International Journal of Coal Geology 76 (2008) 86–97

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International Journal of Coal Geology

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Biodegradation of sedimentary organic matter associated with coalbed methane inthe Powder River and San Juan Basins, U.S.A.

Michael Formolo a,⁎, Anna Martini b, Steven Petsch a

a Department of Geosciences, University of Massachusetts, Amherst, MA, 01003-9297, USAb Department of Geology, Amherst College, Amherst, MA, 01002, USA

⁎ Corresponding author. Current address: DepartmePlanck-Institute for Marine Microbiology, Celsiusstr. 1, D+49 421 2028 655.

E-mail address: mformolo@mpi-bremen.de (M. Form1 http://www.eia.doe.gov/.

0166-5162/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.coal.2008.03.005

A B S T R A C T

A R T I C L E I N F O

Article history:

The Powder River Basin and Received 1 August 2007Received in revised form 29 February 2008Accepted 12 March 2008Available online 29 March 2008

Keywords:San Juan BasinPowder River BasinBiodegradationSedimentary organic matterCoalbed methane

San Juan Basin, U.S.A., are two of the most productive coalbed methane reservesin the world. Of particular interest is the microbial biodegradation of coal beds associated with this naturalgas production. Biogenic methane production is indicated as a significant component to the total gasresources in the San Juan Basin, and as the nearly sole source for the shallow coals of the Powder River Basin.Molecular and isotopic signatures indicate a microbial origin for the gas. Geochemical characteristics offormation waters, such as elevated alkalinity and 13C-enriched dissolved inorganic carbon (DIC), furthersupport extensive microbial degradation of coal organic matter associated with methanogenesis. Extractableorganic matter isolated from coals in both basins point to patterns of hydrocarbon biodegradation in coalsrestricted to specific depths. To some extent, biodegradation patterns are similar to those observed inmethanogenic, biodegraded black shales of the mid-continent of the United States. Specifically, both coalsand shales exhibit near-quantitative removal of straight-chain and acyclic isoprenoid hydrocarbons.However, loss of aromatic hydrocarbons in the coals proceeds prior to the extensive removal of thesaturated hydrocarbons, in contrast to what is conventionally observed in biodegraded petroleum systems orin black shales. In addition, previous thermal maturation histories in both the Fruitland and Fort Unioncoalbed methane systems have little impact on more recent hydrocarbon biodegradation. Instead, localizedhydrologic conditions and subsurface geology likely play important roles in controlling the extents ofbiodegradation and methanogenesis. These results suggest that biodegradation of hydrocarbons coupledwith methanogenesis may develop regardless of organic matter source across a range of inherited thermalmaturities.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Economic production of natural gas from coal beds began in theUnited States in the 1930s, but in recent decades coalbed gas devel-opment has increased dramatically and now accounts for approxi-mately 9.3% of the annual production of dry gas in the United States(Ayers, 2002; Energy Information Agency, 20061). Two prolific coalbedgas reservoirs in the United States are the Cretaceous Fruitland For-mation of the San Juan Basin and the Paleocene Fort Union Formationof the Powder River Basin. Understanding the subsurface processesleading to the formation of coalbed methane is important for thepresent production and future exploration of these deposits. Twomechanisms form coalbed gas: biological processes (i.e. methano-genesis) or geologic processes (i.e. thermogenic gas production). In

nt of Biogeochemistry, Max--28359 Bremen, Germany. Tel.:

olo).

l rights reserved.

this study, we examine coalbed gas reserves that are biogenic ormixed biogenic/thermogenic in origin. Microbial methanogenesis isthe main source for gas in the Powder River Basin (Ayers, 1986;Gorody, 1999; Flores et al., 2008-this volume) and a significantaddition to the thermogenic gas found in the San Juan Basin (Rice etal., 1989; Clayton et al., 1991; Kaiser et al., 1991; Scott et al., 1991;Michael et al., 1993; Rice, 1993; Scott et al., 1994; Zhou et al., 2005).

The dominant coalbed methane producing formation in thePowder River Basin is the Paleocene Fort Union Formation. The FortUnion Formation consists of the Tullock, Lebo Shale, and TongueRiver Members (Fig. 1A). Lithologically the Tullock Member containsevidence of deposition within fluvial environments; the Member isdominated by sandstone, mudstone, shale, and coal (Flores andEtheridge, 1985; Flores et al., 2008-this volume). Overlying theTullock Member is the Lebo Shale Member, which contains abundantshale, mudstones, siltstones, and sandstones, while containing minoramounts of coal. The dominant coal-bearing Member is the TongueRiver Member. The Tongue River contains 32 coals seams that rangein thickness from ameter to tens of meters (Ayers, 1986; Montgomery,1999). High-resolution lithological and stratigraphic information for

Fig. 1. Generalized stratigraphic column for the Powder River Basin (A) and San Juan Basin (B).

87M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

the Powder River Basin is available in Ayers and Kaiser (1994), Tyleret al. (1995), and Stricker et al. (2007). Recent data from closelyspaced CBM wells show Tongue River coal beds to be very variablelaterally from a few meters thick to more than 60-m thick withinseveral kilometers (Flores, 2004).

The principal methane producing coal formations in the San JuanBasin are contained in the Upper Cretaceous Fruitland Formation (Fig.1B). The Fruitland Formation consists of interbedded coal, sandstone,and shales, with some coalbeds reaching 30 m in thickness (Fassett,1988; Ayers and Kaiser, 1994; Pashin, 1998; Craigg, 2001, Brister andHoffman, 2002; Lucas et al., 2006). The coal-bearing FruitlandFormation formed in coastal plain settings associated with the LateCretaceousWestern Interior seaway (Fassett and Hinds, 1971; Ayers etal., 1991). Ayers and Ambrose (1990), Ayers et al. (1991), RobinsonRoberts and McCabe (1992), and Pashin (1998) provide high-resolution stratigraphic cross-sections of the San Juan Basin and theFruitland Formation.

In both Powder River and San Juan Basins, stimulation of biogenicgas generation has been linked to incursion of meteoric water (Kaiseret al.,1991; Scott et al.,1991; Rice,1993; Scott et al.,1994;Montgomery,1999; Zhou et al., 2005). Regional hydrology may be an importantmechanism stimulating biogenic gas production in the San Juan andPowder Rivers Basins (Flores et al., 2008-this volume), similar to theshale-gas reserves in the Michigan Basin (Martini et al., 1996, 1998;McIntosh et al., 2004). While geochemical and isotopic indicatorsestablish that gas in these basins contains microbial methane (Rice etal., 1989; Rice, 1993; Scott et al., 1994; Gorody, 1999), little isunderstood regarding consumption of coal organic matter leading tomethanogenesis or the signatures of hydrocarbon biodegradationassociated with biological methane generation.

