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Geochemistry of coalbed gas – A review
International Journal of Coal Geology 35 Ž1998. 159–173
Geochemistry of coalbed gas – A review J.L. Clayton
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U.S. Geological SurÕey, DenÕer, CO 80225, USA Received 1 November 1996; accepted 22 April 1997
Abstract Coals are both sources and reservoirs of large amounts of gas that has received increasing attention in recent years as a largely untapped potential energy resource. Coal mining operations, such as ventilation of coalbed gas from underground mines, release coalbed CH 4 into the atmosphere, an important greenhouse gas whose concentration in the atmosphere is increasing. Because of these energy and environmental issues, increased research attention has been focused on the geochemistry of coalbed gas in recent years. This paper presents a summary review of the main aspects of coalbed gas geochemistry and current research advances. q 1998 Elsevier Science B.V. Keywords: coal; gas; carbon isotope; methane; pyrolysis
1. Introduction Coals serve as both sources and reservoirs of substantial quantities of hydrocarbon and CO 2 gas. Long known for explosion and outburst hazards in underground mining operations, coalbed gas has recently been recognized as an important energy resource. For example, in the United States alone, in-place coalbed gas resources are estimated at about 11.3 trillion cubic meters Žapproximately 400 trillion cubic feet. ŽICF Resources, 1990.. Although coals and coaly organic matter dispersed in shales are also now recognized as potential sources of liquid hydrocarbons, this paper focuses solely on coals as sources of gas. For a review of coals as sources of oil the reader is referred to Boreham and Powell Ž1991.. In addition to its importance as an energy resource, CH 4 is an important greenhouse gas whose atmospheric concentration has been increasing at a rate of about 1% annually in the past 15–20 years ŽRasmussen and Khalil, 1981; Steele et al., 1987; Blake and )
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0166-5162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 6 - 5 1 6 2 Ž 9 7 . 0 0 0 1 7 - 7
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Rowland, 1988; Tyler, 1991. and has more than doubled in the past several hundred years to a current average global value of about 1.7 ppmv ŽTyler, 1991.. Data are scarce on coalbed CH 4 release into the atmosphere from ventilation of underground coal mines or through natural processes. Particularly lacking are data relating to the geochemical, geological, and anthropogenic factors that influence CH 4 emissions from coal beds. Improved understanding of the geochemical processes that control the occurrence and composition of coalbed gas is an important component not only for remediation of potential environmental or mining hazards, but also for exploration and development strategies for utilizing coalbed gas as an energy resource. The purpose of the present paper is to provide a brief literature review of coalbed gas geochemistry — processes that affect its occurrence and composition, and environmental considerations of coalbed gas in the atmosphere. Much more detailed treatment of the topics summarized in this paper can be found in the references given throughout the paper.
2. Chemical and isotopic composition of coalbed gas Coalbed gas consists of hydrocarbons ŽC 1 to C 4 . in variable proportions ŽC 2q from 0 to about 70%., CO 2 in amounts from 0 to greater than 99%, and occasionally small percentages of nitrogen ŽJuntgen and Karweil, 1966; Karweil, 1969; Colombo et al., ¨ 1970; Juntgen and Klein, 1975; Smith et al., 1985a,b; Rice, 1993; Smith and Pallasser, ¨ 1996. and O 2 , H 2 , and He ŽBarns and Edmonds, 1990.. In most cases, CH 4 is the dominant component of coalbed gas in high-volatile bituminous and higher rank coals, with secondary amounts of higher molecular weight hydrocarbons and CO 2 ŽRice, 1993. ŽFigs. 1 and 2.. Small amounts of H 2 S are sometimes generated ŽFig. 1., although like CO 2 , its high solubility in water may facilitate movement of H 2 S out of the coal. Similarly, nitrogen is quite mobile and can escape because of its small molecular diameter, volatility, and low affinity for sorption in coal compared to CH 4 . The relative proportions of CH 4 and higher carbon-number hydrocarbons, or ‘dryness’ of the gas Ži.e., C 1rC 2q ., depends mainly on: Ž1. the mechanism of gas generation Žbacterial versus thermogenic.; Ž2. elemental composition of the macerals contained in the coal, especially hydrogenrcarbon ratio Že.g. Rice et al., 1989.; Ž3. thermal maturity of the coal; and, Ž4. possible retention of higher carbon number Ži.e., ) C 1 . n-alkanes in the coal matrix at low thermal maturities ŽRigby and Smith, 1982.. Coalbed gas contains the greatest proportions of CH 4 Ži.e., most ‘dry’. at high and low ranks, with highly variable hydrocarbon composition at intermediate coal ranks ŽFigs. 1 and 2.. The carbon isotopic composition of coalbed CH 4 varies widely and is related to the process of formation, thermal maturation level ŽFig. 3., and to the maceral composition of the coal. Although the d13 C values of coal macerals may vary by about 3.5‰ ŽSchwartzkopf, 1984; referenced in Whiticar, 1992, 1996., which could be reflected in a small variation in isotopic composition of CH 4 generated, much greater ranges in isotopic composition are associated with different processes of formation and thermal maturation. For example, a range of nearly 30‰ may occur between different pathways
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Fig. 1. Calculated amounts of gases generated for Type I or II Žsapropelic. and Type III Žhumic. organic matter based on changes in elemental composition during coalification Žlitersrkilogram.. Bacterial CH 4 is shown schematically in the upper part Žlow temperature. part of the diagram, but absolute quantity may be highly variable. Modified from Hunt Ž1979. and Rice Ž1993..
Fig. 2. Comparison of gas wetness ŽC 2q . versus thermal maturity Žvitrinite reflectance, %R o . for representative coalbed gases in the United States and Canada Žfrom Rice, 1993..
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Fig. 3. d13 C versus d D for CH 4 in coalbed gases in the United States and Poland showing compositional fields for gases of various origins. Modified from Whiticar et al. Ž1986., Rice Ž1993., Kotarba and Rice Ž1995a,b., and Whiticar Ž1996.. Polish lignite CH 4 data ŽLIG. from Clayton et al. Ž1995.. USB, Upper Silesian Coal Basin, Poland; LSB, Lower Silesian Coal Basin, Poland; SJB, San Juan Basin, U.S.A.; PRB, Powder River Basin, U.S.A.
of bacterial CH 4 production and 13 C enrichment as much as about 25‰ may occur with increasing maturation Žcoalification. due to the kinetic isotope effect ŽFig. 3.. In a compilation of data from coals worldwide, Rice Ž1993. reported d13 C values for coalbed CH 4 from about y80 to y17‰ and d D from y333 to y117‰. Relatively few data have been published for ethane, which has d13 C values from about y23 to y33‰ ŽRice, 1993, and references therein.. d13 C values reported for CO 2 in coal beds are from about y27 to q19‰ ŽRice et al., 1993..
3. Origins of coalbed gas 3.1. Thermal generation of hydrocarbon gases Gas content of a coal is a critical component in determining whether a coal bed is an economically viable gas resource ŽKuuskraa and Boyer, 1993.. Coalbed gas prospect evaluation, reduction of mining hazards, and resource assessment depend on the ability to predict gas content of a coal in advance of drilling or mining. Hydrocarbons in coalbed gas are derived from either thermal breakdown of kerogen or bacterial generation via demethylation of organic molecules or CO 2 reduction. Gases can be distinguished according to origin by their carbon and hydrogen isotopic compositions ŽFig. 3..
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Thermogenesis of hydrocarbon gases begins at coal rank approximately equivalent to vitrinite reflectance Ž%R o . about 0.6 Žhigh-volatile bituminous C; about 1108C in Fig. 1. and continues throughout coalification. In order to predict CH 4 content of coals, researchers have proposed several models of gas generation in coals using numerical methods and laboratory simulations. Important early models were those of Juntgen and ¨ Karweil Ž1966., Karweil Ž1969., and Juntgen and Klein Ž1975. who calculated quantities ¨ of CH 4 generated and stored in coals based on observed changes in elemental composition during coalification and presumed stoichiometry of the expelled products. These models, discussed in detail by Levine Ž1993., have been quoted many times in the literature and form the basis for curves of gas yields from coals as shown in Fig. 1. The CH 4 generation models of Juntgen and co-workers are compared with other models by ¨ Meissner Ž1984. ŽFig. 4.. The curves in Fig. 4 and other literature estimates suggest that coals are capable of generating between about 100 to 300 lrkg coal ŽJuntgen and ¨ Karweil, 1966; Juntgen and Klein, 1975; Hunt, 1979, Levine, 1987; Rice, 1993., ¨ although the actual yield in nature has been estimated at between 150 and 200 lrkg coal ŽRice, 1993., depending on variables such as elemental or maceral composition, gas product composition, and starting maturation level Žrank. ŽJuntgen and Karweil, 1966; ¨ Hunt, 1979; Levine, 1987.. In recent years, several researchers have attempted to quantify coalbed gas generation using laboratory heating experiments to simulate the natural process of gas generation and expulsion from coals. Fig. 5 summarizes results of several laboratory heating experiments on CH 4 yields and gas wetness as a function of experimental temperature. Laboratory-generated coal gases at all experimental temperatures often contain significantly more C 2q hydrocarbons Ži.e. are ‘wetter’. than field samples of coal gases, collected from gas-producing wells or from coal seams in underground mines ŽFig. 5b..
Fig. 4. CH 4 generation in coals versus vitrinite reflectance comparing various generation models. Generation curves A and B are from Juntgen and Karweil Ž1966. and Juntgen and Klein Ž1975., respectively. Modified ¨ ¨ from Meissner Ž1984..
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With a few exceptions, experimental CH 4 yields tend to be significantly less than predicted by models Že.g., Karweil, 1969; Juntgen and Klein, 1975; Meissner, 1984. for ¨ equivalent maturities measured by indirect indicators such as vitrinite reflectance or temperature. For example, in Fig. 5a, an experimental temperature of about 4008C would result in a measured vitrinite reflectance of approximately 1.8–2.0% Ždepending on the type of vitrinite in the coal., or low-volatile bituminous to semi-anthracite rank,
Fig. 5. CH 4 yield Žlrkg. Ža. and gas wetness ŽC 2q . Žb. versus temperature for laboratory pyrolysis experiments on coals and lignites run under both anhydrous and hydrous conditions for variable lengths of time. Data are normalized to coal sample weight Žopen symbols., organic carbon content Žsolid symbols., or organic matter content ŽLewan, 1993.. Polish coal and lignite data are from Kotarba and Lewan Žunpublished data.. From Wood Ž1951., Kim Ž1974., Chung and Sackett Ž1979., Rohrback et al. Ž1984., Higgs Ž1986., Garcıa-Gonzalez ´ ´ et al. Ž1993., Lewan Ž1993., Mukhopadhyay et al. Ž1993., Hill et al. Ž1994., Andresen et al. Ž1995..
