carbon cycling in the pliocene velenje coal basin, slovenia, inferred from stable carbon isotopes

International Journal of Coal Geology 89 (2012) 70–83

Contents lists available at SciVerse ScienceDirect

International Journal of Coal Geology

j ourna l homepage: www.e lsev ie r .com/ locate / i j coa lgeo

Carbon cycling in the Pliocene Velenje Coal Basin, Slovenia, inferred from stablecarbon isotopes

Tjaša Kanduč a,⁎, Miloš Markič b, Simon Zavšek c, Jennifer McIntosh d

a Department of Environmental Sciences, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Sloveniab Geological Survey of Slovenia, Dimičeva 14, SI-1000 Ljubljana, Sloveniac Velenje Coal Mine, Partizanska 78, 3320 Velenje, Sloveniad University of Arizona, Department of Hydrology and Water Resources, 1133 E. James E., Rogers Way, Tucson, AZ, USA

⁎ Corresponding author. Tel.: +386 1 5885 238; fax:E-mail address: [email protected] (T. Kanduč

0166-5162/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.coal.2011.08.008

a b s t r a c t
a r t i c l e i n f o
Article history:Received 15 December 2010Received in revised form 19 August 2011Accepted 22 August 2011Available online 27 August 2011

Keywords:Stable isotopesCarbon cyclingLigniteCoalbed gasGroundwaterVelenje Basin

Stable isotopes of carbon were used to trace organic and inorganic carbon cycles and biogeochemical process-es, especially methanogenesis within different geologic substrates of the Pliocene lignite-bearing VelenjeBasin in northern Slovenia. Lithotypes of lignite, coalbed gases, calcified woods (xylites), carbonate-rich sed-iments, and groundwaters were investigated. Carbon isotope (δ13C) values of the different lignite lithotypesranged from −28.1 to −23.0‰, with the variability likely a function of the original isotopic heterogeneity ofthe source plant materials and subsequent biogeochemical processes (i.e. gelification, fusinitization, mineral-ization of organic matter) during the early stage of biomass accumulation and diagenesis. In the lignite seam,CO2 and CH4 were the major gas components with small amounts of N2. The carbon isotope values of CO2

(δ13CCO2) and CH4 (δ13CCH4) were highly variable, ranging from−9.7 to 0.6‰ and−70.5 to−34.2‰, respec-tively. Carbon dioxide is likely sourced from amixture of in situmicrobial activity and external CO2, while CH4

is dominantly sourced from microbial methanogenesis, with possible addition of thermogenic gas from dee-per formations, and the influence of microbial oxidation of methane. Calcified xylites enriched with 13C (δ13Cvalues up to 16.7‰) indicate that microbial methanogenesis was active during formation of the basin. Theδ13CDIC values (from −17.4 to −3.2‰) of groundwaters recharging the basin from the Triassic aquifer areconsistent with degradation of organic matter and dissolution of dolomite. Groundwaters from the Pliocenesandy and Lithotamnium carbonate aquifers have δ13CDIC values (from −9.1 to 0.2‰) suggestive of degrada-tion of organic matter and enrichment via microbial reduction of CO2.

+386 1 5885 346.).

rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Understanding the sources and sinks of carbon in the environmentis the basis for prediction of global climate change, and carbon iso-topes have been shown to be particularly useful tracers of the carboncycle (Berner, 1996, 2003; Chang and Berner, 1999; Clark and Fritz,1997; Scholes et al., 2009). The isotopic composition of carbon inlow-rank coals may be used to reconstruct past changes in the globalcarbon cycle (Arens et al., 2000; Lücke et al., 1999). Plants use atmo-spheric CO2 to produce carbohydrates (CH2O) during photosynthesis(O'Lary, 1988); although, it is difficult to determine the specific pro-cess governing the δ13C value of bulk organic matter given that

different environmental factors such as atmospheric CO2, the type ofplant material, bacterial degradation and the temperature of forma-tion can influence the δ13C value of coal (Dupouey et al., 1993; Schleser,1995).

There are three main sources of hydrocarbon gases and CO2 insedimentary basins: abiogenic, microbial and thermogenic gas(Scott, 1993). In general, thermogenic gases are typically associatedwith high rank coal, whereas microbial gases are typically associatedwith low rank coals and could have been produced throughoutthe basin history, as long as the coalbeds were never pasteurized.Abiogenic sources of gas are typically found in the deep subsurface(Sherwood Lollar et al., 2006). A number of chemical and isotopicmodels have been developed to distinguish sources of natural gas inorganic-rich reservoirs, such as coalbeds (Aravena et al., 2003; Kotarba,1990; Rice et al., 1989; Schoell, 1983; Smith and Pallaser, 1996). Stablecarbon and hydrogen isotope analyses of hydrocarbon gases and CO2

can be applied to identify the origin of natural gases, their migrationpathways and accumulation processes (Clayton, 1998; Whiticar,

http://dx.doi.org/10.1016/j.coal.2011.08.008

mailto:[email protected]

http://dx.doi.org/10.1016/j.coal.2011.08.008

http://www.sciencedirect.com/science/journal/01665162

71T. Kanduč et al. / International Journal of Coal Geology 89 (2012) 70–83

1999). Microbial gas is dominantly generated via acetate fermentationand/or CO2 reduction (Whiticar, 1999; Whiticar et al., 1986).

A consortium of microorganisms (e.g. acetogenic bacteria) de-grade fatty acid products from coal to produce acetate, with CO2

and H2 as by-products (Kotelnikova, 2002). The products of thesereactions support a variety of methanogens. Some methanogens uti-lize acetate to produce CO2 and methane, according to the followingreactions:

CH3COOH→CH4 þ CO2 Acetate fermentation: ð1Þ

While other methanogens use hydrogen gas to reduce CO2:

CO2 þ 4H2→CH4 þ 2H2O CO2 reduction ð2Þ

Fig. 1. A Geological map of the Velenje Basin area (adapted from Brezigar et al. (1987)) wilimestone (L), and Triassic (T1, T2 and T2,3) dolostones (prevailing) and limestones. Other Trdo not include significant aquifers. For explanation of lithologies please also see the stratigrand Pre-Pliocene basement (after Brezigar, 1986; Brezigar et al., 1987).

or

HCO−3 þ 4H2→CH4 þ 2H2Oþ OH− CO2 reduction: ð3Þ

The reaction pathway, i.e. acetate fermentation or CO2 reduction,will affect the isotopic composition of methane and dissolved inor-ganic carbon (DIC) differently. The best diagnostic tool to identifythe dominant methanogenic pathway appears to be 13C fractionationbetween coexisting carbon dioxide (CO2) and methane (CH4) (Conrad,2005; Whiticar et al., 1986). This “apparent” fractionation factor(αCO2–CH4) is more than 1.06 for CH4 produced via CO2 reduction,whereas for acetate fermentation it is generally lower than about 1.06(Conrad, 2005; Lansdown et al., 1992; Whiticar et al., 1986). Other pro-cesses can also affect the δ13C values ofmethane, including bacterial ox-idation ofmethane (Clark and Fritz, 1997).Methane oxidation reactions

th cross section NNE–SSW. Main aquifers are Pliocene (Pl-1, -2 and -3), Lithotamniumiassic and Paleozoic (Pz) lithologies are composed of impermeable strata and thereforeaphic column on B. B Schematic stratigraphic column of the Plio–Quaternary Basin fil
l

Fig. 1. (continued).

72 T. Kanduč et al. / International Journal of Coal Geology 89 (2012) 70–83


can proceed according to a number of reactions, although the two prin-cipal electron acceptors are O2 and SO4

2−, according to following reac-tions:

CH4 þ 2O2→HCO−3 þ H2O þ Hþ ð4Þ

1

2

4

6

7

3

5

j.v.3123-7

j.v.3233 j.v.3100

jpk-21

j.v.3134

j.v.3133

j.v.3096

j.v.3135

j.v.3101

j.v.3231 j.v.3099

j.v.1021

j.v.3298

j.v.3343

jpk-19

B181B182

B185

B186

-35 c

jpk-20

504400 504900 505400 505900

135600

136100

136600

137100

137600

138100

carbonateslignitecoalbed gases

A

BV 29

BV 27

V12 zV 12 v G1A j.v. 3121

J.V. 3143

j.v. 783j.v. 3136 j.v. 3048

j.v. 3047j.v. 3045R

V 11 nj.v. 661

j.v. 659

j.v 785/6j.v 785/5

4400 4900 5400 5900

5600

6100

6600

7100

7600

8100

PRELOGE

A

B

B

Fig. 2. A Map of sampling locations of coalbed gas, different lithotypes of lignite and carbonalap with j.v. 3123-7, and boreholes of coalbed gases jpk-20 overlap with jpk-21, and jpk-22 o(locations 3 and 4 and locations 2, 5 and 7). B Map of groundwater sampling locations. Depthare also labeled in Fig. 1A. On profiles A–B groundwaters with labels R and j.v. 3045 overl2343/1, 2, j.v. 2377/1, 3 and j.v. 2378/1, 4 overlap.

CH4 þ SO2−4 →HCO−

3 þ HS− þ H2O: ð5Þ

This produces a positive shift in δ13CCH4 and a corresponding de-crease in δ13C values of DIC (Barker and Fritz, 1981).

jpk-22

B184

jpk-5

B183

jpk-23

B18B24

B79

B138

506400 506900 507400 507900

00 100 200 300 400 500 (m)

j.v.2373

j.v.2346j.v.2343

j.v. 2341

j.v.2347

j.v. 2391

j.v.2343

j.v. 2360

j.v.2378j.v.2378

j.v.2377/1j.v.2377/3

j.v. 2434

6400 6900 7400 7900

KALE

C

D

00 100 200 300 400 500 (m)

te-rich sediments. At the scale of A the boreholes of different lithotypes of lignite over-verlap with jpk-23. Also some locations where carbonate-rich strata were taken overlapabove sea level of sampling locations is presented in Table 4. The profiles A–B and C–D

ap, and j.v 785/5 and j.v. 785/6 overlap. On profiles C–D groundwaters with labels j.v.

image of Fig.�2

Table 1Macropetrographic description, and δ13C and δ15N values of lignite from boreholes J. V.3123-7, D70(1–4), L45 (2), J. V. 115(1–4) in the Velenje Basin.

