Active Microbial Methane Production and Organic Matter Degradation in a Devonian Black Shale

Anna M. Martini¹, Steven T. Petsch², Jennifer McIntosh³, and Klaus Nusslein^2
1 Amherst College, Amherst, MA
² University of Massachusetts, Amherst, MA
³ Johns Hopkins University, Baltimore, MD

Microbial methane in organic-carbon rich fractured black shales and coal beds accounts for a significant and growing percentage of global natural gas production. These distinctive reserves are unusual, in that they are located at relatively shallow depths and are associated with active groundwater flow regimes. In many shale gas and coal-bed reserves, indications of very low organic matter thermal maturity cause a solely thermogenic origin for the gas to be questioned. Instead, geochemical and isotopic indicators reveal that much of the gas is microbial in origin, possibly generated in the recent geologic past or even continuing on to the present day. Microbial methanogenesis and other associated metabolic processes impart distinct chemical and isotopic signatures on subsurface dissolved gases and formation waters. In this study, major cation and anion analyses are shown to be consistent with prior results demonstrating recharge of dilute meteoric waters and mixing within saline formation waters in Devonian black shales along the northern margin of the Michigan Basin (USA). Formation water chemistry is coupled with stable carbon isotopic analyses of dissolved inorganic carbon and C1-C3 hydrocarbons and with deuterium isotopic analyses of methane and co-produced waters to demonstrate that a considerable flux of microbial methane generation has occurred in the deep subsurface of this sedimentary basin. Organic matter degradation and methanogenesis in this environment operate in the absence of terminal electron acceptors such as sulfate that could support anaerobic heterotrophy. Instead, a model of subsurface microbial methanogenesis is presented in which anaerobic biodegradation of shale hydrocarbons yields acetate and H2, which in turn serve with dissolved CO2 as the principal substrates of methanogenesis.