Reports: ND251001-ND2: Compound-Specific Isotopes of Methanogenic Precursors in Coal: Laboratory, Field and Modeling Studies

Jennifer McIntosh, University of Arizona

There is significant academic and commercial interest in identifying key pathways of methanogensis in microbial coalbed methane (CBM) systems. Improved understanding of microbial methane and its precursors derived from coal provides fundamental insight into carbon cycling and may be used to stimulate methanogenic systems to yield additional microbial gas. This research seeks to improve understanding of methane precursor utilization and the carbon isotope mass balance during methanogenesis through a combination of field sampling, laboratory investigations, novel compound-specific isotope measurements of acetate, and mass balance modeling.

Field sampling of water and gas: Twenty water and 19 gas samples were collected from producing wells and monitoring wells along a ~25 km long NW-SE transect from the edge of the Powder River Basin (PRB) to the center. The monitoring wells belong to networks maintained by the U.S. Geological Survey and the Montana Bureau of Mines and Geology. These samples bracket a key interface between shallow, sulfate-reducing waters, and deeper methanogenic waters. This sampling plan complements previous studies of both the marginal and basin-center environments with their distinctive methane and CO2 isotope signatures.

A wide range of carbon and hydrogen isotope ratios were observed in the PRB, expressed below in standard delta notation, including variation from edge to center of basin: δ13CCH4 -78.2 to -56.2 per mil, δ13CCO2 -24.7 to 4.7 per mil, δ13CDIC -17.5 to 17.5 per mil, δ2HCH4 -327 to -249 per mil, and δ2H of water -165 to ‑129 per mil. Multiple hypotheses are possible from interpretation of carbon isotope signatures. For example, variations in δ13C may record competition between sulfate reduction and methanogenesis, with their different fractionation factors, rather than recording a change of methanogenic pathways (acetoclastic methanogenesis vs. CO2 reduction). More process-specific information on the utilization of acetate by methanogens or other anaerobes may be derived from compound-specific tracers involving methane precursors, such as δ13C of acetate.

Laboratory incubations: A group of incubations was performed in collaboration with Elizabeth Jones, microbiologist at the U.S. Geological Survey. The objective was to supplement field-collected water and gas samples with experiments in which the metabolic pathway is better constrained. These incubations combined coal, a nutrient solution, and a microbial consortium containing known methanogens. Preliminary results suggest that small quantities of methane were generated over the course of 2 months. A headspace gas sample yielded methane with δ13C of -49 per mil, higher than values observed in the PRB field area, and little acetate (low µM range) was detected in these samples. In contrast, samples treated with 2‑bromoethanesulfonic acid (BES; an inhibitor of methanogens) yielded acetate in excess of the methane generated in BES-free samples (high µM to low mM range). Preliminary δ13C of acetate in these BES-controlled incubations (‑45 to ‑30 per mil) is more negative than bulk coal (‑26 to ‑24 per mil), which could be consistent with acetogenesis having occurred in the BES-treated experiments.

Compound-specific carbon isotope analysis of acetate: In a new collaboration with Neal Blair, of Northwestern University, validating a method for compound-specific δ13C determination of acetate in waters coexisting with microbial gas is in progress. This method involves (1) preconcentration of low-acetate water samples to yield sufficient carbon for analysis; (2) separation of acetate from the sample using headspace solid-phase microextraction (HS-SPME); and (3) analysis of δ13C on the separated acetate by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). Because no internationally-recognized carbon isotope standards exist for acetate, an integral part of this work involves calibration of commercially available acetate salts against a well-characterized acetate salt utilized in previous studies by the Blair group. Initial efforts in the Northwestern University Stable Isotope Laboratory have demonstrated precise and accurate measurement of δ13C of acetate in standard acetate solutions and samples down to ~500 µM using HS-SPME. Initial experiments with the HS-SPME procedure also suggest that subsequent efforts to analyze low-acetate samples will focus on longer HS‑SPME extraction time and/or additional sample preconcentration.

Mass balance modeling: This work has proceeded in two directions: First, a critical review of the application of C & H isotope tracers in biogenic gas systems is underway. This review has numerous implications for the interpretation of methanogenic systems and outlines improved practices in the application of isotopic fingerprints to mass-balance models. Secondly, a new model will be used to describe the PRB in a subsequent publication. Initial efforts are focused on mass balance constraints imposed by the biodegradation of easily metabolized organic matter by highly fractionating vs. less fractionating processes, that is, methanogenesis vs. competing processes such as sulfate reduction. These variations alone could be responsible for the observed differences in δ13C of methane and CO2.

Presentation of results: Initial results were presented at the Goldschmidt geochemistry conference (Montreal, June 2012), at which PI Jennifer McIntosh and postdoctoral research associate David Vinson co-convened a session on "Water-rock-microbial interactions in energy systems". Another presentation is forthcoming at the Geological Society of America conference (Charlotte, NC, November 2012). Vinson is first author and will have presented both of these conference presentations. Two publications, with Vinson as first author, are in preparation: (1) a critical review on the application of carbon and hydrogen isotope systematics to microbial gas systems derived from biodegradation of coal, oil, and shale; and (2) a refined carbon isotope model applying mass balance concepts of methane and its precursors including the competing role of sulfate reduction. This model will be applied to our results from the PRB and will be applicable to other microbial gas systems.

Mentoring and career development: Postdoctoral research associate Vinson has been supported by PRF funding. This project has allowed Vinson to expand his scientific background in low-temperature geochemistry to the biogeochemistry of coal biodegradation and methanogenesis, and Vinson has benefited from numerous mentoring and career advancement opportunities. This research has provided opportunities for Vinson to collaborate with energy-oriented geochemists and biogeochemists by applying models and techniques pioneered in research of aquatic sediments to energy systems. Vinson has also provided mentoring and to five MS and PhD students in McIntosh's research group doing research in microbial methanogenesis and other areas of low-temperature geochemistry.