Methanogenic pathways of coal-bed gas in the Powder River Basin, United States: The geologic factor

Abstract

Coal-bed gas of the Tertiary Fort Union and Wasatch Formations in the Powder River Basin in Wyoming and Montana, U.S. was interpreted as microbial in origin by previous studies based on limited data on the gas and water composition and isotopes associated with the coal beds. To fully evaluate the microbial origin of the gas and mechanisms of methane generation, additional data for 165 gas and water samples from 7 different coal-bed methane-bearing coal-bed reservoirs were collected basinwide and correlated to the coal geology and stratigraphy.

The C₁/(C₂ + C₃) ratio and vitrinite reflectance of coal and organic shale permitted differentiation between microbial gas and transitional thermogenic gas in the central part of the basin. Analyses of methane δ¹³C and δD, carbon dioxide δ¹³C, and water δD values indicate gas was generated primarily from microbial CO₂ reduction, but with significant gas generated by microbial methyl-type fermentation (aceticlastic) in some areas of the basin. Microbial CO₂ reduction occurs basinwide, but is generally dominant in Paleocene Fort Union Formation coals in the central part of the basin, whereas microbial methyl-type fermentation is common along the northwest and east margins. Isotopically light methane δ¹³C is distributed along the basin margins where δD is also depleted, indicating that both CO₂-reduction and methyl-type fermentation pathways played major roles in gas generation, but gas from the latter pathway overprinted gas from the former pathway. More specifically, along the northwest basin margin gas generation by methyl-type fermentation may have been stimulated by late-stage infiltration of groundwater recharge from clinker areas, which flowed through highly fractured and faulted coal aquifers. Also, groundwater recharge controlled a change in gas composition in the shallow Eocene Wasatch Formation with the increase of nitrogen and decrease of methane composition of the coal-bed gas.

Other geologic factors, such as burial, thermal and maturation history, lateral and vertical continuity, and coalification of the coal beds, also played a significant role in controlling methanogenic pathways and provided new perspectives on gas evolution and emplacement. The early-stage gas produced by CO₂ reduction has mixed with transitional thermogenic gas in the deeper, central parts of the Powder River Basin to form ‘old’ gas, whereas along the basin margins the overprint of gas from methyl-type fermentation represents ‘new’ gas. Thus, a clear understanding of these geologic factors is necessary to relate the microbiological, biogeochemical, and hydrological processes involved in the generation of coal-bed gas.

Introduction

Coal-bed methane (CBM), or coal-bed natural gas, is an unconventional gas that has become an emerging and important energy resource worldwide. It forms in both low and high-rank coals, mainly in the subbituminous and bituminous ranks, respectively. The Energy Information Administration (2005) reported that CBM accounted for 8% of U.S. natural gas production in 2003, following almost three decades of developmental history. Coal-bed methane production in the U.S also accounts for more than 70% of the world's (CBM) production, which is more than 59 Gm³. The remainder being produced (in descending order of production) by Australia, India, Canada, China, United Kingdom, Columbia, Russia, Ukraine, and Austria, (Flores, 2006). These countries have produced for about a decade, primarily from shallow to deep subsurface coal beds and secondarily from mineable coal beds in advance of coal mining or from abandoned coal mines. Historically, coal-mine methane has caused mine outbursts or explosions in many countries, especially China, resulting in thousands of lost lives (Flores, 1998). Methane emanating from underground and from surface coal mines is also a major contributor to greenhouse gas emissions.

Gas produced from bituminous and subbituminous coals has traditionally been grouped as either thermogenic, microbial, or a mixture based on the main process involved in its generation — that is, by temperature and microbial activity, respectively. Understanding the generation of the gas requires some knowledge of the formation of coal and the processes of peatification and coalification. Coal is a combustible rock composed of about 50% or more organic matter by weight or 70% by volume (Schopf, 1956). This ‘hard metamorphosed rock’ of organic matter was originally accumulated by the degradation, decomposition, and alteration of plant remains forming peat in tropical and temperate swamps and/or bogs (Taylor et al., 1998). The warmer temperatures typical of peat swamp and/or bog settings promote higher average plant growth and subsequent degradation as well as more intensive microbial activity accompanied by quicker chemical decomposition. The peatification process includes microbial and chemical changes of the organic matter that forms hydrogen-rich humic substances; these changes are influenced by oxygen supply, peat temperature, and alkaline environments. Carbon content of the peat increases with depth of the peatigenic layer particularly in the form of cellulose and hemicellulose oxygen-rich substances that are decomposed microbiologically, resulting in enriched carbon-rich lignin and humic acids (Taylor et al., 1998). Increased compression of peat caused by autocompaction and compaction by burial of overlying sediments during substrate subsidence results in decreased moisture content accompanied by increased carbon content (dry-ash-free basis) and absence of free cellulose.

Coalification is the transformation of peat at depths via chemical and physical alterations. Geologic time, combined with increasing temperature and pressure, progressively converts the organic matter deposit into higher rank such as lignite, subbituminous, bituminous, and anthracite coal. During coalification from low- to high-rank coals, diagenesis, catagenesis, and metagenesis increase calorific value and fixed carbon, decrease volatile matter and moisture content, and generate natural gas (Tissot and Welte, 1984). Organic matter becomes progressively more aromatic with expulsion of aliphatic components through generation of gases and liquids; condensation of solid residual humic products also takes place (Taylor et al., 1998, Faiz, 2004). In addition, depending on the parent organic substance, the coalification process is known to generate condensate and oil (Boreham and Powell, 1993, Mukhopadhyay and Hatcher, 1993).

Early- and late-stage microbial and thermogenic gases and liquids, such as CH₄, C₂H₆, CO₂, and H₂O as well as higher hydrocarbon gases (e.g. ethane, propane, butane) and liquid hydrocarbons, are produced during peatification and coalification (Rice, 1993, Faiz, 2004). Extrapolation of degassing curves from long-term experimental pyrolysis of peats and coals by van Heek et al. (1971) indicated that, during early coalification, water is first lost at 20–50 °C, followed by methane from residual lignin at 30–70 °C, then by carbon dioxide at 70–100 °C, and lastly by large quantities of methane at 160–200 °C. Significant volumes (100–300 cm³/g of coal) of methane are generated from coals (e.g. bituminous) during coalification (Juntgen and Karweil, 1966, Rice, 1993) but most of the early-stage microbial and thermogenic gas is lost, when not sealed and trapped, by migration into surrounding reservoirs during burial, as well as by escape into the atmosphere during uplift and erosion of the basinal sedimentary rocks. Thus, deformation and erosion as well as the geology and stratigraphy of the coal-bed reservoirs and surrounding rock units play a major role in the preservation and accumulation of early-stage gases. More importantly, these geologic events play a key role in the timing of the generation, migration, and accumulation of late-stage gases such as microbial gas (Rice and Flores, 1990, Rice and Flores, 1991, Rice, 1993, Flores, 2004).

The purposes of this paper are to: (1) test the presumptive microbial origin of gas in subbituminous coal in the Powder River Basin, which is a prolific coal-bed gas producer (Fig. 1); (2) compare results of our work to previous investigations by Rice, 1993, Gorody, 1999; (3) expand the work on isotopic composition and fractionation of deuterium (D) between water and methane (CH₄) and of carbon (C) between carbon dioxide (CO₂) and methane in order to differentiate the origins and mixtures of microbial and thermogenic gases; and (4) determine the role of coal geology and stratigraphy on the origin and methanogenic pathways of coal-bed gas generation, evolution, and emplacement.