Geochemical Significance of n-AlkaneCompositional-Trait Variations in Coals

Charles R. Nelson*,†

Basic Research Group, Gas Research Institute, Chicago, Illinois 60631

Wenbao Li, Iulia M. Lazar, Kristine H. Larson, Abdul Malik, andMilton L. Lee*,‡

Department of Chemistry, Brigham Young University, Provo, Utah 84602

Received July 7, 1997

The compositional traits of C9-34 n-alkanes were measured in supercritical CO2 extracts from14 U.S. coals of varied geologic ages and wide thermal maturity range (lignite through low-volatile bituminous). The analysis data exhibit no unique component depletion pattern signaturesdiagnostic of known types of postgeneration physical, chemical, or microbiological degradationprocesses that commonly affect crude oil in sedimentary rocks. The C9-34 n-alkanes in Paleoceneand Upper Cretaceous age coals exhibit bimodal carbon-number distribution profiles that stronglyresemble those of the biogenic n-alkanoic acids present in brown coals. The compositional traitsimilarities between these n-alkanes and n-alkanoic acids and the covariance of the bulk coalorganic matter atomic oxygen-to-carbon (O/C) ratios and carbon-preference index (CPI) valuesoffer tangible evidence for the existence of a genetic linkage between these two series ofcompounds. Our analysis results indicate that the C2-5 alkanes and C6+ hydrocarbons in coalsattain their maximum abundances over the thermal maturity interval from 0.50 to 0.72% R0,which, in turn, strongly suggests that these two groups of compounds are formed concurrentlyby similar overall reaction processes during coal maturation. The compositional traits of theC4-5 alkanes and C9-34 n-alkanes in coals appear to uniquely mimic those of the alkane productsformed by mineral-catalyzed defunctionalization and cracking of n-alkanoic acids, which suggeststhat mineral catalysis rather than temperature-controlled thermolysis may be a critical variablecontrolling the formation and compositional traits of natural gas and C9+ n-alkanes during coalmaturation. These mechanistic insights should be useful to those seeking to formulate improvedgeochemical models for predicting hydrocarbon evolution during coal maturation.

Introduction

Coals are organic matter-rich sedimentary rockswhose geological evolution generates a complex hydro-carbon mixture composed of alkane gases and crude oiltype constituents.1-8 Geochemical models of n-alkaneevolution during coal maturation implicitly assume apreserved genetic linkage between their observed com-positional traits and those of their presumed biogenic

precursors.2-6 This assumption may not be universallyapplicable, since n-alkane compositional traits arevulnerable to alteration by postgeneration physical,chemical, and microbiological degradation processes.5,6The possible occurrence of such compositional traitalteration was recently inferred from a depleted C15+n-alkane anomaly in solvent extracts obtained from SanJuan Basin Fruitland Formation coal.9-11 One way tosubstantiate this interpretation is to demonstrate con-current depletion of light oil (C6-14) n-alkanes.5,6 How-ever, during the solvent extraction of hydrocarbons fromcoals and other sedimentary rocks, the detection andquantification of C6-14 hydrocarbons generally sufferbecause of their preferential susceptibility to volatilityfractionation or loss.6,12In this study, coal samples from various locations and

different geologic ages were extracted using neat su-percritical CO2. This technique allows the recovery of

* To whom correspondence should be addressed.† E-mail: cnelson@gri.org.‡ E-mail: milton_lee@byu.edu.(1) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1967, 31,

2389-2397.(2) Brooks, J. D.; Gould, K.; Smith, J. W. Nature 1969, 222, 257-

259.(3) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1969, 33,

1183-1194.(4) Bartle, K. D.; Jones, D. W.; Pakdel, H.; Snape, C. E.; Calimli,

A.; Olcay, A.; Tugrul, T. Nature 1979, 277, 284-287.(5) Hunt, J. M. Petroleum Geochemistry and Geology; Freeman: San

Francisco, 1979.(6) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence;

Springer-Verlag: Berlin, 1984.(7) Rice, D. D.; Clayton, J. L.; Pawlewicz, M. J. Int. J. Coal Geol.

1989, 13, 597-626.(8) Clayton, J. L. In Hydrocarbons from Coal; Law, B. E., Rice, D.

D., Eds.; American Association of Petroleum Geologists: Tulsa, 1993;pp 185-201.

(9) Clayton, J. L.; Rice, D. D.; Michael, G. E. Org. Geochem. 1991,17, 735-742.

(10) Michael, G. E.; Anders, D. E.; Law, B. E. Org. Geochem. 1993,20, 475-498.

(11) Scott, A. R.; Kaiser, W. R.; Ayers, W. B., Jr. Am. Assoc. Pet.Geol. Bull. 1994, 78, 1186-1209.

