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Controls on Silurian Reef Distribution Allow for Cost-Effective Exploration, Clinton and Washington Counties, Illinois



Thomasson Partner Associates, Denver, CO,


Illinois State Geological Survey, Champaign, IL


     Production of significant volumes of oil has been associated with Silurian reefs in Clinton and Washington counties, Illinois. More than (??) million barrels of oil have been produced from reef facies and more than (??) million barrels have been produced from shallower, units draped over the reefs in these two counties. Early exploration methods for Silurian reefs included geomorphology, gravity, and coal structure maps identifying shallow structures associated with the underlying reefs. As the large or easily identified features have been drilled, new reefs have been more difficult to find.

     A new exploration model was developed to assist in locating unidentified reefs. This model defines the southern boundary of Silurian reef development based on a previously mapped western shelf margin. Localization of reef development on this shelf is controlled, at least in part, by basement faults. Although published seismic data are limited, those reefs with available seismic control are associated with a sub-Silurian fault. Faults may have controlled subtle topography providing more favorable habitat for reef growth or ground water discharge supplying nutrients necessary for reef formation. This model can be used to identify areas of likely reef occurrence on the shelf.

     Aeromagnetic data, a cost-effective exploration tool, were used to construct a map of basement fabric/faults. Using the fault map and the geometry of the shelf margin, twelve leads were identified. Reprocessing old seismic lines (another cost-effective technique) provided additional clues to the possible location of reefs in the lead areas. These leads were finally evaluated with acquisition of carefully located, short 2-D seismic lines (<10 miles/lead). Two anomalies were identified and drilling confirmed two Silurian reefs.