In this study, we examine solvent-extractable organic matterisolated from samples of the Fruitland coal (San Juan Basin) and FortUnion coal (Powder River Basin) to determine the relative abundanceof saturated and aromatic hydrocarbons in these coals, and toevaluate the source, maturity and biodegradation of coals associatedwith coalbed methanogenesis in the Powder River and San JuanBasins. Calculation of biodegradation indices for hydrocarbon com-pound classes allows us to describe the molecular signatures ofhydrocarbon biodegradation in coalbed methane systems. By estab-lishing similar thermal histories and organic matter sources any

differences in our biodegradation ratios are the result of biodegrada-tion and not source or thermal influences.

2. Methods

Coal samples from the Powder River and San Juan Basin (Fig. 2)were collected from the USGS Core Repository in Denver, Colorado.Sample locations and USGS library reference numbers are listed inTable 1. The Fort Union Formation and the Fruitland Formation weretargeted in the Powder River and San Juan Basins, respectively, asthese are the two dominant coal coal-bearing formations associatedwith gas production. During sub-sampling of the cores, particularattention was given to trying to select the least weathered coals bysampling from the center of the core when possible. Samples werechosen to reflect a range of burial depths and thermal maturitiesacross each basin.

Total organic carbon (TOC) concentrations were determined usinga Costech ECS140 Elemental Analyzer. Bulk organic matter δ13Ccompositions were determined by online combustion using a CostechECS140 EA interfaced to a Thermo Delta 5 isotope-ratio mass spectro-meter. Analyses were run in triplicate and are reported relative to theVienna PDB in standard permil (‰) notationwith a precision of ±0.3‰.

Vitrinite reflectance (Ro) values in percent (%) were obtained onpolished, vacuum epoxy fixed, chips of coal from seven samplesrepresenting six different well locations. The surfacewas examined forvitrinite using a petrographic microscope and ten counts were madefor 12–19 sites per chip and values are reported as averages ±s.d. (1σ).Though the maturity rank of the coals are low, vitrinite reflectancedata can provide adequate determination of maturity (Canónico et al.,2004; Sýkorová et al., 2005.

To isolate extractable organic matter, 6 to 10 g of crushed FortUnion and Fruitland coals were solvent extracted in 3:1 dichloro-methane (DCM): methanol (MeOH) for 72 h in a Soxhlet apparatus.Total extractable organic matter (EOM) was determined by gravi-metric measurements following solvent extraction. The extracts wereseparated on silica gel into aliphatic (F1), aromatic (F2), and polarfractions (F3), by the successive elution with hexane, hexane:dichloromethane (1:1), and dichloromethane:methanol (1:1), respec-tively (Brocks et al., 2005). Fractions were dried under a purified N2

stream at room temperature.

Fig. 2. Map of the sample locations. Numbers refer to depth (m) of sample. (A) The Powder River Basin; (B) The San Juan Basin.

88 M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

Fractions F1 and F2 were redissolved in hexane and analyzed ona HP 6890 gas chromatograph (GC) interfaced to a HP 5973 mass-selective detector, using helium as a carrier gas. The GC wasequipped with a split/splitless injector and a 30 m Hewlett-PackardHP-5MS fused silica capillary column (0.25 mm i.d., 0.25 μm filmthickness). For F1, the GC temperature program was held isothermalat 60 °C for 1.5 min followed by an increase of 20 °C/min until 130 °Cand then a 2 °C/min increase to 300 °C, then held isothermal for10.0min. For F2, the GC temperature programwas initially set at 60 °Cfor 2.0 min and followed by an increase of 20 °C/min until 150 °C andthen followed by a 2 °C/min increase until 320 °C, then held

Table 1Sample depth, organic geochemistry, source indicators, and δ13C(‰)V-PDB of the Fort Union

Formation Latitude (°N)/Longitude (°W) USGS reference # Depth (m) TOCa wt.% EO

San Juan BasinFruitland 36°20′18.24″/107°38′17.16″ S057 132 33.94Fruitland 36°50′34.80″/107°43′55.20″ D395 750 63.03Fruitland 36°50′34.80″/107°43′55.20″ D395 760 60.15Fruitland 36°40′54.48″/107°12′23.40″ C590 1190 70.86

Average 57.00

Powder River BasinFort Union 45°30′36.72″/106°52′39.72″ S939 49 57.49Fort Union 45°30′36.72″/106°52′39.72″ S939 146 59.22Fort Union 44°60′15.48″/105°33′20.88″ S739 223 58.08Fort Union 44°60′15.48″/105°33′20.88″ S739 224 63.36Fort Union 43°51′6.12″/105°29′43.80″ S352 266 57.84Fort Union 44°45′41.04″/106°5′18.96″ S794 429 59.06

Average 59.18

a TOC=total organic carbon.b EOM=extractable organic matter.c CPI=2(C23+C25+C27+C29/[C22+2(C24+C26+C28)+C30].d OEPx=[C(x −2)+6Cx+C(x+ 2)]/[4C(X −1)+4C(x +1)].e Pr/Ph=pristane/phytane.

isothermal for 10 min. For both analyses, the mass-spectrometersource was operated at 250 °C in EI-mode at 70 eV ionization energy.Identification of compounds was based on interpretation of gaschromatograph-mass spectra and comparison with published massspectra and retention times (Figs. 3 and 4). Quantification ofindividual compounds was achieved using extracted ion chromato-grams and peak area integration, using m / z=57 (alkanes and acyclicisoprenoids), m / z=191 (hopanes), m / z=128, 156, 170, 184 (alkylatednaphthalenes), and m / z=178, 192, 206, 220, 234 (alkylated phenan-threnes) (Peters and Moldowan, 1993; Fisher et al., 1996, 1998;Ahmed et al. 1999; Ahmed and Smith, 2001; Peters et al., 2005).

and Fruitland coals

Mb (mg)/g TOC δ13C (‰)V-PDB CPIc OEP(17)d OEP(29)d Pr/Phe

101.3 −26.2 2.20 1.65 3.68 9.20111.19 −26.6 1.14 1.98 1.85 7.72109.18 −26.2 1.15 6.47 1.62 6.64101.98 −26.7 −26.8 −26.8 1.05 2.14 1.72 −

Average −26.53±0.3

60.54 −25.8 2.46 2.15 3.87 4.17202.74 −24.9 2.17 1.21 3.12 4.9537.3 −24.8 2.83 1.21 3.67 4.2674.45 −24.1 0.94 1.29 – 2.33

104.6 −24.1 2.44 1.35 3.64 5.9857.64 −22.7 2.82 1.06 3.84 5.45

Average −24.4±1.1

Fig. 3. Representative total ion current [TIC] chromatograms obtained from GC-MS analysis of saturated and aromatic hydrocarbon fractions isolated from selected depths of theFruitland Coal, San Juan Basin. UCM=unresolved complex mixture.

