoclimatic conditions. The findings, no doubt could serve as a template for a sequence stratigraphic model, generally beyond the resolution of seismic stratigraphy. Keywords: drilling regime, roller cone bit, wear, reduced penetration rate, change of the material properties. Izvleček Z uporabo alg luknjičark, apnenčastih nanofosilov in palinomorf so bile na območju zahodne delte reke Niger izvedene visoko resolucijske biostratigrafske preiskave morske vrtine STEP-1. Namen preiskave je bilo prepoznavanje biostratigrafskih območij, starosti ter paleookoljskih in paleoklimatskih rekonstrukcij. Iz 609 m dolgega preiskovanega odseka vrtine (18292438 m) je bilo uporabljenih 50 vzorcev za preiskave alg luknjičark ter apnenčastih nanofosilov ter 25 vzorcev za palinološke preiskave na intervalih dolžin 12 m in 24 m. Uporabljene so bile standardne laboratorijske tehnike priprave za tri skupine mikrofosilov. Zaradi pojavljanja nekaterih oblik, kot so Globigerina praebulloides, Haplophragmoides spp, Bolivina scalprata miocenica, Valvulina flexilis in Cyclammina cf. minima sta bili identificirani dve planktonski ter ena bentonska cona alg luknjičark. To sta coni: lower N18, Upper N17 (zgodnji Pliocen, pozni Pliocen) in cona Cyclammina minima (pozni Miocen). Za nanofosile sta bili prepoznani dve bioconi, in sicer NN12 (cona Ceratolithus cristatus) in NN11 (cona Discoaster berggrenii). Omenjeni coni sta bili dodeljeni zgodnjemu Pliocenu ter poznemu Miocenu. Ostale forme vključujejo: Discoaster pentaradiatus, Sphenolithus 136 abies in Ceratolithus armatus. Echitricolporites spinosus/ cona P800 je bila dodeljena sklopom palinomorfa in je bila datirana kot pozni Miocen zaradi kvantitativnega pojavljanja Cyperaceaepollis spp. Štiri identificirani glavni zgoščeni odseki vključujejo intervale 1926 m, 1987 m, 2097 m in 2316 m, ki so bili datirani kot 5.0 Ma, 5.8 Ma, 6.3 Ma in 7.0 Ma. Na osnovi bentonske vrste alg luknjičark in palinološkega morskega indeksa, je plitvo morsko območje izbrano za preučevan interval ki je nastal pod mokrimi in suhimi paleoklimatskimi pogoji. Izsledki preiskave lahko brez dvoma služijo kot osnova za zaporedni stratigrafski model, ki v splošnem presega ločljivost seizmične stratigrafije. Ključne besede: biostratigrafija, paleoklima, paleookolje, palinološki morski indeks Introduction The Niger Delta Basin is one of the rift-generated sedimentary basins on the continental margin of the Gulf of Guinea in Nigeria [1]. The study area lies approximately between latitudes 4°23'30"N and 6°24'00"N and longitudes 2°45'30"E and 6°46'30"E (Figure 1). As the Niger delta basin matures, most of its subsurface uncertainties lie at reservoir scale, and hence the need for application of biostratigraphy. Oil companies which invest hugely in this high-risk business of oil exploration have undoubtedly gained much from biostratigraphic studies. Apart from assigning ages to rocks, the prediction of water depths and palaeodepositional environments is vital for the understanding and deduction of deposition-al models with reasonable productiveness [2]. Previous studies include those on foraminifera biostratigraphy from offshore Western Niger Delta [3]. Fajemila [3] recognized five foramin-iferal zones and inferred that the sediments were of normal salinity and belonged to early Pliocene to late Miocene age. Sanuade [4] carried out calcareous nanno-fossil biostratigraphy of Well 'K-2', in the deep offshore, Niger Delta. Two major zones were identified, while one condensed section was believed to be associated with 2.0 Ma maximum flooding surface (MFS). Aturamu [6] reported on integrated biostratigraphic studies using foraminiferal and palynomorph assemblages within the Niger Delta. They established two planktonic fora-miniferal zones, Globigerinoidies obliquus and Globigerinoides ruber, aged Miocene-Pliocene and four benthic foraminiferal biozones. They proposed Echiperioporities cf. estelae, Psilatricolpites okeizeis and Foeveotricolpites sp. palynological zones which are correlatable with the Pantropical zones. Ukpabi and Osterloff [7] RMZ - M&G | 2020 | Vol. 67 | pp. 135-148 integrated microflora and nannoflora for biostratigraphic and palaeoenvironmental study of the Niger Delta sediments. They documented the occurrence of Belskipolliselegans which revealed the influence of climate, sea-level changes and/or sediment influx in the Niger Delta during the middle Miocene. The trans-gressive and regressive trends were interpreted and correlated based on the general distribution of Belskipolliselegans within the palynological zones in the middle Miocene Agbada Formation of the Niger Delta Basin. The present study gives an account of integrated foraminiferal, calcareous nannofossils and palynostratigraphy of the STEP-1 well, offshore western Niger delta, in order to deduce the age of the penetrated interval, and provide detailed information on the paleoenvironment of deposition and palaeoclimatic conditions of the sediments. Such information, no doubt, could serve as a sequence stratigraphic model, generally beyond the resolution of seismic stratigraphy. Geologic setting and stratigraphy The Niger Delta basin is situated between latitudes 3° and 6°N and longitudes 5° and 8°E in the Gulf of Guinea on the margin of West Africa, and is one of the largest deltaic systems in the world. Doust and Omatsola [8] and Short and Stauble [9] classified the subsurface Niger Delta into three stratigraphic units from the oldest to the youngest: Akata, Agbada and Benin Formations (Figure 2). The Akata Formation is the basal unit of the Tertiary Delta Complex and comprised shales, clays and silts at the base of the known delta sequence. They contain a few Nton, M. E. and Famori, S. O. 137 Geological Map or Nigeria Legend Ternary Hcftcanf Hnilgççne Cretaceous und ■ "J sil m Fr^carr.trian =updMiaa-Water _I Quaternary andTertiary SO KUomitira Figure 1: Geological map of Western Niger Delta showing the approximate study area (modified from [5]). AÖE FORMATION UTMOLOGY »icon 3ÎDr«*NT«tv ÇTCLE EnVIRMIMeUT HOÎ.OŒNE \ v 3ENI« PLEISTOCENE ^ \ V• ■ % ' ' s S N m 9 e ID z u PI.IOCENE \ \ * - ' . ". * . " »J CONTINENTAL o MJ MIOCENE J \ \ a \ * OU30CENE \ AGBADA E V in «Ü Iti o o SOtENÈ r-r-z--- o s m A »EGBF3SI0N Z IM TRANSfTIONAL TO MiftiNE *t _ ÄKATA \ O - - _—_—_— S X PALEOCENE S8 m « i r»AHSO«e$i»0« WAR|Nt Figure 2: Stratigraphie succession of the