Mechanisms and Kinetics of Reactions Leading to Natural Gas

VOLUME 14, NUMBER 2

MARCH/APRIL 2000

© Copyright 2000 American Chemical Society

Reviews Mechanisms and Kinetics of Reactions Leading to Natural 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 oil and gas at typical low-temperature conditions of coal maturation. These data indicate that gas formation rates from hydrocarbon thermolysis are several orders of magnitude too low to have generated known coal-seam natural gas reserves, assuming bulk first-order kinetics defined by a single activation energy and preexponential factor. By assuming distributed activation energies, thermal cracking of liquid hydrocarbons and coal kerogen to methane can occur at sufficiently high rates to produce commercial quantities over long periods of geologic time. Acid-mineralcatalyzed cracking, transition-metal-catalyzed hydrogenolysis of liquid hydrocarbons, and transition-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 predicted from thermolysis; moreover, the gaseous products for metal-catalyzed hydrogenolysis of hydrocarbon liquids and for CO2 hydrogenation are nearly the same as those of typical natural coalbed gases, while gases from thermal and catalytic cracking differ from most coalbed gases. The available data are most consistent with a model involving thermal and catalytic cracking of kerogen 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 hydrogenation.

Introduction Coal seam reservoirs are important potential sources of natural gas. In fact, worldwide resources are estimated to be about 3000-12000 trillion cubic feet (TCF), with U.S. reserves alone estimated to be 90-400 TCF.1-3 While enormous, relatively little of this resource * 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 Methane Extraction; IEA Coal Research: London, 1995; pp 22-29.

is being produced, in good part due to the absence of reliable gas sweet-spot indicators. Accordingly, the potential for expanded production is considerable, if more reliable geologic markers could be discovered. It is commonly assumed that oil and hydrocarbon gases were formed in coal seams by thermolysis (crack(2) Murray, D. K. Coalbed Methane in the USA: Analogues for Worldwide 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 Methane Resources of the United States: AAPG Studies in Geology Series # 17; Rightmire, C. T., Eddy, G. E., Kirr, J. N., The American Association of Petroleum Geologists, Tulsa, OK, 1984; Chapter 1, pp 1-13.

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

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ing) of coal organic matter;4-11 this model is supported by experiments showing high rates of gas and oil production during pyrolysis of sedimentary organic matter at temperatures of 200-250 °C.12-13 Recently however, the reliability of the thermogenic model for oilgas and coal-gas generation has been questioned14-17 because (1) thermolysis of model organic compounds is too slow to account for the present reserves even over time periods of hundreds of millions of years,18-20 (2) the hydrocarbon product distribution obtained during thermolysis of model organic compounds is much different than natural gas;15,18,19,21-23 and (3) results of artificial maturation experiments indicate that clays participate in hydrogen exchange of aromatic compounds and apparently catalyze hydrocarbon formation from sedimentary organic matter.24-25 This suggests that mineral catalysis may play a crucial role in hydrocarbon gas formation during coal maturation. This paper identifies the key types of molecular transformations that mineral catalysis might impact and assesses the potential effects of this catalysis on gas generation during coal maturation. Method, Justification, and Limitations Proposed, abiogenic methane formation mechanisms from the scientific literature were identified. Rate constants and activation energies were calculated from the available rate data for specified experimental conditions. The rate constants were than recalculated to catagenic temperatures (50-200 °C). The potential contribution of each mechanistic route to methane formation was then evaluated on the basis of kinetics and methane selectivity. Extrapolation of literature data to geologic conditions required four important assumptions: (1) about 10 wt (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, 20822102. (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 During Coal 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 Indicators for 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, 135144. (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.

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% of the carbon in the coal is converted to methane during maturation, (2) coal has a 20 wt % kerogen content of which 80 wt % can be thermally or catalytically converted to liquids and/or gases, (3) coal has a typical porosity of 20%, and (4) one-third of the pore volume is occupied by liquid or vapor hydrocarbons and the remaining by water vapor and gases. The first assumption of 10 wt % conversion of hydrocarbons to methane was based on an estimate of a typical gas generation potential of 150 cm3 (STP) g-coal-1 during maturation of a U.S. carboniferous coal,26 which corresponds to 0.080 g-C g-coal-1 or 0.11 mol-CH4 mol-C -1 in the coal. The second assumption of 20 wt % kerogen content is based on the arithmetic mean of kerogen content determined by us for two Valencia Canyon coals. While Waples27 states that “[c]oal is best thought of as a special kind of kerogen that is relatively undiluted by mineral matter...”, kerogen, in the context of this paper, is defined as “the... organic material of sedimentary rocks which is insoluble in organic and inorganic solvents”.28 Hutton et al.29 add further that this “definition for kerogen has grown to include all solid organic matter in all sedimentary rocks...[including,]...oil shales, coal, clastic sedimentary rocks, and metamorphic rocks which contain solid organic matter...” While imprecise and problematic, we have adopted this definition because the kerogens listed in Tables 2, 4, 5, and 7, under the subheading of “Isolated Type III Kerogens” are best defined in this manner. These kerogens are the dried, residual material remaining from coal after an HF/HCl treatment followed by two extractions. These kerogens were included in the data set so that inferences could be made as to the presence and absence of the bitumen and mineral matter. Unfortunately, the residual masses were not reported. Indeed, Caldier et al.30 state that very little “attention has been paid to the quantitative determination of kerogen in source rocks...” “There are problems [however,] when defining kerogen as the ‘insoluble organic matter in sedimentary rocks’. Solubility is...[dependent on] the solvent, and the solvents used have varied widely”.29 Waples27 further adds, “Because this is an operational definition, the exact quantity and chemical composition of kerogen will depend on many factors, such as the solvent used in extraction, the length of time used for the extraction, and the particle size to which the rock was ground before it was extracted. Significant problems can therefore arise when data from two laboratories are compared.” Notwithstanding these problems, we define the isolated coal kerogens on a procedural basis. Specifically, the coal was ground to pass through a 200 mesh sieve, then dried in a vacuum oven overnight at room (26) Rice, D. D. Composition and Origins of Coalbed Gas. In Hydrocarbons 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, 14781488. (30) Caldiero, L.; Chiaramonte, M.; Pellegrin, L.; Rausa, R. Fuel 1992, 71, 277-281.

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temperature, then exhaustively extracted with THF in a Soxhlet apparatus. The coal was then treated with a 20% HCl solution for 2 h at room temperature, then a 20% HCl/48% HF solution for 3 days at room temperature, then again with a 20% HCl solution for 2 h at room temperature. The coal was then exhaustively extracted again with THF in a Soxhlet unit and dried in a vacuum oven overnight at room temperature. The residual dry mass was quantified and defined to be the coal kerogen. While the kinetic calculations in this study were derived from coal or material thought to be ubiquitous to coal-bearing strata, occasionally we cite other studies that utilized Type I and/or II kerogens, especially in clarifying probable oil generation mechanisms. Admittedly, these kerogen types are chemically distinct from Type III kerogens. Differences have been noted in solvent swelling,31 thermal cracking,32 and selective catalytic chemical degradation33 experiments. However, Radke et al.,34 citing the work of others, point out that “[s]oluble organic matter of a petroleum-like gross composition is generated from coals as a byproduct of the coalification process during subbituminous and high volatile rank stages. This process shows similarities with the maturation of [marine-based] organic matter in petroleum source beds.” The third assumption is confirmed by measured porosities of 15-20% for several U.S. coals of low to medium rank,35 the largest porosity fraction consisting of macropores having diameters of 30-300 nm.36,37 The fourth assumption is reasonable since the molecular fraction 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 low rank are readily accommodated by the macropores. However, Vahrman39 infers that a significant quantity of additional hydrocarbons may be contained in the micropores and/or as clathrate entities entrapped by a macromolecular moiety or moieties. Levine40 furthermore, citing the work of others, states that “...loss of primary microporosity in the oil window may be due to ‘plugging’ by hydrocarbons generated during coalification.” While the concept of three-dimensional, interconnected pores has been challenged,40,41 Levine comments that it is nevertheless “... a useful conceptual model to (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, 695702. (34) Radke, M.; Schaefer, R. G.; Leythaeuser, D.; Teichmu¨ller, 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, 272277. (37) Parkash, S.; Chakrabartty, S. K. Int. J. Coal Geol. 1986, 6, 5570. (38) Cook, A. C. Oil Occurrence, Source Rocks and Generation History of Some Coal-Bearing Tertiary Basins; Plenary Lecture at the International Conference on Coal Seam Gas and Oil, Brisbane Australia, 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.; The American 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.

