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The Lower Devonian Oriskany (Ridgeley) Sandstone is an important deep exploration target in the Appalachian basin. In outcrop, the Oriskany is typically a calcareous quartzarenite with few discernible lithologic variations. Petrographic examination of two drill cores of the Oriskany-Needmore Shale interval from south-central Somerset County indicates that identifiable lithofacies of the upper Oriskany Sandstone exist in the subsurface. Oriskany lithofacies are defined as silica-cemented quartzarenite, sandy biosparite, calcareous-cemented quartz wacke, and coquinoid calcareous-cemented quartzarenite. These lithofacies are interpreted as depositional features of a shallow marine sand-bar complex, corresponding to central-bar, bar-margin, interbar, and storm-generated sheet sand (tempestite) depositional units. Paleoenvironmental interpretations are supported by diagnostic trace-fossil assemblages and coarsening-upward grain size trends indicative of vertically stacked marine bars. Correlation of gamma-ray logs from Oriskany wells across Somerset County suggests that these marine bars are laterally discontinuous and may change abruptly in thickness over relatively short distances. The basinal Needmore Shale overlies the Oriskany in this area, indicating a deepening of the depositional setting after Oriskany deposition.
Although vertically oriented fractures were observed in all the Oriskany lithofacies and Needmore Shale, most of the fractures are healed by secondary calcite. Fracture porosity occurs primarily in the silica-cemented quartzarenite lithofacies, or central-bar paleoenvironmental unit. The quartzarenite lithofacies is recognizable on gamma-ray logs by its blocky, low API unit signature at the top of sequences that exhibit a downward increase in shale content. As the presence of fracture porosity is important in Oriskany natural gas production in this region, central-bar units are primary targets for exploration.
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