Engelder, Terry¹, Gary Lash², Staci Loewy³
(1) Penn State University, University Park, PA
(2) State University of New York – College at Fredonia, Fredonia, NY
(3) The University of Texas, Austin, TX

ABSTRACT: The Catskill Delta Complex: An Ancient Hydrocarbon System with Black Shale Acting as Source, Reservoir, and Seal during Multiple Phases of Pressure Generation

There is a close association between joint sets and their host black shale within the Devonian Catskill delta complex of the Appalachian Basin. Surface morphology on these joints, mainly repeated cycles of propagation and arrest, is characteristic of natural hydraulic fractures. We hypothesize that thermal maturation was responsible for this pore-pressure-induced fracturing within the Devonian black shale and that there was more than one phase of pressure generation within the delta complex depending on position and timing of burial. The initial pressure-generation mechanism was compaction disequilibrium as recorded using shale density, a chlorite fabric, and the anisotropy of magnetic susceptibility but disequilibrium compaction does not drive pore pressure to the fracture gradient. In the deeper, more proximal portion of the delta complex to the east, black shale reached a thermal maturation, as indicated by EASY%Ro modeling, during burial associated with the late stages of the Acadian Orogeny (i.e., early Mississippian time). During maturation an ENE joint set propagated within the black shale and some bounding beds. Another phase of pressure generation by tectonic compaction during the Alleghanian Orogeny (i.e., late Carboniferous time) led to joint propagation through the delta complex. However, burial by late Alleghanian sedimentation led to thermal maturation in the more distal portions of the delta to the west as indicated by the propagation of another ENE joint set restricted to black shale. In sum, well-developed joints associated with black shale are pre-Alleghanian in the deep, proximal delta and late- to post-Alleghanian in the distal portion.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.