Impact of Pleistocene glaciation on midcontinent sedimentary basin fluids: Reorganization of salinity structure and generation of microbial gas

Melting of the Laurentide Ice Sheet during the Pleistocene profoundly altered regional-scale groundwater flow in the low-lying interior of the North American craton. Glacial meltwater recharge into underlying sedimentary basins suppressed basinal brine salinity to great depths and introduced surface microbial communities into fractured, organic-rich Upper Devonian shales, creating a unique class of natural gas deposits along the shallow basin margins. This dissertation investigates the impact of Pleistocene glaciation and microbial methanogenesis on the hydrogeochemistry of the New Albany and Antrim shales and adjacent regional aquifer systems in the Illinois and Michigan basins respectively.; Various geochemical and hydrogeologic approaches were employed in this study on fluids and gas, including stable and radiogenic isotope, and elemental geochemistry, and mass balance calculations. These investigations have culminated in an areally extensive database on formation water and gas geochemistry that is used to make a compelling case for reorganization of basinal-scale flow systems in the Illinois and Michigan basins by Pleistocene glaciation. This work has important implications for sedimentary basin fluid migration, economic accumulations of microbial gas, microbial modification of formation waters, and residence times of freshwater resources.; Glacial meltwaters selectively invaded the permeable Silurian-Devonian aquifer system along the Illinois and Michigan basin margins and migrated into overlying black shales, driven by ice-induced hydraulic loading. This large influx of freshwaters occurred against strong salinity gradients, disrupting relatively stagnant basinal brines, penetrating to 1 km depth, and creating a strong disequilibrium pattern in fluid salinity. Along the northern margin of the Michigan Basin, freshwaters also dissolved significant quantities of halite in evaporite-bearing Silurian-Devonian carbonates. Economic deposits of microbial gas are predominately found in areas associated with freshwater recharge through Silurian-Devonian carbonates. Microbial methanogenesis subsequently altered the elemental and stable isotope composition of shale formation waters and gas. Glacial meltwaters remain at shallow depths in the carbonate aquifers and shales along the basin margins, confined by impermeable drift and Mississippian shales, and extensively altered by water-rock interactions. Major differences in the architecture and fluid composition of the sedimentary basins controlled the extent of freshwater invasion, occurrence of microbial methane, and reorganization of the freshwater-saline water interface.