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Identification of microbial and thermogenic gas components from Upper Devonian black shale cores, Illinois and Michigan basins

¹ Department of Geology, Amherst College, Amherst, Massachusetts 01002; ammartini{at}amherst.edu
² Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109; lmwalter{at}umich.edu
³ Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85716; mcintosh{at}hwr.arizona.edu

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 associate professor of geology at Amherst College. Her research interests include unconventional natural gas plays, isotopic tracing of microbial pathways, 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 emphasis on carbon transformations and mineral mass transport.

Jennifer C. McIntosh received her B.A. degree in geology-chemistry from Whitman College and her M.S. degree and her Ph.D. in geology from the University of Michigan (2000 and 2004). She is currently an assistant professor at the University of Arizona. Her research interests include hydrogeochemical controls on microbial methane generation in organic-rich sediments, origin and transport of saline fluids, and impacts of past climate change on modern groundwater resources.

Differentiation of microbial versus thermogenic methane in coalbed and black shale accumulations can affect strategies for exploration and may influence the total gas content in a given area. Early identification of these processes from crushed core materials, even before formation fluids and produced gas samples are available, could permit a more efficient and cost-effective exploration. Total gas contents and compositional and isotopic data from New Albany Shale core materials are presented, which delineate regional occurrence of microbial, thermogenic, and mixed gas generation in the Illinois Basin. These trends are consistent with those identified from detailed prior studies of produced gas and water chemistry from the same locations. The most useful markers for microbial gas in crushed core gases are elevated CO₂ contents characterized by high values (>5). Core gas analyses from wells in which microbial gas is identified commonly have significantly more total gas absorbed than do core samples from wells producing gases solely of thermogenic origin. These observations are independent of variations in sample depth and organic carbon content in a given core. Thus, this integrated case study of core and produced gases in the Illinois Basin illustrates that the areas containing microbial gas, in addition to early thermogenic gas, may be more productive than pure thermogenic zones for these early to immature unconventional gas deposits.