Location, depth ofthe borehole

Macroscopic lignite description δ13C (‰) δ15N (‰)

j.v. 3123-7, 0.3 m Xylite −24.2 2.4j.v.3123-7, 0.5 m Poorly gelified detrital lignite −27.3 3.3j.v.3123-7, 1.8 m Semidegradofusitic lignite −25.2 4.6j.v.3123-7, 2.5 m Xylite −23.0 2.0j.v.3123-7, 7.1 m Xylite −23.6 3.9j.v.3123-7, 8.3 m Moderately gelified detrital lignite −28.1 3.2j.v. 3123-7, 9.8 m Poorly gelified detrital lignite −27.4 3.3j.v.3123-7, 12.3 m Strongly gelified detrital lignite −28.0 1.8j.v.3123-7, 14 m Xylite −27.5 3.9j.v.115(-90)1, 3 m Poorly gelified detrital lignite −27.0 3.4j.v.115 (-90)3, 3m Strongly gelificated detrital lignite −27.6 2.6L45(2), 2.5 m Strongly gelified detrital lignite −27.5 2.6D70 (2), 7.8 m Strongly gelified detrital lignite −28.2 2.3D70(1), 2.5 m Strongly gelified detrital lignite −28.5 2.7D70(3), 13.5 m Poorly gelified detrital lignite −27.5 3.0D70(4), 11.5 m Moderately gelified detrital lignite −28.0 2.9

Table 2Chemical and isotopic characteristics of coalbed seam gases, Velenje Basin.

Sampledenotation

Location CH4

(vol. %)CO2

(vol. %)N2

(vol. %)δ13CCO2(‰)

δ13CCH4

(‰)αCO2–

CH4

j.v. 3134 Preloge,South

34.7 65.3 0.0 −9.7 −45.0 1.037


34.3 65.7 0.0 −8.2 −34.2 1.027

j.v. 3096 Preloge,NW

89.7 10.4 0.0 −2.3 −53.8 1.054


47.9 52.0 0.1 −5.9 −50.0 1.046


37.8 62.2 0.0 −2.0 −50.9 1.052


22.3 77.7 0.0 −9.6 −39.2 1.031


71.9 28.1 0.0 −1.0 −49.2 1.051


55.2 44.8 0.0 −5.2 −58.5 1.057


61.2 38.8 0.0 −4.5 −58.5 1.057

j.v. 1021 Pesje 37.5 59.8 2.7 −6.0 −48.8 1.045j.v. 3298 Preloge,

NW41.7 42.2 16.1 −7.6 −49.4 1.044

jv3343 Preloge,NW

77.5 18.5 4.0 0.6 −68.9 1.075

jpk-19 Preloge,NW

35.7 64.4 0.0 −3.9 −45.2 1.043

jpk-5 Preloge,South

45.2 49.4 5.4 −1.5 −63.1 1.066

B181 Preloge,South

46.3 43.5 10.1 −8.2 −66.7 1.063

B182 Preloge,South

44.5 47.1 8.4 −3.4 −66.5 1.068

B183 Pesje 44.8 47.3 7.9 −3.5 −66.2 1.067B184 Pesje 46.9 47.2 5.9 −3.4 −66.6 1.068B185 Preloge,

NW44.3 47.9 7.8 −3.5 −66.1 1.067

B186 Preloge,NW

46.5 44.5 9.1 −3.4 −65.0 1.066

−35 c Pesje 29.6 70.4 0.0 −4.4 −59.1 1.058jpk-22 Pesje 33.1 66.9 0.0 −0.7 −42.6 1.044jpk-23 Pesje 38.7 61.3 0.0 0.4 −49.4 1.052jpk-21 Preloge,

NW37.8 62.2 0.0 0.1 −43.0 1.045

jpk-20 Preloge,NW

38.4 61.6 0.0 −0.7 −47.0 1.049

B18 Škale 23.5 76.4 0.1 −2.9 −67.9 1.070B24 Škale 12.2 85.1 2.7 −2.4 −61.9 1.064B79 Škale 25.0 75.0 0.0 −3.4 −64.1 1.065B138 Škale 58.2 41.9 0.0 −4.0 −70.5 1.072


Diagenetically precipitated authigenic minerals are common incoal as pore fillings and as thin sheets along cleats and fractures.The most common diagenetic minerals occurring in coals are sul-fides (e.g. pyrite, sphalerite, chalcopyrite), carbonates (e.g. calcite,ankerite, siderite), clay minerals (e.g., kaolinite), and quartz (Budaiet al., 2002; Daniels et al., 1996; Pitman et al., 2003). The spatial dis-tribution of diagenetic minerals along cleats and fractures resultsfrom preferential migration of groundwater flow and the associatedtransport of solutes (Daniels et al., 1996), and the precipitation ofcalcite cements may be driven by addition of DIC from microbialmethanogenesis (Solano-Acosta et al., 2008). Therefore, diageneticmineralization in coal can have diagnostic value for identifyingpaleohydrological and microbial conditions (Solano-Acosta et al.,2008).

This present study analyzed themolecular and isotopic compositionof various inorganic and organic carbon sources in the coal-bearingVelenje Basin in Slovenia (Fig. 1A) to evaluate inputs and biogeochem-ical processes affecting the carbon cycle, includingmicrobial generationof methane in coal seams. Materials analyzed include: 1) lignite of dif-ferent macroscopic varieties (i.e. lithotypes), 2) “free” coalbed seamgases and gases from aquifers, 3) authigenic carbonate-rich lenses andcalcified xylite, and 4) groundwaters from Triassic, Lithotamnium andPliocene aquifers.

2. Site description

The Velenje Basin is located in the northeastern part of Slovenia(Fig. 1A), at the junction of the west–northwest to east–southeasttrending Šoštanj fault and the east–west trending Periadriatic zone,and is bounded to the south by the Smrekovec fault segment(Fig. 1A). The Periadriatic lineament and the Šoštanj and Smrekovecfaults were formed after collision of the Adriatic and European conti-nental plates 35 Ma ago (Fodor et al., 1998). The origin of the VelenjeBasin is related to the transtension between these two fault systems.In the Pre-Pliocene basement of the basin, Triassic carbonates anddolomites prevail on the northeastern side of the Velenje fault,while Oligocene to Miocene clastic strata, consisting predominantlyof marls, sandstones and volcanoclastics are dominant on thesouth-western side of the fault (Fig. 1A, B).

The basin fill (Fig. 1B) is up to 1000 m thick and shows a typicalcontinental fill-up succession, ranging from terrestrial to lacustrineclastic sediments (Brezigar, 1986). The lower part of the Pliocenecoal-bearing strata consists of alternating shales, clayey coal and lig-nite, and is up to 50 m thick. Above, a uniform lignite seam (up to160 m thick) has developed. The strata, with lacustrine character

above the lignite seam, are up to 350 m thick and consist of clays,marls, and silts. This succession is overlain with a 90 m thick sandy–silty formation with several lignite lenses. The uppermost part ofthe basin fill consists of terrestrial silts, overlain by recent fluvialsediments (Fig. 1B).

The alkaline, calcium-rich environment during formation ofVelenje Basin also caused a relatively high degree of gelification,which is significantly higher than the degree of gelification observedin other lignites (e.g. from lignites of the Lower Rhine bay; Markičand Sachsenhofer, 1997). Recent investigations of biomarkers inVelenje lignite by Bechtel et al. (2003, 2008) indicate that gelificationof plant tissues might have been governed by the activity of anaerobicrather than by aerobic bacteria. In addition, carbon cycling during bio-geochemical decomposition of plant tissue by bacteria is expected tohave affected the δ13C values of the coal (Bechtel et al., 2008).

In the Velenje coal basin, the shallow aquifers are located in Qua-ternary and Pliocene clastic sediments. Pliocene aquifers, found in thePreloge coalmine, are further divided into: 1) aquifers right above thecoal (Pl 1), 2) aquifers 20–80 m above the coal (Pl 2), and 3)

Table 3Macropetrological description of carbonate samples from lignites, with calculatedpaleotemperatures of precipitation from isotope values after Kim and O'Neil (1997).Samples marked with * were analyzed by SEM microscopy.

Macroscopic description of the sample,location on the map on Fig. 2A

δ13CCaCO3

(‰)δ18OCaCO3

(‰)δ18OVSMOW

(‰)

Calcified xylitea, location 1 16.2 −7.2 22.010 cm strata in tiny detrital lignite, location 2 −2.4 −11.3 19.3Completely calcified xylite, location 3 16.6 −7.6 22.6Completely calcified xylite, location 4 15.9 −7.3 22.15 cm strata of calcarenite in tiny detritallignitea, location 5

10.3 −9.2 20.7

Completely calcified xylite, location 6 15.3 −7.1 22.85–10 cm lense of calcarenite in tiny detritallignite, location 7

8.6 −7.8 21.0

5 cm strata of calcarenite in tiny detritallignite, location 1

11.5 −10.4 20.5

Completely calcified xylite, location 1 16.4 −8.2 22.7Completely calcified xylite, location 1 16.7 −8.3 38.15 cm strata of calcarenite in tiny detritallignite, location 1

−2.1 −11.7 19.2

Completely calcified xylite, location 1 16.4 −8.8 22.1Completely calcified xylite, location 1 12.6 −8.6 22.4

a In situ mapping and SEM microscopy.


uppermost Pliocene aquifers (Pl 3) (Jamnikar and Fijavž, 2006). Thenorthern part of the Preloge coalmine, on the southern side of theVelenje Fault, is underlain by the Lithotamnium limestone aquifer ofMiocene age. The Škale coalmine located in the northwestern partof the Velenje basin contains a Triassic aquifer composed of Scythianand Anisian age limestone and dolomite (Fig. 1A, B).

3. Materials and methods

3.1. Sample collection

The Velenje coalmine in the Velenje Basin is separated into twoparts: the still active Preloge coalmine and the Škale coalmine,which was closed for mining in 2008 (Fig. 2A). In the western partof the Preloge mine-workings, at the locality of a gas outburst inFebruary 2003 (long-wall lignite excavation at the level of −90 m),numerous lignite samples (16) were taken from the J.V.3123-7,D70(1–4), L45(2) and J.V.115(1–4) boreholes, which were drilledimmediately after the above-mentioned event.

