Original scientific paper Received: Accepted: DOI: 10.1515/rmzmag-2016-0019 Spatial distribution and origin of coalbed gases at the working faces of the Velenje Coal Basin, Slovenia, since the year 2000 Prostorska porazdelitev in izvor premogovnih plinov raziskanih iz odkopnih območij iz Velenjskega premogovnega bazena, Slovenija od leta 2000 Tjaša Kanduč1, *, Simon Zavšek2, Sergej Jamnikar2, Timotej Verbovšek3 1 Jožef Stefan Institute, Jamova 39, SI - 1000 Ljubljana, Slovenia 2 Velenje Coal Mine, Partizanska cesta 78, 3320 Velenje, Slovenia 3 Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana, Privoz 11, 1000 Ljubljana, Slovenia * tjasa.kanduc@ijs.si Abstract Geochemical and isotopic monitoring of coalbed gases at the excavation fields of mining areas in Velenje Coal Basin, Slovenia, has been ongoing since the year 2000 with the aim of obtaining better insights into the distribution and origin of coalbed gases. Results from the mining areas Pesje and Preloge (active excavation fields) are presented here from the year 2000 up to the present. Composition and origin of coalbed gases were determined using mass spectrometry at the Jožef Stefan Institute. From a larger database of geochemical samples, 119 samples were used for analysis and spatial presentation in a geographical information system (GIS) environment. We have used geochemical (CH4, CO2 and N2) and isotopic (513CCO2 and 513CCH4) tracers for geochemical and isotopic characterisation of coalbed gases from the active excavation fields. Concentrations of CO2 and the carbon dioxide-methane indices in the southern part of the basin are higher than in the northern part of the basin due to the vicinity of the active Šoštanj Fault. The value of 513CCH4 at the active excavation field indicates a bacterial origin, with values greater than -50%o, and only some boreholes show elevated 513CCH4 quantities as a consequence of the CO2 reduction process in Velenje Coal Basin. The value of S13CCO1 indicates the bacterial and endogenic origin of carbon. Key words: distribution of coalbed gases, gas origin, JamTveg GIS, excavation fields, Velenje Coal Basin Izvleček Geokemični in izotopski monitoring premogovnih plinov na aktivnih odkopih v rudarskih območjih Velenjskega premogovnega bazena poteka od leta 2000 z namenom pridobiti vpogled v porazdelitev in sestavo premogovnih plinov ter njihov izvor. V tem prispevku predstavljamo geokemične (CH4, CO2, N2) in izotopske (513CCO2, 513CCH4) parametre iz rudarskih območij Pesje in Preloge, kjer potekajo aktivni odkopi od leta 2000 do danes. Sestavo in izvor premogovnih plinov smo določili z metodami masne spektrometrije na Institutu Jožef Stefan. Iz večje baze plinskih vzorcev je bilo izbranih 119 vzorcev, ki so bili analizirani in nato prikazani v GIS okolju. Uporabili smo geokemične (CH4, CO2, N2) in izotopske sledilce (513CCO2, 513CCH4) za geokemično in izotopsko karakterizacijo premogovnih plinov iz aktivnih odkopov. Koncentracije CO2 in CDMI indeksa v južnem delu bazena so višje v bližini Šoštanjskega preloma v primerjavi s severnim delom bazena. 513CCH4 na aktivnih odkopih nakazuje bakterijski izvor z vrednostmi nižjimi od -50%o, samo v nekaterih vrtinah so bile zaznane višje vrednosti 513CCH4, kar je posledica CO2 redukcijskih procesov v Velenjskem premogovnem bazenu. 