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GEOLOGIC NOTE |
1 Department of Geosciences, State University of New York–College at Fredonia, Fredonia, New York 14063; Lash{at}fredonia.edu
2 Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802; engelder{at}geosc.psu.edu
Gary received his B.S. degree from Kutztown State University and his M.S. degree and his Ph.D. from Lehigh University. Before working in the fractured Upper Devonian shales of the western New York state region of the Appalachian basin, he was involved in stratigraphic and structural investigations of thrusted Cambrian–Ordovician deposits of the central Appalachians.Terry received his B.S. degree from Pennsylvania State University, where he joined the faculty after tours at Texas A&M University (Ph.D.) and the Lamont-Doherty Geological Observatory (postdoctoral study). After collaborating with him on brittle fracture and earth stress, his former students have moved on to companies including Anadarko, Atlas Western, British Petroleum, Chevron, Exxon-Mobil, Marathon, Royal Dutch Shell, Schlumberger, Shell U.S.A., and Texaco.
Horizontal bitumen-filled microcracks are common within clay laminae of the finely laminated organic carbon-rich shale in the lower half of the heavily jointed Upper Devonian Dunkirk Shale, western New York state. Such cracks are not found higher in the Dunkirk Shale, where moderate bioturbation resulted in a relatively porous and permeable microfabric. Horizontal microcracks in a hydrocarbon source rock that carries regional vertical joints indicating a horizontal least principal stress owe their presence to material properties of the fractured shale and the magnitude and orientation of the crack-driving stress during kerogen maturation. Three material properties favored the horizontal initiation of microcracks in the Dunkirk Shale: (1) the abundance of flat kerogen grains oriented parallel to layering; (2) a marked strength anisotropy in large part caused by the laminated nature of the rock; and (3) the tight, strongly oriented planar clay-grain fabric produced by gravitational compaction of flocculated clay at shallow-burial depth. The latter was especially important to sustaining elevated pore pressure, the crack-driving stress, which was generated by the conversion of kerogen to bitumen. Poroelastic deformation of the low-permeability laminated shale pressurized by catagenesis, perhaps enhanced by compaction disequilibrium prior to kerogen conversion, elevated the in-situ horizontal stress in excess of the vertical stress, which remained constant during pore-pressure buildup, thereby favoring the propagation of microcracks in the horizontal plane.
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