Geochemical Journal, Vol. 39, pp. 397 to 409, 2005

Origin and distribution of coalbed gases from the Velenje basin, Slovenia

TJASA KANDUC1* and JOZE PEZDIC2

1Jozef Stefan Institute, Jamova 39, SI - 1000 Ljubljana, Slovenia 2Department of Geology, University of Ljubljana, Askerceva 12, SI - 1000, Ljubljana, Slovenia

(Received March 30, 2004; Accepted October 5, 2004)

Coalbed gases in the Velenje basin are highly variable in both their concentrations and stable isotope composition.

Major gas components are CO2 and methane. The CDMI carbon dioxide-methane index [CDMI = (CO2/(CO2 + δ13 δ13 CH4))·100(%)] varies from 0 to 98.8%, CCO2 from –34.1 to 2.9‰ and CCH4 from –74.0 to –34.7‰. According to the geochemical results (recalculated on an air-free basis) for coalbed gases, several types of origin of CO2 and methane could be recognized: thermogenic methane, endogenic and thermogenic CO2, microbial methane and CO2. The phenomenon of thermogenic methane in the lignite seam could be explained by microbial CO2 reduction and/or methane oxidation proc- esses, causing enrichment in the heavy 13C isotope of methane. Considering geological events at the time of formation of the lignite seam in the Velenje basin, it was found that most of the gas in the lignite seam is of bacterial origin, indicating

CO2 reduction processes mixed with external CO2. A considerable fraction of the CO2 in the lignite seam is of external origin. A crucial factor determining the formation of coalbed gases in the Velenje basin was bacterial activity.

The distribution of coalbed gases (CO2 and methane) is a result of the different physicochemical properties of CO2 and methane. Methane accumulates at the subsurface of the Velenje basin, while carbon dioxide remains adsorbed on the

coalbed surface and in micropores of the coal structure. CO2 is the major gas component in the lignite seam and represents a permanent danger for gas outbursts. Isotopic fractionation due to migration of methane from the lignite seam to the

surface is reflected in isotopically light methane in the water saturated layers. CO2 accumulating at the subsurface of the Velenje basin is mainly derived from hinterland water and oxidation of methane.

Keywords: coalbed gas, carbon isotopes, gas origin, gas migration

duced at the early stages of the coalification process (Pa- INTRODUCTION tience, 2003). Geochemical investigations designed to help predict, This paper attempts to explain some possible inter- prevent, and manage coal mine gas outbursts and to study pretations of gas origin and the distribution of coalbed their origin and mechanisms have been described (Flores, gases (CO2 and methane) accumulating in the lignite seam 1998). and the subsurface of the Velenje lignite coalmine from A numbers of models were developed to describe wells. Considerations of gas origin are based on the con- sources of hydrocarbon gases and CO2 in natural gas pools centrations of gases, and stable carbon isotope studies of (Schoell, 1983; Kotarba, 1990; Smith and Pallaser, 1996; methane and CO2. Also an A–B geochemical profile was Aravena et al., 2003). Stable carbon isotope analyses of designed to determine the genetic types and lateral dis- hydrocarbon gases and carbon dioxide can be applied to tribution of CO2 and methane in the Velenje basin. identify the origin of natural gases, their migration path- ways and accumulation processes (Clayton, 1998). Large GEOLOGICAL SETTING volumes of coalbed gases are generated during the coalification process, but gases produced by other proc- The concentrations and isotopic composition of esses may also occur in coal basins (Rice et al., 1989). coalbed gases accumulating in fractures and microporous Three main source categories for gas can be defined: structures of the lignite seam might be related to tectonic abiogenic, microbial and thermogenic (Scott, 1993). In events before and at the time of formation of the Velenje general, thermogenic gases have been associated with high basin. rank coal, whereas microbial gases are thought to be pro- The Velenje coal basin is situated in the NE part of Slovenia (Fig. 1). It is located at the junction of the WNW- ESE-trending Sostanj fault and the E-W trending *Corresponding author (e-mail: [email protected]) Periadriatic zone, bounded to the south by the Smrekovec Copyright © 2005 by The Geochemical Society of Japan. fault segment (Fig. 1). The Periadriatic lineament was

397 V 15° C . K IT A H A R A V AN A N K JE Velenje E m I t s ( P gr A L ) ab 46° en SLO CRO

V ELE NJE S fau . K SMR lt A R EKO A VEC V A fault B N K E B m t oun s d ar y o f th e li N gni te s A eam

G DO OR B EN Velenje RN 3 km JE A - ä O fa ä TA ult NJ blo äOä ck TAN Plio-Quaternary (Velenje basin) J fa Gravel, sand, clay, coal ult Middle Miocene Conglomerate, sandstone, marl Triassic Early Miocene Dolomites and limestones Sandstone, marl and dacific tuff Late Paleozoic. Shales, Oligo-Miocene Sandstones, conglomerates, limestones Andesitic tuffs and clastic sediments Cross-section A-B See Fig.8 Oligo-Miocene Tonalite PAL Periadriatic lineament