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describe pores as physical features of the coal which may be either vacant or filled with a resident pore-filling substance.” The comparisons in this work of rates and selectivities for methane production include data obtained in both batch (closed) and flow (open) systems. There are no fundamental problems in comparing rates for a single stoichiometric reaction obtained in these two different kinds of reactors (systems) as long as the comparisons are made at the same temperatures, reactant partial pressures, and conversions as well as similar reaction and residence times.42 If these conditions are met for a network of parallel single reactions, product selectivities may differ, but not substantially so. For example, Schenk and Horsfield43 pyrolyzed Torcian shale samples in both open and closed systems utilizing heating rates of 0.1, 0.7, and 5.0 K min-1. They reported “...very similar petroleum potential vs activation energy distributions [resulted],...[and] that bulk petroleum generation is simulated equally well using either closed- or opened-system pyrolysis...[and that] the predicted temperature ranges of oil and gas formation under geological heating conditions are unaffected by the open or closed nature of the method used for kinetic model calibration.” It was further reported, however, that the closed system produced lower yields of petroleum relative to the open system, due to either secondary cracking or tar condensation in the analytical equipment. Careful analysis of the paper further suggests, however, that other extraneous confounding factors may also have contributed to the low yields. Reasonably good agreements between rates and selectivities for olefin hydrogenolysis and CO2 hydrogenation in both batch reactors44,45 and flow systems46,47 have also been reported. However, rate and selectivity data obtained for a sequence of reactions (i.e., kerogen f oil f gas) are very different when measured in a lowresidence-time open system relative to those obtained in a closed, high-residence-time system. Accordingly, this paper only examines and compares parallel, singletype reactions, (i.e., kerogen f oil, oil f gas, and kerogen f gas). A study conducted by Monthioux et al.48 is often referenced to illustrate the differences produced by open- and closed-system pyrolysis. Mahakam delta coals were subjected to open- and closed-system pyrolysis and the products compared with CCl4-extracted bitumen. It was concluded that the closed-system more closely approximated natural bitumen evolution when compared with the open-system, as the open-system produced different hydrocarbon distributions and a pre(42) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; John Wiley 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 Natural Gas During Coal Maturation. In Coalbed Methane: Scientific, Environmental and Economic Evaluation; Mastalerz, M., Glikson, M., Golding, S. D., Kluwer Academic Publishers: Norwell, MA, 1999; pp 279-296. (46) Weatherbee, G. D.; Bartholomew, C. H. Catalysis 1984, 87, 352-362. (47) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994; pp 526-555. (48) Monthioux, M.; Landais, P.; Durand, B. Org. Geochem. 1985, 10, 299-311.

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ponderance of alkenes. However, the heating rate of the open-system was 25 °C min-1 (no final temperature was reported), 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 and heating rate are key variables in coal devolatization.” The effect of heating rate has been empirically observed. For example, while rapid heating rates often lead to higher tar yields, the H/C ratio of the tar is concurrently degraded. Thus, the impact of the open or closed nature of the system on bitumen generation in the Monthioux et al. experiment is masked by differences in heating rates utilized. The problem is further complicated with Tmax not being reported. While recognizing heating rates and peak temperature to be important, these variables, being outside our control in the published studies, were ignored for pragmatic purposes. We note that while Price50 believes most systems are more accurately modeled as closed, Monthioux and Landais51 acknowledge “that a wide range of more or less open maturation systems could be encountered in nature.” Specific examples of such systems would include the Hitra formation, Haltenbanken area, offshore Norway,52 the Saar basin in Germany,53 the San Juan basin in New Mexico and Colorado,54 the Greater Green River basin in Wyoming,55 and the Talang Akar formation in Java, Indonesia.56 Nevertheless, considerable debate continues in this regard.57,58 Since the publication of Lewan et al.59 in 1979, demonstrating the generation of pyrolysates similar to natural crude oil from shales, and that of Lewan60 in 1985 claiming geological simulation of petroleum generation via hydrous pyrolysis, considerable debate has arisen as to if and/or how water impacts geological processes.61,62 Michels et al.63 point out, however, that “[a] survey of the existing literature...[reveals]...contradictions [which] may be related to the nature of the experimental setups used,” from which it can be inferred that some results are experiment specific (i.e., artifacts); thus, exact extrapolations to natural systems are not possible at our current stage of knowledge. Nevertheless, the preponderance of evidence supports the assertion that water is an important medium; however, its (49) Smith, K. L.; Smoot, L. D.; Fletcher, T. H.; Pugmire, R. J. The Structure 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.; Go¨tz, 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) Garcıá-Gonza´lez, M.; MacGowan, D. B.; Surdam, R. C. Mechanisms of Petroleum Generation from Coal, as Evidenced from Petrographic and Geochemical Studies: Examples from Almond Formation Coals in the Greater Green River Basin in Fiftieth Anniversary Field Conference, Wyoming Geological Association Guidebook; 1993, pp 311323. (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, 22072210. (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, 123134. (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.

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precise role in petroleum and gas generation remains uncertain.62 While its consideration in the chemical alteration of organic sedimentary matter is generally beyond the scope of this study, some important mechanistic aspects will be discussed. Coalbed methane can be formed by either biogenic or thermogenic means. Biogenic methane is formed during peatification via microbial activity64 and in coals not mature enough to produce methane thermogenically.65 Thermogenic methane is generated during the later stages of coalification by the thermal transformation of organic matter and accounts for the largest amounts of coalbed methane.64 Differentiation between biogenic and thermogenic methane is based on the isotopic 13C/12C ratio. The general observed trend is toward isotopically heavier natural gas with increased maturity and burial depth.66 It is generally accepted that isotopically heavy methane (δ13C > -50 %) is thermogenic gas. Methane with δ13C values between -50 and -60 ‰ is both biogenic and thermogenic, and isotopically light methane (δ13C < -60 ‰) is biogenic gas. Figure 1 displays the δ13C ranges of methane from several coal formations.67-71 Thus, as illustrated in the figure, the majority of coalbed methane is apparently thermogenic gas. This paper deals soley with thermogenic coalbed methane. Smith et 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 coalification depends on the geothermal history, i.e., on the rock temperature, especially the maximum temperature, and to a lesser degree on the duration of heat exposure.” Such a time/temperature model implies that the coalification process is kinetically rate-limited and is not equilibrium driven.40 While there is considerable debate as to the accuracy of this model, it is widely utilized in approximating real basin processes.40 The validity of this model is a principal focus of this study. Results Our survey of the literature uncovered relevant rate data for the formation of methane and other light hydrocarbons and, in some cases, liquid hydrocarbons from (1) thermal cracking of model liquid hydrocarbons, model polymers, paraffinic oil, and aromatic oil; (2) lowtemperature pyrolysis of various coal and coal kerogens; (3) acid-mineral cracking of model liquid hydrocarbons; (4) hydrogenolysis of 1-octadecene on a sedimentary rock containing transition-metal compounds; (5) hydrogenolysis of C3-C6 hydrocarbons on supported iron and nickel metal catalysts; (6) CO2 hydrogenation on supported iron and nickel metal catalysts; and (7) steam reforming of hydrocarbons on nickel catalysts. From these data, (64) Rogers, R. E. Coalbed Methane: Principles & Practice; PTR Prentice 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.

