HR: 16:20h
AN: H14D-02 INVITED    [Abstracts]
TI: Paleohydrologic controls on methanogenesis in organic-rich saline aquifers
AU: * McIntosh, J
EM: mcintosh@hwr.arizona.edu
AF: University of Arizona, Department of Hydrology and Water Resources, Tucson, AZ 85721, United States
AU: Petsch, S
EM: spetsch@geo.umass.edu
AF: University of Massachusetts-Amherst, Department of Geosciences, Amherst, MA 01003, United States
AU: Schlegel, M
EM: meschleg@hwr.arizona.edu
AF: University of Arizona, Department of Hydrology and Water Resources, Tucson, AZ 85721, United States
AU: Osborn, S
EM: sosborn@hwr.arizona.edu
AF: University of Arizona, Department of Hydrology and Water Resources, Tucson, AZ 85721, United States
AB: Freshwater recharge into the margins of sedimentary basins, during periods of continental glaciation, stimulated microbial methane generation in organic-rich shales and coal beds, by significantly diluting the ambient formation water salinity. Subglacial recharge may have also transported microorganisms and nutrients into the subsurface environment. Methane is generated by a diverse consortium of both acetoclastic and CO2-reducing methanogenic Archaea, and adsorbed onto the organic matter. These shallow methane accumulations account for ~20% of the total U.S. natural gas production. Anaerobic microbial metabolism of shales and coals is in part controlled by the volume of pore waters and fluid composition, amount of extractable organic matter and intermediary substrates, reservoir temperature, and mass transport processes that provide essential rock-derived nutrients and organic acids. Methanogens are most active in low salinity environments (<2.5 mol/L Cl) with no SO4, and at the interfaces between confining units and adjacent aquifers where diffusion dominates. Microbial degradation of organic matter generates high alkalinity concentrations, which may induce calcite precipitation in shale fractures and coal cleats, which can in turn modify the subsurface hydrology. Microbial methanogenesis also imparts a strong control on the cycling of carbon, H2, and other elements in the subsurface environment. This presentation will focus on the timing of recharge and establishment of microbial communities within the Upper Devonian black shales, Pennsylvanian coal beds, and overlying glacial drift in the Illinois Basin, and the importance of continued groundwater flow on active methane generation and accumulation. There is an approximately 65-70 per mil depletion in 13C of CH4, relative to the precursor CO2 in the Upper Devonian shales, Pennsylvanian coals, and glacial drift. In addition, there is a linear correlation between the dD values of co- produced formation waters and CH4. Isotope mass-balance modeling results confirm that these isotopic shifts can be produced by coupled acetate fermentation and CO2-reduction. The lowest d13C values for CO2 and CH4 are found in the shallow glacial drift (-14 to 8 per mil, -80 to -68 per mil, respectively), where the permeable aquifers are a relatively open system, rapidly flushed by modern recharge. In contrast, the deep Upper Devonian shales have relatively positive d13C values for CO2 and CH4 (6 to 20 per mil, -56 to -50 per mil, respectively), indicating that methane has been generated over relatively long time scales (at least since the Late Pleistocene) in a closed system. The Pennsylvanian coal beds have intermediary d13C values for CO2 and CH4 (-8 to 11 per mil, -66 to -56 per mil, respectively), and contain Holocene groundwaters. Understanding the hydrobiogeochemical processes active within fractured shales and coal beds is important for energy resources, as well as CO2 sequestration.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0448 Geomicrobiology
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1807 Climate impacts
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2007 Fall Meeting