Fig. 1. Geological map of the Velenje basin area (adapted from Brezigar et al., 1987).

formed after collision of the Adriatic and European con- calcium-rich environment resulted in high pH values and tinental plates 35 Ma ago. In the Oligocene the heavier is responsible for high sulphur lignites, up to 4% on a dry oceanic plate sank into the mantle, commenced partial basis. The alkaline, calcium-rich environment also caused melting along the fracture, with lifting of tonalite and a relatively high degree of gelification, which is signifi- granite intrusions along the Periadriatic lineament. The cantly higher than the degree of gelification in the lignites Sostanj and Smrekovec faults were generated due to the from the Lower Rhine bay (Markic and Sachsenhofer, collision of continental plates (Fodor et al., 1998). The 1997). Gelification as a biogeochemical reaction in early origin of the basin is probably related to the transtension degradation of organic matter is also reflected in lower between these two fault systems. The basin has a half- δ13C values of lignite in comparison with ungelified sam- graben geometry. In the pre-Pliocene basement of the ples (Kanduc et al., 2004). The upper contact of the lig- basin, Triassic limestones and dolomites prevail on the nite seam is sharp and covered by a thin layer of marls northeastern side of the Velenje fault. Oligocene to with lacustrine molluscs. The strata with lacustrine char- Miocene clastic strata, consisting predominantly of marls, acter above the lignite seam are up to 350 m thick and sandstones and volcanoclastics prevail on the southwest- consist of clays, marls, and silts, locally interweaving with ern side. sands of fluvial origin. This succession is overlain with a The basin fill (Fig. 2) is up to 1000 m thick and shows 90 m thick sandy-silty formation with several lignite a typical continental fill-up succession, ranging from ter- lenses of swampy character. The uppermost part of the restrial to lacustrine clastic sediments (Brezigar, 1986). basin fill consists of terrestrial silts, overlain by recent The lower part of the Pliocene coal-bearing strata con- fluvial sediments (Fig. 2). sists of alternations of shales, clayey coal and lignite, and is up to 50 m thick. Above, an up to 160 m thick uniform EXPERIMENTAL lignite seam has developed. The thickness of the seam decreases towards the basin margins where the seam Sample collection pinches out and interweaves with alluvial fans. The pet- Sampling of coalbed gas was performed during 1999– rographic composition of the lignite indicates an envi- 2002, in the coalbed seam and at the subsurface from wells ronment with the following main units: wet forest swamp, in the NW and the central, central west and central north dry forest swamp, bush moor, wet forest swamp, fen. A part of the Velenje basin. Depths above sea level of the

398 T. Kanduc and J. Pezdic evacuated ampoules using a metal capillary tube and plas-

Lithologic tic syringe. Free gas includes both the volatiles filling Lithology

description Age the pores and cracks within the coal structure and some Pollen LITH. UNITS Depositional Depositional environment gas degassed from the coal during drilling and sampling Thickness (m) Thickness RECENT Sand and gravel 5 HOLO- terres- (Kotarba, 2001). Boreholes were drilled in pillar coal to 15 CENE trial Green and a depth of 3 m. After drilling, a capillary tube was in- yellow sandy 40 silts, serted in the borehole. Free gas emitted from boreholes thin lignite bed 50 VILLA- FRAN- CHIAN marshy terrestrial EARLY QUATERNARY was collected in a 50 ml plastic syringe and then trans- ferred to 12 ml evacuated ampoules with septum. After Silts and sands 100 sampling free gas from boreholes, the ampoules were ” stored at normal atmospheric conditions until analysis. Lignite samples (Table 1) from pillar coal ahead of Massive clay- 100 stone and sands the working face and the carbonate forming pre-Pliocene

Fagus flora Fagus

UPPER UPPER PLIOCENE basement were also taken for better understanding of the “ Massive and origin of CO2 and methane. Also water from wells laminated

120 lacustrine dewatering Pliocene strata was sampled from the coal claystone 13 PLIOCENE HANGING WALL HANGING PLIOCENE and sands mine in order to determine δ C of dissolved inorganic

” δ13 carbon ( CDIC). Claystone 30 Marl 5 Analytical procedures Determination of the concentrations of methane, CO , Main lignite 160 2 seam nitrogen, oxygen and argon was performed using a homemade NIER mass spectrometer. The method of sin- marshy Clay, lignite flora Taxodium 50 “ COAL HORIZON COAL coaly clay gular decomposition of the matrix was used, to obtain simultaneous analysis of the gases. The precision of the ± Green sandy method was 3%. 250 PLIOCENE silts The isotopic composition of methane and CO2 was

FOOT WALL FOOT determined using a Europa 20-20 continuous flow iso- tope ratio mass spectrometer (IRMS) with an ANCA-TG Silty sediments with coarse preparation module. Gas samples were flushed with a 150 terrestrial clastics continuous flow of helium across two chemical traps. First

