Cypress Sandstone Reservoir Characterization Across the Clay City Anticline, Richland and Clay Counties, Illinois

Abstract

The Upper Mississippian Cypress Sandstone is the most prolific siliciclastic unit in the Illinois Basin and commonly produces from southwest trending lenticular reservoirs. The boundaries of Cypress with the overlying Beech Creek Limestone and the underlying Ridenhower Formation are conformable. However, in places thick Cypress sandstone cut into Ridenhower that commonly consists of shale and discontinuous sandstone and limestone beds. A persistent paleosol horizon is present near the top of Cypress recording a subaerial unconformity. Detailed well to well correlation and lithofacies analysis in the deeper part of the basin (Fairfield Basin) have indicated that the Cypress (over 60 meters thick) commonly consists of lenticular sandstone bodies interbedded with shale. In places, shale and siltstone are the dominant lithology or the only lithology present.

In Richland and Clay Counties, along the Clay City Anticline, Cypress oil production is from the upper part of amalgamated thick sand bodies (mainly on anticlinal closures) and from porous lenticular and compartmentalized sandstones developed in the upper part of the formation. Sandstone bodies consist of fine to medium grained sublitharenite to quartzarenite attaining an average porosity of over 18 percent. The Cypress succession comprises: (1) a mainly deltaic unit in which prodelta mudstone passing upward into distal to proximal coarsening-upward distributary channel mouth-bar sandstones or blocky to fining-upward sandstone lenses interpreted as distributary channel fill deposit (2) a shallow marine succession of mudstone to mature sandstone interpreted as offshore bar or shoreface deposit and (3) major lenticular, multistory sand bodies at several horizons with shale partings or shale interbeds displaying blocky or bell shaped profile. These sand bodies cut down several meters into the succession and the lowermost horizon may reach the limestone or shale of the underlying Ridenhower Formation. They may correlate with the paleosol horizons reported within the Cypress and are interpreted as incised-valley fills that cut into the previous deposits during fourth-order sea level falls.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016