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Fig. 5 Žcontinued..
corresponding to CH 4 generation levels of about 122 lrkg according to most models ŽFigs. 1 and 4.. Yet, with the exception of 5 analyses in Fig. 5a, no experiments yielded more than 150 lrkg even at temperatures up to 6008C, and most experiments obtained about 50 lrkg or less, depending on specific experimental conditions. On the other hand, measured gas content of coals is as much as 50–100 lrkg less than model predictions and most experimental yields for thermal maturities corresponding to low-volatile bituminous or greater Žabout 4008C in Fig. 5a.. For example, coals of the Black Warrior Basin, U.S., which are important commercial coalbed gas producers, contain a maximum of about 31 lrkg of total gas at coal ranks of low- to medium-volatile bituminous Žabout 1.2 to 1.8% R o . ŽMcFall et al., 1986.. Central Appalachian coals ŽPennsylvanian. have gas content similar to Black Warrior coals, but other U.S. coals have much lower gas content at equivalent ranks, as low as about 9–10 lrkg ŽKuuskraa and Boyer, 1993.. Martınez ´ et al. Ž1996. calculated that Cretaceous coal in the Fruitland Formation, San Juan Basin, U.S., should generate about 80 lrkg of CH 4 at R o less than 1.2%, based on comparison of Rock–Eval pyrolysis previous modeling by Higgs Ž1986.. In fact, San Juan Basin coals at depths up to 2,500 contain less than 15 lrkg total gas ŽKuuskraa and Boyer, 1993.. Paradoxically, some experimental results indicate that coal Ži.e., coaly or Type III organic matter. actually generates less gas per gram of carbon than more hydrogen-rich
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Types I and II organic matter ŽLewan, 1993.. In hydrous pyrolysis experiments, Lewan Ž1993. obtained gas yields from Types I and II kerogens of about 45 and 142 to 182 lrkg at experimental temperatures of 350 and 4008C, respectively, about 5 to 6.5 times greater than that of Type III kerogen Žyield approximately 28 lrkg. at thermal maturities equivalent to low-volatile bituminous rank Ž1.8%R o . Žabout 1758C in Fig. 1.. Some generated CH 4 may be retained in pore spaces in the coal instead of escaping as a free gas phase and thus would not be detected in the experiments. Garcıa-Gonzalez ´ ´ et al. Ž1993. obtained approximately 13 lrkg expelled CH 4 during hydrous pyrolysis experiments on the Cretaceous Almond Formation coal, Greater Green River Basin, and calculated that an approximately 13.6 lrkg of additional CH 4 was not released from the coal, but was stored in pore spaces. These experimental results indicate that commercial gas accumulations in coals are not caused by particularly high gas generation potential of coals relative to Types I or II organic matter, but rather the high concentration of total organic carbon and a high gas-to-oil ratio in the hydrocarbon generation product. There are a number of possible reasons for the discrepancies between gas generation models or experimental results and actual measured gas content of coals. In the case of the models, the amount of hydrocarbon gas generated depends strongly on the assumed reaction stoichiometry. For example, by varying the relative amounts of H 2 O, CO 2 , and CH 4 in the reaction product, Levine Ž1993. calculated that cumulative CH 4 generated using the model of Juntgen and Karweil Ž1966. can range from 116 to 280 lrkg for ¨ lignite through anthracite coals. Stoichiometric calculations are further complicated for coals that generate substantial quantities of C 2q and liquid hydrocarbons, such as the Fruitland Formation coals in the San Juan basin, U.S. ŽRice et al., 1989. or Mahakam delta coals ŽDurand and Paratte, 1983. and others. Additional data are needed to clarify the relationships between coal composition Žmaceral, elemental. and composition of the generated products in order to further develop maturationrgas generation models that can be applied to coals of varying original compositions. For purposes of predicting gas content of coals, particularly at intermediate thermal maturities, an equally important consideration is the kinetics of gas generation in coals. Variations in reaction kinetics have magnified Žexponential. consequences in the amount of CH 4 or other gaseous products generated, particularly for coals of low to intermediate rank where reaction rates are probably highest. An additional complication with both field determinations of coal gas content and experimental studies is that gas may be retained within the coal matrix to varying degrees depending on the sample collection and laboratory measurement procedures. In the strict sense, the amount of gas generated in a coal includes both expelled gas existing as a free phase in the reaction chamber and that retained within the coal matrix. Gas retained in the coal matrix can include both conventional storage in micropores or fractures in the coal and that held by adsorption on organic matter surfaces or possibly within the molecular structure of the coal organic matter ŽMeissner, 1984., although the details of CH 4 retention in coals are not well understood. In general, the quantity of matrix gas depends on several factors including those intrinsic to the coal itself, those dependent upon the experimental conditions, and those related to the method by which matrix gas is determined preceding and following the heating experiment. Among the variables influencing gas expulsion or retention from coal during laboratory heating
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experiments are coal particle size ŽMoffat and Weale, 1955; Airey, 1968; Yee et al., 1993., presence or absence of water, maceral composition, pressure, and level of thermal maturation. Higher amounts C 2q in coalbed gases measured during laboratory experiments compared to field data may be due to several factors. First, the presence of abundant liptinites or hydrogen-rich vitrinites may correlate with high C 2q content. No systematic laboratory heating experiments have been published that test for this type of correlation, although Rice et al. Ž1989. observed a correlation between the presence of hydrogen-rich vitrinite Ždesmocollonite. and high C 2q content in coalbed gases produced from the Fruitland Formation, San Juan basin. Secondly, Mukhopadhyay et al. Ž1993. postulated that some loss of CH 4 may occur in hydrous experiments due to higher water solubility compared to higher hydrocarbons, although presumably this would be a relatively minor effect. Alternatively, Andresen et al., 1995 argued that in natural systems migration fractionation increases the amount of CH 4 relative to the C 2q fraction, an effect that does not occur in a closed laboratory pyrolysis system. Another reason for higher proportions of CH 4 in gas produced from coals compared with laboratory experiments is that the desorption isotherm for CH 4 is lower than that of C 2q hydrocarbons ŽWhiticar, 1996.. Laboratory experiments run at temperatures higher than those of typical coalbed reservoirs may desorb and contain relatively greater amounts of the C 2q fraction than production samples. 3.2. CO2 sources and bacterial CH4 Although much of the CH 4 in coalbed gas is derived from thermogenic processes, a significant proportion is sometimes derived from bacterial methanogenesis Že.g., Smith and Pallasser, 1996.. Conventional Ži.e., sandstone reservoir. gas deposits of biogenic CH 4 may have been generated during or shortly after the time of deposition of the reservoir and source rocks ŽRice and Claypool, 1981.. It has been proposed that biogenic CH 4 in coals may have been generated not only during early Žpeat or lignite. stages of coal formation Ž‘early stage’ biogenic CH 4 . but also may have formed in the past tens of thousands to millions of years BP Ž‘late stage’ biogenic CH 4 . due to invasion of meteoric water ŽKotarba and Rice, 1995a,b. which provides nutrients necessary for bacterial growth. Preservation of biogenic CH 4 in conventional deposits Žsandstone reservoir. requires rapid sedimentation that inhibits escape of the newly-formed CH 4 ŽRice and Claypool, 1981.. CH 4 generated in peats may be retained in pore spaces or by sorption ŽWhiticar, 1996., but it is unclear whether or not overlying clays or other lithologies would be sufficiently thick or impermeable at the time of biogenic CH 4 generation to contribute to preservation ŽKotarba and Rice, 1995a.. In field measurements of CH 4 in Poland peat fields to depths of about 2 m, we found highly variable CH 4 concentrations, from about 4 ppm to 42% Žvrv. ŽKotarba et al., 1997.. The high CH 4 concentrations were bacterial in origin Ž d13 C s y70‰., preserved beneath an impermeable clay layer. Further, a surface lignite mine in Poland contained significant quantities of Ž‘late stage’. bacterial CH 4 Ž d13 C s y72‰; Fig. 3. related to local hydrological conditions allowing meteoric water recharge in the lower part of the lignite deposit ŽClayton et al., 1995.. Apart from these limited studies, few data are presently
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available documenting the timing of CH 4 generation in coals and it is uncertain if early stage bacterial CH 4 can be preserved. CO 2 is sometimes a major or dominant component of coalbed gas. The origin of CO 2 in coals is of more than solely academic interest because outbursts in underground coal mines are frequently related to high CO 2 concentrations ŽKotarba and Rice, 1995a,b. and high CO 2 content diminishes the value of coalbed gas as an energy source. Sources of CO 2 in coalbed gases are: Ž1. decarboxylation reactions of kerogen and soluble organic matter during burial heating of the coal, Ž2. mineral reactions such as thermal decomposition or dissolution of carbonates or other metamorphic reactions, Ž3. bacterial oxidation of organic matter ŽCarothers and Kharaka, 1980; James and Burns, 1984; Whiticar et al., 1986., and Ž4. deep-seated sources such as magma chambers or the upper crust ŽSmith et al., 1985a; Smith and Pallasser, 1996.. The isotopic composition of CO 2 in conjunction with the CO 2 –CH 4 index wCO 2rŽCO 2 q CH 4 . in %; CDMIx is often used to infer the origin of CO 2 in coal gases ŽKotarba and Rice, 1995a,b; Smith and Pallasser, 1996., an approach used also in conventional gas deposits ŽClayton et al., 1990.. Isotopically heavy CO 2 Ž d13 C up to about q18‰. is interpreted to be a by-product of preferential utilization of isotopically light CO 2 during microbial generation of CH 4 via reduction of CO 2 . Such isotopically heavy CO 2 in coal gas is typically associated with relatively low CO 2 content ŽCDMI less than about 5%.. In contrast, isotopically lighter CO 2 Ž d13 C about y5 to y10‰. is characteristic of coal gases with high CO 2 content ŽCDMI up to 99%., indicative of external CO 2 sources. CO 2 derived from thermocatalytic transformation of kerogen or soluble organic matter in coals is recognizable by very 13 C-depleted isotopic signatures Žless than about y10‰, sometimes as light as y28‰ or lower. and constitutes variable proportions of coal gases. High CO 2 content Ž) 90%. is sometimes associated with relatively wet Žhigh C 2q . coal gases, due possibly to selective solution of CH 4 which has a lower sorption coefficient than C 2q hydrocarbons ŽGould et al., 1987; Rice and Kotarba, 1993.