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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 Reactions under Geologic Conditions (180 °C)a reaction

rate, 180 °C (gmeth/gcoal y)a

t (10% conv.)b

Thermal

thermal cracking, n-C6 thermal cracking, n-C16H34 thermal cracking, aromatic oil thermal cracking, paraffinic oil

1.8 × 10-12 1.1 × 10-12 1.3 × 10-10 7.9 × 10-13

acid mineral cat. cracking, C16 to C1 sedim. rock hydrogenol., C18)c hydrogenolysis n-C5, Ni hydrogenolysis C18), Ni hydrogenolysis n-C5, Fe hydrogenolysis C18), Fe hydrogenolysis C12,)d Fed hydrogenolysis C12), Fee CO2 hydrogenation, Ni CO2 hydrogenation, Fe CO2 hydrogenation, Fe

Catalytic 2.1 × 10-6 3.7 × 10-5 5.6 × 10-4 1.8 × 10-4 1.8 × 10-4 1.8 × 10-5 1.7 × 10-5 2.9 × 10-5 5.4 × 10-1 1.1 × 10-1 1.3 × 10-2

9.2 × 107 y 2.0 × 107 y 8.1 × 105 y 1.3 × 108 y 10 y 146 d 91 d 29 d 285 d 299 d 9.6 y 5.6 y 2h 8h 36 h

Eact (kcal/mol)

ref

69 60 65 69

72 73 74 74

35 31 31 50 25

73 15 47 44 47 44 45 45 75 46 45

19 15 17

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 (or liquid) 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 be first-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.

low-temperature rates of methane production for thermal and catalytic reactions and activation energies for conditions applicable to coal beds were calculated to the extent allowed by the published information. Representative methane formation rates at 180 °C, estimated times for reaching 10% conversion, and activation energies for the relevant thermal and catalytic reactions are summarized in Table 1. Conversion times of 50% for catalytic reactions and thermal decomposition 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 at temperatures below 300 °C. It is evident that, under these low temperature conditions, thermal cracking of mineral-free aromatic and paraffinic oils to methane occurs at negligibly slow rates relative to those for acid catalytic cracking or catalytic hydrogenolysis. Indeed, times for 10% conversion of model compounds or paraffinic oil by thermolysis at 180 °C are on the order of tens to hundreds of millions of years. On the other hand, acid-mineral-catalyzed cracking of n-C16, Fe- or Ni-catalyzed hydrogenolysis of

240


Figure 2. Times for 50% conversion of hydrocarbons or CO2 to methane as a function of temperature for thermolysis of n-C5, acid-catalyzed cracking of n-C16, hydrogenolysis of n-C6, and CO2 hydrogenation.

C3-C18 hydrocarbons, and Fe- or Ni-catalyzed CO2 hydrogenation occur at rates that are 5-12 orders of magnitude higher. For example, assuming only 100 ppm of Fe metal, 10% conversion of n-pentane to methane by 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 beds by Fe- (or Ni-) catalyzed hydrogenolysis of liquid hydrocarbons or CO2 methanation is a likely scenario, while thermolysis of liquid hydrocarbons is unlikely to contribute measurably to coal-bed methane, even over long periods of geologic time at relatively high coal-bed temperatures, e.g., 150-190 °C.76 Nevertheless, as illustrated in Table 2, pyrolysis of lignite or isolated kerogen to methane occurs at geologically significant rates at 180 °C, (i.e., 10% conversion as low as 1500 y). It is interesting to note that the isolated kerogens show conversion times nearly identical to those of mineral-containing coals. These results are in agreement with Reynolds and Burnham80 who, comparing pyrolysis kinetics and oil generation curves of shales with their corresponding isolated kerogens, found that “kerogen isolation does little to affect...kinetic parameters.” It has been argued however, that this observation may be explained by metal catalysis in view of the difficulty of completely removing metallic minerals, e.g., organometallic complexes or suspended, colloidal Fe/SiO2 or Fe/Al2O3.45 In other words, the results of Reynolds and Burnham80 implicitly assume that the kerogen concentrates obtained were mineral-free (or near mineral-free), and thus no minerals were present (in significant quantities) to catalyze kerogen degradation reactions. However, Bartholomew et al.45 speculate that the isolated kerogen could still contain significant (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, 6776. (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, 163173. (80) Reynolds, J. G.; Burnham, A. K. Org. Geochem. 1995, 23, 1119.

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quantities of mineral matter to catalyze thermal degradation reactions. There is evidence to lend credence to this viewpoint. For example, Waples27 states that isolation “...procedures produce a kerogen concentrate which also contains other resistant minerals...[not amenable to chemical removal].” Berkel and Filby81 also state that irrespective “...of the technique used, the kerogen can never be isolated mineral-free.” However, both Waples and Berkel and Filby add further that the majority of the remaining minerals is pyrite. While pyrite is expected to be more or less catalytically inactive, Berkel and Filby point out that the pyrite may still contain significant quantities of other elements, including Ni. While the speculative explanation of Bartholomew et al. has credibility, it is complicated by two issues. First, while it appears that minerals were present in the isolated shale kerogen concentrates, it is not known if they were present in significant quantities to catalyze the formation of oil. Second, the argument does not consider the molecular structure of the shale kerogen itself, which would also impact pyrolysis behavior.82 Thus, even if, by analogy, minerals were present in significant quantities in the whole coals listed in Table 2 to catalyze methane formation reactions, this effect could be masked by the molecular structure of the coal kerogen. Table 3 lists the kinetic parameters for the pyrolysis of model polymers of coal kerogen83-86 and the time required to produce threshold commercial reserves (300 scf ton-1, 9 cm3 g-coal-1)77 of methane gas (Note that this minimum threshold amount is about 15 times lower than the typical amount of methane production expected during catagenesis.26). Activation energies range from 49 to 62 kcal mol-1, while production times range from hundreds to billions of years, which not only indicates that molecular structure heavily influences rate, but also suggests that direct conversion of kerogen to natural gas can occur independent of mineral catalysis. We emphasize however, that no one compound adequately models Type III kerogen. Polymers are inherently homogeneous while coals are hetergeneous.87 Thus, it is difficult to make inferences regarding coal kerogen on the basis of the data for any single polymer. While literature data rule out any practical contribution of liquid hydrocarbon thermolysis to methane at normal coalbed temperatures and depths, the data do suggest the possible contribution of kerogen thermolysis to methane, albeit ambiguously. The data, however, also strongly establish the possibility of thermal decomposition of Type III kerogens to liquid hydrocarbons (Table (81) Berkel, G. J. V.; Filby, R. H. Determination of the MineralFree Nickel and Vanadium Contents of Green River Oil Shale Kerogen. In Geochemical Biomarkers; Yen, T. F.; Moldowan, J. M., Harwood Academic Publishers: Switzerland, 1988; pp 89-114. (82) Tegelaar, E. W.; Noble, R. A. Org. Geochem. 1994, 22, 543574. (83) Solomon, P. R. Synthesis and Study of Polymer Models Representative of Coal Structure; Gas Research Institute Annual Report for Contract No. 5081-260-0582, April 1983. (84) Behar, F.; Vandenbroucke, M. Org. Geochem. 1987, 11, 1524. (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.

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Table 2. Summary of Calculated Methane Formation Rates from Literature Data for Coals and Isolated Kerogen under Geologic Conditions (180 °C) coal/kerogen ND lignitec Upper Freeportd Wyodakd Illinois No. 6d Pittsburgh No. 8d Pocahontas No. 3d Blind Canyond Lewiston-Stocktond Beulah-Zapd

rate, 180 °C (gmeth/gcoal y) Coalsb

9.8 × 10-10 3.2 × 10-9 2.6 × 10-8 3.1 × 10-6 2.5 × 10-6 8.7 × 10-8 1.1 × 10-5 1.9 × 10-6 4.6 × 10-7

t (10% conv.) (y)a

Eact (kcal/mol)

ref

3.8 × 105 5.0 × 106 6.0 × 105 5.2 × 103 6.3 × 103 1.8 × 105 1.5 × 103 8.5 × 103 3.4 × 104

54 63 63 50 49 53 47 50 55

77 78 78 78 78 78 78 78 78

Isolated Type III Kerogense Gippsland Eocene 2h 10 h 100 h 1000 h Rocky Mountain 2h 10 h 100 h 1000 h Wilcox 2h 10 h 100 h 1000 h

3.3 × 10-6 8.9 × 10-7 2.9 × 10-7 7.2 × 10-8

2.6 × 104 9.4 × 104 2.9 × 105 1.2 × 106

55

79

2.8 × 10-6 1.0 × 10-6 3.0 × 10-7 6.4 × 10-8

3.0 × 104 8.3 × 104 2.8 × 105 1.3 × 106

55

79

3.1 × 10-6 7.3 × 10-7 1.6 × 10-7 3.8 × 10-8

2.7 × 104 1.1 × 105 5.2 × 105 2.2 × 106

55

79

a y ) years. b Mineral-containing coals; mean activation energies from energy distributions were utilized. c North Dakota lignite Tertiary coal. d Argonne National Laboratory Premium coal. e Rate extrapolated from 300 to 180 °C using Eact) 55 kcal/mol from which preexponential 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

a

model compound

Eact (kcal/mol)

rateb (gmeth/gcoal y)

time (y)c (300 scf ton-1)

poly(p-xylene) poly(1,3-dimethylenenaphthalene) poly(1,4-dimethylenenaphthalene) poly(dimethylenedurene)

62 58 56 49

8.8 × 10-13 9.0 × 10-10 2.2 × 10-9 9.8 × 10-6

7.6 × 109 7.5 × 106 3.0 × 106 6.9 × 102

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 at 180 °Ca reaction substrate poly(p-xylene) poly(1,3-dimethylenenaphthalene) poly(1,4-dimethylenenaphthalene) poly(dimethylenedurene)