(12) Hunt, J. M. Science 1984, 226, 1265-1270.

277Energy & Fuels 1998, 12, 277-283

S0887-0624(97)00112-6 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 02/24/1998

C6-14 hydrocarbons without subsequent loss (evapora-tion) during the removal of the extraction agent.

Experimental Section

The coals evaluated in this study came from sedimentsranging in geological age from Paleocene to Pennsylvanian (seeTable 1) and from subsurface deposits unaffected by naturalweathering processes13 and ranged in thermal maturity fromlignite through low-volatile bituminous rank (74.1% to 91.8%carbon, dry, mineral matter free basis). Proximate, ultimate,carbon isotopic, vitrinite reflectance, and maceral analyseswere performed according to ASTM or other standard proce-dures by commercial testing or university laboratories.Coal samples were dynamically extracted with neat super-

critical CO2 (2 h, 120 °C, and 200 atm) using the apparatusshown in Figure 1 and described in detail elsewhere.14 Ap-proximately 1 g of powdered coal (ground to pass a 200-meshscreen) was placed in an extraction cell (3.5-mL volume) andsandwiched between silanized, washed glass beads of 250-µmouter diameter to minimize dead volume. A Lee ScientificSeries 600 high-pressure syringe pump was used to introduceSFC-grade CO2 into the extraction cell. The extraction wasessentially complete after 1 h, as evidenced by a negligiblechange in yield with longer extraction time. The extractedanalytes were collected in 0.5 mL of cold (-5 °C) carbondisulfide.Soxhlet solvent extraction and chemical class fractionation

procedures were used to isolate the C15+ hydrocarbon compo-

nents.15,16 The supercritical fluid and solvent-extracted ana-lytes were analyzed by gas chromatography/mass spectrometryusing a Hewlett-Packard 5890 gas chromatograph and aHewlett-Packard 5970 mass-selective detector. The extractcompound quantitation, identification, and carbon-preferenceindex (CPI) analysis were performed using the proceduresdescribed in detail previously.15-17

Results and Discussion

Bulk Extracts. Gross compositions for the super-critical CO2-extracted C6+ hydrocarbons from 14 U.S.coal samples are shown in Figure 2. The coal-sampleorigins are described in Table 1. The C6+ hydrocarboncontent ranges between 0.02 and 0.27 wt % of the bulkcoal organic matter. The predominant compoundsinclude a homologous series of C9-C24-34 n-alkanes, C6+branched and alkylcyclic alkanes, alkylbenzenes, andalkylnaphthalenes. The aliphatic-to-aromatic ratios ofthe extracts typically ranged from 3 to 8.Numerous factors can potentially affect the content

and compositional traits of petroleum-like hydrocarbonsin coals. It is generally acknowledged that the hydrogencontent of the bulk coal organic matter and the abun-dances of different macerals (vitrinite, liptinite, andinertinite), in particular the abundance of the hydrogen-

(13) Nelson, C. R. In Chemistry of Coal Weathering; Nelson, C. R.,Ed.; Elsevier: New York, 1989; pp 1-32.

(14) Li, W.; Lazar, I. M.; Wan, Y. J.; Butala, S. J.; Shen, Y.; Malik,A.; Lee, M. L. Energy Fuels 1997, 11, 945-950.

(15) Chang, H.-C. K.; Nishioka, M.; Bartle, K. D.; Wise, S. A.;Bayona, J. M.; Markides, K. E.; Lee, M. L. Fuel 1988, 67, 45-57.

(16) Carlson, R. E.; Critchfield, S.; Vorkink, W. P.; Dong, J.-Z.;Pugmire, R. J.; Lee, M. L.; Zhang, Y.; Shabtai, J.; Bartle, K. D. Fuel1992, 71, 19-29.

(17) Garcia-Gonzales, M.; Surdam, R. C.; Lee, M. L. Am. Assoc. Pet.Geol. Bull. 1997, 81, 62-81.

Table 1. Coal Sample Location and Analytical Data

samplelocation

(county, state) age, rankg

Vitrinitereflectance(% R0)

