Stratigraphic studies of Middle and Upper Devonian rocks in West Virginia suggest that thick zones of interbedded siltstone and shale present between the Devonian Shale gas fields in southwestern West Virginia, and the gas fields in Upper Devonian sandstones and siltstones in central and eastern West Virginia may contain important reserves of natural gas. Wells drilled into these zones of interbedded siltstone and shale should be completed in the same way as wells drilled into the black shales; i.e., the entire zone should be fractured, not selected siltstones.
During much of Middle and Late Devonian time, the Appalachian Basin was covered by a shallow epicontinental sea. This sea was bordered on the east by a land area that supplied the large amount of clastic sediments that formed the Catskill clastic wedge in the eastern part of the sea. To the west, north, and south, the sea encroached upon low-lying parts of the interior of the continent. Figure 1 parts of the interior of the continent. Figure 1 shows the location of the Appalachian Basin and some of the structural elements associated with it.
For the purpose of this paper, rocks in the Cat-skill clastic wedge are divided into three facies:
gray shale and sandstone; and
dark-gray shale and siltstone.
Facies 1 contains red, gray, and green shale, siltstone, sand-stone, and conglomerate deposited in terrestrial and near-shore marine environments. This facies forms the Hampshire Formation and the uppermost part of the Greenland Gap Group. The facies is present in the eastern and upper part of the clastic wedge (Fig. 2). The Fifth, Bayard, Gantz, Fifty-foot, and Gordon are some of the sandstones in this facies. They have produced significant amounts of oil and gas in northern produced significant amounts of oil and gas in northern West Virginia.
Facies 2 contains gray shale, sandstone, and some siltstone. These rocks were deposited in a shallow to moderately deep marine environment west of facies 1. This facies roughly corresponds to the "Chemung facies" of older usage. Most of the Greenland Gap Group belong to this facies. The Speechley, Balltown, Warren, Bradford, Riley, and Benson are some of the sandstones and siltstones present in this facies. They have produced significant amounts of gas throughout the produced significant amounts of gas throughout the "Benson trend" in Barbour, Upshur, and adjacent counties in West Virginia. Many of the sandstones and siltstones within this facies appear to be turbidites deposited from turbidity currents that flowed west into the shallow epicontinental sea.
Facies 3 contains dark-gray shale and siltstone deposited in the deepest part of the epicontinental sea, and west of facies 2. The siltstones in this facies appear to be turbidites deposited from turbidity currents that reached the deepest part of the basin. In outcrops in easters-West Virginia, this facies is characterized by the Brallier Formation. Intervals of interbedded siltstone and shale within this facies make up the tight siltstones that may be important sources of natural gas.
Facies 3 intertongues westward into a black shale facies. The black shale facies contains organic-rich black shale, greenish-gray shale, gray shale, and minor amounts of limestone deposited in the shallow to moderately deep marine environment present on the western side of the epicontinental sea. The Ohio Shale and the Chattanooga Shale are examples of the black shale facies. The Devonian Shale gas production of southwestern West Virginia is from these rocks. Figure 2 shows an idealized interpretation of the relations between these facies.
The Upper Devonian rocks in the Appalachian Basin were deposited in a rising sea that transgressed westward onto the interior of the continent.