4. Environmental effects of coalbed gas CH 4 is a strong infrared absorber and important greenhouse gas in the atmosphere. It is the most abundant hydrocarbon in the atmosphere. Studies of gases trapped as bubbles in ice cores indicate that the atmospheric concentration of CH 4 has more than doubled over the past 200–300 years, from a preindustrial average of about 0.6–0.7 ppmv to approximately 1.7 ppmv currently ŽTyler, 1991.. CH 4 plays a role in several processes that affect the chemistry and radiation budget of the atmosphere. On an atomic basis, CH 4 has about 25–30 times the warming effect of CO 2 , and about 70 times on a weight basis ŽKeihl and Dickinson, 1987.. Donner and Ramanathan Ž1980. calculated that an atmospheric CH 4 concentration increase of 1.3 ppmv causes an elevation in the average global surface temperature of about 1.38C. Because CH 4 has an atmospheric residence time of only about 10 years ŽFung et al., 1991., an estimated 10% reduction of the annual emission rate is estimated to be sufficient to stabilize atmospheric concentrations ŽHogan et al., 1991..
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CH 4 in the atmosphere reacts with hydroxyl radicals ŽOH P. in an initial reaction step of a series of reactions involving ozone ŽO 3 ., H 2 O, hydrogen oxides ŽHO x ., nitrous oxides, formaldehyde, hydrogen, chlorine, and other species that affect the H 2 O and O 3 concentration and overall oxidizing power of the atmosphere ŽTyler, 1991.. CH 4 reaction with chlorine produces hydrochloric acid ŽHCl., an important reaction because, unlike free chlorine, HCl does not destroy ozone. Other atmospheric gases, such as CH 3 Cl, CH 3 Br, CHClF3 , CH 2 Cl 2 , CHCCl 3 , and SO 2 , are affected directly or indirectly by CH 4 and OH P levels ŽTyler, 1991.. All of these gases play important direct or indirect greenhouse roles and in the overall physics and chemistry of the atmosphere and have the effect of amplifying the greenhouse effect of atmospheric CH 4 . Coal mining, natural gas ventilation, and gas transmission losses combined account for approximately 50–114 Tg Žteragrams, 10 12 g. out of a total annual global CH 4 flux estimated at 540 Tg from all sources ŽKhalil et al., 1985; Stevens and Englekemeir, 1988; Cicerone and Oremland, 1988; Hogan et al., 1991.. Some studies have estimated that losses of CH 4 during coal mining alone could account for as much as half of the total fossil CH 4 source ŽQuay et al., 1991.. This estimate of CH 4 flux from fossil fuels is obviously not well constrained and it is unknown exactly what the contribution is from coal beds, either through natural processes or associated with mining activities. In the United States, CH 4 emissions from underground coal mines have increased due to a combination of mining of gassier coal seams and improved or increased CH 4 drainage technology ŽDiamond, 1993.. Because of increasing energy demand, particularly in countries where population is increasing, coal production is likely to increase over the next few decades, which could lead to increased atmospheric CH 4 flux from coals. Although the amount of CH 4 emitted from coal beds due to human activities and natural geochemical or geological processes is not well documented, safety ventilation of underground mines is clearly the single largest source of atmospheric CH 4 from coals ŽClayton et al., 1995.. Estimates of CH 4 emissions from coal mining are typically derived by comparing curves relating gas content of coal versus coalification rank multiplied by tonnage mined for coal at various ranks Že.g., Barns and Edmonds, 1990; Boyer et al., 1990., an approach which depends on accurate determination or modeling of coal CH 4 content and understanding of controls on CH 4 retention in coal. We observed 136,000 lrmin from a single ventilation shaft in one mine in the Black Warrior Basin, U.S. ŽClayton et al., 1993.. However, natural leakage of gas from buried coals along faults or fractures, or other permeable migration routes may also constitute a significant atmospheric CH 4 source on a global scale ŽClayton et al., 1993, 1995.. For example, at a surface exposure of a fault zone in the Black Warrior basin we found a CH 4 flux of 1,000 kgryr ŽClayton et al., 1993. and 13 = 10 6 kgryr from one location along a stream bed in the San Juan basin ŽClayton et al., 1995.. In coal mining regions of Poland, soil gas samples taken at depths of about 2.0 m contained from - 1.0 to about 6,000 ppm CH 4 ŽKotarba et al., 1997.. The high CH 4 soil concentrations in Poland are due probably to increased CH 4 flux from underlying coal mines that have been recently closed and no longer operate ventilation shafts. The higher of these soil CH 4 concentrations are high enough to constitute an explosion hazard if mechanisms for accumulation are present, such as underground structures. Atmospheric CH 4 concentrations in Polish coal-mining areas
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are as high as about 10 ppmv ŽKotarba et al., 1997., nearly 5 times ambient values. Although these previous studies were limited in scope, they indicate that additional field studies of CH 4 flux are needed to better understand the role of coal and mining activities in global warming and the global CH 4 cycle. 5. Conclusions Much is now known about the isotopic and chemical composition of coalbed gas and the general processes, if not some of the details, of gas generation. However, as coal mining is likely to increase and coalbed gas is likely to become an increasingly important energy source in future years, further research is needed on the geochemistry of coalbed gases, particularly in the areas of gas generation and retention, primary migration, and producibility. Continued development of effective safety ventilation technologies and procedures for underground mines requires enhanced understanding of CH 4 generation, retention, and migration in coals. As noted by Diamond Ž1993., mining delays have been experienced in mines in the United States due to higher than expected CH 4 content in certain situations. Resource assessment of coalbed gas is dependent on prediction of gas content and migration. Areas of possible fruitful investigation are controls on quantity or composition of gas generated Žsuch as maceral or elemental composition., all factors that affect primary and secondary migration, including external geologic factors such as tectonic stress ŽLittke and Leythaeuser, 1993., and kinetics of gas generation from coals of varying composition. Moreover, estimates of global atmospheric CH 4 flux from coals are order-of-magnitude estimates based on extrapolations from limited data sets. Additional research is needed to determine the influence of various geochemical and geological factors that control CH 4 flux, so that models can be developed that take into account controlling factors for particular regions, thus allowing more meaningful global estimates.
References Airey, E.M., 1968. Gas emission from broken coal. An experimental and theoretical investigation. Int. J. Rock Mechanics Mining Sci. 5, 475–494. Andresen, B., Throndsen, T., Raheim, A., Bolstad, J., 1995. A comparison of pyrolysis products with models ˚ for natural gas generation. Chem. Geol. 126, 261–280. Barns, D.W., Edmonds, J.A., 1990. An evaluation of the relationship between and production and use of energy and atmospheric methane emissions, U.S. Department of Energy Report TR047, DE-AC06-76RLO1830, 143 p. Blake, D.R., Rowland, F.S., 1988. Contintuing worldwide increase in tropospheric methane, 1978–1987. Science 239, 1129–1131. Boreham, C.J., Powell, T.G., 1991. in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, Oklahoma, pp. 133–157. Boyer, C.M, Kelafant, J.R., Kuuskraa, V.A., Manger, K.C., Kruger, D., 1990. Methane emissions from coal mining – issues and opportunities for reduction, U.S. Environmental Protection Agency EPAr400r9-90008, 131 p. Carothers, W.W., Kharaka, Y.K., 1980. Stable carbon isotopes of HCOy 3 in oil-field waters: implications for the origin of CO 2 . Geochim. Cosmochim. Acta 44, 323–332.
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Chung, H.M., Sackett, W.M., 1979. Use of stable carbon isotope compositions of pyrolytically derived methane as maturity indices for carbonaceous materials. Geochim. Cosmochim. Acta. 43, 1979–1988. Cicerone, R.J., Oremland, R.S., 1988. Biogeochemical aspects of atmospheric methane. Global Biogeochem. Cycles 2, 299–327. ´ 1990. Generation and migration of hydrocarbon gas and Clayton, J.L., Spencer, C.W., Koncz, I., Szalay, A., carbon dioxide in the Bekes ´ ´ basin, southeastern Hungary. Org. Geochem. 15, 233–247. Clayton, J.L., Leventhal, J.S., Rice, D.D., Pashin, J.C., Mosher, B., Czepiel, P., 1993. Atmospheric methane flux from coals – preliminary investigation of coal mines and geologic structures in the Black Warrior basin, Alabama, in: The Future of Energy Gases, U.S. Geological Survey Professional Paper 1570, pp. 471–492. Clayton, J.L., Leventhal, J.S., Rice, D.D., Kotarba, M., Korus, A., 1995. Atmospheric methane flux from U.S. and Polish coals, in: Grimalt, J.O., Dorronsoro, C. ŽEds.., Organic Geochemistry, Developments and Applications to Energy, Climate, Environment and Human History, Selected Papers from the 17th International Meeting on Organic Geochemistry, Donostia-San Sebastian, Spain, pp. 641–643. Colombo, U., Gazzarrini, F., Gonfiantini, R., Kneuper, G., Teichmuller, M., Teichmuller, R., 1970. Carbon isotope study on methane from German coal deposits, in: Hobson, G.D., Speers, G.C. ŽEds.., Advances in Organic Geochemistry 1966, Pergamon Press, Oxford, pp. 1–26. Diamond, W.P., 1993. Methane control for underground coal mines, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, OK, pp. 237–267. Donner, L., Ramanathan, V., 1980. Methane and nitrous oxide – Their effects on the terrestrial climate. J. Atmospheric Sci. 37, 119–124. Durand, B., Paratte, M., 1983. Oil potential of coals: A geochemical approach, in: Brooks, J. ŽEd.., Petroleum Geochemistry and Exploration of Europe, Blackwell, Oxford, pp. 255–265. Fung, I., John, J., Lerner, J., Matthews, E., Prather, M., Steele, L.P., Fraser, P.J., 1991. Three-dimensional model synthesis of the global methane cycle. J. Geophys. Res. 96, 13033–13065. Garcıa-Gonzalez, M., MacGowan, D.B., Surdam, R.C., 1993. Coal as a source rock of petroleum and gas – a ´ ´ comparison between natural and artificial maturation of the Almond Formation coals, greater Green River basin in Wyoming, in: The Future of Energy Gases, U.S. Geological Survey Professional Paper 1570, pp. 405–436. Gould, K.W., Hargraves, A.J., Smith, J.W., 1987. Variation in the composition of seam gases issuing from coal. Bull. Aust. Inst. Mining Metall. 292, 69–73. Higgs, M.D., 1986. Laboratory studies into the generation of natural gas from coals, in: Brooks, J., Goff, J.C., van Hoorn, B. ŽEds.., Habitat of Palaeozoic gas in N.W. Europe, Geological Society Special Publication, No. 23, London. pp 113–120. Hill, R.J., Jenden, P.D., Tang, Y.C., Teerman, S.C., Kaplan, I.R., 1994. Influence of pressure on pyrolysis of coal, in: Mukhopadhyay, P.K., Dow, W.G. ŽEds.., Vitrinite Reflectance as a Maturity Parameter, ACS Symposium Series 570, American Chemical Society, Washington, p. 160–193. Hogan, K.B., Hoffman, J.S., Thompson, A.M., 1991. Methane on the greenhouse agenda. Nature 354, 181–182. Hunt, J.M., 1979. Petroleum Geochemistry and Geology, W.H. Freeman and Company, San Francisco. ICF Resources, 1990. The United States coalbed methane resource, Quarterly Review of Methane from Coal Seams Technology, Vol. 7, pp. 10–28. James, A.T., Burns, B.J., 1984. Microbial alteration of subsurface natural gas accumulations. Bull. AAPG 68, 957–960. Juntgen, H., Karweil, J., 1966. Gasbildung and gasspeicherung in steinkohlenflozen, Part I and II: Erdol ¨ ¨ and Kohle Petrochem., 19, 251–258, 339–344. Juntgen, H., Klein, J., 1975. Entstehung von erdgas gus kohligen sedimenten. Erdol ¨ ¨ und Kohle Petrochem. 1, 52–69. Karweil, J., 1969. Aktuelle problem der geochemic der kohle, in: Schenk, P.A., Havernaar, I. ŽEds.., Advances in Geochemistry 1968, Pergamon Press, Oxford, pp. 59–84. Khalil, M.A.K., Rasmussen, R.A., Shearer, M.J., Ge, S., Rau, J.A., 1985. Causes of increasing atmospheric methane: Depletion of hydroxyl radicals and the rise of emissions. Atmosphere Environ. 19, 397–407. Keihl, J.T., Dickinson, R.E., 1987. A study of the radiative effects of enhanced atmospheric CO 2 and CH 4 on early earth temperatures. J. Geophys. Res. 92, 2991–2998.
