ANATOMY OF STYLES FOR ACADIAN GRAVITATIONAL FAULT DEVELOPMENT AND HYDROCARBON MIGRATION IN THE WESTERN APPALACHIAN FORELAND BASIN OF PENNSYLVANIA AND WEST VIRGINIA

Spectacular imaging in 3D and 2D seismic data reveals details of the complicated geometries and timing of structural elements in the western part of the PA/WV Appalachian Plateau. These structures are consistent with Late Devonian extension/sliding/slumping of the Upper Silurian/Devonian section easterly toward the basin axis in response to gravitational/vertical loading. Prominent back-rotated blocks over locally downslope flow-thickened Silurian units, and hinterland (easterly) verging thrusts all initiated in Late Devonian, based on growth fault geometries. Normal faults are associated with the early slumping. This “early” deformation may be represented by deformation zones observed in Upper Devonian outcrops in western NYS that typically exhibit ductile to brittle transition deformation features.

Near the major faults J1 fracture trajectories rotate into orthogonality with the fault strike, indicating these particular faults were “open” when J1 developed; the J1 fractures would have delivered hydrocarbon to these faults, which in turn may have provided hydrocarbon migration pathways to the Elk and Bradford sands, as well as surface seeps. Subsidence models suggest the faulted area entered the oil window in Late (neo) Acadian, and the gas window in early Alleghanian.

Part of the Acadian structural system in a 3D seismic survey includes 1) an upslope zone-of-removal where the basal decollement cut a linear, 1050 m wide trough ~ 65m downsection into the Silurian Vernon shale, 2) an adjacent down-slope, slide-thickened, Vernon/Lower Salina section that exhibits spectacular box folds, kink folds, and disharmonic folds that indicate multiple decollement and thrust ramps primarily in the Vernon, and 3) steeply dipping faults with down-on-the-east throw that accommodated slump block back-rotation on the Vernon. A Late Devonian age is suggested for the decollement because Upper Devonian Elk and Bradford sands onlap and infill the drape-syncline above the zone-of-removal and basement fault-influenced paleoslope changes occurred then. The thick F-salt section does not display significant thinning or thickening in the zone of removal or in the slide-thickened area of the back-rotated blocks, but is internally deformed and does contribute to core-thickening in thrust fault-associated anticlines.