Niger Delta [7]. streaks of sand, possibly of turbiditic origin [10]. The Agbada Formation is the hydrocarbon-prospective sequence, a paralic clastic sequence which lies above the Akata Formation. The upper part of the Agbada Formation often has sand percentages ranging from 50% to 75%, becoming increasingly sandy towards the overlying Benin Formation, while the basal part is more of shale sequence [8]. The Benin Formation is the freshwater-bearing formation in the Niger Delta. It comprises the top part of the Niger Delta Basin, from the Benin-Onitsha area in the north to beyond the present coastline [9]. Biostratigraphic studies of STEP-1 well, offshore, Western Niger Delta 138 Materials and methods Materials used in this study include ditch cuttings and gamma ray log. Because of the proprietary nature of the well, the exact location information was not provided, hence it was code-named STEP-1 well. The samples were selected and composited at intervals of 12 m and 24 m, respectively, which was then prepared and analysed based on fauna and flora contents. The biostratigraphic analyses were carried out while the other analysis was achieved using the STRATABUG software. Lithologie description The lithologic description was carried out by examining the samples under a hand lens in order to identify the constituents, describe and name the lithology. This was supported by the GR log to complement the lithologic description based on its deflection away from the Shale Baseline (SBL). Sample preparation The ditch cuttings were prepared for fora-minifera, calcareous nannofossils and palyno-morph contents using standard preparatory methods. Fifty samples were selected at 12 m intervals for foraminifera preparation; the standard weight (20 g] of each sample was soaked overnight to ensure proper disaggregation and liberation of microfossils present within the cuttings. The disaggregated samples were then washed under a shower of water over a 63 ^m sieve, rinsed in liquid detergent to remove drilling mud and dried over a hot plate. The forms were picked with a picking needle under an Olympus binocular microscope. Preparation and identification of individual foraminifera were guided following the works of [11-13], among others. Fifty samples were selected at 12 m interval for calcareous nannofossils; about 2 g of each of the samples were scraped onto a glass microscope slide. The slides were labelled sequentially and two blobs of Norland Optical Adhesive were affixed onto the cover-slip. The slides were dried and later studied under a transmitting light microscope in polarized light. This was done following Martini [14]. Twenty-five samples were selected and composited at 24 m intervals for palynological analysis. A constant weight (30 g] of each sample was initially given a 5% dilute hydrochloric acid treatment to remove carbonates prior to complete digestion in concentrated hydrofluoric acid (HF] under a fume cupboard. The samples were decanted thrice at an interval of 1 h each through the Branson Sonifier and with the aid of a 5 ^m sieve to filter away the inorganic matter. A LOCTITE mounting medium was used for the residues, which are the pal-ynomorphs. Identification and preparation of the specimen was done following Germeraad et al. [15]. Results and discussion Lithologic description The lithostratigraphic section of the studied well was produced from ditch sample descriptions and the deflection away from the shale base line on the gamma ray log. The total thickness of the analysed sample is 609.6 m (i.e. between 1,828.8 m and 2,438.4 m intervals]. From the lithologic description, the samples are mainly shale with a little sand. The shale is fissile, greyish to black, while the sand is fine to medium grained. The observation revealed that the studied well is largely made up of a sequence of fine-grained shale alternating with fine- to medium-grained sandstone in the lower part while the upper part is mainly shale (Figure 3]. Foraminifera biostratigraphy The well recorded fairly rich and diverse assemblages of planktonic and benthonic foramin-ifera at the upper part (1,902-2,109 m], with 55 species recorded. Of these, 22 (40%) species are calcareous, while 27 (49%) are arenaceous. Two foraminiferal "zones" were recognized in the studied section. The Cenozoic chronostrati-graphic scheme of Berggreni et al. [13] and the Global Sequence Cycle Chart of Hardenbol et al. [16] were adopted for this study. The zones are characterized briefly below.\ RMZ - M&G | 2020 | Vol. 67 | pp. 135-148 Nton, M. E. and Famori, S. O. 139 Figure 3: Lithological description of STEP-1 well. 1. Globorotalia tumida/Cyclammina minima zone Stratigraphic interval: 1,841-1,987 m Equivalent planktic foraminiferal zone: "Lower" N18-"Upper" N17 zone. Age: Late Miocene (5.99-5.47 Ma) Diagnosis: The top of this zone is placed at the 5.47 Ma MFS, recognized at 1,926 m, while the base is defined by the 5.8 Ma MFS, recognized at 1,987 m (Figure 4). The First Downhole Occurrence (FDO) of the zonal marker, Cyclammina cf. minima was recorded at 1,926 m (Table 1). The zone correlates with the "Lower" N18-"Upper" N17 planktic foraminiferal zone of Berggreni et al.[13] and Hardenbol et al. [16]. The age is late Miocene. 2. Globoquadrina dehiscens/Haplophragmoides narivaensis zone Stratigraphic interval: 1,987-2,438 m. Equivalent planktic foraminiferal zone: "Middle-Lower" N17 zone. Age: Late Miocene Diagnosis: Undiagnosed Calcareous nannofossil biostratigraphy The results of calcareous nannofossil analysis show high abundance and diversity of species (Figure 5). Biozonation and age determination of the well was based largely on calcareous nannofossils assemblages, abundance and diversity. The chronostratigraphic scheme adopted follows the usage of the worldwide zo-nation schemes of Okada and Bukry [17] and Haq et al. [18]. Considerable effort was made to identify and define zonal tops with the FDOs of diagnostic marker species, abundance and species diversity peak as these form the most reliable events [14]. The highest nannofossil peaks were dated using important marker species such as Sphenolithus abies, Discoaster pentara-diatus, Ceratolithus cristatus, Ceratolithus ar-matus, Discoaster quinqueramus and Discoaster berggrenni (Table 2). The stratigraphic distribution of the recorded species along with the significant datum, MFSs, nannofossil zones and age interpretations are presented in distribution charts (Figure 5). The highlights of the results are summarized below. Biostratigraphic studies of STEP-1 well, offshore, Western Niger Delta 140 W-s® «Jira : tTEF-1 STEP-1 C«YI»i. AK LJWIE3 UfiOl f B-..I J I 3 | i | ! II I I s I I Pi Mil itiii! jl 1!.!. til i il jl. E | — - I = 1 jL_ ■ liL-klikT-^. B"F*J* rjajHiudMii ■ lififetaWS- ■■zz?. ■ ■ ■ ■ ■ - I -- i r I! " LL 1 li11 'l .:.. ' 'I i "l" "1......... = V : '. 