Coal seam gases collected from Bowen Basin cores have moderately negative methane carbon isotope compositions (-51 ± 9 per mil) which overlap the ...
SOURCE AND TIMING OF COAL SEAM GAS GENERATION IN BOWEN BASIN COALS
S.D. GOLDING, K.A. BAUBLYS, M. GLIKSON, LT. UYSAL Department of Earth Sciences, The University of Queensland, QLD 4072 C.J. BOREHAM Petroleum and Marine Division, AGSO, GPO Box 378, Canberra, ACT 2601
Abstract Coal seam gases collected from Bowen Basin cores have moderately negative methane carbon isotope compositions (-51 ± 9 per mil) which overlap the published range for Australian coal seam methane of -60 ± 11 per mil. No systematic relationship between coal rank and methane o13C value is apparent. A thermogenic origin for methane has been assigned when its carbon isotope composition is heavier than -60 per mil, although biogenic methane generated in closed systems may have similar o13 C values from -60 to -40 per mil depending on the methanogenic pathway and the carbon isotope composition of the source. Subordinate inputs from biogenic methane could account for some of the isotopic variability of the desorbed methane; however, a good correlation between desorbed methane volumes and bitumen/pyrobitumen content suggests that much of the methane sorbed in the coal was produced by secondary cracking of bitumen. Bowen Basin methane o13C values are typically some 20 to 30 per mil lighter than vitrinite and inertinite o13C values. Carbon isotope compositions ofvitrinites and inertinites in subbituminous coals become less negative with increasing rank as a result of the preferential loss of the lighter isotope of carbon during maturation. Vitrinite reflectance and maceral carbon isotope compositions often display an anomolous trend for coals in the high to medium volatile bituminous rank as a result of the presence of adsorbed isotopically light methane. Thus, a sharp distinction in isotope systematics distinguishes coals that are within peak oil generation from those that are at or below the threshold of oil generation. Bitumen may show equal or higher concentrations within inertinite cell cavities as in vitrinite cleats and affect carbon isotope compositions ofvitrinites and inertinites. Compositional data for desorbed coal seam gases from the Bowen Basin show that ethane and the other wet gases are a minor component. On the other hand, high concentrations of wet gases are produced during pyrolysis of coals. This discrepancy between the proportion of wet-gas components produced during pyrolysis and that observed in many naturally matured coals may be the result of preferential migration of wet gas components. Alternatively, the wet gas components may be thermally cracked to methane at higher maturation levels or diluted by additional methane produced by secondary cracking of bitumen. Previous vitrinite reflectance and clay mineral diagenesis studies indicate that thermal 257 M. Mastalerz et al. (eds.), Coalbed Methane: Scientific, Environmental and Economic Evaluation © Springer Science+Business Media Dordrecht 1999
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maturation of the Late Permian coals in the central and northern Bowen Basin occurred largely as a result of a short-lived hydrothermal event in the Late Triassic rather than during maximum burial in the Middle Triassic as previously thought. Textural relationships at a variety of scales and the observation that coals in proximity to Cretaceous intrusions are highly mineralised with carbonates and sulfides suggest several periods of hydrothermal activity. It is concluded, therefore, that thermal maturation of coal in the Bowen Basin to form oil and gas was caused predominantly by transient thermal and fluid flow events in the Mesozoic. High temperatures associated with transient hydrothermal events and the potential for secondary crackin& of bitumen may provide an explanation for anomolous gas compositions and isotope systematics.