BASEL BEDS BASEL water was removed and then CO2 was directly analyzed 13 PRE-PLIOCENE BASEMENT for C content. For methane measurements, first CO2 was removed and then methane was combusted over hot 10% Fig. 2. Schematic stratigraphic column of the Plio-Quaternary platinum CuO (1000°C). The methane completely con- 13 basin fill (after Brezigar, 1986; Markic and Sachsenhofer, 1997). verted to CO2 was then directly analyzed for C content. The stable carbon isotope data are presented in the δ-no- tation relative to VPDB (Vienna PDB) and expressed in ‰ (Coplen, 2002). The analytical precision for analysis sampling points are given in Tables 1 and 2. of carbon was estimated to be ±0.2‰. The sampling procedure consisted of three types, each The stable isotope composition of carbon in lignite with its own purpose (sampling type A, B, C3). samples was determined using Europa 20-20 continuous Sampling type A was performed to detect regional flow IRMS ANCA-SL preparation module. Lignite sam- changes in gas composition during the years 1999–2000 ples were first ground and homogenized. 1 mg of sample in the coal seam, and type B was performed to determine was weighed in a tin capsule for carbon analysis. Sam- gas composition from pillar coal ahead of the working ples for carbon analysis were pre-treated with 1 M HCl face in the years 1999–2002 (Table 1). Sampling type C3 to remove carbonates. The sample residues were washed was performed in wells, dewatering different aquifers: in distilled water, dried and homogenized. The isotopic Triassic carbonates, Pliocene sandstones and Quaternary composition of carbon was determined after combustion sands, clays, gravels, from the subsurface of the Velenje of the capsules in a hot furnace (temperature 1000°C). coalmine (Table 2). Generated products were reduced in a Cu tube (600°C), Free gas (in continuation coalbed gas) for geochemical where excess O2 was absorbed. H2O was trapped on a analyses was collected from boreholes after drilling and drying column composed of MgClO4. NBS 22 (oil) ref- tightly closed wells from the subsurface of the mine in erence standard was used to relate the analytical results

Origin and distribution of coalbed gases from the Velenje basin, Slovenia 399 Table 1. Molecular and isotopic composition of CO2 and methane in the lignite seam of the Velenje basin

δ13 δ13 α δ13 Sample Depth above sea level CH4 CO2 N2 CDMI CCO 2 CCH 4 CO 2 - CH 4 Clig n ite (m) (vol.%) (vol.%) (vol.%) (%) (‰)(‰)(‰)

A7 –43 12.5 48.3 39.2 79.5 –11.4 –60.4 1.052 A3 –48 6.0 48.9 45.1 89.1 –10.5 –44.8 1.036 A12 –99 8.8 67.3 23.9 88.5 –12.2 –42.0 1.031 A6 –117 10.7 73.6 15.7 87.4 –10.0 –37.0 1.028 A11 20.3 12.9 32.5 54.6 71.5 –11.9 –61.4 1.053 Ba –82 12.9 83.5 3.6 86.6 –9.6 –46.2 1.038 Be –98 17.7 72.7 9.6 80.4 –8.9 –53.0 1.047 B3 –4.5 1.7 92.7 5.6 98.2 –4.1 –73.8 1.075 B4 –85 34.2 65.8 0.0 65.8 –4.4 –37.1 1.034 B7 –85 4.2 95.0 0.8 95.7 –8.4 –53.8 1.048 B12 –149.8 15.1 84.9 0.0 84.9 –5.7 –56.4 1.054 B18 72 23.5 76.4 0.1 76.5 –2.9 –67.9 1.070 B24 65 12.2 85.1 2.7 87.4 –2.4 –61.9 1.064 B26 –92 6.1 89.6 4.3 93.6 –10.1 –47.1 1.039 –26.3 B30 –85 1.1 18.0 4.3 94.2 –5.6 –70.2 1.070 B49 –66 53.8 46.2 0.0 46.2 –8.8 –52.0 1.046 –28.2 B66 –132.9 7.1 92.9 0 92.9 –11.4 –34.7 1.024 B67 43.1 8.9 91.1 0 91.1 –6.0 –45.9 1.042 –23.2 B68 –18.4 49.6 50.4 0 50.4 –9.2 –47.3 1.040 B71 –85.9 17.4 82.6 0 82.6 –11.4 –44.3 1.034 –25.3 B84 20.9 14.4 85.6 0 85.6 –4.2 –62.3 1.062 B120 –34.5 64 36.0 0 36.0 –10.0 –53.5 1.046 –27.3 B122 45.7 37 63.3 0 63.1 –5.0 –71.7 1.072 –27.8 B124 42.8 7.5 66.4 26.1 89.9 –1.8 –66.2 1.069 B125 –88 4.3 95.7 0 95.7 –0.9 –57.5 1.060 –25.8 B126 –16.5 49.8 50.2 0 50.2 –4.4 –62.4 1.062 B134 –18.2 1.2 98.7 0 98.8 –1.6 –58.7 1.061 –26.3 B139 –19.1 3.3 96.7 0 96.7 2.9 –49.1 1.055

