Abstract:
Seven of 28 wells penetrating the Trenton Limestone in West Virginia have reported shows of natural gas, enough to continue industry’s interest in this potential reservoir. The formation consists of thin limestones inter-bedded with shales and bentonites. Sediments were deposited on a ramp that sloped eastward from a shallow platform in northwest Ohio into the foreland basin of Virginia, and the unit forms a wedge-shaped mass that thickens into the basin. Limestones of the upper ramp were deposited on sand shoals (skeletal grainstones) and restricted flats (lime mudstones), which passed downslope into skeletal patches of a deep, muddy environment (packstones and wackestones). Rapid downwarping of the carbonate ramp produced a major transgression, and deeper limestone facies migrated westward during time. Bentonites spread across the region from distant volcanic islands. In West Virginia, a lower bentonite package is present in the Black River Limestone to the southwest, whereas an upper package occurs in the Trenton to the east and north. This distribution indicates that the site of volcanic activity shifted during time. A growing orogenic source area shed terrigenous sediment into the basin and onto the Trenton ramp. Initially, these muddy influxes came from the north and from the south, but terrigenous mud eventually swamped the entire ramp and carbonate sedimentation then came to an end. Trenton Limestones have only minor intergranular porosity because of abundant mud matrix and cementation. The only significant contribution to porosity is in fractures. Thus, wells with Trenton shows are characterized by high initial potential that decreases rapidly.
