Collaborative Research: Active subsurface methanogenesis and anaerobic organic matter decomposition in a Devonian black shale

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ABSTRACTMicroorganisms employ many novel strategies to derive energy and obtain nutrients, and in doing so alter the chemistry of their environments in ways that are significant for formation and transformation of geologic materials. We propose to investigate one such strategy: microbial methane generation in sedimentary basins. Stable isotopic signatures of CH4, CO2 and H2O in shale formation waters indicate a microbial origin for several natural gas reserves. However, these signatures leave intriguing issues unaddressed. Anaerobic hydrocarbon degradation proceeds by many metabolic pathways including sulfate, nitrate, and iron reduction, fermentation, and methanogenesis, and has been recognized in petroleum reservoirs, polluted aquifers and deep marine sediments. This research will expand understanding of methane generation by exploring anaerobic decomposition of organic matter in subsurface sedimentary rocks leading to economic methane accumulations.The Antrim shale (Late Devonian, Michigan USA) is one of the largest shale gas reserves in North America. Isotopic studies have established that this gas is dominantly microbial in origin. This project will focus on a N-S transect of wells through the main gas-producing trend, sampling formation waters, archived cuttings, and freshly drilled well cores. Prior research on Antrim formation waters by Martini and co-workers provides a solid framework for understanding the subsurface physical and chemical environments within the region. A positive working relationship with energy companies and land owners in northern Michigan allows us an unusual degree of access to the field site, including opportunities to collect fresh material during drilling and to use poorly-producing wells as natural laboratories.Intellectual Merit. This research integrates analytical approaches from aqueous geochemistry, organic geochemistry, thermodynamics, molecular biology and microbiology. We will synthesize our results into a conceptual model that addresses key research questions including: How do analyses of temperature, salinity, nutrients and metabolic substrates reveal thermodynamic and environmental constraints on the activity of these organisms? What does chemical characterization of bitumens, kerogen, and dissolved organic matter reveal about specific sources of organic matter that fuel methane generation in this subsurface environment?What are the 16S and functional gene molecular phylogenies of active microorganisms within Antrim Shale formation waters, and what metabolic roles do these organisms play?Preliminary efforts have determined a high diversity of methanogenic Archaea in formation waters from two Antrim wells, including an entirely new cluster of mcrA genotypes and novel 16S sequences within the Methanomicrobiales, Methanosarcinales, and Methanobacteriales. 16S rRNA-based Bacterial diversity is more limited, confined to sequences closely related to acetate-producing Acetobacterium and within the Syntrophomoadaceae and Syntrophobacteraceae. Surprisingly, no DNA evidence for sulfate- or metal-reducing bacteria has yet been detected. Thus rather than a suite of electron acceptors (O2, NO3, Fe, Mn and SO4) followed by fermentation and methanogenesis (such as found in modern sediments), the subsurface methanogenic environment of the Antrim may more closely resemble documented syntrophic communities in which hydrocarbons are decomposed to acetate and H2, a reaction that is energetically favored only when acetate and H2 are in turn rapidly consumed by methanogens. Our research will explore both of these hypotheses.The broader impacts of this research are varied and far reaching. This study will define an unusual microbial community relevant to human activities. It will also detail how microbial activity in rocks may confound and overprint ancient molecular biosignatures. In addition, the study will shed light on a poorly-constrained source of reduced gases to Earth's atmosphere, important for understanding controls on atmospheric composition, carbon cycling and carbon transformations in earth surface reservoirs. Our collaborative research will increase opportunities in education and training, including a new Five Colleges Biogeochemistry course. We are also initiating partnership with Gaylord High School, the regional public high school serving residents of our field area. Because the gas industry is a large part of the local economy, it will be valuable for students in this modest-income region to gain hands-on experience with scientific endeavors occurring in their backyard. Lastly, by disseminating our results widely, we will improve understanding of gas reserve origins, indices for exploration, and controls on reserve production.