Sampling of coalbed gases from the exit and delivery roadwayswas performed during 1999–2002 in the Škale mining area andfrom the Pesje and Preloge mining areas from exit and deliveryroads and working faces during 2002 and 2010. Sampling locationsare presented on Fig. 2A (Kotarba, 2001). Boreholes were drilled inpillar coal to a depth of 3 or 15 m. After drilling, a capillary tubewas inserted into the borehole. Free gas emitted from the boreholewas collected in a 50 ml plastic syringe and then transferred to a12 ml evacuated glass ampoule with a septum, and stored at normalatmospheric conditions until analysis.

Carbonate samples from pillar coal ahead of the working face(Fig. 2A) were also taken for a better understanding of the origin ofmethane in the coal seam.

Most studies of microbial methanogenesis in organic-rich reser-voirs utilize both formation water and gas samples to decipher the or-igin of methane (Aravena et al., 2003; Bates et al., 2011; Martini et al.,1996, 1998), but we were not able to sample formation waters due todewatering of water recharging the lignite seam to prevent water in-side the working mine. Thirty-one groundwater samples were col-lected from dewatering objects in October 2003 from Plioceneaquifers (16 samples from Pl 1 and Pl 2 aquifers), Triassic aquifers(13 samples) and Litotamnium aquifers (2 samples), and analyzedfor geochemical and stable isotopic parameters (locations shown inFig. 2B). Temperature and pH were measured in the field. Sample

aliquots collected for chemical and water isotope analyses werepassed through a 0.45 μm nylon filter into HDPE bottles and kept re-frigerated until analyzed. Samples for cation analyses were acidifiedwith HNO3. Samples for δ13CDIC analyses were also filtered and storedin glass bottles with septa caps and no headspace.

3.2. Analytical methods

3.2.1. SolidsA macroscopic description of the lignite samples in terms of litho-

types was carried out following the criteria of Markič and Sachsenhofer(1997), prior to any isotopic analysis.

Carbonate samples (calcified xylite and limestone lenses) wereexamined to determine their qualitative and quantitative composi-tion using a JEOL JSM 5800 electron microanalyzer scanning electronmicroscope energy dispersive X-ray spectroscopy (SEM/EDXS) at theGeological Survey of Slovenia, Ljubljana and at the Department ofCeramics at the Jožef Stefan Institute.

3.2.2. Concentration measurements on fluids and gasesTotal alkalinity in groundwater samples was measured within

24 h of sample collection by Gran titration (Gieskes, 1974) with a pre-cision of ±1%. Concentrations of dissolved Ca2+, Mg2+, Na+, K+ andSi were determined using a Jobin Yvon Horiba ICP-OES with an ana-lytical precision of ±2%. Anions (SO4

2−, NO3−, Cl−) were analyzed on

a Dionex ICS-2500 IC with an analytical precision of ±2%. Concentra-tions of DIC were determined on a UIC Coulometrics CO2 coulometerwith a precision of ±2%. Dissolved organic carbon (DOC) concentra-tions were measured using high temperature platinum-catalyzedcombustion followed by infrared detection of CO2 (Shimadzu TOC-5000A) with a precision of ±2%.

Thermodynamic modeling was used to calculate pCO2 values andthe saturation state of calcite (SIcalcite) using pH, alkalinity, tempera-ture, and major ion chemistry as inputs to the PHREEQC speciationprogram (Parkhurst and Appelo, 1999).

Determination of the concentrations of CH4, CO2, nitrogen, oxygenand argon was performed using a homemade NIER mass spectrome-ter (Kanduč and Pezdič, 2005). The method of singular decompositionof the matrix was used, to obtain simultaneous analysis of the gases.The precision of the method was ±3%. To correct for any possibleair contamination of the capillary system and evacuated ampoulesduring sampling, sample results were recalculated on an air-freebasis. The percentage of oxygen in the sampled ampoules was usedto calculate the amount of nitrogen, according to the ratio in air(N2/O2=3.7) considering Dalton's law (Atkins, 1994).

3.2.3. Isotope measurements (solids, fluids, gases)The stable isotope composition of carbon and nitrogen in different

lignite lithotype samples was determined using a Europa 20-20 IRMSANCA-SL preparation module. Lignite samples were first ground andhomogenized, and 1 mg of sample was weighed in a tin capsule forcarbon analysis and 10 mg for nitrogen analysis. Samples for carbonanalysis were pre-treated with 1 M HCl to remove carbonates. Thesample residues were washed in distilled water, dried and homoge-nized. The isotopic compositions of nitrogen and carbon were deter-mined after combustion of the capsules in a hot furnace(temperature 1000 °C). Generated products were reduced in a Cutube (600 °C), where excess O2 was absorbed. H2O was trapped on adrying column composed of MgClO4. NBS 22 (oil) and IAEA N-1 (am-monium sulfate) reference standards were used to relate the analyti-cal results to the VPDB (Vienna Pee Dee Belemnite; Coplen, 1996) andAIR standards. Sample reproducibility for was ±0.2‰ for carbon, and0.3‰ for nitrogen.

Carbonate samples from carbonate-rich strata and calcified xylitewere first roasted at 350 °C (Krantz et al., 1987) and then ground toa powder in an agate mortar. δ18O and δ13C values were determined

Table 4Chemical and isotopic data for groundwaters from the Triassic, Lithotamnium, and Pliocene aquifers.

Samplinglocation

Geology Depthabovesealevel z(m)

T(°C)

pH Totalalkalinity(meq/l)

Ca2+

(mM)Mg2+

(mM)Na+

(mM)K+

(mM)Si(mM)

SO42−

(mM)NO3

−

(mM)Cl−

(mM)DOC(mM)

δ13CDIC

(‰)pCO2

(ppm)δ18O(‰)

SIcalcite δ13CCH4(‰)

BV 29 Pliocene 1,2 417 19.0 7.00 19.9 2.18 4.43 3.02 0.14 1.25 n.a. 0.02 0.10 0.60 −2.5 100,000 −10.6 0.3 −69.2BV 27 Pliocene 1,2 413 16.7 7.12 11.9 1.76 2.39 1.65 0.08 0.95 n.a. n.a. 0.07 0.33 −3.2 45,709 −10.7 0.2V12 z Pliocene 1,2 387 18.6 6.70 32.4 3.61 8.46 4.55 0.19 1.37 n.a. n.a. 0.19 0.58 −3.3 309,030 −11.3 0.4V12 v Pliocene 1,2 385 18.6 6.50 27.4 3.05 6.89 3.66 0.16 1.26 n.a. n.a. 0.15 n.a. −2.9 426,580 −11.2 0.1G1A Pliocene 1 −61.2 18.6 6.50 4.44 5.45 12.4 0.35 0.86 1.11 n.a. 0.51 n.a. −9.1 −11.4 −70.2j.v. 3121 Pliocene 1 −60.0 20.1 6.62 32.9 2.86 8.51 8.56 0.14 1.91 n.a. n.a. 0.22 0.58 −2.4 389,045 −11.1 0.2 −65.3j.v. 3143 Pliocene 1 −45.0 20.1 6.35 33.5 4.17 8.16 7.30 0.14 1.71 n.a. n.a. 0.20 0.69 0.2 724,436 −10.8 0.1 −44.1j.v. 783 Pliocene 1 −71.5 20.5 6.45 52.2 4.12 15.5 13.1 0.18 1.79 0.20 n.a. 0.41 1.22 −2.6 870,964 −10.7 0.3 −62.4j.v. 3136 Pliocene 1 −63.9 20.5 6.49 38.2 3.18 10.1 9.98 0.15 1.84 n.a. 0.08 0.23 0.81 −1.7 588,844 −10.4 0.1 −58.3j.v. 3048 Pliocene 1 −62.0 19.5 6.51 26.0 2.52 7.07 5.19 0.11 1.65 n.a. 0.01 0.17 0.46 −3.6 398,107 −10.5 0.0 −66.8j.v. 3047 Pliocene 1 −79.0 21.4 6.43 43.1 3.63 12.6 10.3 0.16 1.81 0.01 0.01 0.41 1.05 −3.1 776,247 −10.0 0.2j.v. 3045 Pliocene 1 −91.2 20.6 6.36 32.6 2.79 10.0 6.86 0.14 1.76 0.01 n.a. 0.21 n.a. −5.5 691,831 −10.4 −0.1R Pliocene 1 −60.0 20.8 6.53 51.6 3.60 16.3 12.8 0.20 1.91 n.a. n.a. 0.40 n.a. −4.8 707,946 −10.5 0.3 −70.1V11 n Pliocene 1,2 373 20.1 7.38 62.0 4.25 21.7 11.3 0.26 1.72 n.a. n.a. 0.54 n.a. −3.3 114,815 −10.4 1.3j.v. 661 Pliocene 1 −107 20.8 7.01 59.8 4.50 20.8 11.5 0.25 1.83 0.01 0.08 0.53 1.00 −3.5 263,027 −10.4 0.9j.v. 659 Pliocene 1 −106 21.0 6.93 44.3 3.53 13.6 9.47 0.19 1.82 n.a. n.a. 0.33 0.95 −2.6 245,471 −10.9 0.7j.v 785/6 Limnestone

litotamnium−150 30.2 7.18 39.9 1.19 1.29 54.0 0.50 0.22 n.a. n.a. 2.08 12.7 −2.6 151,356 −10.7 0.6 −50.6

j.v 785/5 Limnestonelitotamnium

−150 30.0 7.30 38.1 1.29 1.22 50.7 0.48 0.23 n.a. n.a. 2.01 n.a. −1.9 109,648 −10.7 0.7

j.v.2346 TriassicAnisian

121 17.7 7.06 5.94 2.03 1.54 0.07 0.01 0.16 1.98 0.01 0.05 0.98 n.a. 26,915 −9.7 −0.1

j.v.2343/1 TriassicAnisian

121 17.1 7.13 5.88 1.99 1.50 0.07 0.01 0.15 0.78 0.01 0.05 0.13 −11.0 22,909 −9.5 0.0

j.v. 2341 TriassicAnisian

122 16.0 7.11 6.68 2.70 1.46 0.10 0.01 0.16 0.64 n.a. 0.10 0.21 −17.4 26,303 −9.6 0.1

j.v.2347 TriassicAnisian

73.7 18.9 6.90 5.79 2.68 1.76 0.17 0.03 0.17 1.06 0.01 0.12 n.a. −8.7 38,019 −9.6 −0.2

j.v. 2391 TriassicAnisian

85.6 14.2 7.08 5.57 1.68 1.34 0.11 0.01 0.11 1.81 0.01 0.11 n.a. −12.8 23,442 −9.5 −0.2

j.v.2343/2 TriassicAnisian

121 16.4 7.11 5.79 2.15 1.46 0.07 0.01 0.14 0.35 0.05 0.06 n.a. −12.5 22,909 −9.1 0.0

j.v. 2360 TriassicScythian

27.0 20.6 6.94 5.56 3.19 2.32 1.57 0.08 0.19 0.56 n.a. 0.24 0.19 −9.3 33,884 −9.6 −0.1

j.v.2378/1 TriassicScythian

30.2 20.1 6.74 6.66 4.46 3.16 1.51 0.08 0.17 3.61 n.a. 0.15 n.a. −8.3 61,660 −10.1 −0.1