Eact (kcal/mol)

rate, 180 °C (goil /gcoal y)

rate, 180 °C (gmeth/gcoal y)

ratio (goil /gmeth)

Kerogen Model Polymers 62 9.8 × 10-11 58 8.8 × 10-9 56 5.1 × 10-8 49 7.6 × 10-5

8.8 × 10-13 9.0 × 10-10 2.2 × 10-9 9.8 × 10-6

110 10 23 8

Isolated Type III Kerogens Gippsland Eocene 2h 10 h 100 h 1000 h Rocky Mountain 2h 10 h 100 h 1000 h Wilcox 2h 10 h 100 h 1000 h a

55

5.8 × 10-4 3.7 × 10-5 2.2 × 10-6 6.7 × 10-8

3.3 × 10-6 8.9 × 10-7 2.9 × 10-7 7.2 × 10-8

180 42 8 1

55

3.2 × 10-4 7.1 × 10-6 1.1 × 10-6 7.9 × 10-8

2.8 × 10-6 1.0 × 10-6 3.0 × 10-7 6.4 × 10-8

110 7 4 1

55

3.7 × 10-4 4.0 × 10-5 5.9 × 10-6 4.4 × 10-7

3.1 × 10-6 7.3 × 10-7 1.6 × 10-7 3.8 × 10-8

120 55 37 12

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

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

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

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

rate,a 180 °C Thermal

thermal cracking, n-C6 to C1 thermal cracking, n-C16 to C1 thermal cracking, aromatic oil

1.8 × 10-12 1.1 × 10-12 1.3 × 10-10

t (10% conv.) (y)b

Eact (kcal/mol)

ref

9.2 × 107 2.0 × 107 8.1 × 105

69 60 65

72 73 74

Type III Coal Kerogens to C12+c Gippsland Eocene 2h 10 h 100 h 1000 h Rocky Mountain 2h 10 h 100 h 1000 h Wilcox 2h 10 h 100 h 1000 h acid mineral cat. cracking, C16 to C1 hydrogenolysis C18) to C1, Fe CO2 hydrogenation, Fe

5.8 × 10-4 3.7 × 10-5 2.2 × 10-6 6.7 × 10-8

1.5 × 102 2.2 × 103 3.8 × 104 1.3 × 106

55

79

3.2 × 10-4 7.1 × 10-6 1.1 × 10-6 7.9 × 10-8

2.6 × 102 1.2 × 103 7.8 × 104 1.2 × 106

55

79

3.7 × 10-4 4.0 × 10-5 5.9 × 10-6 4.4 × 10-8

2.3 × 102 2.1 × 103 1.4 × 104 1.9 × 106

55

79

10 0.8 4.1 × 10-3

35 50 17

73 44 45

Catalytic 2.1 × 10-6 1.1 × 10-5 1.3 × 10-2

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 onethird-filled with HC vapor and 50% H2O; 700 ppm H2; 100 ppm surface Fe; reaction assumed to be first-order. b y ) years. c Rate extrapolated from 300 to 180 °C using Eact) 55 kcal/mol.

the data. First, the more condensed the model polymer is, the less is its oil/gas ratio. This would suggest that the more condensed a Type III kerogen becomes, the more gas-prone it is likely to be, and indeed isolated Type III kerogen data indicate that with increasing maturity, there is a commensurate decrease in the oil/ gas production ratio. Specifically, as coal kerogen matures, it becomes increasingly condensed in structure and increasingly gas-prone. However, with increasing condensation there is also a commensurate decrease in 10% conversion times for the production of both oil and gas. While the Wilcox coal kerogen predominately produces liquid hydrocarbons throughout all maturation stages of the experiment, the Gippsland and Rocky Mountain coal kerogens produce both methane and liquid hydrocarbons on a near 1:1 mass basis when the coal is most mature. Reported %R0 values for the Gippsland and Rocky Mountain coal kerogens after 1000 h pyrolysis time were 0.75 and 1.2, respectively. These would correspond approximately to the kerogens attaining high and medium volatile bituminous ranks.40 However, Levine40 cautions that “there is...considerable natural variability in humanite/vitrinite reflectance that is due to factors other than thermal history...[and] ...stratigraphic variations in reflectance do not necessarily imply differences in rank per se, but might reveal differences in...[organic matter]...type or [an] early maturation history.” Irrespective of what the rank might be, it is evident from Table 4 that sufficient gas generation from the thermal decomposition of kerogen does not occur until it has reached a relatively high level of maturity. However, at that stage the production rate of hydrocarbons is low and the resulting input of additional oil and gas would appear to be small. Nevertheless, in a geological time frame the input could be significant.

Table 5 lists selected thermolytic and catalytic cracking reactions for the formation of oil and methane gas. Thermal cracking reactions of liquid hydrocarbons to methane are characterized by relatively high activation energies of 60-69 kcal/mol (see Tables 1 and 5); this observation is consistent with what one might predict for thermal scission of C-C bonds. On the other hand, activation energies for catalytic cracking and syntheses reactions are generally substantially lower (15-50 kcal/ mol), as one might expect for catalytic C-C or C-O bond breaking. The consequence of these lower activation energies are substantially smaller conversion times. Thermal cracking of kerogen to oil, however, involves activation energies of intermediate ranges leading to very significant geological rates (see Table 5). For example, 10% conversion times for the thermal cracking of coal kerogens of an immature rank to C12+ liquids are as small as 150 years at 180 °C. Accordingly, these data indicate that thermolytic conversion of kerogen to oil, followed by transition-metal catalyzed reactions to gas is a viable scenario. Pelet et al.88 allude to such a sequence, concluding that the thermal degradation sequence of kerogen may be more appropriately modeled as kerogen f asphaltenes f resins f heavy hydrocarbons f light hydrocarbons; nevertheless, they state explicitly that resins and asphaltenes “are not obligatory intermediates...[but that]...[h]ydrocarbons can be produced directly from kerogen. [T]he relative importance of [the] asphaltenes and resins can be different in different situations.” While our models have assumed bulk first-order kinetics defined by single preexponential factors and activation energies, “the highly disordered structure of kerogen [results in] each individual bond [having] its (88) Pelet, R.; Behar, F.; Monin, J. C. Org. Geochem. 1986, 10, 481498.

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Table 6. Summary of Calculated Methane Formation Rates from Literature Data for Thermal and Catalytic Reactions for Distributed Activation Energies and a Geothermal Gradient of 9 °C My-1 a Eact (kcal/mol)

reaction

A (y-1) Thermal

thermal cracking, n-C6 thermal cracking, n-C16H34 thermal cracking, aromatic oil thermal cracking, paraffinic oil

69 60 65 69

acid mineral cat. cracking, C16 to C1 sedim. rock hydrogenolysis, C18) hydrogenolysis n-C5, Ni hydrogenolysis n-C5, Fe hydrogenolysis C18), Fe CO2 hydrogenation, Ni CO2 hydrogenation, Fe CO2 hydrogenation, Fe

35 31 31 25 23 19 15 17

Catalytic

normal distribution t (10% conversion)b σ ) 4%c σ ) 6%c

2.5 × 1024 4.7 × 1020 2.5 × 1024 2.4 × 1024

4.9 × 106 y 4.2 × 106 y 2.0 × 106 y 4.9 × 106 y

3.6 × 106 y 3.0 × 106 y 8.9 × 105 y 3.6 × 106 y

7.9 × 1014 2.4 × 1014 3.9 × 1014 1.6 × 1011 1.6 × 1010 9.2 × 1011 2.7 × 109 3.1 × 109

58 y 2.0 y 1.2 y 3.1 y 3.1 y 4.7 h 15 h 6d

23 y 336 d 208 d 1.8 y 1.9 y 3.2 h 12 h 4d

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.