Beulah-Zapa Mercer, ND Paleocene, lig 0.28Wyodak-Andersona Campbell, WY Paleocene, sub 0.31Illinois No. 6a St. Clair, IL Pennsylvanian, hvb 0.46Blind Canyona Emery, UT Cretaceous, hvb 0.50Pittsburgh No. 8a Greene, PA Pennsylvanian, hvb 0.72Intermediate Fruitlandb La Plata, CO Cretaceous, hvb 0.72Lewiston-Stocktona Logan, WV Pennsylvanian, hvb 0.77Basal Fruitlandb La Plata, CO Cretaceous, hvb 0.79Basal Fruitlandc La Plata, CO Cretaceous, hvb 0.86Upper Freeporta Indiana, PA Pennsylvanian, mvb 0.99Pottsvilled Jefferson, AL Pennsylvanian, mvb 1.28Pocahontas No. 3a Buchanan, VA Pennsylvanian, lvb 1.42Pottsvillee Jefferson, AL Pennsylvanian, mvb 1.43Pottsvillef Jefferson, AL Pennsylvanian, mvb 1.46

a Premium coal sample from Argonne National Laboratory. b Valencia Canyon Southern Ute No. 32-1 well, San Juan Basin (32 T33NR11W). c Southern Ute Tribal H-1 well, San Juan Basin (18 T32N R10W). d Pratt seam, Corehole C-6, Rock Creek Site, Warrior Basin (7T18S R5W). e Mary Lee seam, Corehole C-6, Rock Creek Site, Warrior Basin (7 T18S R5W). f Black Creek seam, Corehole C-6, RockCreek Site, Warrior Basin (7 T18S R5W). g lig ) lignite; sub ) subbituminous; hvb ) high volatile bituminous; mvb ) medium volatilebituminous; lvb ) low volatile bituminous.

Figure 1. Dynamic supercritical CO2-extraction apparatus.

Figure 2. Supercritical CO2 extract yields as a function ofthe coal thermal maturity (vitrinite reflectance, % R0).

278 Energy & Fuels, Vol. 12, No. 2, 1998 Nelson et al.

rich maceral liptinite, exert strong controls on the abilityof coals to generate oil and gas during thermal matura-tion.18 However, both pyrolysis and field studies indi-cate that no simple, universally applicable correlationexists between the atomic hydrogen-to-carbon (H/C)ratio or the maceral composition of coals and the amountand compositional traits of oil products except when theatomic H/C ratio is greater than about 0.9 or theliptinite content is greater than 20-25%.18-25 The suiteof coals studied here have atomic H/C ratios of <0.9 andare composed predominantly of huminite/vitrinite (∼75-90%) macerals.The bulk extract yield data shown in Figure 2 do not

covary continuously with increasing thermal maturitybut instead exhibit a pronounced maximum over thevitrinite reflectance interval from 0.50 to ∼0.72% R0,similar to the trends observed for other coals.26,27 Thesharp drop in extract yields for coals having vitrinitereflectance values greater than∼0.72% R0 is interpretedas reflecting a change from predominantly oil-genera-tion reactions to predominantly oil-to-gas cracking reac-tions.26 The data-variation trends in Figure 2 revealthat coals of similar thermal maturities generally hadsimilar C6+ oil content and compositions irrespective oftheir geologic age. These data-variation trends aresignificant from a geologic interpretation perspective,since they strongly suggest that thermal maturity is thedominant variable affecting the gross C6+ hydrocarboncontent variation trends of the suite of coals examinedin this study.C9+ n-Alkanes. In the supercritical CO2 extracts the

n-alkane carbon chain lengths range from C9 to C24-34,whereas in the Soxhlet solvent extracts they range fromC15 to C35 with one anomalous exception, which isdiscussed separately below. Histograms of the n-alkanecarbon chain length predominance in the supercriticalCO2 extracts from Paleocene and Upper Cretaceous agecoals (see Figure 3) exhibit bimodal distributions withmaxima centered at C13-15 and C25-27. There is also anincreasing overall abundance of shorter chain lengthn-alkanes with increasing thermal maturity, which issimilar to trends observed for other coals and is inter-preted as reflecting the progressive generation of n-alkanes.26 The bimodal C9-34 n-alkane compositionalprofiles shown in parts C and D of Figure 3 replicateones observed in chromatograms of oils derived fromcoaly organic matter sources7-10 or obtained by closed-system pyrolysis of coal.23,25 This bimodal carbonnumber profile also mimics the compositional trait

profiles exhibited by the biogenic n-alkanoic acidsextracted from brown coals (see Figure 4).28,29

A simple carbon-number correspondence exists be-tween the C9+ n-alkanes present in the supercritical CO2extracts and the n-alkanoic acids present in brown coalsexcept for a one carbon-number shift. The dominant

(18) Hunt, J. M. Org. Geochem. 1991, 17, 673-680.(19) Bertrand, P.; Behar, F.; Durand, B. Org. Geochem. 1985, 10,

601-608.(20) Horsfield, B.; Yordy, K. L.; Crelling, J. C. Org. Geochem. 1988,

13, 121-129.(21) Radke, M.; Willsch, H.; Teichmuller, M. Org. Geochem. 1990,

15, 539-563.(22) Littke, R.; Leythaeuser, D.; Radke, M.; Schaefer, R. G. Org.

