Geological Characterization and CO2 EOR Potential of the Silurian Racine Formation in Forsyth Field, Macon County, Illinois

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Forsyth field in Macon County, central Illinois was discovered in 1963 and has produced chiefly from a dolomitized carbonate reservoir in the upper part of the Middle Silurian Racine Formation. Following early development in the 1960s and 70s, a second phase of development took place in the early 1980s when a large number of wells were drilled resulting in a sharp increase in production. A total number of 89 oil wells were drilled in the field, 83 of which were completed. The field has produced close to 750,000 bbl; 40 wells are currently producing at a rate of approximately 20 bbl of oil/day. The main focus of this study was characterization of the Racine reservoir to evaluate its potential for CO2 EOR using the available subsurface data. The Racine Formation in Forsyth Field is characterized by layers of limestone, dolomite, silty argillaceous limestone/dolomite, and shale. The main producing horizon covers an area of over 2,000 acres and is a lenticular dolomite reservoir capped by an impermeable transgressive limestone marker. The reservoir constitutes the upper part of a small scale cycle and occurs within the highstand systems tract of a depositional sequence, suggesting sea level fluctuations as the primary control for early dolomitization. The Racine reservoir displays lateral thickness variation and reaches a maximum thickness of nearly 12 feet. Examination of porosity logs from a number of wells indicate maximum dolomite porosity of 24% and average porosity of 16%. Low initial production (mainly under 100 barrels of oil) and below average cumulative primary production (less than 10,000 barrels per well) along with wireline log data suggest a very low permeability for the Forsyth reservoir. The reservoir has produced less than 10% of its original oil in place (OOIP), and has never been waterflooded. OOIP of over 8,000 Mbbl and the proximity of the field to the Archer Daniels Midland (ADM) facility in Decatur, which could provide commercial CO2, make the Racine reservoir a potential target for CO2-EOR.
Original languageEnglish (US)
StatePublished - 2015


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