STRATIGRAPHIC ARCHITECTURE AND RESERVOIR CHARACTERIZATION OF THE SILURIAN RACINE FORMATION, FORSYTH FIELD, CENTRAL ILLINOIS

The Silurian (Wenlock-Ludlow) Racine Formation is the major oil-producing unit in the southeastern margin of the Sangamon Arch, central Illinois. The Racine succession comprises two third-order depositional sequences consisting chiefly of interbedded limestone, dolomite, and silty argillaceous limestone/dolomite. The focus of this study is stratigraphic analysis and reservoir characterization of the Racine at Forsyth Field, Macon County using the available subsurface data. Discovered in 1963, Forsyth Field has produced over 700,000 barrels of oil from dolomite reservoirs in the upper part of the Racine Formation.

Detailed stratigraphic analysis and well-to-well correlation of the Racine at Forsyth Field indicated development of dolomite reservoirs in the highstand package of the upper Racine sequence. The reservoirs constitute the upper part of small-scale shallowing upward cycles. They are generally lenticular bodies encased in impermeable limestone. The main producing horizon covers an area of roughly 2,000 acres (809 ha) and is a lenticular dolomite reservoir capped by an impermeable transgressive limestone marker. It reaches a maximum net thickness of nearly 12 feet (3.66 m) and is composed of two juxtaposed lenses trending in a northeast-southwest direction paralleling the trend of the Sangamon Arch. Neither core nor core analysis data are available; however, based on wireline logs, reservoir porosity ranges between 8 and 18%. Low initial production (mainly under 100 barrels of oil per day) and low cumulative production (nearly 10,000 barrels per well) suggest a low permeability for the Forsyth reservoir. The reservoir has an abrupt contact with the capping limestone facies, which suggest sea level fluctuation as the primary control for early dolomitization. A combination of depositional and diagenetic processes and updip pinch out of the reservoir against the Sangamon Arch was responsible for Petroleum entrapment. The Racine at Forsyth has produced less than 10% of its original oil in place and has never been waterflooded. Original oil in place of nearly 8 million barrels and the proximity of the field to the Archer Daniels Midland Company facilities, which could provide commercial CO_2, make the Racine reservoir a potential target for CO₂-EOR.