89M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

Source and maturity indices were calculated following Peters et al.(2005) and references therein. Source indices include carbon-preference index (CPI) and odd-over-even predominance (OEP).Terrestrial land plants contribute leaf waxes comprising odd carbonnumber n-alkanes, and organic matter sources dominated byterrestrial land plants will result in elevated (N1) CPI and OEPratios (Tissot and Welte, 1984). CPI was integrated over the entirerange of available n-alkanes, while OEP ratios were separated intoshort (OEP17) and long (OEP29) chain n-alkanes to address the odd-over-even preference in specific n-alkane chain lengths. Pristane/Phytane (Pr/Ph) ratios were also measured to address depositionalconditions.

In addition to the vitrinite reflectance analyses, molecularmaturity indices include hopane stereoisomer ratios, specifically theratio of C27 17α(H)-trisnorhopane (Tm) to C27 18α(H)-trisnorhopaneII (Ts) (Siefert and Moldowan, 1978) and the isomerization of C31–C34

17α(H)21β(H)-homohopanes. Generally, these ratios increase withincreasing thermal maturity, (Ensminger et al., 1974; Siefert andMoldowan, 1978; Peters and Moldowan, 1993; Peters et al., 2005). Theisomerization of C31-17α(H)21β(H)-homohopanes is governed by therates of generation and thermal degradation of these hopanes(Farrimond et al., 1998), however, in these relatively immature

coals it is likely that minimal thermal degradation has occurredtherefore maintaining the integrity of the molecular signatures. Inaddition, due to the influences of source facies on Ts and Tm ratios itwas necessary to integrate multiple maturity indicators to adequatelydetermine the thermal maturity of the coals.

Utilizing the biodegradation scheme of Peters and Moldowan(1993) and Peters et al. (2005) it was determined that the most bio-resistant compounds present in our samples are the C31-homoho-panes. Biodegradation indices were calculated based on loss of themore susceptible compounds relative to the sum of the more resistantC31-17α(H)-21β(H) 22S and 22R hopanes as follows:

Biodegradation Index =X

C31 � hopanesð Þ=X

compound sð Þ of interestð Þ:ð1Þ

For our calculations, greater values of the biodegradation indexindicate increased relative removal of the compound(s) of interest, i.e.greater biodegradation. Biodegradation was evaluated for the follow-ing: nC15–nC19 alkanes, nC20–nC24 alkanes, nC25–nC30 alkanes, threeacyclic isoprenoids (norpristane, pristane, phytane), nC18–nC27

Fig. 4. Representative total ion current [TIC] chromatograms obtained from GC-MS analysis of saturated and aromatic hydrocarbon fractions isolated from selected depths of the FortUnion Coal, Powder River Basin. UCM=unresolved complex mixture.

90 M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

alkylcyclohexanes, methylnaphthalene (MN), dimethylnaphthalene(DMN), trimethylnaphthalene (TrMN), tetramethylnaphthalene(TeMN), phenanthrene (P), methylphenanthrene (MP), dimethylphe-nanthrene (DMP), trimethylphenanthrene (TrMP), and tetramethyl-phenanthrene (TeMP). Since the possibility of comparing differentcompound classes, e.g. saturated versus aromatic hydrocarbons, maybe subjected to biases from potential differences in the responsefactors of mass fragment ions we also assessed the biodegradation ofaromatic hydrocarbons by taking the ratios of less-susceptiblearomatic compound classes over more-susceptible compound classes.This approach is similar to the approach by Ahmed et al. (1999) wherethe authors compared ratios of compounds that had varying suscept-ibility to biodegradation. The additional aromatic biodegradationindex is:

Aromatic Index

¼X

less susceptible compound sð Þð Þ=X

more susceptible compounds sð Þð Þð2Þ

And similar to the Biodegradation Index, increased values indicatethe preferential removal of the more easily biodegraded compound.

3. Results

3.1. Source parameters

Results for the Fort Union coal and Fruitland coal are tabulated inTable 1. Fort Union TOC values range from 57.49 to 63.36 wt.%(average=59.18 wt.%, n=7). Fruitland TOC values range from 33.94 to70.86 wt.% (average=57.00 wt.%, n=4), with the lowest concentrationmeasured at 132-m depth. Total EOM (mg)/g TOC values in the FortUnion coal range are highly variable and range from 37.3 to202.74 mg/g TOC. Total EOM (mg)/g TOC values in the Fruitlandcoal exhibit little variability and range from 101.3 to 111.19 mg/g TOC.Bulk organic matter δ13C values for the Fort Union coal range from−22.7 to −25.8‰ (average=−24.4‰±1.1‰). Bulk organic matter δ13Cvalues for the Fruitland have an average value of −26.5±0.3‰, and arange of −26.2 to −26.7‰.

Biomarker indices recording the source input include CPI, OEP17,and OEP29. CPI values for the Fort Union coal range between 2.17 and2.83, with an anomalous value of 0.94 at 224 m. Fort Union coal OEP17values range between 1.06 and 1.35, except for the shallowest depth(49 m), which has a value of 2.15. Fort Union coal OEP29 values range

Fig. 5. Comparison of source andmaturity parameters versus biodegradation indices forthe Fruitland and Fort Union Formations coals. (A) Comparison of organic matter sourcerevealed by carbon preference index values (CPI) versus hydrocarbon biodegradationindices for Fruitland and Fort Union Coals. R2 values from linear regression analysis areprovided in figure legend. (B) Comparison of Ts/(Ts+Tm) maturity index values versushydrocarbon biodegradation indices for Fruitland and Fort Union Coals. R2 values fromlinear regression analysis are provided in figure legend.

91M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

from 3.12 to 3.87. Pr/Ph ratios throughout the core and range from 4.17to 5.98, with one anomalous value of 2.33 at 224 m.

A CPI value of 2.20 was calculated for the shallowest Fruitland coalsample (132 m), while the other three depths have values between1.05 and 1.15. Fruitland coal OEP17 values are similar to those from theFort Union coal, and range between 1.65 to 2.14; however, at a depth of760 m the value increases to 6.47. The Fruitland coal OEP29 range from1.62 to 3.68 and the Fruitland Pr/Ph ratios range from 6.64 to 9.20.

