MAJOR GEOCHEMICAL MODIFICATION OF FORMATION WATERS ALONG SEDIMENTARY BASIN MARGINS: ROLE OF MICROBIAL METHANOGENESIS

Pleistocene glacial meltwater invasion to the Illinois and Michigan basin margins profoundly changed the salinity structure of basinal fluids and induced significant biogeochemical modification of formation water elemental and isotope geochemistry. We show that formation waters in Late-Devonian black shales along the northern margins of these basins were dramatically affected. The recharge induced chemical modification of shale-hosted fluids and was essential in generating economically viable deposits of microbial methane.

Geochemical analysis of fluids and gas from organic-rich shales reveal geochemical trends indicative of methanogenesis. The hydrogen isotope composition of the CH₄ is directly related to the isotopic composition of the formation water. The hydrogen isotopic composition of CH₄ and associated fluids in the Devonian shale reservoirs are within the known equilibrium range for methanogenesis. The Ca/Mg ratios of shale formation waters are significantly affected by microbial processing within these relatively shallow hydrogeochemical settings. As alkalinity increases, via a host of microbial metabolic pathways, fluids become supersaturated with respect to calcite. The carbon isotope values for dissolved inorganic carbon (DIC) in the shale formation waters increase with increasing microbial reduction of the remaining carbon source. Subsequent calcite precipitation removes Ca and DIC from the fluid, increasing the Mg/Ca ratio in the water. The interface with down-dip basinal brines is sharply delineated by a change to fluid chemistries with higher Ca/Mg and lower DIC.

Microbial sulfate reduction can inhibit methanogenesis where sulfate concentrations are sufficiently high. Although sulfate reduction also produces DIC, the carbon isotope value of this DIC is isotopically depleted, contrary to the isotopic enrichment seen in these basin margin settings. Siluro-Devonian and Mississippian brines in the Illinois Basin show a likely pattern of sulfate reduction, with increasing alkalinity and decreasing carbon isotope values. This is not the case in the Michigan Basin where high Ba concentrations in formation waters buffers sulfate to low concentrations by insoluble barite precipitation.