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Abstract: Geneva Dolomite (Middle Devonian) Reservoirs: Marion County, Illinois

SEYLER, BEVERLY
Illinois State Geological Survey, Champaign, IL

The Geneva Dolomite in Marion County, Illinois is a recent exploration target that has generated much interest. New wells have produced in excess of 300 barrels of oil per day at depths of 3000 ft or less in the Geneva Dolomite, a basal member of the Middle Devonian Grand Tower Formation. Study of reservoirs in the Geneva Dolomite at Raccoon Lake, Sandoval, and Patoka Oil Fields shows that pronounced closure, fracturing, and secondary porosity from dolomitization, solution and karsting are key elements needed for reservoir development and entrapment of petroleum.

Draping of younger Middle Devonian strata over Silurian reefs has resulted in approximately 100 ft 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. Examination of core from reservoirs in the Geneva Dolomite in Patoka Field shows vuggy, sucrosic and some sandy dolomite, and collapse breccia filling solution cavities, a feature associated with karsted terrains. The Middle Devonian Geneva Dolomite unconformably overlies Lower Devonian strata in Marion County, Illinois. Exposure along a sequence boundary in the Middle Devonian is suggested as a mechanism for development of these dolomite reservoirs.

AAPG Search and Discovery Article #90930©1998 AAPG Eastern Section, Columbus, Ohio