Geochem. 1990, 16, 247-258.(23) Lu, S.-T.; Kaplan, I. R. Am. Assoc. Pet. Geol. Bull. 1990, 74,

163-173.(24) Teerman, S. C.; Hwang, R. J. Org. Geochem. 1991, 17, 749-

764.(25) Mukhopadhyay, P. K.; Hatcher, P. G.; Calder, J. H. Org.

Geochem. 1991, 17, 765-783.(26) Radke, M.; Schaefer, R. G.; Leythaeuser, D.; Teichmuller, M.

Geochim. Cosmochim. Acta 1980, 44, 1787-1800.(27) Teichmuller, M.; Durand, B. Int. J. Coal Geol. 1983, 2, 197-

230.

(28) Komori, Y.; Itoh, H.; Ouchi, K. Fuel 1990, 69, 1362-1369.(29) Chaffee, A. L.; Perry, G. J.; Johns, R. B.; George, A. M. In Coal

Structure; Gorbaty, M. L., Ouchi, K., Eds.; American Chemical SocietyAdvances in Chemistry Series 192; American Chemical Society:Washington, DC, 1981; pp 113-131.

Figure 3. Histograms of n-alkane carbon chain lengthpredominance in supercritical CO2 extracts of selected coals:(A) Beulah-Zap, CPI ) 2.70; (B) Wyodak-Anderson, CPI ) 2.44;(C) Blind Canyon, CPI ) 1.56; (D) Intermediate Fruitland, CPI) 1.22.

Figure 4. Histogram of n-alkanoic acid carbon chain lengthpredominance in Yallourn brown coal (data from ref 28).

Significance of Variation in Coals Energy & Fuels, Vol. 12, No. 2, 1998 279

n-alkanes in Figure 3 have an odd number of carbonatoms, whereas the dominant n-alkanoic acids in Figure4 have an even number of carbon atoms. It seemsrather unlikely that the carbon-number correspondencebetween these n-alkanes and n-alkanoic acids is purelyfortuitous. A more likely explanation is that there is astrong genetic linkage between these two series ofcompounds.The C23-33 n-alkanes in the supercritical CO2 extracts

from the low-rank lignite (Figure 3A) and subbitumi-nous (Figure 3B) coals also exhibit a very strong odd-to-even carbon-number predominance ratio or carbon-preference index (CPI) value, which progressivelydiminishes with increasing coal thermal maturity (partsC and D of Figure 3). Artificial maturation experimentsdemonstrate that long-chain n-alkanoic acids undergodefunctionalization and cracking reactions when heatedat 200-300 °C in the presence of coal-derived mineralmatter, clays such as bentonite and montmorillonite,and calcite.2,3,30-33 The n-alkanoic acids did not yieldsignificant amounts of hydrocarbons when heated undercomparable conditions without the mineral catalysts.The products from these mineral-catalyzed reactionsincluded C1-5 alkanes and a homologous series ofshorter-chain n-alkanes whose CPI values progressivelydecrease with increasing artificial maturation time.30,31Clays and calcite are the dominant mineral constitu-

ents in most coals.34 If n-alkanes in coals are primarilyformed by mineral-catalyzed defunctionalization andcracking of n-alkanoic acids, then the atomic oxygen-to-carbon (O/C) ratio of the bulk coal organic matter andCPI value of the C15+ n-alkanes should covary in asystematic way with increasing thermal maturity of thecoal. Figure 5 reveals that the atomic O/C ratios andCPI values for the coals studied here exhibit significantcovariance over a broad range of thermal maturity. Thiscovariance strongly suggests the coupled progression ofa distinctive precursor-product transformation duringthe natural-coal maturation process.Anomalous C15+ n-Alkane Data. Figure 6 shows

the chromatograms for the aliphatic fraction Soxhletand supercritical CO2 extracts for a San Juan BasinFruitland Formation coal sample. Peak identifications

are given in Table 2. A distinctive compositional traitof the Soxhlet extract chromatogram (Figure 6A) is thecomplete absence of C15+ n-alkanes and the isoprenoidspristane and phytane. This compositional-trait featureis unique and replicates the one previously observed inseveral Fruitland Formation coal samples.9,10 However,low concentrations of C9-24 n-alkanes are clearly presentin the supercritical CO2 extract chromatogram (Figure6B).Previous interpretations of the geochemical signifi-

cance of the depleted C15+ n-alkane content anomaly inFruitland Formation coal hypothesized postgenerationdepletion by gas solution stripping, biodegradation, ora combination of thermal degradation and migrationfractionation.9-11 However, the data in these previousstudies did not clearly confirm these mechanisms andare open to alternative interpretations. Other processesthat can affect the compositional traits of hydrocarbonsin sedimentary rocks are aerial oxidation, water wash-

(30) Jurg, J. W.; Eisma, E. Science 1964, 144, 1451-1452.(31) Shimoyama, A.; Johns, W. D. Nature 1971, 232, 140-144.(32) Shimoyama, A.; Johns, W. D. Geochim. Cosmochim. Acta 1972,

36, 87-91.(33) Johns, W. D. Annu. Rev. Earth Planet. Sci. 1979, 7, 183-198.(34) Renton, J. J. In Coal Structure; Meyers, R. A., Ed.; Academic

Press: New York, 1982; pp 283-326.