1: Hi:" i 1 -- | i E f It ■ '' {■'............. , V 1 --- 1 k !' —. . : f t !' i ......L.I .....f ....!.........i f U L, 1 he It i' " — = Figure 4: Foraminiferal distribution chart of STEP-1 well. Table 1: Foraminferal biostatigraphy of STEP-1 well Depth Age (m) Planktonic zone Benthonic foram zone Bioevents 1,853 1,926 Early Pliocene N18 Indeterminate FDO Valvulina flexilis Cyclamina cf. minima 5.0 Ma MFS 1,987 2,097 2,438 Cyclamina cf. minina Late Pliocene N17 and older Peak fauna abundance 5.8Ma MFS Indeterminate 1. Interval: 1,841-1,926 m Zone: NN12 Age: Early Pliocene Diagnosis: This interval is dated based on the presence of Ceratolithus cristatus at 1,902 m and the presence of Ceratolithus armatus at 1,914 m. These records of nannofossils represent the early Pliocene. 2. Interval: 1,926-2,438 m Zone: NN11 Age: Late Miocene Diagnosis: This interval is characterized by an increase in nannofossil abundance and diversity. Its nannofossil peak at 1,963 m represents the 5.8 Ma MFS [18] late Miocene NN11 zone. This is confirmed by the FDO of Discoster quinqueramus at 1,939 m. The RMZ - M&G | 2020 | Vol. 67 | pp. 135-148 presence of Discoaster berggrenii at 2,292 m further confirmed the 7.0 Ma MFS [18] age assigned to the lower section. Palynostratigraphy Twenty-five palynomorphs were selected at 24 m intervals. The palynomorphs are well preserved and fairly diverse (Figure 6]. These include Zonocostites ramonae, Monoporites annulatus, Sapotaceae, Retitricolporites irregularis, Pachydemites diederixi, Acrostichum aureum, Pteris spp, Laevigatosporites spp, Verucatosporites spp, Stereisporites spp, Cyperaceaepollis spp, Sapotaceae and Psilatricolporites crassus. Dinoflagellate cysts recorded are Selenopemphix spp, Polysphaeridium zoharyi and Leiosphaeridia spp. Nton, M. E. and Famori, S. O. 141 Figure 5: Calcareous nannofossil distribution chart of STEP-1 well. Table 2: Calcareous nannofossils biostratigraphy of STEP-1 well Depth (m) Downhole occurrence of calcareous nannofossils Age (Ma) Haq et al. (1988) NP zones (Martini, 1971) Inferred relative ages 1,841 1,902 1,914 1,926 1,939 1,963 2,292 2,316 2,438 TD First sample analysed Presence of Ceratolithus cristatus Presence of Ceratolithus cristatus FDO: Discoaster quinqueramus Maximum flooding surface FDO: Discoaster berggrenii Maximum flooding surface, Presence: Discoaster quinqueramus 5.8 7.0 NN 12 NN 11 Late Miocene to early Pliocene The STEP-1 sediments are assigned to the section within the Pantropical Echitricolporites spinosus zone of Germeraad et al. [15] and the P800 zone of Evamy et al. [1]. The zone was further subdivided into P840 and P830 sub-zones. Details of the zone and sub-zone identified are represented in Tables 3 and 4, and explained briefly below. 1. Zone: P800 Sub-zone: P840 Interval: 1,829-2,204 m Discussion: The top of this sub-zone is placed at 1,829 m of the first sample analysed. The base is defined by the quantitative occurrence of Cyperaceaepollis spp at 2,204 m. The P840 sub-zone is further confirmed and characterized by the presence of Stereisporites Biostratigraphic studies of STEP-1 well, offshore, Western Niger Delta 142 Figure 6: Palynomorphs distribution chart of STEP-1 well. Table 3: Palynostratigraphy of STEP-1 well Depth (m) series Sub-series Gemeraad et al. (1968) Evamy et al. (1978) Bioevents Zone Sub-zone 1,829 1,981 2,134 2,204 2,286 2,438 Miocene Late Miocene Echitricolporites Spinosus zone P800 P840 P 830 Quantitative base occurrence of Cyperaceaepollis spp spp, abundant mangrove pollen Zonocostites ramonae and Monoporites annulatus. The age assigned to this sub-zone is late Miocene. 2. Zone: P800 Sub-zone: P830 Interval: 2,204-2,438 m Discussion: The top of this sub-zone is placed at 2,204 m defined by the quantitative base occurrence of Cyperaceaepollis spp while the base was not penetrated. This sub-zone is further characterized by the presence of Cyperaceaepollis spp, Stereisporites spp and Racemonocolpites hians. The age of this subzone is late Miocene. Age deductions The ages for this well were established using three bioevents (Table 5]: these include recorded peak faunal abundance (MFS), sequence boundary (SB] and the occurrence of some index fossils such as Cyclammina cf. minima and Discoaster beggrenii. The chronostratigraphic position of STEP-1 well is given in Table 6. Palaeoenviroment of deposition 1. The presence of some benthonic foramin-ifera such as Uvigerina auberiana, Lenticulina spp, Valvulineria spp, Haplophragmoides spp, Cyclamina cf. inina and Bolivina scalprata RMZ - M&G | 2020 | Vol. 67 | pp. 135-148 Nton, M. E. and Famori, S. O. 143 m 3 -a 15 3 -a to to o CJ u c CJ Ph PH & o Q O ^ 00 Ph 2 2 m oo Ln C ns -Ö C -Q ns "ro C ,ns t4—I ^ ns CJ Ph O O 00 Ph O J3 O £ -S C« Co 00 2 z < w u z < N CJ c iy u t: o ns ;=; W Ph o o o o o O LTl O LTl O O0 '"l ^H" ^H" cm" CM" CM" o CJ u c aj m in ns o CJ w ns -Q CJ > ^ Oü Q Si a c tu s- tu -a s- £ tO Q O O fO 00 Ph Z Z t^ _ aj £ = 2 Z ns O Z < m in W o o LTl O CM" cm" cm" ns o o o o LTl O LTl O CM" C-0 t"0 cm" cm" cm" cm" indicates a shallow marine environment (inner neritic). 