Mechanisms and Kinetics of Reactions Leading to Natural Gas Formation during Coal Maturation

Reviews

Mechanisms and Kinetics of Reactions Leading toNatural Gas Formation during Coal Maturation

Steven J. M. Butala,†,‡ Juan Carlos Medina,† Terrence Q. Taylor,†Calvin H. Bartholomew,† and Milton L. Lee*,†

Department of Chemistry and Biochemistry, and Department of Chemical Engineering,Brigham Young University, Provo, Utah, 84602-5700

Received April 27, 1999. Revised Manuscript Received September 27, 1999

Kinetic data from the literature were used to predict formation rates and product yields of oiland gas at typical low-temperature conditions of coal maturation. These data indicate that gasformation rates from hydrocarbon thermolysis are several orders of magnitude too low to havegenerated known coal-seam natural gas reserves, assuming bulk first-order kinetics defined bya single activation energy and preexponential factor. By assuming distributed activation energies,thermal cracking of liquid hydrocarbons and coal kerogen to methane can occur at sufficientlyhigh rates to produce commercial quantities over long periods of geologic time. Acid-mineral-catalyzed cracking, transition-metal-catalyzed hydrogenolysis of liquid hydrocarbons, and transi-tion-metal-catalyzed CO2 hydrogenation form gas at very high rates at geologic temperatures.Rates of gas production in these reactions are orders of magnitude higher than those predictedfrom thermolysis; moreover, the gaseous products for metal-catalyzed hydrogenolysis of hydro-carbon liquids and for CO2 hydrogenation are nearly the same as those of typical natural coalbedgases, while gases from thermal and catalytic cracking differ from most coalbed gases. Theavailable data are most consistent with a model involving thermal and catalytic cracking ofkerogen to oil followed by iron- and nickel-metal-catalyzed hydrogenolysis of oil to natural gas.In CO2-containing coal gases, natural gas may also be formed by iron-catalyzed CO2 hydrogena-tion.

Introduction

Coal seam reservoirs are important potential sourcesof natural gas. In fact, worldwide resources are esti-mated to be about 3000-12000 trillion cubic feet (TCF),with U.S. reserves alone estimated to be 90-400TCF.1-3 While enormous, relatively little of this resource

is being produced, in good part due to the absence ofreliable gas sweet-spot indicators. Accordingly, thepotential for expanded production is considerable, ifmore reliable geologic markers could be discovered.

It is commonly assumed that oil and hydrocarbongases were formed in coal seams by thermolysis (crack-

* Corresponding author.† Brigham Young University.‡ Current address: Donald P. and Katherine B. Loker Hydrocarbon

Research Institute, University of Southern California, Los Angeles,California, 90089-1661.

(1) Davidson, R. M.; Sloss, L. L.; Clarke, L. B. Coalbed MethaneExtraction; IEA Coal Research: London, 1995; pp 22-29.

(2) Murray, D. K. Coalbed Methane in the USA: Analogues forWorldwide Development in Coalbed Methane and Coal Geology; Gayer,R., Harris, I., Eds.; The Geological Society, Special Publication No. 109,London, 1996; pp 1-12.

(3) Rightmire, C. T. Coalbed Methane Resource in Coalbed MethaneResources of the United States: AAPG Studies in Geology Series # 17;Rightmire, C. T., Eddy, G. E., Kirr, J. N., The American Associationof Petroleum Geologists, Tulsa, OK, 1984; Chapter 1, pp 1-13.

VOLUME 14, NUMBER 2 MARCH/APRIL 2000

© Copyright 2000 American Chemical Society

10.1021/ef990076k CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 02/15/2000

ing) of coal organic matter;4-11 this model is supportedby experiments showing high rates of gas and oilproduction during pyrolysis of sedimentary organicmatter at temperatures of 200-250 °C.12-13 Recentlyhowever, the reliability of the thermogenic model for oil-gas and coal-gas generation has been questioned14-17

because (1) thermolysis of model organic compounds istoo slow to account for the present reserves even overtime periods of hundreds of millions of years,18-20 (2)the hydrocarbon product distribution obtained duringthermolysis of model organic compounds is much dif-ferent than natural gas;15,18,19,21-23 and (3) results ofartificial maturation experiments indicate that claysparticipate in hydrogen exchange of aromatic com-pounds and apparently catalyze hydrocarbon formationfrom sedimentary organic matter.24-25 This suggeststhat mineral catalysis may play a crucial role inhydrocarbon gas formation during coal maturation. Thispaper identifies the key types of molecular transforma-tions that mineral catalysis might impact and assessesthe potential effects of this catalysis on gas generationduring coal maturation.

Method, Justification, and Limitations

Proposed, abiogenic methane formation mechanismsfrom the scientific literature were identified. Rateconstants and activation energies were calculated fromthe available rate data for specified experimental condi-tions. The rate constants were than recalculated tocatagenic temperatures (50-200 °C). The potentialcontribution of each mechanistic route to methaneformation was then evaluated on the basis of kineticsand methane selectivity.

Extrapolation of literature data to geologic conditionsrequired four important assumptions: (1) about 10 wt

% of the carbon in the coal is converted to methaneduring maturation, (2) coal has a 20 wt % kerogencontent of which 80 wt % can be thermally or catalyti-cally converted to liquids and/or gases, (3) coal has atypical porosity of 20%, and (4) one-third of the porevolume is occupied by liquid or vapor hydrocarbons andthe remaining by water vapor and gases.

The first assumption of 10 wt % conversion of hydro-carbons to methane was based on an estimate of atypical gas generation potential of 150 cm3 (STP)g-coal-1 during maturation of a U.S. carboniferouscoal,26 which corresponds to 0.080 g-C g-coal-1 or 0.11mol-CH4 mol-C -1 in the coal. The second assumptionof 20 wt % kerogen content is based on the arithmeticmean of kerogen content determined by us for twoValencia Canyon coals. While Waples27 states that“[c]oal is best thought of as a special kind of kerogenthat is relatively undiluted by mineral matter...”, kero-gen, in the context of this paper, is defined as “the...organic material of sedimentary rocks which is insolublein organic and inorganic solvents”.28 Hutton et al.29 addfurther that this “definition for kerogen has grown toinclude all solid organic matter in all sedimentaryrocks...[including,]...oil shales, coal, clastic sedimentaryrocks, and metamorphic rocks which contain solidorganic matter...” While imprecise and problematic, wehave adopted this definition because the kerogens listedin Tables 2, 4, 5, and 7, under the subheading of“Isolated Type III Kerogens” are best defined in thismanner. These kerogens are the dried, residual materialremaining from coal after an HF/HCl treatment followedby two extractions. These kerogens were included in thedata set so that inferences could be made as to thepresence and absence of the bitumen and mineralmatter. Unfortunately, the residual masses were notreported. Indeed, Caldier et al.30 state that very little“attention has been paid to the quantitative determi-nation of kerogen in source rocks...”

“There are problems [however,] when defining kero-gen as the ‘insoluble organic matter in sedimentaryrocks’. Solubility is...[dependent on] the solvent, and thesolvents used have varied widely”.29 Waples27 furtheradds, “Because this is an operational definition, theexact quantity and chemical composition of kerogen willdepend on many factors, such as the solvent used inextraction, the length of time used for the extraction,and the particle size to which the rock was groundbefore it was extracted. Significant problems can there-fore arise when data from two laboratories are com-pared.” Notwithstanding these problems, we define theisolated coal kerogens on a procedural basis. Specifi-cally, the coal was ground to pass through a 200 meshsieve, then dried in a vacuum oven overnight at room

(4) Philippi, G. T. Geochim. Cosmochim. Acta 1965, 29, 1021-1049.(5) James, A. T. Am. Assoc. Pet. Geol. Bull. 1983, 67, 1176-1191.(6) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence;

Springer: New York, 1978; pp 157, 163, 179, and 183.(7) Kissin, Y. V. Geochim. Cosmochim. Acta 1987, 51, 2445-2457.(8) Takach, N. E.; Barker, C.; Kemp, M. K. Am. Assoc. Pet. Geol.

Bull. 1987, 71, 322-333.(9) Barker, C. Am. Assoc. Pet. Geol. Bull. 1990, 74, 1254-1261.(10) Ungerer, P. Org. Geochem. 1990, 16, 1-25.(11) Hunt, J. M. Org. Geochem. 1991, 17, 673-680.(12) Harwood, R. J. Am. Assoc. Petrol. Geol. Bull. 1977, 61, 2082-

2102.(13) Tannenbaum, E.; Kaplan, I. R. Nature 1985, 317, 708-709.(14) Mango, F. D. Geochim. Cosmochim. Acta 1992, 53, 553-555.(15) Mango, F. D.; Hightower, J. W.; James, A. T. Nature 1994, 368,

536-538.(16) Shock, E. L. Nature 1994, 368, 499-500.(17) Nelson, C. R.; Li, W.; Lazar, I. M.; Larsen, K. H.; Malik, A.;

Lee, M. L. Energy Fuels 1998, 12, 277-283.(18) Jackson, K. J.; Burnham, A. K.; Braun, R. L.; Knauss, K. G.

Org. Geochem. 1995, 23, 941-953.(19) Butala, S. J.; Medina, J. C.; Lee, M. L.; Felt, S. A.; Taylor, T.

Q.; Andrus, D. B.; Bartholomew, C. H.; Yin, P.; Surdham, R. C.Catalytic Effects of Mineral Matter on Natural Gas Formation DuringCoal Maturation; GRI-97/0213, Gas Research Institute, 1997.

(20) Butala, S. J.; Medina, J. C.; Lee, M. L.; Taylor, T. Q.; Andrus,D. B.; Bartholomew, C. H.; Yin, P.; Surdham, R. C. Chemical Indicatorsfor Mineral-Catalyzed Coal Seam Gas Producibility Sweet Spots;Annual Report for 1997, Gas Research Institute, March 5, 1998.

(21) Evans, R. J.; Felbeck, G. T., Jr. Org. Geochem. 1983, 4, 135-144.

(22) Espitalie, J.; Ungerer, P.; Irwin, I.; Marquis, F. Org. Geochem.1988, 13, 893-899.

(23) Horsfield, B.; Schenk, H. J.; Mills, N.; Weite, D. H. Org.Geochem. 1991, 19, 191-204.

(24) Alexander, R.; Kagi, R. I.; Larcher, A. V. Geochim. Cosmochim.Acta 1982, 46, 219-222.

(25) Tannenbaum, E.; Kaplan, I. R. Geochim. Cosmochim. Acta 1985,49, 2589-2604.

(26) Rice, D. D. Composition and Origins of Coalbed Gas. InHydrocarbons from Coal: AAPG studies in geology #38; Law, B. E.,Rice, D. D., Eds.; The American Association of Petroleum Geologists,Tulsa, OK, 1993; Chapter 7, pp 159-184.

(27) Waples, D. Organic Geochemistry for Exploration Geologists;Burgess Publishing Company: Minneapolis, MN, 1981; pp 14, 15, 20,33, 43, 62, 68.

(28) Kvenvolden, K. A. In Geochemistry of Organic Molecules:Benchmark Papers in Geology; Vol. 52, Kvenvolden, K. A., Ed.; Dowden,Hutchinson, & Ross, Inc.: Stroudsburg, PA, 1980; p 75.

(29) Hutton, A.; Bharati, S.; Robl, T. Energy Fuels 1994, 8, 1478-1488.

(30) Caldiero, L.; Chiaramonte, M.; Pellegrin, L.; Rausa, R. Fuel1992, 71, 277-281.

236 Energy & Fuels, Vol. 14, No. 2, 2000 Reviews

temperature, then exhaustively extracted with THF ina Soxhlet apparatus. The coal was then treated with a20% HCl solution for 2 h at room temperature, then a20% HCl/48% HF solution for 3 days at room temper-ature, then again with a 20% HCl solution for 2 h atroom temperature. The coal was then exhaustivelyextracted again with THF in a Soxhlet unit and driedin a vacuum oven overnight at room temperature. Theresidual dry mass was quantified and defined to be thecoal kerogen.

While the kinetic calculations in this study werederived from coal or material thought to be ubiquitousto coal-bearing strata, occasionally we cite other studiesthat utilized Type I and/or II kerogens, especially inclarifying probable oil generation mechanisms. Admit-tedly, these kerogen types are chemically distinct fromType III kerogens. Differences have been noted insolvent swelling,31 thermal cracking,32 and selectivecatalytic chemical degradation33 experiments. However,Radke et al.,34 citing the work of others, point out that“[s]oluble organic matter of a petroleum-like grosscomposition is generated from coals as a byproduct ofthe coalification process during subbituminous and highvolatile rank stages. This process shows similaritieswith the maturation of [marine-based] organic matterin petroleum source beds.”

The third assumption is confirmed by measuredporosities of 15-20% for several U.S. coals of low tomedium rank,35 the largest porosity fraction consistingof macropores having diameters of 30-300 nm.36,37 Thefourth assumption is reasonable since the molecularfraction of medium to low rank coals contains 3-10 wt% moisture and 3-20 wt % occluded hydrocarbons.26

Cook38 believes that the oil and gas generated at lowrank are readily accommodated by the macropores.However, Vahrman39 infers that a significant quantityof additional hydrocarbons may be contained in themicropores and/or as clathrate entities entrapped by amacromolecular moiety or moieties. Levine40 further-more, citing the work of others, states that “...loss ofprimary microporosity in the oil window may be due to‘plugging’ by hydrocarbons generated during coalifica-tion.” While the concept of three-dimensional, intercon-nected pores has been challenged,40,41 Levine commentsthat it is nevertheless “... a useful conceptual model to

describe pores as physical features of the coal which maybe either vacant or filled with a resident pore-fillingsubstance.”

The comparisons in this work of rates and selectivitiesfor methane production include data obtained in bothbatch (closed) and flow (open) systems. There are nofundamental problems in comparing rates for a singlestoichiometric reaction obtained in these two differentkinds of reactors (systems) as long as the comparisonsare made at the same temperatures, reactant partialpressures, and conversions as well as similar reactionand residence times.42 If these conditions are met for anetwork of parallel single reactions, product selectivitiesmay differ, but not substantially so. For example,Schenk and Horsfield43 pyrolyzed Torcian shale samplesin both open and closed systems utilizing heating ratesof 0.1, 0.7, and 5.0 K min-1. They reported “...verysimilar petroleum potential vs activation energy distri-butions [resulted],...[and] that bulk petroleum genera-tion is simulated equally well using either closed- oropened-system pyrolysis...[and that] the predicted tem-perature ranges of oil and gas formation under geologi-cal heating conditions are unaffected by the open orclosed nature of the method used for kinetic modelcalibration.” It was further reported, however, that theclosed system produced lower yields of petroleum rela-tive to the open system, due to either secondary crackingor tar condensation in the analytical equipment. Carefulanalysis of the paper further suggests, however, thatother extraneous confounding factors may also havecontributed to the low yields.

Reasonably good agreements between rates and se-lectivities for olefin hydrogenolysis and CO2 hydrogena-tion in both batch reactors44,45 and flow systems46,47 havealso been reported. However, rate and selectivity dataobtained for a sequence of reactions (i.e., kerogen f oilf gas) are very different when measured in a low-residence-time open system relative to those obtainedin a closed, high-residence-time system. Accordingly,this paper only examines and compares parallel, single-type reactions, (i.e., kerogen f oil, oil f gas, andkerogen f gas).

A study conducted by Monthioux et al.48 is oftenreferenced to illustrate the differences produced byopen- and closed-system pyrolysis. Mahakam delta coalswere subjected to open- and closed-system pyrolysis andthe products compared with CCl4-extracted bitumen. Itwas concluded that the closed-system more closelyapproximated natural bitumen evolution when com-pared with the open-system, as the open-system pro-duced different hydrocarbon distributions and a pre-

(31) Larsen, J. W.; Li, S. Org. Geochem. 1997, 26, 305-309.(32) Behar, F.; Vandenbroucke, M.; Tang, Y.; Marquis, F.; Espitalie,

J. Org. Geochem. 1997, 26, 321-339.(33) Boucher, R. J.; Standen, G.; Eglinton, G. Fuel 1991, 70, 695-

702.(34) Radke, M.; Schaefer, R. G.; Leythaeuser, D.; Teichmuller, M.

Geochim. Cosmochim. Acta 1980, 44, 1787-1800.(35) White, W. E.; Bartholomew, C. H.; Hecker, W. C.; Smith, D.

M. Adsorpt. Sci. Technol. 1990, 7, 180-209.(36) Gan, H.; Nandi, S. P.; Walker, P. L., Jr. Fuel 1972, 51, 272-

277.(37) Parkash, S.; Chakrabartty, S. K. Int. J. Coal Geol. 1986, 6, 55-

70.(38) Cook, A. C. Oil Occurrence, Source Rocks and Generation

History of Some Coal-Bearing Tertiary Basins; Plenary Lecture at theInternational Conference on Coal Seam Gas and Oil, BrisbaneAustralia, March 23-25, 1998, Handbook and Abstracts, p 13.

(39) Vahrman, M. Chem. Br. 1972, 8, 16-24.(40) Levine, J. R. Coalification: The Evolution of Coal as Source

Rock and Reservoir Rock for Oil and Gas in Hydrocarbons from Coal:AAPG Studies in Geology #38; Law, B. E., Rice, D. D., Eds.; TheAmerican Association of Petroleum Geologists, Tulsa, OK, 1993;Chapter 3, pp 39-77.

(41) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9,324-330.

(42) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; JohnWiley and Sons: NY, 1972.

(43) Schenk, H. J.; Horsfield, B. Geochim. Cosmochim. Acta 1993,57, 623-630.

