Abstract
In southwest Virginia, the Wildcat Valley Sandstone of Early Devonian age is present in several fault belts of the Valley and Ridge province and underlies the Cumberland Plateau in Wise, Dickenson, and Buchanan Counties. In the southeastern part of its depositional basin, it is predominantly a medium- and coarse-grained marine sandstone about 15 m thick. Northwestward beneath the plateau, it thickens and becomes predominantly limestone.
At the surface, most of the original carbonate cement has been removed, and the sandstone is extremely porous and permeable, in some places even unconsolidated. Some or most of this solution could have taken place in the long interval of Middle Devonian time after cessation of Wildcat Valley sedimentation and before invasion of the Late Devonian seas in which the overlying Chattanooga (black) Shale was deposited.
In the Cumberland Plateau, deep cable-tool wells have penetrated the Wildcat Valley Sandstone below the zone of recent weathering. Laboratory analysis of cuttings from several of these wells indicates that much of the carbonate in the Wildcat Valley Sandstone is impoverished in carbon 13, and hence resembles a freshwater carbonate more than a marine carbonate. Removal of original marine carbonate cement and replacement with freshwater carbonate are indicated. Thus, in this region, pore spaces that formed as a result of subaerial weathering in Middle Devonian time largely seem to have been refilled by freshwater carbonate. In the Valley and Ridge province, however, where the formation is almost entirely sandstone, porosity that formed in Middle Devonian time may have been retained. Hence, the formation could be an excellent reservoir for migrating hydrocarbons.
Three formations contain probable source beds for the generation of gas, of which the black shale of the Chattanooga is thickest and most promising. Both fault and anticlinal traps, or combinations of the two, may be present. However, drilling will be needed to determine whether gas indeed has entered and been trapped in the Wildcat Valley Sandstone in the Valley and Ridge province.
