7. Hydrologic and Geochemical Investigations into Groundwater Conditions Favorable for Microbial Generation of Methane in Organic-Rich Deposits

Natural gas generated from microbial activity (microbial methanogenesis) in organic-rich formations (coal, black shale, petroleum) represents an increasingly important energy resource.  In the past, producers have tended to ignore microbially-derived natural gas accumulations, in favor of larger thermogenic reservoirs, because they were considered too small to be economic.  However, the increasing demand for natural gas has encouraged producers to begin targeting these smaller, and often shallower, microbial deposits.  It is estimated that methane from microbial activity (methanogenesis) accounts for about 20% of the world's natural gas resources. 

Biogenic methane is actively generated in-situ by a consortium of microorganisms that break down organic matter to produce CO₂ and CH₄.  Hence, microbial CH₄ generation is directly coupled to formation water chemistries, available organic substrates, and recharge conditions, and represents a possible renewable resource.  Examples of microbially-generated natural gas deposits are found in the organic-rich Antrim shale deposits in the Michigan Basin and the extensive coal deposits in the Powder River Basin, Wyoming.  In addition, significant coal-bed gas resources exist in subsurface Wilcox Group and younger (Paleocene-Eocene) coal beds found across much of the Gulf Coastal Plain, and preliminary geochemical and isotopic work by the U.S. Geological Survey (USGS) and others has shown that Wilcox coal gas in north Louisiana originates from microbial methanogenesis, as recently as the Late Pleistocene.

Although a considerable body of research exists on identification of microbial versus thermogenic gases from isotopic and molecular studies, less is known about the ground-water hydrologic and geochemical conditions that are required to establish, maintain and promote biogenic methane generation in the subsurface.  Several studies have addressed the geochemical and isotopic nature of ground water associated with coals and black shales bearing biogenic methane.  These studies indicate that formation waters are devoid or nearly so of nitrate and sulfate ions, are depleted in calcium and magnesium, and are enriched in alkalinity with positive carbon isotope values (Bartos and Ogle, 2002; Van Voast, 2003; McIntosh and others, 2004).  In addition, methanogens are strongly inhibited by high salinities (>2 mole/L Cl) often present in sedimentary basins (for example, McIntosh and others, 2004; Waldron and others, 2007).  Non-biogenic methane bearing coals and shales, on the other hand, commonly contain substantial amounts of sulfate and higher concentrations of calcium and magnesium.  These contrasting water chemistries have been associated in only a very general manner with ground-water flow and residence times, and are too infrequently tied to produced gas chemistry (e.g. δD of CH₄ and H₂O, δ¹³C of DIC and CH₄) and microbiology.  Specific ground-water flow paths, rates of ground-water movement, and associated major- and minor-element chemistry and nutrient transport that might control sustained in-place biogenic gas generation has not been investigated.   Such studies are needed to better understand the conditions associated with biogenic gas generation and to prepare for future field experiments that test the possibilities of enhanced biogenic gas generation.  Research under this Opportunity will be conducted in concert with ongoing USGS field and laboratory studies on microbiology, organic geochemistry, and geology of methanogenic processes. 

Postdoctoral research will include investigation of ground-water flow, residence time tracers, and major and trace element chemistry at one or two focused field investigation sites (for example, Gulf Coast, Powder River Basin, Williston Basin) to answer a series of questions using water chemistry and ground-water flow and transport modeling to couple with on-going microbiology and geochemistry studies.  Example research questions include: How rapidly is ground-water transport occurring through the cleat or fracture networks under natural (ambient) conditions versus past recharge conditions (for example, the Last Glacial Maximum)?  When were these microbial communities established in the subsurface and how long has biogenic CH4 been generated?  What trace metals and nutrients are enriched or depleted in biologically active zones?  How rapidly does diffusive transport of key chemical constituents occur into the coal or shale matrix from the fracture network and/or adjacent more permeable formations (for example, sands)?  What level of Cl- and electron acceptors, such as SO₄, NO₃, and Fe is inhibitory for methanogenesis? The final intended outcome of the research will be, in conjunction with geologists, geochemists, and microbiologists, to establish the field conditions that are necessary for renewable biogenic gas generation in organic-rich beds.

References

Bartos, T.T., and Ogle, K.M., 2002, Water quality and environmental isotopic analyses of ground-water samples collected from the Wasatch and Fort Union Formations in areas of coalbed methane development−implications to recharge and ground-water flow, eastern Powder River Basin, Wyoming: U.S. Geological Survey Water-Resources Investigations Report 02-4045, 87 p.

McIntosh, J.C., Walter, L.M., and Martini, A.M., 2004, Extensive microbial modification of formation water geochemistry−case study from a Midcontinent sedimentary basin, United States: Geological Society of America Bulletin, v. 116, no. 5, p. 743-759.

Van Voast, W.A., 2003, Geochemical signature of formation waters associated with coalbed methane: American Association of Petroleum Geologists Bulletin, v. 87, no. 4, p. 667-676.

Waldron, P.J., Petsch, S.T., Martini, A.M., and Nüsslein, K., 2007, Salinity constraints on subsurface archaeal diversity and methanogenesis in sedimentary rock rich in organic matter: Applied and Environmental Microbiology v. 73, p. 4171–4179.

Proposed Duty Station: Reston, VA; Denver, CO; Tucson, AZ

Areas of Ph.D.: Geology, geochemistry, microbiology

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Hydrologist, Research Chemist, Research Microbiologist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): Peter Warwick, (703) 648-6469, pwarwick@usgs.gov; Arthur Clark, (303) 236-5793, aclark@usgs.gov; Jennifer McIntosh (University of Arizona), (520) 626-2282, mcintosh@hwr.arizona.edu; Edwin Weeks, (303) 236-4981, epweeks@usgs.gov

Human Resources Office contact: Brian Arnold-Renicker, (703) 648-7468, brenicke@usgs.gov

Summary of Opportunities