Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Fractures, Veins, Fluid Migration and Hydrocarbon Generation in the Utica Shale, Northern Appalachian Basin, New York
(1) Geology, Colgate University, Hamilton, NY.
The Upper Ordovician Utica Shale is a widespread hydrocarbon source rock in the Appalachian Basin, and an active (in Quebec) and potential target for shale gas development. Analysis of field exposures in the Mohawk Valley and Tug Hill Plateau regions 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 formed at relatively shallow burial depths. Dilational jogs in E-W Mode 2 fractures host calcite veins with hydrocarbon staining, plus 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 document 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 (Th ≈105-185 C, TMice = -0.5 to -4.5 C) and 
stable
isotope
(δ13Ccalcite= -3 to +15 PDB; δ18Ocalcite = -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.
The types and orientations of fractures in units overlying the basal Utica are markedly different from the Flat Creek Member, suggesting that fracturing and fluid expulsion in the Flat Creek were relatively early burial phenomena. 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.