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In southwestern Virginia, west of New River and east of East Stone Gap, the Lower Devonian Rocky Gap Sandstone and Huntersville Formation have the best potential as hydrocarbon reservoirs. Both units have a large areal extent and a combined thickness locally exceeding 150 ft (45 m).
In outcrops formed by the present erosional cycle, the Rocky Gap Sandstone is a poorly indurated and friable sandstone. Secondary porosity could have formed also by leaching during erosional cycles both before and after deposition of the Oriskany Sandstone. Combined with primary and fracture porosity, the Rocky Gap Sandstone appears to have good reservoir potential.
All of the exposures of the Huntersville Chert are fractured. The thrust faulting of southwestern Virginia could have led to the development of significant fracture permeability and porosity in the subsurface. Like the Huntersville Chert in West Virginia, the unit could become an important gas producer in southwestern Virginia.
Both the Tonoloway Limestone and the Millboro Shale are excellent source beds for hydrocarbons. In western counties, the Onondaga Limestone also smells highly petroliferous after fracturing. Conodont color alteration index (CAI) maps of Silurian through Middle Devonian rocks in the Appalachian basin indicate that the rocks are above the upper limit of thermal maturity for gas.
Unconformities throughout the area have the potential for stratigraphic traps. More detailed seismic surveys of the area can help to define structural and stratigraphic traps that are capped by the Millboro-Chattanooga Shale.
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