513CCO2 kaže na bakterijski in endogeni izvor ogljika. Ključne besede: porazdelitev premogovnih plinov, izvor plinov, JamTveg GIS, aktivni odkopi, Velenjski premogovni bazen Mli'di'M'l-ll © 2016 T. Kanduc, S. Zavsek, S. Jamnikar, T. Verbovsek, published by De Gruyter Open. This work is licensed under the Creative Commons Attributio Broughttoyouby| National& UniversityLibrary Authenticated Download Date | 4/18/17 12:32 PM Introduction The distribution and origin of coalbed gases (CO2 and CH4] in Velenje Basin are related to the geological composition of the basin. Geochemi-cal processes have occurred in the basin during its formation (Kanduč and Pezdič, 2005; Kan-duč et al., 2012]. The geological characteristics of the Velenje Basin are thus crucial for the understanding and interpretation of the distribution of coalbed gases. The aim of this study is to present the concentrations (CO2 and CH4) and isotopic compositions of carbon in both the gases (513CCO2 and 5 13CCH4], as well as their spatial distributions. Because the concentrations of coalbed gases change with advance of the working face, we have included available data from the year 2000. Kočar et al. (1987, 1989] investigated elements of safe longwall top coal caving (LTCC) under water-bearing strata at Velenje Coal Mine. Research into the dynamics of longwall excavation and its effects on overburden strata resulted in the formulation of the Criteria of Safe Excavation under water-bearing strata. Technical documentation related to mining operations and technical solutions were assembled according to the guidelines of the Criteria of Safe Excavation under water-bearing strata by the designers at Velenje Coal Mine (Lenart et al., 1996]. Geological setting Mining in the Velenje, with one of the thickest single coal seams in the world, has been active for about 140 years. The Velenje coal seam is located in Northern Slovenia near the town of Velenje. The Velenje Basin (Figure 1] formed in the Pliocene to Pleistocene times by polyphase dextral strike-slip faulting at the junction of three tectonic units: the Central Karavanke Mountains, the Southern Karavanke Mountains and the Gorenje-Šoštanj block (Brezigar et al., 1987]. The Velenje Basin is filled with Plio-Pleistocene sediments of terrestrial coarse-grained clastic beds with fine-grained lacustrine clastic sediments, summing to more than 1000 m thickness. Along the Smrekovec fault at the southern edge of the Basin, andesite bodies lie at the bottom of the Basin. On the northern parts of the Velenje Basin, in the footwall near the Velenje fault, lie Triassic dolomites and limestones (Brezigar et al., 1987]. The coal is classified as lignite (Markic and Sachsenhofer, 1997] and lies in a single stratum between the Pliocene clastic sediments. The coal body is lens shaped, about 8.3 km long and 2.5 km wide. Under the lignite seam, the Pliocene strata are up to 50 m thick, consisting of shales, clay, coal and lignite. Above the Pliocene strata lie green sandy silts up to 250 m thick. The lignite seam, whose thickness can reach 160 m, lies above the footwall. Above the coal seam, a thin layer of marls with lacustrine molluscs is detected, comprising up to 350-m-thick lacustrine strata consisting of clays, marls and silts. These strata are overlain with a 90-m-thick sandy-silty formation. The uppermost part of the basin consists of terrestrial silts, overlain by recent fluvial sediments (Brezigar et al., 1987] (Figure 1]. The area of the Velenje Basin was formed by many geological processes, such as magmatic activity, strike-slip reverse faulting and erosion. Heat flow studies suggest that heat flows were elevated in the Velenje area during Oligocene times, as a result of Smrekovec volcanism (Brezigar et al., 1987]. The geochemical and isotopic characterisation of coalbed gases from different excavation