PATHWAY OF MICROBIAL METHANOGENESIS VARIES WITH FORMATION WATER SALINITY IN A COALBED METHANE RESERVOIR

Subsurface microbial communities can degrade complex organic matter in coal-bearing strata and generate methane, the primary component of natural gas. By learning about these communities and controls on their activity, we may be able to develop strategies to stimulate their growth and boost energy recovery.

In this study, we consider environmental controls on microbial populations in methane-producing, coal-bearing strata of the Cherokee basin in southeast Kansas, USA. Pennsylvanian-age strata in the basin contain numerous thin (0.4-1.1 m) coalbeds with marginal thermal maturities (0.5-0.7 %R_o) that are interbedded with shale and sandstone. We collected gas, water, and microbe samples from 16 commercial coalbed methane wells for geochemical and microbiological analysis.

Gas geochemistry and archaeal community composition indicate that hydrogenotrophic methanogenesis is the primary source of methane in the coalbeds. Gas dryness values [C1/(C2+C3)] averaged 2640 and carbon and hydrogen isotope ratios of methane differed from those of carbon dioxide and water, respectively, by an average of 65‰ and 183‰. Methanogen sequences dominate the archaeal community in each sample (average 91%), and few archaeal sequences overall (average 4.2%) were classified within Methanosarcinales, an order that contains methanogens capable of forming methane from acetate or methyl-containing C1 compounds.

Although hydrogenotrophs were dominant overall, our results indicate that the proportion of methane that was generated by acetoclastic methanogens increases with the solute content of formation water. The total dissolved solids (TDS) content of water samples varied from 34.9 to 91.3 g L^-1. As TDS increases, carbon and hydrogen isotope ratios drift toward values consistent with acetoclastic methanogenesis and the relative abundance of sequences classified within Methanosarcinales increases significantly. Most of the sequences classified within Methanosarcinales (92%) were further classified within Methanosaeta, a genus of acetoclastic methanogens. Consistent with these trends, beta diversity analyses show that formation water solute content is a major control on archaeal diversity.