Am. Martini et al., GENETIC AND TEMPORAL RELATIONS BETWEEN FORMATION WATERS AND BIOGENIC METHANE - UPPER DEVONIAN ANTRIM SHALE, MICHIGAN BASIN, USA, Geochimica et cosmochimica acta, 62(10), 1998, pp. 1699-1720
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.