As production from younger strata has continued to decline, exploratory activity in the 4 east-central states (Illinois, Indiana, Kentucky, and Tennessee) has turned to the Lower Ordovician and Cambrian. Deep Knox tests and basement tests are being drilled at an increasing rate throughout the area. A discovery in the Cambrian Tomstown Dolomite at 7,598 ft in Boyd County, eastern Kentucky, provides the deepest as well as the oldest production in the 4-state region. New drilling has shown that the so-called "deep Trenton" production in part of the old northeastern Indiana field really came from an erosional remnant of Knox Dolomite (Cambrian-Lower Ordovician) capped by tight Middle Ordovician beds. Because trapping conditions parallel those in the Ohio Trempealeau productiv areas, the prospective area for such Knox reservoirs is now extended to much of eastern Indiana. Free oil has been found several hundred feet within the Knox in an Illinois test which, although not commercial, shows that probably indigenous Lower Ordovician oil can occur deep in the Illinois basin.

Oil production in the 4 states was 86.7 million bbl, down more than 4% from 1966. Gas production, nearly all from Kentucky, increased slightly to a record high of 89.4 Bcf. The total number of wells drilled for oil and gas was 3,294, decreased 20% from 1966. All classes of well completions shared in the decline. The 160 successful exploratory tests failed to add significantly to actual or anticipated reserves.

Regulations concerning permits, filing, and spacing will go into effect in Tennessee in 1968.