3.2. Maturity indices

Vitrinite reflectance and molecular maturity indices for the FortUnion and Fruitland coals are tabulated in Table 2. Vitrinite reflectance(Ro) data from the Fort Union coals range from 0.45 to 0.53%(average=0.49%). Ts/(Ts+Tm) ratios in the Fort Union coal are 0.20to 0.25 in the shallowest four samples and decrease to 0.13 and 0.16 inthe deepest two samples. Fort Union coal C31 22S/22R ratios areextremely variable and range from 0.03 to 0.82. Fort Union coal C32maturity ratios are between 0.56 and 0.74 with an outlying value of0.02 at 224 m. The C33 ratios determined in the Fort Union coal are0.43, 0.02, 0.03, and 0.01, with increasing depth. Fort Union coal C34

hopane ratios ranged from 0.29 to 0.68.Ro data was calculated for two of the Fruitland Formation coals:

Rovalues are 0.55 and 0.82%. Ts/(Ts+Tm) ratios in the Fruitland coalrange from 0.07 to 0.94. The Fruitland coal C31 22S/22R ratio is 0.26 at132 m and range between 0.58–0.59 in deeper samples, similar toresults reported by Michael et al. (1993). Fruitland coal C32 ratios aresimilar to the C31 ratios, equaling 0.13 at 132 m and ranging from 0.58to 0.59 in deeper samples. The Fruitland coal C33 ratio is 0.52 at 132 m,then ranges from 0.58–0.60 at depth. The C34 hopane ratio could notbe determined for the shallowest sample of Fruitland coal but rangedfrom 0.60 to 0.61 in the samples from greater depth.

3.3. Biodegradation indices

To assess biodegradation, the abundance of compound(s) ofinterest relative to the summed abundance of the C3117α(H)21β(H)-hopanes 22S and 22R isomers was calculated. C3117α(H)21β(H)-hopanes were chosen as non-degraded reference compounds as theseare known to be highly resistant to biodegradation (Peters andMoldowan, 1993; Peters et al., 2005). Similar source inputs andthermal histories suggest that the relative inputs and generation orthermal degradation of the hopane isomers should be consistent inthe samples from their respective basins. The pristane/nC17 andphytane/nC18 ratios were also calculated as parameters for degrada-

Table 2Vitrinite reflectance and hopane maturity biomarker indices

Maturity indices

Formation Depth (m) Ro (ave.±s.d.) Ts/(Ts+Tm)a S/(S+R)b

C31 C32 C33 C34

Fruitland 132 0.55±0.04 0.37 0.26 0.13 0.52 –

Fruitland 750 0.82±0.06 0.07 0.59 0.59 0.60 0.61Fruitland 760 0.94 0.59 0.59 0.58 0.60Fruitland 1190 0.26 0.58 0.58 0.58 0.61Fort Union 49 0.53±0.04 0.25 0.82 0.58 0.43 0.29Fort Union 146 0.45±0.05 0.25 0.05 0.74 0.03 0.68Fort Union 223 0.48±0.04 0.25 0.06 0.50 0.02 0.46Fort Union 224 0.20 0.03 0.02 0.03 0.47Fort Union 266 0.47±0.04 0.13 0.79 – 0.01 0.45Fort Union 429 0.52±0.04 0.16 0.08 0.56 – –

a Ts/(Ts+Tm)=C2718α(H)-trisnorhopane II (Ts)/(C2718α(H)-trisnorhopane II (Ts)+C27

17α(H)-trisnorhopane(Tm)).b 22S/(22S + 22R) = 17α(H),21β(H)-22S homohopane/[17α(H),21β(H)-22S

homohopane+17α(H),21β(H)-22R homohopane].

tion of methyl-branched versus straight-chain saturated hydrocar-bons. These ratios can be used as biodegradation indicators if sampleshave similar sources and experienced similar thermal histories.Importantly, regression analysis revealed no discernible relationshipsbetween source or maturity parameters and biodegradation indices ineither the Fort Union or Fruitland coals (Fig. 5).

In addition to the C31-hopane biodegradation index, the relativebiodegradation of aromatic hydrocarbons was determined by compar-ing aromatic compounds more susceptible to biodegradation toaromatic compounds that are less susceptible from the samecompound classes. This calculation was performed to assure thatbiases from variations in the mass fragment responses betweensaturated and aromatic hydrocarbons would be not be a factor.

3.3.1. Saturated hydrocarbonsBiodegradation indices for saturated hydrocarbons are tabulated in

Table 3. Elevated biodegradation indices of n-alkanes and acyclicisoprenoids, specifically norpristane, pristane, and phytane, in theFruitland coal was calculated at 132 and 760 m depths (Fig. 6A),representing biodegradation at these depths. Values in the Fort Union

Table 3Biodegradation indices for saturated hydrocarbons

n-alkanes

Formation Depth (m) Pristane/nC17a Phytane/nC18

b ∑(nC15–nC19)c ∑(nC20–nC24)d ∑(nC25–nC30)e ∑(acyclic isoprenoids)f ∑(n-alkyl-(CH))g

Fruitland 132 6.84 0.77 3.19 1.45 0.50 2.20 26.68Fruitland 750 2.39 0.55 0.27 0.22 0.20 0.28 0.53Fruitland 760 10.83 15.30 6.35 8.52 1.01 1.01 –

Fruitland 1190 0.71 – 0.06 0.04 0.07 0.06 0.08Fort Union 49 2.62 1.03 2.37 1.34 0.66 1.64 –

Fort Union 146 7.46 1.62 15.62 9.79 6.70 8.05 –

Fort Union 223 0.94 0.33 1.00 1.29 1.18 2.17 –

Fort Union 224 0.95 0.62 6.61 8.16 9.01 8.60 –

Fort Union 266 1.69 0.46 0.51 0.66 0.69 0.87 –

Fort Union 429 2.86 0.62 1.29 0.93 0.41 1.26 –

a Pr/n-C17=pristane/C17 n-alkane.b Ph/n-C18=phytane/C18 n-alkane.c ∑(nC15-nC19)=(∑C31-hopanes) / (∑(nC15–nC19).d ∑(nC20–nC24)=(∑C31-hopanes) /∑(nC20–nC24).e ∑(nC25–nC30)=(∑C31-hopanes) /∑(nC25–nC30).f ∑(acyclic isoprenoids)=(∑C31-hopanes) /∑(norpristane+pristane+phytane).g ∑(n-alkyl-(CH))=(∑C31-hopanes) /∑(n-alkylcyclohexanes).

92 M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

coal are highest at depths of 146 and 224m, indicating biodegradationlocated at these depths (Fig. 6B).

