The basal sediments of the Devonian clastic wedge in Pennsylvania consist of a series of black, radioactive, organic-rich shales interbedded with non-black shales and siltstones. Three major black shales are present in the subsurface as indicated by stratigraphic cross sections. These units are the Middle Devonian Marcellus shale facies, and the Upper Devonian Rhinestreet, and Dunkirk shale facies. Mapping of these facies indicates that they are distributed in three generally overlapping belts paralleling the regional strike. The Marcellus facies attains its maximum development in the eastern Appalachian Plateau. This unit, the oldest and deepest of the Devonian black shales, has had numerous shows of gas in wells drilled through it. However, gas has not been produced fro it in commercial quantities. The Rhinestreet facies is best developed in the northwest. It is younger and shallower than the Marcellus and has produced commercially from wells in Beaver County. The Dunkirk facies is restricted to the northwestern part of the state where it reaches its greatest accumulation along the margin of Lake Erie. This youngest and shallowest of the major black shale facies has produced commercially since the early 1800s. All three facies have the potential to produce natural gas in commercial quantities. However, owing to drilling economics, the Dunkirk facies appears to have the most immediate potential because of its shallower depths and production history. The Rhinestreet facies is also considered to have immediate potential. The Marcellus facies does not appe r to be an attractive primary target at this time because of its great depth, but it could be a good secondary drilling objective.

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