CARBON ISOTOPE MODELING OF METHANOGENIC COAL BIODEGRADATION: METABOLIC PATHWAYS, MASS BALANCE, AND THE ROLE OF SULFATE REDUCTION, POWDER RIVER BASIN, USA

Improved understanding of in situ coal biodegradation using stable isotope signatures, by identifying metabolic pathways and/or mass balance effects, may facilitate a sustainable source of microbial coalbed methane (CBM) derived from unmined coal. Microbial CBM provides an alternative to coal, with its large greenhouse gas emissions and other environmental impacts, when used for electricity generation. This study examines a large microbial CBM system, the Powder River Basin (PRB) of Montana and Wyoming, where significant variations in H and C isotope signatures of methane have been documented previously. Water and gas were sampled in 2011 from production and monitoring wells in the northwestern PRB.

With δ²H_CH4 from -327‰ to -249‰ and δ¹³C_CH4 from -78.2 to -56.2‰, PRB samples generally plot in or near the “methyl-type fermentation” field of Whiticar (Chem. Geol., v. 161 p. 291-314, 1999), whereas none plot exclusively in the “carbonate reduction” field. However, this fingerprinting approach may be misleading: (1) δ²H_CH4 is likely affected by exchange with water (δ²H -165 to -129‰), and indeed apparent ε²H_H2O-CH4 near 160‰ is normally attributed to CO₂ reduction, not acetate fermentation; and (2) δ¹³C_CH4 alone is not highly diagnostic of metabolic pathways; for example, as a C_org source undergoes methanogenic degradation, >20‰ variations in δ¹³C_CH4 (and δ¹³C_CO2) can be caused by mass balance effects.

The difference between δ¹³C_CH4 and δ¹³C_CO2 (as α¹³C_CO2-CH4 = (δ¹³C_CO2+1000)/(δ¹³C_CH4+1000)) varies locally within the PRB, and may indicate a transition from acetoclastic methanogenesis to CO₂ reduction as suggested in prior studies. However, δ¹³C can be affected by non-methanogenic processes. For example, sulfate reduction may: (1) Contribute CO₂ relatively unfractionated from source C_org; and (2) Cause a smaller proportion of CO₂ to undergo CO₂ reduction because sulfate reduction produces CO₂ while oxidizing electron donors (e.g. H₂). Given inconsistencies between isotopic fingerprints in this system and possible mass balance effects on δ¹³C, pathway-independent tracers (e.g. α¹³C_CO2-CH4) may record the favorability or progressive extent of methanogenesis rather than changing metabolic pathways. Acetate utilization may be better defined by tracers such as compound-specific C isotopes of acetate.