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

Clk )

∫EE

( ) (

f

0

)

E h act - Eact 1 exp -2σ σx2π

2

(∫

exp -

t

A exp 0 i

( )) -Eact

RT(t)

dt dE

where Clk is the fraction of thermally labile kerogen remaining at time t. Incorporated within this model is a simple linear geothermal temperature gradient, T(t) ) T0 + Rt, where T0 denotes the initial temperature and R denotes the temperature increase rate. Quigley and Mackenzie76 demarcated temperature ranges for oil and gas genesis as “oil (and gas) formation from labile kerogen breakdown, 100-150 °C; gas formation from refractory kerogen breakdown, 150-220 °C; oil-to-gas cracking, 150-190 °C.” Accordingly, T0 values of 100 and 150 °C were assigned for oil and gas generation, respectively. Conservative heating rates have been estimated to be 1-10 °C megayears-1.90 Thus, R was assigned a value of 9 °C megayears-1 for our computations. The limits of integration, Ef and E0, were chosen to include g 99.9% of the area under the Gaussian curve. The mean values chosen were those listed in Table 1. Standard deviation values (σ) were chosen to be 4 and 6% of the mean value according to actual observed values of Burnham et al.78 The equations were solved numerically. Table 6 lists the recalculated 10% conversion times for methane production from the major reactions previously examined, but now assuming normally distributed activated energies. Three of the four thermal cracking reactions have 10% conversion times about an order of magnitude less than when calculated with a single activation energy under isothermal conditions (see Table 1). The aromatic oil cracking time, however, is increased by a factor of about 2.5 for σ ) 4%, and is about the same for σ ) 6%. It is also interesting to note that while (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 1 shows aromatic oil cracking 3 orders of magnitude faster relative to the paraffinic oil, under the constraints listed in Table 6 the 10% conversion time difference dropped to factors of 2.45 and 4.0 for σ )4 and 6%, respectively. Times for 10% conversion during the catalytic reactions increased, albeit insignificantly. From Table 6, it can be seen that the 10% conversion times for the thermal cracking of n-hexane, n-hexadecane, and paraffinic oil are considerably reduced relative to the values listed in Tables 1 and 2 by assuming distributed activation energies. Thus, even though the initial temperature of the gradient is 30 °C lower than the isothermal temperature, distributed activation energies apparently reduce conversion times. It could be argued, however, that the introduction of a temperature gradient likewise affects conversion times, since in 4.9 megayears (during the thermal cracking of n-hexane), the temperature of the bed is assumed to increase from 150 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 for the observed reductions in conversion time is the distribution of activation energies. In a geological context, this reduction in 10% conversion times can have important consequences. For example, while the 92 megayears required for the 10% conversion of n-hexane via thermal cracking under the constraints listed in Table 1 is unreasonably long to be considered as a viable contribution 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 difference at σ ) 6%. However, the temperatures reached by the system would be 168 and 158 °C, respectively. Thus, while there is no observed reduction in conversion time at σ ) 6%, there is a compensatory effect of the distribution in that a greater amount of cracking takes place at lower temperatures. It is also interesting to note that the distributed activation energies result in no significant 10% conversion time differences between the model alkanes and paraffinic oil. On the other hand, all of the catalytic reactions are so fast that even though there is an increase in 10% conversion times, (probably

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

reaction

Eact (kcal/mol)

ND lignitee Upper Freeportf Wyodakf Illinois No. 6f Pittsburgh No. 8f Pocahontas No. 3f Blind Canyonf Lewiston-Stocktonf Beulah-Zapf

54 63 63 50 49 53 47 50 55

normal distribution t (10% conversion)b c σ ) 4% σ ) 6%c

A (y-1) Coalsd 3.1 × 1019 3.9 × 1022 3.3 × 1023 3.3 × 1019 6.3 × 1018 2.9 × 1019 2.9 × 1018 1.2 × 1019 1.8 × 1021

1.4 × 106 y 3.0 × 106 y 1.6 × 106 y 4.3 × 104 y 5.1 × 104 y 9.0 × 105 y 1.2 × 104 y 7.0 × 104 y 2.8 × 105 y

5.5 × 105 y 1.8 × 106 y 6.8 × 105 y 1.1 × 104 y 1.3 × 104 y 3.1 × 105 y 3.2 × 103 y 1.8 × 104 y 6.6 × 104 y

Isolated Type III Kerogens Gippsland Eocene 2h 10 h 100 h 1000 h Rocky Mountain 2h 10 h 100 h 1000 h Wilcox 2h 10 h 100 h 1000 h

55

1.4 × 1021 3.8 × 1020 1.3 × 1020 3.1 × 1019

2.1 × 105 y 6.0 × 105 y 1.2 × 106 y 2.1 × 106 y

5.1 × 104 y 1.7 × 105 y 4.4 × 105 y 1.1 × 106 y

55

1.2 × 1021 4.3 × 1020 1.3 × 1020 2.8 × 1019

2.5 × 105 y 5.5 × 105 y 1.2 × 106 y 2.2 × 106 y

6.1 × 104 y 1.5 × 105 y 4.3 × 105 y 1.2 × 106 y

55

1.3 × 1021 3.2 × 1020 7.0 × 1019 1.7 × 1019

2.2 × 105 y 6.8 × 105 y 1.5 × 106 y 2.6 × 106 y

5.4 × 104 y 2.0 × 105 y 6.7 × 105 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. Tertiary coal. f Argonne National Laboratory Premium coal.

owing to the lower initial temperature), in a geological time frame the effects of distributed activation energies are insignificant. Table 7 lists the 10% conversion times for mineralcontaining coals and isolated kerogens to methane for the same distributed geologic model as used for Table 6. Relative to those listed in Table 2, conversion times in Table 7 of all the coals with exception of the Upper Freeport coal are observed to increase. Conversion times for the Illinois No. 6, Pittsburgh No. 8, Blind Canyon, Lewiston-Stockton, and Beulah-Zap coals increase by factors of 8 and 2 for σ ) 4 and 6%, respectively. Conversion times for all of the isolated kerogens in the immature stage likewise increase by factors of 8 and 2 for σ ) 4 and 6%, respectively. At the latest maturation stage, 10% conversion times of the Gippsland and Rocky Mountain coal kerogens are greater by a factor of only 2 at a σ value of 4% and nearly identical at σ ) 6%. The conversion time of the Wilcox coal is nearly identical at 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 reaction or a step in a complex process [such as coal maturation] involves free-radicals or not. The ways of answering this question depend on whether the radical intermediate is persistent, stable, or short-lived. In the first two cases direct observation by EPR [electron paramagnetic resonance spectroscopy] is the most rapid and the most conclusive.”91 In other words, it can be inferred that if the formation of oil and gas during coal maturation occurred by thermolysis via homolytic cleavage of C-C bonds in the coal matrix, then long-lived paramagnetic (91) Fossey, J.; Lefort, D.; Sorba, J. Free Radicals in Organic Chemistry; John Wiley & Sons: New York, 1995; Chapter 17, p 201.

d

Mineral-containing coals. e North Dakota lignite

centers should be detected by EPR in coal samples. Indeed, Waples27 points out that “[i]n some kerogen molecules,...free-radicals may be stabilized by delocalization over an extended aromatic system.” Convincing evidence is presented by Lewis and Singer.92 Naphthalene and anthracene were pyrolyzed to pitches at 400 °C. Paramagnetic center numbers exceeding 1019 g-pitch-1 were detected. Various trimers, tetramers, and pentamers were also observed. Various structures inferred by X-ray diffraction studies suggested large, condensed systems. More important, however, the nature of these stable radicals was determined. They found that “[s]ubstantial evidence supports the contention that the stable free radicals formed during the pyrolysis of polynuclear aromatic compounds are odd-alternate hydrocarbon radicals.” For example, pyrolysis of acenaphthylene and dihydronaphthalene at 400 °C produced the phenalenyl radical. They add further, “These radicals exhibit considerable stability at elevated temperatures because of the ease of delocalization of the unpaired electron.” While large concentrations of free radicals have been found in lignin,93 a precurser of vitrinite,49 and in green leaves and dried vegetation,94 Duchesne et al.94 point out that “[t]his would seem to imply that the free radicals in plants may be trapped and stabilized after the plants have dried. However, the fact that their concentration is definitely lower than in coals and even (92) Lewis, I. C.; Singer, L. S. Thermal Conversion of Polynuclear Aromatic Compounds to Carbon in Polynuclear Aromatic Compounds; Advances in Chemistry Series 217; Ebert, L. B., Ed., American Chemical 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.

