Microbial methane from subsurface organic-rich units such as coals and shale support approximately 5% of the United States and Canada's energy needs. In the deep subsurface, microbial methane is formed by the metabolism of primarily CO2, H2, and acetate by methanogens. These metabolites are the by-products of multi-step biodegradation of complex organic matter by microbial consortia. This study investigates microbial methane in the Illinois Basin, which is present in organic-rich shallow glacial sediments (surficial), Pennsylvanian coals (up to 600 m depth), and the Upper Devonian New Albany Shale (up to 900 m depth). Findings from the study show that hydrogeochemical conditions are favorable for methanogenesis in each reservoir, with a decrease in groundwater flushing rates corresponding to a decrease in average reservoir depth and an increase in carbon isotopic fractionation. The deeper reservoirs (coals and shale) were paleopasteurized, necessitating re-inoculation by methanogens. The microbes were likely advectively transported from shallow sediments into the coals and shale, where areas of microbial methanogenesis correlate with freshwater recharge. The recharge in the shale was primarily sourced from paleoprecipitation with minor contributions from glacial meltwater during the Pleistocene (4He ages). All areas sampled in the shale were affected by Pleistocene recharge, however groundwater ages in areas of microbial methanogenesis are younger (average 0.33 Ma) than areas with thermogenic methane (average 1.0 Ma). Estimates of in-situ microbial methane production rates for the shale (10-1000 TCF/Ma) are 104-106 times slower than laboratory rates. Only limited biodegradation is observed in the shale. In-situ stimulation of methane production may be most effective if aimed at increasing production of the supporting microbial consortia as well as methanogens. Trace metal concentrations in the shale are below known levels of inhibition or enhancement, with the exception of Fe, suggesting that microbial methanogenesis is not repressed by any of the measured trace metals and may be improved with the addition of Ag, Co, Cr, Ni, and Zn.

Natural gas is an important transitional energy source to replace more carbon intensive coal combustion in the face of climate change and increasing global energy demands. A significant proportion of natural gas reserves (~20%) were recently generated by microorganisms that degrade organic-rich formations (i.e. coal, shale, oil) in-situ to produce methane. Recent studies have shown that these microbial communities may be potentially stimulated to generate more methane to extend the lifetime (~10 years) of existing biogenic gas wells. This dissertation investigates how microbial coalbed methane (CBM) systems are impacted by geochemical conditions, microbial community composition, and groundwater recharge. The first study is a review and synthesis of existing basic research and commercial activities on enhancement of microbial CBM generation, and identification of key knowledge gaps that need to be addressed to advance stimulation efforts. The second study couples water and gas geochemistry with characterization of microbial communities in coalbeds in the Powder River Basin (PRB), Wyoming to investigate the influence of microbiology on water and gas geochemistry. Geochemistry results indicated that nutrients are likely source in situ from coal, and that all sulfate must be removed from the system before methanogenesis will commence. Increased archaeal (i.e. methanogens) diversity was observed with decreasing sulfate concentration, while sulfate reducing bacterial communities were different in wells with high sulfate concentrations (sulfate reducing conditions) when compared to wells with low sulfate concentrations (methanogenic conditions). The third study uses noble gases to constrain the residence time of groundwater associated with CBM in the PRB. Measured diffusional release rates of 4He from PRB coals were ~800 times greater than typical rates observed in sandstone or carbonate aquifers, and measured 4He values far exceeded expected values from in-situ decay of U and Th. Groundwater 4He residence times ranged from <1 to ~800 years using the measured diffusion rates versus ~130 to 190,000 years using a standard model. Coal waters with the longest residence time had the highest alkalinity concentrations, suggesting greater extents of microbial methanogenesis, although there was no relationship between groundwater "age" and methane concentrations or isotopic indicators of methanogenesis. Constraining the relationship between microbial activity (e.g. mechanisms of coal biodegradation and methane generation), environmental geochemical conditions, and groundwater flow is important to better understand subsurface hydrobiogeochemical processes and to ensure the success of future projects related to stimulation of microbial CBM.