(44) Mango, F. D. Org. Geochem. 1996, 24, 977-984.(45) Bartholomew, C. H.; Butala, S. J.; Medina, J. C.; Lee, M. L.;

Taylor, T. Q.; Andrus, D. B. Mineral-Catalyzed Formation of NaturalGas During Coal Maturation. In Coalbed Methane: Scientific, Envi-ronmental and Economic Evaluation; Mastalerz, M., Glikson, M.,Golding, S. D., Kluwer Academic Publishers: Norwell, MA, 1999; pp279-296.

(46) Weatherbee, G. D.; Bartholomew, C. H. Catalysis 1984, 87,352-362.

(47) Somorjai, G. A. Introduction to Surface Chemistry and Cataly-sis; Wiley: New York, 1994; pp 526-555.

(48) Monthioux, M.; Landais, P.; Durand, B. Org. Geochem. 1985,10, 299-311.

Reviews Energy & Fuels, Vol. 14, No. 2, 2000 237

ponderance of alkenes. However, the heating rate of theopen-system was 25 °C min-1 (no final temperature wasreported), while the closed system was heated to 250,300, 375, 400, 425, and 450 °C in a 24 h time period.Smith et al.49 state that “[p]eak temperature andheating rate are key variables in coal devolatization.”The effect of heating rate has been empirically observed.For example, while rapid heating rates often lead tohigher tar yields, the H/C ratio of the tar is concurrentlydegraded. Thus, the impact of the open or closed natureof the system on bitumen generation in the Monthiouxet al. experiment is masked by differences in heatingrates utilized. The problem is further complicated withTmax not being reported. While recognizing heating ratesand peak temperature to be important, these variables,being outside our control in the published studies, wereignored for pragmatic purposes.

We note that while Price50 believes most systems aremore accurately modeled as closed, Monthioux andLandais51 acknowledge “that a wide range of more orless open maturation systems could be encountered innature.” Specific examples of such systems would in-clude the Hitra formation, Haltenbanken area, offshoreNorway,52 the Saar basin in Germany,53 the San Juanbasin in New Mexico and Colorado,54 the Greater GreenRiver basin in Wyoming,55 and the Talang Akar forma-tion in Java, Indonesia.56 Nevertheless, considerabledebate continues in this regard.57,58

Since the publication of Lewan et al.59 in 1979,demonstrating the generation of pyrolysates similar tonatural crude oil from shales, and that of Lewan60 in1985 claiming geological simulation of petroleum gen-eration via hydrous pyrolysis, considerable debate hasarisen as to if and/or how water impacts geologicalprocesses.61,62 Michels et al.63 point out, however, that“[a] survey of the existing literature...[reveals]...contra-dictions [which] may be related to the nature of theexperimental setups used,” from which it can be inferredthat some results are experiment specific (i.e., artifacts);thus, exact extrapolations to natural systems are notpossible at our current stage of knowledge. Neverthe-less, the preponderance of evidence supports the asser-tion that water is an important medium; however, its

precise role in petroleum and gas generation remainsuncertain.62 While its consideration in the chemicalalteration of organic sedimentary matter is generallybeyond the scope of this study, some important mecha-nistic aspects will be discussed.

Coalbed methane can be formed by either biogenic orthermogenic means. Biogenic methane is formed duringpeatification via microbial activity64 and in coals notmature enough to produce methane thermogenically.65

Thermogenic methane is generated during the laterstages of coalification by the thermal transformation oforganic matter and accounts for the largest amounts ofcoalbed methane.64 Differentiation between biogenic andthermogenic methane is based on the isotopic 13C/12Cratio. The general observed trend is toward isotopicallyheavier natural gas with increased maturity and burialdepth.66

It is generally accepted that isotopically heavy meth-ane (δ13C > -50 %) is thermogenic gas. Methane withδ13C values between -50 and -60 ‰ is both biogenicand thermogenic, and isotopically light methane (δ13C< -60 ‰) is biogenic gas. Figure 1 displays the δ13Cranges of methane from several coal formations.67-71

Thus, as illustrated in the figure, the majority of coalbedmethane is apparently thermogenic gas. This paperdeals soley with thermogenic coalbed methane. Smithet al.49 state that “the main catagenic coalification[factor] of...organic peat is likely...[to be] heat.” Levine40

states that “[f]or the most part, the degree of coalifica-tion depends on the geothermal history, i.e., on the rocktemperature, especially the maximum temperature, andto a lesser degree on the duration of heat exposure.”Such a time/temperature model implies that the coali-fication process is kinetically rate-limited and is notequilibrium driven.40 While there is considerable debateas to the accuracy of this model, it is widely utilized inapproximating real basin processes.40 The validity ofthis model is a principal focus of this study.

Results

Our survey of the literature uncovered relevant ratedata for the formation of methane and other lighthydrocarbons and, in some cases, liquid hydrocarbonsfrom (1) thermal cracking of model liquid hydrocarbons,model polymers, paraffinic oil, and aromatic oil; (2) low-temperature pyrolysis of various coal and coal kerogens;(3) acid-mineral cracking of model liquid hydrocarbons;(4) hydrogenolysis of 1-octadecene on a sedimentary rockcontaining transition-metal compounds; (5) hydrogenol-ysis of C3-C6 hydrocarbons on supported iron and nickelmetal catalysts; (6) CO2 hydrogenation on supportediron and nickel metal catalysts; and (7) steam reformingof hydrocarbons on nickel catalysts. From these data,

(49) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire, R. J. TheStructure and Reaction Processes of Coal; Plenum Press: New York,1994; pp 33, 47, 55, 84, 214.

(50) Price, L. C. Geochim. Cosmochim. Acta 1993, 57, 3261-3280.(51) Monthioux, M.; Landais, P. Fuel 1987, 66, 1703-1708.(52) Hvoslef, S.; Larter, S. R.; Leythaeuser, D. Org. Geochem. 1988,

13, 525-536.(53) Pickel, W.; Gotz, G. K. E. Org. Geochem. 1991, 17, 695-704.(54) Clayton, J. L.; Rice, D. D.; Michael, G. E. Org. Geochem. 1991,

17, 735-742.(55) Garcıa-Gonzalez, M.; MacGowan, D. B.; Surdam, R. C. Mech-

anisms of Petroleum Generation from Coal, as Evidenced from Petro-graphic and Geochemical Studies: Examples from Almond FormationCoals in the Greater Green River Basin in Fiftieth Anniversary FieldConference, Wyoming Geological Association Guidebook; 1993, pp 311-323.

(56) Noble, R. A.; Wu, C. H.; Atkinson, C. D. Org. Geochem. 1991,17, 363-374.

(57) Burnham, A. K. Geochim. Cosmochim. Acta 1998, 62, 2207-2210.

(58) Lewan, M. D. Geochim. Cosmochim. Acta 1998, 62, 2211-2216(59) Lewan, M. D.; Winters, J. C.; McDonald, J. H. Science 1979,

203, 897-899.(60) Lewan, M. D. Philos. Trans. R. Soc. London A 1985, 315, 123-

134.(61) Mansuy, L.; Landais, P. Energy Fuels 1995, 9, 809-821.(62) Lewan, M. D. Geochim. Cosmochim. Acta 1997, 61, 3691-3723.(63) Michels, R.; Landais, P.; Torkelson, B. E.; Philp, R. P. Geochim.

Cosmochim. Acta 1995, 59, 1589-1604.

(64) Rogers, R. E. Coalbed Methane: Principles & Practice; PTRPrentice Hall: Englewood Cliffs, NJ, 1994; pp 16, 65.

(65) Rice, D. D.; Claypool, G. E. Am. Assoc. Pet. Geol. Bull. 1981,66, 5-25.

(66) Rice, D. D.; Clayton, J. L.; Pawlewicz, M. J. Int. J. Coal Geol.1989, 13, 597-626.

(67) Schoell, M. Geochim. Cosmochim. Acta 1980, 44, 649-661.(68) Jenden, P. D.; Newell, K. D.; Kaplan, I. R.; Watney, W. L. Chem.

Geol. 1988, 71, 117-147.(69) Fuex, A. N. J. Geochem. Explor. 1977, 7, 155-188.(70) Stahl, W. H.; Carey, B. D., Jr. Chem. Geol. 1975, 16, 257-267.(71) Rice, D. D.; Flores, R. M. Am. Assoc. Pet. Geol. Bull. 1990, 74,

1343-1343.


low-temperature rates of methane production for ther-mal and catalytic reactions and activation energies forconditions applicable to coal beds were calculated to theextent allowed by the published information.

Representative methane formation rates at 180 °C,estimated times for reaching 10% conversion, andactivation energies for the relevant thermal and cata-lytic reactions are summarized in Table 1. Conversiontimes of 50% for catalytic reactions and thermal decom-position of n-hexane to methane are shown in Figure 2.Data for steam reforming are not included since meth-

ane formation rates were estimated to be negligible attemperatures below 300 °C.

It is evident that, under these low temperatureconditions, thermal cracking of mineral-free aromaticand paraffinic oils to methane occurs at negligibly slowrates relative to those for acid catalytic cracking orcatalytic hydrogenolysis. Indeed, times for 10% conver-sion of model compounds or paraffinic oil by thermolysisat 180 °C are on the order of tens to hundreds of millionsof years. On the other hand, acid-mineral-catalyzedcracking of n-C16, Fe- or Ni-catalyzed hydrogenolysis of

Figure 1. δ13C1 (‰) for various basins. Data from refs 67-71.

Table 1. Summary of Calculated Methane Formation Rates from Literature Data for Thermal and Catalytic Reactionsunder Geologic Conditions (180 °C)a

reactionrate, 180 °C

(gmeth/gcoal y)a t (10% conv.)bEact

(kcal/mol) ref

Thermalthermal cracking, n-C6 1.8 × 10-12 9.2 × 107 y 69 72thermal cracking, n-C16H34 1.1 × 10-12 2.0 × 107 y 60 73thermal cracking, aromatic oil 1.3 × 10-10 8.1 × 105 y 65 74thermal cracking, paraffinic oil 7.9 × 10-13 1.3 × 108 y 69 74

Catalyticacid mineral cat. cracking, C16 to C1 2.1 × 10-6 10 y 35 73sedim. rock hydrogenol., C18

)c 3.7 × 10-5 146 d 31 15hydrogenolysis n-C5, Ni 5.6 × 10-4 91 d 31 47hydrogenolysis C18

), Ni 1.8 × 10-4 29 d 50 44hydrogenolysis n-C5, Fe 1.8 × 10-4 285 d 25 47hydrogenolysis C18

), Fe 1.8 × 10-5 299 d 44hydrogenolysis C12,)d Fed 1.7 × 10-5 9.6 y 45hydrogenolysis C12

), Fee 2.9 × 10-5 5.6 y 45CO2 hydrogenation, Ni 5.4 × 10-1 2 h 19 75CO2 hydrogenation, Fe 1.1 × 10-1 8 h 15 46CO2 hydrogenation, Fe 1.3 × 10-2 36 h 17 45

a Units of g-methane per g-coal or source rock per year; assumes 20% porosity and that pores are one-third filled with HC vapor (orliquid) or 1% CO2 and 4% H2 (remainder is water); coal is assumed to contain 100 ppm of surface Ni or Fe; reaction is assumed to befirst-order. b y ) years, d ) days, h ) hours. c C18

) ) 1-octadecene; C12) ) 1-dodecene. d Excess liquid 1-dodecene. e H2/1-dodecene ) 5.


C3-C18 hydrocarbons, and Fe- or Ni-catalyzed CO2hydrogenation occur at rates that are 5-12 orders ofmagnitude higher. For example, assuming only 100 ppmof Fe metal, 10% conversion of n-pentane to methaneby hydrogenolysis occurs in about 91 days and 10%conversion of CO2 to methane occurs in only 8-36 h (180°C). In other words, production of methane in coal bedsby Fe- (or Ni-) catalyzed hydrogenolysis of liquid hy-drocarbons or CO2 methanation is a likely scenario,while thermolysis of liquid hydrocarbons is unlikely tocontribute measurably to coal-bed methane, even overlong periods of geologic time at relatively high coal-bedtemperatures, e.g., 150-190 °C.76

Nevertheless, as illustrated in Table 2, pyrolysis oflignite or isolated kerogen to methane occurs at geologi-cally significant rates at 180 °C, (i.e., 10% conversionas low as 1500 y). It is interesting to note that theisolated kerogens show conversion times nearly identicalto those of mineral-containing coals. These results arein agreement with Reynolds and Burnham80 who,comparing pyrolysis kinetics and oil generation curvesof shales with their corresponding isolated kerogens,found that “kerogen isolation does little to affect...kineticparameters.” It has been argued however, that thisobservation may be explained by metal catalysis in viewof the difficulty of completely removing metallic miner-als, e.g., organometallic complexes or suspended, col-loidal Fe/SiO2 or Fe/Al2O3.45 In other words, the resultsof Reynolds and Burnham80 implicitly assume that thekerogen concentrates obtained were mineral-free (ornear mineral-free), and thus no minerals were present(in significant quantities) to catalyze kerogen degrada-tion reactions. However, Bartholomew et al.45 speculatethat the isolated kerogen could still contain significant

quantities of mineral matter to catalyze thermal deg-radation reactions. There is evidence to lend credenceto this viewpoint. For example, Waples27 states thatisolation “...procedures produce a kerogen concentratewhich also contains other resistant minerals...[notamenable to chemical removal].” Berkel and Filby81 alsostate that irrespective “...of the technique used, thekerogen can never be isolated mineral-free.” However,both Waples and Berkel and Filby add further that themajority of the remaining minerals is pyrite. Whilepyrite is expected to be more or less catalyticallyinactive, Berkel and Filby point out that the pyrite maystill contain significant quantities of other elements,including Ni.

While the speculative explanation of Bartholomew etal. has credibility, it is complicated by two issues. First,while it appears that minerals were present in theisolated shale kerogen concentrates, it is not known ifthey were present in significant quantities to catalyzethe formation of oil. Second, the argument does notconsider the molecular structure of the shale kerogenitself, which would also impact pyrolysis behavior.82

Thus, even if, by analogy, minerals were present insignificant quantities in the whole coals listed in Table2 to catalyze methane formation reactions, this effectcould be masked by the molecular structure of the coalkerogen.

Table 3 lists the kinetic parameters for the pyrolysisof model polymers of coal kerogen83-86 and the timerequired to produce threshold commercial reserves (300scf ton-1, 9 cm3 g-coal-1)77 of methane gas (Note thatthis minimum threshold amount is about 15 times lowerthan the typical amount of methane production expectedduring catagenesis.26). Activation energies range from49 to 62 kcal mol-1, while production times range fromhundreds to billions of years, which not only indicatesthat molecular structure heavily influences rate, butalso suggests that direct conversion of kerogen tonatural gas can occur independent of mineral catalysis.We emphasize however, that no one compound ad-equately models Type III kerogen. Polymers are inher-ently homogeneous while coals are hetergeneous.87

Thus, it is difficult to make inferences regarding coalkerogen on the basis of the data for any single polymer.

While literature data rule out any practical contribu-tion of liquid hydrocarbon thermolysis to methane atnormal coalbed temperatures and depths, the data dosuggest the possible contribution of kerogen thermolysisto methane, albeit ambiguously. The data, however, alsostrongly establish the possibility of thermal decomposi-tion of Type III kerogens to liquid hydrocarbons (Table

(72) Domine, F. Org. Geochem. 1991, 17, 619-634.(73) Goldstein, T. P. Am. Assoc. Pet. Geol. Bull. 1983, 67, 152-159.(74) Ungerer, P.; Behar, F.; Villalaba, M.; Heum, O. R.; Audiber, A.

Org. Geochem. 1987, 13, 857-868.(75) Weatherbee, G. D.; Bartholomew, C. H. Catalysis 1981, 68, 67-

76.(76) Quigley, T. M.; Mackenzie, A. S. Nature 1988, 333, 549-552.(77) Tang, Y.; Jenden, P. D.; Nigrini, A.; Teerman, S. C. Energy Fuels

1996, 10, 659-671.(78) Burnham, A. K.; Oh, M. S.; Crawford, R. W.; Samoun, A. M.

Energy Fuels 1989, 3, 43-55.(79) Lu, S.; Kaplan, I. R. Am. Assoc. Pet. Geol. Bull. 1990, 74, 163-

173.(80) Reynolds, J. G.; Burnham, A. K. Org. Geochem. 1995, 23, 11-

19.

(81) Berkel, G. J. V.; Filby, R. H. Determination of the Mineral-Free Nickel and Vanadium Contents of Green River Oil Shale Kerogen.In Geochemical Biomarkers; Yen, T. F.; Moldowan, J. M., HarwoodAcademic Publishers: Switzerland, 1988; pp 89-114.

(82) Tegelaar, E. W.; Noble, R. A. Org. Geochem. 1994, 22, 543-574.

(83) Solomon, P. R. Synthesis and Study of Polymer Models Repre-sentative of Coal Structure; Gas Research Institute Annual Report forContract No. 5081-260-0582, April 1983.

(84) Behar, F.; Vandenbroucke, M. Org. Geochem. 1987, 11, 15-24.

(85) Mann, A. L.; Patience, R. L.; Poplett, L. J. F. Geochim.Cosmochim. Acta 1991, 55, 2259-2268.

(86) Squire, K. R.; Solomon, P. R.; Carangelo, R. M.; DiTaranto, M.B. Fuel 1986, 65, 833-843.

(87) Powell, T. G.; Boreham, C. J.; Smyth, M.; Cook, A. C. Org.Geochem. 1991, 17, 375-394.