CDMI = [CO2/(CO2 + CH4)]·100%. α δ13 δ13 CO2-CH4 = (1000 + CCO2)/(1000 + CCH4).

to the VPDB standard. Sample reproducibility for carbon Velenje basin range from 1.1 to 53.8% and CO2 from 18 was ±0.2‰. to 98.7% (Table 1). In wells at the subsurface of the coal- Carbonate samples forming the pre-Pliocene basement mine methane concentrations vary from 43.5 to 100% and were ground before analysis and homogenized. They were CO2 from 0 to 56.6% (Table 2). reacted with 100% phosphoric acid by the method of The CDMI carbon dioxide-methane index [(CO2/CO2 McCrea, (1950). The measurement of CO2 liberated from + CH4)·100%)], is normally used for interpretation of the carbonates was the same as described for CO2 measure- origin of CO2. In the Velenje basin it varies from 0 to ments above. 98.8% (Tables 1 and 2); in the lignite seam the CDMI δ13 Before CDIC analysis, the water was filtered (0.45 index varies from 36.0 to 98.8% (Table 1) and in wells at µm PTFE filter) in situ by syringe into 12 ml evacuated the subsurface of the coalmine from 0 to 56.5% ampoules pre-filled with HgCl2 in order to avoid second- (Table 2). Coalbed gases in the lignite seam also contain ary biological fractionations. Samples were stored at 4°C, surplus concentrations of nitrogen (recalculated on an air- leaving no headspace. For extraction, an aliquot of sam- free basis); concentrations of nitrogen in the lignite seam ple was acidified under vacuum with saturated H3PO4. vary from 0 to 54.6% (Table 1). δ13 δ13 The resulting CO2 was trapped with liquid nitrogen and Values of CCH4 and CCO2 in the Velenje basin analyzed as described for CO2. vary from –74.0 to –34.7‰ and from –34.1 to 2.9‰ δ13 δ13 (Tables 1 and 2). In the lignite seam CCH4 and CCO2 vary from –73.8 to –34.7‰ and from –12.2 to 2.9‰ (Ta- RESULTS δ13 ble 1). At the subsurface of the mine at wells CCH4 and δ13 Chemistry and stable isotope data of coalbed gas CCO2 vary from –74.0 to –44.1‰ and from –34.1 to Methane concentrations in the lignite seam of the –1.1‰ (Table 2).

400 T. Kanduc and J. Pezdic Table 2. Molecular and isotopic composition of CO2 and methane from wells at the subsurface of the Velenje basin

δ13 δ13 α δ13 Sample Depth above sea level Location Aquifer CH4 CO2 CDMI CCO2 CCH4 CO2-CH4 CDIC (m) (vol.%) (vol.%) (%) (‰)(‰) C3.5 413.3 Gaberke Pliocene 55.5 44.5 44.5 −9.8 −44.1 1.036 −3.2 C3.10 419.9 Gorice Pliocene sandstone 81.0 19.0 19.0 −11.4 −74.0 1.068 −2.5 C3.13 391.5 Gaberke Pliocene sandstone 98.5 1.5 1.5 −5.9 −70.2 1.069 C3.16 389.2 Gaberke Trias 67.0 33.0 33.0 −4.3 −51.5 1.050 C3.18 415.5 Ravne Pliocene 100.0 0 0 n.a. −69.2 n.a. C3.19 376.2 Ravne Quaternary 90.8 9.2 9.2 −1.1 −65.3 1.069 C3.22 372.9 Druzmirje Pliocene 43.5 56.5 56.5 −11.0 −71.4 1.065 −2.9 C3.24 379.8 Gaberke Pliocene 55.3 44.7 44.7 −13.2 −69.3 1.060 C3.25 388.2 Gaberke Pliocene sandstone 90.5 9.5 9.5 −9.5 −70.1 1.065 C3.29 380.7 Gaberke Pliocene 50.7 49.3 49.3 −34.1 −66.8 1.035 C3.32 375.1 Gaberke Pliocene 66.3 33.7 33.7 −8.7 −58.3 1.053 n.a. - not analyzed.