30.2 19.8 6.61 7.65 4.63 2.36 0.91 0.17 0.16 5.37 0.01 n.a. 0.48 −10.1 97,724 −9.7 −0.1


33.6 15.3 6.91 6.89 3.68 1.73 0.18 0.02 0.15 n.a. n.a. 0.12 0.17 −8.6 41,687 −9.8 0.0


33.6 18.7 6.58 10.6 3.70 2.38 1.51 0.07 0.14 2.07 n.a. n.a. 0.68 −3.2 141,254 −9.3 −0.1

j.v.2373 TriassicScythian

85.9 18.4 7.08 9.88 2.37 2.02 0.42 0.03 0.16 n.a. n.a. 0.03 0.18 −8.4 42,658 −9.5 0.2

j.v. 2434 TriassicScythian

50.0 20.0 6.66 n.a. 2.97 1.64 3.98 0.11 0.15 1.44 n.a. 0.19 n.a. −8.1 −9.6 n.a.

n.a.—not analyzed.Locations are plotted on Fig. 2B.


using a dual inlet Varian Mat 250 Isotope Ratio Mass Spectrometer.Two to three milligrams of carbonate sample was transformed intoCO2 by reaction with anhydrous H3PO4 at 55 °C under vacuum(McCrea, 1950). CO2 was then extracted from the reaction mixtureand trapped in a glass tube cooled with liquid N2. NBS 18 and NBS19 were used as international standards to report all isotopic signa-tures in ‰ relative to VPDB. Precision as determined by repeated an-alyses of the KH-2 working standard was ±0.05‰ for δ13C and±0.1‰ for δ18O. Measured δ18OVPDB values ranged from −11.7 to−7.1‰ and were recalculated to the VSMOW (Vienna standardmean ocean water) reference standard to enable the comparisonwith data from other coal basins (Illinois and Black Warrior Basins)according to the equation (O′Neil, 1979):

δ18OVSMOW ¼ 1:03039·δ18OVPDB þ 30:39: ð6Þ

The stable isotope composition of dissolved inorganic carbon(δ13CDIC) was determined with a Europa Scientific 20-20 continuousflow IRMS using an ANCA-TG preparation module. Phosphoric acid(100%) was added (100–200 μl) to a septum-sealed vial that wasthen purged with pure He. The water sample (6 ml) was injectedthrough the septum and headspace CO2 was measured (modifiedafter Miyajima et al.(1995), Spötl(2005)). In order to determine theoptimal extraction procedure for groundwater, a standard solutionof Na2CO3 (Carlo Erba) with a known δ13CDIC of −10.8±0.2‰ wasprepared with a concentration of either 4.8 meq/l (for samples withan alkalinity above 2 meq/l) or of 2.4 meq/l (for samples with alkalin-ity below 2 meq/l). Precision was ±0.2‰ for δ13CDIC.

The isotopic composition of oxygen in water (δ18O) was measuredafter equilibration with reference CO2 at 25 °C for 24 h (Epstein andMayeda, 1953). The measurement was performed on a Varian MAT250 mass spectrometer. Stable isotope results for oxygen are reported

-50

-40

-30

-20

-10

0

10

20

-90 -80 -70 -60 -50 -40 -30

δ13C

CO

2 (‰

)

δ13CCH4 (‰)

αC = 1.04αC = 1.055αC = 1.09

Fig. 3. Interpretation of the origin of methane in the lignite seam using δ13CCO2 versusδ13CCH4 values after Whiticar et al. (1986) with lines of equal carbon isotopic fraction-ation (αC) between methane and carbon dioxide as reference with values of αC=1.04through 1.09 according to Eq. (7).


using conventional delta (δ) notation as δ18O, in permil (‰) relativeto VSMOW. The precision of the measurements was ±0.1‰ for δ18O.

The isotopic composition of CH4 and CO2 was determined using aEuropa 20-20 continuous flow isotope ratio mass spectrometer (CF-IRMS) with an ANCA-TG preparation module. Gas samples wereflushed with a continuous flow of He across two chemical traps.First water was removed and then CO2 was directly analyzed for 13Ccontent. For CH4 measurements, first CO2 was removed and thenCH4 was combusted over hot 10% platinum CuO (1000 °C). The CH4

completely converted to CO2 was then directly analyzed for 13C con-tent. The stable carbon isotope data are presented in the δ notationrelative to the VPDB standard and expressed in ‰ (Coplen, 1996).The analytical precision for analysis of carbon was ±0.2‰.

4. Results

4.1. Lithotypes and isotopic composition of lignite

According to the macroscopic description of the lignite samples,the following lithotypes were distinguished: xylite, poorly gelifiedfine detrital lignite, moderately gelified fine detrital lignite, stronglygelified fine detrital lignite, fusite and semifusinitic lignite. δ13Cvalues of organic carbon in different lithotypes of lignite varied from−28.5‰ in strongly gelified lignite to −23.0‰ in xylite, while δ15Nvalues of total nitrogen ranged from 1.8‰ in strongly gelified detritallignite to 4.6‰ in semi fusinitic lignite (Table 1).

4.2. Chemical and isotopic composition of coalbed gases

Results for the chemical and isotopic compositions of coalbedgases from the Preloge, Pesje and Škale mines are presented inTable 2. Major gas components in the coal seams were CO2 and CH4,which ranged from 10.4 to 85.1 vol.% and from 12.2 to 89.7 vol.%, re-spectively. Coalbed gases in the lignite seam also contained excess ni-trogen (recalculated on an air-free basis), up to 16.1% aboveatmospheric values. Values of δ13CCO2 and δ13CCH4 from coal seamgas ranged from −9.7 to 0.6‰ and from −70.5 to −34.2‰,respectively.

4.3. Associated carbonate minerals

A macroscopic description of analyzed carbonate-rich lenses andcalcified xylite is presented in Table 3. δ13C values of carbonate-rich

lenses and calcified xylite ranged from −2.4 to 16.7‰. Most of thecarbonate samples were enriched in 13C, and only two sampleswere depleted, compared to marine carbonates with isotopic compo-sition of ~0‰ (Hoefs, 2009).

4.4. Major element and stable isotope geochemistry of groundwaters

From the geochemical and stable isotope results, there appear tobe three distinct aquifers within the Velenje Basin: 1) a Pliocene aqui-fer with an alkalinity of 11.9 to 62.0 meq/l, Ca2+ concentrations from1.76 to 4.50 mM, Mg2+ from 2.39 to 21.74 mM, Na+ from 1.65 to13.09 mM, Si from 0.86 to 1.91 mM, δ13CDIC values from −9.1 to0.2‰, and δ18O values from −11.3 to −10‰. 2) a Triassic aquiferwith an alkalinity of 5.56 to 10.6 meq/l, Ca2+ concentrations from1.68 to 4.63 mM, Mg2+ from 1.34 to 3.16 mM, Na+ from 0.07 to3.98 mM, Si from 0.11 to 0.19 mM, δ13CDIC values from −17.4 to−3.2‰, and δ18O values from −10.1 to −9.1‰. 3) A Lithotamniumaquifer with an average alkalinity of 39.0 meq/l, a Ca2+ concentrationof 1.24 mM, a Mg2+ value of 1.26 mM, a Na+ value of 52.4 mM, a Sivalue of 0.23 mM, a δ13CDIC value of −2.2‰, and δ18O values from−10.1 to −9.1‰ (Table 4).

Calculated CO2 partial pressures (pCO2) varied from 22909 ppm to870964 ppm, which is from 57- to 2180-fold supersaturated relativeto atmospheric CO2 (around 400 ppm) (Clark and Fritz, 1997). Thecalcite saturation index (SIcalcite=log ([Ca2+]·[CO3

2−] /Kcalcite) wasgenerally well above equilibrium (SIcalcite=0), indicating that calcitewas supersaturated and precipitation was thermodynamically favor-able in groundwater from the Pliocene and Lithotamnium aquifers,while groundwater from the Triassic aquifers was under-saturatedor close to saturation with respect to calcite (Table 4).

Dissolved Organic Carbon (DOC) concentrations in the Pliocene(0.33–1.22 mM) and Triassic aquifers in the Velenje Basin were low(range from 0.13 to 0.98 mM, Table 4), compared to organic-richshales in the Michigan Basin, where DOC concentrations up to70 mM have been reported (Martini et al., 1996), but within therange of coalbeds in the Powder River Basin (0.09–1.53 mM; Bateset al., 2011). An elevated concentration of DOC (12.7 mM) was mea-sured in the Lithotamnium limestone aquifer, which is probably relat-ed to higher organic acid production, and/or limited consumption bymicroorganisms. The isotopic composition of methane (δ13CCH4) fromPliocene aquifers ranged from −70.2 to −44.1‰ (Table 4).

