Origin of Early Overpressure in the Upper Devonian Catskill Delta Complex, Western New York State

Lash, Gary G.¹, David Blood² (1) SUNY Fredonia, Fredonia, NY (2) Chesapeake Appalachia, LLC, Charleston, WV

The Upper Devonian Rhinestreet black shale of the western New York state region of the Appalachian Basin has experienced multiple episodes of overpressure generation manifested by at least two sets of natural hydraulic fractures. These overpressure events were thermal in origin and induced by the generation of hydrocarbons during the Alleghanian orogeny close to or at the Rhinestreet's ~3.1 km maximum burial depth. Analysis of differential gravitational compaction strain of the organic-rich shale around embedded carbonate concretions that formed within a meter or so of the seafloor indicates that the Rhinestreet shale was compacted ~58%. Compaction strain was recalculated to a paleoporosity of 37.8%, a value well in excess of that expected for burial > 3 km. The paleoporosity of the Rhinestreet shale suggests that porosity reduction caused by normal gravitational compaction of the low-permeability carbonaceous sediment was arrested at some depth shy of its maximum burial depth by pore pressure in excess of hydrostatic. The depth at which the Rhinestreet shale became overpressured, the paleo-fluid retention depth, was estimated by use of published normal compaction curves and empirical porosity-depth algorithms to fall between 850 and 1,380 m. Early and relatively shallow overpressuring of the Rhinestreet shale likely originated by disequilibrium compaction induced by a marked increase in sedimentation rate in the latter half of the Famennian stage (Late Devonian) as the Catskill Delta Complex prograded westward across the Appalachian Basin in response to Acadian tectonics. The regional Upper Devonian stratigraphy of western New York state indicates that the onset of overpressure occurred at a depth of ~1,100 m, well in advance of the Rhinestreet shale's entry into the oil window during the Alleghanian orogeny.