Stable isotope data of the Velenje lignite, water and pre- activity were considered for interpretation of gas origin Pliocene carbonates (Tables 1 and 2). The data on geochemical analysis of The lignite samples collected from exit and delivery coalbed gases from the lignite seam were also processed roadways show an isotope range from –28.2 to –23.2‰ by cluster analysis using the statistical program TEACH (Table 1). Higher δ13C values (up to –23.2‰) of Velenje ME to determine boundaries between different origins of lignite macro petrologically belong to the xylite lithotype methane and CO2 (Figs. 3b and 5b). To correct for any and lower values (up to –28.2‰) belong to highly gelified possible air contamination of the capillary system and detrital lignite. The gelification process in Velenje lig- evacuated ampoules during sampling, sample results were nite leads to enrichment with the light carbon isotope recalculated on an air-free basis. The percentage of oxy- (Kanduc et al., 2004). gen in the sampled ampoules was used to calculate the δ13 Measured CDIC values from wells C3.5, C3.10 and amount of nitrogen, according to air ratio (N2/O2 = 3.7) C3.22 range from –3.5 to –2.5‰ (Table 2). considering the Dalton’s law (Atkins, 1994). Only sam- Measured δ13C values forming the pre-Pliocene base- ples containing less than 2% of oxygen were considered ment from the Velenje basin are around –3‰ (Fig. 8). in this study. δ13 The high variability of gas concentrations and CCH4 and δ13C values in the Velenje basin might be the ef- DISCUSSION CO2 fect of multiple origins of the gases and/or mixing and/or Origin of methane and CO2 from the Velenje basin differences in physicochemical properties of methane and It must be noted that the isotopic composition of CO2 during migration. Diagrams (Figs. 3a, 4, 5a and 6) coalbed gases (CO2 and methane) in natural gas basins is used for interpretation of gas origin (after Schoell, 1983; a result of several factors: the composition of the source Kotarba, 2001) were adapted to the events occurring in organic matter, the temperature of coal formation, Eh, pH the Velenje basin. δ13 δ13 conditions determining microbiological activity and so- The results for CCH4 and CCO2 in the Velenje called secondary processes, i.e., diffusion, migration, basin indicate the following successive origins of meth- δ13 adsorption, desorption, mixing of gases of different ori- ane: thermogenic with CCH4 from –50 to –34.7‰ δ13 gin (Kotarba, 2001) and also recent biogenic activity (Aravena et al., 2003), microbial with CCH4 less than (Kotelnikova, 2002; Aravena et al., 2003). All these proc- –50‰ and a mixed origin between these two (Figs. 3a δ13 δ13 esses could influence CCO2 and CCH4 values and and 4). camouflage gas origin. In the lignite seam samples can be divided into sev- Secondary processes due to mining activity in the eral groups based on the results of cluster analysis of the δ13 δ13 Velenje coalmine can influence the distribution of CO2, parameters CCO2 and CCH4, indicating different proc- δ13 δ13 methane concentrations, CCO2 and CCH4 (Kanduc et esses in coalbed gas formation (Figs. 3a and b): micro- al., 2003). Therefore, only data on gas concentrations and bial methane and CO2 (fermentation and CO2 reduction δ13 δ13 CCO2 and CCH4 values from sites under an excavated processes), mixing between thermogenic and microbial area which had already reached consolidation or on open- methane and thermogenic methane. ing new roadways with null secondary effects of mining It is known that thermogenic methane generated dur-

Origin and distribution of coalbed gases from the Velenje basin, Slovenia 401 (a) -20 fermentation+CO2 reduction

CO2 reduction, thermogenic CO2 -30 CO2 reduction

-40 thermogenic methane, endogenic and ) microbial CO2 ‰ ( -50 CH4 C 13 δ -60

oxidation of -70 CO2 reduction methane

-80 -14 -12 -10 -8 -6 -4 -2 0 2 4 δ13 CCO2 (‰)

(b)

δ13 δ13 Fig. 3. (a) Interpretation of the origin of methane and CO2 in the lignite seam using CCO2 versus CCH4 after Gutsalo and Plotnikov (1981), Schoell (1983), Kotarba and Rice (2001) adapted to geological events in the Velenje basin; CO2 reduction and oxidation of methane cause enrichment with 13C in methane; boundaries indicate different origins of coalbed gases (see (b)). (b) Dendrogram illustrating the results of cluster analysis. The association of samples at a low value of the distance coefficient means a high similarity between the respective entities. Different groups of coalbed gases show ranges of δ13C values for methane and CO2 from various sources modified from Gutsalo and Plotnikov (1981), Kotarba (1988), Kotarba and Rice (2001): ther- δ13 δ13 δ13 mogenic coalbed gases with CCH4 from –60 to –20‰ and CCO2 from –25 to –7‰, microbial methane with CCH4 between δ13 δ13 –80‰ to –70‰, microbial CO2 with CCO2 values from –25 to +18‰, endogenic methane and CO2 with C values from –40 to –20‰ and –10 to –5‰.

402 T. Kanduc and J. Pezdic 0

-10

-20

) -30 ‰ ( -40

CH4 oxidation of methane C 13

δ -50 decomposition of isotopic fractionation -60 organic matter of methane from lignite seam due to -70 migration

-80 -40 -35 -30 -25 -20 -15 -10 -5 0 δ13 CCO2 (‰) δ13 δ13 Fig. 4. Interpretation of the origin of methane and CO2 at the subsurface of the Velenje coalmine using CCO2 versus CCH4; fractionation of methane from the lignite seam to the surface due to migration causes depletion with 13C isotope; oxidation of methane causes enrichment in 13C isotope in residual methane. Compositional fields modified from Gutsalo and Plotnikov (1981) and Kotarba (1988).