5. Discussion

5.1. Isotopic composition of lignite

The isotopic composition of lignite in the Velenje Basin varieddepending on the lithotype and appears to be related to biogeochem-ical processes, such as gelification or degradofuzinitization/minerali-zation, which occurred during peat accumulation, especially inplaces with xylite-rich accumulations. These locations were chaotic,with stumpy xylite remnants of broken trees, unevenly layered treetrunks, and many vacinities (Markič and Sachsenhofer, 2010).According to Diessel (1992) gelification is the result of biochemicalprocesses during the earliest stage of coal diagenesis (early coalifica-tion) up to the conversion of vegetal matter into peat and lignite.Degradofusinitization (mineralization) is degradation of organic mat-ter under slightly oxic conditions and the activity of bacteria or fungiis mostly not visible macroscopically. Morphologically (under a mi-croscope), degradofusinite resembles a transition between textinite(nonoxidised) and fusinite (severely oxidized) tissue (Markič andSachsenhofer, 2010). A consequence of the formation of degradofusi-nite is a release of volatiles (gases) and mineralization (enrichment inresidual mineral matter due to oxidation/degradation of organic mat-ter) (Markič and Sachsenhofer, 2010). Fusite originates from intensiveoxidation, such as surface burning of trees or already accumulated


biomass. True fusite-rich horizons are not known in the Velenje ligniteand therefore only local fires, due to e.g. lightning strikes can be consid-ered (Markič and Sachsenhofer, 2010). The lower δ13C values observedin the strongly gelified lignite samples (ranging from −28.5to −27.5‰), compared to the ungelified samples, are likely a result ofearly biodegradation of organic matter (Table 1). Gelified lithotypes oflignite were formed in anoxic conditions (Kanduč et al., 2005).

Nitrogen (N) is an element that is sensitive to bacterial activity incoals. It is a constituent of proteins; therefore it is enriched in micro-organisms and marine water plants. As a result of metabolism andafter the death of microorganisms, N is concentrated in the sedimentand as a nutrient again enhances vegetation growth and bacterial ac-tivity (Zhu et al., 2000). According to Stach et al. (1982), Diessel(1992), Taylor et al. (1998), and Zhu et al. (2000), mineralization oforganic matter is caused by various bacteria in an alkaline environ-ment and is reflected in δ15N values higher than in their plant precur-sors. It is also well known that mineralization is more pronounced inaerobic environments, leading to formation of semifusinitic and relat-ed inertinitic components in coals. In contrast, strongly gelified detri-tal lignites formed in an anaerobic environment are characterized bythe lowest δ15N values among the analyzed lithotypes (Table 1). Deg-radation of organic matter (gelification and degradofusinitization/mineralization) of organic matter accompanied by microbial activityalso played a significant role in microbial gas genesis in the VelenjeBasin.

5.2. Origin of methane in relation to the formation of the Velenje Basin

Carbon isotope values of CH4 and CO2 in the Velenje Basin arequite variable, ranging from −70.5 to −34.2‰ and −9.7 to +0.6‰,respectively (Table 2). Past researchers have used stable isotope ra-tios of CH4, CO2, and H2O to decipher the origin of methane and dis-tinguish pathways of microbial methane generation (i.e. acetatefermentation versus CO2 reduction) (Bates et al., 2011; Flores, 2004;Flores et al., 2008; Jahren et al., 2004; Rice et al., 2008; Schoell,1980; Waldron et al., 2007; Warren et al., 2004; Whiticar, 1999;Whiticar et al., 1986). Whiticar et al. (1986) suggested that variationsin the carbon isotopic fractionation factors (α) between CO2 and CH4

(Eq. (7)) could be utilized to distinguish between CH4 generated viaCO2 reduction and acetate fermentation:

α13CCO2–CH4 ¼ ðδ13CCO2 þ 1000Þ=ðδ13CCH4 þ 1000Þ: ð7ÞGas samples with αCO2–CH4 values N1.06 are typically representa-

tive of methanogenic environments dominated by CO2 reduction,whileαCO2–CH4 values b1.06 are characteristic of acetate fermentation(Whiticar, 1999; Whiticar et al., 1986). A plot of δ13CCO2 versusδ13CCH4 for Velenje Basin gas samples (Fig. 3), shows that sampleshave relatively low δ13CCH4 values (b−50‰ for 19 of 29 samples) in-dicative of microbial methane (Whiticar et al., 1986). In addition,samples plot within both the CO2 reduction and acetate fermentationfields, with αCO2–CH4 values ranging from 1.027 to 1.075, suggestingthe presence of both methanogenic pathways. Previous studies inthe Velenje Basin have reported δ13CCO2 values from −34.1 to 2.9‰,which suggest both abiogenic and biogenic generation of CO2 (Kandučand Pezdič, 2005). The relatively invariant δ13CCO2 values of samplescollected as part of this study (−9.7 to+0.6‰) may indicate a relative-ly large reservoir of CO2 that has not been significantly depleted bymethanogens.

Three Velenje Basin gas samples are significantly enriched in 13C,with δ13CCH4 and δ13CCO2 values up to −34.2‰ and 0.6‰, respective-ly (Fig. 3). If methane was generated via CO2 reduction from a reser-voir of isotopically enriched CO2, then methane with δ13C valuesas high as −40‰ could have been generated, as observed in otherstudies (Kotelnikova, 2002). Alternatively, thermogenic gas from dee-per formations could have migrated into the shallow lignite seams

and mixed with microbial gas. Also microbial oxidation of methanecould cause CH4 to become enriched in 13C, as observed in otherorganic-rich reservoirs (Martini et al., 2003).

Microbial methane may have been generated in the lignite fromthe time of deposition up to the present because of the lack of highpaleotemperatures, which would have sterilized the lignite, andearly gas may have been lost because of insufficient compaction ofbasin sediments (Kanduč and Pezdič, 2005). Therefore, other carbonphases were analyzed to better constrain the timing and mechanismsof methane generation in the Velenje Basin.

5.3. Isotopic equilibration between oxygen in minerals and water duringmineralization and its implications for coalbed methane occurrence

Lignites in the Velenje Basin are Ca-rich type coals (Markič andSachsenhofer, 1997) and the replacement of xylite with calcite isoften observed at the working faces (Fig. 4A). Mapping of the workingfaces and SEM/EDX microscopy of calcified xylite and carbonate-richlenses is presented in Fig. 4B and C. The mineralogical composition(SEM/EDX spectrum) of the calcified xylite and carbonate-rich lensesindicates pure calcite precipitation without magnesium (Fig. 4B andC). The carbonate-rich lenses and calcified xylite observed at long-walls were reported to be syngenetic as a consequence of depositionof calcitic sediment above a relatively large wood remnant exposedfrom the peat layer, with tree trunks still in their vertical (growth)position (Markič, 2009). Degradation of wood remnants was thesource of CO2 for calcite deposits, while Ca, in addition to other cat-ions, originated from the marls, dolostones and carbonate-richrocks, shales, magmatic and volcanoclastics rocks from the hinterlandof the peat accumulation area (Hamrla, 1952; Markič, 2009).

The isotopic composition of carbonates lenses and calcified xylitesmay record various biological and hydrologic processes that occurredduring deposition. For example, carbonates with δ13C values from−9to −8‰ may reflect the uptake of plant respired CO2 during watermigration/recharge (Candy, 2009). Lacustrine carbonates tend tohave δ13C values close to ~0–1‰ due to the equilibration of lakewaters with atmospheric CO2. Elevated δ13C values in carbonates(as high as 25‰) have been shown to be the result of microbialmethanogenesis (Pitman et al., 2003). The majority of carbonatesamples associated with lignites in the Velenje Basin have δ13Cvalues from −2.1‰ to 16.7‰ (Fig. 5), which suggests a lacustrinedepositional environment with isotopically enriched CO2 generatedvia microbial CO2 reduction.

Two carbonate samples had relatively low δ13C values, from−2.4‰ to −2.1‰, which could be attributed to CO2 sourced fromdegradation of wood (lignite) with an average δ value of −26.8‰(Table 1).

The presence of minerals in coal cleats may be significant formethane generation and extraction from coalbeds as mineral fillingsaffect fluid flow and permeability, and the isotopic composition ofauthigenic calcite is diagnostic of the onset of microbial methanogen-esis relative to calcite mineralization (Budai et al., 2002; Solano-Acosta et al., 2008). Cleat filling minerals reduce the permeability ofcoal for gas migration, which may inhibit gas extraction; thereforeoutburst of CO2 and CH4 could be expected during excavation ofcoal at locations where cleat fillings are more pronounced.

Calcified xylite and carbonate-rich lenses in the Velenje Basin areenriched in 13C, in addition to CO2 gas from the Preloge and Škalemines (Fig. 2), which suggests that methanogens partially depletedthe individual CO2 pools before or during the time of calcite forma-tion. δ13C values from the Velenje coalmine, ranging from 8.6 to16.7‰, are comparable with calcite in coal cleats from the Black War-rior Basin, Alabama, which was interpreted to have been precipitatedin the presence of microbial methanogenesis via CO2 reduction (Pitmanet al., 2003). In contrast, calcite data from the Illinois Basin coals are dis-tinctly different from the BlackWarrior andVelenje basins, and showno


isotopic evidence for microbial methanogenesis via CO2 reduction(Fig. 5; Ambers, 1993; Solano-Acosta et al., 2008). The age of calcifiedxylite and carbonate-rich lenses was not determined as part of this

Fig. 4. A Mapping of working faces in the Preloge coalmine. B Calcified xylite in the Prelogwood tissue. Dark bends are lignin rich cell walls (Markič, 2009). C SEM microscopy of carwhich prevail, represent pure calcite, the darker grains belong to alumosilicates (albite) awith corresponding spectrum represents pure calcite.

study;microbial methanogenesismight have been active during forma-tion of the basin or after it, if there was continuous groundwater circu-lation in the basin.

e mine on different scales; calcite crystallization within “channels” (tracheides) of thebonate-rich lenses in fine detrital lignite matrix from the Preloge mine; lighter grains,nd the dark matrix belongs to organic matter (fine detrital lignite). The marked grain

C

Fig. 4 (continued).


