--> Abstract: Prolific Reservoirs In The Geneva Dolomite (Middle Devonian), West-Central Illinois Basin, by B. Seyler and J. P. Grube; #90928 (1999).

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SEYLER, BEVERLY and JOHN P. GRUBE
Illinois State Geological Survey, Champaign, IL

Abstract: Prolific Reservoirs in the Geneva Dolomite (Middle Devonian), West-Central Illinois Basin

The Geneva Dolomite, a basal member of the Middle Devonian Grand Tower Formation, in west-central Illinois basin is an exploration target that has recently generated much interest. New wells have produced in excess of 300 barrels of oil per day at depths of 3000 - 4000 feet. A study of reservoirs in the Geneva Dolomite at Raccoon Lake, Sandoval, Patoka, and St. James oil fields shows that pronounced closure, fracturing, and development of secondary porosity which include dolomitization, solution and possible karsting are needed for reservoir development and entrapment of petroleum.

Draping of younger Middle Devonian strata over Silurian reefs has resulted in approximately 100 feet of closure in Sandoval and Raccoon Lake fields. Each of these fields produces from Silurian and Devonian rocks. Patoka field is a deeper seated structure with primary production in the Ordovician Trenton Limestone. St. James field is an anticline which produced from younger Mississippian strata prior to the discovery of Devonian reservoirs. Examination of core from reservoirs in the Geneva Dolomite shows vuggy, sucrosic and some sandy dolomite, and collapse breccia filling solution cavities, a feature associated with karstified carbonates. We suggest that alteration caused by post depositional subaerial exposure of Geneva carbonates is a viable mechanism that explains the brecciation and enhanced porosity and permeability. Therefore, the top of the Geneva may define a sequence boundary. Mapping suggests that Devonian age structures are an element that aids porosity enhancement and more recent structural movement has created the pronounced closure needed for petroleum entrapment.

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