Influence of groundwater flowpaths, residence times and nutrients on the extent of microbial methanogenesis in coal beds: Powder River Basin, USA

Water and natural gas samples were collected from coalbed methane wells and a surface coal mine in the Powder River Basin (PRB) and analyzed for solute chemistry, isotopes, and gas composition to test hypotheses about the timing and source of recharge, importance of nutrient influx, and extent of microbial methanogenesis in coalbeds. Recharge to the Wyodak–Anderson coal zone in the central portions of the PRB appears to be from the southern basin margin based on δ18O–H2O values and hydraulic gradients. Coal zones along the eastern margin, near the coal outcrop, represent mixing between surface water and deeper circulating groundwater. Coalbeds along the northwestern basin margin may contain high elevation recharge from the Bighorn Mountains, with flow patterns likely affected by local faults. Detectable 14C (0.39 to 4.13 pmC) in coal waters throughout the PRB indicates they were recharged < 50,000 BP. Correlation of δD–CH4 and δD–H2O values suggests that methane has accumulated since the Late Pleistocene. Nutrient concentrations (C, N and P species) are low throughout the PRB and do not correlate to groundwater flowpaths or methanogenic pathways; nutrients may be sourced from in-situ leaching of coals. Carbon isotope values of CH4 and CO2 likely indicate the extent of methanogenesis (i.e. early versus late stage), rather than the dominant metabolic pathway (i.e. acetate fermentation versus CO2 reduction). Coal gasses from the northwestern margin of the basin are isotopically depleted and suggest “early stage” methanogenesis, where the reactant carbon reservoir has not been significantly fractionated due to relatively rapid groundwater flow. Coal gasses from the central basin and southeastern margin are isotopically enriched and suggest “late stage” methanogenesis where the reactant carbon reservoir has been significantly depleted likely due to longer groundwater residence times.

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► Carbon isotopes of CH4 and CO2 reflect early versus late stage methanogenesis.
► Metabolic pathways of methanogenesis are not related to nutrient concentrations.
► Oxygen and H isotopes of coal waters indicate regional groundwater flowpaths.
► Coal waters throughout the Powder River Basin are likely < 50,000 years old.