2. With land-derived palynomorphs such as Monoporites annulatus, Laevigatosporites spp, Zonocostites ramonae, Acrostichum aureum, Pachydemites diederixi, Pteris spp, Striatricolpites catatumbus and Cyperaceaepollisspp (from 1,853-2,438 m). It can be deduced that the sediments were deposited within a shallow marine environment. Palaeoclimatic deductions Palynofossils are preserved mainly in continental basins. The characteristics of great quantity, wide distribution and different preserved lithofacies are the unique advantage of these fossils [19], For this reason, palynomorphs are now very important for reconstructing palae-oclimatic conditions at the time of sediments deposition. The climate of an area is reflected by its vegetation type [20]. The three important palynomorphs used for this study are Monoporites annulatus, Zonocostites ramonae and Acrostichum aureum. Monoporites annulatus abundance is often used as an indicator of a large degree of landscape openness [21, 22], and increased aridity [23, 24]. Zonocostites ra-monae has been used as an indicator for mangrove vegetation and hence, wet climatic condition [25]. Acrostichum aureum is used as an indicator of wet climatic condition [26]. From Figure 7, the dominance in abundance of Monoporites annulatus over the relative abundance of Acrostichum aureum favours a dry climatic condition for the interval 1,8532,048 m. While the sediments are believed to have been deposited in the Savanna setting, interval 2,073-2,146 shows alternating wet and dry climate. From Figure 8, the dominance in abundance of Acrostichum aureum over Monoporites annulatus from interval 2,170-2,438 m indicates a wet climatic condition; hence, they are of mangrove species, although interval 2,2432,265 m relatively shows alternating wet and dry climatic conditions. Figure 9 shows photomicrographs of foraminifera, calcareous nan-nofossil and Palynomorps recovered from the STEP-1 well. Biostratigraphic studies of STEP-1 well, offshore, Western Niger Delta 144 Table 5: Age deductions for STEP-1 well Depth (m) Age (Ma) Microfossil Bioevents 1,926 1,963 1,987 2,G97 2,316 5.G 5.8 5.8 6.3 7.G Foraminifera Nannofossils Foraminifera Nannofossils Nannofossils FDO Cyclammina cf. minima Maximum flooding surface (MFS) Peak faunal abundance (MFS) Sequence boundary MFS Table 6: Chronostratigraphic position of STEP-1 well Ma Period Standard chronostratigraphy Epoch Age/stage This study 4 5 6 7 8 9 1G Quaternary Neogene Holocene Pleistocene Pliocene Miocene Taranian Ionian Calabrian Gelasian Piacenzian Zanclean Messinian Tortonian STEP-1 well Summary and conclusions Foraminifera, calcareous nannofossils and pal-ynomorph integrated biostratigraphic studies of the STEP-1 well in the offshore of Niger Delta Basin has resulted in the identification of biostratigraphic zones, determination of ages as well as reconstruction ofthe palaeoenvironment and palaeoclimatic conditions. The presence of some benthonic foraminifera such as Uvigerina auberiana, Lenticulina spp, Valvulineria spp, Haplophragmoides spp, Cyclamina cf. inina and Bolivina scalprata indicates a shallow marine environment of deposition (inner neritic). Two nannofossil zones subdivided into late Miocene and early Pliocene are confirmed by the FDO of Discoster quinqueramus at 1,939 m. The presence of Discoaster berggrenni at 2,292 m further confirmed the 7.0Ma MFS age assigned to the interval. The late Miocene age also corroborates with the appearance of the zonal marker, Cyclammina cf. minima, which was recorded Figure 7: Plot of A. aureum/M. annulatus against depth (1,853-2,146 m). at 1,914 m. The Palynological Zonation of the well and their stratigraphic distribution based on the zonation schemes of Germeraad et al. [15] and Evamy et al. [1] are broadly assigned to the Echitricolporites spinosus and P800 zone, respectively. The latter was further subdivided into P840 and P830 sub-zones. The assigned age is late-late Miocene. The boundary between P830 and P840 was at 2,204 m, which was marked by the Quantitative base RMZ - M&G | 2020 | Vol. 67 | pp. 135-148 Nton, M. E. and Famori, S. O. 145 Figure 8: Plot of A. aureum/M. annulatus against depth (2,170-2,438 m). occurrence of Cyperaceaepollis spp. The four major condensed sections identified within the studied section of the well which include intervals at 1,926, 1,987, 2,097 and 2,316 m have been dated 5.0, 5.8, 6.3 and 7.0 Ma, respectively, based on the identified foraminiferal and calcareous nannofossil zones with their equivalent bioevents. This study reveals that the well penetrated a major sedimentary formation in the Niger Delta, which conforms to the paralic 1. Uvigerina peregrina 2. Bolivina scalptrata miocenico 3. Lagen asp. 4. Cyclammina cancellata 5. Lenticulina sp. 6. Cibicides sp 7. Planulina arinimensis 8. Lenticulina inornata 9. Valvulineriaspp. 10. Cyclammina cf. minima 11. Globigerina praebulloides (1) Leiosphaeridia sp (2) Acrostichum aureum (3) Pachydermites diederixi (4&5) Psilatricolporites crassus (6) Microforaminiferal wall linings (7) Laevigatosporites sp (8) Monoporites annulatus (9) Pteris sp (10) Verrucatosporites sp Figure 9: Continued Biostratigraphic studies of STEP-1 well, offshore, Western Niger Delta 146 1 «4 u in. Ar 18 £ 0 (1) Discoaster quinqueramus (2) Discoasterpentaradiatus (3) Discoaster brouweri (4) Sphenolithus abies (5) Discoaster berggrenii (6) Pontosphaera multipora (7) Helicosphaera carteri (8) Reticulofenestra pseudoumbilicus (9) Ceratolithus armatus (10) Ceratolithus cristatus (11) Discoaster quinqueramus (12) Pontosphaera japonica Figure 9: Photomicrographs offoraminifera, calcareous nannofossil and Palynomorps recovered from STEP-1 well. sequence of Agbada Formation. The palynoflo-ra Monoporites annulatus, Laevigatosporites sp. among others suggests a shallow marine environment of deposition, while the palaeoclimat-ic setting indicated both wet (1,853-2,146 m) and dry (2,170-2,438 m) conditions. The integration of the three microfossil groups for biostratigraphic studies has produced a detailed analysis in the recognition and interpretation of absolute biosignals. Acknowledgements The authors appreciate the efforts of Mrs Adebiyi E. Abidemi from Mosunmolu Ltd. for her support and the staff of Crystal Age Ltd, Lagos. References [1] Evamy, B.D., Harremboure, J., Kamerling, P., Knaap, W.A., Molloy, F.A., Rowlands, P.H. (1978): Hydrocarbon habitat of the Tertiary Niger Delta. American Association of Petroleum Geologists Bulletin, 62, pp. 277-298. [2] Singh, A. (2008): Micropaleontology in petroleum exploration, Seventh International Conference and Exposition on Petroleum Geophysics Bulletin, pp. 288-294. RMZ - M&G | 2020 | Vol. 67 | pp. 135-148 Nton, M. E. and Famori, S. O. 147 [3] Fajemila, O.T. (2012): Foraminifera biostratigraphy and paleoenvironmental studies of two wells from offshore western Niger delta. Ife Journal of Science, 14(2), pp. 369-384. [4] Sanuade, O.A. (2014): Calcareous nannofossil biostratigraphic analysis of well 'K-2', deep offshore Niger Delta, Nigeria. Advances in Research, 2(12), pp. 696-711. [5] Obaje, N.G. (2009): Geology and mineral resources of Nigeria, lecture notes in earth sciences 120. Journal of Geoscience and