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Table 8. Number of Paramagnetic Centers in Coals of Various Carbon Content total carbon (%)

aromaticity (fa)

paramagnetic centers (number/gram)

ref

67 77 78 82 83 85 89 89 90 93 94 95 96 97

- - -a 0.69 - - -a - - -a 0.75 0.76 0.81 - - -a - - -a 0.88 0.91 0.95 0.98 0.99

1.2 × 1018 2.5 × 1018 3.9 × 1018 5.1 × 1018 3.6 × 1018 5.3 × 1018 7.2 × 1018 1.0 × 1019 1.4 × 1019 1.8 × 1019 2.5 × 1019 2.3 × 1019 1.5 × 1019 2.2 × 1019

96 97 96 96 97 97 97 96 96 97 97 97 97 97

a

- - -Denotes unavailable data.

than in lignites...shows that other factors intervene in these rocks.” Paramagnetic centers in coal were first reported by Uebersfeld et al.95 in 1954. Table 8 lists the concentrations of paramagnetic centers found in coals of various carbon content. The data suggest both a trend and a correlation with increasing carbon content and aromaticity. Specifically, as carbon content and aromaticity increase to approximately 94 and 91%, respectively, the number of paramagnetic centers likewise increases. Further maturation leads to a decrease in free-radical concentrations. This trend agrees with the observations of Retcofsky et al.98 who, in analyzing a series of 18 U.S. coals and a peat, found that “[t]he concentrations of unpaired electrons in the coals rise exponentially with increasing rank up to about 94% C after which there is a large decrease.” However, “...small quantitative differences...” have been noted in the trend. One might surmise that the decrease in radical density with increasing maturity (%C > 94) might be due to greater condensation which brings the radicals into closer proximity, leading to bond formation via pairing. Table 949,99,100 lists the number of aromatic carbons per cluster, number of paramagnetic centers, and spectroscopic splitting factors (g-values) for the Argonne Premium Coals. A free electron has a g-value of 2.0023. Free radicals with g-values close to 2.0023 may be considered to be in extended π-orbitals while larger g-values imply a more restrictive environment. The AC/ Cl and g-values reported follow the systematic rank change well, i.e., AC/Cl increases while g-values decrease with increasing rank. This is also in agreement with the observations of Retcofsky et al.98 who also found decreasing g-values with increasing rank. Such (95) Uebersfeld, J.; E Ä 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 Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds., American Chemical Society: Washington, DC, 1993; Chapter 33, pp 629-643. (100) Buckmaster, H. A.; Kudynsha, J. Dynamic In Situ 9-Ghz Electron Paramagnetic Resonance Studies of Argonne Premium Coals in Magnetic Resonance of Caronaceous Solids; Botto, R. E.; Sanada, Y., Eds., American Chemical Society: Washington, DC, 1993; Chapter 26, pp 483-506.

a well-defined trend however, is not evident in the mean radical density data. Furthermore, the variances about the means are large. Because these calculations assume normally distributed observations arising from random errors, and because these data were calculated from observations made in independent studies, it is obvious then that interlaboratory systematic errors have been introduced. Silbernagel and Botto99 appear to concur with this conclusion, stating that these samples “...have been treated in a very uniform manner up to the time when they were opened in the individual laboratories. Once the samples were opened, the variety of different handling procedures that were employed undoubtedly resulted in alterations of the samples from their pristine state because of the loss or addition of water or such incidental oxidation as may have occurred.” However, even observations made by single laboratories do not seem to follow any systematic variation with rank. This has been attributed to “...the presence of magnetic transition-metal ions, principally iron, that are in intimate contact with the organic matter in the coal.” While Silbernagel and Botto,99 citing the work of others, recognize that concentrations of iron ions can be high at times, they subsequently imply that they cannot account for the total EPR signal. Duchesne et al.,94 citing others, likewise conclude “...that the number of paramagnetic...[centers in coal] is so high that it is out of the question to attribute these as being due to the existence of... iron.” Furthermore, while Retcofsky et al.101 have noted that “[o]ne group of investigators...has proposed that charge-transfer complexes rather than 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 gas and oil formation in coal seams, it is equally important to consider if the product distributions observed for thermal or catalytic routes are consistent with those measured 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 produces only 23% methane along with significant fractions of C2-C7 hydrocarbons (see Figure 3 and Table 10); while no rate data are available at lower temperatures expected in coal beds (100-200 °C) for liquid hydrocarbon thermolysis (rates are too low to permit measurement within a reasonable time frame), the fraction of methane is expected to be significantly lower and the fraction of liquid hydrocarbons significantly higher under these milder conditions.18,23,74 Products of catalytic cracking (acidic) at 190 °C are characterized by gas and liquid hydrocarbons having a maximum in the distribution around C4 to C5 with only 8% methane (see Figure 3 and Table 10). Pyrolysis of coal kerogens, with exception of the Cherry Canyon coal kerogen, produces methane con(101) Retcofsky, H. L.; Thompson, G. P.; Hough, M.; Friedel, R. A. Electron Spin Resonance Studies of Coals and Coal-Derived Asphaltenes. In Organic Chemistry of Coal; ACS Symposium Series 71, Larsen, J. W., Ed.; American Chemical Society, Washington, DC, 1978; pp 142155. (102) Nishioka, M.; Gebhard, L. A.; Silbernagel, B. G. Fuel 1991, 70, 341-348.

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Figure 8 contains the frequency histograms of the branched C4 and C5 hydrocarbons/normal C4 and C5 hydrocarbons ratios for various thermogenic natural gas basins. The majority of observed ratios with the exception of the Cherokee, Forest City, and San Juan Basins are less than unity. For the above-named basins, however, a significant number of observed ratios are greater than unity, indicating extensive branching which could have been caused by the formation of a carbocation intermediate. This would tend to indicate that while for the majority of basins acid-mineral catalysis is not the main route for natural gas formation, there can be instances where the input from this route can be significant. Figure 3. Product distributions for thermal cracking and catalytic cracking (on Houndry M-46) of 1-octadecene at 500 and 190 °C. Data from ref 15.

tents at the low end of observed values from U.S. coal mines (70-84%). The data also indicate some general, albeit counter-intuitive, trends. Figures 4-6 illustrate the products for the pyrolysis of the Gippsland Eocene, Rocky Mountain, and Wilcox coal kerogens. As pyrolysis times increase, methane content decreases while ethane and propane concentrations increase. This trend is especially prevalent in the Wilcox data where the methane content shows an overall decrease of 16%, the ethane content an overall increase of 9%, and a propane increase of 4%. This observation is inconsistent with what one would expect from thermolysis. Indeed, one would expect an increase in methane content with respect to pyrolysis time as evidenced by the fact that older deposits usually contain a preponderance of lighter hydrocarbons. However, metal catalysis arguments can be employed to explain this observation. It was mentioned earlier that Bartholomew et al.45 speculated that isolated kerogens could still contain significant quantities of metallic minerals. Thus, it follows that these minerals could be involved in the formation of liquid and gaseous hydrocarbons. Such could be inferred in this case because the loss of methane selectivity with increasing pyrolysis time is indicative of catalyst deactivation. Specifically, a methanation catalyst undergoing deactivation via carbon deposition loses both activity and methane selectivity with increasing time. Thus, if metallic catalysts were still present in the isolated coal kerogens, the loss of methane selectivity with increasing time would be predicted due to deactivation via formation of coke-like, carbonaceous material forming on the active catalytic sites. Indeed, if such is the case, then metal catalysis would not only explain the very similar 10% conversion times observed between the whole coals and isolated kerogens (see Tables 2 and 7), but would also indicate that only trace amounts of metal would be necessary to produce copious amounts of methane, a fact evidenced by our kinetic calculations (see Tables 1, 5, and 6). Furthermore, product compositions for Feor Ni-catalyzed olefin hydrogenolysis measured by Mango et al.15 and CO2 methanation measured by Medina et al.103 are methane rich (90-94 mol %) and closely match those for U.S. coal gases (see Figure 7 and Table 10).

Discussion Plausible reactions for producing natural gas in coal beds must meet the following criteria: (1) produce methane at significant rates under geologic conditions within geologically significant times for coal maturation (i.e., less than 1-10 million years) and (2) produce gases rich in methane. According to Hunt,104 methane formation from catagenesis is significant in the range of 100-200 °C with a maximum at about 150-160 °C corresponding to a depth of around 4500 m.75,106 Maximum generation of methane apparently occurs in medium-volatile bituminous and low-volatile bituminous coals104 and is high for coals having a reflectance (Ro) higher than about 2%.105 These conclusions are supported by the recently developed model of Tang et al.77 indicating that T > 120 °C and Ro g 0.9% are required for a minimum threshold methane production of 300 ft3 ton-1. Accordingly, the most important methane-producing reactions should occur at temperatures in the range of 120-180 °C. While the current paradigm asserts that coalbed gas is produced by thermal decomposition of light oils,10,11 a careful analysis of rate data from the previous literature (see Tables 1 and 2 and Figure 1) provides evidence that reaction rates for thermal cracking of light-medium hydrocarbons are orders of magnitude too slow to have produced significant quantities of methane within coal maturation times, assuming isothermal conditions and bulk first-order kinetics characterized by a single activation energy. If thermal cracking of light-medium hydrocarbons could be characterized by distributed activation energies, methane generation would still be time limited even at higher temperatures (see Table 6). However, the environment causing these hydrocarbons to acquire distributed, thermal decomposition activation energies is ambiguous. On the other hand, hydrocarbon decomposition or methanation reactions catalyzed by acidic or transitionmetal minerals could have produced known reserves of coalbed gas within days to thousands of years, i.e. well within maturation times, independent of temperature and distributed activation energies (see Tables 1 and 6 (103) Medina, J. C.; Butala, S.; Bartholomew, C. H.; Lee, M. L. Fuel 2000, 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 Occurrence, Second Revised and Enlarged Edition; Springer-Verlag: New York, 1984; pp 70, 215, 247.