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Kim, A.G., 1974. Low-temperature evolution of hydrocarbon gases from coal, U.S. Bureau of Mines, Report of Investigations 7965, 23 p. Kotarba, M.J., Rice, D.D., 1995a. Composition and origin of coalbed gases in the Lower Silesian coal basin, NW Poland, in: Report from Research Cooperation within the U.S.-Polish Maria Sklodowska-Curie Joint Fund II, Origin and Habitat of Coal Gases in Polish Basins, Isotopic and Geological Approach, pp. 69–78. Kotarba, M.J., D.D., Rice, 1995b. Composition and origin of coalbed gases in the Upper Silesian and Lublin coal basins, Poland, Report from Research Cooperation within the U.S.-Polish Maria Sklodowska-Curie Joint Fund II, Origin and Habitat of Coal Gases in Polish Basins, Isotopic and Geological Approach, pp. 79–89. Kotarba, M.J., Clayton, J.L., Korus, A., Rice, D., 1997. Near-surface and atmospheric CH 4 concentration in Polish coal mining districts, in preparation. Kuuskraa, V.A., Boyer, C.A., 1993. Economic and parametric analysis of coalbed methane, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, Oklahoma, pp. 373–394. Levine, J.R., 1987. Influence of coal composition on the generation and retention of coalbed natural gas, Proceedings of 1987 Coalbed Methane Symposium, pp. 15–18. Levine, J.R., 1993. Coalification: The evolution of coal as source rock and reservoir rock for oil and gas, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, Oklahoma, pp. 39–77. Lewan, M.D., 1993. Hydrocarbon gas generation from different kerogen types subjected to hydrous pyrolysis, American Chemical Society, Division of Geochemistry, Inc., 206th ACS National Meeting Abstracts, Chicago, Illinois, 22-27 August 1993, Abstract No. 32. Littke, R., Leythaeuser, D., 1993. Migration of oil and gas in coals, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, Oklahoma, pp. 219–236. Martınez, L., Suarez-Ruiz, I., Jimenez, A., 1996. Application of geochemical and petrographic parameters to ´ ´ ´ the study of gas genesis in coals, in: Gomez Cortes, ´ Luna, M.E., Martınez ´ ´ A. ŽEds.., Memorias del V Congreso Latinoamericano de Geoquimica Organica, Cancun, Mexico, p. 195–197. ´ McFall, K.S., Wicks, D.E., Kuuskraa, V.A., 1986. A geologic assassment of natural gas from coal seams in the Warrior basin of Alabama, Gas Research Institute Topical Report 86r0272, 80 p. Meissner, F.F., 1984. Cretaceous and lower Tertiary coals as sources for gas accumulations in the Rocky Mountain area, in: Woodward, J., Meissner, F.F., Clayton, J.L. ŽEds.., Hydrocarbon Source Rocks of the Greater Rocky Mountain Region. Rocky Mountain Association of Geologists, Denver, pp. 401–431. Moffat, D.H., Weale, K.E., 1955. Sorption by coal of methane at high pressures. Fuel. 34, 449–462. Mukhopadhyay, P.K., Calder, J.H., Hatcher, P.G., 1993. Geological and physicochemical constraints on methane and C 6q hydrocarbon generation capabilities and quality of Carboniferous coals, Cumberland basin, Nova Scotia, Canada, in: Chiang, S.-H. ŽEd.., Proceedings Tenth Annual International Pittsburgh Coal Conference – Coal-Energy and the Environment, September 20–24, 1993, Pittsburgh, pp. 1074–1079. Quay, P.D., King, S.L., Stutsman, J., Wilbur, D.O., Steele, L.P., Fung, I., Gammon, R.H., Brown, T.A., Farwell, G.W., Grootes, P.M., Schmidt, F.H., 1991. Carbon isotopic composition of atmospheric CH 4 : Fossil and biomass burning source strengths. Global Biogeochem. Cycles 5, 25–47. Rasmussen, R.A., Khalil, M.A.K., 1981. Atmospheric methane ŽCH 4 .: Trends and seasonal cycles. J. Geophys. Res. 86, 9826–9832. Rice, D.D., Claypool, G.E., 1981. Generation, accumulation, and resource potential of biogenic gas. AAPG Bull. 65, 5–25. Rice, D.D., Clayton, J.L., Pawlewicz, M.J., 1989. Characterization of coal-derived hydrocarbons and sourcerock potential of coal beds, San Juan basin, New Mexico and Colorado, U.S.A. Int. J. Coal Geol. 13, 597–626. Rice, D.D., Law, B.E., Clayton, J.L., 1993. Coalbed gas – an undeveloped resource, in: The Future of Energy Gases, U.S. Geological Survey Professional Paper 1570, pp. 389–404. Rice, D.D., 1993. Composition and origins of coalbed gas, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, OK, pp. 159–184. Rice, D.D., Kotarba, M.J., 1993. Origin of Upper Carboniferous coalbed gases, Lower and Upper Silesian coal basins, Poland, Proceedings of the 1993 International Coalbed Methane Symposium, The University of Alabama, Tuscaloosa. pp. 649–652.
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Rigby, D., Smith, J.W., 1982. A reassessment of stable carbon isotopes in hydrocarbon exploration. Erdol ¨ und Kohle Erdgas Petrochem. 35, 415–417. Rohrback, B.G., Peters, K.E., Kaplan, I.R., 1984. Geochemistry of artificially heated humic and sapropelic sediments – II: Oil and gas generation. AAPG Bull. 8, 961–970. Schwartzkopf, T., 1984. Kohlenstoff- und Wasserstoffisotope in Kohlen und ihren Nebengesteinen: Diplomarbeit Ruhr Universtitat ¨ Bochum, 39 p. Smith, J.W., Gould, K.W., Hart, G.H., Rigby, D., 1985a. Isotopic studies of Australian natural and coal seam gas. Bull. Aust. Inst. Mining Metall. 290, 43–51. Smith, J.W., Rigby, D., Gould, K.W., Hart, G., Hargraves, A.J., 1985b. An isotopic study of hydrocarbon generation processes. Organic Geochem. 8, 341–347. Smith, J.W., Pallasser, R.J., 1996. Microbial origin of Australian coalbed methane. AAPG Bull. 80, 891–897. Steele, L.P., Fraser, P.J., Rasmussen, R.A., Khalil, M.A.K., Conway, T.J., Crawford, A.J., Gammon, R.H., Masarie, K.A., Thoning, K.W., 1987. The global distribution of methane in the troposphere. J. Atmos. Chem. 5, 125–171. Stevens, C.M., Englekemeir, A., 1988. Stable carbon isotopic composition of methane from some natural and anthropogenic sources. J. Geophy. Res. 93, 725–733. Tyler, S.C., 1991. The global methane budget, in: Rogers, J.E., Whitman, W.B. ŽEds.., Microbial Production and Consumption of Greenhouse Gases: Methane, Nitrogen Oxides, and Halomethanes, American Society for Microbiology, Washington, pp. 7–38. Whiticar, M.J., Faber, E., Schoell, M., 1986. Biogenic methane formation in marine and freshwater environments, CO 2 reduction vs. acetate fermentation – isotopic evidence. Geochim. Cosmochim. Acta 50, 693–709. Whiticar, M.J., 1992. Stable isotope geochemistry of coals, humic kerogens, and related natural gases, The Canadian Coal and Coalbed Methane Geoscience Forum Proceedings, Parksville, B.C., February 2–5, 1992, Alberta Research Council, pp. 149–172. Whiticar, M.J., 1996. Stable isotope geochemistry of coals, humic kerogens and related natural gases. Int. J. Coal. Geol. 32, 191–215. Wood, W.L., 1951. The production of gaseous hydrocarbons from coals and their tars, in: Sell, G. ŽEd.., Oil Shale and Cannel Coal, Vol. 2, Proceedings of the Second Oil Shale and Cannel Coal Conferences, Glasgow, July, 1950, The Institute of Petroleum, London. pp. 691–709. Yee, D., Seidle, J.P., Hanson, W.B., 1993. Gas sorption on coal and measurement of gas content, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, OK, pp. 203–218.
