Abstract

High-quality 3D seismic data in Clearfield County, central Pennsylvania, reveal subsurface structural details of regional flat-top, box-shaped folds, convergent reverse faults, and cross-regional lineaments in the hinge of the Pennsylvanian salient. Cross-stratal variations in structural relief, bed curvature, and seismic facies indicate that the deformational intensity increases from below the Salina (Upper Silurian) to the Onondaga (Middle Devonian) and then decreases from the Onondaga to the Elk (Upper Devonian). The pre-Onondaga isochron thickness sees an increasing trend but the post-Onondaga isochron thickness sees a decreasing trend on the anticlinal axes. The seismically imaged subsurface structures are consistent with outcrop and topographic observations, wireline and formation microimager (FMI) logs, and gravity and magnetic intensity measurements. The Marcellus Shale (Middle Devonian) gas exploration and production data indicate that the operational activity and gas productivity drop significantly in Clearfield County and other counties in the hinge of the Pennsylvanian salient. We interpret that the regional folds and faults and cross-regional lineaments are detachment and wrench faults that were most likely caused by gravitational sliding above the Salina (salt) during the Acadian (Middle Devonian-Early Mississippian). We infer that the co-occurrence of the regional and the cross-regional faults, particularly the cross-regional wrench faults, might have had a significant impact on the Marcellus Shale reservoir integrity and gas productivity in the hinge of the Pennsylvania salient. These observations and interpretations demonstrate the dynamic interplay among tectonic deformation, syntectonic deposition, and shale gas retention, thus entailing significant geologic, economic, and environmental implications in the Appalachian Basin.

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