The Fruitland coals exhibit a series of n-alkylcyclohexanes,specifically nC18–nC27 cyclohexanes. This series of n-alkylcyclohex-anes were not observed in the Fort Union coal. In the Fruitland coals,

Fig. 6. Calculated biodegradation indices for the saturated hydrocarbons for theFruitland Coal and Fort Union Coal. Short-chain n-alkanes=nC15–nC19. Medium-chainn-alkanes=nC20–nC24. Long-chain n-alkanes=nC25–nC30. Acyclic isoprenoids includenorpristane, pristane, and phytane.

similar to both the n-alkanes and acyclic isoprenoids, the n-alkylcyclohexanes show variations in the biodegradation index withdepth (Table 3). At 132 m the value is 26.68, at 750 m the value is 0.53,and at 1302 m the value is 0.08. Biodegradation indices for saturatedhydrocarbons exhibit minima for the deepest samples of both the FortUnion and Fruitland coals (429 m and 1190 m, respectively).

3.3.2. Aromatic hydrocarbonsVariations in the distribution of the aromatic hydrocarbons also

highlight the occurrence of biodegradation in these coals. Calculatedbiodegradation and aromatic indices for aromatic hydrocarbons areprovided in Tables 4 and 5. In the Fruitland coal of the San Juan Basin,biodegradation indices calculated for the naphthalene and phenan-threne series show elevated biodegradation at 132, 750, and 760 m(Fig. 7A and B). The results and depth distributions from the aromaticindex are similar to the results from the biodegradation index (Tables4 and 5). It is apparent from these two calculated indices that there hasbeen preferential removal of the more susceptible compounds.Interestingly, at a depth of 1190m in the Fruitland there are extremelyelevated TMN/DMN ratios, this could be an artifact of an increasedthermal maturity.

Values for naphthalenes suggest biodegradation throughout mostof the Fort Union coal samples (Fig. 8A). There were no identifiableMNs in the Fort Union coal and only minimal DMNs. Only threemeasurements could be made for the DMNs; however the biode-gradation indices are elevated at 223 m compared to the deepersamples. The TrMN and TeMN show elevated biodegradation at 146and 224 m, similar to the saturated hydrocarbons. Biodegradationindices calculated for Fort Union coal phenanthrenes show patternssimilar to the naphthalenes (Fig. 8B). Phenanthrene biodegradationindices in the Fort Union are high for most of the sample locations,with the most elevated biodegradation indices at 146 and 224 m.

As observed for saturated hydrocarbons, the biodegradation indexfor aromatic hydrocarbons exhibit minima for the deepest samples ofboth the Fort Union and Fruitland coals (429 m and 1190 m,respectively). The aromatic index, similar to the biodegradationindex, indicates the preferential removal of the more susceptiblecompounds in both the naphthalene and phenanthrene compoundclasses (Table 5). However, at a depth of 429 m there appears to beelevated TMN/DMN and MP/P ratios, these may be the result of localheterogeneities or a variation in thermal maturity, though the latter isunlikely based upon other results.

Though the two separate indices compare different compoundclasses they both suggest there is the preferential removal of the more

Table 4Biodegradation indices for aromatic hydrocarbons

Naphthalenes Phenanthrenes

Formation Depth (m) ∑MNa ∑DMNb ∑TrMNc ∑TeMNd ∑Pe ∑MPf ∑DMPg ∑TrMPh ∑TeMPi

Fruitland 132 – – 43.59 38.90 63.92 14.81 9.62 17.35 41.19Fruitland 750 9.36 1.34 10.29 0.21 0.22 0.10 0.07 0.11 0.45Fruitland 760 – – 1.26 1.40 46.18 0.62 0.41 0.59 4.09Fruitland 1190 – – 0.52 0.47 0.34 0.09 0.06 0.10 0.39Fort Union 49 – – 7.48 4.20 11.53 0.98 2.23 – –

Fort Union 146 – – 8.75 5.60 15.41 1.44 1.08 1.75 –

Fort Union 223 – 13.73 0.57 0.73 2.37 0.68 0.38 0.83 –

Fort Union 224 – – 14.31 5.20 13.08 3.54 1.34 3.08 15.18Fort Union 266 – 2.76 0.22 0.35 – – – – –

Fort Union 429 – 1.51 0.10 0.43 0.90 0.12 0.09 0.25 –

a ∑MN=(∑C31-hopanes) / (∑methylnaphthalene).b ∑DMN=(∑C31-hopanes) / (∑dimethylnaphthalenes).c ∑TrMN=(∑C31-hopanes) / (∑trimethylnaphthalenes).d ∑TeMN=(∑C31-hopanes) / (∑tetramethylnaphthalenes).e ∑P=(∑C31-hopanes) / (∑phenanthrene).f ∑MP=(∑C31-hopanes) / (∑methylphenanthrene).g ∑DMP=(∑C31-hopanes) / (∑dimethylphenanthrenes).h ∑TrMP=(∑C31-hopanes) / (∑trimethylphenanthrenes).i ∑TeMP=(∑C31-hopanes) / (∑tetramethylphenanthrenes).

93M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

susceptible compounds when compared to a more bio-resistantcompound from the same class.

4. Discussion

4.1. Biodegradation of coal associated with coalbed methane

Microbial methanogenesis is themain source for gas in the PowderRiver Basin (Ayers,1986; Gorody,1999; Flores et al., 2008-this volume)and a significant addition to the thermogenic gas found in the San JuanBasin (Clayton et al., 1991; Kaiser et al., 1991; Michael et al., 1993; Scottet al., 1994; Zhou et al., 2005). Though the evidence and mechanismsinvolved in the production of coalbed methane have been intensivelystudied, few studies have focused on the signatures of organic matterbiodegradation preserved in the coals (Michael et al., 1993; Curry et al.,1994; Ahmed et al., 1999; Ahmed and Smith, 2001). Qualitativeobservations, specifically the generation of the unresolved complexmixture, of the total ion chromatograph of the saturated hydrocarbonsindicate that both the Fruitland coal in the San Juan Basin and the FortUnion coal in the Powder River Basin have experienced some degreeof biodegradation (Figs. 3 and 4). These chromatographs aredominated by a bimodal distribution and an increase in the UCM,qualitative indications of biodegradation. Unimodal distributions in

Table 5Aromatic indices for naphthalene and phenanthrene compound classes

Formation Depth(m)

TMN/DMNa

TeMN/TMNb

MP/Pc

DMP/MPd

TrMP/DMPe

TeMP/TrMPf

Fruitland 132 – 1.12 4.32 1.54 0.55 0.42Fruitland 750 7.00 0.47 2.30 1.30 0.69 0.24Fruitland 760 – 0.90 74.13 1.50 0.71 0.14Fruitland 1190 58.89 1.13 3.69 1.54 0.63 0.25Fort Union 49 – 1.78 11.78 0.44 – –