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

rankc

AC/Cld

paramagnetic centers (number/g-OM)e

spectroscopic splitting factorf

spectroscopic splitting factorg

Beulah-Zap Wyodak Illinois No. 6 Blind Canyon Lewiston-Stockton Pittsburgh No. 8 Upper-Freeport Pocahontas No. 3

ligA subC hvCb hvBb hvAb hvAb mvb lvb

9 14 15 15 14 16 18 20

1.85 ( 1.13 2.00 ( 1.09 0.88 ( 0.51 1.43 ( 1.11 2.10 ( 1.50 1.25 ( 0.59 1.13 ( 0.70 2.12 ( 1.29

2.0035 2.0035 2.0032 2.0032 2.0030 2.0029 2.0028 2.0028

2.0034 2.0034 2.0030 2.0030 2.0028 2.0027 2.0026 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 ) lignite A, 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 the Compositions of Coalbed Gas mol % of gaseous product reaction )

°Ca

thermolysis, C18 at 500 acid mineral cracking, C18) at 190 °Cb pyrolysis of isolated coal kerogens Cherry Canyon, 310 °C for 1 weekc Gippsland Eocene, 300 °Cd 2h 10 h 100 h 1000 h Rocky Mountain, 300 °Cd 2h 10 h 100 h 1000 h Wilcox, 300 °Cd 2h 10 h 100 h 1000 h catalytic hydrogenolysis C18), 190 °C on Fee C12), 180 °C on Fef catalytic hydrogenation CO2, 180 °C on Fef coalbed gas, Piceanceg coalbed gas, Mary Leeh

C1

C2

C3

C4

C5

C6

C7

23 8

40 17

10 10

14 25

7 16

4 11

1 13

48

15

3

5

22

2

5

83 80 80 72

6 9 9 13

5 7 6 8

2 2 2 4

4 1 1 2

82 81 81 74

7 10 10 14

4 5 5 6

3 2 2 3

3 1 1 2

84 82 76 68

6 8 12 17

6 7 8 10

1 1 2 4

2 2 1 2

90 86

4 14

94 90 96

3

1

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 H . f Data from ref 45. g Data 2 from 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.

and Figure 2). Studies conducted by Medina et al.103,106 confirm that rates of CO2 methanation and olefin hydrogenolysis are very significant in the presence of reduced, dispersed iron and nickel under conditions simulating coalbeds. For example, 20-50% conversion of CO2 or 1-dodecene occurs on 10% Fe/SiO2 in 40-60 h at 180 °C; moreover, when extrapolated to reactant and catalyst concentrations at the lower end of what might be expected for a coal bed (e.g., only 100 ppm surface iron), 10% conversion times of 1-10 months for olefin hydrogenolysis and 2-36 h for CO2 hydrogenation are predicted. The rate of methane production reported by Medina et al.106 for 1-dodecene hydrogenolysis on reduced 10%

Fe/silica at 180 °C is in excellent agreement with that reported by Mango44 for 1-octadecene hydrogenolysis on reduced Fe(AcAc)3 (see Table 1). Rates of CO2 hydrogenation of 10% Fe/silica obtained by Medina et al.103 in a batch system are in fair agreement with those extrapolated from the higher temperature data of Weatherbee and Bartholomew46 obtained in a flow reactor for 15% Fe/silica. The data reported by Medina et al. are, to our best knowledge, the first to be obtained at low temperatures in a closed system more representative of geologic conditions. Thus, while Mango and Hightower107 have suggested oil hydrogenolysis catalyzed by transition-metal minerals as a unique route for 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. Acta 1997, 61, 5347-5350.

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

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Figure 6. Product distributions for the Wilcox isolated coal kerogen at pyrolysis times of 2, 10, 100, and 1000 h. Data from ref 79.

Figure 7. Product distributions for catalytic hydrogenolysis of 1-octadecene and CO2 hydrogenation on iron catalysts at 190 and 180 °C, respectively, compared with composition of a typical coalbed gas. Data from ref 15, 64, 90, 106. Figure 5. Product distributions for the Rocky Mountain isolated coal kerogen at pyrolysis times of 2, 10, 100, and 1000 h. Data from ref 79.

these results suggest an additional route for “dry” natural gas. Specifically, CO2 methanation on Ni and Fe minerals is an additional scenario in basins that contain high concentrations of CO2 and H2. In examining light hydrocarbons (C1 to C14) from various well cuttings, Hunt108 found that compounds containing tertiary carbons have a greater concentration in shallower depths (lower temperatures), while compounds containing quaternary carbons appear in greater concentrations in the older, deeper sediments (higher temperatures). Mango109 states that Hunt interpreted these results as indicating that acid-catalyzed carbocation mechanisms are preferred in the shallower regions with free-radical cracking mechanisms competing at greater depths. This would suggest oil generation via mineral-acid-catalyzed cracking at shallow depths with free-radical thermolytic generation of oil dominating at the greater depths. An earlier publication from Hunt104 confirms this interpretation. He states, when making reference to petroleum generation from marine (108) Hunt, J. M. Science 1984, 226, 1265-1270. (109) Mango, F. D. Org. Geochem. 1997, 26, 417-440.

sediments, that “[p]etroleum hydrocarbons are cracked from...kerogen-mineral complex[es] by mechanisms that require apparent[ly] [low] activation energ[ies]...to break the bonds. Catalytic cracking appears to be the dominant process in petroleum generation in the subsurface temperature range up to about 125 °C...Thermal cracking becomes increasingly important at higher temperatures.” These works thus imply that oil generation from coal kerogen is acid-mineral catalyzed at lower, subsurface temperatures; however copious amount of oil can be produced by thermolysis alone at higher temperatures. This interpretation, however, contradicts the results of Reynolds and Burnham80 who found few differences between oil curves from whole shales and their corresponding isolated kerogens. Heteńyi110 found that pyrolyzing Type III/a and Type III/b coal kerogens with montmorillonite slightly reduced oil production while heating the kerogens with calcite slightly enhanced oil production. Espitalie´ et al.111 found, however, that pyrolyzing “mixtures of kerogens with various minerals...[resulted]...in retention of heavy hydrocarbon products issued...[from]...kerogen pyrolysis...[and thus,]...[t]he experimental procedure (110) Heteńyi, M. Org. Geochem. 1995, 23, 121-127. (111) Espitalie´, J.; Madec, M.; Tissot, B. Am. Assoc. Pet. Geol. Bull. 1980, 64, 59-66.

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

separates the lighter hydrocarbons [lighter than C15] from total hydrocarbons, ...showing that the decreased hydrocarbon yield from rocks as compared to kerogen is principally due to retention of the heaviest hydrocarbons [in the minerals.]” Indeed, Heteńyi110 acknowledges that many “authors consider clays and especially montmorillonite to be very active in terms of adsorption of products, and also in terms of their ability to crack organic matter at high temperatures.” Thus, the results of Reynolds and Burnham80 may be explained by retention of hydrocarbons by the mineral matrix of the shale. It is interesting that even model coal compounds covalently bonded to a less active substrate result in apparent catalytic transformations. For example, Buchanan III et al.112 reported that surface-immobilized 1,2diphenylethane on amorphous silica underwent thermolysis four-times faster than gaseous 1,2-diphenylethane at 350-400 °C, and that the resulting products were also altered notably. It was further reported by Buchanan and Biggs 113 that thermolysis rates of 1,3diphenylpropane increased as surface coverage on silica increased. It is generally held that shales are predominately oil sources while coals are primarily sources of gas.11 For example, while Waples27 acknowledges that “[s]mall amounts of fat and waxes, particularly those that occur in leaf coatings, pollens, and tree resins, may in some (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.

cases be converted to oil,” he adds, “[m]ost terrestrial plants contain too little lipid material...to form oilgenerative kerogens.” He confirms this by stating, “The relative proportions of gas and oil will depend on the kerogen type. Type I kerogens may give as much as 80% oil, while Type III kerogens may only yield 10% oil and 90% gas.” However, Horsfield et al.114 point out that “[s]trict adherence to an elemental classification scheme whereby hydrogen-rich kerogens (Type I and II) are considered oil-prone...and hydrogen-poor kerogen (Type III) is considered mainly gas-prone can cause problems. This applies particularly in fluviodeltaic-lacustrine systems where...oil can be generated from terrigenous organic matter...and...the theoretical[ly] significant hydrocarbon-generating potential of hydrogen-rich...[kerogens]...has been questioned.” When correlations have been established between petroleum reserves and terrestrial source rocks, bulk maceral compositional arguments have been employed to explain the observed connection. For example, Snowdon115 states “[i]n order for coal or Type III OM to function as a source rock for liquid hydrocarbons (oil and condensates), the organic hydrogen content must be enriched...[in]...liptinitic macerals such as sporinite and cutinite and especially resinite.” Indeed, the Gippsland Eocene isolated coal kerogen, which in this study had the highest calculated oil/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 (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.

