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Counter-Regional Detachment Structures in Southwestern Pennsylvania, Central Appalachian Basin: Implications for Marcellus Shale Gas Exploration and Production

Abstract

Recently acquired 3D seismic data in southwestern Pennsylvania to the west of the central Appalachian Basin demonstrate that the structures in the upper Paleozoic (above Silurian Salina salt) section are dominated by the east-vergent folds and thrusts. Lateral variation in bed curvature indicates that the magnitude of local contraction increases from the west to the east. Vertical variation in bed curvature indicates that the magnitude of overall contraction increases from the lower Paleozoic (pre-salt) section to the upper Paleozoic (post-salt) section. These observations suggest that the tectonic transport direction of the upper Paleozoic section was dominantly toward the southeast at approximately 110°, with the Salina salt serving as the primary detachment horizon and the Marcellus shale as the secondary one. The geometry and kinematics of the counter-regional folds and thrusts contrast with those of the regional west-vergent folds and thrusts that have been commonly observed to the east of the central Appalachian Basin. Preliminary observations of the Marcellus shale gas production, from both local and regional perspectives, hint at a possible relationship of the gas productivity to the occurrence of natural faults and fractures, particularly of the cross-strike faults and fractures in the Appalachian basin. The relatively high gas productivity and the overall quiescence of major cross-strike discontinuities in the study area contrast with the relatively low gas productivity and overall activity of major cross-strike discontinuities in other areas. We interpret that natural faults and fractures could cause the sustained release of the “trapped” gas in shale reservoirs, and that natural faults and fractures can either enhance or reduce the gas productivity depending on their scale, orientation, and mode. We conclude that it is critically important to delineate natural faults and fractures with distinct scale, orientation, and mode in gas shale reservoirs.