Fort Union 146 – 1.56 10.68 1.34 0.61 –

Fort Union 223 24.10 0.78 3.50 1.77 0.46 –

Fort Union 224 – 2.75 3.69 2.64 0.44 0.20Fort Union 266 12.41 0.63 1.36 0.83 – –

Fort Union 429 14.80 0.24 7.23 1.40 0.35 –

a TMN/DMN=Trimethylnaphthalene/Dimethylnaphthalene.b TeMN/TMN=Tetramethylnaphthalene/Trimethylnaphthalene.c MP/P=Methylphenanthrene/Phenanthrene.d DMP/MP=Dimethylphenanthrene/Methylphenanthrene.e TrMP/DMP=Trimethyylnaphthalene/Dimethylnaphthalene.f TeMP/TrMP=Tetramethylnaphthalene/Trimethylnaphthalene.

coals would bemore indicative of increased thermal maturity (Ahmedet al., 1999). These characteristics are further indication that theseobservations are not the result of thermal maturity variations andsuggest biodegradation.

Fig. 7. Calculated biodegradation indices for the aromatic hydrocarbons from theFruitland Coal, San Juan Basin.MN=methylnaphthalenes. DMN=dimethylnaphthalenes.TrMN=trimethylnaphthalenes. TeMN=tetramethylnaphthalenes.P=pheneanthrenes.MP=methylphenanthrenes. DMP=dimethylphenanthrenes. TrMP=trimethylphenan-threnes. TeMP=tetramethylphenanthrenes.

Fig. 8. Biodegradation indices for thearomatichydrocarbons for theFortUnionCoal, PowderRiver Basin. DMN=dimethylnaphthalenes. TrMN=trimethylnaphthalenes. TeMN=tetra-methylnaphthalenes. P=pheneanthrenes. MP=methylphenanthrenes. DMP=dimethylphe-nanthrenes. TrMP=trimethylphenanthrenes. TeMP=tetramethylphenanthrenes.

94 M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

Biodegradation in petroleum reservoirs (Head et al., 2003;references therein) has beenwell established; however, the anaerobicbiodegradation of sedimentary organic matter associated withmethanogenesis and coalbed methane are less well known. Recently,a study by Formolo et al. (2008) demonstrated that biodegradation ofshale hosted organic matter, leading to the production of biogenic gas,in the Antrim Formation is similar to what is observed for petroleumreservoirs, though few studies have addressed the biodegradation ofcoals associated with coalbed methane (Michael et al., 1993; Ahmedet al., 1999; Ahmed and Smith, 2001). To address biodegradationsignatures, it is necessary to assess the source inputs and thermalhistories of these basins so we can determine that any measurabledifferences are due to external processes such as biodegradation.

4.2. Source of organic matter

The δ13C isotopic composition of the Fruitland coals fall within arelatively narrow range, −25.9 to −26.8‰ (Table 1), indicating similarorganic matter sources throughout the Fruitland coal. These resultsare similar to δ13C reported by Michael et al. (1993) for the Fruitlandcoals.

The isotopic compositions of the Fort Union coals are also relativelyconstant, −24.4 to −25.8‰, except for a 13C-enriched value (−22.7‰)measured in the deepest portion of the Formation (Table 1). Overall,the isotopic compositions of the bulk organic matter in both basinsindicate a consistent input of similar organic matter within eachspecific basin and our samples.

Inputs from the waxes of terrestrial land plants result in CPI andOEP values greater than 1.0, or an odd-over-even predominance (Brayand Evans, 1961; Eglington and Hamilton, 1967; Scalan and Smith,1970; Tissot and Welte, 1984). The CPI and OEP(29) for both the FortUnion and Fruitland coals suggest a similar input of long chain n-alkanes derived from terrestrial higher plants, with modification dueto post-depositional maturation and biodegradation.

4.3. Thermal maturity

The rank of the Fort Union coals fall into the lignite tosubbituminous A over the entire basin (Law et al., 1991; Rice, 1993;Stricker et al., 2007, Flores et al., 2008-this volume), making these thelowest grade coals in the United States to actively be exploited as anatural gas reserve. Stricker et al. (2007) shows an increase in coalrank (subbituminous C to A) from the margins to the central part ofthe Powder River Basin. The coals in the San Juan Basin are higher inrank than those of the Powder River Basin. The Fruitland coals increasein rank from the southwest to northeast portion of the basin, fromsubbituminous C to medium- and low-volatile bituminous, respec-tively (Rice, 1983; Rice et al., 1989; Law, 1992; Rice, 1993; Scott et al.,1994). Although these ranks of coals are well established, determiningthe individual thermal maturities of the coals analyzed in this study isimportant prior to accessing the level of biodegradation. Extremelyhigh and anomalous maturities could influence the abundance ofaliphatic and aromatic hydrocarbons and could lead to falsedeterminations of biodegradation levels. All maturity indicators arelisted in Table 2.

Fort Union and Fruitland coals Ro values differ as expected. FortUnion coal Ro values are indicative of immature to transitionalmaturities and the Fruitland Ro values suggest transitional to maturelevels of thermal maturity. These vitrinite reflectance data are con-sistent with the thermal histories of the Powder River and San JuanBasins (Nuccio, 1990; Michael et al., 1993; Rice, 1993; Stricker et al.,2007; Flores et al., 2008-this volume). As expected, Ts/(Ts+Tm)maturity ratios also differ. During increasing thermal maturation, Tmis less stable andwill decrease in relative concentration compared toTs(Waples and Machihara, 1991), thereby increasing the ratio. Becauseorganic matter source inputs are similar within the respective basins,these ratios record maturity or other changes, such as biodegradation.The San Juan Basin Fruitland coals showelevated Ts/(Ts+Tm) ratios forthree of the four coals. However, there is no apparent relationshipbetween depth and increasing maturity, potentially indicating thatdifferent portions of the basin have experienced variable localizedthermal histories. In contrast to the Fruitland coals, Ts/(Ts+Tm) ratiosin the Powder River Basin Fort Union coal are uniformand low in all thesamples. These indicate the low-grade nature of the coal, consistentwith the inferred geologic history of the Powder River Basin (Nuccio,1990; Rice, 1993; Stricker et al., 2007; Flores et al., 2008-this volume).

