MOLECULAR, MICROBIAL AND GEOCHEMICAL SIGNATURES OF HYDROCARBON BIODEGRADATION AND METHANOGENESIS IN COALS OF THE POWDER RIVER BASIN

The Powder River Basin of eastern Wyoming and SE Montana is estimated to contain over 14 trillion cubic feet of natural gas that has been generated and stored in extensive coals deposited in the early Cenozoic. Along the eastern margin of the basin, incursion of meteoric water into the subsurface is associated with methane that is classified as microbial in origin based on isotopic characteristics of gas and co-produced waters, including a linear relationship between hydrogen isotope ratios of formation waters and co-produced CH₄. Geochemical characteristics of formation waters and gases, such as elevated alkalinity (up to 22 meq L^-1) and ¹³C-enriched DIC (up to +20 ‰), further support extensive microbial degradation of coal organic matter associated with methanogenesis. A limited number of wells associated with poor CH₄ production exhibit measurable sulfate concentrations, and ¹³C-depleted DIC (as low as -20 ‰). Extractable organic matter isolated from coals across the basin points to more extensive biodegradation in shallow coals compared with material retrieved from deeper in the basin. Patterns of hydrocarbon biodegradation are broadly similar to anaerobic petroleum biodegradation and to methanogenic environments in Paleozoic black shales of the mid-continent of the USA, including extensive loss of straight chain and acyclic isoprenoid hydrocarbons. However, unlike typical petroleum biodegradation, concurrent biodegradation of aromatic hydrocarbons such as methylated naphthalenes and phenanthrenes is also indicated. Viable cells and nucleic acids recovered from water-producing wells suggest an active subsurface community within shallow Powder River Basin coals, with cell counts ranging between 10³-10⁴ cells mL^-1 formation water. Sequence analysis of Bacterial and Archaeal 16S rRNA genes reveals a diverse assemblage of mainly fermentative bacteria coupled with acetoclastic and CO₂-reducing methanogenic Archaea.