STRATIGRAPHY AND RESERVOIR CHARACTERIZATION OF THE SILURIAN RACINE FORMATION IN HARRISTOWN OIL FIELD, CENTRAL ILLINOIS

The Middle Silurian Racine Formation is the major oil-producing unit in the southeastern flank of the Sangamon Arch, northwest of the Illinois Basin. It has produced over 30 million barrels of oil since early 1920s. The objective of this study was to establish stratigraphic variation and reservoir characteristics of the Racine Formation in Harristown Field, central Illinois. The Racine in the study area consists mainly of limestone, dolomite and silty argillaceous dolomitic limestone/dolomite. Its upper contact with the Upper Devonian New Albany Shale is unconformable and its lower contact with lower Middle Silurian Joliet Formation is sharp. The formation is the only petroleum producer in Harristown Field, which has produced over 600,000 barrels of oil since 1954. It encompasses three productive dolomite horizons that commonly grade laterally and vertically into impermeable limestone facies.

The uppermost reservoir interval is of very limited lateral extent. The middle reservoir is the most extensive body in the upper part of the Racine Formation. It covers an area of over 1600 acres (647 hectares) reaching a maximum thickness of 12 ft. (3.6 m). One well produces from a 20 ft. (6 m) thick interval encountered in the lower part of the formation. This zone is coeval with the prolific patch reef interval in the neighboring fields. However, most wells drilled thus far have not tested this horizon. Reservoir porosity, based on density log and limited number of core analysis data, ranges between 8 and 20 percent. Reported maximum permeability (3 md) and drill stem test results in the area indicate low reservoir permeability. However, majority of wells attained commercial production following a low volume hydraulic fracturing treatment. Developed in a gently sloping ramp setting, the reservoir facies in the Racine constitute the upper part of small-scale shallowing upward cycles. Gradual loss of porosity laterally, gradual upward increase in porosity, and sharp upper contact of reservoir with capping limestone facies suggest sea level fluctuations as the primary control for dolomitization. The Racine reservoirs have never been water flooded; proximity of Harristown Field to a commercial source of CO₂ makes the Racine a potential target for CO₂-EOR, which could result in significant oil recovery and storage of carbon dioxide.