Acadian hinterland-vergent detachment structures in the southwestern Appalachian Plateau: Implications for the Marcellus Shale gas exploration and production

Abstract

Three-dimensional seismic data in southwestern Pennsylvania in the Appalachian Plateau demonstrate that the structural style in the Devonian section is dominated by east-vergent folds and reverse faults, which contrasts with that in the Valley and Ridge Province where west-vergent folds and thrusts dominate. Vertical (cross-stratal) variations in fold curvature and fault throw indicate that the intensity of shortening increases from the Salina (Upper Silurian) to the Onondaga (Middle Devonian) and then decreases from the Onondaga to the Elk (Upper Devonian). Lateral (along-stratal) variations in fold curvature and fault throw indicate that the folds and faults tend to propagate in the cross-strike and along-strike directions. Isochron thickness below the Onondaga increases on the anticlinal, up-thrown side of the faults, whereas isochron thickness above the Onondaga increases to the synclinal, down-thrown side of the faults. In concert with seismic structure and isochron thickness, seismic facies see vertical and lateral variations that are spatially and temporally related to folds and faults. Four years of gas production data from the Middle Devonian Marcellus Shale show that the gas productivity drops near the regional reverse faults, whereas regional drilling patterns from a broader perspective of the Plateau reveal operational gaps near major cross-regional wrench faults. These observations are indicative of the dynamic interplay among hinterland-vergent detachment deformation, syntectonic sedimentation, and shale gas preservation during the Acadian (Middle Devonian–Early Mississippian).