FRACTURES, VEINS AND FLUID MIGRATION GENERATION IN THE UTICA SHALE AND IMPLICATIONS FOR TRENTON-BLACK RIVER HYDROTHERMAL DOLOMITE RESERVOIRS, NORTHERN APPALACHIAN BASIN, NEW YORK

The Upper Ordovician Utica Shale is a widespread hydrocarbon source rock in the Appalachian Basin. Analysis of field exposures in the Mohawk Valley of New York State demonstrate that the lower interval of the Utica (high TOC Flat Creek Member) is characterized by E-W Mode 2 (strike-slip) fractures, bed-parallel thrusts and N-S Mode 1 (tensile) fractures. Dilational jogs in Mode 2 fractures host calcite veins with hydrocarbon stains, and methane-dominated and low-salinity aqueous fluid inclusions. Mode 1 fractures host calcite veins, and sand injectite dikes sourced from volcanic ash within the Utica, and sand and dolomite sourced from underlying Paleozoic strata and faulted Proterozoic basement. Horizontal calcite veins in the Flat Creek Member document high fluid pressures and/or relatively low confining pressure during vein formation. These features indicate active seismic pumping of fluid and sediment slurry during fracturing, and are linked to the diagenetic fluid systems that gave rise to hydrothermal dolomite and quartz/bitumen mineralization in units that underlie the Utica. Fluid inclusion (T_h ≈105-185°C, T_Mice = -0.5 to -4.5 C) and stable isotope (δ¹³C_calcite =+1 to +15 PDB; δ¹⁸O_calcite = -9 to -11 PDB) data indicate that vein generation occurred during hydrocarbon maturation and that vein-forming fluids were mainly derived from within the Flat Creek Member. A mixture of Utica Shale-derived fluids and more saline brines sourced from basement fracture systems formed the hydrothermal waters that drove development of hydrothermal dolomite reservoirs in the Trenton-Black River interval that immediately underlies the Utica. Vertical flux and mixing of fluids was driven by active seismic pumping in fault systems during evolution of the Taconic foreland basin.

The types and orientations of fractures in the upper Utica and overlying units are markedly different from the Flat Creek Member, suggesting that fracturing and fluid expulsion in the Flat Creek Member were relatively early burial phenomena. Hydrocarbon maturation during early burial may have been facilitated by basement-derived hydrothermal fluids. Later burial and fracturing of the Utica occurred after deposition of overlying Silurian strata and permitted up-migration of dry gas into Silurian sandstone reservoirs.