COUPLED HYDROLOGY AND BIOGEOCHEMISTRY OF PALEOCENE-EOCENE COAL BEDS, NORTHERN GULF OF MEXICO

Twenty-three formation waters, gas, and microbial samples were collected from coal beds and adjacent sandstones in the Middle and Lower Wilcox Group in northern Louisiana to constrain the hydrology and biogeochemistry of an emerging coal bed methane play. The thin Wilcox Group coals are thermally immature (lignite to high volatile C bituminous, ~6 m maximum thickness) and interbedded with shales and permeable sandstones.

Fluids in the Lower Wilcox aquifer generally flow up dip from the south (>1700 m depth) to the north (<300 m depth), driven by topography, salinity, and pressure gradients. Na-Cl-Br relations and O/H isotopes of Wilcox formation waters suggest mixing of freshwater with halite-derived brines. Cl values increase from 0.4 to 2.3 mol/L with depth and proximity to salt domes. There is no detectable SO₄ or acetate. High alkalinity concentrations (up to 48 meq/L), and δ¹³C values of dissolved inorganic carbon (up to 20‰, VPDB) and CO₂ (up to 14‰) indicate microbial methanogenesis. The δ¹³C and δD values of CH₄, and carbon isotope fractionation of CO₂ and CH₄, suggest CO₂ reduction is the dominant microbial pathway. Lower Wilcox Group waters contain <0.01 percent modern carbon (A¹⁴C). Despite corrections for additional sources of carbon, these fluids have a relatively long residence time (>16ka). The covariance of δD values of co-produced H₂O and CH₄ link the onset of microbial methanogenesis to at least the Late Pleistocene. High carbon isotope values of C₂ and C₃ (up to –21‰, and -12‰, respectively), and the depletion of n-alkanes in associated oils (from published data), suggest the presence of anaerobic microbial oxidation.

Microbial cell densities of porewaters in the shallow Lower Wilcox coals (<1 km depth) averaged 28x10³ cells/mL, compared to 1x10³ in the overlying Middle Wilcox sandstones. Microorganisms were not detected in fluids from the deeper Lower Wilcox sandstones (>1 km depth), indicating that microbial CH₄ likely migrated from adjacent coals. Shallow Lower Wilcox coal beds have low salinity porewaters, large microbial populations, and ¹³C enriched CH₄ and CO₂ compared to the deeper oil-associated Wilcox coals and adjacent sandstones. This is likely due to the more open groundwater flow system along the basin margin and increased nutrient transport into the organic-rich sediments.