Coalbed methane (CBM) represents a small but significant portion of the world’s annual natural gas production, and it has been estimated that up to 20% of CBM was generated by microbes. Recently, significant research has been conducted to assess the feasibility of stimulating microbial CBM production to generate new natural gas resources. However, the connections between redox conditions in coal beds, microbial community composition, biodegradation pathways and mechanisms for microbial methane generation are still poorly understood. To investigate these relationships, this study collected geochemistry and microbiology samples from monitoring and CBM production wells completed in single coal seams along several transects in the Powder River Basin where redox conditions transition from sulfate reduction to methanogenesis. In wells where microbial methanogenesis is the primary redox process, associated waters are typically dominated by Na+ and HCO3-, with SO42- concentrations &lt;0.1 mM. In sulfate reducing wells, Na+ is also the dominant cation, but high HCO3- concentrations are coupled with SO42- concentrations ranging from ~2-25 mM. Archaeal diversity was greatest in wells with low sulfate concentrations and relatively higher methane concentrations. δ34S values in coal waters ranged from -0.3‰ to 34.4‰, but the range was significantly smaller in samples in which SO42- concentrations were &gt;10 mM (5.5‰ to 7.5‰). This suggests that the source of sulfur in wells with high SO42- concentrations is organic sulfur from coals, while there is a strong influence of bacterial sulfate reduction on isotope values in wells with lower sulfate concentrations. δ13C values of CH4 were generally around -60‰ in CBM production wells, with slightly more negative values (-70‰ and -85‰) in monitoring wells. The monitoring wells sampled are closer to recharge areas compared to CBM production wells, and so lower δ13C values may be indicative of the onset of methanogenesis in these areas. On-going work will compare these results to microbial analyses to further investigate the linkages between biogeochemical processes and redox conditions in coal.

Relationship between coal biodegradation, microbial methane generation, and redox conditions in coalbeds indicated by coupled water, gas, and microbial analyses

249th National Meeting (2015)

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