Figure 5. Variation of bulk coal organic matter atomic O/Cratios and C15+ n-alkane carbon-preference index (CPI) valuesas a function of thermal maturity.

Figure 6. Gas chromatograms of Southern Ute Tribal H-1well (Basal Fruitland) coal. (A) Soxhlet tetrahydrofuranextract, aliphatic fraction. Chromatographic conditions are thefollowing: SE-54-coated fused-silica capillary column (0.25-µm film thickness), 50 m × 200 µm i.d., temperature program-ming from 100 to 320 °C at 2.5 °C min-1 after a 2-min initialisothermal period, splitless injection. (B) Supercritical CO2

extract. Chromatographic conditions are the following: SE-54-coated fused-silica capillary column (0.25-µm film thick-ness), 25 m × 200 µm i.d., temperature programming from 40to 300 °C at 2.5 °C min-1 after a 10-min initial isothermalperiod, splitless injection. Peak identifications are given inTable 2.

Table 2. Compounds Identified by GC/MS in the CO2Extract of Southern Ute Tribal H-1 Well

peakno. compd(s)

peakno. compd(s)

o n-aliphatic hydrocarbons 5 C2-benzene∆ branched aliphatic 6 C3-cyclohexane

hydrocarbons 7 C3-benzene1 C1-cyclohexane 8 C4-benzene2 C3-cyclopentane 9 C6-benzene3 toluene 10 C2-naphthalene4 C2-cyclohexane 11 C3-naphthalene

280 Energy & Fuels, Vol. 12, No. 2, 1998 Nelson et al.

ing, evaporative fractionation, molecular diffusion, andmineral-catalyzed cracking. Direct experimental evi-dence is required to identify these postgenerationcompositional-trait alteration processes, since theoreti-cal models cannot be used to predict their occurrence.The bulk organic matter comprising coal undergoes

progressive aerial oxidation at outcrops or when storedin contact with air.13 This aerial oxidation decreasesboth the total yield and average molecular weight of thesolvent-extract components but has only a negligibleeffect on the compositional traits of C15+ n-alkanes.35,36Thus, the depleted C15+ n-alkane content anomalyobserved in Figure 6A cannot be explained by aerialoxidation of the coal.The depletion of light oil n-alkanes up to about n-C15

is regarded as a diagnostic signature for postgenerationalteration of crude oil in sedimentary rocks by suchprocesses as evaporative fractionation,37 water wash-ing,38 molecular diffusion,12,39 gas solution stripping,40and anaerobic biodegradation.41 The compositional traitprofiles for the n-alkanes in the supercritical CO2extracts from the coals studied here do not exhibit thisdistinctive depletion pattern signature.Although the aerobic biodegradation of crude oil in

sedimentary rocks can completely remove all n-alkanesover the range C4-24 and higher, this degradationprocess can only occur when the hydrocarbon-bearingrock contains oxygenated meteoric water.5,6,42-44 Sub-surface coal-weathered zones resulting from downwardinfiltration of oxygenated meteoric water only extendto distances tens of meters from outcrop surfaces.13,36The chromatograms shown in Figure 6 are from a drillcore coal sample obtained from a site 12 km from theformation outcrop and a subsurface depth of over 900m. Most significantly, the supercritical CO2-extractedC6+ n-alkanes (see Figure 6B) do not exhibit anycompositional depletion pattern signature diagnostic ofaerobic biodegradation.Mineral Catalysis. Hydrocarbon generation during

coal maturation is commonly modeled as being a tem-perature-controlled thermal cracking (thermolysis) reac-tion.5,6 However, if hydrocarbon generation during coalmaturation is controlled by mineral-catalyzed reactions,then the catalytic properties of the indigenous mineralmatter might be a limiting factor on the content andcompositional traits of n-alkanes in coals. The C15+-n-alkane-depleted Fruitland Formation coal (see Figure6) has a vitrinite reflectance value of 0.86% R0, indicat-ing an adequate thermal maturity for generation of C15+n-alkanes5,6 but not for their subsequent depletion bythermal cracking.45