fields, with their advance from the Velenje Basin to the boreholes, in the period 2000-2012 has been partly published elsewhere (Kanduc, 2004, Kanduc and Pezdic, 2005; Kanduc et al., 2011; Kanduc et al., 2012, Sedlar et al., 2014, Kanduc et al., 2015]. Apart from the gas analysis, several investigations have also been performed on the hydrogeochemical and isotopic processes occurring in groundwater (Kanduc et al., 2014; Urbanc and Lajlar, 2002; Veselic and Pezdic, 1998]. Formation waters in Velenje Basin are not trapped within the coal seam, because large amounts of groundwater are extracted from the Velenje Basin aquifers by intensive pumping (strata de-watering] to facilitate underground mining of coal (Vukelic et al., 2016]. The groundwater recharging the basin is therefore not in direct contact with the coal seam. Prior geochemical research on coalbed gas The coalbed gas in the Velenje Basin consists of, on average, a mixture of CO2 and CH4. Lignite at 215 Figure 1: Location of Velenje Basin with geological profile, adapted from Brezigar (1987). the Velenje Coal Mine has an average coal gas mixture of approximately CO2:CH4 > 2:1 (Kanduc et al., 2011, Kanduc et al., 2015]. This ratio is related to the excavation technique known worldwide as the Velenje mining method (Jero-mel et al., 2010; Sedlar et al., 2014]. The ratio changes with advance of the working face, and sometimes N2 is in excess (Kanduc et al., 2011, Kanduc et al., 2015]. When the advance rate of the longwall face is slow (less than 3 m/day), gas can escape more slowly under the high pressure in the virgin coal. In general, investi- gations have shown an opposite trend between CO2 and CH4 concentrations (R2 = 0.99) with the advancement of the working faces (Kanduc et al., 2011, Kanduc et al., 2015). Due to outbursts of coal and gas in the roadways at the working face (Vizintin et al., 2016), the origin of CO2 needed to be established. We have used the ratio of the concentrations of CO2 and CH4, known as carbon dioxide-methane index (CDMI) or the Australian index. This index is defined as CDMI = [CO2/(CO2+CH4)] and is expressed in percentage. The isotopic composition of CO2 Figure 2. Map of sampling locations of coalbed gases from the lignite seam, from active working faces, from the year 2000. Numbers indicate the consecutive numbers of samples (refer Table I). Shaded relief is provided by light detection and ranging (LiDAR) data from the Slovenian Environment Agency, available at http://gis.arso.gov.si/evode/profile.aspx?id=atlas_voda_ Lidar@Arso). in conjunction with the CDMI is often used to infer the origin of CO2 in coal gases (Kotarba, 2001; Smith and Pallasser, 1996]. This index was therefore introduced to decipher the origin of CO2 in the Velenje Basin, as in the Bow-en and Sydney Basins, Australia (Smith and Gould, 1980; Faiz and Hendry, 2006]. Four different types of origin of CO2 are known: endogenic CO2, CO2 originating from carbonates, CO2 of microbial origin and CO2 originating from headwaters recharging the basin (Kanduc and Pezdic, 2005]. Materials and methods Sampling locations Sampling of coalbed gas was performed by an operator, e.g. a miner responsible for the underground coalbed gas monitoring. Free gas includes the volatiles filling the pores and the cracks within the coal structure, as well as some gas degassed from the coal during drill- ing and sampling (Kotarba, 2001]. Short (3 m) and long boreholes (25 m) were used for coal gas concentration monitoring; the design of a 25-m-long borehole is described by Jamnikar (2016). Since the year 2000, the