δ13 δ13 ing coalification processes has a CCH4 value from –50 ized by values of CCH4 from –74 to –50‰ (Figs. 3a α to –20‰ (Rice and Kotarba, 1993) and its isotopic com- and 4). The carbon isotope fractionation factor ( CO2-CH4) position depends mainly on the genetic type of source between CO2 and methane was found to range between δ13 coal. Values of CCH4 from –50 to around –34.7‰ in 1.048 and 1.079 and is dependent on the growth tempera- the Velenje basin suggest it to be thermogenic methane. ture of microorganisms (Botz et al., 1996). In our study α It is known that thermogenic methane is characterized by CO2-CH4 ranged from 1.024 to 1.075 in the lignite seam the presence of heavier hydrocarbons in intermediate and from 1.035 to 1.069 at the subsurface of the Velenje ranks of coal with vitrinite reflectance (R0) > 0.6%, and basin (Tables 1 and 2). Such microbial methane can be is generated from coal at temperatures of more than 110°C produced by two processes: by methane fermentation and/ (Stach et al., 1982). Since the R0 value of lignite is around or by reduction of CO2 at the time of formation of the 0.3 and the temperature of lignite formation did not ex- Velenje basin (early stage methane) or recently (late stage ceed 40°C (Markic and Sachsenhofer, 1997), the ther- methane). These bacteria could still be active and gener- mogenic methane in the Velenje basin could not be the ate late stage microbial methane. Gas generated during result of the coalification process of Pliocene lignites. the peat and lignite stages of biochemical coalification Lignite coal is generated in early diagenesis by biochemi- (early stage microbial gas) cannot be retained in large cal degradation of organic matter (Stach et al., 1982). One volumes within the coal structure because of the insig- explanation of the source of methane (Figs. 3a and 4) nificant compaction of sediments. The portion of early which results in an enrichment of residual methane in the stage biogenic methane retained in the lignite seam re- 13 12 C isotope and depletion of C in the generated CO2 is mains unknown and mainly depends on the degree of the microbial oxidation process. Another explanation of compaction of the sediments (Kotarba, 1990). The mi- thermogenic methane should also be considered: the CO2 crobial late stage origin of methane is related to shallow reduction process. It must be noted that generation of bio- depths (600 m), zones that are highly permeable with genic methane requires an anoxic environment, contain- many fault systems, which enable the inflow of meteoric ing available CO2 and low sulfate concentrations water and accelerate microbial processes. In contrast, (Whiticar et al., 1986). It is known that the isotopic com- water-unsaturated soils are known to bear methane-con- position of methane formed by CO2 reduction is governed suming (methanotrophic) bacteria. Methanotrophic bac- δ13 by the C value of the original CO2 (Whiticar et al., teria are active in contact zones between aerobic and 1986; Wallin et al., 1995). anaerobic conditions (Kotelnikova, 2002). These bacte- Microbial methane in the Velenje basin is character- ria either oxidize atmospheric methane which diffuses into

Origin and distribution of coalbed gases from the Velenje basin, Slovenia 403 (a) 4 endogenic CO2+reduction CO2 with lower CDMI index 2 endogenic CO2+CO2 reduction with higher CDMI index thermogenic CO2+ endogenic CO2+ CO2 reduction with 0 higher CDMI index thermogenic CO2+endogenic CO2 with lower CDMI index -2 ) ‰

( -4 CO2

C -6 13 δ -8

-1 0

-1 2

-1 4 30 40 50 60 70 80 90 100 CDMI (vol. %)

Fig. 5. (a) Interpretation of the origin of CO2 in the lignite seam using CDMI index versus CO2 concentrations from the Velenje basin after Schoell (1983), Jenden et al. (1993), Kotarba and Rice (2001) adapted to geological events in the Velenje basin, different boundaries indicate different origins of coalbed gases (see (b)). (b) Dendrogram illustrating the result of cluster analy- sis. The association of samples at a low value of the distance coefficient means a high similarity between the respective entities. δ13 Different groups of coalbed gases show ranges of C values for CO2 originating from various sources modified from Jenden et δ13 al. (1993), Kotarba and Rice (2001): microbial CO2 with CCO2 from –25 to +18‰ and low CDMI index, thermogenic CO2 with δ13 δ13 CCO2 from –25 to –7‰ and CDMI index from 10 to 50, alteration of marine carbonates with CCO2 from –3 to 5‰, endogenic δ13 CO2 with CCO2 from –5 to –10‰ and CDMI index more than 85%.

