DETAILED STRUCTURE STUDY OF THE UTICA, SCHENECTADY, AND TRENTON-BLACK RIVER IN CORE 75-NY-2 IN THE MOHAWK VALLEY, NY

The Ordovician section in the Mohawk Valley has served as a model for Taconic plate tectonics, for Trenton-Black River gas production, and now for Utica black shale gas production. We conducted a detailed fracture analysis on a 635m (2084ft) unoriented core (75-NY-2, near Charlton, NY; housed at NY State Museum) that penetrates the succession from the Ordovician Schenectady Formation down to Grenville basement. The detailed analysis of kinematic indicators (e.g., calcite-filled rhombochasms, slickensides, vein offsets, etc.) provide structural guidelines for future gas production from the Utica Shale, and constraints on timing of fluid migration and faulting.

We measured 455 fractures and faults in the core, including shallow dipping fractures (<10⁰), intermediate dipping fractures (10⁰ to 45⁰), steeply dipping fractures (>45⁰), normal and thrust faults,. The Schenectady and Frankfort formations in the upper 152m (500 ft) are highly fractured and exhibit zones of steeply dipping (70-85¢ª), soft-sediment-deformed beds. Brittle structures are concentrated in soft-sediment-deformed zones, and include 94 slickensided/fiber calcite surfaces coated with anthraxolite. Strongly deformed intervals range from a few meters to more than 12m thick with tops at 174m, 186m, 192m, 216m (within Frankfort Fm.), and 278m (within Schenectady Fm.) above the Knox unconformity. These deformed intervals are within the mid Katian (late Caradocian) Diplacanthograptus spiniferus graptolite zones (~452-450Ma). A limited number of fracture intersections include steeply to shallowly dipping (<30⁰) calcite-filled fractures that abut horizontal, calcite filled fractures, and steeply dipping fractures that cut horizontal calcite filled fractures. These intersections are consistent with the sequence of events proposed from our oriented horizontal core in the Trenton-Black River where a set of horizontal veins represent Taconic unloading. The Utica Shale (base at 154m above the Knox unconformity) is characterized by horizontal laminations with no visible soft sediment deformation; the few fractures in the Utica are generally steeply dipping. Stable isotope analyses on samples from veined fractures are consistent with our fracturing sequence: strong fracturing and deformation during Schenectady-Frankfort time.