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 4.13pmC) in coal waters throughout the PRB indicates they were recharged <50,000BP. 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.