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Abstract: Facies Variations Of The Cisco Sandstone, Eastern Shelf, Midland Basin, Noland County, Texas

CUYLER, CHRISTOPHER ROBERT, Mobil Exploration and Producing, U.S. Inc.;JAMES 0. JONES, The University of Texas at San Antonio

Upper Pennsylvanian paralic sandstone units are important oil and gas reservoirs within the Eastern Shelf of the Midland Basin. In Nolan County, Texas, such reservoirs have produced approximately 120 million barrels of oil and 500 bcf of natural gas. One of these, the Cisco Sandstone is a prolific oil and gas reservoir.

The Cisco Sandstone exhibits slightly differing environments of deposition, differences in the sedimentologic history, complex stratigraphic relationships, and rapid facies changes laterally.

An examination of the Cisco Sandstone was conducted using electrical log analysis, mud log data, stratigraphic cross sections, isopach maps, sandstone body orientation maps and petrographic data. Sandstone bodies were found to exhibit differing electrical log patterns which represents lateral facies changes. Stratigraphically, Cisco Sandstone bodies occupied slightly differing positions within the stratigraphic column and "overlapped" or "shingled". In places, two different sandstone bodies overlapped and resulted in a thickened Cisco Sandstone unit.

Previous studies have suggested a deltaic depositional environment for the Cisco Sandstone. However, sandstone bars, oriented parallel to strike were mapped and were found to be over six miles long and extended as much as three miles in a dip direction. Therefore, at this location, the Cisco Sandstone is a barrier bar complex.

AAPG Search and Discovery Article #90936©1998 AAPG Southwest Section Meeting, Wichita Falls, Texas