Figure 2. Times for 50% conversion of hydrocarbons or CO2

to methane as a function of temperature for thermolysis ofn-C5, acid-catalyzed cracking of n-C16, hydrogenolysis of n-C6,and CO2 hydrogenation.


4). Indeed, it can be seen from the oil/gas ratios thatliquid hydrocarbons are the preferred product for bothmodel polymers and isolated kerogens. This appears to

be especially true for the kerogens in the early matura-tion stages (%R0 e 0.45) where all oil/gas ratios aregreater than 100. There are distinguishing features in

Table 2. Summary of Calculated Methane Formation Rates from Literature Data for Coals and Isolated Kerogen underGeologic Conditions (180 °C)

coal/kerogenrate, 180 °C(gmeth/gcoal y)

t (10% conv.)(y)a

Eact(kcal/mol) ref

Coalsb

ND lignitec 9.8 × 10-10 3.8 × 105 54 77Upper Freeportd 3.2 × 10-9 5.0 × 106 63 78Wyodakd 2.6 × 10-8 6.0 × 105 63 78Illinois No. 6d 3.1 × 10-6 5.2 × 103 50 78Pittsburgh No. 8d 2.5 × 10-6 6.3 × 103 49 78Pocahontas No. 3d 8.7 × 10-8 1.8 × 105 53 78Blind Canyond 1.1 × 10-5 1.5 × 103 47 78Lewiston-Stocktond 1.9 × 10-6 8.5 × 103 50 78Beulah-Zapd 4.6 × 10-7 3.4 × 104 55 78

Isolated Type III Kerogense

Gippsland Eocene2 h 3.3 × 10-6 2.6 × 104 55 7910 h 8.9 × 10-7 9.4 × 104

100 h 2.9 × 10-7 2.9 × 105

1000 h 7.2 × 10-8 1.2 × 106

Rocky Mountain2 h 2.8 × 10-6 3.0 × 104 55 7910 h 1.0 × 10-6 8.3 × 104

100 h 3.0 × 10-7 2.8 × 105

1000 h 6.4 × 10-8 1.3 × 106

Wilcox2 h 3.1 × 10-6 2.7 × 104 55 7910 h 7.3 × 10-7 1.1 × 105

100 h 1.6 × 10-7 5.2 × 105

1000 h 3.8 × 10-8 2.2 × 106

a y ) years. b Mineral-containing coals; mean activation energies from energy distributions were utilized. c North Dakota lignite Tertiarycoal. d Argonne National Laboratory Premium coal. e Rate extrapolated from 300 to 180 °C using Eact) 55 kcal/mol from whichpreexponential factors were determined; data correspond to 2, 10, 100, and 1000 h pyrolysis time; rates changed during the experiment.

Table 3. Times for Kerogen Model Polymers Conversion to 300 scf/ton at 180 °Ca

model compoundEact

(kcal/mol)rateb

(gmeth/gcoal y)time (y)c

(300 scf ton-1)

poly(p-xylene) 62 8.8 × 10-13 7.6 × 109

poly(1,3-dimethylenenaphthalene) 58 9.0 × 10-10 7.5 × 106

poly(1,4-dimethylenenaphthalene) 56 2.2 × 10-9 3.0 × 106

poly(dimethylenedurene) 49 9.8 × 10-6 6.9 × 102

a Data from ref 86; gas composition was not reported. b Assumes gas is 100% CH4. c y ) years.

Table 4. Ratios of Oil-Production Rate vs Gas-Production Rate of Kerogen Model Polymers and Isolated Kerogens at180 °Ca

reaction substrateEact

(kcal/mol)rate, 180 °C(goil /gcoal y)

rate, 180 °C(gmeth/gcoal y)

ratio(goil /gmeth)

Kerogen Model Polymerspoly(p-xylene) 62 9.8 × 10-11 8.8 × 10-13 110poly(1,3-dimethylenenaphthalene) 58 8.8 × 10-9 9.0 × 10-10 10poly(1,4-dimethylenenaphthalene) 56 5.1 × 10-8 2.2 × 10-9 23poly(dimethylenedurene) 49 7.6 × 10-5 9.8 × 10-6 8

Isolated Type III KerogensGippsland Eocene

2 h 55 5.8 × 10-4 3.3 × 10-6 18010 h 3.7 × 10-5 8.9 × 10-7 42100 h 2.2 × 10-6 2.9 × 10-7 81000 h 6.7 × 10-8 7.2 × 10-8 1

Rocky Mountain2 h 55 3.2 × 10-4 2.8 × 10-6 11010 h 7.1 × 10-6 1.0 × 10-6 7100 h 1.1 × 10-6 3.0 × 10-7 41000 h 7.9 × 10-8 6.4 × 10-8 1

Wilcox2 h 55 3.7 × 10-4 3.1 × 10-6 12010 h 4.0 × 10-5 7.3 × 10-7 55100 h 5.9 × 10-6 1.6 × 10-7 371000 h 4.4 × 10-7 3.8 × 10-8 12

a Data from refs 79,86; gas composition was assumed to be 100% CH4.


the data. First, the more condensed the model polymeris, the less is its oil/gas ratio. This would suggest thatthe more condensed a Type III kerogen becomes, themore gas-prone it is likely to be, and indeed isolatedType III kerogen data indicate that with increasingmaturity, there is a commensurate decrease in the oil/gas production ratio. Specifically, as coal kerogen ma-tures, it becomes increasingly condensed in structureand increasingly gas-prone. However, with increasingcondensation there is also a commensurate decrease in10% conversion times for the production of both oil andgas.

While the Wilcox coal kerogen predominately pro-duces liquid hydrocarbons throughout all maturationstages of the experiment, the Gippsland and RockyMountain coal kerogens produce both methane andliquid hydrocarbons on a near 1:1 mass basis when thecoal is most mature. Reported %R0 values for theGippsland and Rocky Mountain coal kerogens after 1000h pyrolysis time were 0.75 and 1.2, respectively. Thesewould correspond approximately to the kerogens at-taining high and medium volatile bituminous ranks.40

However, Levine40 cautions that “there is...considerablenatural variability in humanite/vitrinite reflectance thatis due to factors other than thermal history...[and]...stratigraphic variations in reflectance do not neces-sarily imply differences in rank per se, but might revealdifferences in...[organic matter]...type or [an] earlymaturation history.” Irrespective of what the rank mightbe, it is evident from Table 4 that sufficient gasgeneration from the thermal decomposition of kerogendoes not occur until it has reached a relatively high levelof maturity. However, at that stage the production rateof hydrocarbons is low and the resulting input ofadditional oil and gas would appear to be small.Nevertheless, in a geological time frame the input couldbe significant.

Table 5 lists selected thermolytic and catalytic crack-ing reactions for the formation of oil and methane gas.Thermal cracking reactions of liquid hydrocarbons tomethane are characterized by relatively high activationenergies of 60-69 kcal/mol (see Tables 1 and 5); thisobservation is consistent with what one might predictfor thermal scission of C-C bonds. On the other hand,activation energies for catalytic cracking and synthesesreactions are generally substantially lower (15-50 kcal/mol), as one might expect for catalytic C-C or C-Obond breaking. The consequence of these lower activa-tion energies are substantially smaller conversion times.Thermal cracking of kerogen to oil, however, involvesactivation energies of intermediate ranges leading tovery significant geological rates (see Table 5). Forexample, 10% conversion times for the thermal crackingof coal kerogens of an immature rank to C12+ liquidsare as small as 150 years at 180 °C. Accordingly, thesedata indicate that thermolytic conversion of kerogen tooil, followed by transition-metal catalyzed reactions togas is a viable scenario. Pelet et al.88 allude to such asequence, concluding that the thermal degradationsequence of kerogen may be more appropriately modeledas kerogen f asphaltenes f resins f heavy hydrocar-bons f light hydrocarbons; nevertheless, they stateexplicitly that resins and asphaltenes “are not obligatoryintermediates...[but that]...[h]ydrocarbons can be pro-duced directly from kerogen. [T]he relative importanceof [the] asphaltenes and resins can be different indifferent situations.”

While our models have assumed bulk first-orderkinetics defined by single preexponential factors andactivation energies, “the highly disordered structure ofkerogen [results in] each individual bond [having] its

(88) Pelet, R.; Behar, F.; Monin, J. C. Org. Geochem. 1986, 10, 481-498.

Table 5. Comparison of Oil-to-Gas and Kerogen-to-Oil Rates for Thermolysis versus Catalysis under GeologicConditions (180 °C)

reaction rate,a 180 °C t (10% conv.) (y)bEact

(kcal/mol) ref

Thermalthermal cracking, n-C6 to C1 1.8 × 10-12 9.2 × 107 69 72thermal cracking, n-C16 to C1 1.1 × 10-12 2.0 × 107 60 73thermal cracking, aromatic oil 1.3 × 10-10 8.1 × 105 65 74

Type III Coal Kerogens to C12+c

Gippsland Eocene2 h 5.8 × 10-4 1.5 × 102 55 7910 h 3.7 × 10-5 2.2 × 103

100 h 2.2 × 10-6 3.8 × 104

1000 h 6.7 × 10-8 1.3 × 106

Rocky Mountain2 h 3.2 × 10-4 2.6 × 102 55 7910 h 7.1 × 10-6 1.2 × 103

100 h 1.1 × 10-6 7.8 × 104

1000 h 7.9 × 10-8 1.2 × 106

Wilcox2 h 3.7 × 10-4 2.3 × 102 55 7910 h 4.0 × 10-5 2.1 × 103

100 h 5.9 × 10-6 1.4 × 104

1000 h 4.4 × 10-8 1.9 × 106

Catalyticacid mineral cat. cracking, C16 to C1 2.1 × 10-6 10 35 73hydrogenolysis C18

) to C1, Fe 1.1 × 10-5 0.8 50 44CO2 hydrogenation, Fe 1.3 × 10-2 4.1 × 10-3 17 45

a Rate has units of grams-methane or grams-oil per gram-coal or source rock per year; assume 20% porosity and that pores are one-third-filled with HC vapor and 50% H2O; 700 ppm H2; 100 ppm surface Fe; reaction assumed to be first-order. b y ) years. c Rateextrapolated from 300 to 180 °C using Eact) 55 kcal/mol.


own environment,...[leading to]...a final continuum ofbond energies.”89 Accordingly, we have calculated 10%conversion times assuming a Gaussian distribution ofactivation energies utilizing the function

where Clk is the fraction of thermally labile kerogenremaining at time t. Incorporated within this model isa simple linear geothermal temperature gradient, T(t)) T0 + Rt, where T0 denotes the initial temperature andR denotes the temperature increase rate. Quigley andMackenzie76 demarcated temperature ranges for oil andgas genesis as “oil (and gas) formation from labilekerogen breakdown, 100-150 °C; gas formation fromrefractory kerogen breakdown, 150-220 °C; oil-to-gascracking, 150-190 °C.” Accordingly, T0 values of 100and 150 °C were assigned for oil and gas generation,respectively. Conservative heating rates have beenestimated to be 1-10 °C megayears-1.90 Thus, R wasassigned a value of 9 °C megayears-1 for our computa-tions. The limits of integration, Ef and E0, were chosento include g 99.9% of the area under the Gaussiancurve. The mean values chosen were those listed inTable 1. Standard deviation values (σ) were chosen tobe 4 and 6% of the mean value according to actualobserved values of Burnham et al.78 The equations weresolved numerically.

Table 6 lists the recalculated 10% conversion timesfor methane production from the major reactions previ-ously examined, but now assuming normally distributedactivated energies. Three of the four thermal crackingreactions have 10% conversion times about an order ofmagnitude less than when calculated with a singleactivation energy under isothermal conditions (see Table1). The aromatic oil cracking time, however, is increasedby a factor of about 2.5 for σ ) 4%, and is about thesame for σ ) 6%. It is also interesting to note that while

Table 1 shows aromatic oil cracking 3 orders of magni-tude faster relative to the paraffinic oil, under theconstraints listed in Table 6 the 10% conversion timedifference dropped to factors of 2.45 and 4.0 for σ )4and 6%, respectively. Times for 10% conversion duringthe catalytic reactions increased, albeit insignificantly.

From Table 6, it can be seen that the 10% conversiontimes for the thermal cracking of n-hexane, n-hexade-cane, and paraffinic oil are considerably reduced relativeto the values listed in Tables 1 and 2 by assumingdistributed activation energies. Thus, even though theinitial temperature of the gradient is 30 °C lower thanthe isothermal temperature, distributed activation en-ergies apparently reduce conversion times. It could beargued, however, that the introduction of a temperaturegradient likewise affects conversion times, since in 4.9megayears (during the thermal cracking of n-hexane),the temperature of the bed is assumed to increase from150 to 194 °C. However, when examining the data for σ) 6%, the maximum temperature would be only 182 °C.Thus, it appears that the main factor responsible forthe observed reductions in conversion time is thedistribution of activation energies. In a geologicalcontext, this reduction in 10% conversion times can haveimportant consequences. For example, while the 92megayears required for the 10% conversion of n-hexanevia thermal cracking under the constraints listed inTable 1 is unreasonably long to be considered as a viablecontribution to gas formation during coal maturation,even in a geological time frame, the 3.6-4.9 megayears(Table 6) may not be.

For the aromatic oil, there is an increase in 10%conversion time for σ ) 4%, and no significant differenceat σ ) 6%. However, the temperatures reached by thesystem would be 168 and 158 °C, respectively. Thus,while there is no observed reduction in conversion timeat σ ) 6%, there is a compensatory effect of thedistribution in that a greater amount of cracking takesplace at lower temperatures. It is also interesting to notethat the distributed activation energies result in nosignificant 10% conversion time differences between themodel alkanes and paraffinic oil. On the other hand,all of the catalytic reactions are so fast that even thoughthere is an increase in 10% conversion times, (probably

(89) Ungerer, P.; Pelet, R. Nature 1987, 327, 52-54.(90) Gretener, P. E.; Curtis, C. D. Am. Assoc. Pet. Geol. Bull. 1982,

66, 1124-1149.

Table 6. Summary of Calculated Methane Formation Rates from Literature Data for Thermal and Catalytic Reactionsfor Distributed Activation Energies and a Geothermal Gradient of 9 °C My-1 a

normal distributiont (10% conversion)b

reactionEact

(kcal/mol) A (y-1) σ ) 4%c σ ) 6%c

Thermalthermal cracking, n-C6 69 2.5 × 1024 4.9 × 106 y 3.6 × 106 ythermal cracking, n-C16H34 60 4.7 × 1020 4.2 × 106 y 3.0 × 106 ythermal cracking, aromatic oil 65 2.5 × 1024 2.0 × 106 y 8.9 × 105 ythermal cracking, paraffinic oil 69 2.4 × 1024 4.9 × 106 y 3.6 × 106 y

Catalyticacid mineral cat. cracking, C16 to C1 35 7.9 × 1014 58 y 23 ysedim. rock hydrogenolysis, C18

) 31 2.4 × 1014 2.0 y 336 dhydrogenolysis n-C5, Ni 31 3.9 × 1014 1.2 y 208 dhydrogenolysis n-C5, Fe 25 1.6 × 1011 3.1 y 1.8 yhydrogenolysis C18

), Fe 23 1.6 × 1010 3.1 y 1.9 yCO2 hydrogenation, Ni 19 9.2 × 1011 4.7 h 3.2 hCO2 hydrogenation, Fe 15 2.7 × 109 15 h 12 hCO2 hydrogenation, Fe 17 3.1 × 109 6 d 4 d

a Data from references listed in Table 1; initial temperature of geothermal gradient is 150 °C. b y ) years, d ) days, h ) hours. c σ )4 or 6% of the mean value.

Clk ) ∫E0

Ef( 1

σx2π)exp(Eh act - Eact

-2σ )2

exp(-∫0

tAi exp(-Eact

RT(t))dt)dE


owing to the lower initial temperature), in a geologicaltime frame the effects of distributed activation energiesare insignificant.

Table 7 lists the 10% conversion times for mineral-containing coals and isolated kerogens to methane forthe same distributed geologic model as used for Table6. Relative to those listed in Table 2, conversion timesin Table 7 of all the coals with exception of the UpperFreeport coal are observed to increase. Conversion timesfor the Illinois No. 6, Pittsburgh No. 8, Blind Canyon,Lewiston-Stockton, and Beulah-Zap coals increase byfactors of 8 and 2 for σ ) 4 and 6%, respectively.Conversion times for all of the isolated kerogens in theimmature stage likewise increase by factors of 8 and 2for σ ) 4 and 6%, respectively. At the latest maturationstage, 10% conversion times of the Gippsland and RockyMountain coal kerogens are greater by a factor of only2 at a σ value of 4% and nearly identical at σ ) 6%.The conversion time of the Wilcox coal is nearly identicalat a σ value of 4% and larger by a factor of 1.5 when σ) 6% at the latest maturation stage.

“The question often arises as to whether a reactionor a step in a complex process [such as coal maturation]involves free-radicals or not. The ways of answering thisquestion depend on whether the radical intermediateis persistent, stable, or short-lived. In the first two casesdirect observation by EPR [electron paramagnetic reso-nance spectroscopy] is the most rapid and the mostconclusive.”91 In other words, it can be inferred that ifthe formation of oil and gas during coal maturationoccurred by thermolysis via homolytic cleavage of C-Cbonds in the coal matrix, then long-lived paramagnetic

centers should be detected by EPR in coal samples.Indeed, Waples27 points out that “[i]n some kerogenmolecules,...free-radicals may be stabilized by delocal-ization over an extended aromatic system.” Convincingevidence is presented by Lewis and Singer.92 Naphtha-lene and anthracene were pyrolyzed to pitches at 400°C. Paramagnetic center numbers exceeding 1019

g-pitch-1 were detected. Various trimers, tetramers, andpentamers were also observed. Various structures in-ferred by X-ray diffraction studies suggested large,condensed systems. More important, however, the na-ture of these stable radicals was determined. They foundthat “[s]ubstantial evidence supports the contention thatthe stable free radicals formed during the pyrolysis ofpolynuclear aromatic compounds are odd-alternate hy-drocarbon radicals.” For example, pyrolysis of acenaph-thylene and dihydronaphthalene at 400 °C produced thephenalenyl radical. They add further, “These radicalsexhibit considerable stability at elevated temperaturesbecause of the ease of delocalization of the unpairedelectron.”