1. Introduction Methane is generated from organic matter including coals mainly as a result of biological decomposition or thermal maturation (cf. Schoell, 1980). During early diagenesis, the most important mechanisms of methane generation are microbially-mediated acetate fermentation and the reduction of carbon dioxide (Rice and Claypool, 1981; Whiticar et al., 1986; Jenden and Kaplan, 1986). With increasing temperature and pressure, thermal degradation and cracking reactions produce carbon dioxide and methane (e.g., Schoell, 1980; Rice.and Claypool, 1981). Methane is the main constituent of thermogenic gas from overmature source rocks (Hunt, 1996). Original thermogenic gases in coals may be modified at shallow levels by secondary processes such as oxidation of the wet gas component or mixing with biogenic gases (e.g., Rice, 1993; Scott et al., 1994). In addition, magmatic carbon dioxide or other natural gases may be adsorbed by coal seams resulting in complex gas mixtures (e.g., Smith et al., 1982, 1985). Stable carbon isotope compositions have been routinely used to establish the origin and follow the evolution of natural gases and petroleum and coal seam methane. Artificial maturation studies on a variety of organic components show that there can be up to a -30 per mil difference between the parent organic matter and the lightest methane; however, the carbon isotope composition of methane varies significantly depending on the mechanism of formation and the heterogeneity of the methane-generating moieties (Sackett, 1978; Chung and Sackett, 1980; Rooney eta/., 1995). A microbial origin for methane is generally associated with carbon isotope values less than -60 per mil, attributed to the large kinetic isotope effect during methanogenesis (Hunt, 1996). A thermogenic origin for methane has been assigned when its carbon isotope composition is heavier than -60 per mil, although biogenic methane generated in closed systems may have similar () 13C values from -60 to -40 per mil depending on the methanogenic pathway and the carbon isotope composition of the source. The source of coal seam gas in Australian Permo-Triassic coal basins is a subject of controversy. Most studies support thermal cracking of coal macerals through burial effect as the trigger and source of coal seam gases (e.g., Mallett eta/., 1990). Recently, bacterial reduction of carbon dioxide by methanogens was put forward as a major process
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responsible for coal seam gas generation in Sydney and Bowen Basin coals (Smith eta/., 1992; Smith and Palasser, 1996). These authors have worked extensively on isotopic characterisation of Australian coals and coal seam gases and were the first to recognize that there were two major sources of carbon dioxide in these gases, one of seam gas origin and the other of magmatic origin (e.g., Smith eta/., 1982, 1985). Smith and coworkers have generated a large isotopic data base for coal seam gas from the Sydney Basin and to a lesser extent the Bowen Basin; however, sample selection was driven largely by interest in outbursting in underground coal mines. Their conclusion that much of the methane in shallow coal seams was of microbial origin was based on anomolously large carbon isotope fractionations between methane and ethane as well as light carbon isotope compositions less than -60 per mil for some methane samples. Because several sources and processes may be involved in gas generation, the current study used a range of techniques to establish the thermal history of the coals as well as maceral composition, gas chemistry and isotope composition. Aspects of this work have been presented elsewhere and in this volume (Glikson et al., 1995, 1998; Golding et al., 1998; Boreham et al., 1998).
2. Methods of Study 2.1 SAMPLING The samples for this study were collected from the Rangal, Fort Cooper and Moranbah Coal Measures in the central and northern Bowen Basin and from the Baralaba Coal Measures in the southernmost part of the basin (Theodore area) (Fig. 1). Desorption of gases was carried out on core samples which had been immediately stored after drilling in stainless steel cylinders. Cores were desorbed over a time of 600 hours. 2.2 ORGANIC PETROLOGY Organic petrology and specialised characterisation of coals was undertaken using facilities in the Department of Earth Sciences and The Centre for Microscopy and Microanalysis at The University of Queensland. Organic petrology was carried out on polished coal blocks in reflected white light, and fluorescence mode, using an MPV-2 photornicroscope at wave length of 546nm. The same coal blocks were used for maceral analysis and reflectance determinations. 2.3 ELEMENTAL ANALYSIS Elemental analysis was carried out routinely on separate coal lithotypes using a Carlo Erba Analyser Model 1106 in the Department of Chemistry, The University of Queensland. 2.4 BITUMEN EXTRACTION Bitumen extraction from coal cores for stable isotope analysis was carried out in the Department of Earth Sciences, The University of Queensland using chloroform in a
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I'- I Coa.l Measures Figure I . The distribution of coal measures and sample localities in the Bowen Basin (open squares with numbers correspond to Q series samples in Table I).
