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Authors

Julian Michaels

Abstract

The Upper Ordovician Utica Formation of the Appalachian Basin is a potential target for natural gas development. This black shale outcrops throughout the Mohawk Valley in Central New York State. The lower Flat Creek Member is characterized by E-W Mode 2 (strike-slip) fractures, bedparallel thrusts, and N-S Mode 1 (tensile) fractures. E-W fractures and dilational jogs host the majority of calcite veins, some with hydrocarbon staining and methane-dominated low-salinity aqueous fluid inclusions. Mode 1 fractures host calcite veins and sand injectite dikes sourced from sand and dolomite sourced from underlying Paleozoic strata and Proterozoic basement. Volcanic ash beds from within the Utica are also a source of material for sand injectites. These features promote the hypothesis that faulting was active at the time of burial and seismic pumping may have allowed vertical migration of fluids from overlying and underlying units. Horizontal veins also promote this hypothesis, as low confining pressures and/or high fluid pressure would allow fracturing and vein precipitation.

The migration of multiple fluids is evidenced by stable isotope data for carbon and oxygen (δ13Ccalcite =-11 to +15 PDB; δ18Ocalcite = -6 to -12 PDB) and fluid inclusion data (Th ≈105-185°C, TMice = -0.5 to -4.5 C), and indicate that vein generation occurred during hydrocarbon maturation, and that vein-forming fluids were mainly derived from within the Flat Creek Member. Multiple events and mixtures of fluids caused the variation in isotope values and suggest mixing of seismically pumped fluids in fault systems during the evolution of the Taconic foreland basin.

The upper Utica (Indian Castle Member) and overlying units show different fracture patterns than the Flat Creek Member and generally lack mineralization, suggesting relatively early burial likely caused the fracturing and fluid expulsion in the Flat Creek Member. Basement-derived hydrothermal fluids may have facilitated hydrocarbon maturation during the early burial and diagenesis in the Flat Creek Member. Later burial and fracturing events in the Utica occurred after deposition of Silurian sandstone strata and permitted up-migration of dry gas into sandstone reservoirs.

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