More than half of the sedimentary rocks of the Illinois basin are of Cambrian and Ordovician ages. Within the area of the basin where younger Paleozoic rocks already have produced about 2.9 billion barrels of oil, these older sediments still remain virtually unexplored. Three holes have tested the Cambrian and Lower Ordovician rocks within the productive region. The Galena (Trenton) near the top of the Ordovician is productive on the western and northeastern flanks of the basin, but has been tested by less than one well per thousand square miles in the deep part of the basin and on the southern flank.

Cambrian and Ordovician rocks probably are more than 6,000 feet thick in much of the deep part of the basin. They thicken and become finer-grained and darker basinward, indicating that a basin structure was present during early Paleozoic time. The rocks appear to be entirely marine; they contain brines whose salinity is more than 10 per cent. Dolomite, sandstone, shale, and limestone are present in that order of abundance.

Indications of hydrocarbons in these beds on the basin flanks have been slight. Sparse seismic data in the basin and drilling in neighboring provinces indicate that structures beneath the pre-Middle Ordovician unconformity are complex and correspond only in part to those in the younger rocks. Drilling depths of 6,000-14,000 feet, which would be required to test these older rocks, are not great by modern standards.

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