Geochemistry of coalbed gas – A review J.L. Clayton
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U.S. Geological SurÕey, DenÕer, CO 80225, USA Received 1 November 1996; accepted 22 April 1997
Abstract Coals are both sources and reservoirs of large amounts of gas that has received increasing attention in recent years as a largely untapped potential energy resource. Coal mining operations, such as ventilation of coalbed gas from underground mines, release coalbed CH 4 into the atmosphere, an important greenhouse gas whose concentration in the atmosphere is increasing. Because of these energy and environmental issues, increased research attention has been focused on the geochemistry of coalbed gas in recent years. This paper presents a summary review of the main aspects of coalbed gas geochemistry and current research advances. q 1998 Elsevier Science B.V. Keywords: coal; gas; carbon isotope; methane; pyrolysis
1. Introduction Coals serve as both sources and reservoirs of substantial quantities of hydrocarbon and CO 2 gas. Long known for explosion and outburst hazards in underground mining operations, coalbed gas has recently been recognized as an important energy resource. For example, in the United States alone, in-place coalbed gas resources are estimated at about 11.3 trillion cubic meters Žapproximately 400 trillion cubic feet. ŽICF Resources, 1990.. Although coals and coaly organic matter dispersed in shales are also now recognized as potential sources of liquid hydrocarbons, this paper focuses solely on coals as sources of gas. For a review of coals as sources of oil the reader is referred to Boreham and Powell Ž1991.. In addition to its importance as an energy resource, CH 4 is an important greenhouse gas whose atmospheric concentration has been increasing at a rate of about 1% annually in the past 15–20 years ŽRasmussen and Khalil, 1981; Steele et al., 1987; Blake and )
Fax: q1-303-2363202; E-mail: [email protected]
0166-5162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 6 - 5 1 6 2 Ž 9 7 . 0 0 0 1 7 - 7
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Rowland, 1988; Tyler, 1991. and has more than doubled in the past several hundred years to a current average global value of about 1.7 ppmv ŽTyler, 1991.. Data are scarce on coalbed CH 4 release into the atmosphere from ventilation of underground coal mines or through natural processes. Particularly lacking are data relating to the geochemical, geological, and anthropogenic factors that influence CH 4 emissions from coal beds. Improved understanding of the geochemical processes that control the occurrence and composition of coalbed gas is an important component not only for remediation of potential environmental or mining hazards, but also for exploration and development strategies for utilizing coalbed gas as an energy resource. The purpose of the present paper is to provide a brief literature review of coalbed gas geochemistry — processes that affect its occurrence and composition, and environmental considerations of coalbed gas in the atmosphere. Much more detailed treatment of the topics summarized in this paper can be found in the references given throughout the paper.
2. Chemical and isotopic composition of coalbed gas Coalbed gas consists of hydrocarbons ŽC 1 to C 4 . in variable proportions ŽC 2q from 0 to about 70%., CO 2 in amounts from 0 to greater than 99%, and occasionally small percentages of nitrogen ŽJuntgen and Karweil, 1966; Karweil, 1969; Colombo et al., ¨ 1970; Juntgen and Klein, 1975; Smith et al., 1985a,b; Rice, 1993; Smith and Pallasser, ¨ 1996. and O 2 , H 2 , and He ŽBarns and Edmonds, 1990.. In most cases, CH 4 is the dominant component of coalbed gas in high-volatile bituminous and higher rank coals, with secondary amounts of higher molecular weight hydrocarbons and CO 2 ŽRice, 1993. ŽFigs. 1 and 2.. Small amounts of H 2 S are sometimes generated ŽFig. 1., although like CO 2 , its high solubility in water may facilitate movement of H 2 S out of the coal. Similarly, nitrogen is quite mobile and can escape because of its small molecular diameter, volatility, and low affinity for sorption in coal compared to CH 4 . The relative proportions of CH 4 and higher carbon-number hydrocarbons, or ‘dryness’ of the gas Ži.e., C 1rC 2q ., depends mainly on: Ž1. the mechanism of gas generation Žbacterial versus thermogenic.; Ž2. elemental composition of the macerals contained in the coal, especially hydrogenrcarbon ratio Že.g. Rice et al., 1989.; Ž3. thermal maturity of the coal; and, Ž4. possible retention of higher carbon number Ži.e., ) C 1 . n-alkanes in the coal matrix at low thermal maturities ŽRigby and Smith, 1982.. Coalbed gas contains the greatest proportions of CH 4 Ži.e., most ‘dry’. at high and low ranks, with highly variable hydrocarbon composition at intermediate coal ranks ŽFigs. 1 and 2.. The carbon isotopic composition of coalbed CH 4 varies widely and is related to the process of formation, thermal maturation level ŽFig. 3., and to the maceral composition of the coal. Although the d13 C values of coal macerals may vary by about 3.5‰ ŽSchwartzkopf, 1984; referenced in Whiticar, 1992, 1996., which could be reflected in a small variation in isotopic composition of CH 4 generated, much greater ranges in isotopic composition are associated with different processes of formation and thermal maturation. For example, a range of nearly 30‰ may occur between different pathways
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Fig. 1. Calculated amounts of gases generated for Type I or II Žsapropelic. and Type III Žhumic. organic matter based on changes in elemental composition during coalification Žlitersrkilogram.. Bacterial CH 4 is shown schematically in the upper part Žlow temperature. part of the diagram, but absolute quantity may be highly variable. Modified from Hunt Ž1979. and Rice Ž1993..
Fig. 2. Comparison of gas wetness ŽC 2q . versus thermal maturity Žvitrinite reflectance, %R o . for representative coalbed gases in the United States and Canada Žfrom Rice, 1993..
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Fig. 3. d13 C versus d D for CH 4 in coalbed gases in the United States and Poland showing compositional fields for gases of various origins. Modified from Whiticar et al. Ž1986., Rice Ž1993., Kotarba and Rice Ž1995a,b., and Whiticar Ž1996.. Polish lignite CH 4 data ŽLIG. from Clayton et al. Ž1995.. USB, Upper Silesian Coal Basin, Poland; LSB, Lower Silesian Coal Basin, Poland; SJB, San Juan Basin, U.S.A.; PRB, Powder River Basin, U.S.A.
of bacterial CH 4 production and 13 C enrichment as much as about 25‰ may occur with increasing maturation Žcoalification. due to the kinetic isotope effect ŽFig. 3.. In a compilation of data from coals worldwide, Rice Ž1993. reported d13 C values for coalbed CH 4 from about y80 to y17‰ and d D from y333 to y117‰. Relatively few data have been published for ethane, which has d13 C values from about y23 to y33‰ ŽRice, 1993, and references therein.. d13 C values reported for CO 2 in coal beds are from about y27 to q19‰ ŽRice et al., 1993..
3. Origins of coalbed gas 3.1. Thermal generation of hydrocarbon gases Gas content of a coal is a critical component in determining whether a coal bed is an economically viable gas resource ŽKuuskraa and Boyer, 1993.. Coalbed gas prospect evaluation, reduction of mining hazards, and resource assessment depend on the ability to predict gas content of a coal in advance of drilling or mining. Hydrocarbons in coalbed gas are derived from either thermal breakdown of kerogen or bacterial generation via demethylation of organic molecules or CO 2 reduction. Gases can be distinguished according to origin by their carbon and hydrogen isotopic compositions ŽFig. 3..
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Thermogenesis of hydrocarbon gases begins at coal rank approximately equivalent to vitrinite reflectance Ž%R o . about 0.6 Žhigh-volatile bituminous C; about 1108C in Fig. 1. and continues throughout coalification. In order to predict CH 4 content of coals, researchers have proposed several models of gas generation in coals using numerical methods and laboratory simulations. Important early models were those of Juntgen and ¨ Karweil Ž1966., Karweil Ž1969., and Juntgen and Klein Ž1975. who calculated quantities ¨ of CH 4 generated and stored in coals based on observed changes in elemental composition during coalification and presumed stoichiometry of the expelled products. These models, discussed in detail by Levine Ž1993., have been quoted many times in the literature and form the basis for curves of gas yields from coals as shown in Fig. 1. The CH 4 generation models of Juntgen and co-workers are compared with other models by ¨ Meissner Ž1984. ŽFig. 4.. The curves in Fig. 4 and other literature estimates suggest that coals are capable of generating between about 100 to 300 lrkg coal ŽJuntgen and ¨ Karweil, 1966; Juntgen and Klein, 1975; Hunt, 1979, Levine, 1987; Rice, 1993., ¨ although the actual yield in nature has been estimated at between 150 and 200 lrkg coal ŽRice, 1993., depending on variables such as elemental or maceral composition, gas product composition, and starting maturation level Žrank. ŽJuntgen and Karweil, 1966; ¨ Hunt, 1979; Levine, 1987.. In recent years, several researchers have attempted to quantify coalbed gas generation using laboratory heating experiments to simulate the natural process of gas generation and expulsion from coals. Fig. 5 summarizes results of several laboratory heating experiments on CH 4 yields and gas wetness as a function of experimental temperature. Laboratory-generated coal gases at all experimental temperatures often contain significantly more C 2q hydrocarbons Ži.e. are ‘wetter’. than field samples of coal gases, collected from gas-producing wells or from coal seams in underground mines ŽFig. 5b..
Fig. 4. CH 4 generation in coals versus vitrinite reflectance comparing various generation models. Generation curves A and B are from Juntgen and Karweil Ž1966. and Juntgen and Klein Ž1975., respectively. Modified ¨ ¨ from Meissner Ž1984..
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With a few exceptions, experimental CH 4 yields tend to be significantly less than predicted by models Že.g., Karweil, 1969; Juntgen and Klein, 1975; Meissner, 1984. for ¨ equivalent maturities measured by indirect indicators such as vitrinite reflectance or temperature. For example, in Fig. 5a, an experimental temperature of about 4008C would result in a measured vitrinite reflectance of approximately 1.8–2.0% Ždepending on the type of vitrinite in the coal., or low-volatile bituminous to semi-anthracite rank,
Fig. 5. CH 4 yield Žlrkg. Ža. and gas wetness ŽC 2q . Žb. versus temperature for laboratory pyrolysis experiments on coals and lignites run under both anhydrous and hydrous conditions for variable lengths of time. Data are normalized to coal sample weight Žopen symbols., organic carbon content Žsolid symbols., or organic matter content ŽLewan, 1993.. Polish coal and lignite data are from Kotarba and Lewan Žunpublished data.. From Wood Ž1951., Kim Ž1974., Chung and Sackett Ž1979., Rohrback et al. Ž1984., Higgs Ž1986., Garcıa-Gonzalez ´ ´ et al. Ž1993., Lewan Ž1993., Mukhopadhyay et al. Ž1993., Hill et al. Ž1994., Andresen et al. Ž1995..
