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.