length of boreholes has changed and has varied between 3 and 25 m (Table 1). From a larger database of gas samples, 119 samples were included for this study. As several measurements were taken at the same locations during the investigated period, only the first measurements were taken at any one location. Locations are presented in Figure 2. After drilling, the capillary tube was inserted in boreholes. "Free gas" (Kotarba, 2001) emitted from the borehole was collected in a 50 mL plastic syringe, then transferred to a 12 mL ampoule, which was flushed with coalbed gas and filled under pressure. After sampling of the "free gas" from the boreholes, the ampoules were stored under normal atmospheric conditions until analysis. Seven ampoules were sampled at each location; the third ampoule was t. KcSroughtZovyou SjydmtokoeaJ Verdeiveksity Library Authenticated analysed for chemical composition (CO2, CH4, O2, N2 and Ar), and other ampoules were used for determination of the isotopic compositions of carbon in CH„ (S13CM„) and carbon dioxide 4 v CH4-/ (813Cco2). Data to be analysed were recorded in an Excel spreadsheet, with names of the boreholes, their technical data and geochemical measurements. A local coordinate system was used for the boreholes, with D48/GK geodetic datum as a base, but with all but the last four digits truncated. Therefore, the local coordinates had to be transformed back to the georeferenced system to be used in the geographical information system (GIS) environment. Analytical procedures Concentrations of CH4, CO2, O2, N2 and Ar were determined using a homemade mass Nier spectrometer. The method is described in detail in the studies by Kanduc et al. (2011, 2015] and Sedlar et al. (2014). Isotopic compositions of CH4 and CO2 were determined using a Europa 20-20 continuous flow isotope ratio mass spectrometer with an automated nitrogen and carbon analyser (ANCA) - trace gas (TG) preparation module. First, water was removed and CO2 was then analysed directly for 513CCO2. For CH4 measurements, CO2 was first removed and then the CH4 was combusted over hot 10% platinum CuO (1000°C). The CH4, completely converted to CO2, was then analysed directly for the isotopic composition of carbon (513C). Working standards calibrated to International Atomic Energy Agency (IAEA) reference materials were used, with values of -4.3%o, 3.2%o and -5.2% for CO2 and values of -53.4% and -47.5% for CH4 relative to Vienna Pee Dee Belemnite (VPDB). The analytical precision for carbon isotope composition is estimated to be ±0.2% for CO2 and ±0.6% for CH4. Stable carbon isotopes are presented in the 5 notation relative to VPDB standards and are expressed in parts per million (Coplen, 1996) as follows (O'Neil, 1979): 3C )s=- R -R R •1000 [%0] where Rs = 13C/12C ratio in the sample and Rrm 13C/12C ratio in the reference material. Results and discussion Due to air contamination within boreholes (ventilation of the coalmine) and the capillary system, samples were reassessed on an air-free basis. The percentage of oxygen in the sampled ampoules was used to calculate the amount of nitrogen, according to the ratio in air (N2/ O2) considering Dalton's law (Atkins, 1994); CH4 migrates faster and CO2 slower. Results show that the major gas components were CO2 and CH . 