While large concentrations of free radicals have beenfound in lignin,93 a precurser of vitrinite,49 and in greenleaves and dried vegetation,94 Duchesne et al.94 pointout that “[t]his would seem to imply that the freeradicals in plants may be trapped and stabilized afterthe plants have dried. However, the fact that theirconcentration is definitely lower than in coals and even

(91) Fossey, J.; Lefort, D.; Sorba, J. Free Radicals in OrganicChemistry; John Wiley & Sons: New York, 1995; Chapter 17, p 201.

(92) Lewis, I. C.; Singer, L. S. Thermal Conversion of PolynuclearAromatic Compounds to Carbon in Polynuclear Aromatic Compounds;Advances in Chemistry Series 217; Ebert, L. B., Ed., AmericanChemical Society: Washington, DC, 1988; Chapter 16, pp 269-285.

(93) Steelink, C. Geochim. Cosmochim. Acta 1964, 28, 1615-1622.(94) Duchesne, J.; Depireux, J.; van der Kaa, J. M. Geochim.

Cosmochim. Acta 1961, 23, 209-218.

Table 7. Summary of Calculated Methane Formation Rates from Literature Data for Coals and Isolated Kerogen forDistributed Activation Energies and a Geothermal Gradient of 9 °C My-1 a

normal distributiont (10% conversion)b

reactionEact

(kcal/mol) A (y-1) σ ) 4%c σ ) 6%c

Coalsd

ND lignitee 54 3.1 × 1019 1.4 × 106 y 5.5 × 105 yUpper Freeportf 63 3.9 × 1022 3.0 × 106 y 1.8 × 106 yWyodakf 63 3.3 × 1023 1.6 × 106 y 6.8 × 105 yIllinois No. 6f 50 3.3 × 1019 4.3 × 104 y 1.1 × 104 yPittsburgh No. 8f 49 6.3 × 1018 5.1 × 104 y 1.3 × 104 yPocahontas No. 3f 53 2.9 × 1019 9.0 × 105 y 3.1 × 105 yBlind Canyonf 47 2.9 × 1018 1.2 × 104 y 3.2 × 103 yLewiston-Stocktonf 50 1.2 × 1019 7.0 × 104 y 1.8 × 104 yBeulah-Zapf 55 1.8 × 1021 2.8 × 105 y 6.6 × 104 y

Isolated Type III KerogensGippsland Eocene

2 h 55 1.4 × 1021 2.1 × 105 y 5.1 × 104 y10 h 3.8 × 1020 6.0 × 105 y 1.7 × 105 y100 h 1.3 × 1020 1.2 × 106 y 4.4 × 105 y1000 h 3.1 × 1019 2.1 × 106 y 1.1 × 106 y

Rocky Mountain2 h 55 1.2 × 1021 2.5 × 105 y 6.1 × 104 y10 h 4.3 × 1020 5.5 × 105 y 1.5 × 105 y100 h 1.3 × 1020 1.2 × 106 y 4.3 × 105 y1000 h 2.8 × 1019 2.2 × 106 y 1.2 × 106 y

Wilcox2 h 55 1.3 × 1021 2.2 × 105 y 5.4 × 104 y10 h 3.2 × 1020 6.8 × 105 y 2.0 × 105 y100 h 7.0 × 1019 1.5 × 106 y 6.7 × 105 y1000 h 1.7 × 1019 2.6 × 106 y 1.5 × 106 y

a Data from references listed in Table 2. b y ) years. c σ ) 4 or 6% of the mean value. d Mineral-containing coals. e North Dakota ligniteTertiary coal. f Argonne National Laboratory Premium coal.


than in lignites...shows that other factors intervene inthese rocks.”

Paramagnetic centers in coal were first reported byUebersfeld et al.95 in 1954. Table 8 lists the concentra-tions of paramagnetic centers found in coals of variouscarbon content. The data suggest both a trend and acorrelation with increasing carbon content and aroma-ticity. Specifically, as carbon content and aromaticityincrease to approximately 94 and 91%, respectively, thenumber of paramagnetic centers likewise increases.Further maturation leads to a decrease in free-radicalconcentrations. This trend agrees with the observationsof Retcofsky et al.98 who, in analyzing a series of 18 U.S.coals and a peat, found that “[t]he concentrations ofunpaired electrons in the coals rise exponentially withincreasing rank up to about 94% C after which there isa large decrease.” However, “...small quantitative dif-ferences...” have been noted in the trend. One mightsurmise that the decrease in radical density withincreasing maturity (%C > 94) might be due to greatercondensation which brings the radicals into closerproximity, leading to bond formation via pairing.

Table 949,99,100 lists the number of aromatic carbonsper cluster, number of paramagnetic centers, andspectroscopic splitting factors (g-values) for the ArgonnePremium Coals. A free electron has a g-value of 2.0023.Free radicals with g-values close to 2.0023 may beconsidered to be in extended π-orbitals while largerg-values imply a more restrictive environment. The AC/Cl and g-values reported follow the systematic rankchange well, i.e., AC/Cl increases while g-values de-crease with increasing rank. This is also in agreementwith the observations of Retcofsky et al.98 who alsofound decreasing g-values with increasing rank. Such

a well-defined trend however, is not evident in the meanradical density data. Furthermore, the variances aboutthe means are large. Because these calculations assumenormally distributed observations arising from randomerrors, and because these data were calculated fromobservations made in independent studies, it is obviousthen that interlaboratory systematic errors have beenintroduced. Silbernagel and Botto99 appear to concurwith this conclusion, stating that these samples “...havebeen treated in a very uniform manner up to the timewhen they were opened in the individual laboratories.Once the samples were opened, the variety of differenthandling procedures that were employed undoubtedlyresulted in alterations of the samples from their pristinestate because of the loss or addition of water or suchincidental oxidation as may have occurred.” However,even observations made by single laboratories do notseem to follow any systematic variation with rank. Thishas been attributed to “...the presence of magnetictransition-metal ions, principally iron, that are inintimate contact with the organic matter in the coal.”

While Silbernagel and Botto,99 citing the work ofothers, recognize that concentrations of iron ions canbe high at times, they subsequently imply that theycannot account for the total EPR signal. Duchesne etal.,94 citing others, likewise conclude “...that the numberof paramagnetic...[centers in coal] is so high that it isout of the question to attribute these as being due tothe existence of... iron.” Furthermore, while Retcofskyet al.101 have noted that “[o]ne group of investiga-tors...has proposed that charge-transfer complexes ratherthan stable free-radicals may be responsible for the[EPR signals]”, Nishioka et al.102 citing others, state,“A large number of solid charge-transfer complexes,formed from nonradical electron-donors and -acceptors,are diamagnetic.”

While this section has focused thus far on rates of gasand oil formation in coal seams, it is equally importantto consider if the product distributions observed forthermal or catalytic routes are consistent with thosemeasured in coal seams. Methane concentrations in U.S.coal mines are typically 85-95 mol %. In contrast,thermal cracking of 1-octadecene at 500 °C producesonly 23% methane along with significant fractions ofC2-C7 hydrocarbons (see Figure 3 and Table 10); whileno rate data are available at lower temperaturesexpected in coal beds (100-200 °C) for liquid hydrocar-bon thermolysis (rates are too low to permit measure-ment within a reasonable time frame), the fraction ofmethane is expected to be significantly lower and thefraction of liquid hydrocarbons significantly higherunder these milder conditions.18,23,74 Products of cata-lytic cracking (acidic) at 190 °C are characterized by gasand liquid hydrocarbons having a maximum in thedistribution around C4 to C5 with only 8% methane (seeFigure 3 and Table 10).

Pyrolysis of coal kerogens, with exception of theCherry Canyon coal kerogen, produces methane con-

(95) Uebersfeld, J.; EÄ tienne, A.; Combrisson, J. Nature (London)1954, 174, 614.

(96) Friedel, R. A.; Breser, I. A. Science 1959, 130, 1762-1763.(97) Yen, T. F.; Erdman, G. J.; Saracemo, A. J. Anal. Chem. 1962,

34, 694-700.(98) Retcofsky, H. L.; Stark, J. M.; Friedel, R. A. Anal. Chem. 1968,

40, 1699-1704.(99) Silbernagel, B. G.; Botto, R. E. Advanced Magnetic Resonance

Techniques Applied to Argonne Premium Coals in Magnetic Resonanceof Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds., AmericanChemical Society: Washington, DC, 1993; Chapter 33, pp 629-643.

(100) Buckmaster, H. A.; Kudynsha, J. Dynamic In Situ 9-GhzElectron Paramagnetic Resonance Studies of Argonne Premium Coalsin Magnetic Resonance of Caronaceous Solids; Botto, R. E.; Sanada,Y., Eds., American Chemical Society: Washington, DC, 1993; Chapter26, pp 483-506.

(101) Retcofsky, H. L.; Thompson, G. P.; Hough, M.; Friedel, R. A.Electron Spin Resonance Studies of Coals and Coal-Derived Asphalt-enes. In Organic Chemistry of Coal; ACS Symposium Series 71, Larsen,J. W., Ed.; American Chemical Society, Washington, DC, 1978; pp 142-155.

(102) Nishioka, M.; Gebhard, L. A.; Silbernagel, B. G. Fuel 1991,70, 341-348.

Table 8. Number of Paramagnetic Centers in Coals ofVarious Carbon Content

total carbon (%) aromaticity (fa)paramagnetic centers

(number/gram) ref

67 - - -a 1.2 × 1018 9677 0.69 2.5 × 1018 9778 - - -a 3.9 × 1018 9682 - - -a 5.1 × 1018 9683 0.75 3.6 × 1018 9785 0.76 5.3 × 1018 9789 0.81 7.2 × 1018 9789 - - -a 1.0 × 1019 9690 - - -a 1.4 × 1019 9693 0.88 1.8 × 1019 9794 0.91 2.5 × 1019 9795 0.95 2.3 × 1019 9796 0.98 1.5 × 1019 9797 0.99 2.2 × 1019 97

a - - -Denotes unavailable data.


tents at the low end of observed values from U.S. coalmines (70-84%). The data also indicate some general,albeit counter-intuitive, trends. Figures 4-6 illustratethe products for the pyrolysis of the Gippsland Eocene,Rocky Mountain, and Wilcox coal kerogens. As pyrolysistimes increase, methane content decreases while ethaneand propane concentrations increase. This trend isespecially prevalent in the Wilcox data where themethane content shows an overall decrease of 16%, theethane content an overall increase of 9%, and a propaneincrease of 4%. This observation is inconsistent withwhat one would expect from thermolysis. Indeed, onewould expect an increase in methane content withrespect to pyrolysis time as evidenced by the fact thatolder deposits usually contain a preponderance of lighterhydrocarbons. However, metal catalysis arguments canbe employed to explain this observation. It was men-tioned earlier that Bartholomew et al.45 speculated thatisolated kerogens could still contain significant quanti-ties of metallic minerals. Thus, it follows that theseminerals could be involved in the formation of liquidand gaseous hydrocarbons. Such could be inferred inthis case because the loss of methane selectivity withincreasing pyrolysis time is indicative of catalyst deac-tivation. Specifically, a methanation catalyst undergoingdeactivation via carbon deposition loses both activityand methane selectivity with increasing time. Thus, ifmetallic catalysts were still present in the isolated coalkerogens, the loss of methane selectivity with increasingtime would be predicted due to deactivation via forma-tion of coke-like, carbonaceous material forming on theactive catalytic sites. Indeed, if such is the case, thenmetal catalysis would not only explain the very similar10% conversion times observed between the whole coalsand isolated kerogens (see Tables 2 and 7), but wouldalso indicate that only trace amounts of metal wouldbe necessary to produce copious amounts of methane, afact evidenced by our kinetic calculations (see Tables1, 5, and 6). Furthermore, product compositions for Fe-or Ni-catalyzed olefin hydrogenolysis measured byMango et al.15 and CO2 methanation measured byMedina et al.103 are methane rich (90-94 mol %) andclosely match those for U.S. coal gases (see Figure 7 andTable 10).

Figure 8 contains the frequency histograms of thebranched C4 and C5 hydrocarbons/normal C4 and C5hydrocarbons ratios for various thermogenic natural gasbasins. The majority of observed ratios with the excep-tion of the Cherokee, Forest City, and San Juan Basinsare less than unity. For the above-named basins,however, a significant number of observed ratios aregreater than unity, indicating extensive branchingwhich could have been caused by the formation of acarbocation intermediate. This would tend to indicatethat while for the majority of basins acid-mineralcatalysis is not the main route for natural gas formation,there can be instances where the input from this routecan be significant.

Discussion

Plausible reactions for producing natural gas in coalbeds must meet the following criteria: (1) producemethane at significant rates under geologic conditionswithin geologically significant times for coal maturation(i.e., less than 1-10 million years) and (2) produce gasesrich in methane.

According to Hunt,104 methane formation from cat-agenesis is significant in the range of 100-200 °C witha maximum at about 150-160 °C corresponding to adepth of around 4500 m.75,106 Maximum generation ofmethane apparently occurs in medium-volatile bitumi-nous and low-volatile bituminous coals104 and is highfor coals having a reflectance (Ro) higher than about2%.105 These conclusions are supported by the recentlydeveloped model of Tang et al.77 indicating that T > 120°C and Ro g 0.9% are required for a minimum thresholdmethane production of 300 ft3 ton-1. Accordingly, themost important methane-producing reactions shouldoccur at temperatures in the range of 120-180 °C.While the current paradigm asserts that coalbed gas isproduced by thermal decomposition of light oils,10,11

a careful analysis of rate data from the previousliterature (see Tables 1 and 2 and Figure 1) providesevidence that reaction rates for thermal cracking oflight-medium hydrocarbons are orders of magnitudetoo slow to have produced significant quantities ofmethane within coal maturation times, assuming iso-thermal conditions and bulk first-order kinetics char-acterized by a single activation energy. If thermalcracking of light-medium hydrocarbons could be char-acterized by distributed activation energies, methanegeneration would still be time limited even at highertemperatures (see Table 6). However, the environmentcausing these hydrocarbons to acquire distributed,thermal decomposition activation energies is ambigu-ous. On the other hand, hydrocarbon decomposition ormethanation reactions catalyzed by acidic or transition-metal minerals could have produced known reserves ofcoalbed gas within days to thousands of years, i.e. wellwithin maturation times, independent of temperatureand distributed activation energies (see Tables 1 and 6

(103) Medina, J. C.; Butala, S.; Bartholomew, C. H.; Lee, M. L. Fuel2000, 79, 89-93.

(104) Hunt, J. M. Petroleum Geochemistry and Geology; W. H.Freeman and Company, San Francisco, 1979; pp 145, 148, 163, 165,172.

(105) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occur-rence, Second Revised and Enlarged Edition; Springer-Verlag: NewYork, 1984; pp 70, 215, 247.

Figure 3. Product distributions for thermal cracking andcatalytic cracking (on Houndry M-46) of 1-octadecene at 500and 190 °C. Data from ref 15.


and Figure 2). Studies conducted by Medina et al.103,106

confirm that rates of CO2 methanation and olefinhydrogenolysis are very significant in the presence ofreduced, dispersed iron and nickel under conditionssimulating coalbeds. For example, 20-50% conversionof CO2 or 1-dodecene occurs on 10% Fe/SiO2 in 40-60h at 180 °C; moreover, when extrapolated to reactantand catalyst concentrations at the lower end of whatmight be expected for a coal bed (e.g., only 100 ppmsurface iron), 10% conversion times of 1-10 months forolefin hydrogenolysis and 2-36 h for CO2 hydrogenationare predicted.

The rate of methane production reported by Medinaet al.106 for 1-dodecene hydrogenolysis on reduced 10%

Fe/silica at 180 °C is in excellent agreement with thatreported by Mango44 for 1-octadecene hydrogenolysis onreduced Fe(AcAc)3 (see Table 1). Rates of CO2 hydro-genation of 10% Fe/silica obtained by Medina et al.103

in a batch system are in fair agreement with thoseextrapolated from the higher temperature data ofWeatherbee and Bartholomew46 obtained in a flowreactor for 15% Fe/silica. The data reported by Medinaet al. are, to our best knowledge, the first to be obtainedat low temperatures in a closed system more represen-tative of geologic conditions. Thus, while Mango andHightower107 have suggested oil hydrogenolysis cata-lyzed by transition-metal minerals as a unique routefor production of high methane content natural gas,

(106) Medina, J. C.; Butala, S.; Bartholomew, C. H.; Lee, M. L.Geochim. Cosmochim. Acta 2000, 64, 57-63.

(107) Mango, F. D.; Hightower, J. W. Geochim. Cosmochim. Acta1997, 61, 5347-5350.

