Coupled hydrology and biogeochemistry of Paleocene–Eocene coal beds, northern Gulf of Mexico

Jennifer C. McIntosh; Peter D. Warwick; Anna M. Martini; Stephen G. Osborn

doi:10.1130/b30039.1

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Coupled hydrology and biogeochemistry of Paleocene–Eocene coal beds, northern Gulf of Mexico

Jennifer C. McIntosh·Peter D. Warwick·Anna M. Martini·Stephen G. Osborn

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Abstract

Thirty-six formation waters, gas, and microbial samples were collected and analyzed from natural gas and oil wells producing from the Paleocene to Eocene Wilcox Group coal beds and adjacent sandstones in north-central Louisiana, USA, to investigate the role hydrology plays on the generation and distribution of microbial methane. Major ion chemistry and Cl − Br relations of Wilcox Group formation waters suggest mixing of freshwater with halite-derived brines. High alkalinities (up to 47.8 meq/L), no detectable SO 4 , and elevated δ 13 C values of dissolved inorganic carbon (up to 20.5‰ Vienna Peedee belemnite [VPDB]) and CO 2 (up to 17.67‰ VPDB) in the Wilcox Group coals and adjacent sandstones indicate the dominance of microbial methanogenesis. The δ 13 C and δD values of CH 4 , and carbon isotope fractionation of CO 2 and CH 4 , suggest CO 2 reduction is the major methanogenic pathway. Geochemical indicators for methanogenesis drop off significantly at chloride concentrations above ∼1.7 mol/L, suggesting that high salinities inhibit microbial activity at depths greater than ∼1.6 km. Formation waters in the Wilcox Group contain up to 1.6% modern carbon (A 14 C) to at least 1690 m depth; the covariance of δD values of co-produced H 2 O and CH 4 indicate that the microbial methane was generated in situ with these Late Pleistocene or younger waters. The most enriched carbon isotope values for dissolved inorganic carbon (DIC) and CO 2 , and highest alkalinities, were detected in Wilcox Group sandstone reservoirs that were CO 2 flooded in the 1980s for enhanced oil recovery, leading to the intriguing hypothesis that CO 2 sequestration may actually enhance methanogenesis in organic-rich formations.

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Source-reported bounding extent: 31.015278981711266° to 33.02708758002874° latitude; -94.031982421875° to -89.307861328125° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Jennifer C. McIntosh, Peter D. Warwick, Anna M. Martini, Stephen G. Osborn. 2010-03-29. Coupled hydrology and biogeochemistry of Paleocene–Eocene coal beds, northern Gulf of Mexico. https://doi.org/10.1130/b30039.1

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