The Tar Springs Sandstone along the southwestern flank of the Illinois basin is one of a series of Mississippian Chester clastic formations comprising a major deltaic complex. The Tar Springs was deposited in a slowly subsiding, intracratonic basin by a major river system, the Michigan River system of D. H. Swann.

The Tar Springs deltaic deposits are the principal reservoir in the 9-mile-long, 1-3-mile-wide producing trend formed by the Benton, Orient, and West Frankfort fields in south-central Franklin County. In this north-south oriented trend, the Tar Springs Sandstone is at an average depth of 2,050 feet and lies between two widespread, shallow marine, impermeable limestones. The reservoir is made up of very fine-grained to fine-grained sandstone laid down in overlapping and coalescing fan-shaped buildups and in lenticular bodies. Individual sand buildups are partially separated vertically and laterally from one another by impermeable siltstone and shale. The sandstone was probably deposited by shifting distributary channels. The siltstone and shale are probably quiet water, interdistributa y deposits.

Oil accumulations in the Benton-Orient-West Frankfort trend are primarily structurally controlled; however, stratigraphic variations influence the over-all distribution of hydrocarbons. All the Tar Springs accumulations lie on a broad, north-south trending anticline of moderate closure. Local folding and warping of the anticline combined with lateral and vertical facies change from sandstone to shale determine the size, shape, and position of the oil pools.

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