The oxygen isotopic fractionation process (Δ18O=δ18Omineral

−δ18Owater (‰)) during calcite crystallization at thermodynamicequilibrium depends primarily on temperature and has been used ex-tensively for isotopic paleothermometry (Valley and Cole, 2011).Pressure is not an important factor, as exemplified by Clayton et al.(1975), who studied the fractionation factors of the calcite–water sys-temwithin a range of 100 to 2000 MPa. The following equation repre-sents the relationship between oxygen isotopic fractionation of waterand minerals as a function of temperature, according to equilibriumcalibrations for calcite (Kim and O'Neil, 1997):

103 ln αðcalcite–waterÞ ¼ δ18Ocalcite−δ18Owater ¼ 18:03 � 103T−1−32:42

ð8Þ

where α is the isotopic fractionation factor (Criss, 1999) and T is thetemperature in K.

Different scenarios were proposed by Solano-Acosta et al. (2008)for Pennsylvanian coals in the Illinois Basin, such as low temperatureand δ18Owater~−6.2‰, intermediate temperature and δ18Owater~−1.25‰, intermediate temperature and δ18Owater~+3‰, high tem-perature and δ18Owater~+7.5‰. In our study, the most probable

18

19

20

21

22

23

24

25

26

27

-20 -10 0 10 20 30

δ18O

VS

MO

W (‰

)

δ13CVPDB (‰)

Solano-Acosta etal., 2008

Ambers, 1993 Pitman et al.,2003

This study

Illinois Basin, Indiana

Black Warrior Basin, Alabama

Velenje Basin, Slovenia

Fig. 5. δ18Ocalcite (VSMOW) versus δ13Ccalcite (VPDB) values from calcified xylite andcarbonate-rich lenses from the Preloge mine. Data from the Velenje Basin are distinctlydifferent from the Illinois Basin, Indiana/Illinois but comparable to the Black WarriorBasin, Alabama, where calcite reflects 13C enrichment of residual carbon dioxide as aresult of preferential utilization of 13CO2 during microbial methanogenesis via CO2

reduction.

scenario for the Velenje Basin (low temperature and δ18Owater~−6.2‰) was taken into account.

According to Eq. (8) the average temperature of carbonate precip-itation for calcified xylite is estimated to be 17.3 °C and between 32.5and 34.7 °C for carbonate-rich lenses. The data indicate that methano-gens are functional over the temperature range of 22–65 °C in situ(Gorden and Fliermans, 1980) and it was found recently that metha-nogens are active even at higher temperatures up to 80 to 100 °C(Schlegel et al., 2011), suggesting that methanogenesis in the VelenjeBasin was limited to locations where T conditions were more than22 °C, and hydrogeochemical conditions (e.g. availability of bio-degradable organic matter, lack of sulfate, and anoxic conditions)were suitable.

5.4. Carbon sources and controls on δ13CDIC of basin groundwaters

Groundwater in the Triassic aquifer, beneath the lignite in theVelenje Basin, is enriched in Ca2+, Mg2+ and HCO3

−, and hasCa2++Mg2+ versus alkalinity concentrations of approximately1:2, similar to surface waters in Slovenia (Kanduč et al., 2007a).This suggests that weathering of carbonates is the primary contrib-utor to major ion chemistry of groundwater in the Triassic aquifer(Kanduč et al., 2007a). Furthermore, the Mg2+/Ca2+ ratio ofgroundwater in the Triassic aquifer is higher than 0.5, indicatingthat dolomite weathering dominates over calcite (Fig. 6B). In con-trast, groundwater in the Pliocene and Lithotamnium aquifers devi-ates from the 1:2 Ca2++Mg2+ versus alkalinity line (Fig. 6A),indicating that weathering of carbonates contributes minimally to thechemistry of these Mg2+–Na+-rich waters.

From Fig. 6B it can be seen that groundwaters belonging to theTriassic aquifer have similar chemical composition to surface waters,while groundwaters belonging to the Pliocene aquifer have higherK+, Na+ and Si concentrations probably from weathering of feld-spars in sand, marl and mud sediments within the Pliocene aquifer.According to the X-ray diffraction results, clays and sands in thePliocene aquifer are composed of the minerals quartz (SiO2), cal-cite (CaCO3), muscovite (KAl(Si3Al)O10(OH,F)), montmorillonite(CaO2(Al,Mg)2Si4O10), goethite (FeOOH), dolomite (CaMg(CO3)2),albite (NaAlSi3O8), kaolinite (Al2Si2O5(OH)4) and pyrite (FeS2)(Ranzinger, 2003).

Controls on the carbon isotope value of dissolved inorganic carbon(DIC) in groundwater include the degree to which carbonic acidweathering takes place under open or closed system conditions,whether the parent material is silicate or carbonate, and subsequentmicrobial activity. The sources of carbon dissolved in groundwaterare soil CO2, geogenic/magmatic CO2 (from deep crustal or mantlesources), living and dead organic matter in soils and rocks, methane,

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

Ca2+

+Mg

2+ (

mM

)

Alkalinity (meq/l)

Pliocene aquifers

Lithotamniumaquifers

Triassic aquifers

2 HCO3-= Ca2++Mg2+A

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Mg

2+ (m

M)

Ca2+ (mM)

Mg2+/Ca2+=1

Mg2+/Ca2+=0.75

Mg2+/Ca2+=0.5

Mg2+/Ca2+=0.33

Mg2+/Ca2+<0.1

Calcite only

Dolomite only

B

Fig. 6. A) Geochemical parameters: Ca2++Mg2+ versus alkalinity concentrations withthe 1:2 line indicating weathering of carbonates in Triassic aquifers, while samplesfrom the Pliocene and Lithotamnium aquifers plot below the 1:2 line, indicating differ-ent sources of solutes, B) Mg2+ versus Ca2+ ratios indicating the dominance of dolo-mite water–rock interactions in the Triassic aquifers.


and carbonate minerals (Clark and Fritz, 1997). Each of these sourceshas a different carbon isotopic composition and contributes to thetotal DIC in various proportions. Fig. 7 indicates the processesinfluencing the δ13CDIC value in groundwaters. An average δ13C of

-21.0

-19.0

-17.0

-15.0

-13.0

-11.0

-9.0

-7.0

-5.0

-3.0

-1.0

1.0

3.00 10 20 30

δ13C

DIC

(‰

)

Alkalin

nonequilibrium cfrom soil zone w

open system eqdegradation of o

microbial CO

1

2

Fig. 7. δ13CDIC values versus alkalinity with lines indicating likely processes occurring in thetion of DIC with soil CO2 originating from degradation of organic matter with δ13C=−26.6‰soil zone CO2 with a δ13CCO2 of −26.6‰. Higher δ13CDIC values of groundwater from the Pl

−26.6‰ was assumed to calculate the isotopic composition of DICderived from in-stream respiration. Open system equilibration ofDIC with CO2 enriches DIC in 13C by about 9‰ (Mook et al., 1974),thus yielding the estimate of −16.6‰ shown in Fig. 7 (line 1). Thisopen system equilibration assumes an unlimited reservoir of CO2

that imprints its isotopic composition and associated isotope shiftson the DIC species present: −1.1‰, +9‰ and 11‰ for H2CO3,HCO3

− and CO32−, respectively, at an average water temperature of

12 °C (Mook et al., 1974).Measured δ13C values of Triassic carbonate rocks in the Velenje

Basin, beneath the Pliocene coal-bearing formations, were around−3‰ (Kanduč and Pezdič, 2005). Nonequilibrium dissolution of car-bonates with one part of DIC originating from soil CO2 (−26.6‰), andthe other from carbonates, produces an intermediate δ13CDIC value of−11.8‰ in groundwater (line 2 on Fig. 7). Groundwaters in the Trias-sic aquifer have similar δ13CDIC values as surface waters in Sloveniaand fall around the line of nonequilibrium carbonate dissolution in-duced by carbonic acid produced from the soil zone in temperate for-ested climate zones (Kanduč et al., 2007a,b, 2010). δ13C of soil CO2 issimilar to the average value of lignite (−26.8‰). Groundwaters inthe Pliocene and Lithotamnium aquifers have higher δ13CDIC values(−9.1 to 0.2‰) than the Triassic aquifer (−3.2 to −17.4‰), whichcould be attributed to microbial methanogenesis, causing an enrich-ment in 13C. The high alkalinity values of groundwater in the Plioceneand Lithotamnium aquifers are similar to groundwater in the ElkValley coal field, the Powder River Basin coalbeds, and the MichiganBasin Antrim Shale associated with microbial methane. However, theδ13CDIC values of Velenje Basin groundwater are lower (less than+0.2‰, compared to −9.9 to +32.6‰ in the Elk Valley coalfield,Powder River Basin coals, and Antrim Shale; Aravena et al., 2003;Bates et al., 2011; Martini et al., 1998), which is consistent with thelower δ13CCO2 values observed. As suggested previously, a relativelylarge input of CO2 with a constant δ13CCO2 value, and slow conversionof CO2 to CH4 may keep δ13C values of CO2 and DIC low. Alternative-ly, bacterial oxidation of methane could explain the low δ13C valuesof CO2 and DIC observed in the Pliocene and Lithotamnium aquiferscompared to other organic-rich reservoirs, and the relatively positiveδ13C values of CH4 observed in select samples (N−50‰). Bacterialoxidation of methane produces a positive shift in δ13CCH4 and corre-sponding 13C depletion in CO2 and DIC (Barker and Fritz, 1981).

40 50 60 70

ity (meq/l)

Plioceneaquifers

Lithotamniumaquifers

Triassic aquifers

arbonate dissolution by carbonic acid produced ith a δ13CCO2

of -26.6 ‰

uilibration of DIC with soil CO2 originating from rganic matter with δ13CCO2

of -26.6‰

2 reduction

Velenje Basin aquifers. These include values calculated for: (1) open system equilibra-and (2) nonequilibrium carbonate dissolution (−3‰) by carbonic acid produced from

iocene and Lithotamnium aquifers could be attributed to microbial methanogenesis.


6. Conclusions

• The concentration and isotope values of various organic and inorgan-ic carbon substrates (lignite, coal gases, calcified xylites, carbonate-rich sediments, and groundwater) were investigated to constrainmajor biogeochemical processes, including methanogenesis, occur-ring in the coal-bearing Velenje Basin.

• Different lithotypes of lignite have δ13C values from −28.5 to−23.0‰ and δ15N values from 1.8 to 4.6‰ and indicate that bothoriginal floristic ingredients as well as biogeochemical processes(gelification and degradofuzunitisation/mineralization) were ofgreat importance in the early diagenesis of the Velenje lignite andgas formation.

