Geochemical constraints on microbial methanogenesis in an unconventional gas reservoir: Devonian Antrim shale, Michigan
Conference
·
OSTI ID:425717
- Univ. of Michigan, Ann Arbor, MI (United States); and others
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 (R{sub o} = 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. {sup 14}C 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 ({delta}{sup 13}C = -49 to -59{per_thousand}) has been used to suggest that the gas is of early thermogenic origin. However, the highly positive carbon of co-produced CO{sub 2} gas ({delta}{sup 13}C {approximately} +22{per_thousand}) and DIC in associated Antrim brines ({delta}{sup 13}C = +19 to +31{per_thousand}) are consistent with bacterially mediated fractionation. The correlation of deuterium in methane ({delta}D = -200 to -260{per_thousand}) with that of the co-produced waters (SD = -20 to -90176) suggests that the major source of this microbial gas is via the CO{sub 2} 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.
- OSTI ID:
- 425717
- Report Number(s):
- CONF-960527--
- Country of Publication:
- United States
- Language:
- English
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