Soxtech System HT2. Bitumen content was quantified by gravimetric methods for extractable fraction, and by petrological methods using fluorescence mode for all bitumen before and after extraction. 2.5 STABLE ISOTOPE ANALYSIS Stable isotope analyses were undertaken in the Stable Isotope Geochemistry Laboratory, The University of Queensland and in the Australian Geological Survey Organisation Isotope and Organic Geochemistry Unit. Separate vitrinite and inertinite fractions as well as separated bitumens were combusted with cuprous oxide in sealed vi cor tubes at 1040°C to produce carbon dioxide for carbon isotope analysis (Grady eta/., 1984). Carbon isotopic ratios were measured on a Micromass 602E dual inlet isotope ratio mass spectrometer. Methane gas was quantitatively converted to C0 2 after passing through a Finnigan combustion interface at 1000°C. The C0 2 was introduced via an open split into a Finnigan 252 continuous flow isotope ratio mass spectrometer and isotopically analysed using the m/z 44 response and m/z 45/44 ratio trace. Carbon isotope analyses are reported in per mil relative to PDB with analytical uncertainties of better than ±0.2 (2 SD) per mil.
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3. Results and Discussion 3.1 NATIVE GAS DESORPTION WITH COAL RANK AND CHARACTER Gas distribution patterns in the Bowen and Sydney Basin are known to be complex; however, gas contents generally correlate with coal rank at the regional scale (e.g., Bocking and Weber, 1993). Our desorption studies conftrm the importance of rank but also suggest that maceral composition is an important indicator of probable gas content. Although there is a general increase in gas content with rank, the highest and lowest gas contents are for the highest rank coals (Fig. 2). This anomaly suggests that other factors such as coal composition are more important than previously believed. Petrological studies of Bowen Basin coals indicate two major groups of maceral populations dominated respectively by vitrinite and inertinite (Glikson et al., 1995). In addition, heavy oils generated from Bowen Basin coals during maturation are retained in microcleats and inertinite cell cavities as the secondary maceral bitumen. There is a good correlation between native gas desorption and bitumen content although the rank parameter is also highlighted (Fig. 3). 3.2 CARBON ISOTOPES The carbon isotope compositions of coal seam gases desorbed from Bowen Basin coal cores as well as vitrinite, inertinite and bitumen separated from these coals are given in Table 1. The o13C values of methane typically vary systematically through the desorption, with the earliest desorbed gases several per mil lighter than those subsequently desorbed from the same core (e.g., 9407Q005V02 C2/CH 4 and C3/CH 4 ; -45 and -42 respectively). The o13 C values of the coal seam methane range from -67 to -36 per mil, with a mean of -51±9 per mil which overlaps the published range for Australian coal seam methane of60±11 per mil (Smith et al., 1992). The methane o13 C values are typically some 20 to 30 per mil lighter than the vitrinite and inertinite o13 C values; however, no systematic relationship between coal rank and methane o13 C value is apparent (Table 1). Increasing rank (and decreasing H/C) is followed generally by isotopically 'heavier' (i.e., less negative) vitrinites in sub-bituminous coals before oil generation (Fig. 4). The simplest explanation for the trend of heavier carbon isotope compositions of the major maceral groups with maturity is that the lighter isotope of carbon was preferentially lost during maturation as a result of Rayleigh fractionation (cf. Clayton, 1991 ). Studies of kerogen in both contact and regionally metamorphosed sediments show similar changes in carbon isotope composition with maturity as measured by vitrinite reflectance (Simoneit et al., 1981; Clayton and Bostick, 1985). In samples where bitumen concentrations are relatively low, inertinites have less negative carbon isotope compositions than those of the corresponding vitrinites (Fig. 5). Differentiation between inertinite and vitrinite carbon isotope compositions is often masked, however, by higher bitumen concentrations in the former. Carbon isotopic compositions of extracted bitumens are similar to or lighter than those of the associated vitrinites (Table 1), explaining why a high bitumen concentration in inertinite reduces the
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