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

Brian Slater, Richard Nyahay, and Langhorne B. Smith
New York State Museum, Albany, NY

Field relations, petrography, and geochemical analysis of Lower Paleozoic carbonate outcrops in the Mohawk Valley of New York suggest that they have undergone significant fault-related hydrothermal diagenesis. This study focuses on an outcrop where field mapping, a 3D-Ground penetrating radar survey, petrography and geochemistry all show a clear link between wrench faulting, dolomitization, brecciation and other hydrothermal alteration. It is a scaled analog for Trenton Black River hydrothermal dolomite reservoirs of the eastern United States. The outcrop is in the Lower Ordovician Tribes Hill Formation, which is time-equivalent to the upper Beekmantown, Knox, Arbuckle and Ellenburger Groups to the south and west. This outcrop may help better understand brecciated and dolomitized oil and gas reservoirs in these units as well.

The outcrop has an en echelon fault, fracture and fold pattern and a similar, though not identical en echelon distribution of dolomite. A 3D Ground penetrating radar survey of the quarry floor has helped to map out faults, fractures, anticlines, synclines and the extent of dolomitization. Most of the dolomitization occurs in fault-bounded synclines or “sags”. Dolomite only occurs around the faults and is absent away from faults and fractures. Saddle dolomite-cemented breccias occur at the tips of the dolomitized bodies. There is no evidence of karst at this outcrop, the breccias were produced by faulting and thermobaric fluid flow. Evidence for fault-related hydrothermal alteration includes: fracture-, vug- and pore-filling saddle dolomite, chalcedony, anthraxolite, calcite, sphalerite and pyrite, matrix dolomitization around faults and fractures and extensive brecciation and fracturing.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005