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Two cores from the Lower Devonian Oriskany Sandstone underlying the Allegheny Plateau in south-central Somerset County, Pennsylvania, contain fractures that may either be filled with epigenetic minerals or unfilled. The 1 Sipe core, taken from an unproductive well drilled on the crest of an anticlinal structure, is characterized by numerous discontinuous vertical fractures. The 1 Romesburg core, taken from a productive well drilled on the flank of an anticlinal structure, contains numerous fractures that lie parallel with bedding planes. The walls of these fractures consist of smooth slickensided surfaces.
Quartz and ferroan-calcite crystal filling fractures contain numerous hydrocarbon-bearing fluid inclusions. The distribution of these fluid inclusions within most minerals allowed a determination to be made regarding the relative times of migration of fluid hydrocarbon phases in the subsurface with respect to the paragenetic sequence of mineralization events.
Analysis of fluid inclusions indicates that fractures were opened at 22,000 ft (6,700 m) and remained open throughout an extended period of uplift. Furthermore, inclusions contain hydrocarbon-rich fluids that are comparable to reservoir hydrocarbons in the nearby Shamrock field. This relationship implies that hydrocarbons that currently exist in reservoirs were conducted along fractures that were once open.
Fractures crosscut diagenetic features, indicating that diagenesis, for the most part, preceded fracture events. Although they differ in origin and orientation, fractures characterizing Oriskany strata were healed by a consistent sequence of epigenetic minerals.
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