• δ13C values of CO2 and CH4 indicate that the majority of methanewas generated via microbial methanogenesis. Carbon isotope frac-tionation factors (αCH4–CO2 from 1.027 to 1.075) suggest both ace-tate fermentation and CO2 reduction as important methanogenicpathways. Select samples with higher δ13CCH4 values could haveresulted from significant depletion of the CO2 pool by methanogens,addition of thermogenic methane that migrated from deeper stratain the basin, and/or oxidation of methane during migration.

• δ13C and δ18O values of calcified xylite and carbonate-rich sedimentsfrom the Velenje Basin constrain paleotemperatures to 32.5–34.7 °C,and 17.3 °C, respectively) and reveal relatively low temperatureconditions during mineralization. Elevated δ13C values from 8.6 to16.7‰ indicate that calcite precipitation was driven by microbialmethanogenesis via CO2 reduction, consistent with observationsfrom coals in the Black Warrior Basin in Alabama. In addition, thealkaline, Ca-rich, anoxic, moderate temperature conditions duringformation of the lignite basin likely promoted microbial activity.

• Major ion chemistry of groundwaters in the Velenje Basin indicatesthree distinct aquifers. The concentration of solutes decreasesaccording to the sequence HCO3

−NMg2+NNa+NCa2+ in the Plio-cene aquifer, Na+NHCO3

−NMg2+ in the Lithotamnium aquifer andHCO3

−NCa2+NMg2+ in the Triassic aquifer. Alkalinity in the Plio-cene aquifer reached up to 62.0 mM, in the Lithotamnium aquifer39.9 mM and in the Triassic aquifer 10.6 mM. Higher δ13CDIC values(up to 0.2‰) in the Pliocene and Lithotamnium aquifers could beattributed to microbial methanogenesis via CO2 reduction. δ13CCH4values range from −70.2 to −44.1‰ in the Pliocene and Triassicaquifers and indicate the presence of microbial methane. Theδ13CCH4 values found in dewatering objects indicate microbialmethane (δ13CCH4 values below −50‰), in addition to methaneinfluenced by a mixture of thermogenic methane, and/or bacterialoxidation of methane (δ13CCH4 values above −50‰). Highly alka-line waters (alkalinity up to 62.0 mM), high pCO2 and a SIcalciteindex above 0 (indicating supersaturation) in the Pliocene aquifersprovide further evidence of microbial methanogenesis. Althoughthe analysis of formation waters in coal seams in the Velenje Basinwas not possible (dry coalbed seam due to dewatering of waterrecharging the basin), we were still able to find evidence of micro-bial methane by tracing the overall carbon cycle of materials withinand adjacent to the coal seams.

• Methane and CO2 are potent greenhouse gases and represent po-tential risk for hazardous outbursts in Velenje Basin coalmine.Therefore monitoring of methane and CO2 concentrations in rela-tion to structure at working faces is essential to predict and preventsuch events. Current emissions of greenhouse gases, especiallymethane, in the Velenje Basin are related to lignite exploitation;this methane could be considered as an additional source of energyin the future to reduce fugitive emissions from lignite.

Acknowledgements

This study was conducted in the framework of project Z1-2052funded by the Slovenian Research Agency (ARRS) and the Velenje

Coalmine. The authors are also grateful to Mr. Marko Ranzinger, Mr.Igor Medved, Mr. Robert Lah and Mr. Stojan Žigon for technical sup-port, assistance in the field sampling and laboratory analyses.

References

Ambers, C.P., 1993. The nature and origin of very well crystallized kaolinite. IndianaUniversity, Bloomington, IN. Ph.D. dissertation, 493pp.

Aravena, R., Harrison, S.M., Barker, J.F., Abercrombie, H., Rudolph, D., 2003. Origin ofmethane in the Elk Valley coalfield, southeastern British Columbia, Canada. Chem-ical Geology 195, 219–227.

Arens, N.C., Jahren, A.H., Amundson, R., 2000. Can C3 plants faithfully record the carbonisotopic composition of atmospheric carbon dioxide? Paleobiology 261, 137–164.

Atkins, P.W., 1994. Physical Chemistry, fifth ed. Oxford Univ. Press, Oxford.Barker, J.F., Fritz, P., 1981. Carbon isotope fractionation during microbial methane oxi-

dation. Nature 293, 289–291.Bates, B.L., McIntosh, J.C., Lohse, K.A., Brooks, P.D., 2011. Influence of groundwater flow-

paths, residence times and nutrients on the extent of microbial methanogenesis incoal beds: Powder River Basin, USA. Chemical Geology 284, 45–61.

Bechtel, A., Sachenhofer, R.F., Markič, M., Gratzer, R., Lücke, A., Püttman, W., 2003.Paleoenvironmental implications from biomarker and stable isotope investigationson the Pliocene Velenje lignite seam (Slovenia). Organic Geochemistry 34,1277–1298.

Bechtel, A., Gratzer, R., Sachsenhofer, R.F., Gusterhuber, J., Lücke, A., Püttmann, W.,2008. Biomarker and carbon isotope variation in coal and fossil wood of CentralEurope through the Cenozoic. Palaeogeography, Palaeoclimatology, Palaeoecology262, 166–175.

Berner, R.A., 1996. Biological aspects of the long term carbon cycle. Bulletin de InstitutOceanographique Monaco 14 (4), 11–22.

Berner, R.A., 2003. The long-term carbon cycle, fossil fuels and atmospheric composi-tion. Nature 426, 323–326.

Brezigar, A., 1986. Coal seam of the Velenje coalmine. Geologija 28, 319–336 (inSlovene).

Brezigar, A., Ogorelec, B., Rijavec, L., Mioč, P., 1987. Geologic setting of the Pre-Pliocenebasement of the Velenje depression and its surroundings. Geologija 30, 31–65 (inSlovene).

Budai, J.M., Martini, A.M., Walter, L.M., Ku, T.C.W., 2002. Fracture-fill calcite as a recordof microbial methanogenesis and fluid migration: a case study from the DevonianAntrim Shale, Michigan Basin. Geofluids 2, 163–183.

Candy, I., 2009. Terrestrial and freshwater carbonates interglacial deposits, UK: micro-morphology, stable isotope composition and palaeoenvironmental significance.Proceedings of the Geologists' Association, 120, 1, pp. 49–57.

Chang, S.B., Berner, R.A., 1999. Coal weathering and the geochemical carbon cycle. Geo-chimica et Cosmochimica Acta 63 (19/20), 3301–3310.

Clark, I., Fritz, P., 1997. Environmental Isotopes in Hydrology. Lewis Publishers, NewYork.

Clayton, J.L., 1998. Geochemistry of coalbed gas—a review. International Journal of CoalGeology 35, 159–173.

Clayton, R.N., Goldsmith, J.R., Karel, K.J., Mayeda, T.K., Newton, R.C., 1975. Limits on theeffect of pressure on isotopic fractionation. Geochimica et Cosmochimica Acta 39,1197–1201.

Conrad, R., 2005. Quantification of methanogenic pathways using stable carbon isoto-pic signatures: a review and a proposal. Organic Geochemistry 36, 739–752.

Coplen, T.B., 1996. New guidelines for reporting stable hydrogen, carbon and oxygenisotopes ratio data. Geochimica et Cosmochimica Acta 60, 390–3360.

Criss, R.E., 1999. Principles of Stable Isotope Distribution. Oxford University Press, NewYork.

Daniels, E.J., Marshak, S., Altaner, S.P., 1996. Use of clay-mineral alteration patterns todefine syntectonic permeability of joints (cleat) in Pennsyvanian anthracite coal.Tectonophysics 263, 123–136.

Diessel, C.F.K., 1992. Coal-Bearing Depositional Systems. Springer-Verlag, Berlin.Dupouey, J.L., Leavitt, S., Choisnel, E., Jourdain, S., 1993. Modeling carbon isotope frac-

tionation in tree rings based on effective evapotranspiration and soil water status.Plant, Cell & Environment 16, 939–947.

Epstein, S., Mayeda, T., 1953. Variations of 18O contents of water from natural sources.Geochimica et Cosmochimica Acta 4, 213–224.

Flores, R.M., 2004. Chapter 2-coalbed methane in the Powder River Basin, Wyomingand Montana: an assessment of the Tertiary–Upper Cratecous coalbed methanetotal petroleum system. Total Petroleum System and Assessment of Coalbed Gasin the Powder River Basin Province, Wyoming and Montana, U.S. Geol. Surv. DigitalData Series DDS-69-C.

Flores, R.M., Rice, C.A., Stricker, G.D., Warden, A., Ellis, M.S., 2008. Methanogenic path-ways of coalbed gas in the Powder River Basin, United States: the geologic factor.International Journal of Coal Geology 76, 52–75.

Fodor, L., Jelen, B., Martan, E., Skaberne, D., Car, J., Vrabec, M., 1998. Miocene–Pliocenetectonic evolution of the Slovenian Periadriatic Fault: implications for Alpine–Carphatian extrussion models. Tectonics 17, 690–709.

Gieskes, J.M., 1974. The alkalinity total carbon dioxide system in seawater. In:Goldberg, E.D. (Ed.), Marine Chemistry of the Sea. John Wiley and Sons,New York, pp. 123–151.

Gorden, R.W., Fliermans, C.B., 1980. Methanogenesis in thermal reactor effluents.Journal of Thermal Biology 5, 169–177.

Hamrla, M., 1952. Poročilo o petrografski preiskavi velenjskega lignita. Poročilo, arhivGeoZS (C-II-30d/a3-10/5). 24 pp. (in Slovene).

Hoefs, J., 2009. Stable Isotope Geochemistry, sixth ed. Springer-Verlag, Berlin.


Jahren, A.H., LePage, B.A., Werts, S.P., 2004. Methanogenesis in Eocene Arctic soils in-ferred from δ13C of tree fossil carbonates. Palaeogeography, Palaeoclimatology,Palaeoecology 214, 347–358.

Jamnikar, S., Fijavž, O., 2006. Hydrogeological report for year 2006. PremogovnikVelenje d.d, p. 21 (in Slovene).