Table 9. Number of Aromatic Carbons per Cluster, Paramagnetic Centers and Spectroscopic Splitting Factors for theArgonne Premium Coalsa

Argonne Premiumcoalb rankc AC/Cld

paramagneticcenters

(number/g-OM)espectroscopic

splitting factorfspectroscopic

splitting factorg

Beulah-Zap ligA 9 1.85 ( 1.13 2.0035 2.0034Wyodak subC 14 2.00 ( 1.09 2.0035 2.0034Illinois No. 6 hvCb 15 0.88 ( 0.51 2.0032 2.0030Blind Canyon hvBb 15 1.43 ( 1.11 2.0032 2.0030Lewiston-Stockton hvAb 14 2.10 ( 1.50 2.0030 2.0028Pittsburgh No. 8 hvAb 16 1.25 ( 0.59 2.0029 2.0027Upper-Freeport mvb 18 1.13 ( 0.70 2.0028 2.0026Pocahontas No. 3 lvb 20 2.12 ( 1.29 2.0028 2.0026

a Data from refs 49, 99, 100. b Lewiston-Stockton and Pittsburgh No. 8 coals have carbon contents of 82.6 and 83.2%, respectively,which dictated the order of their listing in this table; some references in ref 41 list Lewiston-Stockton coal as mvb in rank. c ligA ) ligniteA, subC ) subbituminous, hvCb ) high volatile bituminous C, hvBb ) high volatile bituminous B, hvAb ) high volatile bituminous A,mvb ) medium volatile bituminous, lvb ) low volatile bituminous. d Number of aromatic carbons per cluster. e Mean ( 1 s.d. (×1019);observations reported in units of spins cm-3 not included; g-OM ) grams organic matter. f Data reported on an “as received” basis;uncertainty of these values is 10-5. g Data reported on a dry basis; uncertainty of these values is 10-5.

Table 10. Gaseous Product Distributions for Thermal and Catalytic Cracking of Hydrocarbons Compared with theCompositions of Coalbed Gas

mol % of gaseous product

reaction C1 C2 C3 C4 C5 C6 C7

thermolysis, C18) at 500 °Ca 23 40 10 14 7 4 1

acid mineral cracking, C18) at 190 °Cb 8 17 10 25 16 11 13

pyrolysis of isolated coal kerogensCherry Canyon, 310 °C for 1 weekc 48 15 3 5 22 2 5Gippsland Eocene, 300 °Cd

2 h 83 6 5 2 410 h 80 9 7 2 1100 h 80 9 6 2 11000 h 72 13 8 4 2

Rocky Mountain, 300 °Cd

2 h 82 7 4 3 310 h 81 10 5 2 1100 h 81 10 5 2 11000 h 74 14 6 3 2 1

Wilcox, 300 °Cd

2 h 84 6 6 1 210 h 82 8 7 1 2100 h 76 12 8 2 11000 h 68 17 10 4 2

catalytic hydrogenolysisC18

), 190 °C on Fee 90 4C12), 180 °C on Fef 86 14

catalytic hydrogenationCO2, 180 °C on Fef 94 3coalbed gas, Piceanceg 90coalbed gas, Mary Leeh 96 1

a Data from ref 15; heated in glass for 1 h. b Data from ref 15; Houndry cracking catalyst, M-46; reaction time, 24 h. c Data from ref 12.d Data from ref 79. e Data from ref 15; catalysis by a carbonaceous rock (Monterey Formation) with 1 atm H2. f Data from ref 45. g Datafrom ref 64; D Coal Seam, Piceance Basin; also contains an additional 6 mol % CO2 and 1 mol % C3+ hydrocarbons. h Data from ref 64;Mary Lee Seam, Warrior Basin; also contains an additional 3 mol % N2 and 0.1 mol % CO2 and 0.01 mol % H2.


these results suggest an additional route for “dry”natural gas. Specifically, CO2 methanation on Ni andFe minerals is an additional scenario in basins thatcontain high concentrations of CO2 and H2.

In examining light hydrocarbons (C1 to C14) fromvarious well cuttings, Hunt108 found that compoundscontaining tertiary carbons have a greater concentrationin shallower depths (lower temperatures), while com-pounds containing quaternary carbons appear in greaterconcentrations in the older, deeper sediments (highertemperatures). Mango109 states that Hunt interpretedthese results as indicating that acid-catalyzed carbo-cation mechanisms are preferred in the shallowerregions with free-radical cracking mechanisms compet-ing at greater depths. This would suggest oil generationvia mineral-acid-catalyzed cracking at shallow depthswith free-radical thermolytic generation of oil dominat-ing at the greater depths. An earlier publication fromHunt104 confirms this interpretation. He states, whenmaking reference to petroleum generation from marine

sediments, that “[p]etroleum hydrocarbons are crackedfrom...kerogen-mineral complex[es] by mechanisms thatrequire apparent[ly] [low] activation energ[ies]...to breakthe bonds. Catalytic cracking appears to be the domi-nant process in petroleum generation in the subsurfacetemperature range up to about 125 °C...Thermal crack-ing becomes increasingly important at higher temper-atures.” These works thus imply that oil generation fromcoal kerogen is acid-mineral catalyzed at lower, sub-surface temperatures; however copious amount of oil canbe produced by thermolysis alone at higher tempera-tures. This interpretation, however, contradicts theresults of Reynolds and Burnham80 who found fewdifferences between oil curves from whole shales andtheir corresponding isolated kerogens.

Hetenyi110 found that pyrolyzing Type III/a and TypeIII/b coal kerogens with montmorillonite slightly re-duced oil production while heating the kerogens withcalcite slightly enhanced oil production. Espitalie etal.111 found, however, that pyrolyzing “mixtures ofkerogens with various minerals...[resulted]...in retentionof heavy hydrocarbon products issued...[from]...kerogenpyrolysis...[and thus,]...[t]he experimental procedure

(108) Hunt, J. M. Science 1984, 226, 1265-1270.(109) Mango, F. D. Org. Geochem. 1997, 26, 417-440.

(110) Hetenyi, M. Org. Geochem. 1995, 23, 121-127.(111) Espitalie, J.; Madec, M.; Tissot, B. Am. Assoc. Pet. Geol. Bull.

1980, 64, 59-66.

Figure 4. Product distributions for the Gippsland Eoceneisolated coal kerogen at pyrolysis times of 2, 10, 100, and 1000h. Data from ref 79.

Figure 5. Product distributions for the Rocky Mountainisolated coal kerogen at pyrolysis times of 2, 10, 100, and 1000h. Data from ref 79.

Figure 6. Product distributions for the Wilcox isolated coalkerogen at pyrolysis times of 2, 10, 100, and 1000 h. Data fromref 79.

Figure 7. Product distributions for catalytic hydrogenolysisof 1-octadecene and CO2 hydrogenation on iron catalysts at190 and 180 °C, respectively, compared with composition of atypical coalbed gas. Data from ref 15, 64, 90, 106.


separates the lighter hydrocarbons [lighter than C15]from total hydrocarbons, ...showing that the decreasedhydrocarbon yield from rocks as compared to kerogenis principally due to retention of the heaviest hydrocar-bons [in the minerals.]” Indeed, Hetenyi110 acknowl-edges that many “authors consider clays and especiallymontmorillonite to be very active in terms of adsorptionof products, and also in terms of their ability to crackorganic matter at high temperatures.” Thus, the resultsof Reynolds and Burnham80 may be explained byretention of hydrocarbons by the mineral matrix of theshale.

It is interesting that even model coal compoundscovalently bonded to a less active substrate result inapparent catalytic transformations. For example, Bucha-nan III et al.112 reported that surface-immobilized 1,2-diphenylethane on amorphous silica underwent ther-molysis four-times faster than gaseous 1,2-diphen-ylethane at 350-400 °C, and that the resulting productswere also altered notably. It was further reported byBuchanan and Biggs 113 that thermolysis rates of 1,3-diphenylpropane increased as surface coverage on silicaincreased.

It is generally held that shales are predominately oilsources while coals are primarily sources of gas.11 Forexample, while Waples27 acknowledges that “[s]mallamounts of fat and waxes, particularly those that occurin leaf coatings, pollens, and tree resins, may in some

cases be converted to oil,” he adds, “[m]ost terrestrialplants contain too little lipid material...to form oil-generative kerogens.” He confirms this by stating, “Therelative proportions of gas and oil will depend on thekerogen type. Type I kerogens may give as much as 80%oil, while Type III kerogens may only yield 10% oil and90% gas.” However, Horsfield et al.114 point out that “-[s]trict adherence to an elemental classification schemewhereby hydrogen-rich kerogens (Type I and II) areconsidered oil-prone...and hydrogen-poor kerogen (TypeIII) is considered mainly gas-prone can cause problems.This applies particularly in fluviodeltaic-lacustrinesystems where...oil can be generated from terrigenousorganic matter...and...the theoretical[ly] significant hy-drocarbon-generating potential of hydrogen-rich...[kero-gens]...has been questioned.” When correlations havebeen established between petroleum reserves and ter-restrial source rocks, bulk maceral compositional argu-ments have been employed to explain the observedconnection. For example, Snowdon115 states “[i]n orderfor coal or Type III OM to function as a source rock forliquid hydrocarbons (oil and condensates), the organichydrogen content must be enriched...[in]...liptinitic mac-erals such as sporinite and cutinite and especiallyresinite.” Indeed, the Gippsland Eocene isolated coalkerogen, which in this study had the highest calculatedoil/gas production ratio in the initial stages of pyrolysis(goil /ggas ) 180, see Table 4), was rich in liptinite (8%liptinite, 92% vitrinite, HI ) 237.5) relative to the other

(112) Buchanan, A. C., III; Dunstan, T. D. J.; Douglas, E. C.;Poutsma, M. L. J. Am. Chem. Soc. 1986, 108, 7703-7715.

(113) Buchanan, A. C., III; Biggs, C. A. J. Am. Chem. Soc. 1989, 54(4), 517-525.

(114) Horsfield, B.; Yordy, K. L.; Crelling, J. C. Org. Geochem. 1988,13, 121-129.

(115) Snowdon, L. R. Org. Geochem. 1991, 17, 743-747.

Figure 8. Frequency histograms for C4 and C5 branched/C4 and C5 normal hydrocarbon ratios for various thermogenic coalbedbasins. Data from ref 67-70.


two coal kerogens. However, Lu and Kaplan79 indicate“...that the relative abundances of various macerals [inthe Gippsland Eocene, Rocky Mountain and Wilcox coalkerogens were]...not critical to the hydrocarbon-generat-ing potential.” They add, “Rather, the amount of dis-persed algae and bacterial material (unidentified) in thecoal probably is more significant. Depositional environ-ment of the source organic material also may beimportant because it could control preservation oforganic constituents.”

Powell et al.87 did find broad trends in the relativeamounts of liptinite, vitrinite, and inertinite maceralsand hydrocarbon potential of Australian coals. However,the large observed variance made firm petrographicalcorrelations impossible. Bertrand et al.116 studied 14humic coal samples and likewise could find no correla-tions between maceral composition and petroleum po-tential, even between coals of the same rank, addingfurther that “[t]he diversity in oil potential must there-fore, to a large extent, be attributed to a geochemicaldiversity of macerals from the vitrinite group,...[whichsaid]...diversity is probably greater than recognized bypresent-time petrographic techniques.” It should benoted, however, that such an argument presupposesrank is the only variable and the main source ofvariance must therefore be within the vitrinite maceralcomposition beyond the identification ability of present-day petrography and further ignores the inherentheterogeneous nature of coal in general, even within thesame seam. Thus, while the observed lack of correlationby Bertrand et al. agrees with the observation of Powellet al., the explanation appears suspect.

Cook38 states that the effectiveness of coal to generateoil depends more upon the presence of labile materialin the coal than upon the abundance of liptinite. Whileambiguous in terms of what constitutes “labile mate-rial,” such an explanation may provide insight as to whythe Wilcox coal kerogen was primarily oil-generativethroughout the entire pyrolysis experiment. Under thisassertion one could speculate that the Wilcox coal hadgreater amounts of labile material, which could be theunidentified algae or bacterial material previouslymentioned. Such an interpretation however, becomescomplicated by the fact that the Wilcox coal was lowerin rank (lignite, R0 ) 0.32%) relative to the other coalkerogens pyrolyzed. Indeed, in a point that shall beemphasized later, oil generation from coal is stronglyrank-dependent. In summary then, despite the apparentlack of agreement regarding its causes and inability topredict oil generation, there appears to be evidence tosupport the assertion that coal and coal kerogens cangenerate oil.

There are apparently two kinds of acidic mineralsavailable for catalytic cracking of hydrocarbons in coalbeds; (1) silica-aluminas or aluminosilicates, whichsurface acid strength can, according to Brooks,117 beequivalent to 0.1 to 1.0 M HCl,178 and (2) natural clayminerals such as bentonite and montmorillonite. Bothkinds are found in the coals and surrounding strata.119

Generation of hydrocarbon gases and condensates dur-ing catalytic cracking by these and closely relatedminerals under geologically relevant conditions has beendemonstrated in a number of studies, e.g., refs 25, 73,119. While catalytic cracking on acidic minerals occursat rates comparable to those of alkane/alkene hydro-genolysis (see Tables 1 and 2, and Figure 2), theexpected product compositions do not resemble typicalcoalbed gas (see Table 10 and Figures 3 and 7) in thatthe methane contents are low. Thus, while catalyticcracking may contribute to the formation of methane,hydrogenolysis reactions on transition metals appear tobe necessary to realize high-methane-content gases thatresemble typical, natural, coalbed gas. Similarly, whileformation of hydrocarbon liquids by thermolysis occursat geologically relevant rates (see Tables 4 and 5), thesehydrocarbons would have to be further processed bymetal-catalyzed hydrogenolysis to form typical coalbedgas.

Significant input from acid-mineral catalysis wouldseem to depend on, among other factors, the presenceof water in the correct amount. Between the layers ofsmectite clays are countercations that neutralize thenegative charge of the tetrahedral silicate sheets.120

According to Johns,121 the Brønsted acidity arises fromthe polarization dissociation of water by the exchange-able cations, i.e.,

where M is the cation, x is the number of coordinatedwater molecules to the cation, and K is the dissociationconstant for the water-cation system. Johns121 alsoreports that “[t]he proton-donating ability of the clay...[depends on three]...factors: (1) the polarizing effect ofthe exchangeable cation (increase with increasing chargeand decreasing charge); (2) The number of exchangeablecations, n; and (3) the source of isomorphous replace-ment which gives rise to interlayer charge (greateracidity associated with tetrahedral rather than octahe-dral substitution).”

When the clays contain more than a monomolecularlayer of water, however, the polarizing effect becomesdispersed and the pH rapidly approaches that of water.Conversely, as the water content becomes low, thedissociation constant of adsorbed water can increase by6 orders of magnitude relative to bulk water.121 In anillustrative experiment,121 Behenic acid was heated at250 °C over calcium montmorillonite and excess water.(Experiments with adsorbed water alone were notperformed). Branched/normal mass ratios for C14-C28were determined to be 0.114 and 0.113 for 25 and 150h reaction times, respectively. In view of this result, i.e.,insignificant branched products, it was concluded thata carbocation intermediate was not formed, suggestinginstead that the reaction proceeded through a free-radical mechanism. The aluminosilicates, in conjunction

(116) Bertrand, P.; Behar, F.; Durand, B. Org. Geochem. 1986, 10,601-608.

(117) Brooks, B. T. Origins of Petroleum. In The Chemistry ofPetroleum Hydrocarbons: Volume 1, Brooks, B. T.; Boord, C. E.; Hurtz,S. S., Jr., Schmerling, L., Eds., Reinhold Publishing Corporation: NewYork, 1954; Chapter 6, pp 83-102.

(118) Farneth, W. E.; Gorte, R. J. Chem. Rev. 1995, 95, 615-635.(119) Saxby, J. D.; Chatfield, P.; Taylor, G. H.; Fitzgerald, J. D. Org.

Geochem. 1992, 18, 373-383.(120) Brady, N. C. The Nature and Properties of Soil: 10th ed.;

Macmillan Publishing Company: New York, 1990; Chapter 7, pp 186-192.

(121) Johns, W. D. Annu. Rev. Earth Planet. Sci. 1979, 7, 183-198.

n[M(H2O)x]z+ {\}

Kn[M(H2O)x-1(OH)](z-1)+ + nH+


with providing Brønsted acid sites, can also provideLewis acid sites allowing for hydride transfer. This leadsto the formation of a carbenium ion which then under-goes â-scission to form an alkene and a smaller carbe-nium ion.122

Farrauto and Bartholomew122 point out that silica-aluminas and acidic natural clays have the samestructural features as zeolites. In other words, it is thecharge imbalance in the silica matrix by aluminum andother metals that gives rise to the Brønsted and Lewisacidities in these materials. Thus, it follows that dis-cussions of zeolitic acid reactions would be applicableto aluminosilicates and acidic natural clays, albeit theacidity of these materials is lower relative to the zeolites.

In a published theoretical review of zeolitic Brønstedacid site reactivity, van Santen and Kramer,123 inmaking reference to alkane activation via nonclassicalcarbonium ion formation, state that while “...the inter-mediates formed in acid-catalyzed reactions in zeolitesare very similar to those formed in [homogeneous]superacids, the energetics of their formation is verydifferent.” In other words, formation of carbonium ionsfrom alkane molecules necessitates overcoming theenergy of the strong proton-zeolite bond. Citing others,van Santen and Kramer123 add, “Reactions that proceedvia carbonium ion intermediates have high activationenergies and hence will be suppressed at lower temper-atures in fav[or] of...hydride transfer reaction[s].” Thisis important because carbonium ions decompose to forman alkane and a smaller carbenium ion, or decomposeinto hydrogen gas and a carbenium ion. This latterdecomposition route, as shall be pointed out later, is aproposed source for gaseous H2 for transition-metal-catalyzed reactions. Thus, the implication is that whilereaction initiation may occur via carbonium ion forma-tion, chain propagation will be through hydride transferreactions, thus limiting H2 generation. A study con-ducted by Shertugde et al.124 would tend to support thisimplication. These workers cracked isobutane and n-pentane on dealuminated faujasites at high tempera-tures in a flow reactor under differential conditions (e.g.,conversions were <1%). They state, “These reactionswere initiated by the protonation of C-H and C-Cbonds by the Brønsted acid sites. The pentacoordinatedcarbonium ions thus formed decomposed into carbeniumions and the products of the initiation reactions, viz.,H2, CH4, and C2H6. These carbenium ions propagatedchain reactions, mainly by isomerization followed byhydride transfer. Disproportionation occurred concomi-tantly.” Chain propagation lengths varied from 3 to 15.

