The Salem oil field, Marion County, Illinois, was discovered by The Texas Company on July 1, 1938. During the week of January 7, 1939, the field ranked seventh in the United States on the basis of daily production. Twelve months after discovery the field had produced 20,080,000 barrels of oil.

All producing formations of the Salem field are Mississippian in age and include the Benoist sand, the Aux Vases sand, and the McClosky oolitic limestone. At present the Benoist sand is the principal formation being developed.

A geologic study of the Illinois basin indicates early periods of structural adjustment. Marked zones of influence are thought to have been established by these movements. In certain areas of the basin late Mississippian and early Pennsylvanian structural developments have a close relationship to these early established zones.

Subsurface mapping of the Salem field on the Benoist sand and the McClosky oolitic limestone indicates close similarity in structural definition. No increase of structural relief with depth is apparent on the data supplied by present drilling.

The Salem structure covers an area of approximately 14 square miles with a closure of more than 200 feet as mapped on the Benoist sand.