Kanduč, T., Pezdič, J., 2005. Origin and distribution of coalbed gases from the VelenjeBasin, Slovenia. Geochemical Journal 39, 397–409.

Kanduč, T., Markič, M., Pezdič, J., 2005. Stable isotope geochemistry of different litho-types of the Velenje lignite (Slovenia). Geologija 48, 83–92.

Kanduč, T., Szramek, K., Ogrinc, N., Walter, L.M., 2007a. Origin and cycling of riverineinorganic carbon in the Sava River watershed (Slovenia) inferred from major sol-utes and stable carbon isotopes. Biogeochemistry 86, 137–154.

Kanduč, T., Ogrinc, N., Mrak, T., 2007b. Characteristics of suspended matter in the riverSava watershed, Slovenia. Isotopes in Environmental and Health Studies 43,369–385.

Kanduč, T., Jamnikar, S., McIntosh, J., 2010. Geochemical characteristics of surface andgroundwaters in the Velenje Basin, Slovenia. Geologija 53 (1), 37–46.

Kim, S.T., O'Neil, J.R., 1997. Equilibrium and nonequilibrium oxygen isotope effects insynthetic carbonates. Geochimica et Cosmochimica Acta 61, 3461–3475.

Kotarba, M., 1990. Isotopic geochemistry and habitat of the natural gases from theUpper Carboniferous Zacler coal-bearing formation in the Nowa Ruda coal district(Lower Silesia, Poland). Organic Geochemistry 16, 549–560.

Kotarba, M.J., 2001. Composition and origin of coalbed gases in the Upper Silesian andLublin basins, Poland. Organic Geochemistry 32, 163–180.

Kotelnikova, S., 2002. Microbial production and oxidation of methane in deep subsur-face. Earth-Science Reviews 58, 367–395.

Krantz, D.E., Williams, D.F., Jones, D.S., 1987. Ecological and paleoenvironmental infor-mation using stable isotope profiles from living and fossil mollusks. Palaeogeogra-phy, Palaeoclimatology, Palaeoecology 58, 249–266.

Lansdown, J.M., Quay, P.D., King, S.L., 1992. CH4 production via CO2 reduction in a tem-perate bog: a source of 13C-depleted CH4. Geochimica et Cosmochimica Acta 56,3493–3503.

Lücke, A., Helle, G., Schleser, G.H., Figueiral, I., Mosbrugger, V., Jones, T.P., Rowe, N.P.,1999. Environmental history of the German Lower Rhine Embayment during theMiddle Miocene as reflected by carbon isotopes in brown coals. Palaeogeography,Palaeoclimatology, Palaeoecology 154, 339–352.

Markič, M., 2009. Petrology and genesis of the Velenje lignite. Ph.D. Dissertation.Ljubljana, 208 pp.

Markič, M., Sachsenhofer, R.F., 1997. Petrographic composition and depositional envi-ronments of the Pliocene Velenje lignite seam (Slovenia). International Journal ofCoal Geology 33, 229–254.

Markič, M., Sachsenhofer, R.F., 2010. The Velenje lignite—its petrology and genesis.Geological Survey of Slovenia. 218 pp.

Martini, A.M., Budai, J.M., Walter, L.M., Schoell, M., 1996. Microbial generation of eco-nomic accumulations of methane within a shallow organic-rich shale. Nature383, 155–157.

Martini, A.M., Walter, L.M., Budai, J.M., Ku, T.C.W., Kaiser, C.J., Schoell, M., 1998. Geneticand temporal relations between formation waters and biogenic methane: UpperDevonian Antrim Shale, Michigan Basin, USA. Geochimica et Cosmochimica Acta62 (10), 1699–1720.

Martini, A.M., Walter, L.M., Ku, T.C.W., Budai, J.M., McIntosh, J.C., Schoell, M., 2003. Mi-crobial production and modification of gases in sedimentary basins: a geochemicalcase study from a Devonian shale gas play, Michigan Basin. AAPG Bulletin 87 (8),1355–1375.

McCrea, J.M., 1950. On the isotopic chemistry of carbonates and a paleotemperaturescale. Journal of Chemical Physics 18, 849.

Miyajima, T., Yamada, Y., Hanba, Y.T., 1995. Determining the stable isotope ratio of totaldissolved inorganic carbon in lake water by GC/C/IRMS. Limnology and Oceanogra-phy 40, 994–1000.

Mook, W.G., Bommerson, J.C., Staverman, W.H., 1974. Carbon isotope fractionation be-tween dissolved bicarbonate and gaseous carbon dioxide. Earth Planetary ScienceLetters 22, 169–176.

O'Lary, M.H., 1988. Carbon isotopes in photosynthesis. Bioscience 38, 328–336.O′Neil, J.R., 1979. Stable isotope geochemistry of rocks and minerals. In: Jager, E.,

Hunzinger, J.C. (Eds.), Lectures in Isotope Geology. Springer-Verlag, Berlin,pp. 235–263.

Parkhurst, D.L., Appelo, C.A.J., 1999. User's guide to PHREEQC for Windows (ver-sion 2)—a computer program for speciation, batch-reaction, one-dimensionaltransport, and inverse geochemical calculations. Water-Resources Investiga-tions Report, pp. 99–4259.

Pitman, J.K., Pashi, J.C., Hatch, J.R., Goldhaber, M.B., 2003. Origin of minerals in jointand cleat systems of the Pottville Formation, Black Warrior Basin, Alabama: impli-cations for coalbed methane generation and production. American Association ofPetroleum Geologists Bulletin 87, 713–731.

Ranzinger, M., 2003. X-ray diffraction of clays in the Velenje coalmine. 16 pp. (inSlovene), project work.

Rice, D.D., Clayton, J.L., Pawlewicz, M.J., 1989. Characterization of coal-derived hydro-carbons and source-rock potential of coal beds, San Juan Basin, New Mexico andColorado, U.S.A. International Journal of Coal Geology 13, 597–626.

Rice, R.M., Flores, G.D., Stricker, G.D., Ellis, M.S., 2008. Chemical and stable evidence forwater/rock interaction and biogenic origin of coalbed methane, for Union forma-tion, Powder River, Wyoming and Montana, U.S.A. International Journal of CoalGeology 76, 76–85.

Schlegel, M.E., McIntosh, J.C., Bates, B.L., Kirk, M.F., Martini, A.M., 2011. Comparisonof fluid geochemistry and microbiology of multiple organic-rich reservoirs inthe Illinois Basin, USA: evidence of controls on methanogenesis and microbialtransport. Geochimica et Cosmochimica Acta 75, 1903–1919.

Schleser, G.H., 1995. Parameters determining carbon isotope ratios in plants. In:Frenzel (Ed.), Problems of Stable Isotopes in Tree-Rings, Lake Sediments andPeat-Bogs as Climatic evidence for the Holocene.-ESF Spec Iss., 15, pp. 71–96.

Schoell, M., 1980. The hydrogen and carbon isotopic composition of methane from nat-ural gases of various origins. Geochimica et Cosmochimica Acta 44, 649–661.

Schoell, M., 1983. Genetic characterization of natural gases. American AssociationPetroleum Geological Bulletin 67, 2225–2238.

Scholes, R.J., Monteiro, P.M., Sabine, C.L., Canadell, J.G., 2009. Systematic long-term ob-servations of the global carbon cycle. Trends in Ecology & Evolution 24, 427–430.

Scott, A.R., 1993. Composition and origin of coalbed gases from selected Basins in theUnited States. Proc. of Internat. Methane Symp, pp. 207–222.

Sherwood Lollar, B., Lacrampe-Couloume, G., Slater, G.F., Ward, J., Moser, D.P., Gihring,T.M., Lin, L.-H., Onstott, T.C., 2006. Unravelling abiogenic and biogenic sources ofmethane in the Earth's deep subsurface. Chemical Geology 226, 328–339.

Smith, J.W., Pallaser, R., 1996. Microbial origin of Australian coalbed methane.American Association of Petroleum Geologists 80, 891–897.

Solano-Acosta, W., Schimmelmann, A., Mastalerz, M., Arango, I., 2008. Diageneticmineralization in Pennsylvanian coals from Indiana, USA: 13C/12C and 18O/16Oimplications from cleat origin and coalbed methane generation. InternationalJournal of Coal Geology 73, 219–236.

Spötl, C., 2005. A robust and fast method of sampling and analysis of δ13C of dissolvedinorganic carbon in ground waters. Isotopes in Environmental and Health Studies41, 217–221.

Stach, E., Mackowsky, M.Th., Teichmüller, M., Taylor, G.H., Chandra, D., Teichmüller, R.,1982. Stach's Textbook of Coal Petrology, Gebruder Borntraeger. 535 pp.

Taylor, G.H., Teichmüller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. OrganicPetrology. Gebrüder Borntraeger, Berlin.

Valley, J.W., Cole, R.D. (Eds.), 2011. Stable isotope geochemistry. In: Reviews inMineralogy and Geochemistry, vol. 43. The Mineralogical Society of America,Washington, D.C.

Waldron, P.J., Petsch, S.T., Martini, A.M., Nüsslein, K., 2007. Salinity constraints on sub-surface archaeal diversity and methanogenesis in sedimentary rock rich in organicmatter. Applied and Environmental Microbiology 73, 4171–4179.

Warren, E., Bekins, B.A., Godsy, E.M., Smith, V.K., 2004. Inhibition of acetoclastic metha-nogenesis in crude oil and creosote-contaminated groundwater. BioremediationJournal 8, 1–11.

Whiticar, M.J., 1999. Carbon and hydrogen isotope systematics of bacterial formationand oxidation of methane. Chemical Geology 161, 291–314.

Whiticar, M.J., Faber, E., Schoell, M., 1986. Biogenic methane formation in marine andfresh water environment, CO2 reduction vs. acetate fermentation—isotopic evi-dence. Geochimica et Cosmochimica Acta 50, 693–709.

Zhu, Y., Shi, B., Fang, C., 2000. The isotopic composition of molecular nitrogen: implica-tions on their origins in natural gas accumulations. Chemical Geology 164,321–330.

carbon cycling in the pliocene velenje coal basin, slovenia, inferred from stable carbon isotopes

Documents