The Upper Devonian Antrim Shale in the Michigan Basin is an economically significant source of microbially produced methane along the basin margins where meteoric recharge has been focused. Oxygen and hydrogen stable isotope compositions of Antrim formation waters show that fresh waters, recharged from Pleistocene glaciation and modern precipitation, suppressed basinal brine salinity to great depths and enhanced methanogenesis. This paper presents results of integrated elemental and isotope analyses of Antrim Shale formation waters from the margins and center of the Michigan Basin, focusing on solute sources and geochemical modifications associated with regionally extensive microbial methanogenesis. Cl-Br-Na systematics reveal that salinity is controlled not only by mixing between variable amounts of basinal brine and meteoric water, but also by halite dissolution where fluids recharged through underlying Devonian carbonate aquifers with localized evaporite deposits.

Divalent cations, carbonate system parameters, and carbon isotope compositions of dissolved inorganic carbon have been systematically and profoundly altered by microbial methanogenesis. Large decreases in formation water Ca/Mg and Ca/Sr ratios accompany increasing carbonate alkalinity values in areas with high rates of microbial gas production. Thermodynamic and reaction-path modeling show that these changes are consistent with calcite precipitation during progressive microbial methanogenesis. Similar variations in fluid chemistry are evident in databases from other sedimentary basins containing black shales and coal beds associated with microbial gas. Microbial methanogenesis may play an important role in the geochemical evolution of divalent cation relationships in crustal fluids and should be considered in models of formation water origin and evolution.

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