HYDROGEOCHEMICAL CONSTRAINTS ON PLEISTOCENE GLACIAL MELTWATER INVASION INTO MIDCONTINENT, U.S. REGIONAL AQUIFER SYSTEMS: IMPACT ON SALINITY STRUCTURE AND GENERATION OF MICROBIAL GAS

Pleistocene glaciation profoundly altered regional-scale groundwater flow in the low-lying interior of the North American craton. Glacial meltwater recharge into regional aquifer systems in Midcontinent sedimentary basins disrupted strong, and likely long-lived salinity gradients. Recharge of glacial meltwater through regionally-extensive Silurian-Devonian carbonate aquifers is of special significance because it has introduced microbial communities into overlying fractured, organic-rich Upper Devonian shales, producing a unique class of natural gas deposits.

The detailed anatomy of recharge paths was investigated along the northern margin of the Michigan Basin. Here, the Upper Dev. black shale has been extensively drilled for natural gas (>10,000 wells). Together with fluid chemistry from subjacent Sil.-Dev. aquifers, this large dataset provides important constraints on the salinity structure and sources of solutes. Using Na-Cl-Br mass-balances, spatial variations in salinity, and geologic cross-sections, major recharge pathways were delineated and relative volumes of glacial meltwater were quantified. The majority of Antrim Shale fluids owe >60% of their salinity to halite dissolution and contain >80% (by vol.) meteoric waters, indicating freshwater selectively recharged the evaporite-bearing Sil.-Dev. carbonates along the basin margin and migrated upward into the shale. This large influx of meteoric waters occurred against sharp salinity gradients (<0.5 to >5 M Cl-), to great depths (up to 300 m beneath the shale subcrop), and over considerable lateral distances (≈50 km). Economic deposits of microbial gas are found primarily in areas inundated by freshwater recharge through the underlying Sil.-Dev. aquifers.

These results can be extrapolated to the larger glaciated Midcontinent region, where the Sil.-Dev. carbonate subcrop is continuous along the Illinois, Michigan, and Appalachian basin margins, and is overlain by Upper Dev. black shales. Major differences in the hydrostratigraphy and formation water geochemistry of these basins controlled the extent of glacial meltwater invasion, location of the freshwater-saline water interface, and occurrence of microbial gas.