--> ABSTRACT: Geochemical Constraints on Microbial Methanogenesis in an Unconventional Gas Reservoir: Devonian Antrim Shale, MI, by A. M. Martini, J. M. Budai, L. M. Walter, T. Ku, and M. Schoell; #91019 (1996)

Datapages, Inc.Print this page

Geochemical Constraints on Microbial Methanogenesis in an Unconventional Gas Reservoir: Devonian Antrim Shale, MI

A. M. Martini, J. M. Budai, L. M. Walter, T. Ku, and M. Schoell

The Upper Devonian Antrim Shale is a self-sourced, highly fractured gas reservoir. It subcrops around the margin of the Michigan Basin below Pleistocene glacial drift, which has served as a source of meteoric recharge to the unit. The Antrim Shale is organic-rich (>10% total organic carbon), hydrogen-rich (Type I kerogen) and thermally immature (R0 = 0.4 to 0.6). Reserve estimates range from 4-8 Tcf, based on assumptions of a thermogenic gas play.

Chemical and isotopic properties measured in the formation waters show significant regional variations and probably delineate zones of increased fluid flow controlled by the fracture network. 14C determinations on dissolved inorganic carbon indicate that freshwater recharge occurred during the period between the last glacial advance and the present. The isotopic composition of Antrim methane (^dgr13C = -49 to -59^pmil) has been used to suggest that the gas is of early thermogenic origin. However, the highly positive carbon of co-produced CO2 gas (^dgr13C +22^pmil) and DIC in associated Antrim brines (^dgr13C= +19 to +31^pmil) are consistent with bacterially mediated fractionation. The correlation of deuterium in methane (^dgrD = - 00 to -260^pmil) with that of the co-produced waters (^dgrD = -20 to -90^pmil) suggests that the major source of this microbial gas is via the CO2 reduction pathway within the reservoir. Chemical and isotopic results also demonstrate a significant (up to 25%) component of thermogenic gas as the production interval depth increases.

The connection between the timing of groundwater recharge, hydrogeochemistry and gas production within the Antrim Shale, Michigan Basin, is likely not unique and may find application to similar resources elsewhere. Biologic production of methane is observed in oil fields today undergoing freshwater injection to enhance oil recovery. Microbes introduced with the water colonize the reservoir and may produce methane of economic importance. Our findings may stimulate a broader view of unconventional resources and suggest new geochemical approaches to recognizing biogenic gas deposits.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California