However, a study conducted by Krannila et al.,125 incracking n-butane on an HZSM-5 zeolite, providescontradicting evidence. They state that “[t]he results areconsistent with the occurrence of two simultaneousmechanisms: (1) a monomolecular mechanism proceed-ing through the pentacoordinated carbonium ion formedby protonation of the n-butane at the two position and

(2) a bimolecular hydride transfer proceeding throughcarbenium ion intermediates. The former...[was foundto] proceed...almost to the exclusion of the latter in thelimit approaching zero n-butane conversion.”

Kazansky et al.126 studied the protolytic cracking ofethane on high silica zeolites via ab initio quantummechanical calculations. Remarkably, they found thatthe transition state was only ∼8 kcal/mol less stablethan the adsorbed carbonium ion, suggesting by Ham-mond’s postulate127 that the transition state and car-bonium ion have only minor structural differences. Theyconclude that “...there is no doubt that both adsorbednonclassical carbonium ions resulting from...protontransfer and the transition state of protolytic crackingof paraffins represent...highly excited unstable com-plexes and do not exist on the surface of the catalyst asreal intermediates. They should be...considered as short-living complexes resulting from the interaction of ad-sorbed molecules with thermally excited acid hydroxylgroups or from collisions of gaseous molecules with suchexcited OH groups...if the reaction takes place atelevated temperature.”

A study conducted by Henderson et al.128 neverthelessdoes suggest that clay minerals catalyze cracking reac-tions of n-alkanes at geologically relevant temperatures.They heated n-octacosane at 200 °C for 1000 h in avacuum and found the conversion to be <1 wt %.However, when n-octacosane was heated at 200 °C for170 h on bentonite clay, the conversion was ∼1 wt %.Product distributions were obtained at 375 °C. Ther-molysis of n-octacosane produced alkanes, alkenes, andaromatic constituents. Evidently, little methane wasproduced as evidenced by the fact that little n-hepta-cosane was found in the product mixture. However, “-[t]he effect of the bentonite at this temperature was tobring about the conversion of more than 90 [wt %]...ofthe n-octacosane ...[into an] insoluble black carbon-aceous material.” What was soluble contained largeconcentrations of olefinic and aromatic constituents.

Olefins appear to be very reactive on zeolites. In astudy conducted by van den Berg et al.,129 ethene,propene, isobutene, and isopentene were reacted onzeolite HZSM-5 at 27 and 100 °C. Analyses wereperformed using 13C NMR. At 27 °C, the olefins wereoligomerized into linear alkanes while at 100 °C somebranched compounds were observed. The linear alkaneformation at the lower temperature was attributed tothe specific pore structure of the zeolite resulting inpreferential adsorption of linear alkanes. Differentialscanning calorimetry of hexane vs isohexane revealed∆(∆H) ) ∼-5.5 kcal/mol. It was speculated that at thehigher temperature, surface catalytic reactions occurredleading to branched products. Ethene produced hydro-carbons ranging from C23 to C27. The remaining olefinsproduced hydrocarbons ranging from C8 to C12. Twoadditional observations were reported. First, no NMRsignals were detected in the 160-250 ppm range rela-tive to TMS (trimethylsilane), indicating very low car-

(122) Farrauto, R. J.; Bartholomew, C. H. Fundamentals of Indus-trial Catalytic Processes; Blackie Academic & Professional: New York,1997; pp 57-115, 538-543.

(123) van Santen, R. A.; Kramer, G. J. Chem. Rev. 1995, 95, 637-660.

(124) Shertugde, P. V.; Marcelin, G.; Sill, G. A.; Hall, W. K. J. Catal.1995, 95, 637-660.

(125) Krannila, H.; Haag, W. O.; Gates, B. C. J. Catal. 1992, 135,115-124.

(126) Kazansky, V. B.; Senchenya, I. N.; Frash, M.; van Santen, R.A. Catal. Lett. 1994, 27, 345-354.

(127) Hammond, G. S. J. Am. Chem. Soc. 1955, 77, 334-338.(128) Henderson, W.; Eglinton, G.; Simmonds, P.; Lovelock, J. E.

Nature 1968, 219, 1012-1016.(129) van den Berg, J. P.; Wolthuizen, J. P.; Clague, A. D. H.; Hays,

G. R.; Huis, R.; van Hoof, J. H. C. J. Catal. 1983, 80, 130-138.


benium ion concentrations. This observation is in agree-ment with Aronson et al.130 who adsorbed and reactedtert-butyl alcohol on HZSM-5. Likewise, “[n]o spectralfeatures were observed in the region expected forcarbenium ions, even though the chemistry observed forthe adsorption intermediate was consistent with...andsimilar to that observed following olefin adsoption...”These results indicate that charged hydrocarbons arehighly unstable, lending credence to the Kazansky etal.126 study. Also reported by van den Berg et al.129 wereno observable signals for olefinic or aromatic carboncompounds. Thus, all the products were paraffiniccompounds.

In a follow-up study, van den Berg et al.131 demon-strated that these adsorbed zeolitic oligomers start tocrack at 127 °C, while at 217 °C “...the rate of crackingequals the rate of oligomerization.” GC analysis of theproducts at the latter temperature showed primarilylinear hydrocarbons with a substantial branched hy-drocarbon concentration. Citing others, the importanceof Lewis-acid sites were also emphasized.

These results are important because they suggest thatalkenes produced during coal maturation are short-livedin the presence of aluminosilicates and acidic naturalclays. For example, it was previously implied that the10% conversion times listed in Table 6 for the thermaldecomposition of n-hexane, n-hexadecane, and paraffinicoil to methane were not necessarily long in a geologicaltime frame. While the product distribution suggests thisis not a primary route to natural gas formation, itcannot be excluded as a contributory mechanism. In-deed, Henderson et al.,128 citing others, state “...thatthermolytic cracking must be at least a contributoryfactor in the genesis of [light] petroleum, and one whichbecomes of increasing importance with increasing age,depth of burial and temperature.” Under low pressures,the Rice-Kossiakoff model132 predicts that thermolysisof n-alkanes will produce a series of olefinic compoundswith some methane and ethane arising from successiveC-C bond cleavage. Jones et al.133 showed that athigher pressures, intermolecular reactions become in-creasingly important, resulting in higher alkane/alkeneratios in the product; however, the concentration ofalkenes can still be significant. In either case, thermaldecomposition of liquid hydrocarbons can be expectedto produce light olefinic compounds,133 (i.e., ethene,propene, and butenes), and their presence in naturalgas would be indicative of C-C bond homolysis. Theresults of the van den Berg et al.129,131 studies suggest,however, that these thermolysis indicators would berapidly oligomerized by silica-aluminas and naturalacid clays in coalbeds and surrounding strata into liquidalkanes, essentially masking thermolysis behavior.

Thermolysis can also generate alkenes larger thanC4.128 Mango et al.,15 citing others, adds that “...labora-tory maturation of source rocks under open condition-s...generates both hydrogen...and R-alkenes as major

products.” Because entire source rocks were presumablybeing maturated, we cannot speculate if thermolysis ormineral catalysis, or both were involved in the genera-tion of the alkenes and hydrogen. Mango et al.15 addfurther, however, again citing others, “Moreover, thehydrogenation of alkenes has been proposed as thecritical step in the formation of alkanes under closedconditions...” This statement implies that transition-metal-catalyzed hydrogenation of R-alkenes is respon-sible for alkane formation. We speculate though, thatunder very mild conditions, larger alkenes could betransformed into other hydrocarbons by aluminosili-cates and acidic natural clays. Indeed, Henderson etal.,128 citing others, state that “[o]lefins...might wellundergo isomerization, rearrangement, cyclization, andhydrogen transfer in the presence of...[clay minerals].”

It was implied earlier that clays and other sedimen-tary minerals could catalyze kerogen cracking reactions.Such was inferred from the work of Hunt104,108 and byexplaining the contradicting results of Reynolds andBurnham72 in terms of heavy hydrocarbon retention.Indeed, adsorption of organic compounds on clay miner-als has been documented.134,135 However, Waples27

states that “[q]uestions about whether mineral catalysisactually is important in subsurface kerogen catagenesis,and about whether clay minerals or carbonates are moreeffective catalysts have not yet been answered.” Forexample, Eglinton et al.136 pyrolyzed Type II kerogen,with and without water, in the presence and absenceof montmorillonite, kaolinite, illite, calcite, and limonite.At 280 °C, the kerogen mixed with calcite and kaolinitegenerated the largest amount of extractable material.Also at 280 °C the kerogen pyrolyzed alone underanhydrous conditions generated more extractable mate-rial than kerogen pyrolyzed alone under water, andunder water with montmorillonite, illite, and limonite.At 330 °C, the kerogen pyrolyzed alone under watergenerated the largest amount of extractable material,followed by kerogen heated with kaolinite and calcite.Eglinton et al.136 state that “...montmorillonite, illite,and limonite do not appear to enhance generation [ofextractable material] at the lower pyrolysis tempera-ture...[nor] at 330 °C [because they] either actuallypromote cracking of material [to gaseous products]or...suppress [material] generation.”

Horsfield and Douglas137 pyrolyzed various isolatedcoal macerals under anhydrous conditions, alone andwith various minerals. They also pyrolyzed vitriniticsediments of equivalent rank. In general, the C1-n-C6fraction increased while the n-C15+ fraction decreasedwhen the macerals were heated with minerals. Thesame observation was made when the sediments werecompared with the isolated coal macerals. They specu-lated that the role of the minerals “...might simply bethat they afford preferential retention of ...[n-C15+]compounds...in the...pores long enough to allow second-ary fragmentation to smaller molecules, and so pro-duce...an enhanced...[C1-n-C6 fraction], compared to that

(130) Aronson, M. T.; Gorte, R. J.; Farneth, W. E.; White, D. J. Am.Chem. Soc. 1989, 111, 840-846.

(131) van den Berg, J. P.; Wolthuizen, J. P.; van Hoof, J. H. C. J.Catal. 1983, 80, 139-144.

(132) Kossiakoff, A.; Rice, F. O. J. Am. Chem. Soc. 1943, 65, 590-595.

(133) Jones, E. G.; Balster, L. M.; Balster, W. J.; Striebich, R. C.Effects of Pressure on Supercritical Pyrolysis of n-Paraffins. Prepr.Pap.sAm. Chem. Soc., Div. Petroleum Chem. 1999, 44 (3), 394-397.

(134) Hoffman, R. W.; Brindley, G. W. Geochim. Cosmochim. Acta1960, 20, 15-29.

(135) Brindley, G. W.; Bender, R.; Ray, S. Geochim. Cosmochim. Acta1963, 27, 1129-1137.

(136) Eglinton, T. I.; Rowland, S. J.; Curtis, C. D.; Douglas, A. G.Org. Geochem. 1986, 10, 1041-1052.

(137) Horsfield, B.; Douglas, A. G. Geochim. Cosmochim. Acta 1980,44, 1119-1131.


of the...[coal macerals] alone.” Condensation reactionsof the higher molecular weight fraction was also specu-lated. They cautioned “...however, that total productyields were not measured for the suite of sediments andkerogens, so...[there is] no direct evidence to support themechanism[s] proposed.” Morever, few inferences canbe made due to the unusually high temperaturesutilized (Tmax ) 800 °C).

Kawamura et al.138 studied the generation of organicacids generated from isolated Type I and Type IIkerogens pyrolyzed with and without “...water andminerals (montmorillonite, illite, and calcite).” Thestudy concluded that “...a combination of water and clayminerals (montmorillinite or illite) enhances the releaseof organic acids from kerogen...” However, the acidsrecovered from the water and calcite mixture were lessrelative to the amount of acids recovered from thekerogen and water mixture alone. This observation canbe explained by the catalytic decomposition of the acidson the calcite. Shimoyama and Johns139 heated docosano-ic acid and nonadecanoic acid with calcite at 250 °C for50 and 300 h. The data “...show...that at 250 °C CaCO3promotes formation of substantially greater amounts ofhydrocarbons than are produced by thermal degradationin the absence of CaCO3.” The major product formedwas the n-alkane resulting from cleavage at the â-posi-tion (i.e., an alkane containing two less carbons thanthe parent acid.) They also reported that “[t]he 50h...heating experiment did not produce detectableamounts of n-alkanes with chain lengths greater thanthose of the fatty acid. However, longer chain n-alkaneswere produced by heating for 300 h..., revealing minorpolymerization reactions.” In contrast, when the acidswere heated with montmorillonite, the major productwas the n-alkane resulting from decarboxylation (i.e.,an alkane with one less carbon than the parent acid.)Linear alkanes having longer chain lengths than theoriginal acids were detected at both 50 and 300 h.Similar to the observations of Henderson et al.,128

Shimoyama and Johns also reported that “...much of theconsumed fatty acid[s]...[were] converted to a brown,insoluble, kerogen-like material. It is possible that someof the alkanes produced may [have been]...trapped inthis material and not released by the extraction proce-dure.”

Jurg and Eisma140 heated docosanoic acid at 200 °Cover bentonite clay with and without water. Underanhydrous conditions, n-C21 was the major alkanedetected. Alkanes containing carbon chains longer thanthe parent acid were also detected. Both of theseobservations are in agreement with Shimoyama andJohns. However, they also collected C3-C6 hydrocarbonsunder liquid nitrogen. Propene, butenes, and penteneswere the major volatile species after 89 h pyrolysis time.As mentioned earlier, these olefins could have originatedfrom carbenium ion intermediates which underwentâ-scission to form an alkene and a smaller carbeniumion. However, after 760 h heating time, the alkenes wereonly minor constituents. This would lend credence toan earlier assertion that light alkenes could react on

clay minerals to form saturated hydrocarbons. When theacid was heated on the clay with water, “...only verysmall quantities of the hydrocarbons C1 to C6 weredetected, whereas in the higher molecular-weight frac-tions, ...n-alkanes up to C29 were detected.” No hydro-carbons were produced when the acid was heated alone.Furthermore, very little branching was noted in thehydrocarbons produced under water relative to thehydrocarbons produced under anhydrous conditions.These results would tend to support Henderson et al.,128

who, citing others, state, “Water is present in sedimen-tary deposits during diagenesis, but it is evident from-...laboratory experiments that water does not inhibitthese alteration processes, though it does affect thecomposition of the products.” Fatty acids have also beenimplicated in petroleum genesis.141-143

Huang144 models hydrocarbon generation from coalas occurring in 3 stages. The first two stages areprimarily oil-generative while the last stage is primarilygas-generative. Huang145 asserts that the first oilgeneration stage is during diagenesis (R0 ) ∼0.2-0.5%).At this stage, the coal kerogen is believed to be tooimmature to generate oil, “...as proved by the fact that[the] hydrogen content of the young kerogen [is] etherconstant or increases slightly.”145 Thus, the source ofthe oil “...is mainly derived directly from decarboxylationof nonhydrocarbon[s].”145 While Huang does not definewhat constitutes “nonhydrocarbons,” we speculate itcould include any species containing carboxyl groups(i.e., organic acids and esters). Linear146 and aromatic147

organic acids have been identified in low rank coals andBoudou et al.148 have reported that “[t]he decarboxyla-tion and dehydroxylation [of coal] before pyrolysisresulted in a substantial increase and acceleration ofoil formation.”

Brooks and Smith149 present evidence in support ofthis early oil generation model. They state, “Long-chainparaffins and other hydrocarbons are formed from waxand cuticle constituents in brown and subbituminouscoals with increasing depth of burial. The formationtakes place mainly at an early stage of diagenesis, asthe carbon content increases from about 70% to 82% inthe transformation to high-volatile bituminous coal.”The study focused on ester waxes as a possible paraffinsource. Crystalline esters extracted from a brown coalwere heated at 330 °C for 5 days under water. Thisexperiment yielded 10% even-carbon-numbered n-al-kanes. However, when the experiment was repeatedwith either kaolinite or bentonite clay, a 30% yield ofn-alkanes with a smooth molecular distribution wasrealized. Interestingly, Brooks and Smith also reportedthat the “...addition of bentonite to the coal beforeheating with water to 330 °C had almost no effect on

(138) Kawamura, K.; Tannenbaum, E.; Huizi, B. J. Geochem. J.1986, 20, 51-59.

(139) Shimoyama, A.; Johns, W. A. Geochim. Cosmochim. Acta 1972,36, 87-91.

(140) Jurg, J. W.; Eisma, E. Science 1964, 144, 1451-1452.

(141) Cooper, J. E.; Bray, E. E. Geochim. Cosmochim. Acta 1963,27, 1113-1127.

