Typical Stratigraphic Entrapment in Southern Illinois

John Avila

The Chesterian Series occupies the upper part of the Mississippian System in the southern half of Illinois. It extends from the major unconformity at the base of the Pennsylvanian System to the top of the lower Renault Limestone, which is the top of the Valmeyeran Series of middle Mississippian age.

The Chesterian Series consists of alternating limestones, shales, and lenticular sand bodies. The sedimentation in this series is very complex due to the large deltaic systems prograding from north-northeast to south-southwest. The delta built up into a deeper marine environment as the river mouth continued to move laterally and expanded its channels and distributaries.

Distribution of clastic sediments was further affected by periodic and differential subsidence of the basin and regression of the delta system.

The Galatia field, located in Saline County, Illinois, is a typical example of a stratigraphic reservoir found in the Chesterian intervals. Thirty oil wells have been drilled and completed in the Waltersburg (a distributory channel) and in the upper Tar Springs (a channel sand of upper Chester). Two other oil wells are producing from the Aux Vases Sandstone. Primary production from the two Chesterian reservoirs from October 1983 to December 31, 1987, has been 442,400 bbl of oil, and at the end of the year production was 211 BOPD.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.