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Fig. 5 Žcontinued..
corresponding to CH 4 generation levels of about 122 lrkg according to most models ŽFigs. 1 and 4.. Yet, with the exception of 5 analyses in Fig. 5a, no experiments yielded more than 150 lrkg even at temperatures up to 6008C, and most experiments obtained about 50 lrkg or less, depending on specific experimental conditions. On the other hand, measured gas content of coals is as much as 50–100 lrkg less than model predictions and most experimental yields for thermal maturities corresponding to low-volatile bituminous or greater Žabout 4008C in Fig. 5a.. For example, coals of the Black Warrior Basin, U.S., which are important commercial coalbed gas producers, contain a maximum of about 31 lrkg of total gas at coal ranks of low- to medium-volatile bituminous Žabout 1.2 to 1.8% R o . ŽMcFall et al., 1986.. Central Appalachian coals ŽPennsylvanian. have gas content similar to Black Warrior coals, but other U.S. coals have much lower gas content at equivalent ranks, as low as about 9–10 lrkg ŽKuuskraa and Boyer, 1993.. Martınez ´ et al. Ž1996. calculated that Cretaceous coal in the Fruitland Formation, San Juan Basin, U.S., should generate about 80 lrkg of CH 4 at R o less than 1.2%, based on comparison of Rock–Eval pyrolysis previous modeling by Higgs Ž1986.. In fact, San Juan Basin coals at depths up to 2,500 contain less than 15 lrkg total gas ŽKuuskraa and Boyer, 1993.. Paradoxically, some experimental results indicate that coal Ži.e., coaly or Type III organic matter. actually generates less gas per gram of carbon than more hydrogen-rich
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Types I and II organic matter ŽLewan, 1993.. In hydrous pyrolysis experiments, Lewan Ž1993. obtained gas yields from Types I and II kerogens of about 45 and 142 to 182 lrkg at experimental temperatures of 350 and 4008C, respectively, about 5 to 6.5 times greater than that of Type III kerogen Žyield approximately 28 lrkg. at thermal maturities equivalent to low-volatile bituminous rank Ž1.8%R o . Žabout 1758C in Fig. 1.. Some generated CH 4 may be retained in pore spaces in the coal instead of escaping as a free gas phase and thus would not be detected in the experiments. Garcıa-Gonzalez ´ ´ et al. Ž1993. obtained approximately 13 lrkg expelled CH 4 during hydrous pyrolysis experiments on the Cretaceous Almond Formation coal, Greater Green River Basin, and calculated that an approximately 13.6 lrkg of additional CH 4 was not released from the coal, but was stored in pore spaces. These experimental results indicate that commercial gas accumulations in coals are not caused by particularly high gas generation potential of coals relative to Types I or II organic matter, but rather the high concentration of total organic carbon and a high gas-to-oil ratio in the hydrocarbon generation product. There are a number of possible reasons for the discrepancies between gas generation models or experimental results and actual measured gas content of coals. In the case of the models, the amount of hydrocarbon gas generated depends strongly on the assumed reaction stoichiometry. For example, by varying the relative amounts of H 2 O, CO 2 , and CH 4 in the reaction product, Levine Ž1993. calculated that cumulative CH 4 generated using the model of Juntgen and Karweil Ž1966. can range from 116 to 280 lrkg for ¨ lignite through anthracite coals. Stoichiometric calculations are further complicated for coals that generate substantial quantities of C 2q and liquid hydrocarbons, such as the Fruitland Formation coals in the San Juan basin, U.S. ŽRice et al., 1989. or Mahakam delta coals ŽDurand and Paratte, 1983. and others. Additional data are needed to clarify the relationships between coal composition Žmaceral, elemental. and composition of the generated products in order to further develop maturationrgas generation models that can be applied to coals of varying original compositions. For purposes of predicting gas content of coals, particularly at intermediate thermal maturities, an equally important consideration is the kinetics of gas generation in coals. Variations in reaction kinetics have magnified Žexponential. consequences in the amount of CH 4 or other gaseous products generated, particularly for coals of low to intermediate rank where reaction rates are probably highest. An additional complication with both field determinations of coal gas content and experimental studies is that gas may be retained within the coal matrix to varying degrees depending on the sample collection and laboratory measurement procedures. In the strict sense, the amount of gas generated in a coal includes both expelled gas existing as a free phase in the reaction chamber and that retained within the coal matrix. Gas retained in the coal matrix can include both conventional storage in micropores or fractures in the coal and that held by adsorption on organic matter surfaces or possibly within the molecular structure of the coal organic matter ŽMeissner, 1984., although the details of CH 4 retention in coals are not well understood. In general, the quantity of matrix gas depends on several factors including those intrinsic to the coal itself, those dependent upon the experimental conditions, and those related to the method by which matrix gas is determined preceding and following the heating experiment. Among the variables influencing gas expulsion or retention from coal during laboratory heating
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experiments are coal particle size ŽMoffat and Weale, 1955; Airey, 1968; Yee et al., 1993., presence or absence of water, maceral composition, pressure, and level of thermal maturation. Higher amounts C 2q in coalbed gases measured during laboratory experiments compared to field data may be due to several factors. First, the presence of abundant liptinites or hydrogen-rich vitrinites may correlate with high C 2q content. No systematic laboratory heating experiments have been published that test for this type of correlation, although Rice et al. Ž1989. observed a correlation between the presence of hydrogen-rich vitrinite Ždesmocollonite. and high C 2q content in coalbed gases produced from the Fruitland Formation, San Juan basin. Secondly, Mukhopadhyay et al. Ž1993. postulated that some loss of CH 4 may occur in hydrous experiments due to higher water solubility compared to higher hydrocarbons, although presumably this would be a relatively minor effect. Alternatively, Andresen et al., 1995 argued that in natural systems migration fractionation increases the amount of CH 4 relative to the C 2q fraction, an effect that does not occur in a closed laboratory pyrolysis system. Another reason for higher proportions of CH 4 in gas produced from coals compared with laboratory experiments is that the desorption isotherm for CH 4 is lower than that of C 2q hydrocarbons ŽWhiticar, 1996.. Laboratory experiments run at temperatures higher than those of typical coalbed reservoirs may desorb and contain relatively greater amounts of the C 2q fraction than production samples. 3.2. CO2 sources and bacterial CH4 Although much of the CH 4 in coalbed gas is derived from thermogenic processes, a significant proportion is sometimes derived from bacterial methanogenesis Že.g., Smith and Pallasser, 1996.. Conventional Ži.e., sandstone reservoir. gas deposits of biogenic CH 4 may have been generated during or shortly after the time of deposition of the reservoir and source rocks ŽRice and Claypool, 1981.. It has been proposed that biogenic CH 4 in coals may have been generated not only during early Žpeat or lignite. stages of coal formation Ž‘early stage’ biogenic CH 4 . but also may have formed in the past tens of thousands to millions of years BP Ž‘late stage’ biogenic CH 4 . due to invasion of meteoric water ŽKotarba and Rice, 1995a,b. which provides nutrients necessary for bacterial growth. Preservation of biogenic CH 4 in conventional deposits Žsandstone reservoir. requires rapid sedimentation that inhibits escape of the newly-formed CH 4 ŽRice and Claypool, 1981.. CH 4 generated in peats may be retained in pore spaces or by sorption ŽWhiticar, 1996., but it is unclear whether or not overlying clays or other lithologies would be sufficiently thick or impermeable at the time of biogenic CH 4 generation to contribute to preservation ŽKotarba and Rice, 1995a.. In field measurements of CH 4 in Poland peat fields to depths of about 2 m, we found highly variable CH 4 concentrations, from about 4 ppm to 42% Žvrv. ŽKotarba et al., 1997.. The high CH 4 concentrations were bacterial in origin Ž d13 C s y70‰., preserved beneath an impermeable clay layer. Further, a surface lignite mine in Poland contained significant quantities of Ž‘late stage’. bacterial CH 4 Ž d13 C s y72‰; Fig. 3. related to local hydrological conditions allowing meteoric water recharge in the lower part of the lignite deposit ŽClayton et al., 1995.. Apart from these limited studies, few data are presently
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available documenting the timing of CH 4 generation in coals and it is uncertain if early stage bacterial CH 4 can be preserved. CO 2 is sometimes a major or dominant component of coalbed gas. The origin of CO 2 in coals is of more than solely academic interest because outbursts in underground coal mines are frequently related to high CO 2 concentrations ŽKotarba and Rice, 1995a,b. and high CO 2 content diminishes the value of coalbed gas as an energy source. Sources of CO 2 in coalbed gases are: Ž1. decarboxylation reactions of kerogen and soluble organic matter during burial heating of the coal, Ž2. mineral reactions such as thermal decomposition or dissolution of carbonates or other metamorphic reactions, Ž3. bacterial oxidation of organic matter ŽCarothers and Kharaka, 1980; James and Burns, 1984; Whiticar et al., 1986., and Ž4. deep-seated sources such as magma chambers or the upper crust ŽSmith et al., 1985a; Smith and Pallasser, 1996.. The isotopic composition of CO 2 in conjunction with the CO 2 –CH 4 index wCO 2rŽCO 2 q CH 4 . in %; CDMIx is often used to infer the origin of CO 2 in coal gases ŽKotarba and Rice, 1995a,b; Smith and Pallasser, 1996., an approach used also in conventional gas deposits ŽClayton et al., 1990.. Isotopically heavy CO 2 Ž d13 C up to about q18‰. is interpreted to be a by-product of preferential utilization of isotopically light CO 2 during microbial generation of CH 4 via reduction of CO 2 . Such isotopically heavy CO 2 in coal gas is typically associated with relatively low CO 2 content ŽCDMI less than about 5%.. In contrast, isotopically lighter CO 2 Ž d13 C about y5 to y10‰. is characteristic of coal gases with high CO 2 content ŽCDMI up to 99%., indicative of external CO 2 sources. CO 2 derived from thermocatalytic transformation of kerogen or soluble organic matter in coals is recognizable by very 13 C-depleted isotopic signatures Žless than about y10‰, sometimes as light as y28‰ or lower. and constitutes variable proportions of coal gases. High CO 2 content Ž) 90%. is sometimes associated with relatively wet Žhigh C 2q . coal gases, due possibly to selective solution of CH 4 which has a lower sorption coefficient than C 2q hydrocarbons ŽGould et al., 1987; Rice and Kotarba, 1993.
