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ABSTRACT: Sedimentology of the Berea Sandstone (Early Mississippian) and the Bedford Shale (Latest Devonian), Williams Field, Michigan

David A. Balthazor

The Berea Sandstone (Early Mississippian) and the underlying Bedford Shale (latest Devonian) occur in the eastern half of the Michigan basin. Previous workers have shown that these formations are genetically related and have a lobate geometry which thins toward the basin center. Previous work indicates that the Berea-Bedford Sequence was deposited as a constructive, fluvial-dominated, epeiric sea deltaic complex. Other recent work suggests the Berea/Bedford succession in Williams field of Midland and Bay Counties, Michigan, is a tidally dominated strand-plain, barrier-bar environment of deposition.

For this study, six lithofacies were described from nine conventional cores from the Berea-Bedford Sequence of Williams field and correlated to approximately 150 well logs. These facies are interpreted as deltaic deposits comprising prodelta (Bedford), delta-front sandstone, destructive marine mudstone, inactive distributary channel-fill, interdistributary bay, crevasse splay, and a destructive (transgressive) marine sandstone.

Approximately 2 million bbl of oil has been produced from the Berea Sandstone of Williams field since its discovery in 1980. The delta-front sandstone facies is the main reservoir within the field. A secondary reservoir occurs in the destructive marine sandstone facies. Hydrocarbon trapping is both structural and stratigraphic. A northwest-plunging anticline within the field provides most of the closure. Impermeable inactive distributary channel-fill facies oriented northeast-southwest occur as lateral equivalents to the delta-front sandstone. These passive channel-fill facies compartmentalize the reservoir sandstone from northeast to southwest. Variations in formation pressures and production are attributed to the stratigraphic compartmentalization of the field by facies associated w th a lobate river-dominated deltaic depositional system.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990