Extent and limits of biodegradation by in situ methanogenic consortia in shale and formation fluids

Consortia of microbes degrade recalcitrant organic-matter in deep subsurface reservoirs, such as shales and coals, under anaerobic conditions into simple C molecules such as CO₂ and acetate. These substrates are subsequently metabolized by methanogens into economic quantities of natural gas in sedimentary basins world-wide. This study explores organic matter in the Devonian New Albany Shale (Illinois Basin, USA) and associated fluids to investigate the extent of organic matter biodegradation, and evaluate the potential for stimulating in situ gas production. Identification of labile compound classes such as n-alkanes, fatty acids, and phenols in produced waters of the New Albany Shale, and low biodegradation indices in the shale core samples indicate limited biodegradation. Together with detectable acetate concentrations (up to 225.1μM), these observations suggest that both the supporting microbial consortia and methanogens are limited in extent and activity. By comparison, the New Albany Shale is much less biodegraded than the microbial CH₄-producing Michigan Basin Antrim Shale, Powder River Basin coals, or San Juan Basin coals. In the New Albany Shale, the extent of biodegradation generally becomes more varied with higher salinities, suggesting diverse microbial adaptations to degrade OM at high salinities. Enhancement of in situ CH₄ production may be most effective if targeted at stimulating production of the supporting microbial consortia as well as methanogens.