the soil, or methane which migrates from coal into the Pallasser, 1996). δ13 atmosphere (Whalen et al., 1990). Late stage microbial Values of CCO2 from the Velenje basin indicate en- methane could be present from pillar coal in the mine at dogenic CO2, thermogenic CO2, CO2 originating from shallow depths (200 m), where the circulation of mete- carbonates, CO2 of microbial origin and mixing between oric waters facilitates microbial processes. In wells at the these gases (Figs. 5a and 6). subsurface of the Velenje basin (samples C3.5, C3.16) In the lignite seam (Figs. 5a and b) most of the CO2 is methanotrophic bacteria (methane oxidizing) could be of a result of mixing of endogenic CO2 and microbial CO2 13 great importance, causing enrichment in C of residual generated by CO2 reduction with higher and lower CDMI methane (Table 2, Fig. 4). Isotopic light methane at the indices, mixing of thermogenic and endogenic CO2, and subsurface of the Velenje basin might be related to the mixing of thermogenic, endogenic and microbial CO2 isotopic fractionation process of methane from the lig- generated by CO2 reduction processes. δ13 nite seam. CCO2 values from –10 to –5‰ are characteristic of The origin of CO2 is interesting due to its relation to coal gases with a high CO2 content (CDMI up to 99%), gas outbursts in coalmines (Smith and Gould, 1980). indicating external CO2 sources. The high CO2 content There are many possible sources of CO2 in coalbed gases: found in many basins appears to be of bacterial origin δ13 (1) decarboxylation reactions of kerogen and soluble or- (Scott, 1993). Typical endogenic values of CCO2 are ganic matter during burial heating of coal; (2) mineral about –7‰ (also today’s atmospheric value), while a reactions such as thermal decomposition or dissolution wider range is obtained due to mixing of gases of differ- of carbonates or other metamorphic reactions; (3) bacte- ent origin and migration through the microporous struc- rial oxidation of organic matter, including methane ture of coals (Smith and Gould, 1980). The high CDMI (Whiticar et al., 1986); and (4) deep-seated sources such index indicating high CO2 concentrations accumulated in as magma chambers or the upper crust (Smith and the lignite seam could be related to the tectonics of the

404 T. Kanduc and J. Pezdic Endogenic CO2 + CO2 reduction with lower CDMI index

Endogenic CO2 + CO2 reduction with higher CDMI index

Endogenic CO2 + CO2 reduction + thermogenic CO2 with higher CDMI index

Thermogenic + CO2 reduction + endogenic CO2 with lower CDMI index

Fig. 5. (continued).

Sostanj and Smrekovec faults (Fig. 1). At the time of for- tion of the carbonates forming the pre-Pliocene basement mation of the Velenje basin active fault systems could of the Velenje basin would give values of generated CO2 have opened fissures and fractures, enabling the inflow in the temperature interval 150–200°C from –4 to –2‰ of endogenic CO2 into the lignite strata. (Bottinga, 1968). The lower is the temperature of decom- It is known that thermogenic CO2 is generated in the position, the lighter is the CO2 derived from carbonates. ° δ13 ° temperature interval from 50–100 C (Stach et al., 1982). CCO2 around –10‰ (temperature of degradation 25 C) δ13 ° Values of CCO2 indicating thermogenic CO2 are esti- to –4‰ (temperature of carbonate degradation 200 C) mated to range from –25 to –10‰. CO2 released during indicates CO2 derived from carbonates. thermal maturation should be characterized by enrichment All types of external CO2 in the lignite seam (endog- in 13C in comparison to the organic matter (Aravena et enic, thermal degradation of carbonates, hinterland wa- al., 2003) from which it originates. The CDMI index in- ter recharging the water saturated roof strata of the Velenje dicating thermogenic CO2 in the Velenje basin is very low basin) could be utilized by microbiological activity, caus- δ13 13 (to 10%). Values of CCO2 from –13.2 to –10‰ in the ing enrichment in the C isotope in residual CO2 since Velenje basin indicate thermogenic CO2, probably derived the formation of the Velenje basin. from the coalification process, but more probably related At the subsurface, CO2 from wells of the lignite coal- to methanogenic CO2 reduction activity causing enrich- mine was found to be of mixed origin, indicating the ori- 13 ment in the C isotope in residual CO2 and retained in gins of coalbed gas to be: free gaseous CO2 derived from the lignite structure since the time of formation of the hinterland water recharging the wells and/or oxidation of basin. methane and decomposition of organic matter (Fig. 6). δ13 δ13 CCO2 derived from carbonates is dependent on the The CDIC values from hinterland water indicate CO2 δ13C value of the carbonates and the temperature of their derived from wells (Table 2). The sample from well δ13 degradation. Triassic carbonates are placed below lignite number C3.29 from a well has a very low CCO2 of strata in the NE part of the Velenje basin. According to –34.1‰ (Table 2, Fig. 6), indicating CO2 from degrada- the results for δ13C of carbonates, thermal decomposi- tion of organic matter, causing enrichment in the light

Origin and distribution of coalbed gases from the Velenje basin, Slovenia 405 0

-5

-10

) -15 ‰ (

-20 CO2 derived from CO2 derived from CO2

C hinterland water hinterland water and/or 13 -25 oxidation of methane δ

-30 decomposition of organic matter -35

-40 0 102030405060 CDMI (%) δ13 Fig. 6. Interpretation of the origin of CO2 from wells at the subsurface of Velenje coalmine using CDMI index versus CCO2 out; δ13 CO2 indicating decomposition of organic matter has CCO2 lower than average organic matter (around –25‰). Boundaries δ13 show ranges of C values for CO2 originating from various sources modified from Jenden et al. (1993).