When a sample of C15+-n-alkane-depleted FruitlandFormation coal (see Figure 7A) from a second site (0.78%R0; 77.5% vitrinite, 10.3% liptinite, and 12.2% inertinite)was treated by mild catalytic hydrogenation16 andconsequently depolymerized, the resultant aliphaticfraction Soxhlet extract chromatogram (see Figure 7B)exhibits the prominent molecular signature of a ho-mologous series of n-alkanes up to n-C32. This demon-stration of C15+ n-alkane generation potential undermild catalytic hydrogenation conditions is significantfrom a geologic interpretation perspective, since itsuggests that the indigenous mineral matter in the coalhad an inherently low catalytic activity for generatingthese n-alkanes.The rate of a catalytic reaction is dependent upon

several critical parameters including the concentrationof the reactants, the activity and concentration of thecatalyst, and the accessibility of the reactants to thecatalyst’s active sites.46 In coal beds, the mineral-matter content and composition are not uniform butvary both vertically and laterally as a function of suchgeologic variables as the mineral-matter grain size, coalthermal maturity and maceral composition, pore waterchemistry, seam thickness, and the nature of thebounding rock lithologies.34

The catalytic activity of clays and other minerals ishighly dependent upon their ionic form and the amountof water that is present.30-33,46-49 The catalytic activityof clays for defunctionalizing and cracking n-alkanoic

(35) Buchanan, D. H.; Osborne, K. R.; Warfel, L. C.; Mai, W.; Lucas,D. Energy Fuels 1988, 2, 163-170.

(36) Lo, H. B.; Cardott, B. J. Org. Geochem. 1995, 22, 73-83.(37) Thompson, K. F. M. Org. Geochem. 1987, 11, 573-590.(38) Lafargue, E.; Barker, C. Am. Assoc. Pet. Geol. Bull. 1988, 72,

263-276.(39) Mackenzie, A. S.; Leythaeuser, D.; Muller, P.; Quigley, T. M.;

Radke, M. Nature 1988, 331, 63-65.(40) Price, L. C.; Wenger, L. M.; Ging, T.; Blount, C. W. Org.

Geochem. 1983, 4, 201-221.(41) Rueter, P.; Rabus, R.; Wilkes, H.; Aeckersberg, F.; Rainey, F.

A.; Jannasch, H. W.; Widdel, F. Nature 1994, 372, 455-458.(42) Bailey, N. J. L.; Jobson, A. M.; Rogers, M. A. Chem. Geol. 1973,

11, 203-221.(43) Deroo, G.; Tissot, B.; McCrossan, R. G.; Der, F. Can. Soc. Pet.

Geol., Mem. 1974, 3, 148-167.(44) James, A. T.; Burns, B. J. Am. Assoc. Pet. Geol. Bull. 1984, 68,

957-960.

(45) Mango, F. D. Nature 1991, 352, 146-148.(46) Goldstein, T. P. Am. Assoc. Pet. Geol. Bull. 1983, 67, 152-159.(47) Tannenbaum, E.; Kaplan, I. R. Nature 1985, 317, 708-709.(48) Tannenbaum, E.; Kaplan, I. R.Geochim. Cosmochim. Acta 1985,

49, 2589-2604.(49) Mango, F. D.; Hightower, J. W.; James, A. T. Nature 1994, 368,

536-538.

Figure 7. Gas chromatograms of Hamilton No. 3 well (BasalFruitland) coal. (A) Soxhlet tetrahydrofuran extract, aliphaticfraction. Chromatographic conditions are as in Figure 6A. (B)Soxhlet tetrahydrofuran extract of depolymerized coal, ali-phatic fraction. Chromatographic conditions are the follow-ing: SE-54-coated fused-silica column, temperature program-ming from 80 to 300 °C at 3 °C min-1.

Significance of Variation in Coals Energy & Fuels, Vol. 12, No. 2, 1998 281

acids decreases as a function of reaction time, and alarge portion of the n-alkanoic acid is converted to akerogen-like material that forms a coating on thesurface of the clay particles.31,32 This suggests that theobserved time-dependent loss of the clay’s catalyticactivity is due to the loss of active-site accessibility.These reactivity trends are significant from a geologicinterpretation perspective, since they suggest that theamount and compositional traits of petroleum-likehydrocarbons generated by the bulk organic matterthroughout a coal bed will differ depending, at least inpart, upon the concentration and catalytic activity of themineral matter with which it is in direct physicalcontact.C1-5 Alkanes. According to traditional petroleum-

