Stratigraphic and Sedimentologic Controls of Gas in Shale--Example from Upper Devonian of Northern Ohio

The gas-bearing Devonian Ohio and Chagrin Shales along Lake Erie have produced marginally commercial volumes of gas for over 100 years. These shales thin depositionally westward from a maximum of 2,000 ft (600 m) at the Pennsylvania-Ohio border to less than 500 ft (180 m) near Sandusky, Ohio. West of Sandusky, they have been truncated by Holocene and preglacial erosion on the Findlay arch. Westward thinning is accompanied by a facies change from gray shale and siltstone in the east to black shale in the west. The Ohio and Chagrin Shales were deposited on a turbidite slope and basin plain and all were derived from the Catskill delta on the east. The Late Devonian epeiric sea was at least 700 ft (215 m) deep and was poorly oxygenated. There are three major lithologies--black bituminous shale, greenish-gray shale, and siltstone. Black bituminous shale is most abundant in the west, whereas gray to greenish-gray shale and siltstone are most abundant in the east, where they constitute 75% of the section. Zones productive of natural gas occur most commonly in the greenish-gray shale and siltstone. These rocks contain abundant thin silt laminae which probably act as permeable conduits and reservoirs for gas. This gas appears to have had its origin in underlying black shales. Sedimentologic study helps explain the origin of both small- and large-scale interbedding of the greenish-gray and black bituminous shale and provides guides for improved gas exploration in the Appalachian basin.