Circular 596: Stratigraphic Architecture and Reservoir Characterization of the Silurian Racine Formation, Forsyth Oil Field, Macon County, Central Illinois



The Middle Silurian Racine Formation is the major oil-producing unit in a number of oil fields in the Mt. Auburn trend of the Sangamon Arch, central Illinois. This study focuses on geological characterization of the Racine Formation at Forsyth Field to evaluate its potential for recovering the remaining oil from the field.

The field has produced more than 750,000 barrels of oil and has never been waterflooded. The reservoir is a lenticular, porous, low-permeability dolomite body reaching a maximum net thickness of nearly 12 feet and an average porosity of 16%. It was deposited in a shallow marine setting displaying a northeast–southwest trend paralleling the paleoshoreline. Petroleum at Forsyth Field was entrapped by depositional and diagenetic processes as well as by an updip pinch-out of the reservoir against the Sangamon Arch.

The original oil in place at Forsyth is calculated at more than 9 million barrels. Poor reservoir performance and below-average cumulative primary production suggest that the dolomite reservoir interval at Forsyth has poor permeability. The wells previously drilled were stimulated with relatively low-volume hydraulic fracturing. Petroleum recovery from the field, however, could be greatly improved by development of the field in the undrilled areas, larger volume hydraulic fracturing, and through enhanced oil recovery (EOR). Forsyth Field is close to a commercial source of carbon dioxide (CO2) making the Racine reservoir a potential target for CO2-EOR, which could result in significant oil recovery and storage of anthropogenic CO2.

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