FAULT-CONTROLLED HYDROTHERMAL DOLOMITE RESERVOIRS IN ORDOVICIAN TRENTON BLACK RIVER CARBONATES OF NEW YORK

Fifteen new gas fields have been discovered in New York State over the past several years in fault-controlled hydrothermal dolomites of the Ordovician Trenton and Black River Groups. Fluid inclusions, stable isotopes, trace elements, strontium isotopes, field relations and petrography all support a hydrothermal origin for the dolomite.

The structural lows are visible on seismic and form around strike-slip faults that were activated during the Ordovician Taconic Orogeny, soon after deposition of the Trenton and Black River Groups. These wrench faults were conduits for upward migrating high-pressure, high temperature fluids that leached and dolomitized clean, permeable limestones in the highstand and early transgressive parts of sequences. Argillaceous limestones and shales, which occur in the middle and upper transgressive portions of sequences, impeded further upward fluid migration. The high-pressure fluids induced brecciation and fracturing in the uppermost parts of the clean limestone and lowermost portions of the argillaceous limestones. The breccias and fractures are partially cemented with saddle dolomite, quartz and calcite. Porosity and permeability occur in matrix dolomite, breccias, fractures, and vugs.

Analysis of various hydrothermal dolomite reservoirs and analogous MVT Ore deposits from around the world shows that they occur in a variety of structural settings. These include wrench faults associated with compressional tectonic events (the TBR play as we know it), fault controlled margins, fault intersections, and in carbonates deposited on newly rifted or tectonically active basement rocks. Some dolomitization and brecciation previously attributed to other processes may be fault-controlled hydrothermal in origin.