Environment Protection, DOI: 10.1007/978-3-540-92685-6. [6] Aturamu, A.O., Ojo, A.O. (2015): Integrated biostratigraphic analysis of the Agbada Formation [Nep-1 Well] offshore, Eastern Niger-Delta Basin, Nigeria, Australian Journal of Biology and Environment Research, 2(1), pp. 1-14. [7] Ukpabi, N., Osterloff, P.L., Oloto, I.N. (2014): Biostratigraphy studies of Miocene sediments in the onshore/offshore area, Niger Delta Basin, Nigeria. International Journal of Science Inventions Today, 3(4), pp. 330-345. [8] Doust, H., Omatsola, E. (1990): Niger Delta. In: Divergent/passive Margin Basins, American Association of Petroleum Geologists Memoir 48, Edwards, J.D., Santogrossi, P.A., (eds.), American Association of Petroleum Geologists: Tulsa, pp. 239-248. [9] Short, K.C., Stauble, A.J. (1967): Outline of the geology of Niger Delta. American Association of Petroleum Geologists Bulletin, 51, pp. 761-779. [10] Avbovbo, A.A. (1978): Tertiary lithostratigraphy of Niger Delta. American Association of Petroleum Geologists Bulletin, 62, pp. 295-300. [11] Blow, W.H. (1969): Late middle Eocene to recent planktonic foraminiferal biostratigraphy. In: Proceedings of the First International Conference on Planktonic Microfossils, Genova, Brönnimann, P., Renz, H.H. (eds.). E.J. Brill, pp. 199-442. [12] Loeblich, A.R. Jr., Tappan, H. (1988): Foraminifera genera and their classification, Vol. 2. Van Nostrand Reinhold Company: New York, pp. 970. [13] Berggreni, W.A., Kent, D.V., Swisher, C.C., Aubry, M. (1998): A revised Cenozoic geochronology and chron-ostratigraphy. DOI:10.2110/pec.95.04.0129 [14] Martini, E. (1971): Standard Tertiary and Quaternary Calcareous Nannoplankton Zonation. In: Proceedings IIPlanktic Conference, Rome, Farinacci, A. (ed.). 1970. 2, pp. 739-785. [15] Germeraad, J.H., Hopping, C.A., Muller, J. (1968): Palynology of tertiary sediments from tropical Biostratigraphic studies of STEP-1 well, offshore, Western Niger Delta areas. Review of Palaeobotany and Palynology, 6, pp. 189-346. [16] Hardenbol, J., Thierry, J., Farley, M.B., Jacquin, T., de Gracuabsjt, P., Vail, P.R. (1998): Mesozoic and Cenozoic sequence chronostratigraphic framework of European basins. In: Mesozoic and Cenozoic sequence stratigraphy of European basins, De Graciansky, P.C., Hardenbol, J., Jacquin, Th., Vail, P.R. (eds.). SEPMSpecial Publication 60. [17] Okada, H., Bukry, D. (1980): Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation [Bukry, 1973, 1975]. Marine Micropaleontology, 5, pp. 321-325, DOI: 10.1016/0377-8398[80]90016-X. [18] Haq, B.V.J., Hardenbol, J., Vail, P.R. (1988): Mesozoic and Cenozoic chronostratigraphy and eustatic cycles. In: Sea level change - an integrated approach, Wilgus et al. (eds.), SEPM Special Publication, No. 42, pp. 71-108. [19] Zhao, J., Wan, X.Q., Xi, D.P., Jing, X., Li, W., Huang, Q.H., et al. (2014): Late cretaceous palynology and paleoclimate change: Evidence from the SK1 [South] core, Songliao Basin, NE China. Science China Earth Sciences, 57(12), pp. 2985-2997, DOI: 10.1007/ s11430-014-4975-4. [20] Samant, B., Phadtare N.R. (1997). Stratigraphic pa-lynoflora of the Early Eocene Rajpardi lignite, Gujarat and the lower age limit of the Tarkeshwar Formation of South Cambay Basin, India. Palaeontographica, B, 245, pp. 1-108. [21] Joly C, Barille L, Barreau M, Mancheron A, Visset L. (2007): Grain and annulus diameter as criteria for distinguishing pollen grains of cereals from wild grasses. Review of Palaeobotany and Palynology, 146, pp. 221-233. [22] Schüler, L., Behling H. (2011): Poaceae pollen grain size as a tool to assess palaeoecological grassland dynamics in South America. Veget Hist Archaeobot, 20, pp. 83-96. [23] Salgado-Labouriau M.L., Rinaldi M., (1990): Measurements of gramineae pollen of the Venezuelan mountains. Revista Brasileira de Biologia, 50, pp. 115-122. [24] Behre K.E. (1981): The interpretation of anthropogenic indicators in pollen diagrams. Pollen Spores, 23, pp. 225-245. [25] Massini Garcia, J.L., Jacobs, B.F., Pan, A., Tabor, N., Kappelman, J. (2006): The occurrence of the fern Acrostichum in Oligocene volcanic strata of the 148 Northwestern Ethiopian Plateau. International Journal of Plant Science, 167(4), pp. 909-918. [26] Morley, R.J. (1995): Biostratigraphy characterization of systems tracts in tertiary sedimentary basins. Proceedings of the International Symposium on Sequence Stratigraphy, Jakarta, S.E. Asia, 1, pp. 49-71. RMZ - M&G | 2020 | Vol. 67 | pp. 135-148 Nton, M. E. and Famori, S. O. 149 Original scientific article Received: Oct 14, 2020 Accepted: Jan 14, 2021 DOI: 10.2478/rmzmag-2020-0015 Fractional Current Flow in the Subsurface Using Electrical Resistivity Method: A Laboratory Approach Podpovršinsko gibanje električnega toka z uporabo električne uporovne metode: laboratorijski pristop James Adeyemo Adegoke1, Gideon Oluyinka Layade2*, Temitayo Kadir1 1Department of Physics, University of Ibadan, Ibadan, Nigeria 2Department of Physics, Federal University of Agriculture, Abeokuta, Nigeria *layadeoluyinka018@gmail.com Abstract The authors describe electrical resistivity method using a laboratory experiment, which was conducted in order to calculate the percentage of current that penetrated each layer of soil arranged in a container using Schlum-berger array. Four soil samples arranged in three different set-ups were used. The apparent resistivity obtained was interpreted using curve matching techniques and WinResist iteration yielding types A curve, H curve and A curve, respectively. The interpreted data gave the resistivity of each layer and its thicknesses. The thicknesses obtained from the interpretation were at variance with the actual thicknesses arranged in the container. A multiplier was obtained which serves as a constant in other to obtain the actual thickness. The effective penetration depth of current was determined through the calculated thickness of each layer and the known electrode spacing (AB). The percentage of current that penetrates the layers was found to depend on the electrode spacing as well as the thickness of that layer. Thus, a layer with relatively small thickness has a small percentage of current passing through it compared to a thicker layer. Key words: resistivity, depth, curve, soil, current. Povzetek Raziskava prikazuje električno uporovno metodo z laboratorijskim eksperimentom, ki je bil izveden z namenom izračuna odstotka