U.S. Geological Survey, Bulletin 1909, Chapter H, 1993

Petrography and Reservoir Geology of Upper Devonian Shales, Northern Ohio

By Ronald F. Broadhead

The Upper Devonian of northern Ohio consists predominantly of shales that have produced marginal quantities of gas for more than 100 years. This shale sequence was deposited in the northwestern part of the Appalachian basin. It thins depositionally westward from 2,000 ft in northeastern Ohio to less than 500 ft in north-central Ohio.

Stratigraphy reflects and east-to-west facies change. In the east, the Chagrin Shale consists of organic-poor gray shales and minor siltstones; it overlies the lower part of the Huron Member of the Ohio Shale, which consists predominantly of organic-rich black shales. Chagrin shales have abundant laminations of quartz silt and were deposited by distal turbidity currents that traveled west down the turbidite slope of the Catskill delta. Chagrin siltstones are thinly bedded distal turbidites. Shales of the lower Huron Member were deposited in distal deeper parts of the Appalachian basin.

In the west, the Cleveland Member of the Ohio Shale overlies the Three Lick Bed of the Ohio Shale, which overlies the Huron Member of the Ohio Shale. These three units consist of laminated organic-rich black shales and laminated to unlaminated organic-poor gray shales. Shales in the Three Lick Bed are siltier and contain thicker silt laminations than shales in the Huron and Cleveland Members. The Three Lick Bed contains minor thin siltstone beds. The laminated organic-poor shales and the siltstones are turbidite deposits in the distal deeper parts of the Appalachian basin.

Gas shows and production are concentrated in the Chagrin Shale and the Three Lick Bed. The abundant thick silt laminations and siltstones in those stratigraphic units probably act as permeable conduits and reservoirs. Gas shows are least abundant in units that have the highest concentrations of organic carbon - the Huron and the Cleveland Members of the Ohio Shale.