geology theory, coals are self-sourced reservoirs fornatural gas formed by temperature-controlled thermoly-sis of the bulk coal organic matter.5,6 The methanerichness (i.e., C1/∑C1-5 ratio) of the natural gas producedfrom Fruitland Formation coal seam reservoirs variesirregularly as a function of the coal thermal maturity(C1/∑C1-5 ) 0.871-0.999).7 The cause of this irregularC1/∑C1-5 ratio variability is not well understood. It hasbeen hypothesized that this variability may be due topostgeneration biodegradation.11 This interpretation isbased, in part, on the assumption that this irregularC1/∑C1-5 ratio variability is a compositional trait anomalyand that the methane richness of natural gas formedduring coal maturation should covary in a unidirectionalway with variation in the thermal maturity of the coal.11The methane-richness values of the natural gas

produced from Fruitland Formation coal seam reser-voirs appear to naturally segregate into three groupsdepending upon the thermal maturity of the coal. Forvitrinite reflectance values between ∼0.50 and ∼0.70%R0, the C1/∑C1-5 ratios range from 0.871 to 0.929,whereas for vitrinite reflectance values of either <0.50%R0 or >0.70% R0, the C1/∑C1-5 ratios range from 0.983to 0.999.7 These C1/∑C1-5 ratio-variation trends closelymimic the trends exhibited by the hydrocarbon gasesgenerated during closed-system pyrolysis of dry peatshown in Figure 8.50 The hydrocarbon gases generatedduring peat pyrolysis initially exhibit a pronouncedmaximum in methane richness (i.e., C1/∑C1-5 ) 0.991at 0.40% R0). Then there is a pronounced minimumover the thermal-maturity interval ∼0.50 to 0.70% R0,which is then followed by a unidirectional increase withprogressive thermal maturity.

The pronounced minimum in methane richness or,alternatively, maximum in C2-5 alkane richness exhib-ited by the hydrocarbon gases generated during peatpyrolysis (see Figure 8) occurs over the same thermalmaturity interval (i.e., 0.50-0.72% R0) that the C6+hydrocarbon-content values of the suite of coals studiedhere exhibit their pronounced maximum (see Figure 2).These data-variation similarities strongly suggest acommon explanation, i.e., that C2-5 alkanes and C6+hydrocarbons are formed concurrently by similar overallreaction processes during the thermal maturation ofcoal. Clearly, it is not necessary to invoke the occur-rence of postgeneration biodegradation to explain theirregular C1/∑C1-5 ratio-variation trends in the naturalgas produced from Fruitland Formation coal seamreservoirs.Pyrolysis studies suggest that mineral catalysis should

exert a significant effect upon the amount and compo-sitional traits of the C1-5 alkanes generated during coalmaturation. The pyrolysis of raw coal and peat gener-ates more hydrocarbon gas than demineralizedcoal,23,50-52 and mineral catalysis affects both the quan-tity and compositional traits of the C1-5 alkanes gener-ated during pyrolysis of n-alkanoic acids,30 kerogen,47,48and hydrocarbons.49,53 Water is also an importantreaction variable that affects the mineral catalyst’sactivity and selectivity for methane and branched-chainC4-5 alkane genesis.30,47-49

When behenic acid was heated for 89 h at 200 °C inthe presence of bentonite clay and water, the C4-5alkane products had branched-to-normal-chain ratios ofabout 0.1 whereas in the absence of water the C4-5alkane products had branched-to-normal-chain ratios of3.5-4.0.30 Closed-system dry pyrolysis (100-1000 h at300 °C) of demineralized subbituminous coal gave C4-5alkanes having branched-to-normal-chain ratios of 0.61-0.73.23 The C4-5 alkanes in natural gas produced fromFruitland Formation coal seam reservoirs exhibit con-siderable branched-to-normal-chain ratio variability:i-C4/n-C4 ) 0.50-1.74, and i-C5/n-C5 ) 0.08-2.00.7 Therelatively high values (i.e., >1.0) of the C4-5 alkanebranched-to-normal-chain ratios in some of these res-ervoir gas samples could be the result, at least in part,of clay catalyst selectivity effects.Methane δ13C Values. The stable carbon isotope

values of the methane in natural gas vary greatlydepending, in part, upon whether the methane wasformed by biogenic or thermogenic reactions.5,7 Meth-ane δ13C values lighter (i.e., more negative) than -55%relative to the carbon isotopic standard Peedee Belem-nite (PDB) are commonly regarded as being derivedfrom biogenic reactions, whereas more positive valuesare regarded as being derived from thermogenic reac-tions.5,7 The stable carbon isotope values for themethane in natural gas produced from Fruitland For-mation coal seam reservoirs (δ13C1 ) -46.6 to -40.5%vs PDB)7,11 vary somewhat irregularly as a function ofcoal maturation, which has been interpreted as beingdue, at least in part, to bacterial modification of themethane isotopic signature through the formation and

(50) Rohrback, B. G.; Peters, K. E.; Kaplan, I. R. Am. Assoc. Pet.Geol. Bull. 1984, 68, 961-970.