električnega toka, ki se je gibal preko posamezne plasti zemljine v posodi, z uporabo Schlumbergerjeve postavitve elektrod. Uporabljeni so bili štirje različni vzorci zemljine s tremi postavitvami. Pridobljena navidezna upornost je bila interpretirana z uporabo tehnike ujemanja krivulj ter WinResist itera-cijskih tipov krivulj A, H in A. Iz interpretacije podatkov so razvidne upornosti posamezne plasti in njena debelina. Pridobljeni rezultati debelin posameznih plasti niso bile skladne z dejanskimi debelinami. Za izračun dejanske debeline je bil zato pridobljen in uporabljen korekcijski faktor, ki služi kot konstanta. Efektivna globina prodiranja električnega toka je bila določena preko izračunane debeline posamezne plasti in znane razdalje med elektrodama (AB). Odstotek električnega toka, ki prodira preko plasti, je bil odvisen od razmika med elektrodama ter debelino plasti. Tako ima plast zemljine z razmeroma majhno debelino v primerjavi z večjo debelino majhen odstotek toka, ki prehaja skoznjo. Ključne besede: upornost, globina, krivulja, zemljina, električni tok. 150 Introduction The electrical resistivity method is a geophysical prospecting method in which electrical current is transmitted through copper electrodes to the ground for the determination of the electrical properties of subsurface [1-3]. Two types of electrodes are involved in the resistivity surveying method and they are current and potential electrodes. A pair of current electrode is used to transfer current into the ground, while the same pair of potential electrode is employed to measure the potential drop [4]. In other words, the resistivity meter calculates the current flow resistance. The distribution of resistivity of the volume of sounding soil is determined by the electrical resistivity surveys. The possible patterns of differences in result provide information on the form of heterogeneities in the subsurface and their corresponding electrical properties [5, 6]. Resistivity measurements are also based on rock composition, interstitial fluids, temperature: and pore geometry. Such parameters influtnce the electrical resistivity of the soil, but in various ways and in varying degrees [7-9]. Electrical geophysical methods allow soil electricrl properties to be measured non-destructive=y, cost-effectively and quickly [10, 11]. Over the years, engineering studies and groundwater exploration are on the increase as a result of geophysical survey. This is made possible due to the swift advances in software development and associated numerical modelling solutions which have shown the way to the development of the present day technology [12-14]. Studies have shown that vertical electrical sounding (VES] and electrical profiling are Che two ways by which electrical resistivity survey can be conducted [6, 15, 16]. VES is applicaCle for the study of horizontal and near-horizontal interfaces. The procedures involve positioning of both the current and potential electrodes at the same relative spacing. The range is gradually expanded around a fixed cenBtal point while electrical profiling is used to assess lateral resistivity variations. The current and potential electrodes are retained and grad ually moved along a profile at a fixed distanc [6, 17, 18]. This research is aimed at determialiuu the relationship that exists between electrode separation and the percentage of current passing at a particular depth based on the arrangement of layers. Theoretical background Resistivity varies with conductivity inversely. A high-conductivity surface therefore has low resistivity and vice versa. The relationship between resistivity (p) and conductivity (s) is ^esentud cs Equation (1]; p = l/o (1) The principle of electrical resistivity is based on Ohms' law which statethat "the current flowing through a metallic conductor is proportional to the potential difference across its end provided other conditions are kept constant". Mathematically, it is defined as V = IR Where V is the Voltage in volts, I is the current in An^sund R is th e resistance in Ohms. The current density (J] and the Electric filed id]ere rulaton thro ugh Ohms' law by Equation id] wliste 10 ic in volts per meter and s is the ednducsivity nf 1he medium in Siemens per mrCer(SUm) J = rE (2) Eht Hjctri c field is the gradient of the scalar Eotential Ec -JJV (3) Thui we inte J = irVV (4) Takingthegradientofbothside V.U= -V.(rVV) (5) But A.J = 0 V.(rVV ) = 0 (6) VaVV + rV 2V = 0 The Equation (6] becomes Laplace if s inthe first term is constant and disappear, that is, the potential is harmonicwhich is stated asfollows: RMZ - M&G | 2020 | Vol. 67 | pp. 149-159 /Sacgede J.A,/_ayclOeG.O., Kadir T. 151 (a) inpUt Figure 1: Electric field lines and equipotential surfaces on the surface of a uniform half-space around a single electrode: (A) hemisphericequipotentialsurfaces, (B) radiallyoutwardfieldlinesaroundasource,and (C) radiallyinwardfieldlinesarounda sink[19]. V 2V = 0 (7) TTjepoieniia/ ojfa single electrode The current ci] in iejecled into e homogeneous half-space, with fspenifie elnctricaleend uctivity toh and sesistiviny Wt The eurrenS jna^^^^ tinditi" Che suaScco art aCl (fiaatdl^ior^ei boC does not; flow thr oush =he surface. The electric field lines ate paeallsl as tnectinreni flow eannormel co the hemisphercesh ap eee quipotential surfaces. Henae, ^ecuseent elensity i as a functi on of radiua y it waitten f e But E = pJ htiirePore, E =] pt (9) 2jir2 NOW1 E = -VV = - — = - (10] ds 2jij2 Integrating Equation (10), the equation for the potential can be stated as follows V=htL 2nr for a point source at the surface of the halfspace. The four-electrode method Among the existing geophysical techniques, theresistivitymethod is generally and mostly used for shallow subsurface studies and groundwater exploration [20]. Four electrodes are used in this method to measure resistivity [21]. One pair of electrodes is used to penetrate the current into the ground and another pair is used to measure the potential difference of hemispheric equipotential surfaces where the ground surface is intersectingthem [22]. This is the conventional way in which resistivity is measuredinuniformhalf space[23].Figure 2 Fi M, r AM" r AN B il <-r, BN r BM Figure2: Generalresistivitymeasurementmethodoffourelectrodes. shows the basic configuration of four electrodes for measuring resistivity. p a = The curre=t electrodes, A