Since 1976, significant new oil reserves have been discovered in the Ewing area of Jefferson and Franklin Counties, Illinois. One new field and three deeper pool discoveries contain oil reserves in excess of 1.2 million bbl. These oil reserves are in Middle Mississippian limestone at depths of less than 4,000 ft (1,219 m).

Spring Garden field, discovered in November 1977, was drilled on a seismic prospect and has 600 acres (240 ha.) under production from the McClosky Limestone Member of the Ste. Genevieve formation. A study of a core from the field indicates the reservoir was deposited in an oolite bar or beach environment similar to present-day deposition at the Lily Bank oolitic shoal in the Bahamas. The trap at Spring Garden field is formed by the updip pinch-out of porous oolitic limestone into a tight micritic lime mud on a structural nose. The field has primary recoverable reserves of 600,000 bbl of oil.

Based on seismic work and well control, deep tests were drilled in three old fields: Bessie, Ewing East, and

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Taylor Hill. All three fields had previously produced oil from the O'Hara Limestone Member of the Ste. Genevieve formation. New production has been established in the three fields from the deeper Salem and Warsaw limestone section. Oil in all three fields is found in a combination of structural and stratigraphic traps. New reserves in Ewing East and Taylor Hill total about 400,000 bbl of oil. The Bessie field is currently in the development stage but preliminary indications show the reserves should greatly exceed those of Taylor Hill and Ewing East.

Recent geophysical work in the Ewing area, employing a portable mini-hole seismic crew, indicates several more untested features which occur along the same Middle Mississippian depositional trend that created the producing facies in the above fields. The new portable mini-hole seismic operation has made it possible to survey areas previously unaccessible by deep hole or vibroseis crews.

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