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Seismic and stratigraphic models resulted from a successful 5-yr seismic and exploration drilling program focusing on the Devonian Oriskany Sandstone in north-central West Virginia. Disciplines applied to the geologic evaluation of the area include analyses of Landsat, synthetic-aperture radar imagery, outcrop, seismic, geophysical well logs, engineering, and reservoir fluid data.
Prospectiveness of the Oriskany, a key producer of natural gas in the northern Appalachian basin, is dependent on three basic factors: (1) the stratigraphic delineation of trends of matrix porosity lenses developed within the main body of the sandstone, (2) seismic definition of anticlinal and related fault closures, and (3) drilling of fracture-prone structural positions to achieve fracture-enhanced reservoirs.
Stratigraphically, the Oriskany Sandstone (Ulsterian Series) is a tight, regionally extensive, transgressive, marine sandstone. The unit ranges in thickness from zero at its pinchout edges on the north, south, and west, to a maximum thickness of 250 ft (76 m) within the study area. Depths vary structurally from 7,000 to 10,000 ft (2,100 to 3,050 m).
Structurally, the area of interest falls between the intraplateau structural front and the Allegheny Front, wherein seismic structural targets are predominantly thrusted cores of concentric folds and imbricately thrusted blocks. Listric faults originate from decollement zones below the Oriskany. Major shear zones are most effectively identified by satellite imagery.
Local recoverable gas reserves from the Oriskany range from 1.5 to 5 bcf per well, with an attained average of 2.25 bcf per well. Wells penetrating fracture-enhanced Oriskany exhibit high rates of natural flow. Reservoir engineering data are effective in defining the extent of fracture envelopes. Tighter sections require acidizing and hydraulic fracturing.
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