Natural gas has been produced commercially in eastern Kentucky (Big Sandy gas field) from the upper Devonian bituminous shale since 1921. More than 3,800 wells have been drilled through this formation, and development continues at the present time. Although gas is produced from other formations, the Devonian shale is responsible for about 80 per cent of the total reserves. The Big Sandy gas field extends through Floyd, Knott, Martin, Pike, Johnson, and Magoffin counties.

The Big Sandy gas field of eastern Kentucky occupies a structural position on the east flank of the Cincinnati arch with regional dip (30-50 feet per mile) and thickening (350-750 feet) east and southeast.

Throughout the field, the Devonian bituminous shale is separated from the Sunbury Mississippian shale by the Berea-Bedford interval of shales, silts, and sandstones.

Gas occurs in the shale in two systems or magnitudes of porosity and permeability: (1) a minor consisting of silty and fine sandy zones in the shale, and (2) a major system provided by joints, fractures, and to a less extent, bedding planes. Accumulation in the latter system is the source of commercial production.

Wells are shot with 80 per cent gelatinated nitroglycerine (occasionally other explosives) throughout the entire shale section from 350 to 750 feet. Such shooting, requiring 3,000-7,000 pounds of explosive, is routine procedure in the field and good wells frequently result when only very light showings or dry holes are encountered while drilling.

Production-rock-pressure decline curves show remarkable flattening after a period of initial drop from flush deliveries. Many wells drilled 15-20 years ago are still good producers. Rock-pressure decline in areas of depletion is nearly uniform and when market demands permitted parts of the field to be shut in for a period of several months, pressures would gradually build up, indicating feeding from the minor to the major permeable systems.

Well spacing at the present time ranges from 2,000 to 3,500 feet, although many examples occur where large wells are offset by dry holes at distances considerably less.