and B act as source v = sirk, respectively. The potential at the electrode M due to the source A is + p/4r while / AM sink B ie m . Tha combinsd poSsnaal (V] at M is (Voi which relaiesthe sesii^'^iaiS-s Pal current (I) and distance (r) between the source en= pa = sink is pvesevted in Equation (11] as; 2Vn A ( V ¡AM MB V ¡AN bn p So, fro m eSe above equation wederive 2Vn (14) (15) VM = fM— pi r i A V rAM rBM y (11) Similaeiy, at N, the combined potential is V =— VL N e 271 i i r r V AN ABNJ P (12) The potential diffeeenee maviureb beawsen M enti IN, Vm + VN iL I r 2tc 1 VrAM A ( i i r r V AN 1BN y (13) Then the resistivity (p) can be determined usingthefollowingformula statedhere: wherekisthegeometricfactor. Apparent resistivity Apparent resistivity is the volumetric average of a heterogeneous half-space, through a complex weighing function depending on the configurations of the electrode[24, 25].On the other hand, true resistivity is the measured' relationship between the current applied and the potential difference of an electrically homogeneous and isotropic half-space for a particular electrode arrangement and spacing [3]. This in situ resistivity is supposed to be homogeneous but not in a real situation, it gives the apparent resistivity of an equivalent uniform half-space and does not represent the ground's true resistivity [19]. The resistivity of the underlined layers is obtained from the measurement taken using a resistivity meter. Thus, apparent resistivity (pa) is obtained by emultiplying measured value by the geometric factors [26] under the assumption that the k RMZ - M&G | 2020 | Vol. 6M | pp. 149-1N9 Adegoke J.A., Layade G.O., Kadir T. 153 A or C1 I ^-w -®- HSH M or P1 N or P2 I I A- s -► B or C2 I J. «- s -* Figure 3: Schlumbergerarrayforresistivitymeasurement,consisting of a pair of current electrodes (A, B) and a pair of potential electrodes(M,N). ground is uniform; therefore, the measured resistivity is only the apparent resistivity and not the ground'strue resistivity. Depending on the resistivity value of the; 1 ayers underlined, the apparent resistivity increases or decreases withelectrode spacing. Schlumberger configurations Schlumgeuggr array used for VES investigation [11, 21], is a type of direct current resistivity survey described by its configuration. It makes use of four collinear electrodes, a pair of curredt electrodes as source and ynother pair of potential electrodes as the receivers. The potential lectrodes are arranged in between the current electrodes; the central point is kept constant while the distance between the current electrodes is increased so that current can penetrate far Oown into the guound while the potential e lectro dti s moved onlywhe n lliii voltage becomes too small to measure [27]. This arrangementenhances thecapacity of the current to penetrat into greater depths, as th conductivity is distributed vertically [19]. The Figure d! showsnhe Schlumberger configuration for resistivity measurement; Let the separations between current and potential nlecarohe be (2s + a) and a. This transforms Equation (14] as follows: rMB - S + a • rAN - S + a • rBN - S P a - Pa = 2Vn J f _!_ - _!_ ' f — - ] c n C rAM rMB c r C AN r BN C 2V n - 1 V _ f 1 1- Ji I 1 v s s + a J I s + a s Ji P a = 2Vn Is + a-s-s + s + a I ;(s + a) Pa = 2V I 2a Pa = Pa = I j s (s + a) 2Vn I s (s + a) I 2a Vn j s (s + a) 1 (16) (17) (18) (19) (20) (21) Materials and method Data acquisition The research procedure was carried out at the geophysicsresearch laboratory located at the Department of Physics, Universityof Ibadan. It was done using a tabletop model system made Fractional Current Flow in the Subsurface Using Electrical Resistivity Method: A Laboratory Approach 154 of a perspex container with a total depth of 29 cm and a length of 120 cm. Four samples of soil were used which were arranged in the transparent perspex container at different proportions. The experiment was done three times with three different arrangements. The arrangement from top to bottom is as follows; — A three-layer soil containing humus (7 cm), sandy clay (10 cm) and sand (12 cm) which all together make a depth of 29 cm. — A three-layer soil consisting of clay (4 cm), sandy clay (10 cm) and sand (15 cm) which all together make a depth of 29 cm. — A two-layer soil consisting of clay (7 cm) and sand (11 cm) which make a depth of 18 cm. In each of the arrangements, the soil samples were compacted one after the other to reduce the pore space in-between the soil particles and the moisture content was regulated. Electrical resistivity survey using Schlumberger array was carried out on each of the arrangements using Geopulse Tigre resistivity meter, first on three-layer arrangements which are humus, sandy-clay, and sand; then, clay, sandy-clay and sand; and lastly, a two-layer arrangement of clay and sand. The current electrode spacing starting from 1.0 cm to 55.0 cm and potential electrode spacing starting from 0.25 cm to 5.0 cm were used for measurement. Copper wire was used as probes in place of the copper electrode. A current of 0.5 mA was sent into the soil through the electrodes placed at two points with equal distance apart from the central point. The potential difference between the two points was measured using the resistivity meter in W-cm. Six measurements were taken in all, two for each arrangement for certainty purpose. Results and discussion Data acquired in centimetres were converted to meters and multiplied with the geometric factor (K) to obtain the apparent resistivity. The AB/2 was plotted against apparent resistivity and interpreted using a 2 layer master curve and WinResist. The mean of the thicknesses of each arrangement was mathematically simulated to Figure 4: The laboratory setup of three soil layers in a container. obtain a constant value of 6.03 which serves as a multiplier to obtaining a value close to the real value. The thicknesses obtained were used to calculate the fraction of current that penetrated each layer using the equation for current (I] dirtribution in a uniform half space as stated In Equatihn (22]; where Ix depends upon th e purr ent elec troO e spacing L: The fractionof currentbetweenanytwo depths is obtained from the difference in the fractions above each depth, calculated with Equation (22). First arrangement The first arrangement contains humus, sandy-clay, and sand. The curve is a type-A curve that showed an increase in the resistivity value with depth in Figures 5 and 6, respectively. This indicates that the resistivity of humus is lower than that of sandy clay and the sandy-clay is lower than that of sand. This result is due to the degree of water saturation in the soil type. This factor depends on the porosity, permeability and the interconnectivity of pore spaces in the rock type. Humus and sandy clay have lower resistivity of 23.15 W-m and RMZ - M&G | 2020 | Vol. 67 | pp. 149-159 /Sacgede J.A,/_ayclOeG.O., Kadir T. 155 'RMS-error: 9.0 GEOPHYSICS LAB 1A Schlumberger Configuration 10"1 10*2 Current Electrode Distance (AB/2) [m] 10*3 No Res Thick Depth 1 31.4 0.9 0.9 2 35.2 1.4 2.3 3 613.5 2.1 4 4 4 24875.3 -.