4 All data on coalbed gas composition from the excavation fields of Velenje Basin are summarised in Table 1. Concentrations vary greatly between the samples, with those of CO2 ranging from 0% to 95.5%, CH4 from 0% to 77.5% and N2 from 0% to 80%. Generally, CO2 was the prevailing gas, because its average value was 59%, with CH4 being 30% and N2 much less, about 11%, of the total composition. The geochemical index CDMI varied from 1% to 100%; stable isotope ratios varied in the following ranges: S13Cm, from -14.80% to +3.19% and 513CCH4 LOz CH4 from -74.90% to -19.82% (Table 1). Using the GIS environment application JamTveg GIS (Verbovsek, 2010; Verbovsek and Vrabec, 2012), we interpreted the distribution of coalbed gases from the mining areas Preloge and Pesje and determined the origin of coalbed gases at the active excavation fields, as well as the exit and delivery roadways. The distribution of 513CCO2 in Velenje Basin is presented in Figure 3A. Values of 513CCO2 up to -14.80% are typical for the degradation of organic matter at the northern part of the basin, while values up to +3.19% are typical for bacterial gas formed via the CO2 reduction process, as found in previous studies (Kanduc et al., 2012) and observed in the southern part of the basin (Figure 3A). For endogenic CO2 (thermal degradation of carbonates), a typical value is -7%. For carbonate dissolution in Velenje Basin, values are typically around -3% (Kanduc et al., 2012). Organic degradation exhibits values of -14%, so the source of CO2 in the Velenje Basin can be attributed to bacterial origin, by CO2 reduction. The distribution of 513CCH4 at the active excavation fields since the year 2000 is shown in Figure 3B. On the northern part of Velenje Basin, Table 1. Geochemical composition of coalbed gases and isotopic composition of carbon in CO2 (ô13CCO2) and CH4 (ô 13CCH4), as well as the CDMI, at the investigated locations. Blank values indicate missing data. No. Date Length (m) Borehole S13C 0 CO2 (<%) S13C 0 CH4 (%o) CDMI (%) ch4 (%) CO2 (%) N2 (%) 1 1.2.2000 3 19A -12.05 -43.06 82.52 17.25 81.44 1.31 2 1.2.2000 12 j.v.780-H/2000(IV) -11.80 -53.53 88.65 10.39 81.18 8.43 3 2.2.2000 15 j.v.780-H/2000(V) -13.67 -54.28 75.28 22.97 69.94 7.09 4 17.5.2000 3 j.v.3008-H/00 -8.50 - 94.61 1.76 30.87 67.37 5 18.5.2000 15 j.v.3007-H/00 -11.70 - 91.16 1.79 18.45 79.76 6 17.7.2000 15 j.v.3010-H/00 -10.00 -56.90 87.71 8.87 63.33 27.8 7 25.7.2000 15 j.v.3011-H/00 -9.10 - 92.53 2.44 30.22 67.34 8 25.7.2000 3 j.v.3013-H/00 -11.30 -49.26 48.38 51.6 48.36 0 9 22.8.2000 15 j.v.3016-H/00 -9.69 -39.85 96.63 1.83 52.45 45.73 10 22.8.2000 15 j.v.3018-H/00 -10.20 - 100.00 0 45.36 54.64 11 26.9.2000 15 j.v.3017-H/00 -11.48 - 91.60 5 54.49 40.5 12 8.11.2000 15 j.v.3019-H/00 - -59.86 71.16 28.38 70.01 1.61 13 2.3.2001 3 j.v.969-T/01 -4.68 -47.90 89.54 7.71 65.97 26.32 14 24.5.2001 3 j.v.971-T/01 -3.55 -68.24 92.88 5.47 71.31 23.22 15 24.5.2001 3 j.v.977-I/01 -3.50 -67.95 92.94 5.39 70.91 23.7 16 7.6.2001 3 j.v.978-V/01 -2.70 -55.54 69.72 28.27 65.1 6.63 17 28.9.2001 st.10 -8.50 -59.70 35.81 64.2 35.81 0 18 28.9.2001 st.11 -7.76 -58.80 53.63 0 0 0 19 28.9.2001 st.3 -8.26 -64.29 96.07 0 0 0 20 28.11.2001 st.11 -9.35 -55.60 50.92 49.08 50.92 0 21 28.11.2001 st.3 -8.40 - 81.15 3.41 81.15 15.44 22 23.1.2002 st.3 -11.40 - 77.10 22.9 77.1 0 23 23.1.2002 st.7 -4.20 -42.50 69.79 0 0 0 24 28.3.2002 st.6 -9.95 -53.50 23.05 76.95 23.05 0 25 28.3.2002 st.7 -5.00 -71.70 47.26 52.74 47.26 0 26 26.4.2002 st.4 -4.40 -62.40 31.60 68.39 31.6 0 27 31.5.2002 st.1 -0.03 -47.70 97.06 2.94 97.06 0 28 31.5.2002 st.11 2.84 -57.70 96.63 3.37 96.63 0 29 31.5.2002 st.4 -5.85 -68.60 55.52 44.48 55.52 0 30 31.5.2002 st.7 -3.31 -68.60 83.44 16.56 83.44 0 31 31.5.2002 st.9 -1.60 -58.70 97.77 2.23 97.77 0 32 28.6.2002 st.13 2.91 -49.10 94.08 5.92 94.08 0 33 28.6.2002 st.7 -4.00 -70.50 41.82 58.18 41.82 0 34 1.7.2002 st.8 -2.07 -40.00 96.68 3.32 96.68 0 35 23.7.2002 st.1 -14.80 -67.05 94.55 5.49 94.55 0 36 23.7.2002 st.4 -4.60 - 92.50 7.5 92.5 0 37 24.7.2002 st.13 -2.15 -55.45 76.24 23.76 76.24 0 38 17.10.2002 15 j.v.3096+5/02 -8.90 -47.40 39.98 60.03 39.98 0 39 17.10.2002 15 j.v.993-+60/02 -3.00 - 100.00 0 77.95 22.05 40 24.10.2002 15 j.v.3101+5/02 -7.10 -29.90 61.50 38.5 61.49 0 41 6.11.2002 15 j.v.994-+-0/02 -2.41 - 98.31 1.45 84.55 14 42 29.1.2003 3 j.v.3119-T/02 -9.10 -50.00 86.58 13.42 86.6 0 43 18.4.2003 15 j.v.3133-H/03 -8.20 -34.20 65.74 34.26 65.74 0 44 18.4.2003 15 j.v.3134-H/03 -9.70 -45.00 65.30 34.7 65.3 0 45 14.6.2003 15 j.v.3135-H/03 -5.90 -32.00 52.10 47.9 52.1 0 No. Date Length (m) Borehole S13C 0 CO2 (<%) S13C 0 CH4 (%o) CDMI (%) ch4 (%) CO2 (%) N2 (%) 46 17.9.2004 15 j.v.3231+8-GM/04 -9.60 -39.20 77.68 22.32 77.67 0 47 28.9.2004 15 j.v.3097+5/02 -2.40 -52.70 32.40 67.6 32.4 0 48 28.9.2004 9 j.v.3099+5/02 -1.00 -49.20 28.10 71.9 28.1 0 49 6.10.2004 15 j.v.3233+2-GM/04 -5.20 -58.50 44.80 55.2 44.8 0 50 6.10.2004 komora -4.50 -58.90 40.70 46.2 40.7 13.1 51 6.10.2004 st.11 -5.00 -54.10 49.90 49.9 50.1 49.9 52 14.10.2004 st.11 -13.00 -51.70 68.80 14.6 68.8 16.6 53 19.1.2005 15 j.v.1021+2-GM/04 -6.20 -54.80 60.35 38.9 59.2 1.9 54 19.1.2005 st.11 -6.20 -44.40 61.33 37.7 59.8 2.5 55 26.1.2005 st.11 -1.50 -56.40 53.80 46.2 53.8 0 56 3.2.2005 st.11 -3.70 -37.40 62.70 37.3 62.7 0 57 18.2.2005 st.11 -2.20 -59.90 1.04 87.9 0.92 11.2 58 14.4.2005 st.11 -4.31 -63.70 25.05 74.95 25.05 0 59 6.5.2005 10 j.v.3268+60-GM/05 -11.90 -25.00 83.61 12.79 65.25 22 60 6.10.2005 15 j.v.3298+2-GM/05 -7.60 -49.40 50.31 41.69 42.21 16.08 61 6.10.2005 15 j.v.3299+60-GM/05 -4.70 -57.20 29.20 70.8 29.2 0 62 20.10.2005 15 j.v.3100+5/02 -4.70 -58.70 25.43 69.78 23.8 6.4 63 20.10.2005 st.11 -8.20 -51.20 53.48 46.52 53.48 0 64 26.10.2005 15 j.v.3292+2-GM/05 -10.30 -56.00 33.33 45.43 22.71 31.84 65 26.10.2005 15 j.v.3300+2-GM/05 -5.10 -63.80 44.95 46.93 38.32 14.74 66 26.10.2005 10 j.v.3301+60-GM/05 -10.50 -56.50 33.92 33.31 17.1 49.6 67 3.2.2006 st.4 -12.40 -74.90 88.33 11.67 88.33 0 68 5.7.2006 15 jpk-1/06+2 -3.16 -68.60 80.15 19.85 80.14 0 69 5.7.2006 10 jpk-2/06+40 -3.08 -70.30 82.29 16.55 76.92 6.51 70 20.9.2006 15 j.v.1059-GM/05 -2.79 - 100.00 0 100 1.34 71 20.9.2006 15 j.v.1060-GM/05 -2.82 - 100.00 0 100 3.68 72 21.2.2007 15 jpk-4/06+20 -3.57 -65.60 76.53 23.22 75.73 1.03 73 1.3.2007 6 jpk-3/06+2 -6.43 -54.40 33.24 66.75 33.24 0 74 20.3.2007 st.4 -3.36 -66.50 51.45 48.55 51.45 0 75 20.3.2007 st.8 -8.23 -66.70 48.43 51.57 48.43 0 76 21.3.2007 st.6 -3.52 -66.20 51.38 48.62 51.38 0 77 21.3.2007 st.7 -3.37 -66.60 50.19 49.81 50.19 0 78 28.3.2007 st.11 -3.54 -66.10 51.94 48.06 51.94 0 79 28.3.2007 st.13 -3.35 -65.00 48.93 51.07 48.93 0 80 17.7.2007 3 j.v. 3343-T/07 0.61 -68.90 19.30 77.48 18.53 3.97 81 10.9.2007 20 jpk-5/07 -1.36 - 90.96 7 70.4 22.6 82 10.9.2007 20 jpk-7/07 -4.16 -74.86 55.16 23.9 29.4 46.7 83 5.10.2007 20 jpk-10/07 -5.04 -63.04 53.93 38.7 45.3 16 84 5.10.2007 10 jpk-11/07 -4.75 -62.99 52.41 40.5 44.6 14.9 85 5.10.2007 10 jpk-12/07 3.19 -62.89 5.21 65.5 3.6 30.9 86 5.10.2007 20 jpk-9/07 -4.83 -57.52 68.17 26.9 57.6 15.5 87 18.7.2008 3 k.-35c (bivsa odvozna) -4.40 -58.20 71.09 28.75 70.71 0.53 88 26.11.2008 15 jpk-18/08 -5.30 -59.10 71.11 28.89 71.1 0 89 26.11.2008 15 jpk-19/08 -5.40 -59.30 71.89 28.11 71.88 0 No. Date Length (m) Borehole S13C 0 CO2 (%o) S13C 0 CH4 (%) CDMI (%) ch4 (%) CO2 (%) N2 (%) 90 26.11.2008 10 jpk-21/08 -11.40 -37.20 73.63 26.37 73.63 0 91 11.12.2008 10 jpk-20/08 -12.00 -38.40 70.59 29.41 70.58 0 92 7.9.2009 20 jpk-22/09-II -0.20 -40.75 53.42 46.58 53.42 0 93 7.9.2009 14 jpk-23/09-II 2.85 -40.62 54.90 45.10 54.90 0 94 8.7.2009 9 jpk-26/09 -1.60 -22.80 64.91 35.09 64.91 0 95 4.2.2009 3 vrtina 1 -10.82 -38.80 83.34 16.66 83.33 0 96 18.2.2010 3 K.