STRATIGRAPHIC VARIABILITY AND RESERVOIR CHARACTERIZATION OF THE MISSISSIPPIAN CYPRESS SANDSTONE IN THE DEEP AREA OF THE ILLINOIS BASIN

The Upper Mississippian Cypress Sandstone is the most prolific petroleum reservoir in the Illinois Basin. It is more than 60 meters thick and its boundaries with the overlying Beech Creek Limestone and the underlying Ridenhower Formation are conformable. Paleosol horizons, including a persistent paleosol near the top of Cypress record subaerial exposure events. Detailed geological characterization of the Cypress Sandstone in the deep part of the Illinois Basin (Fairfield Basin) using subsurface data indicated compartmentalized sandstone reservoir bodies related to shallow marine, deltaic, and fluvial depositional system. In the study area, Cypress oil production is from porous and compartmentalized sandstone lenses developed in the upper part of the formation. In addition, the upper part of amalgamated sand bodies, the ‘thick Cypress’, may be productive mainly on anticlinal closures. Sandstone bodies consist of fine to medium-grained sublitharenite to quartzarenite attaining a permeability of up to 780 md and an average porosity of 18%. Detailed lithofacies analysis indicated that in the east and southeast of the study area shale and siltstone are the dominant lithology, or the only lithology present. Westward, the Cypress consists of lenticular sandstone bodies capped with shale partings or shale interbeds. The Cypress comprises (1) prodelta mudstone, passing upward into distal to proximal coarsening-upward distributary channel mouth-bar sandstones, (2) mudstone to mature sandstone interpreted as tide dominated offshore bar or shoreface deposit, and (3) major lenticular, multistory fluvial sand bodies that are present at several horizons. The multistory sand bodies (‘thick Cypress’) exhibit an overall northeast-southwest paleochannel trend. The sand bodies display blocky or bell shaped geophysical log profile characteristic of fluvial channel fill deposits. In places, channels cut down several meters into the succession and the base locally reaches the limestone or shale of the underlying Ridenhower Formation. These sand bodies are interpreted as incised-valley fill deposits that eroded underlying deposits during the falling stage of fourth-order sea level cycles. These low sea level events may correlate with the time of paleosol development reported within the Cypress Sandstone.