COMPARATIVE STUDY OF THE EFFECT OF PLEISTOCENE RECHARGE ON FORMATION WATER GEOCHEMISTRY IN THE ILLINOIS VS. MICHIGAN BASINS

Pleistocene glaciation reorganized regional flow systems of midcontinent sedimentary basins. In the Michigan and Illinois basins, glacial meltwater preferentially invaded the Siluro-Devonian regional aquifer system and suppressed basinal brine salinity to great depths. The waters migrated via fracture networks into overlying Late Devonian organic-rich shales and promoted microbial methanogenesis. Elemental and stable isotope geochemistry reveals the dilution of brine, dissolution of evaporite and carbonate minerals, and modification of formation water carbonate systematics via microbial methanogenesis. Importantly, the fundamental difference in original brine elemental chemistry and salinity between the Illinois and Michigan basins has marked effects on both the extent of recharge and ensuing chemical modification of formation waters.

Late Devonian shale formation waters along the margins of the Michigan and Illinois basins have high alkalinity (>20 meq/kg) and d¹³C (>10‰) values indicative of methanogenesis. The covariance of dD values for water and methane indicates the microbial gas was produced in-situ with shale waters, which were recharged under cooler climatic conditions.

The Ca/Mg and Ca/Sr ratios of shale formation waters are significantly lower than underlying Devonian brines along the margins of the Michigan Basin. The decrease in Ca/Mg is associated with an increase in alkalinity. Mg and Sr values also appear to have increased, relative to Br. The Illinois Basin shale fluids show similar ratios, however, the differences between shale and Devonian brines are muted relative to the Michigan Basin.

Freshwater recharged at basin margins diluted the brines and dissolved carbonates, enriching formation waters in Ca, Mg, Sr, and HCO₃. Along the northern margin of the Michigan Basin, dilute waters also dissolved halite increasing Cl/Br ratios. During methanogenesis shale waters became supersaturated and precipitated calcite, significantly decreasing Ca and HCO₃ concentrations. Carbonate mineral solubility, largely controlled by PCO₂ and ionic strength, also differs between the Michigan and Illinois basins as well as within the basins due to local methanogenesis.