Integrated Reservoir Characterization of Dolomitized Cambro-Ordovican Beekmantown Carbonates,

Mohawk Valley, New York

By

SMITH, LANGHORNE B., RICHARD D. BRAY, and RICHARD NYAHAY

New York State Museum, Room 3140 CEC, Albany NY, 12230,

GARETH CROSS

876 Natural Science Complex, State University of New York at Buffalo, Buffalo, NY 14260

     Dolomitized Cambro-Ordovician Carbonates of the Beekmantown Group in New York may be an overlooked natural gas resource.  Cores and outcrops from the Mohawk Valley have well-preserved porosity in dolomitized grainstones and dolomite cemented breccias and vugs. Nearby wells have had gas shows from the interval, but there has been little production to date.

     The cyclic Beekmantown carbonates are composed of mottled subtidal wackestones, shallow marine high-energy grainstones and packstones and low-energy, and peritidal algally bound mud-dominated carbonates. All of the carbonates in the Lower Beekmantown Theresa and Little Falls Formations are dolomitized; the overlying Tribes Hill Formation is pervasively dolomitized near normal faults, but is mostly limestone away from the faults. Some of the fine crystalline dolomite of mud-dominated carbonates may be of an early “top-down” origin, but coarse dolomite of the grain-rich carbonates and very coarse saddle dolomite cement in breccias and vugs are clearly of a later “bottom-up” origin.

     Common subaerial karst breccias a have fine micritic matrix and later hydrothermal breccias have a saddle dolomite and calcite matrix. Later hydrothermal “karsting” preferentially affects previously subaerially karsted intervals.

     Reservoir quality is best developed in the dolomitized grain-supported rocks and the hydrothermally fractured intervals.  Coarse dolomite replacing grainstones and packstones have excellent intercrystalline porosity, which is commonly reduced by bitumen. Hydrothermal fracturing created large voids, which are only partially filled by coarse saddle dolomite. The combination of matrix and fracture porosity preserved in Beekmantown carbonates could produce economic rates of natural gas given a good source and trap scenario.