Geochemical Linkages Between Groundwater Flow and Microbial Methane Generation in Shallow Coal Beds and Fractured Black Shales: Northern Gulf of Mexico and Midcontinent U.S. Basins

McIntosh, Jennifer¹, Peter Warwick², Anna Martini³, Stephen Osborn¹ (1) University of Arizona, Tucson, AZ (2) U.S. Geological Survey, Reston, VA (3) Amherst College, Amherst, MA

Microbial methane is actively generated in organic-rich fractured black shales and coal beds, along the shallow margins of sedimentary basins. This study compares co-produced fluid and gas chemistry of the Upper Devonian Antrim and New Albany shales in the Michigan and Illinois basins, respectively, with 21 new analyses of waters and gas in the emerging Gulf Coast coal bed methane play (Paleocene/Eocene Wilcox Group), to better constrain the hydrogeochemical controls on biogenic gas production.

Fluids associated with microbial methane in the Antrim and New Albany shales, and Wilcox coal beds contain high alkalinity concentrations (6-70 meq/kg), with no detectable sulfate or acetate, and have been significantly diluted by freshwater recharge through adjacent aquifers. Produced gases from Wilcox coals and Antrim and New Albany shales have variable gas wetness values (C₁/C₂₊ = 12 to >10,000) and δ¹³C-CH₄ values between -65 and –46‰ VPDB. The carbon and hydrogen isotope relations between co-produced waters, CH₄ and CO₂ indicate that methane was generated in-situ via CO₂ reduction. Thermogenic gases from deeper reservoirs in the Gulf Coast, underlying the Wilcox Group, and Upper Devonian shales in the central Michigan and Illinois basins, display distinct gas isotope and compositional trends (low C₁/C₂₊ values, <12; δ¹³C-CH₄ values from –56 to –38‰; δ¹³C-CO₂ values <-15‰; and no isotopic relation between hydrogen in CH₄ and associated waters). In these instances, formation water salinity and freshwater recharge appear to control the location of the transition zone between dominantly microbial gas at shallow depths and deeper thermogenic gas.