Outcrop Analog For Lower Paleozoic Hydrothermal Dolomite Reservoirs, Mohawk Valley, New York

Nyahay, Richard, Slater, Brian, and Smith, Langhorne B., Reservoir Characterization Group, New York State Museum, 3140 Cultural Education Center, Albany, NY 12230, [email protected]

This study focuses on an outcrop in the Mohawk Valley, New York where field mapping, a 3D-Ground penetrating radar survey, petrography, geochemistry, and cores all show a clear link between wrench faulting, brecciation, and hydrothermal alteration in the Lower Ordovician Tribes Hill Formation (Beekmantown Group). Fault-related hydrothermal alteration features include: fracture-, vug- and pore-filling saddle dolomite, chalcedony, anthraxolite, calcite, sphalerite and pyrite, matrix dolomitization around faults and fractures. It is a scaled analog for Trenton Black River hydrothermal dolomite reservoirs of the eastern United States. This outcrop may help better understand brecciated and dolomitized oil and gas reservoirs in time-equivalent Beekmantown/Knox and Ellenburger Group carbonates as well.

The outcrop has an left-stepping en echelon fault pattern and a similar, though not identical en echelon distribution of elongate dolomite bodies. A 3D Ground penetrating radar survey of the quarry floor has helped to map out faults, fractures, folds and the extent of dolomitization. Most of the dolomitization occurs in normal fault-bounded synclines or “sags”. Dolomite only occurs around the faults and is absent away from faults and fractures. New core data confirms this. The dolomite structures begin and end at saddle dolomite-cemented breccia-tips. The breccias were produced by faulting and thermobaric fluid flow.  We are beginning to map classic strike-slip features: forced folds, Riedel and P-shears, scissor faults, inline grabens, and oblique-slip fault segments. The feature may be part of a larger strike-slip fault system in the Mohawk Valley.