Oil and gas are found in western Kentucky chiefly in the Silurian and Devonian limestones and dolomites close below the Chattanooga shale, and in the sandstones of the Chester series. Most of the oil and gas is produced from the flanks of a large structural basin. The strata beneath the Chattanooga shale dip toward the basin a little more steeply than the overlying beds, and this has had an important influence on the production from beneath the shale. Accumulation in western Kentucky is partly in lenticular reservoirs off structure, and partly on anticlines in the more persistent horizons. Much of the oil and gas in the Silurian and Devonian was probably derived from the bituminous Chattanooga shale. The oil and gas in the Chester evidently originated close to the produci g horizons, and the vertical movement was probably less than 200 feet and the horizontal movement generally only a few miles. Some of the porosity in the limestones and dolomites beneath the Chattanooga shale is caused by solution at the unconformity at the base of the shale, but much of it is not related to this unconformity, and was probably developed before the formations were deeply buried. There has evidently been no general artesian circulation in the producing horizons.