ABSTRACT: Isopach Mapping to Identify Oolite Reservoirs in the Ste. Genevieve Limestone (Mississippian)

KEITH, BRIAN D., and CHARLES W. ZUPPANN, Indiana Geological Survey, Bloomington, IN

Contemporary prospecting in the Illinois basin focuses on the search for stratigraphic traps. Isopach mapping techniques are useful when looking for productive oolite zones within the Ste. Genevieve Limestone (Valmeyeran, Mississippian). The Ste. Genevieve may contain multiple oolite lenses at any given location, and facies mapping is often impossible using only geophysical logs and limited available sample information.

Detailed studies of three Ste. Genevieve oil fields in Indiana show that isopach mapping of various stratigraphic intervals can successfully locate oolite reservoirs. At Folsomville field in Warrick County, regional thinning of the interval from the top of the stratigraphically younger Renault Limestone (Mississippian) to the informal x-marker in the older St. Louis Limestone (Mississippian) reflects a paleostructure that localized the distribution of productive Ste. Genevieve oolite zones. At Vollmer field in Knox County, thinning of the interval from the top of the Renault to the top of the productive oolite reveals the location of the oolite body. At Monroe City field in Knox County, a shale interval within the Ste. Genevieve thins over immediately underlying oolite reservoirs. The thickness of this shale is related to the depositional topography immediately following oolite deposition.

The decision of which isopach intervals should be mapped depends upon specific geological circumstances and may require appreciable trial and error. Careful scrutiny of variations in stratigraphic thickness observed at known fields may identify the best mapping intervals to try first. Isopach mapping can be used as both an exploration technique and as a development tool for defining Ste. Genevieve oolite reservoirs.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)