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The Upper Silurian Williamsport Sandstone at the type section in Grant County, West Virginia, is typically composed of very fine-grained green and brown sandstone with some siltstone and shale. A local carbonate member, the Cedar Cliff Limestone Member, is present in the middle of the formation in nearby outcrops in western Maryland. On the north in Pennsylvania, the Williamsport can be traced into the Moyer Ridge Sandstone Member of the Bloomsburg Formation. Eastward, in the eastern panhandle of West Virginia and Maryland, the Williamsport pinches out in the middle of the nonmarine red Bloomsburg facies. Farther south the Williamsport undergoes a facies change into clean well-sorted mature sandstone. The subsurface continuation of this sandstone extends to the Ohio borde , where it has been found to be an important reservoir for natural gas.
Sandstones of the Williamsport in the subsurface are very fine to fine grained, subrounded to rounded, well sorted, and texturally mature and supermature. In general, sandstone is most abundant in the upper half of the unit, whereas carbonates become interbedded with sandstone layers in the lower half. Syntaxial quartz overgrowths serve as the primary cement in the upper part of the formation, but dolomite becomes important lower in the section. Gypsum, anhydrite, and barite are minor cements. Intergranular porosity is greatest near the top of the formation.
The immature sandstones and siltstones of the Williamsport in northeastern West Virginia probably were deposited on low-energy mud flats in front of the Bloomsburg delta. Sediments were supplied by rivers from source lands farther east in Pennsylvania. The limestones and hematitic beds of the Cedar Cliff Member are interpreted as having been deposited in a lagoon associated with this tidal flat. Farther south, the cleaner, coarser, more mature sandstones were deposited in a barrier island-coastal complex. Regression of the shoreline spread a blanket of sand over the underlying subtidal to intertidal McKenzie Formation. As the shoreline and barrier island complex regressed westward the lagoonal sediments of the Wills Creek Formation were superposed on the clean sand of the Williamsport
Gas accumulation in the Williamsport is due to a combination of stratigraphic and structural trapping. Salt water is present downdip in all fields, and updip porosity and permeability decrease where the sandstone thins westward. Gas flows in this formation are the greatest recorded in the Appalachian basin, making the Williamsport the most important deep target for drillers in West Virginia. Future exploration should examine the possibility that combination stratigraphic and structural traps exist near the eastern edge of the sand body in central West Virginia and near the southwestern sandstone pinchout in south-central West Virginia.
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