Carbon mass balance, isotopic tracers of biogenic methane, and the role of acetate in coal beds: Powder River Basin (USA)

Abstract

A multi-isotope (C, H, O) investigation of biogenic gas (δ13C−CH4, δ13C−CO2, δD−CH4) and coal bed waters (δD, δ18O, δ13C-DIC, δ13C-acetate) was conducted along a ∼25-km basinward sampling transect in the Powder River Basin (PRB) in Wyoming and Montana (USA) to address inconsistencies among isotope fingerprinting techniques and to apply mass balance isotope modeling to biogenic coalbed methane (CBM). Results include the first published compound-specific δ13C-acetate values from a natural biogenic CBM system, providing new constraints on acetate’s role in biogenic CH4 generation. Coal bed water chemistry is anoxic and dominated by Na+ and HCO3−, with high alkalinity concentrations (14.8–33.0meq/L). The deep basin interior environment contains sulfate-free, Na+−HCO3−-dominated waters exhibiting signs of methanogenesis. CH4 and CO2 generally become 13C-enriched from the shallow basin edge to the deep basin interior (δ13C−CH4 −78.2 to −56.2‰ and δ13C−CO2 -24.7 to 4.7‰). Also, compound-specific δ13C-acetate is more positive in the basin interior compared to the basin edge setting (range −34.3 to −15.1‰). Using a steady-state mass balance isotope model, the observed variations in δ13C-CH4 and δ13C-CO2 can be explained in terms of the favorability of methanogenesis (fCH4) relative to heterotrophic bacterially-mediated non-methanogenic pathways, such as bacterial sulfate reduction, which compete with methanogenesis for substrates. At the basin interior, fCH4 exceeds 0.4, approaching its inferred limiting value hypothesized from the oxidation state of low-molecular weight compounds detected in PRB formation waters. Therefore, methanogenesis is likely a dominant biogeochemical pathway consuming substrates in the deeper coal beds. The basinward shift toward 13C-enriched acetate broadly indicates linkage between acetate, CH4, and CO2, and likely records acetate synthesis effects modified by methanogenic fractionation. These results suggest that acetate plays a role in methanogenesis in PRB coal beds (not necessarily a dominant role), while other C isotope and microbial evidence suggests that hydrogenotrophic methanogenesis is also a significant methane-producing pathway. The results of this study broadly suggest multiple viable methanogenic pathways and C cycling that affects acetate, a more complex view of coal bed geochemistry than implied by traditional C and H isotope fingerprinting of accumulated products (CH4, CO2) and coexisting water.