(142) Mair, B. J. Geochim. Cosmochim. Acta 1964, 28, 1303-1321.(143) Shimoyama, A.; Johns, W. D. Nature Phys. Sci. 1971, 232,

140-144.(144) Huang, D. J. Pet. Sci. Eng. 1999, 22, 131-139.(145) Huang, D. J. Pet. Sci. Eng. 1999, 22, 121-130.(146) Snape, C. E.; Stokes, B. J.; Bartle, K. D. Fuel 1981, 60, 903-

908.(147) Hayatsu, R.; Winans, R. E.; Scott, R. G.; Moore, L. P.; Studier,

M. H. Nature 1978, 275, 116-118.(148) Boudou, J. P.; Espitalie, J.; Bimer, J.; Salbut, P. D. Energy

Fuels 1994, 8, 972-977.(149) Brooks, J. D.; Smith, J. W. Geochim. Cosmochim. Acta 1969,

33, 1183-1194.


the yield or molecular distribution of the n-alkanesformed.... Decarboxylation must presumably have takenplace in the absence of the added bentonite...” Whenthey heated the chloroform-insoluble part of the coalwith stearic acid at 300 °C under water for 5 days, a10% yield of n-heptadecane was obtained. No hydrocar-bon was recovered when the stearic acid was heatedalone under water for 5 days. Thus, they speculated thecoal itself acted as a catalyst.

Nelson et al.17 implicitly support the Huang model ofearly oil generation by asserting that “...n-alkanes incoal are primarily formed by mineral-catalyzed defunc-tionalization and cracking of n-alkanoic acids...” Nelsonet al. also acknowledge that “[t]he catalytic activity ofclays and other minerals is highly dependent upon...theamount of water that is present.” Thus, these workersindicate that if water is present in significant quantity,defunctionalization and cracking of the organic acidswill not occur, and indeed, the amount of water expectedto be present during diagenesis is substantial, asevidenced by Huang144 who states that primary migra-tion of the oil is “mainly in water-solution and water-drive[n] form with CO2 [acting] as [a] migration pro-moter.” However, as pointed out earlier, adequateevidence exists to suggest that water does not inhibitthe catalytic alteration of organic acids on clay mineralsurfaces; rather, the product is altered. Thus, with waterpresent in substantial quantities, the expected productsare primarily n-alkanes with a small concentration ofbranched hydrocarbons and very little gas. Huang144,145

asserts, however, that a considerable amount of gas willbe coproduced during this diagenesis alteration. Whileone could argue that this gas is biogenic residue formedduring peatification, the Huang model clearly indicatesa gas of separate, thermogenic origin. Interestingly,Brooks and Smith149 did report gas in all their artificialmaturation experiments, including heating brown coalat only 210 °C for 5 days under water. In several caseswhere the gaseous product was analyzed, it was “...foundto consist mainly of...saturated low molecular weightalkanes from methane to hexane...[with] [o]nly smallamounts of alkenes...” The primary constituent reportedwas propane, followed by methane, then ethane. Dataare insufficient to comment further on this point.

Nelson et al.17 support the n-alkanoic acid genesis ofn-alkanes on the basis of data obtained from supercriti-cal CO2 and Soxhlet extractions. They reported that lowrank coal n-alkane histograms “...exhibit bimodal carbon-number distribution profiles that strongly resemblethose of the biogenic n-alkanoic acids present in browncoals.” They add, “The compositional trait similaritiesbetween these n-alkanes and n-alkanoic acids and thecovariance of the bulk coal organic matter atomicoxygen-to-carbon (O/C) ratios and carbon-preferenceindex (CPI) values offer tangible evidence for theexistence of a genetic linkage between these two seriesof compounds.” They certainly have a point. The study,however, is unclear in the establishment of a key point.It is uncertain whether the paper purports that n-alkanoic acids are the sole precursors of n-alkanes.Admittedly, in regards to one reference they cite, it issuggested that such may not be the case. They pointout that “...an overall [increasing] abundance of shorterchain length n-alkanes with increasing thermal matu-

rity...is interpreted as reflecting the progressive genera-tion of n-alkanes.” Nelson et al. do not comment orexpand upon what is meant by the “progressive genera-tion of n-alkanes.” This is an important point, however,as the Huang model asserts a second oil-generativestage taking place during the vitrinite reflectance rangeof R0 ) ∼0.5-1.2%, with peak oil generation takingplace at ∼0.85% vitrinite reflectance. The model impliesthis latter point to be an inflection point because it ishere that oil generation starts to decrease, and gasproduction starts to increase, attaining maximum gasproduction at R0 ) ∼2%, which is the third stage ofHuang’s hydrocarbon generation model.

Huang also emphasizes three important points re-garding this second oil generation stage. First, the oildoes originate from the coal kerogen. Second, there isno odd-over-even or a far less pronounced odd-over-evenpreference reflected in these oils, and third, this secondoil generation stage produces far more oil relative to thefirst stage. This model seems to agree well with theobservations of Michael et al.,150 who reported that inthe Upper Cretaceous coals, “...the onset of thermalhydrocarbon generation begins at about 0.60% Rm [(Rm) mean random vitrinite reflectance)] and peak genera-tion occurs at about 0.85% Rm.” Radke et al.,34 instudying a series of 26 German coals, reported aninflection point at Rm ) ∼0.9%, which represented “...amaximum in yields of total soluble organic matter...”and the onset of thermal cracking. Radke et al. furtheremphasize the critical point “...that yield and composi-tion of soluble organic matter are strongly controlledby rank.” This is an important point because as men-tioned earlier, this may explain why the Wilcox coalkerogen was primarily oil-generative (see Table 4)throughout the entire artificial maturation experiments.Being lowest in rank, it could be argued that thekerogen simply had not passed through the inflectionpoint in the fixed time of the experiment. Unfortunately,Lu and Kaplan79 did not report a final %R0 value forthe kerogen, so only speculations can be made.

Huang’s model likewise agrees with our earlier as-sertions that R0 g 0.9% was required for minimumthreshold methane production and is high for coalshaving reflectance values higher than about 2%. Datareported in the Nelson et al.17 study likewise lendsupport to the Huang model. The total C6+ wt %supercritical CO2 extract yields have two distinct maximain the range of R0 ) 0.26-0.46%, and R0 ) 0.46-0.72%,both within the range predicted by the model. Ananomalous third maxima is at R0 ) 1.42%. The firstmaxima is centered at R0 ) 0.31% and the second at0.50%. This latter maxima is an unusually low reflec-tance value. Hutton et al.29 citing others, report thatvitrinite reflectance suppression can occur if the coalcontains liptinite. They also reported that “[a] relatedproblem is bitumen impregnation of vitrinite, especiallysamples in the early maturation stage. Bitumen im-pregnation depresses vitrinite reflectance.” Irrespectiveof the postulated problems, it appears evident that theNelson et al. data support the notion of two oil-generative stages. However, Nelson et al. do not com-ment upon this point and the possible different sources

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


of n-alkanes in these two stages. The only sourcementioned is the n-alkanoic acids, thus giving rise tothe previously mentioned uncertainty. This is an im-portant point because if the study does purport thatn-alkanoic acids are the sole precursors to n-alkanes,then the study implicitly assumes that the bimodallydistributed n-alkane fraction obtained via supercriticalCO2 extraction (at 120 °C for 2 h) is representative ofthe entire n-alkane content of the coal. A study con-ducted by Bartle et al.151 would disagree with thisassumption. Coal was extracted with a Soxhlet ap-paratus and by supercritical fluid extraction (SFE,presumably using toluene) at 340, 350, and 400 °C. Itwas first established that n-alkanes were not artificiallygenerated at the high SFE temperatures utilizing excessalkene concentrations as diagnostic indicators of ther-molysis. No significant alkene concentrations weredetected at 340 and 350 °C. As would be expected, asubstantially larger paraffin fraction was obtained bySFE relative to Soxhlet extraction. While the n-alkanedistributions obtained by the two methods from a SigmaSouth African coal showed striking similarities, theratios of various components were different. The n-alkane distributions obtained from a Markham coalwere very different and in all cases the CPI values weredifferent.

Butala152 extracted three Argonne Premium coals(Upper-Freeport, Pittsburgh #8, and Lewiston-Stockton)with benzene in a Soxhlet apparatus for 24 h, and in apressurized fluid extraction (PFE) unit for 24 h at 150°C, using excess phenolic compound concentrations asdiagnostic indicators of thermolysis. This particularclass of compounds was chosen because Butala152 andButala et al.153 argued that excess phenolic compoundsresulting from high-temperature solvolysis are thermo-lyzed fragments from vitrinite, of which lignin is a majorprecursor and thus contains many phenolic moieties.The Soxhlet n-alkane distributions for all three coalswere bimodal. However, the n-alkane distributionsobtained via PFE were markedly different. Because noexcess phenolic compounds were detected, and becausetheoretical calculations estimated that only ∼14.6 pgof oil would be produced by thermolysis,153 it wasconcluded that the PFE unit extracted additional n-alkanes normally inaccessible by conventional extrac-tion methodologies. These results are in agreement withSnape et al.146 who state that conventional “[s]olventextraction of...coals removes small quantities...of straight-chain alkanes which are closely related in compositionto the straight-chain fatty acids and are probablyderived from them during maturation. However, [addi-tional]...experiments...show...[that they]...are not themain source of the relatively large quantity of straight-chain alkanes...” Indeed, Dong et al.154,155 add “...that alarge amount of extractable organic material is trappedin the coal matrix so tightly that it cannot be fullyextracted using ordinary methods.” These conclusions

are closely associated with the “two-phase” model of coalthat is vigorously debated.156 Under this paradigm,studies suggest that the amount of trapped, low molec-ular weight material could be substantial in low rankcoals.157,158

Metal-catalyzed hydrocarbon hydrogenolysis or CO2

hydrogenation requires that gas-phase hydrogen bepresent. While mechanisms of hydrogen formation incoal seam reservoirs are uncertain, coal-gas hydrogencontents are nevertheless generally significant. A surveyof analytical data for natural gases compiled by the U.S.Bureau of Mines from 1917 to 1992 reveals that 253 of1067 gas samples from the Western United States werefound to contain hydrogen gas in the range of 0.1 to 0.3mol %; a few samples contained more than 1% and ashigh as 4%, while a large number of these samples werereported to contain only “trace amounts of H2.”159

However, even “trace amounts” could be significant,since only 500 ppm (0.05%) of hydrogen corresponds toa partial pressure of 0.7 atm in a geologic formation at1400 atm, a concentration adequate for driving catalytichydrogenation or CO2 methanation at the rates esti-mated earlier (Table 1). It should be noted, however,that absence of H2 in natural gas samples does notnecessarily imply that it was not formed, since it couldhave been readily consumed in metal-catalyzed reac-tions. Moreover, hydrogenolysis of liquid hydrocarbonscan proceed in the absence of gas-phase hydrogen,possibly by hydrogen transfer from larger hydrocar-bons.107

Geobel et al.160 state “...that although H2 is usuallythought of as being rare as a constituent of naturalgases, it is actually common in a wide variety ofgeological settings.” We have identified a number ofpotential routes for the formation of gas-phase hydrogenduring coal maturation which include (1) free-radicaldehydrogenation of hydrogenated aromatized clusters,19

(2) acid-mineral catalyzed reactions via carbonium ionformation,122 (3) water-gas-shift reaction (assuming asource of CO), and (4) steam-carbon reaction (whichmight provide CO at extreme temperatures and depths).Butala et al.,19 to a first approximation, estimated H2

generation rates via route 1. The rates were found tobe low under typical coalbed temperatures but becomingsignificant at ∼200 °C. As previously mentioned, route2 may generate some hydrogen gas under typicalcoalbed temperatures, but not a substantial amount.Rate calculations based on available literature data fornickel-catalyzed steam-reforming of hydrocarbons leadus to conclude that this reaction is not a viable route tothe formation of hydrogen at typical coalbed tempera-tures. Geobel et al. point out however, several otherpossible sources of H2. One is the low-temperaturedissociation of water via Fe2+ f Fe3+ redox reactions.For example, serpentinization of ultramafic rocks pro-

(151) Bartle, K. D.; Jones, D. W.; Pakdel, H.; Snape, C. E.; Calimli,A.; Olcay, A.; Tugrul, T. Nature 1979, 277, 284-287.

(152) Butala, S. J. M. Analysis of Hydrocarbons Generated inCoalbeds. Ph.D. Thesis, Brigham Young University, Provo, UT, 1999.

(153) Butala, S. J. M.; Medina, J. C.; Hulse, R. J.; Bartholomew, C.H.; Lee, M. L. Fuel, in press.

(154) Dong, J. Z.; Katoh, T.; Itoh, H.; Ouchi, K. Fuel 1986, 65, 1073-1078.

(155) Dong, J. Z.; Katoh, T.; Itoh, H.; Ouchi, K. Fuel 1987, 66, 1336-1346.

(156) Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein,B. C. Fuel 1986, 65, 155-163.

(157) Juriewicz, A.; Marzec, A.; Pislewski, N. Fuel 1982, 61, 647-650.

(158) Marzec, A.; Juriewicz, A.; Pislewski, N. Fuel 1983, 62, 995-998.

(159) Yin, P. Institute for Energy Research, University of Wyoming,Data extracted from the U.S. Bureau of mines data, February 19, 1998.

(160) Geobel, E. D.; Coveney, R. M., Jr.; Anginu, E. E.; Zeller, E. J.;Dreschhuff, G. A. M. Oil Gas J. 1984, 82, 215-222.


ceeds via

Geobel et al. also state that “[i]t is well-known that themixing of subsurface waters of differing ionizationpotentials can result in a weak electrical current andcontinuous production of H2.” Mantle outgassing viamagma intrusion was also identified as a source ofhydrogen gas. Indeed, it has often been speculated thatshallow coal seams containing thermally mature coalresulted from magma intrusion. Sugisaki et al.161 pointout that shearing of silicate rocks in the presence of H2Owill produce H2 gas. Indeed, they report that substantialamounts of “...H2 is generated from [a] paste made ofnewly pulverized rocks and water.” The speculatedmechanism is tSi• + H2 f tSi-OH + H•, followed by2H• f H2. It is also interesting to note that formationof significant amounts of H2 gas during mild pyrolysisof kerogens and coal is well documented.12,25,79 Thereare, however, too few data to enable drawing definitiveconclusions regarding the viability of any of these routesin coalbed seams. However, it does appear that hydro-gen gas generation could be the rate-limiting step intransition-metal-catalyzed reactions.

The concept of catalytic generation of natural gas incoal mines presupposes that suitable catalysts areavailable in the coal. This work implicates reducedoxides of nickel and iron (see Tables 1, 5, and 6) asimportant minerals for methane production. Most coalscontain significant amounts of iron minerals as pyrites,oxides, carbonates, clay/micas (e.g., illite), and/or orga-nometallic compounds (porphyrins, ferrous acetate, andferrous iron associated with carboxylic groups).162-167

Iron contents of 8 Argonne premium coals range from0.2 to 2.2%, while nickel contents of the same coalsrange from only 4 to 30 ppm.168 Much of the iron in coalis tied up as pyrites, which have no measurable activityfor hydrogenolysis or CO2 hydrogenation. Nevertheless,significant fractions of the iron are available in anumber of coals as reducible oxides, clays, or carbonates.For example, in a Mossbauer study of 10 representativeU.S. coals163 pyrite was found in all 10 coals, illite (amica structure) in 9, and siderite (FeCO3) in 5; pyritecontent was found to range over 19-100 wt %, illite over5-52%, and siderite over 9-79% of the iron minerals;of the iron minerals in a Sewell No. 1 coal, 28.1% wasmagnetite. Thus, 9 of these 10 coals contained oxidesand/or carbonates; 10-90% of the iron minerals, orroughly 200-250000 ppm, was apparently available ina readily reducible form for catalysis.

Calculations in Tables 1 and 6 show that only 100ppm of reduced, surface iron can catalyze high rates of

methane formation by hydrocarbon hydrogenolysis orCO2 methanation at geologic temperatures. Assumingthat iron at low concentrations would be highly dis-persed, i.e., have a significant fraction of iron atomsexposed to the surface (e.g., 5-15%), it is reasonable toconclude that as much as 100 ppm of reduced, surfaceiron could be available in selected coals for catalyzingmethane-forming reactions.

Although nickel occurs in coals at only ppm levels,even 1-10 ppm of reduced, surface nickel could besignificant in decomposing oil to methane at geologicconditions within coal maturation times. For example,Mango and Hightower107 estimate a half-life for lightcrude oil in contact with 1 ppm of active nickel to beapproximately 350000 years at 175 °C and 45000 yearsat 200 °C. Accordingly, nickel catalysis could play a rolein methane formation during coal maturation. However,since iron-catalyzed methane formation by hydrogenoly-sis or CO2 hydrogenation occurs at rates only 1-10times lower than the corresponding nickel-catalyzedreactions at the same catalyst concentration (see Tables1 and 6), and since iron is present at 2-3 orders ofmagnitude higher levels in coal, we expect that catalysisof methane-forming reaction by reduced iron mineralsto be a more likely route.

Catalysis of gas and oil formation in sedimentary rockwould require either a significant dispersed organicmatter (dom) content in the rock or migration of liquidhydrocarbons and light gases from the coal to the nearbyrocks or clay. However, there is lack of agreementregarding the importance of dom in associated clasticrock relative to dom in coalbeds.38 Migration of hydro-carbons from coals of lower rank (vitrinite reflectanceof 0.4 to 0.65%) is considered to be relatively easy;38

however, according to Cook,38 in higher ranks of coal,i.e., those most likely to produce natural gas, oil-likecompounds tend to be trapped within the coals andgradually cracked to gases, which provides an explana-tion as to why commercial quantities of petroleum arenot observed with all coal seams, although our calcula-tions indicate that all coal kerogens can produce sig-nificant quantities of oil (see Tables 4 and 5). WhileCook’s explanation is plausible, several other migratoryfactors need to be considered. For example, if oil is tomigrate from coal, then according to Snowdon,115 30-50 mg HC g TOC -1 needs to be produced to overcomethe expulsion threshold. If large quantities of oil areproduced, significant migration from the bed can occur.Porosity of the coal and surrounding strata wouldlikewise either inhibit or encourage migration. Thus,while it has previously been reported that gas and lightoil formation in surrounding strata is less likely thanwithin the coal seams,45 we conclude that the issue ofmigration likelihood (both horizontal and vertical),needs to be more carefully addressed before suchconclusions can be reached.