- -.1 RMS on smoothed data Figure 5: Resistivity measurement of soil layers against electrode spacing (AB/2) for the first arrangement. 43.8 W-m, respectively, while sand has a higher resistivity of 757.95 W-m. Humus and sandy-clay have more porosity than sand because porosity decreases with grain size but they are less permeable, that is, they allow less water to flow. Sand in its case is less porous than humus and sandy-clay but its permeability is higher, so more water flows through it. It cannot hold water like the two types of rocks discussed earlier. This accounts for its high resistivity. The fourth layer is assumed as the plastic container whose resistivity value is very high and can be taken to be infinity. The thicknesses obtained are 5.0, 7.0 and 12.0 for the first layer, second layer and third layer, respectively, against the real values of 7.0, 10.0 and 12.0. The fraction of current passing through each layer is calculated using Equation (16] and it is 6% for first layer, 8% for second layer and 14% for the third layer Second arrangement The second arrangement contains clay, sandy-clay and sand. The sand was made to serve as an aquifer as the base of the container was sealed and the flow of water out of it was restricted. In Figures 7 and 8, the curve is a type-H curve which demarcates the resistivity of each layer. Clay and sandy-clay have higher resistivity of 122 W-m and 161.7 W-m, respectively, while the resistivity of sand is 27 W-m. These values clearly show that in this particular experiment, the clay and the sandy-clay were less saturated compared to the sand. Sand is a good aquifer because of its high permeability property; therefore, it allows the flow of water. The fourth layer is assumed as the plastic container whose resistivity value is very high and can be taken to be infinity. The thicknesses obtained are 5.0, 11.0 and 34.0 for the first layer, second layer and third layer, respectively, against the real values of 4.0, 10.0 and 15.0. The fraction of current through each layer is calculated using Equation (16] and the values obtained are 6% for first layer, 13% for second layer and 35% for the third layer. Third arrangement The third arrangement contains clay and sand. The curve obtained in Figures 9 and 10 is a type-A curve which shows that there is a direct relationship between the increases of resistivity and depth. The resistivity of clay is 67 W-m while that of sand is 301.15 W-m. The value of resistivity obtained for each layer of rock shows the degree of saturation of each layer, and which indicates that the clay held Fractional Current Flow in the Subsurface Using Electrical Resistivity Method: A Laboratory Approach 156 "RMS-error: 6.9 GEOPHYSICS LAB1B i ■ ■ X 1 : ■ / 1 : * / i , / 1 +/ 1 ■ / Y ........j ........ ........ 10*0 10*1 10A2 Current Electrode Distance (AB/2) [m] 10*3 No Res Thick Depth 1 14.9 0.8 2 524 0.8 3 902.4 2.0 4 28267.2 I 0.8 1.5 3.6 * RMS 01 smoothed data Figure 6: Resistivity measurement (repeated) of soil layers against electrode spacing (AB/2) for the first arrangement. "RMS-error: 6.0 E E a> Oi a < 10*3 10*2 GEOPHYSICS LAB 2A Scblumberoer Confi c uration 10*1 10*0 10*1 10*2 10*3 Current Electrode Distance (AB/2) [m] Figure 7: Resistivity measurement of soil layers against electrode spacing (AB/2) for the second arrangement. RMZ - M&G | 2020 | Vol. 67 | pp. 149-159 /Sacgede J.A,/_ayclOeG.O., Kadir T. 157 •RMS-error: 6.1 HR" GEOPHYSICS LAB 2B Schlumberqer Configuration 10A0 10*1 10A2 Current Electrode Distance CABV2) [m] 10*3 No Res Thick Depth 131.3 161.4 30,2 2810.6 09 0.9 2.0 2.9 6.3 9.2 1 RMS on smoothed data Figure 8: Resistivity measurement (repeated) of soil layers against electrode spacing (AB/2) for the second arrangement. 'RMS-error: 11.4 TW GEOPHYSICS LAB 3A Schlumberger Configuration No Res Thick Depth 1 66.5 1.0 1.0 2 299.2 1.8 2.8 3 52S36.2 -.- -.- * RMS on smoothed data 10A0 10*1 10A2 10A3 Current Electrode Distance [AEU2)[m] Figure 9: Resistivity measurement of soil layers in a container against electrode spacing (AB/2) for the third arrangement. Fractional Current Flow in the Subsurface Using Electrical Resistivity Method: A Laboratory Approach 158 'RMS-error imr 9.9 tz 10*3 10A2 10A1 GEOPHYSICS LAB 3B Schlumberger Configuration 10A0 10A1 10A2 10*3 Current Electrode Distance (AB/2) [m] No Res Thicli Depth 1 69.1 1.1 1.1 2 303.1 2.0 3.1 3 37716.3 -.- -_- 17 RMS on smoothed data Figure 10: Resistivity measurement (repeated) of soil layers in a container against electrode spacing (AB/2) for the third arrangement. more moisture content and therefore is more conductive than the sand. The third layer is assumed to be the plastic container whose resistivity value is very high and can be taken to be infinity. The thicknesses obtained are 6.0 and 11.0 for the first layer and second layer, respectively, against the real values of 7.0 and 12.0. The fraction of current passing through each layer is calculated using Equation (16] and the values are 7% for first layer and 14% for second layer. Conclusion The percentage of current penetrating each layer shows that the effective current penetration depth depends on the spacing of electrodes. When the electrode spacing is increased, the effective current penetration depth is also increased. This is true when compared to Lowrie, 2007 who stated when Z = L/2, 50% of the current penetrates while about 90% of the current flows in the same layer when Z = 3 L. This implies that the percentage of current passing through a layer of shallow thickness is lower than that of passing through a deeper layer. 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