(51) Saxby, J. D.; Bennett, A. J. R.; Corcoran, J. F.; Lambert, D. E.;Riley, K. W. Org. Geochem. 1986, 9, 69-81.

(52) Tang, Y.; Jenden, P. D.; Nigrini, A.; Teerman, S. C. Energy Fuels1996, 10, 659-671.

(53) Mango, F. D. Org. Geochem. 1996, 24, 977-984.

Figure 8. Variation of methane richness (C1/∑C1-5) of ther-mogenic natural gas generated by closed-system pyrolysis ofdry peat (data from ref 50).

282 Energy & Fuels, Vol. 12, No. 2, 1998 Nelson et al.

mixing of isotopically light biogenic methane withisotopically heavier thermogenic methane.11 This in-terpretation is based, in part, on the assumption thatthe observed δ13C1-value irregularity is a compositional-trait anomaly and that thermogenic δ13C1 values shouldcovary in a unidirectional way with the thermal matu-rity of the coal.11Peat is the organic matter precursor of humic coals.6

The thermogenic methane formed by closed-systempyrolysis of dry peat exhibits a bimodal stable carbonisotope evolution profile (see Figure 9).50 Under low-thermal-stress conditions (5000 h at 100 °C) the ther-mogenic methane is isotopically light (δ13C1 ) -68% vsPDB) but becomes progressively heavier (more positiveδ13C1 values) with increasing thermal maturation (loweratomic H/C ratios in Figure 9). The isotopic value ofthe initially formed thermogenic methane (δ13C1 )-68% vs PDB) is significant from a geologic interpreta-tion perspective, since it falls within the range of valuescommonly regarded as signifying a biogenic origin. Athigher levels of thermal maturation there is a reversalof the δ13C1-evolution trend due to dilution by isotopi-cally lighter thermogenic methane. A similar bimodalδ13C1-evolution profile is observed during coal pyroly-sis.54 The δ13C1 values for two natural gas samples fromFruitland Formation coal seam reservoirs are plottednext to the bimodal δ13C1-evolution profile for peatshown in Figure 9. This bimodal δ13C1-evolution profileis significant from a geologic interpretation perspective,since it clearly indicates that thermogenic δ13C1 valuesdo not covary in a unidirectional way with progressivecoal maturation. Thus, it is not necessary to invoke theoccurrence of isotopically light biogenic methane forma-

tion and mixing to explain the stable carbon isotopeproperties of the methane in natural gas produced fromFruitland Formation coal seam reservoirs.

Conclusions

The compositional-trait data for the C9-34 n-alkanesin the suite of 14 U.S. coals studied here exhibit nounique component depletion pattern signatures diag-nostic of known types of postgeneration physical, chemi-cal, or microbiological degradation processes that com-monly affect crude oil in sedimentary rocks. Althoughour results question both the occurrence and importanceof postgeneration compositional-trait alteration of n-alkanes in coals, they do not necessarily imply thatthese processes can never occur. However, before thecompositional-trait alteration process occurrence isinvoked, it is important to first determine whether thereis indeed a compositional-trait anomaly to explain. Thereliable detection of such compositional-trait alterationprocess occurrence requires a very careful analysis,which can be greatly facilitated by supercritical CO2extraction and detailed analysis of the C6+ hydrocar-bons.Hydrocarbon evolution during organic-matter matu-

ration in coals and other sedimentary rocks is clearly acomplex process. Attempts to formulate geochemicalmodels for the hydrocarbon-evolution process are com-plicated by the problem of an excessive number ofvariables. The C2-5 alkanes and C6+ hydrocarbonsattain their maximum abundances over the thermalmaturity interval from 0.50 to 0.72% R0, which, in turn,strongly suggests that these two groups of compoundsare formed concurrently by similar overall reactionprocesses during coal maturation. The compositionaltraits of C4-5 alkanes and C9-34 n-alkanes in coalsappear to uniquely mimic those of the alkane productsformed by mineral-catalyzed defunctionalization andcracking of n-alkanoic acids. Thus, mineral catalysismerits consideration as a master variable capable ofintegrating the many diverse factors that can affect theformation and compositional traits of natural gas andC9+ n-alkanes during coal maturation. These mecha-nistic insights should be useful to those seeking toformulate improved geochemical models for predictinghydrocarbon evolution during coal maturation.

Acknowledgment. This research was funded by theGas Research Institute, Contract Nos. 5088-260-1640and 5091-260-2239.

EF970112K(54) Friedrich, H.-U.; Juntgen, H. Adv. Org. Geochem. 1971, 639-

646.

Figure 9. Variation of thermogenic methane stable carbonisotope composition as a function of progressively increasingpeat thermal maturity (data from ref 50).

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