MULTIPHASE GENERATION OF HYDROCARBONS IN THE DEVONIAN CATSKILL DELTA COMPLEX, WESTERN NEW YORK APPALACHIAN BASIN

There is a strong positive correlation between the density of joints, interpreted to be hydraulic fractures that formed by cyclic growth and arrest, and organic carbon content of Upper Devonian shales of the Catskill Delta Complex of the Appalachian Basin. Although compaction disequilibrium may have affected much of the shale sequence during its burial and compaction, we hypothesize that the additional overpressure required to elevate formation pressures within black shale to the local fracture gradient was largely the result of hydrocarbon generation. We further propose that the timing of hydrocarbon generation was diachronous across the basin and a function of the timing and depth of burial. Thermal (maturation) modeling of the Upper Devonian shale sequence across the delta complex suggests that hydrocarbon generation began near the end of the Acadian Orogeny in the Finger Lakes region (Early Mississippian), resulting in the propagation of ENE-trending joints in black shale. To the west, in the Lake Erie region, Upper Devonian shales had not yet entered the oil window and although compaction disequilibrium may have overpressured the shale sequence, it failed to generate the additional pressures required to hydraulically fracture the rock. However, further burial carried the organic-rich shales in this area of the delta complex into the oil window by the middle or end of the Alleghanian Orogeny (Late Carboniferous) leading to the preferential generation of NW-trending joints in black shale. To the east, this phase of pressure generation was augmented by tectonic compaction of the entire Devonian sediment pile, which enabled hydraulic fractures to propagate through much of the shale sequence regardless of carbon content. ENE joints carried preferentially by black shale units in the distal region of the delta complex appear to have formed during post-Alleghanian uplift of the Appalachian Plateau.