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Trenton-Black River: Introduction and Background

 

Discrete dolostone bodies occur in Upper and Middle Ordovician limestone strata in central Kentucky. These dolostone bodies were mapped by the cooperative U.S. Geological Survey - Kentucky Geological Survey geologic mapping program in the 1970's. The mapping program documented 33 occurrences of isolated dolostone in central Kentucky limestones (Black and others, 1981 (12MB PDF file); Black and Haney, 1975 (26MB PDF file). These isolated dolostone bodies are spatially related to mapped faults, and have been interpreted as products of hydrothermal fluids that moved along fault conduits, replacing adjacent limestone (Black and others, 1981). Veins of barite and sphalerite are also associated with these faults, some of which were commercially mined in the early 1900's (Anderson and others, 1982). The study area lies in central Kentucky, on the eastern flank of the Cincinnati Arch. Bedrock in the study area consists of Middle Ordovician limestones of the High Bridge Group and Lexington Limestone, and several Upper Ordovician limestone formations. The High Bridge Group and Lexington Limestone are equivalent to the Black River Group and Trenton Limestone elsewhere in the Appalachian Basin. Faults exposed at the surface in the study area include the Lexington Fault System, Kentucky River Fault System, and the Irvine-Paint Creek Fault System. These faults extend to basement, and were formed in the Cambrian as part of the Rome Trough rift basin. The faults were reactivated numerous times since the Cambrian, and controlled the emplacement of dolomite in some areas. There is new economic interest in fault-controlled dolomitization as the result of significant natural gas discoveries in the Ordovician Trenton and Black River Formations in central New York and West Virginia. Gas production from these formations is attributed to hydrothermal dolomite localized along faults in New York (Shirley, 2001), and to fractured limestones in West Virginia. In New York, gas reservoirs in the Trenton/Black River play consist of narrow, linear dolostone bodies with intercrystalline to vuggy and cavernous porosity. These dolostones are commonly associated with structural grabens, and solution collapse features have been noted in core and borehole image logs. Well-developed porosity and fracturing of the dolostone result in high production rates.