EXPOSING THE ROOTS OF HYDROTHERMAL DOLOMITE HYDROCARBON RESERVOIRS IN THE ADIRONDACK LOWLANDS: FLUID ALTERATION AND FAULT SYSTEMS

Hydrothermal Dolomite (HTD) reservoir systems remain important exploration and development targets in the Appalachian Basin and elsewhere. HTD reservoirs are characterized by secondary porosity and permeability resulting from dolomitization of carbonates, often along fracture and fault systems. In the northern Appalachian Basin, HTD reservoirs are found in upper Cambrian through middle Ordovician rocks at depths exceeding 3000 meters. Productive hydrocarbon fields are often localized along linear belts that suggest structural control by ‘wrench’ fault systems that may parallel older structures in underlying complexly deformed basement. Understanding of the geometry of the fault systems that lead to fluid flow, reservoir quality related to fracturing and dolomitization, and the timing of reservoir development is limited by the rarity of surface exposures and lack of well-documented modern analogues.

The up-dip edge of Appalachian Basin Paleozoic strata is well-displayed in the St. Lawrence Lowlands and adjacent Adirondack Lowlands Provinces of northern New York State. Recent glaciation has stripped away the thin veneer of Cambrian Potsdam Sandstone and exposed the pre-Paleozoic basement topography. When viewed using region-scale DEMs, bedrock topography defines a basement structural fabric dominated by strike-slip shear zones. In areas with thin, discontinuous caps of Paleozoic sandstone underlain by Proterozoic marble, fluid-related alteration of basement marble is widespread. Features include hydrothermal karst, dolomitization and mineralization, shearing and brecciation, and fluidization and injection of sand into basement. Fluid inclusion and stable isotope data indicate hydrothermal fluids in the range of 140-180oC with salinities 15-22% (NaCl equiv.) that were ‘evolved’ with del 18OSMOW in the range +8 to +11 o/oo. These exposures provide an opportunity to examine the scale and style of structural deformation and hydrothermal alteration in the root zone of these economically important systems.