Economic deposits of microbial methane in Upper Devonian fractured black shales
McIntosh, Jennifer C. 1, Martini, Anna M. 2, Walter, Lynn M. 3, Petsch, Steven T. 4, and Nüsslein, Klaus 5
1Johns Hopkins University, Morton K. Blaustein Department of Earth and Planetary Sciences
2Amherst College, Department of Geology
3University of Michigan, Department of Geological Sciences
4University of Massachusetts-Amherst, Department of Geosciences
5University of Massachusetts-Amherst, Department of Microbiology
Relatively recent microbial activity has generated economic deposits of natural gas within fractured Upper Devonian black shales along the shallow margins of the Michigan and Illinois basins, and potentially within the Appalachian Basin. Meteoric waters recharged regional Silurian-Devonian aquifer systems along the basin margins, during Pleistocene glaciation, and migrated vertically into the overlying organic-rich shales, significantly diluting basinal fluid salinities and creating an environment conducive to microbial methanogenesis. Antrim and New Albany shale formation waters, in the Michigan and Illinois basins, respectively, have positive δ13C values for CO2(g) and dissolved inorganic carbon (DIC, >20‰), and high DIC concentrations (10-70 meq/kg), indicative of methanogenesis. The covariance of δD values for CH4 and H2O indicate methane was generated by CO2 reduction in-situ with dilute fluids and adsorbed onto the organic matrix. Carbon isotope values of CH4 range between suggested field for microbial versus thermogenic gas and in some cases are more positive than basin-centered thermogenic gas plays. Carbon isotope values of ethane and propane increase with decreasing concentration due to microbial oxidation of these thermogenic gas components. Selective enrichment cultures and DNA studies of Antrim Shale fluids show methanogens and acetogens associated with this unique gas resource. The fractured Upper Devonian Ohio Shale in the Appalachian Basin is thermally immature along the western and northern basin margins, and contains up to 11% total organic carbon. It is reasonable to assume that microbial gas has also been generated within the Ohio Shale at shallow depths, given the similar hydrostratigraphy and glacial history.