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two coal kerogens. However, Lu and Kaplan79 indicate “...that the relative abundances of various macerals [in the Gippsland Eocene, Rocky Mountain and Wilcox coal kerogens were]...not critical to the hydrocarbon-generating potential.” They add, “Rather, the amount of dispersed algae and bacterial material (unidentified) in the coal probably is more significant. Depositional environment of the source organic material also may be important because it could control preservation of organic constituents.” Powell et al.87 did find broad trends in the relative amounts of liptinite, vitrinite, and inertinite macerals and hydrocarbon potential of Australian coals. However, the large observed variance made firm petrographical correlations impossible. Bertrand et al.116 studied 14 humic coal samples and likewise could find no correlations between maceral composition and petroleum potential, even between coals of the same rank, adding further that “[t]he diversity in oil potential must therefore, to a large extent, be attributed to a geochemical diversity of macerals from the vitrinite group,...[which said]...diversity is probably greater than recognized by present-time petrographic techniques.” It should be noted, however, that such an argument presupposes rank is the only variable and the main source of variance must therefore be within the vitrinite maceral composition beyond the identification ability of presentday petrography and further ignores the inherent heterogeneous nature of coal in general, even within the same seam. Thus, while the observed lack of correlation by Bertrand et al. agrees with the observation of Powell et al., the explanation appears suspect. Cook38 states that the effectiveness of coal to generate oil depends more upon the presence of labile material in the coal than upon the abundance of liptinite. While ambiguous in terms of what constitutes “labile material,” such an explanation may provide insight as to why the Wilcox coal kerogen was primarily oil-generative throughout the entire pyrolysis experiment. Under this assertion one could speculate that the Wilcox coal had greater amounts of labile material, which could be the unidentified algae or bacterial material previously mentioned. Such an interpretation however, becomes complicated by the fact that the Wilcox coal was lower in rank (lignite, R0 ) 0.32%) relative to the other coal kerogens pyrolyzed. Indeed, in a point that shall be emphasized later, oil generation from coal is strongly rank-dependent. In summary then, despite the apparent lack of agreement regarding its causes and inability to predict oil generation, there appears to be evidence to support the assertion that coal and coal kerogens can generate oil. There are apparently two kinds of acidic minerals available for catalytic cracking of hydrocarbons in coal beds; (1) silica-aluminas or aluminosilicates, which surface acid strength can, according to Brooks,117 be equivalent to 0.1 to 1.0 M HCl,178 and (2) natural clay minerals such as bentonite and montmorillonite. Both kinds are found in the coals and surrounding strata.119 (116) Bertrand, P.; Behar, F.; Durand, B. Org. Geochem. 1986, 10, 601-608. (117) Brooks, B. T. Origins of Petroleum. In The Chemistry of Petroleum Hydrocarbons: Volume 1, Brooks, B. T.; Boord, C. E.; Hurtz, S. S., Jr., Schmerling, L., Eds., Reinhold Publishing Corporation: New York, 1954; Chapter 6, pp 83-102.

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Generation of hydrocarbon gases and condensates during catalytic cracking by these and closely related minerals under geologically relevant conditions has been demonstrated in a number of studies, e.g., refs 25, 73, 119. While catalytic cracking on acidic minerals occurs at rates comparable to those of alkane/alkene hydrogenolysis (see Tables 1 and 2, and Figure 2), the expected product compositions do not resemble typical coalbed gas (see Table 10 and Figures 3 and 7) in that the methane contents are low. Thus, while catalytic cracking may contribute to the formation of methane, hydrogenolysis reactions on transition metals appear to be necessary to realize high-methane-content gases that resemble typical, natural, coalbed gas. Similarly, while formation of hydrocarbon liquids by thermolysis occurs at geologically relevant rates (see Tables 4 and 5), these hydrocarbons would have to be further processed by metal-catalyzed hydrogenolysis to form typical coalbed gas. Significant input from acid-mineral catalysis would seem to depend on, among other factors, the presence of water in the correct amount. Between the layers of smectite clays are countercations that neutralize the negative charge of the tetrahedral silicate sheets.120 According to Johns,121 the Brønsted acidity arises from the polarization dissociation of water by the exchangeable cations, i.e., K

n[M(H2O)x]z+ {\} n[M(H2O)x-1(OH)](z-1)+ + nH+ where M is the cation, x is the number of coordinated water molecules to the cation, and K is the dissociation constant for the water-cation system. Johns121 also reports that “[t]he proton-donating ability of the clay... [depends on three]...factors: (1) the polarizing effect of the exchangeable cation (increase with increasing charge and decreasing charge); (2) The number of exchangeable cations, n; and (3) the source of isomorphous replacement which gives rise to interlayer charge (greater acidity associated with tetrahedral rather than octahedral substitution).” When the clays contain more than a monomolecular layer of water, however, the polarizing effect becomes dispersed and the pH rapidly approaches that of water. Conversely, as the water content becomes low, the dissociation constant of adsorbed water can increase by 6 orders of magnitude relative to bulk water.121 In an illustrative experiment,121 Behenic acid was heated at 250 °C over calcium montmorillonite and excess water. (Experiments with adsorbed water alone were not performed). Branched/normal mass ratios for C14-C28 were determined to be 0.114 and 0.113 for 25 and 150 h reaction times, respectively. In view of this result, i.e., insignificant branched products, it was concluded that a carbocation intermediate was not formed, suggesting instead that the reaction proceeded through a freeradical mechanism. The aluminosilicates, in conjunction (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 186192. (121) Johns, W. D. Annu. Rev. Earth Planet. Sci. 1979, 7, 183-198.

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with providing Brønsted acid sites, can also provide Lewis acid sites allowing for hydride transfer. This leads to the formation of a carbenium ion which then undergoes β-scission to form an alkene and a smaller carbenium ion.122 Farrauto and Bartholomew122 point out that silicaaluminas and acidic natural clays have the same structural features as zeolites. In other words, it is the charge imbalance in the silica matrix by aluminum and other metals that gives rise to the Brønsted and Lewis acidities in these materials. Thus, it follows that discussions of zeolitic acid reactions would be applicable to aluminosilicates and acidic natural clays, albeit the acidity of these materials is lower relative to the zeolites. In a published theoretical review of zeolitic Brønsted acid site reactivity, van Santen and Kramer,123 in making reference to alkane activation via nonclassical carbonium ion formation, state that while “...the intermediates formed in acid-catalyzed reactions in zeolites are very similar to those formed in [homogeneous] superacids, the energetics of their formation is very different.” In other words, formation of carbonium ions from alkane molecules necessitates overcoming the energy of the strong proton-zeolite bond. Citing others, van Santen and Kramer123 add, “Reactions that proceed via carbonium ion intermediates have high activation energies and hence will be suppressed at lower temperatures in fav[or] of...hydride transfer reaction[s].” This is important because carbonium ions decompose to form an alkane and a smaller carbenium ion, or decompose into hydrogen gas and a carbenium ion. This latter decomposition route, as shall be pointed out later, is a proposed source for gaseous H2 for transition-metalcatalyzed reactions. Thus, the implication is that while reaction initiation may occur via carbonium ion formation, chain propagation will be through hydride transfer reactions, thus limiting H2 generation. A study conducted by Shertugde et al.124 would tend to support this implication. These workers cracked isobutane and npentane on dealuminated faujasites at high temperatures in a flow reactor under differential conditions (e.g., conversions were

Mechanisms and Kinetics of Reactions Leading to Natural Gas

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