--> Abstract: Chemistry Of Formation Waters: Unraveling Genesis Of Unconventional Gas Deposits In Black Shales, by L. M. Walter, J. M. Budai, and A. M. Martini; #90928 (1999).
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Previous HitWALTERTop, LYNN M., JOYCE M. BUDAI, and ANNA M. MARTINI
University of Michigan, Ann Arbor, MI

Abstract: Chemistry of Formation Waters: Unraveling Genesis of Unconventional Gas Deposits in Black Shales

A regional study of chemical and isotopic trends in formation waters and gases in the Antrim Shale, Michigan Basin and the New Albany Shale, Illinois Basin, emphasizes the importance of local hydrogeology and has led to a useful approach for characterizing and predicting young gas deposits. In gas fields around the northern margin of the Michigan Basin, formation waters from the Late Devonian Antrim Shale represent a mixture of relatively recently recharged (less than or equal to 22,000 ybp) fresh waters with bromide-rich basinal brines. Near the edge of the Basin where the Antrim Shale subcrops beneath glacial drift deposits, waters are more dilute, have high dissolved inorganic carbon (DIC) with unusually high d13C values, and gas is nearly pure methane. Based on isotopic relations between formation waters and gases, it was concluded that methane from the Antrim Shale is largely the result of microbial activity within the fractured, thermally immature reservoir, during the final stages of glacial retreat. It is likely that repeated glacial advances during the Pleistocene exposed the Antrim Shale at the basin margin, enhanced meteoric recharge into shallow zones of the reservoir, and stimulated episodes of bacterial activity.

Based on these results, a model developed for identifying biogenic gas via characteristic water and gas chemistry was tested in the western, central and southern Michigan Basin, and the Illinois Basin. The New Albany Shale (Late Dev) subcrops around the eastern edge of the Illinois Basin, but has experienced a different glacial history and deeper burial depth than the Antrim Shale in Michigan. The hydrogeology is more complex due to variable cover lithology and basin architecture. Preliminary results indicate both thermogenic and microbial gas sources based on geographic trends in gas compositions. The d13C of DIC and dissolved SO4, in formation waters provide further evidence that microbial methanogenesis has occurred in parts of the New Albany reservoir in the Illinois Basin.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas