Analogs for Fault-Controlled Ordovician Dolomite Reservoirs, Appalachian Basin: Characterization of Central Kentucky Outcrops

Harris, David C. ¹, Wilcox, Clay A. ² and Parris, T.M. ¹

¹Kentucky Geological Survey, 228 Mining and Mineral Resources Bldg., University of Kentucky, Lexington, KY,

²Dept. of Geological Sciences, University of Kentucky, Lexington, KY,

Dolomitized Ordovician carbonates outcropping on the eastern flank of the Cincinnati Arch in central Kentucky are analogs for Trenton/Black River gas reservoirs in the northern Appalachian Basin. The dolomite bodies occur adjacent to faults in the Lexington (Trenton) and Calloway Creek Limestones within the Central Kentucky Mineral District. These shallow dolomites are being characterized with conventional and cathodoluminescent petrography, trace element and stable isotope geochemistry, high-resolution seismic-reflection imaging, and fluid inclusion analyses.

Petrography documented two dolomitization events and several phases of calcite cementation. Early dolomite is fine grained, euhedral, with luminescent zoning defined by variation in Fe and Mn trace element concentration. This dolomite replaced calcite matrix and some skeletal grains. Early dolomite is characterized by average O¹⁸  values of -3.8 ^o/_oo PDB. Later saddle dolomite occurs in vugs and skeletal molds, and replaces grains, and is moderately to strongly ferroan and non-luminescent. O¹⁸  values for saddle dolomite average -7.3 ^o/_oo PDB. The trend toward lighter oxygen isotope values might be due to increasing temperature during dolomitization with late-stage dolomite being precipitated at temperatures higher than expected based on burial history.

Analysis of a 1,005-ft continuous core through the largest outcropping dolomite body indicates that dolomitization decreases with depth. Distribution of dolomite is not directly related to proximity to the boundary fault or depositional facies. Additional geochemical and fluid inclusion data being obtained from this core will allow development of a refined dolomitization model for Ordovician carbonates in central Kentucky, and can be applied to exploration deeper in the basin.