The Basin fields of southeastern Illinois are so named because of being located near the center of the Illinois basin. Production has so far been obtained in Wayne, Clay, and Richland counties in a series of fields extending 45 miles in a general north-south direction. It is highly improbable that the producing areas will be found to connect.

Production was discovered in March, and active development was started in May, 1937. The production from the area to July 1, 1939, was 18,159,275 barrels from 692 wells. Daily average production at that time was 57 barrels per well. Large initial productions are common, and decline is rapid for the first few months. Twenty-acre spacing is prevalent.

Production is from oolitic limestone of the Ste. Genevieve formation (McClosky) except in the North Noble field, where it is from the Cypress (Weiler) sandstone, the former being in the upper part of the Lower Mississippian and the latter in the lower part of the Chester series.

The Cypress is a widespread sandstone but locally subject to lensing into beds of sands and shales. It is believed that Chester sandstone production will become more important as development extends northward. The oolitic character of the Ste. Genevieve is persistent in the area; however, porosity occurs in lenses of variable proportions laterally and vertically. Approximately 10 per cent of the holes drilled within the limits of production are dry because of lack of porosity in the limestone. Porosity is more persistent on the structures than in the synclines. Most of the McClosky porosity is believed to be primary in origin.

All except the Flora field are associated with a general north-south anticline, which has a slight southward plunge into the deeper part of the basin. Accumulation has for the most part been on local closures along the axis, but some production has been obtained from noses and terraces on the flanks on the main feature.

Little is known of the pre-Mississippian structural history. The known period of major deformation was post-Mississippian-pre-Pennsylvanian, but subsequent to this there was recurrent folding.

The relation of the contact of the Ste. Genevieve to the overlying Chester is controversial. However, the data do not indicate an angular unconformity at this contact in the deeper part of the basin.

Small production has been obtained from pre-Mississippian beds on the east and west flanks of the basin; however, since no test below the Mississippian has so far been drilled on structure in the area under consideration, the possibilities for deeper production are highly speculative.