To more accurately constrain the thermal history of these coals theC31–C34 17α (H) 21β(H)–homohopane 22S/(22S+22R) ratios weredetermined. These apparent isomerization ratios are controlled byrelative rates of generation and destruction during thermal matura-tion (Bishop and Abbot, 1993; Farrimond et al., 1998), typicallyreaching equilibrium around values of 0.57–0.62 (Seifert andMoldowan, 1986). Therefore, assuming that the relative rates ofgeneration and destruction are of these hopane isomers are similar,these ratios provide an indication of the thermal history of thesedimentary organic matter. For samples fromwithin basins subjectedto similar thermal histories and sources of organic matter, the relativedegrees of isomerization should be comparable. These ratios maydiffer slightly but should be in relative agreement if an external pro-cess, such as source variation or biodegradation, does not alter therespective hopanes. C31–C34 homohopane ratios for the Fruitland coalare all approximately at equilibrium except for the shallowest coal.The shallowest Fruitland coal (132 m) has a C31 and C32 hopane ratio

95M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

that suggests a much lower maturity than deeper in the core. Thesetrends and values are similar to those observed by Michael et al.(1993) for Fruitland coals. For an unknown reason, the C33 hopaneratio in the shallowest Fruitland coal has surpassed typical equili-brium values. The thermal maturity within the Fruitland coal islower in the shallow coals and increases to equilibrium values withdepth.

The C31–C34 hopane ratios in the Fort Union coal are inconsistent,possibly the consequence of the highly variable, low-grade ranking ofthe coals. Having previously calculated consistent Ts/(Ts+Tm) ratiosthat represent lower thermal maturities it is not apparent why thesematurity indicators should be so variable. The C31 ratios are low in foursamples, ranging from 0.05 to 0.08; however, two anomalous values atdepths of 49 and 266 m were observed. Similar observations weremade for the other indices; they are generally variable, greatlyexceeding equilibrium values or are extremely low, ranging between0.01–0.02. If large scale, regional processes raised the thermalmaturation in portions of the Fort Union Formation it seems logicalthat those same processes would have uniformly altered all the FortUnion coals. These enigmatic values may be the result of localizedheterogeneities in the organic-rich facies or secondary biodegradationinfluences. Even with these inconsistencies, it appears the Fort Unioncoals are relatively thermally immature, particularly with respect tothe Fruitland coals. Overall, the consistent source inputs and thermalmaturities measured in these coals provide a foundation to assess thelevel of biodegradation of the saturated and aromatic hydrocarbonsand to address whether this biodegradation is related to the activemicrobial methanogenesis.

4.4. Effects of biodegradation on saturated hydrocarbons

Conventional hierarchies of petroleum or crude oil biodegradationbegin with the loss of straight chain alkanes, followed sequentially byacyclic isoprenoids, highly-branched and cyclic saturated hydrocar-bons (Peters and Moldowan, 1993; Wegner et al., 2001; Head et al.,2003; Peters et al., 2005). The loss of these GC-resolvable componentscontributes to an increase in the ‘unresolved complex mixture’ (UCM),a common qualitative characteristic of biodegraded bitumen (Connan,1984; Peters and Moldowan, 1993; Peters et al., 2005; Rabus, 2005).Overall, biodegradation in petroleum or crude oil generally proceedswith the removal of saturated hydrocarbons followed by the loss ofaromatic hydrocarbons.

Fort Union coals show elevated biodegradation indices at severaldepths (49, 146, 224 m) for the short-, medium-, and long-chain n-alkanes and acyclic isoprenoids, typical of conventional biodegradedpetroleum. In spite of much higher values for maturity indicators, theFruitland coals follow a pattern similar to the Fort Union coals for theremoval of n-alkanes and acyclic isoprenoids at particular depths (132and 760m). The series of nC18–nC27-alkylcyclohexanes in the Fruitlandcoals may be a product of the thermal alteration of the Fruitland coals,as alkylcyclohexanes can be generated during thermal alteration(Kenig, 2000; Brocks and Summons, 2003). The biodegradationindices for these compounds (Table 3) also show preferential removalin the shallowest coals. The biodegradation of these n-alkylcyclohex-anes is additional evidence that biodegradation can develop andproceed in more thermally-mature coals in a manner similar tothermally immature rocks. The deepest coal in the Fruitland (1190 m)has equilibrated maturity ratios and the lowest biodegradationindices, suggesting that this is the nonbiodegraded parent coalmaterial. A similar unaltered coal is not found in the Fort Union,implying that coals even in the deepest settings have experiencedsome level of biodegradation.

Overall, the biodegradation in the Fort Union and Fruitland coalshas resulted in the removal of n-alkanes and acyclic isoprenoids,which would correspond to a Level of Biodegradation equal to 2–3 onthe scale of Peters and Moldowan (1993), or a level of Slight to

Moderate on the scale of Wegner et al. (2001). The generation andsubsequent biodegradation of the alkylcyclohexanes in the Fruitlandcoal suggest an even higher level of biodegradation than what isindicated by the removal of n-alkanes and acyclic isoprenoids.According to the biodegradation hierarchy of Peters and Moldowan(1993) and Wegner et al. (2001) the removal of alkylcyclohexanesdoes not occur until Level 3–4, or Moderate to Heavy. The highestdegrees of biodegradation are confined to particular coal seams (Table3, Fig. 6), suggesting that zones of biodegradation may ultimately becontrolled by stratigraphic variation in the subsurface. These hetero-geneities in the stratigraphy, including fractures and cleat systems incoals, allowing localized groundwater flow thru particular strata, maystimulate the microbial biodegradation of hydrocarbons.

4.5. Effects of biodegradation on aromatic hydrocarbons

In conventional biodegraded petroleum, the biodegradation ofaromatic hydrocarbons, such as naphthalenes and phenanthrenes,only occurs after there has been major alteration, and sometimescomplete removal, of the n-alkanes (Peters and Moldowan, 1993;Wegner et al., 2001; Head et al., 2003; Peters et al., 2005). However,studies by Ahmed et al. (1999) and Ahmed and Smith (2001) proposedthat the biodegradation of aromatic compounds in coals occurs priorto and at a faster rate than the removal of the n-alkanes.

For the Fruitland and Fort Union coals only the trimethyl- andtetramethylnaphthalenes (TrMN and TeMN, respectively) couldprovide useful biodegradation indices because the other substitutednaphthalenes were not consistently present throughout the coals.Though, when present, dimethylnaphthalene (DMN) biodegradationindices are similar to TrMN and TeMN. It is likely the naphthalene,mono- and dimethylnaphthalenes were at one point present, but havesubsequently been removed by biodegradation in the Fort Union coals.The Fruitland coals indicate extensive removal of naphthalenes in theshallowest depths, and a general decrease in the aromatic biode-gradation indices with depth (Table 4, Fig. 7). The Fort Union also haselevated biodegradation indices for the naphthalenes, though thedepth relationships are not as apparent as those observed in theFruitland (Table 4, Fig. 8). The trimethylnaphthalenes appear morebiodegraded than the tetramethylnaphthalenes in both coals, which isin accordance with the increasing resistance to biodegradation withincreasing alkylation. The biodegradation indices in the phenanthreneseries are almost identical to the naphthalene series.

