HYDROTHERMAL DOLOMITES IN CENTRAL KENTUCKY: POSSIBLE ANALOGS FOR MANY HYDROCARBON RESERVOIRS THROUGHOUT THE EASTERN MIDCONTINENT

The occurrence of discrete dolomite bodies in Upper and Middle Ordovician limestone strata of central Kentucky has been well documented by the cooperative U.S. Geological Survey–Kentucky Geological Survey geologic mapping program in the 1970’s. These isolated dolomite bodies, which occur as both coarsely crystalline dolomite and saddle-shaped dolomite cement and vug linings, are spatially related to mapped faults, and have been interpreted as products of hydrothermal fluids that moved along fault conduits, replacing adjacent limestone. Trace occurrences of Mississippi Valley Type (MVT) ores and small hydrocarbon deposits have been identified in outcrops of the hydrothermal dolomite in central Kentucky.

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 possibly in West Virginia. Gas production from these formations is attributed to hydrothermal dolomite, localized along faults. The hydrothermal dolomite bodies that crop out in central Kentucky may serve as analogs to dolomite reservoirs found in West Virginia, Michigan, New York, and other parts of the eastern Midcontinent. Although we cannot state with certainty that the same processes formed dolomites in both Kentucky and New York, the dolomite bodies share striking similarities in structural control, geometry, and stratigraphic occurrence. Understanding the formation of hydrothermal dolomites in central Kentucky may lead to more accurate reservoir prediction and more discoveries throughout the eastern Midcontinent.

Fluid inclusion, electron microprobe, and stable isotope analyses will be performed on samples taken from outcrops and cores of the hydrothermal dolomite in central Kentucky. The data gathered from these analyses will be used to characterize the dolomitizing fluid and dolomitization process in a three-dimensional model. Furthermore, the relative direction of fluid migration will be investigated by comparing the homogenization temperatures of fluid inclusion from each location. Could the hydrothermal dolomites and MVT ores have precipitated from a single fluid that migrated from the east as a result of the Alleghanian orogeny?