Larsen169 points out that “[m]ost oils, including...-[those] generated from Type III kerogens...[consistprincipally of] linear [hydrocarbons]....” Huang144 statesthat “...oils from coal gathered in reservoirs tend to belight oils. There, contents of saturates can reach over80%, and mono-, bi-, tricyclic light aromatics come next,while heavy aromatics with...[more than four

(161) Sugisaki, R.; Ido, M.; Takeda, H.; Isobe, Y.; Hayashi, Y,Nakamura, N.; Satake, H.; Mizutani, Y. J. Geol. 1983, 91, 238-258.

(162) Lefelhocz, J. F.; Friedal, R. A.; Kohman, T. P. Geochim.Cosmochim. Acta 1967, 31, 2261-2273.

(163) Huffman, G. P.; Higgins, F. E. Fuel 1978, 57, 437-448.(164) Montano, P. A. Mossbauer Spectroscopy and its Chemical

Application. Stevens, J. G., Shenoy, G. K. Eds.; Adv. Chem. Ser. 1981,194, pp 135-175.

(165) Taneja, S. P.; Jones, C. H. W. Fuel 1984, 63, 695-701.(166) Herod, A. J.; Gibb, T. C.; Herod, A. A.; Xu, B.; Zhang, S.;

Kandiyoti, R. Fuel 1996, 75, 437-442.(167) Schafer, H. N. S. Fuel 1977, 56, 45-46.(168) Finkelman, R. B.; Palmer, C. A.; Krasnow, M. R.; Aruscavage,

P. J.; Seller, G. A.; Dulong, F. T. Energy Fuels 1990, 4, 755-766. (169) Larsen, J. W. Personal communication, June, 1999.

6[Mg1.5Fe0.5SiO4] + 7H2O a

3[MgSi2O5(OH)4] + Fe3O4 + H2


rings,]...nonhydrocarbons, and asphaltenes rarely makeactual contributions.” During primary migration, itappears that some compounds are preferentially re-tained within the coal while others are more easilyreleased, which can explain coal-derived oils of theabove-stated composition. For example, Huang144 statesthat “...mono-, bi[-], [and] tricyclic light aromatics canbe expelled...[while] most of the aromatics, especiallypolycyclic hydrocarbons, are...[retained] in [the] coalstrata.” Butala,152 in studying Cretaceous coals andpetroleums in the Greater Green River Basin, concludedthat aromatics were preferentially retained in the coalmatrix over saturated hydrocarbons, in apparent agree-ment with Huang144 who states that “...the expulsionefficiency [of saturated hydrocarbons] can reach 86-87%in the initial and middle stages of hydrocarbon genera-tion, and 91% in the later stage[s]...” Hvoslef et al.52

likewise considered fractionation when attempting toestablish oil genesis from coal-bearing strata in theHitra formation. In this case, it was speculated thatlighter n-alkanes were preferentially released relativeto heavier n-alkanes. They also stated, citing anotherstudy, that “...retention...is dependent [primarily] on thefunctionality and aromaticity of the kerogen,...[and]...thatfunctional retentional effects...[are]...dominant in im-mature samples, whereas steric (or mechanical) reten-tion effects...[dominate]...in the oil window.” Erbatur etal.170 noted “...that for...high rank coals, short andstraight alkyl groups are preferentially transferredto...[a pyridine extraction solution], while long-chainalkyl groups, branched alkyl groups, or alicyclic struc-tures are concentrated in the residues.”

In any catalytic process it is important to know thechemical state of the active catalytic phase. It isespecially important in coalbed methane formation forpurposes of (1) modeling the process and (2) usingcatalytic minerals as fingerprints for natural gas ex-ploration. Mango and co-workers15,44,107 have reportedthat transition-metals oxides, e.g., NiO and Fe3O4, arethe active catalytic phases for hydrogenolysis of liquidhydrocarbons; however, in every instance their catalystprecursor, e.g., NiO/silica and Fe(AcAc)3 were “pre-treated” at 200-400 °C for 24 h in flowing H2, conditionswhich in our experience would effect nearly completereduction of the oxides or salts to the metal. Indeed,Medina et al.,106 citing the work of others, report thatthese conditions effect almost complete reduction (90%)of iron precursors to the metallic state. They also found,using oxygen chemisorption, that NiO/SiO2 at 400 °Cunder flowing H2 was reduced completely to the metallicstate, but even at 200 °C, “the extent of reduction tothe metal state was approximately 29%.” Considerableliterature on nickel and iron-catalyzed CO2 methanationand hydrocarbon hydrogenolysis provides strong evi-dence that metals, rather than their oxides, are theactive phases. Thus, we conclude that the active cata-lytic phases for methane production in coalbeds includewell-dispersed Fe and Ni metals. We propose that oxidesor carbonates, e.g., Fe2O3 or FeCO3, dispersed in thepores of high-surface-area aluminosilicates, clay sili-cates, or silica in the coal, are partially reduced todispersed Fe during coal maturation in the presence of

hydrogen and may be subsequently reoxidized to theoxides or carbonates after mining and exposure to air.Indeed, Medina et al.103,106 found high rates of hydro-genolysis and CO2 hydrogenation on a 10% Fe/silicacatalyst that was reduced at only 200 °C for 96 h.Moreover, a sizable H2 chemisorption uptake (41 µmol/g) was observed, indicating that a significant fractionof the iron had been reduced to the metal.45 WhileMango and Hightower have shown that water partiallyinhibits the hydrogenolysis of petroleum on reduced Ni/silica, i.e., reduces the reaction rate about 30-40%, theyfound the effect to be reversed when water was replacedwith H2, indicating a weak inhibition rather thanoxidation of the metal surface. Interestingly, Medina etal.106 reported no change in methane formation rate viaCO2 methanation when water was added to a Nicatalyst.

We propose a new simplified scheme for coal-bed gasand oil formation (see Figure 9) in which coal kerogenis cracked thermally and/or catalytically by minerals toform oil and hydrogen gas; oil is then hydrocracked tomethane on Fe and Ni metals present in reduced,dispersed coal minerals. Alternatively, CO2 formedduring catagenesis reacts catalytically with hydrogenpresent in coal gas in the presence of metal minerals toform methane. The kerogen to gas thermolytic route,denoted by a question mark, indicates our uncertaintyregarding this route. As previously mentioned, it wasspeculated that metal catalysis could explain the ap-parent generation of methane from kerogen; however,without elemental analysis data for unoxidized coals noconclusions can be drawn. However, if methane gas wasgenerated directly via kerogen thermolysis, we speculateit would principally involve methyl groups attached tothe periphery of aromatized clusters. Spontaneous ho-molytic C-C bond cleavage of these groups appears tobe highly unlikely as the resulting radical would be inan orbital perpendicular to the π-system of the aromaticcluster and thus would not be stabilized. It appearsmore likely that hydrogen-transfer reactions would beinvolved where the radical intermediate could be sta-bilized. Release of the methyl radical would restorearomaticity. The methyl radical could then be cappedwith hydrogen from other molecules or hydroaromaticmoieties in the kerogen. The radical formed by the latterroute could be expected to be long-lived, as evidencedby the radical densities in Tables 8 and 9. Stein171

reported that carbons formed from anthracene polym-erization via pyrolysis were expected to catalyze the

(170) Erbatur, G.; Eubatur, O.; Davis, M. F.; Maciel, G. E. Fuel 1986,65, 1265-1272. (171) Stein, S. E. Carbon 1981, 19, 421-429.

Figure 9. New simplified scheme for coalbed methanegeneration.


reaction. Stein,172 citing others, also reports that meth-oxy groups with an adjacent (ortho) hydroxy groupattached to aromatized clusters can undergo thermolysisto form methane and an R,â-diketo group. It wasreported that the adjacent hydroxy group resulted insignificantly faster thermolysis rates than the methoxygroup alone. Stein, however, demonstrated that this isa bimolecular reaction. A radical removes a hydrogenatom from the hydroxy group resulting in an R-oxyradical intermediate. Apparently then, in a concertedstep, a methyl radical is ejected and the R,â-diketo groupis formed. The activation energy from intermediate toproduct is only ∼30 kcal/mol. Thus, the mechanisticpathway of kerogen to gas thermolysis cannot bediscounted. Also indicated in the simplified scheme isthe oil-to-gas contributory route. As mentioned before,the 10% conversion times of liquid hydrocarbons viathermolysis listed in Table 6 may not necessarily be longin a geological time frame. Thus, this route cannot bediscounted altogether as a contributory route, although,as previously mentioned, this is not a primary route tonatural gas generation, as evidenced by the expectedproduct distribution.

Our scheme is consistent with and in several aspectssimilar to the recently proposed model of Mango109 fornatural gas generation in petroleum source rock. Hismodel asserts Ni metal catalyzed hydrogenolysis ofpetroleum to olefins and olefins to principally methaneat low-temperature geologic conditions. Our calculationsfurther demonstrate that catalytic hydrogenolysis andCO2 hydrogenation are plausible reactions to havegenerated natural gas in coal formations.

Gas formation by CO2 hydrogenation is extremelyrapid, but relies on significant concentrations of hydro-gen and CO2 being present simultaneously. Such asituation is plausible since coal has adequate micro- andmacroporosity for storing significant quantities of oil,H2, CO2, and CH4 gases.38,173 Since CO2 is generated inrelatively large quantities during coal maturation,generation of adequate amounts of hydrogen gas forreducing iron minerals to the metal and reacting withCO2 will probably be the rate-determining process. IfH2 is not generated in significant quantities, then underthis scenario one might expect relatively high amountsof CO2 to be present in the natural gas. The kinetic datareported in this study also indicate that if reduced ironand H2 were present in significant quantities, then acoalbed might have low concentrations of oil and highamounts of methane. Furthermore, if gas can be trappedor adsorbed in tight surrounding formations, it may bepossible to produce more gas than those quantitiesadsorbed in the pores. Under this scenario, the some-times observed methane contents exceeding adsorptioncapacities of the coalbed is explained.

On the other hand, if high concentrations of acidicminerals were present relative to reduced iron, then (atlow water concentrations) one might expect to findsignificant contributions made by the acid-minerals to

the coalbed gas content with commensurate amountsof branched hydrocarbons, as observed in samples fromthe Cherokee, Forest City, and San Juan Basins. Ifwater concentrations are high, high quantities of oil maystill be generated at higher subsurface temperatures viadecarboxylation of organic acids in low rank coals.

If, however, neither acid-minerals or reduced ironspecies are present and conditions are such that theassumption of distributed activation energies is valid,then large quantities of both oil and gas can be gener-ated over long periods of geological time via thermolysis.Furthermore, if conditions are such that gas and oil areretained within the coal, then under this scenario onemight expect a wet gas of relatively low methanecontent.

A note of caution should be made, however, whenapplying and interpreting data based on distributedactivation energies. Braun and Burnham174 point outthat “[t]he typical procedure usually assumes a singlefrequency factor, which is probably not strictly valid.Moreover, independent parallel reactions are assumed,whereas real systems often have competing processes.”This may be problematic when extrapolating over widetemperature ranges. Indeed, Snowdon175 states that “-[with competing] multiple reaction mechanisms, ...onereaction...[may be] controlling the kinetics in one ther-mal regime while a different reaction may be controllingin a different thermal regime.” Irrespective of theseproblems, however, Galimov176 used distributed activa-tion energies to model the Urengoy supergiant gas fieldin the former U.S.S.R, which said model “suggests thatthe supergiant gas accumulations...[were]...more likelyformed from terrestrial organic matter through ther-mochemical reactions at moderate depths and...[were]...not the result of either deep, overmature, orbacteriogenic processes. The...model [further] suggeststhat humic organic matter, in general, has a highmethane-generating capacity at comparatively low ma-turities of organic matter equivalent to 0.5-0.7% on thevitrinite reflectance scale.” However, the high methane-generating capacity of the gas field can also be easilyexplained in terms of metal catalysis, especially at themoderate temperatures implied by Galimov. Metalcatalysis likewise provides an explanation for the highconcentrations of indigenous gas found in the immatureBakken shale formation.177 Indeed, Price and Schoell177

suggest that transition-metal catalysis may play a rolein gas production, acknowledging that the rocks are richin transition metals. However, it should also be notedthat they favor other mechanistic explanations. Themajority of previously reported models of coalbed meth-

(172) Stein, S. E. A Fundamental Chemical Kinetics Approach toCoal Conversion in New Approaches in Coal Chemistry, ASC Sympo-sium Series 169; Blaustein, B. D., Bockrath, B. C.; Friedman, S., Eds.;American Chemical Society: Washington, DC, 1981; pp 97-129.

(173) Beamish, B. B.; Crosdale, P. J.; Moore, T. A. Fundamentalsof Methane Sorption by New Zeland Coals; Lecture at the InternationalConference on Coal Seam Gas and Oil, Brisbane Australia, March 23-25, 1998, Handbook and Abstracts, p 25.

(174) Braun, R. L.; Burnham, A. K. Energy Fuels 1987, 1, 153-161.

(175) Snowdon, L. R. Am. Assoc. Pet. Geol. Bull. 1979, 63, 1128-1138.

(176) Galimov, E. M. Chem. Geol. 1988, 71, 77-95.(177) Price, L. C.; Schoell, M. Nature 1995, 378, 368-371.(178) Apparently, Brooks is attempting to convey the Brønsted

acidity of these materials using a familiar frame of reference, althoughit is unclear whether that conveyance is qualitative or quantitative. Ifit is qualitative, then one could infer that the proton-donating abilityof these minerals is significant, as evidenced by the fact that HCl is astrong, solution-phased acid. Indeed, numerous studies support thenotion that silica-aluminas are strong, solid acids. However, complica-tions arise if the comparison is quantitative. Farneth and Gorte118 statethat one “...real problem is the lack of an acceptable scale of solid aciditycomparable to...scales for aqueous solutions,...[given]...that solvationeffects are extremely important in the overall thermochemistry ofproton transfer.”


ane formation have not considered nor included datafor catalytic reactions. It is hoped that the data setprovided by this work adequately illustrates its impor-tance and will be useful in refining such models asnecessary.

Conclusions

(a) Rates based on kinetic data assembled from theliterature, assuming single activation energies andisothermal conditions, predict that at typical coal-bedtemperatures, gas formation rates from hydrocarbonthermolysis reactions are several orders of magnitudetoo low to account for the formation of known coal-seamnatural gas reserves during coal maturation. A modelassuming distributed activation energies can accountfor formation of significant quantities of methane gasfrom liquid hydrocarbon and kerogen cracking over longperiods of geological time. However, gas produced bythis mechanism does not resemble coalbed natural gasin composition.

(b) Acid-mineral-catalyzed cracking, iron- or nickel-metal-catalyzed hydrogenolysis of liquid hydrocarbons,and iron- or nickel-metal-catalyzed CO2 hydrogenationoccur at rates 3-10 orders of magnitude more rapidthan thermolysis and can account for formation ofenormous reserves of coalbed gas in relatively shortperiods of time.

(c) Product distributions from thermolysis and cata-lytic cracking of liquid hydrocarbons are very differentfrom typical coalbed gas. Nevertheless, thermolysis ofliquid hydrocarbons can account for wet gases and acid-mineral catalysis for highly branched hydrocarbons.However, these products are generally different thannatural coalbed gas. The compositions of gaseous prod-ucts from metal-catalyzed hydrogenolysis and CO2hydrogenation are almost identical to typical composi-tions of coalbed gas.

(d) Coal deposits contain iron and nickel minerals thatcould function as catalysts for hydrocarbon hydro-

genolysis and CO2 hydrogenation although much of theiron in many coals is tied-up as pyrites and has nomeasurable activity for hydrogenolysis or CO2 hydro-genation. Significant fractions of the iron and nickel,nevertheless, are available in a number of coals asreducible oxides, clays, or carbonates.

(c) From the results of this and previous studies, it isconcluded that the active catalytic phases for methaneproduction in coalbeds can include well-dispersed Feand Ni metal crystallites.

(f) Transition-metal catalysis appears to be rate-limited by the production of hydrogen gas.

(g) Assuming distributed activation energies, calcu-lated rates from literature data support the concept thateither metal catalysis or thermolysis can account forenormous reserves found in coalbeds. If thermolysis isimplicated, then production over large periods of geo-logical time must also be assumed in conjunction withappropriate conditions leading to the distributed activa-tion energies. If metal catalysis is implicated, thensufficient hydrogen generation for metal-mineral reduc-tion and reaction must also be accounted for. Thetransition-metal catalysis model further suggests thepossibility of using analyses of coal minerals, e.g., ironoxide, carbonate, and metal contents, for gas resourceexploration.

Acknowledgment. The authors gratefully acknowl-edge the technical assistance of Dr. P. Yin and ProfessorR. Surdam of the Institute of Energy Research, Uni-versity of Wyoming, in obtaining coal data and coal gascompositions and Dr. Charles R. Nelson of the GasResearch Institute in providing useful guidance andperspectives. The authors further acknowledge therelevant and insightful criticisms of John W. Larsen andthe other referees of Energy & Fuels. The authors alsogratefully acknowledge financial support from the GasResearch Institute under Contract No. 5093-260-2764.

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Mechanisms and Kinetics of Reactions Leading to Natural Gas Formation during Coal Maturation