Both the Fort Union and Fruitland coals exhibit biodegradation ofall the alkyl-substituted phenanthrenes, except for the tetramethyl-phenanthrenes in the Fort Union coal which could not be calculateddue to the low abundances (Table 4, Figs. 7 and 8). In both systems, thesame depths exhibit the highest degrees of biodegradation amongsaturated and aromatic compounds. Unlike the naphthalene seriesthere is no apparent increasing resistance to biodegradation thatcorresponds to the level of substitution for the phenanthrene seriesfor the Fruitland coal. The Fort Union coals do show progressively lessbiodegradation with increasing numbers of methyl side groups.

According to the aromatic biodegradation hierarchy established byFisher et al. (1998), the Fort Union and Fruitland coals, which show theremoval and alteration of all the alkyl-substituted naphthalenes andphenanthrenes, would suggest a relatively major level of alteration,ranging from moderate to extensive. The scale of Peters andMoldowan (1993) would suggest similarly moderate to heavy levelsof biodegradation and the extent of biodegradation would bemoderate, level 4 or 5, according to Volkman et al. (1984). Theseclassification schemes of biodegradation hierarchy are based uponpetroleum and crude oil degradation and when utilized to assessbiodegradation of coal may inaccurately represent the actual level ofbiodegradation. Aromatic hydrocarbon biodegradation in petroleumor crude oils indicates extensive biodegradation, resulting in aclassification of moderate to heavy levels, in coals the biodegradation

96 M. Formolo et al. / International Journal of Coal Geology 76 (2008) 86–97

of aromatic hydrocarbons may not be evidence of such extensivebiodegradation but may be indicative of moderate to low levels ofbiodegradation. Based upon our results and the results of Ahmed et al.(1999) and Ahmed and Smith (2001), it may be possible thatcomparing coal and sedimentary organic matter biodegradation topetroleum and crude oils is insufficient and it may be necessary todevelop a specific biodegradation scheme for coals and sedimentaryhosted organic matter.

Though these biodegradation indices suggest the occurrence ofbiodegradation in these coals, the influence or effects of waterwashing cannot be completely excluded. According to Volkman et al.(1984), benzene, toluene, and C2- and C3-alkyl-substituted benzenesare very soluble in water, and napththalene, methylnaphthalene, andphenanthrene are less soluble. In addition, C2-napthalanes or highermolecular weight aromatic hydrocarbons, e.g. trimethylphenan-threne, are even less water soluble and therefore more resistant towater washing. The presence of an extensive suite of alkyl-substitutednaphthalenes and phenanthrenes in both the Fort Union and Fruitlandcoals would suggest minor, if any, water washing has occurred.

Relationships between fine-scale stratigraphy and localizedgroundwater movement may be controlling the observed distributionof biodegraded coals. Biodegradation of both saturated and aromatichydrocarbon appears focused in particular, perhaps shallower, coalseams. The biodegradation of aromatic and saturated hydrocarbons,localized in particular depths, suggest that fine-scale groundwatermovement may ultimately control the stimulation of microbialmethanogenesis. While these observations do not exclude thepotential for water-washing of the coals, pristane/nC17 and phytane/nC18 ratios in both the Fruitland and Fort Union coals suggest anincreasing predominance of isoprenoids as other biodegradationindices increase (Table 3) which is more likely the result ofbiodegradation than water-washing. The occurrence of the morepolar aromatic compounds in both coals suggests limited, if any,removal of compounds by water-washing. Also, the presence ofthermogenically produced gas in the San Juan Basin does not seem toinfluence the biodegradation indices in the shallow Fruitland coals,but the thermal imprints may have masked biodegradation in thedeep coals. Overall, it appears these differences in biodegradationindices reflect hydrocarbon decomposition directly associated withmethanogenesis similar to other field and laboratory based observa-tions (Ahmed et al. 1999; Zengler et al., 1999; Anderson and Lovely,2000; Ahmed and Smith, 2001) in two of the world's largest coalbedmethane reservoirs. The biodegradation signatures preserved in thecoals do not specifically follow the observed biodegradation trends inpetroleum, crude oil, or in shale-hosted gas reservoirs, but insteadshow elevated aromatic compound biodegradation prior to completeor extensive removal of more aliphatic compounds in both thermallymature and thermally immature coal seams.

5. Conclusions

Microbial degradation of organic matter in the subsurface is welldocumented in marine sediments and hydrocarbon reservoirs.However, minimal research been focused on linking subsurface coalbiodegradation to active microbial methanogenesis. Traditionally, gascompositions and formation water geochemistry have been used todetermine the presence of biogenic gas, but the microbial utilizationof sedimentary organic matter involved with these processes remainspoorly understood. Results of this study reveal the following: (1)biodegradation has occurred at depths up to several hundred metersin both the Fort Union coal of the Powder River Basin and the Fruitlandcoal of the San Juan Basin, (2) similar to petroleum reservoirs,signatures of biodegradation in the Fort Union and Fruitland coalsinclude extensive removal of n-alkanes and isoprenoids, (3) incontrast to petroleum and crude oil, polycyclic aromatic hydrocarbonsin the Fort Union and Fruitland coals are altered prior to the extensive,

if not complete, removal of n-alkanes and acyclic isoprenoids, (4)biodegradation of aliphatic and aromatic hydrocarbons can occur, andin a similar fashion, across a range of inherited thermalmaturities, and(5) biodegradation in coalbed methane reservoirs produces similarand recognizable biodegradation signatures that differ from biode-graded petroleum, crude oil, and sedimentary organic matter. Overall,these preliminary results suggest that biodegradation in boththermally immature and mature coals associated with coalbedmethane results in similar molecular signatures. However, in orderto fully understand the mechanisms responsible for the biodegrada-tion of coal and the biomarker signatures that are indicative of theseprocesses more research is required. Specifically, more research isnecessary to link the signatures of organic matter biodegradation toactive microbial methanogenesis.

Acknowledgements

We thank Jeanine Honey at the USGS Core Repository in Denver,Colorado, for her assistance in acquiring samples. We also would liketo thank James H. Ruffin at National Petrographic Service Inc. for thevitrinite reflectance data. Additional thanks goes to Elizabeth Gordonof the University of Massachusetts Amherst Biogeochemistry andStable Isotope Laboratories for analyzing the bulk δ13C values of thecoals. Helpful comments were provided by Dr. Romeo Flores and twoanonymous reviewers. This research was funded by grants from theNational Science Foundation Biogeosciences Program to STP (EAR-0433766) and AMM (EAR-0433801).

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