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AAPG Bulletin, V.
Reinterpreted Oriskany structure at the North Summit field, Chestnut Ridge anticline, Pennsylvania
Robert C. Shumaker1
1Department of Geology and Geography, West Virginia University, P.O. Box 6300, Morgantown, West Virginia, 26506-6300; email: [email protected]
Robert C. Shumaker received an A.B. degree (1953) from Brown University and an M.S. degree and a Ph.D. (1960) in geology from Cornell University. He worked for the Humble Oil and Refining and Exxon companies in their research and exploration departments from 1960 to 1972. He joined the faculty in the Department of Geology and Geography at West Virginia University in 1972, where he was a professor and where he served as associate chairperson of the department from 1984 to 1995. He retired in 1996 and currently is a professor emeritus at West Virginia University.
I thank a number of people and organizations that were important to my research. These include the Consolidated Natural Gas Company (now Dominion Resources Inc.) for releasing subsurface data from the North Summit field; William Staub, formally with Consolidated Natural Gas (CNG), who provided most of the well logs, and Andrea Horton and Richard Cross with CNG, who gave continued support for the research; Goguin Zhou, a former student at West Virginia University, who made a preliminary compilation, evaluation, and interpretation of the log data and who completed analysis of Formation MicroScanner logs for the field; Debbie Benson, who drafted the illustrations; T. H. Wilson, who evaluated the seismic data briefly and who, in turn, gave it to Frederick Schule to model a preliminary interpretation of the North Summit structure at the Tully, Oriskany, and Salina levels; Columbia Natural Resources for release of the seismic line taken from Flaherty (1996); and the reviewers of this article who materially improved its technical and literary quality.
A widely accepted structural model for folds in the outer central Appalachian foreland is partially based on the geologic structure of the North Summit field. The model includes a simple surface anti cline that is detached in Silurian Salina Group salts and cored by imbricated Devonian Tully-Helderberg rocks thrust inward toward a depressed axial low.
New data from wells at North Summit show that the core of the Chestnut Ridge anticline is not filled with imbricated reservoir rocks but that the reservoir is deformed into a series of faulted folds. Gas was trapped by a combination of closure and sealing faults. Space problems within collapsing synclines above competent reservoir rocks, the Huntersville-Helderberg lithostructural unit, are resolved by distortion and evacuation of overlying, incompetent Hamilton rocks. Huntersville-Helderberg rocks deformed into a variety of structural shapes, not solely the imbricated model that traditionally has been applied to Plateau folds of the central Appalachian foreland.
Disparity in relief between pre- and post-Salina rocks indicates that primary detachment occurs in evaporite beds of the Salina Group. Erratic dips in presalt units and unresolved differences in structural relief between synclines flanking the Chestnut Ridge anticline, however, suggest that the basal detachment lies within the Martinsburg-Reedsville shales under the Allegheny Mountains. Regional decollement in Martinsburg shales ends in a triangle zone at the Intra-Plateau Front, at the Chestnut Ridge anticline, whereas the Salina decollement continues westward under the Pittsburgh Plateau.
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