During the 4 years, 1944 to 1947, sixteen oil test wells were drilled in Illinois to the St. Peter sandstone, of which thirteen penetrated to various formations below the St. Peter, two reaching the basement rocks.

Isopach maps of various formations between the St. Peter sandstone and the basement rocks, supplemented with cross sections, show the influence of the Ozark and Wisconsin positive structural areas. Notable variations in the stratigraphic sequence are the disappearance of the Galesville sandstone and the introduction of an oolitic limestone in the Cambrian strata in central Illinois.

Maps show the structure of the Illinois basin on the top of the St. Peter sandstone and the general configuration of the basement surface so far as known. Isocons on the St. Peter map show the total mineral content of water in the St. Peter sandstone.

Some of the geological factors that should be considered in prospecting for oil in these deep strata are discussed.