The Origin of Prolific Reservoirs in the Geneva Dolomite (Middle Devonian), West-Central Illinois Basin

By

SEYLER, BEVERLY, JOHN P. GRUBE, and ZAKARIA LASEMI

Illinois State Geological Survey

     The Geneva Dolomite, commonly the basal member of the Middle Devonian Grand Tower Formation in much of the Illinois Basin, is an exploration target that has recently generated much interest.  A new discovery in the Geneva was completed for up to 3,000 barrels of oil per day at a depth of 4,000 feet in the west-central part of the Illinois Basin. A study of reservoirs in the Geneva Dolomite at Raccoon Lake, Sandoval, Patoka, Miletus and St. James oil fields shows that pronounced structural closure, fracturing, and formation of secondary porosity through dolomitization and dissolution are associated with reservoir development and entrapment of petroleum.

     Draping of younger Middle Devonian strata over Silurian reefs has resulted in approximately 100 feet of closure at Sandoval and Raccoon Lake fields. Patoka field, which overlies a larger, deeper seated structure, also produces from the Ordovician Trenton Limestone. St. James field is an anticline that initially produced from shallower Mississippian strata prior to the discovery of Devonian reservoirs. Examination of core from Geneva Dolomite reservoirs shows the rock to be a brown, vuggy, and sucrosic dolomite. We suggest that post-depositional dolomitization of Geneva carbonates combined with dissolution of fossil material is a viable mechanism to explain the enhanced porosity, permeability, and brecciation found in Geneva Dolomite reservoirs. Mapping suggests that Devonian-age structures, commonly associated with underlying Silurian reefs, are an element that enhances reservoir porosity and more recent structural movement has created the pronounced closure needed for petroleum entrapment.