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Exploration and Development Potential of the Benoist Sandstone in Central Illinois


Leetaru, Hannes E.

Illinois State Geological Survey


     The Mississippian Chesterian Age Benoist (Yankeetown) Sandstone produces oil from numerous stratigraphic and structural traps in Illinois.  The study area encompassed over 7000 square miles in south-central Illinois with over 1800 correlated wireline logs used in the mapping.  Eight different individual Benoist reservoirs were studied for their trapping mechanism and heterogeneity. 

     Regional mapping of the formation suggests that the Benoist was deposited as part of a deltaic system.  A map of the Benoist sandstone isolith shows two distinct linear orientations.  The northwest trending sandstone bodies were deposited as distributary mouth bar-channel sandstones.  The complex facies mosaic of channels and interdistributary bay deposits can form complex reservoirs with numerous reservoir compartments.  The northeast oriented sandstone bodies appear to have been deposited as a series of shoreline strandline systems that formed by coastal processes.  The reservoirs within these strandlines can form stratigraphic traps caused by an updip-pinchout of the reservoir sandstone.

     Regional mapping of the Benoist depositional systems helps differentiate the play into areas with high and low degrees of reservoir compartmentalization.  For development purposes, areas with complex reservoir compartmentalization may have potential infill drilling opportunities because of by-passed oil.  The regional mapping also provides a clue on whether an exploration prospect would have a strong structural or stratigraphic component to the trap.