12 C isotope of generated CO2 accumulating at the sub- There is also evidence that under some circumstances 13 12 surface of the mine. CH4 may migrate faster than CH4 (Galimov, 1967). The observed isotopic composition gradient could also 12 Distribution of methane and CO2 in the Velenje basin be a result of preferential dissolution of the CH4 mol- The distribution of the concentrations and isotopic ecule along its pathway of migration, or preferential ad- 12 compositions of methane and CO2 depends on their mo- sorption of CH4 on the coalbed surface (Bondar, 1987). δ13 δ13 bility. Methane migrates faster than CO2 (Atkins, 1994). No correlation was found between CCO2 and CCH4 The gas content of coal increases with depth as the amount versus depth above sea level of the Velenje basin. After of gas that coal can adsorb increases with pressure migration of methane through open cleats of the lignite (Beamish and Crosdale, 1998). Excavation of coal also seam and desorption into the gaseous phase from the lig- causes migration of gas. In our case the distribution of nite structure due to of subsidence of the seam caused by CO2 and methane changes with depth (Figs. 7 and 8). The mining activity, methane diffuses through the microporous CDMI index is higher in the lignite seam; the concentra- lignite structure and accumulates in sands, clays and marls tions of CO2 increase with depth and of methane de- in the wells and roof strata of the Velenje basin. creases. With regard to gas concentrations measured from Isotopically light methane (microbial methane) in wells wells it must also be noted that CO2 is more soluble (Hen- (Figs. 4 and 8) at the subsurface of the mine could there- ry’s law constant at 298 K 1.25·106 K/Torr) in water than fore be explained by the process of migration through methane (Henry’s law constant at 298 K 3.14·105 K/Torr), cleats, while heavier methane remains adsorbed in the lig- so more methane is derived from wells to the atmosphere. nite seam. It is also known that the sorptive capacity of coal for CO2 The A–B geological profile showing geochemical data is 2 to 3 times greater than for methane (Styles, 1995). on methane and CO2 in the Velenje basin is given in Therefore, CO2 remains preferentially sorbed in the Fig. 8. Values with a high CDMI index (80–100%) and δ13 coalbed structure or preferably dissolves in water. In the CCO2 from –11.2 to –0.9‰ characteristic in the central microporous structure of lignite, where physicochemical part of the Velenje basin in the lignite seam indicate en- conditions of CO2 are below the critical temperature dogenic CO2 and microbial CO2 reduction (Fig. 8). Bro- ° (31.04 C) and critical pressure (72.8 bar), CO2 could be ken arrows indicate the possibility of endogenic CO2 dur- in the liquid state (Atkins, 1994). Any relaxation of gas ing formation of the Velenje basin and active tectonic pressure in the microporous structure causes a sudden phases of the Sostanj fault system. In the northern part of transformation of CO2 to the gaseous state. the upper lignite seam (samples B126, B120, B130) lower

406 T. Kanduc and J. Pezdic CDMI (%)

0 102030405060708090100 500

400

A,B 300 C3 200 ) m ( z 100

0

-100

-200

Fig. 7. CDMI coalbed gas index versus depth above sea level in the Velenje basin.

Fig. 8. Geological cross section A–B through the south–north part of the Velenje depression (adapted from Brezigar, 1986) with geochemical data for coalbed gases; for location of cross-section A–B see Fig. 1.

δ13 CDMI values were obtained (Fig. 8). CCH4 values in- fractionation during migration of methane from the lig- δ13 dicate CO2 reduction processes since CCO2 is heavier nite seam to the surface. The low concentration of CO2 δ13 (up to –4.4‰). In well C3.16 at the northern part of the and its CCO2 values from –11 to –4.3‰ in wells at lo- Velenje basin, thermogenic methane resulting from oxi- cations C3.22 and C3.16 (Fig. 8) means that free gaseous dation of methane to CO2 is indicated. At locations C322 CO2 could be derived from roof clastic strata of the and C3.28 isotopically light methane indicates methane Velenje basin saturated with hydrogen carbonate water, δ13 from the lignite seam and is a result of isotopic which was also confirmed by the CDIC values.

Origin and distribution of coalbed gases from the Velenje basin, Slovenia 407 CONCLUSIONS and Arkadij Popovic for their help in analytical procedures. The critical comments of reviewers Milos Markic and Simon Zavsek Studies of gaseous concentrations and their stable iso- are highly appreciated. We are very grateful to Robert Kuster topic composition allow a tentative interpretation of the for computer assistance. origin of coalbed gas in the Velenje basin. Considering Sincere thanks to Anthony Byrne for improving the Eng- the geochemical results for coalbed gases, it can be con- lish of the manuscript. cluded that coalbed gas from the Velenje basin is of mul- δ13 tiple origin: thermogenic methane with CCH4 from REFERENCES δ13 –50 to –37.7‰, microbial methane with CCH4 from δ13 Aravena, R., Harrison, S. M., Barker, J. F., Abercrombie, H. –74.0 to –50‰, endogenic CO2 with CCO2 around δ13 and Rudolph, D. 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