4. Environmental effects of coalbed gas CH 4 is a strong infrared absorber and important greenhouse gas in the atmosphere. It is the most abundant hydrocarbon in the atmosphere. Studies of gases trapped as bubbles in ice cores indicate that the atmospheric concentration of CH 4 has more than doubled over the past 200–300 years, from a preindustrial average of about 0.6–0.7 ppmv to approximately 1.7 ppmv currently ŽTyler, 1991.. CH 4 plays a role in several processes that affect the chemistry and radiation budget of the atmosphere. On an atomic basis, CH 4 has about 25–30 times the warming effect of CO 2 , and about 70 times on a weight basis ŽKeihl and Dickinson, 1987.. Donner and Ramanathan Ž1980. calculated that an atmospheric CH 4 concentration increase of 1.3 ppmv causes an elevation in the average global surface temperature of about 1.38C. Because CH 4 has an atmospheric residence time of only about 10 years ŽFung et al., 1991., an estimated 10% reduction of the annual emission rate is estimated to be sufficient to stabilize atmospheric concentrations ŽHogan et al., 1991..
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CH 4 in the atmosphere reacts with hydroxyl radicals ŽOH P. in an initial reaction step of a series of reactions involving ozone ŽO 3 ., H 2 O, hydrogen oxides ŽHO x ., nitrous oxides, formaldehyde, hydrogen, chlorine, and other species that affect the H 2 O and O 3 concentration and overall oxidizing power of the atmosphere ŽTyler, 1991.. CH 4 reaction with chlorine produces hydrochloric acid ŽHCl., an important reaction because, unlike free chlorine, HCl does not destroy ozone. Other atmospheric gases, such as CH 3 Cl, CH 3 Br, CHClF3 , CH 2 Cl 2 , CHCCl 3 , and SO 2 , are affected directly or indirectly by CH 4 and OH P levels ŽTyler, 1991.. All of these gases play important direct or indirect greenhouse roles and in the overall physics and chemistry of the atmosphere and have the effect of amplifying the greenhouse effect of atmospheric CH 4 . Coal mining, natural gas ventilation, and gas transmission losses combined account for approximately 50–114 Tg Žteragrams, 10 12 g. out of a total annual global CH 4 flux estimated at 540 Tg from all sources ŽKhalil et al., 1985; Stevens and Englekemeir, 1988; Cicerone and Oremland, 1988; Hogan et al., 1991.. Some studies have estimated that losses of CH 4 during coal mining alone could account for as much as half of the total fossil CH 4 source ŽQuay et al., 1991.. This estimate of CH 4 flux from fossil fuels is obviously not well constrained and it is unknown exactly what the contribution is from coal beds, either through natural processes or associated with mining activities. In the United States, CH 4 emissions from underground coal mines have increased due to a combination of mining of gassier coal seams and improved or increased CH 4 drainage technology ŽDiamond, 1993.. Because of increasing energy demand, particularly in countries where population is increasing, coal production is likely to increase over the next few decades, which could lead to increased atmospheric CH 4 flux from coals. Although the amount of CH 4 emitted from coal beds due to human activities and natural geochemical or geological processes is not well documented, safety ventilation of underground mines is clearly the single largest source of atmospheric CH 4 from coals ŽClayton et al., 1995.. Estimates of CH 4 emissions from coal mining are typically derived by comparing curves relating gas content of coal versus coalification rank multiplied by tonnage mined for coal at various ranks Že.g., Barns and Edmonds, 1990; Boyer et al., 1990., an approach which depends on accurate determination or modeling of coal CH 4 content and understanding of controls on CH 4 retention in coal. We observed 136,000 lrmin from a single ventilation shaft in one mine in the Black Warrior Basin, U.S. ŽClayton et al., 1993.. However, natural leakage of gas from buried coals along faults or fractures, or other permeable migration routes may also constitute a significant atmospheric CH 4 source on a global scale ŽClayton et al., 1993, 1995.. For example, at a surface exposure of a fault zone in the Black Warrior basin we found a CH 4 flux of 1,000 kgryr ŽClayton et al., 1993. and 13 = 10 6 kgryr from one location along a stream bed in the San Juan basin ŽClayton et al., 1995.. In coal mining regions of Poland, soil gas samples taken at depths of about 2.0 m contained from - 1.0 to about 6,000 ppm CH 4 ŽKotarba et al., 1997.. The high CH 4 soil concentrations in Poland are due probably to increased CH 4 flux from underlying coal mines that have been recently closed and no longer operate ventilation shafts. The higher of these soil CH 4 concentrations are high enough to constitute an explosion hazard if mechanisms for accumulation are present, such as underground structures. Atmospheric CH 4 concentrations in Polish coal-mining areas
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are as high as about 10 ppmv ŽKotarba et al., 1997., nearly 5 times ambient values. Although these previous studies were limited in scope, they indicate that additional field studies of CH 4 flux are needed to better understand the role of coal and mining activities in global warming and the global CH 4 cycle. 5. Conclusions Much is now known about the isotopic and chemical composition of coalbed gas and the general processes, if not some of the details, of gas generation. However, as coal mining is likely to increase and coalbed gas is likely to become an increasingly important energy source in future years, further research is needed on the geochemistry of coalbed gases, particularly in the areas of gas generation and retention, primary migration, and producibility. Continued development of effective safety ventilation technologies and procedures for underground mines requires enhanced understanding of CH 4 generation, retention, and migration in coals. As noted by Diamond Ž1993., mining delays have been experienced in mines in the United States due to higher than expected CH 4 content in certain situations. Resource assessment of coalbed gas is dependent on prediction of gas content and migration. Areas of possible fruitful investigation are controls on quantity or composition of gas generated Žsuch as maceral or elemental composition., all factors that affect primary and secondary migration, including external geologic factors such as tectonic stress ŽLittke and Leythaeuser, 1993., and kinetics of gas generation from coals of varying composition. Moreover, estimates of global atmospheric CH 4 flux from coals are order-of-magnitude estimates based on extrapolations from limited data sets. Additional research is needed to determine the influence of various geochemical and geological factors that control CH 4 flux, so that models can be developed that take into account controlling factors for particular regions, thus allowing more meaningful global estimates.
References Airey, E.M., 1968. Gas emission from broken coal. An experimental and theoretical investigation. Int. J. Rock Mechanics Mining Sci. 5, 475–494. Andresen, B., Throndsen, T., Raheim, A., Bolstad, J., 1995. A comparison of pyrolysis products with models ˚ for natural gas generation. Chem. Geol. 126, 261–280. Barns, D.W., Edmonds, J.A., 1990. An evaluation of the relationship between and production and use of energy and atmospheric methane emissions, U.S. Department of Energy Report TR047, DE-AC06-76RLO1830, 143 p. Blake, D.R., Rowland, F.S., 1988. Contintuing worldwide increase in tropospheric methane, 1978–1987. Science 239, 1129–1131. Boreham, C.J., Powell, T.G., 1991. in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, Oklahoma, pp. 133–157. Boyer, C.M, Kelafant, J.R., Kuuskraa, V.A., Manger, K.C., Kruger, D., 1990. Methane emissions from coal mining – issues and opportunities for reduction, U.S. Environmental Protection Agency EPAr400r9-90008, 131 p. Carothers, W.W., Kharaka, Y.K., 1980. Stable carbon isotopes of HCOy 3 in oil-field waters: implications for the origin of CO 2 . Geochim. Cosmochim. Acta 44, 323–332.
J.L. Claytonr International Journal of Coal Geology 35 (1998) 159–173
171
Chung, H.M., Sackett, W.M., 1979. Use of stable carbon isotope compositions of pyrolytically derived methane as maturity indices for carbonaceous materials. Geochim. Cosmochim. Acta. 43, 1979–1988. Cicerone, R.J., Oremland, R.S., 1988. Biogeochemical aspects of atmospheric methane. Global Biogeochem. Cycles 2, 299–327. ´ 1990. Generation and migration of hydrocarbon gas and Clayton, J.L., Spencer, C.W., Koncz, I., Szalay, A., carbon dioxide in the Bekes ´ ´ basin, southeastern Hungary. Org. Geochem. 15, 233–247. Clayton, J.L., Leventhal, J.S., Rice, D.D., Pashin, J.C., Mosher, B., Czepiel, P., 1993. Atmospheric methane flux from coals – preliminary investigation of coal mines and geologic structures in the Black Warrior basin, Alabama, in: The Future of Energy Gases, U.S. Geological Survey Professional Paper 1570, pp. 471–492. Clayton, J.L., Leventhal, J.S., Rice, D.D., Kotarba, M., Korus, A., 1995. Atmospheric methane flux from U.S. and Polish coals, in: Grimalt, J.O., Dorronsoro, C. ŽEds.., Organic Geochemistry, Developments and Applications to Energy, Climate, Environment and Human History, Selected Papers from the 17th International Meeting on Organic Geochemistry, Donostia-San Sebastian, Spain, pp. 641–643. Colombo, U., Gazzarrini, F., Gonfiantini, R., Kneuper, G., Teichmuller, M., Teichmuller, R., 1970. Carbon isotope study on methane from German coal deposits, in: Hobson, G.D., Speers, G.C. ŽEds.., Advances in Organic Geochemistry 1966, Pergamon Press, Oxford, pp. 1–26. Diamond, W.P., 1993. Methane control for underground coal mines, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, OK, pp. 237–267. Donner, L., Ramanathan, V., 1980. Methane and nitrous oxide – Their effects on the terrestrial climate. J. Atmospheric Sci. 37, 119–124. Durand, B., Paratte, M., 1983. Oil potential of coals: A geochemical approach, in: Brooks, J. ŽEd.., Petroleum Geochemistry and Exploration of Europe, Blackwell, Oxford, pp. 255–265. Fung, I., John, J., Lerner, J., Matthews, E., Prather, M., Steele, L.P., Fraser, P.J., 1991. Three-dimensional model synthesis of the global methane cycle. J. Geophys. Res. 96, 13033–13065. Garcıa-Gonzalez, M., MacGowan, D.B., Surdam, R.C., 1993. Coal as a source rock of petroleum and gas – a ´ ´ comparison between natural and artificial maturation of the Almond Formation coals, greater Green River basin in Wyoming, in: The Future of Energy Gases, U.S. Geological Survey Professional Paper 1570, pp. 405–436. Gould, K.W., Hargraves, A.J., Smith, J.W., 1987. Variation in the composition of seam gases issuing from coal. Bull. Aust. Inst. Mining Metall. 292, 69–73. Higgs, M.D., 1986. Laboratory studies into the generation of natural gas from coals, in: Brooks, J., Goff, J.C., van Hoorn, B. ŽEds.., Habitat of Palaeozoic gas in N.W. Europe, Geological Society Special Publication, No. 23, London. pp 113–120. Hill, R.J., Jenden, P.D., Tang, Y.C., Teerman, S.C., Kaplan, I.R., 1994. Influence of pressure on pyrolysis of coal, in: Mukhopadhyay, P.K., Dow, W.G. ŽEds.., Vitrinite Reflectance as a Maturity Parameter, ACS Symposium Series 570, American Chemical Society, Washington, p. 160–193. Hogan, K.B., Hoffman, J.S., Thompson, A.M., 1991. Methane on the greenhouse agenda. Nature 354, 181–182. Hunt, J.M., 1979. Petroleum Geochemistry and Geology, W.H. Freeman and Company, San Francisco. ICF Resources, 1990. The United States coalbed methane resource, Quarterly Review of Methane from Coal Seams Technology, Vol. 7, pp. 10–28. James, A.T., Burns, B.J., 1984. Microbial alteration of subsurface natural gas accumulations. Bull. AAPG 68, 957–960. Juntgen, H., Karweil, J., 1966. Gasbildung and gasspeicherung in steinkohlenflozen, Part I and II: Erdol ¨ ¨ and Kohle Petrochem., 19, 251–258, 339–344. Juntgen, H., Klein, J., 1975. Entstehung von erdgas gus kohligen sedimenten. Erdol ¨ ¨ und Kohle Petrochem. 1, 52–69. Karweil, J., 1969. Aktuelle problem der geochemic der kohle, in: Schenk, P.A., Havernaar, I. ŽEds.., Advances in Geochemistry 1968, Pergamon Press, Oxford, pp. 59–84. Khalil, M.A.K., Rasmussen, R.A., Shearer, M.J., Ge, S., Rau, J.A., 1985. Causes of increasing atmospheric methane: Depletion of hydroxyl radicals and the rise of emissions. Atmosphere Environ. 19, 397–407. Keihl, J.T., Dickinson, R.E., 1987. A study of the radiative effects of enhanced atmospheric CO 2 and CH 4 on early earth temperatures. J. Geophys. Res. 92, 2991–2998.