-120B ODVOZ -6.70 -40.80 93.64 6.36 93.6 0 97 18.2.2010 2.3 K.-120B ODVOZ -6.00 -36.70 88.89 11.11 88.89 0 98 18.2.2010 1.5 K.-120B ODVOZ -7.10 -33.50 90.39 9.61 90.39 0 99 25.3.2010 3 k.-50/B ODVOZ -4.80 -52.80 93.90 6.1 93.9 0 100 25.3.2010 3 k.-50/B ODVOZ -3.50 -43.50 85.13 14.87 85.13 0 101 21.4.2010 3 k.-50/B ODVOZ -7.80 - 95.79 4.21 95.79 0 102 21.5.2010 3 k.-50/B ODVOZ -8.10 - 97.94 2.06 97.94 0 103 18.8.2010 20 jpk-30/10 1.80 -30.97 57.80 42.2 57.8 0 104 27.8.2010 20 jpk 31/10 -9.10 -19.82 69.30 30.7 69.3 0 105 8.9.2010 20 jpk 32/10 -4.80 -61.20 72.70 27.3 72.7 0 106 16.9.2010 25 jpk34/10 -4.70 -59.80 70.25 29.75 70.24 0 107 4.10.2010 25 jpk-28/10 -7.10 -58.20 72.41 27.6 72.44 0 108 4.10.2010 25 jpk-29/10-II -8.70 -48.60 82.10 17.9 82.1 0 109 11.11.2010 20 jpk 35/10 1.00 -66.30 93.10 6.9 93.1 0 110 11.11.2010 20 jpk36/10 -6.20 -57.50 77.30 22.7 77.3 0 111 4.1.2013 25 JPK 62+10° -8.00 -48.20 57.50 42.5 57.5 0 112 22.2.2013 25 JPK 56 -3.40 -42.90 36.40 63.6 36.4 0 113 8.3.2013 25 JPK 63+10° -2.20 -34.40 95.50 4.5 95.5 0 114 3.10.2013 JPK 70+10° -8.30 -51.90 42.31 57.55 42.2 0.27 115 5.12.2013 JPK 73+10° -8.70 -53.10 92.44 7.5 91.7 0.81 116 11.7.2014 25 JPK 77+10° -6.50 -54.20 66.80 31.04 62.44 6.51 117 20.10.2014 25 JPK 78+10° -9.70 -48.40 67.43 29.8 61.7 8.5 118 22.4.2015 25 JPK 82+10° -1.50 -50.20 90.91 9.09 90.9 0 119 21.9.2015 25 JPK 87+10° -4.58 -29.45 67.86 32.38 68.37 0 S13CCH4 values are lower (less than 50%], indicating that the CH4 is bacterial in origin (Whiti-car, 1996, 1999; Scott, 1999; Strapoc et al., 2011]. At the southern part of the basin, values up to -10% are observed, indicating thermogenic CH4 (Kanduc et al., 2012]. Higher S13CCH4 values (values up to -10%, Figure 3B] in low-rank coals in Velenje Basin are attributed to the bacterial activity in a Ca-rich alkaline environment, formed via the CO2 reduction process. Methane in the Velenje Basin was formed via the two methanogenesis pathways (CO2 reduction and acetate fermentation]. These pathways, and the origin of coalbed gas at the different excavation fields, were described in detail in the study by Sedlar et al. (2014). Figure 4 shows the CDMI values, measured at the locations (Figure 4A] and on the interpolated map (Figure 4B]. From the direct values at the locations, CDMI is seen to be lower in the northern part of the Velenje Coal Basin (less than 70%]. The spatial distribution becomes even more visible on the interpolated map, where higher values (greater than 70%] are found in the southern part of the basin close to the Šoštanj Fault (Figure 1]. Even the orientation of the higher values in the direction parallel to faults is visible, so the Šoštanj Fault has a visible influence on the higher CO2 contributions in regard to the CH4 values, as calculated from the CDMI. It has to be emphasised that CO2 is used as a critical parameter for gas outbursts Figure 3: A: 6'3C(^02 and B: 6'3C(^H4 values at the active excavation fields since the year 2000. Topographic maps based on the vector DTK50 data, The Surveying and Mapping Authority of the Republic of Slovenia (http://www.gu.gov.si/en/). Figure 4: A: Carbon dioxide-methane index (CDMI) and B: Interpolated map of CDMI values for the active excavation fields since the year 2000. occurring not just in Velenje Basin, but also in other coal basins (Sydney and Bowen basins, Australia basins). Higher