The bioclastic carbonate play in the Appalachian Basin (referred to by drillers as the "Trenton") includes both gas and oil produced from Middle and Upper Ordovician stratigraphic and combination traps in linear reservoirs often mistaken for reefs. While production from this play is currently reported from nine fields in south-central and eastern Kentucky and five fields in New York, potential reservoir rocks are present throughout most of the Appalachian Basin from New York to Tennessee. Along the western flank of the Appalachian Basin, in the Blue Grass Region of central Kentucky, Middle and Upper Ordovician strata crop out. In the subsurface of south-central Kentucky and eastward into the Appalachian Basin, drillers informally refer to "Trenton," "Sunnybrook," "Leipers," "Granville," "Modoc," and "Anderson" pay zones. The mixed carbonates and shales of the Lexington Limestone through the Drakes Formation of Kentucky consist of well-cemented bioclastic grainstones and mudstones separated by relatively thin calcareous shales deposited in shallow platform, ramp, and peritidal settings. In areas of shoaling, reservoirs developed as discontinuous coquinoid lag deposits that were winnowed from carbonate muds by wave and tidal activity and accumulated as discrete offshore bars, tidal bars, and channel fills. In addition, dolomitized limestone reservoirs have developed along faults and fractures associated with recurrent movement of deep-seated basement faults. Structure mapping on the base of the overlying Devonian shale and the underlying Pencil Cave bentonite indicates the bars and channel fills may be spatially associated with basement structures. The Granville Consolidated Pool in Clinton Connty, Ky., is typical of fields in this play where production is associated with multiple stacked sequences of Offshore bars. An estimated 50 to 100 million cubic feet (MMcf) of gas per year is known to be produced from this field, not accounting for production from an unknown number of domestic supply wells. Monte Carlo simulation was used to estimate the undiscovered recoverable resource for this play to be 127billion cubic feet (bcf) of natural gas. These reservoirs will probably not be an important primary target for future drilling outside of southern and eastern Kentucky, however, because the reservoirs are relatively deep throughout much of the basin and are subtle stratigraphic features that will be difficult to detect using seismic or remote-sensing methods. As exploration and development continues for Lower Ordovician and Upper Cambrian reservoirs, these bioclastic carbonate bars will merit further evaluation.
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This work was funded by the U.S. Department of Energy, Morgantown Energy Technology Center under contract number DE-FC21-91MC28176.
Nuttall, Brandon C., "The Middle and Upper Ordovician Bioclastic Carbonate ("Trenton") Play in the Appalachian Basin" (1996). Kentucky Geological Survey Information Circular. 59.