Oil production in Illinois, Indiana, Kentucky, and Tennessee during 1966 was 89.5 million bbls., a decline of 5.3% from 1965. Gas production, nearly all from Kentucky, increased slightly to a record 84.9 Bcf. Exploratory well completions rose sharply to 1,601, an increase of 48.5%. Much of the increase was due to the drilling of many very shallow holes near the crest of the Cincinnati arch in central Kentucky and central Tennessee. A total of 4,465 wells was drilled or deepened by the oil and gas exploration and production industry in the 4 states.

In Tennessee, 62 tests were drilled in 1966 compared to 20 in 1965, resulting in 1 field discovery, 1 gas development well, 1 oil development well, and 59 dry holes. Three dry holes exceeded the previous depth record for the state.

Exploratory well completions in Kentucky totaled 835, nearly double the 437 reported in 1965. The increase was caused primarily by very shallow drilling in central Kentucky, a result of the 260-ft.-deep production found in 1965 at Sulphur Lick. Although 88 of the exploratory wells were completed as oil or gas producers, no large additional reserves appear to have been discovered. The most significant results were in Camp Breckinridge in western Kentucky, where new fields were discovered and old fields were extended into areas recently opened for drilling.

Indiana showed an increase from 285 to 331 exploratory wells, of which 41 (12.4%) were successful. A Trenton discovery, the first in decades, was made near the northwest corner of the old Lima-Indiana field. No discoveries were made in the Devonian of the Indiana part of the Michigan basin; discoveries in the Mississippian and Pennsylvanian of the Illinois basin were not significant.

In Illinois, the number of exploratory wells increased slightly from 337 to 376, of which 42 (11.2%) were successful. All new production is from zones already known to be productive; none of it appears to be significant. Deepening and new drilling in search of dolomite production from the St. Louis Limestone resulted in 3 additional oil pools in the St. Louis, and in the completion of a considerable number of wells in the shallower Ste. Genevieve and Aux Vases. This activity was in and near Clay City Consolidated and other old fields in the central part of the Illinois basin. More than 20 wells were completed in the Trenton at Hayes. This field, at the north edge of production in the basin, had received little attention since its discovery in 1963.