HYDROGEOCHEMISTRY OF DEVONIAN CARBONATE AQUIFER SYSTEMS IN THE MIDCONTINENT, UNITED STATES

The broad expanse of Devonian carbonates in the midcontinent, recently glaciated, make this region an important area to study the impact of Pleistocene glaciation on regional carbonate aquifers and on gas generation in overlying Late-Devonian shales. Where carbonates are exposed near the surface or overlain by permeable deposits, they provide a broad avenue for freshwater recharge into the Michigan and Illinois basins. This influx of dilute waters suppressed the salinity of Devonian carbonate brines and overlying organic-rich shales, creating a sharp disequilibrium pattern in fluid salinity and an environment conducive for microbial methanogenesis.

Both the Antrim and New Albany shales contain extensive microbial gas deposits associated with dilute waters near the margins of the Michigan and Illinois basins. The chloride content of shale fluids shows relatively dilute waters near the subcrop penetrating to significant depths. The isotopically low d18O and dD values (relative to modern precipitation) for shale formation waters and underlying carbonates indicates the fluids are of Pleistocene origin. The dD values for methane and shale fluids confirm that the microbial gas was produced in-situ with these dilute fluids.

The shale formation water chemistry highlights two major pathways for fluids into Late-Devonian shales: 1) through overlying glacial drift, or Mississippian carbonates (Illinois Basin) and/or 2) through underlying Siluro-Devonian carbonates. In northern Michigan, Siluro-Devonian carbonates contain evaporites. Fluids that encountered these salts have higher Cl/Br ratios than do the Siluro-Devonian brines, which evolved from evaporating seawater. Utilizing elemental and isotopic chemistry of the carbonate and shale formation waters we can delineate recharge pathways for freshwaters into regional carbonate aquifers and gas-producing shales. By sampling glacial drift aquifers overlying the shales and Devonian carbonates, we are also able to investigate vertical recharge of meteoric waters into shale subcrops. Multiple advances and retreats of continental ice sheets appear to have significantly altered regional-scale flow systems and introduced chemically distinct fluids into carbonate aquifers and shales, providing a tracer for fluid migration along basin margins.