GENETIC AND TEMPORAL RELATIONS BETWEEN FORMATION WATERS AND BIOGENIC METHANE - UPPER DEVONIAN ANTRIM SHALE, MICHIGAN BASIN, USA

Controversy remains regarding how well geochemical criteria can distin guish microbial from thermogenic methane. Natural gas in most conventi onal deposits has migrated from a source rock to a reservoir, rarely r emaining associated with the original or cogenetic formation waters. W e investigated an unusual gas reservoir, the Late Devonian Antrim Shal e, in which large volumes of variably saline water are coproduced with gas. The Antrim Shale is organic-rich, of relatively low thermal matu rity, extensively fractured, and is both source and reservoir for meth ane that is generated dominantly by microbial activity. This hydrogeol ogic setting permits integration of chemical and isotopic compositions of coproduced water and gas, providing a unique opportunity to charac terize methane generating mechanisms. The well-developed fracture netw ork provides a conduit for gas and water mass transport within the Ant rim Shale and allows invasion of meteoric water from overlying aquifer s in the glacial drift. Steep regional concentration gradients in chem ical and isotopic data are observed for formation waters and gases; di lute waters grade into dense brines (300,000 ppm) over lateral distanc es of less than 30 km. Radiogenic (C-14 and H-3) and stable isotope (O -18 and D) analyses of shallow Antrim Shale formation waters and glaci al drift groundwaters indicate recharge times from modern to 20,000 yr sp. Carbon isotope compositions of methane from Antrim Shale wells ar e typical of the established range for thermogenic or mixed gas (delta (13)C = -47 to -56 parts per thousand). However, the unusually high de lta(13)C values of CO2 coproduced with methane (similar to+22 parts pe r thousand) and dissolved inorganic carbon (DIC) in formation waters ( similar to 28 parts per thousand) require bacterial mediation. The del ta D values of methane and coproduced formation water provide the stro ngest evidence of bacterial methanogenesis. Methane/[ethane + propane] ratios and delta(13)C values for ethane indicate: (1) the presence of a thermogenic gas component that increases basinward and (2) progress ive bacterial oxidation of ethane as the Antrim Shale subcrop is appro ached. Multiple episodes of Pleistocene glaciation over northern Michi gan appear critical to the development of these gas deposits. Loading of thick ice sheets may have provided hydraulic head that enhanced dil ation of preexisting fractures and influx of meteoric water. The physi cal erosion cycle of repeated glacial advances and retreats exhumed th e Antrim Shale around the northern margin of the Michigan Basin, subje cting it to near-surface physiochemical and biochemical processes. The chemical and hydrologic relations demonstrated in the Antrim Shale re servoir suggest a dynamic connection between Pleistocene glacial histo ry of the midcontinent region and development of recoverable, microbia lly generated natural gas reserves. Copyright (C) 1998 Elsevier Scienc e Ltd.