Controls on the carbon isotopic composition of coalbed biogenic methane and implications for biogas stimulation

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Dr. David Vinson has been awarded an NSF Earth Sciences Postdoctoral Fellowship to implement a research and education program based at Northwestern University. He will investigate stable carbon isotope signatures imparted during methanogenic biodegradation of coal. Natural abundance carbon stable isotope (13C/12C) ratios are valuable signatures of specific pathways and processes associated with methanogenesis. Yet, interpretive uncertainties and discrepancies with microbiological techniques complicate the use of carbon isotopes to diagnose methanogenic pathways of interest (e.g. acetoclastic methanogenesis and CO2 reduction). Other competing biogeochemical processes - in particular sulfate reduction - and mass balance effects may also influence observed carbon isotope signatures. Additional pathway-specific tools and techniques may improve understanding of the utilization of methane precursors, especially acetate. The primary tasks of the research are to adapt methods of analyzing compound-specific and intramolecular carbon isotope ratios of acetate to the low concentrations encountered in microbial gas systems, to analyze these isotope ratios in waters from natural and laboratory-constructed methanogenic systems in order to trace acetate utilization during methanogenesis, and to apply modeling techniques including reactive transport modeling to the mass balance of methanogenic coal biodegradation.Natural gas is an increasingly important energy resource, of which a significant portion is of biogenic origin. The research focuses on microbial methane derived from unmined coal (coalbed methane), and has implications for naturally-occurring and human-stimulated biogas resources derived from other carbon sources. In order to stimulate the native microbial community to yield additional methane, a detailed understanding of natural processes and pathways is required. In this research, the use of a compound-specific isotope tracer technique will promote understanding of the utilization of acetate in methanogenesis or in competing processes that do not yield methane. In other words, the efficiency of natural or modified methane-producing systems may be recorded by the isotope signature of acetate, combined with information on source organic carbon and end products (methane and CO2). Education and outreach activities associated with the postdoctoral fellowship include developing and conducting a biofuels-related summer workshop for K 12 teachers and involving undergraduate students in the research.