concentrations of CO2 and, consequently, higher values of CDMI in relation to the active fault zones were also observed in the Bowen and Sydney basins (Smith and Gould, 1980). Conclusions We used geochemical (CH4, CO2) and isotope tracers (S13CCO1 and S13CCH4) for the geochemical and isotopic characterisation of coalbed gases from the active excavation fields Preloge and Pesje in the Velenje Basin since the year 1000. All georeferenced, geochemical and iso-topic data from the available database have been combined spatially in a GIS environment (JamTveg GIS; Verbovšek, 1010; Verbovšek and Vrabec, 1011) to obtain the distribution of the investigated parameters. The CDMI is introduced as a crucial parameter of zones dangerous for gas outbursts. Areas with high CDMI have, consequently, high CO2 concentrations and are areas with greater gas outburst potential. The CDMI ranges from 0% to 100%. At the northern part of the basin, the CDMI is lower (values less than 70%) and, at the southern part, the value is higher (the vicinity of the Šoštanj Fault), indicating a greater potential for gas outburst. S13CCH4 values range from -70% to -50%, indicating a bacterial gas origin. Higher S13CCH4 values, up to -10%, are characteristic of ther-mogenic gas, but appear in low-coal ranks and are attributable to bacterial gas formed via CO2 reduction. S13CCH4 values around -70% indicate bacterial gas, formed via acetate fermentation. S13CCO1 values range from -14.8% to +4.0%, indicating bacterial gas. Values of -14% are characteristic of organic degradation, while values of +4%, observed at the southern part of the basin, are characteristic of bacterial gas formed via CO2 reduction. From all the geochemical and isotopic results gathered since the year 1000, it can be concluded that the origin of the coalbed gases in Velenje Basin is bacterial (formed via CO2 reduction or acetate fermentation). The distribution of gases is also dependent on the geological characteristics of the basin and its formation. Biogeo-chemical processes such as the gelification and mineralization observed in lignite are typical of Ca-rich alkaline environments and were also confirmed from the stable isotope composition. Moreover, petrified woods (composed mostly of calcite) found in coal seams are typically bacterially degraded, which is reflected in the isotope composition. All these facts confirm the conclusion that the distribution of coalbed gas trapped in the coal matrix originated in the early diagenesis of the 160-m-thick coalbed seam known worldwide as a unique phenomenon. Geochemical and isotopic investigations of coalbed gases have been ongoing since the year 1000 at the active excavation fields in Velenje Coalmine d.d. These investigations provide new understanding and insights into the gas composition, the origin of gas components, gas migration pathways and the distribution of gases, in addition to being important for the development of coalbed methane (CBM) technology and for safety reasons. The spatial distribution of CDMI, in the context of the relative values of CO2 and CH4, clearly shows the influence of the active Šoštanj Fault, as the CDMI values are higher in the southern part of the Velenje Basin close to the fault. In addition, the orientation of higher values is spatially distributed and parallel to the faults, indicating the influence of the CDMI on gas distribution. Further investigations need to include more spatially distributed data for a more sound interpretation and further analysis of data. 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