172
J.L. Claytonr International Journal of Coal Geology 35 (1998) 159–173
Kim, A.G., 1974. Low-temperature evolution of hydrocarbon gases from coal, U.S. Bureau of Mines, Report of Investigations 7965, 23 p. Kotarba, M.J., Rice, D.D., 1995a. Composition and origin of coalbed gases in the Lower Silesian coal basin, NW Poland, in: Report from Research Cooperation within the U.S.-Polish Maria Sklodowska-Curie Joint Fund II, Origin and Habitat of Coal Gases in Polish Basins, Isotopic and Geological Approach, pp. 69–78. Kotarba, M.J., D.D., Rice, 1995b. Composition and origin of coalbed gases in the Upper Silesian and Lublin coal basins, Poland, Report from Research Cooperation within the U.S.-Polish Maria Sklodowska-Curie Joint Fund II, Origin and Habitat of Coal Gases in Polish Basins, Isotopic and Geological Approach, pp. 79–89. Kotarba, M.J., Clayton, J.L., Korus, A., Rice, D., 1997. Near-surface and atmospheric CH 4 concentration in Polish coal mining districts, in preparation. Kuuskraa, V.A., Boyer, C.A., 1993. Economic and parametric analysis of coalbed methane, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, Oklahoma, pp. 373–394. Levine, J.R., 1987. Influence of coal composition on the generation and retention of coalbed natural gas, Proceedings of 1987 Coalbed Methane Symposium, pp. 15–18. Levine, J.R., 1993. Coalification: The evolution of coal as source rock and reservoir rock for oil and gas, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, Oklahoma, pp. 39–77. Lewan, M.D., 1993. Hydrocarbon gas generation from different kerogen types subjected to hydrous pyrolysis, American Chemical Society, Division of Geochemistry, Inc., 206th ACS National Meeting Abstracts, Chicago, Illinois, 22-27 August 1993, Abstract No. 32. Littke, R., Leythaeuser, D., 1993. Migration of oil and gas in coals, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, Oklahoma, pp. 219–236. Martınez, L., Suarez-Ruiz, I., Jimenez, A., 1996. Application of geochemical and petrographic parameters to ´ ´ ´ the study of gas genesis in coals, in: Gomez Cortes, ´ Luna, M.E., Martınez ´ ´ A. ŽEds.., Memorias del V Congreso Latinoamericano de Geoquimica Organica, Cancun, Mexico, p. 195–197. ´ McFall, K.S., Wicks, D.E., Kuuskraa, V.A., 1986. A geologic assassment of natural gas from coal seams in the Warrior basin of Alabama, Gas Research Institute Topical Report 86r0272, 80 p. Meissner, F.F., 1984. Cretaceous and lower Tertiary coals as sources for gas accumulations in the Rocky Mountain area, in: Woodward, J., Meissner, F.F., Clayton, J.L. ŽEds.., Hydrocarbon Source Rocks of the Greater Rocky Mountain Region. Rocky Mountain Association of Geologists, Denver, pp. 401–431. Moffat, D.H., Weale, K.E., 1955. Sorption by coal of methane at high pressures. Fuel. 34, 449–462. Mukhopadhyay, P.K., Calder, J.H., Hatcher, P.G., 1993. Geological and physicochemical constraints on methane and C 6q hydrocarbon generation capabilities and quality of Carboniferous coals, Cumberland basin, Nova Scotia, Canada, in: Chiang, S.-H. ŽEd.., Proceedings Tenth Annual International Pittsburgh Coal Conference – Coal-Energy and the Environment, September 20–24, 1993, Pittsburgh, pp. 1074–1079. Quay, P.D., King, S.L., Stutsman, J., Wilbur, D.O., Steele, L.P., Fung, I., Gammon, R.H., Brown, T.A., Farwell, G.W., Grootes, P.M., Schmidt, F.H., 1991. Carbon isotopic composition of atmospheric CH 4 : Fossil and biomass burning source strengths. Global Biogeochem. Cycles 5, 25–47. Rasmussen, R.A., Khalil, M.A.K., 1981. Atmospheric methane ŽCH 4 .: Trends and seasonal cycles. J. Geophys. Res. 86, 9826–9832. Rice, D.D., Claypool, G.E., 1981. Generation, accumulation, and resource potential of biogenic gas. AAPG Bull. 65, 5–25. Rice, D.D., Clayton, J.L., Pawlewicz, M.J., 1989. Characterization of coal-derived hydrocarbons and sourcerock potential of coal beds, San Juan basin, New Mexico and Colorado, U.S.A. Int. J. Coal Geol. 13, 597–626. Rice, D.D., Law, B.E., Clayton, J.L., 1993. Coalbed gas – an undeveloped resource, in: The Future of Energy Gases, U.S. Geological Survey Professional Paper 1570, pp. 389–404. Rice, D.D., 1993. Composition and origins of coalbed gas, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, OK, pp. 159–184. Rice, D.D., Kotarba, M.J., 1993. Origin of Upper Carboniferous coalbed gases, Lower and Upper Silesian coal basins, Poland, Proceedings of the 1993 International Coalbed Methane Symposium, The University of Alabama, Tuscaloosa. pp. 649–652.
J.L. Claytonr International Journal of Coal Geology 35 (1998) 159–173
173
Rigby, D., Smith, J.W., 1982. A reassessment of stable carbon isotopes in hydrocarbon exploration. Erdol ¨ und Kohle Erdgas Petrochem. 35, 415–417. Rohrback, B.G., Peters, K.E., Kaplan, I.R., 1984. Geochemistry of artificially heated humic and sapropelic sediments – II: Oil and gas generation. AAPG Bull. 8, 961–970. Schwartzkopf, T., 1984. Kohlenstoff- und Wasserstoffisotope in Kohlen und ihren Nebengesteinen: Diplomarbeit Ruhr Universtitat ¨ Bochum, 39 p. Smith, J.W., Gould, K.W., Hart, G.H., Rigby, D., 1985a. Isotopic studies of Australian natural and coal seam gas. Bull. Aust. Inst. Mining Metall. 290, 43–51. Smith, J.W., Rigby, D., Gould, K.W., Hart, G., Hargraves, A.J., 1985b. An isotopic study of hydrocarbon generation processes. Organic Geochem. 8, 341–347. Smith, J.W., Pallasser, R.J., 1996. Microbial origin of Australian coalbed methane. AAPG Bull. 80, 891–897. Steele, L.P., Fraser, P.J., Rasmussen, R.A., Khalil, M.A.K., Conway, T.J., Crawford, A.J., Gammon, R.H., Masarie, K.A., Thoning, K.W., 1987. The global distribution of methane in the troposphere. J. Atmos. Chem. 5, 125–171. Stevens, C.M., Englekemeir, A., 1988. Stable carbon isotopic composition of methane from some natural and anthropogenic sources. J. Geophy. Res. 93, 725–733. Tyler, S.C., 1991. The global methane budget, in: Rogers, J.E., Whitman, W.B. ŽEds.., Microbial Production and Consumption of Greenhouse Gases: Methane, Nitrogen Oxides, and Halomethanes, American Society for Microbiology, Washington, pp. 7–38. Whiticar, M.J., Faber, E., Schoell, M., 1986. Biogenic methane formation in marine and freshwater environments, CO 2 reduction vs. acetate fermentation – isotopic evidence. Geochim. Cosmochim. Acta 50, 693–709. Whiticar, M.J., 1992. Stable isotope geochemistry of coals, humic kerogens, and related natural gases, The Canadian Coal and Coalbed Methane Geoscience Forum Proceedings, Parksville, B.C., February 2–5, 1992, Alberta Research Council, pp. 149–172. Whiticar, M.J., 1996. Stable isotope geochemistry of coals, humic kerogens and related natural gases. Int. J. Coal. Geol. 32, 191–215. Wood, W.L., 1951. The production of gaseous hydrocarbons from coals and their tars, in: Sell, G. ŽEd.., Oil Shale and Cannel Coal, Vol. 2, Proceedings of the Second Oil Shale and Cannel Coal Conferences, Glasgow, July, 1950, The Institute of Petroleum, London. pp. 691–709. Yee, D., Seidle, J.P., Hanson, W.B., 1993. Gas sorption on coal and measurement of gas content, in: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal, AAPG Studies in Geology a38, Tulsa, OK, pp. 203–218.