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An expanded data set for gases produced from the Antrim Shale, a Devonian black shale in the Michigan basin, United States, has allowed for a detailed examination of the related chemical and isotopic compositional changes in the solid-gas-liquid systems that discriminate between microbial and thermogenic gas origin. In the Antrim Shale, economic microbial gas deposits are located near the basin margins where the shale has a relatively low thermal maturity and fresh water infiltrates the permeable fracture network. The most compelling evidence for microbial generation is the correlation between deuterium in methane and coproduced water. Along the basin margins, there is also a systematic enrichment in 13C of ethane and propane with decreasing concentrations that suggests microbial oxidation of these thermogenic gas components. Microbial oxidation accounts not only for the shift in δ13C values for ethane, but also, in part, for the geographic trend in gas composition as ethane and higher chain hydrocarbons are preferentially removed. This oxidation is likely an anaerobic process involving a syntrophic relationship between methanogens and sulfate-reducing bacteria.
The results of this study are integrated into a predictive model for microbial gas exploration based on key geochemical indicators that are present in both gas and coproduced water. One unequivocal signature of microbial methanogenesis is the extremely positive carbon isotope values for both the dissolved inorganic carbon in the water and the coproduced CO2 gas. In contrast, the δ13C value of methane is of limited use in these reservoirs as the values typically fall between the commonly accepted fields for thermogenic and microbial gas. In addition, the confounding isotopic and compositional overprint of microbial oxidation, increasing the values to typically thermogenic values, may obscure the distinction between methanogenic and thermogenic gas.
Anna M. Martini received her B.A. degree in geology from Colgate University, her M.S. degree from Syracuse University (1992), and her Ph.D. from the University of Michigan (1998). She is currently an assistant professor of geology at Amherst College. Her research interests include unconventional natural gas plays, isotopic tracing of microbial interactions, and the geochemistry of saline fluids.Lynn M. Walter received her M.S. degree from Louisiana State University (1978) and her Ph.D. from the University of Miami (1983). She was an assistant professor at Washington University in St. Louis until 1988. She then joined the University of Michigan, where she is now a professor of geological sciences and director of the Experimental and Analytical Geochemistry Laboratory. Her research interests focus on the hydrogeochemistry of near-surface and deeper basin environments, with an emphasis on carbon transformations and mineral mass transport.
Tim Ku received his B.S. degree in geological sciences from the University of Rochester and his M.S. degree and Ph.D in geology from the University of Michigan. He is currently an assistant professor in the Department of Earth and Environmental Sciences at Wesleyan University. His research focuses on biogeochemical processes in terrestrial soils and marine sediments.
Joyce Budai received a B.A. degree in English at the University of Kansas, an M.S. degree in geology at Rice University, and a Ph.D. in geology at the University of Michigan. While a research scientist at the University of Michigan, her interests included the sedimentary history of the Michigan basin and the role of fluid flow during deformation of the Wyoming overthrust belt. Joyce is now working on collaborative programs for faculty at private liberal arts colleges in Michigan, Indiana, and Ohio.
Jennifer McIntosh is currently working on her Ph.D. in geology at the University of Michigan. She received her B.A. degree in geology-chemistry at Whitman College in 1998 and her M.S. degree in geology at the University of Michigan in 2000. Her interests include the hydrogeochemistry of sedimentary basins, the impact of Pleistocene glaciation on regional flow systems, and modification of brines via microbial methanogenesis.
Martin Schoell is an internationally known geochemist who specialized during his 30-year career on isotope geochemistry of oil and natural gas. Martin is the principal author and coauthor of many fundamental papers on gas geochemistry and its application in gas exploration and production. From 1984 to 2001, he was a senior scientist at Chevron and worked mostly in international gas exploration. He now works as an independent consultant.