Dolomitized Reef Buildup in the Silurian Louisville Limestone, Muldraugh Gas Storage Field, Meade County, Kentucky

Abstract

A newly discovered core at the Kentucky Geological Survey’s Earth Analysis Research Library contains a dolomitized carbonate buildup and is the best example of a Silurian reef in the western Kentucky part of the Illinois Basin. The buildup was cored in the Louisville Gas and Electric #3 Lewis well, an observation well located downdip on the west flank of the Muldraugh gas storage field in Meade County. The Muldraugh field is a distinctive shallow dome (500–700 ft deep) with about 220 ft of closure. The structure’s origin has been debated for decades—the center of the field contains an anomalous section of reworked, fragmental, and steeply dipping dolomite (Ordovician Knox?) in several wells directly below the Devonian New Albany Shale. The flanks of the structure contain a typical Middle Devonian and Silurian stratigraphic sequence. The Lewis core is 250 ft in length and starts just below the Devonian New Albany Shale. The cored interval has a remarkably uniform, clean, low gamma-ray log response. The Silurian Waldron Shale and Laurel Dolomite are present below the cored interval and serve as good log markers for correlation. The stratigraphic interval from the Waldron Shale to the New Albany is 100 ft thicker in the Lewis well than in nearby wells. Because the typical Middle Devonian Sellersburg and Jeffersonville Limestones and Silurian Brownsport Formation cannot be identified in the core, the interval is assigned to the Silurian Louisville Limestone. We have identified three lithofacies in the core, all pervasively dolomitized. The lowermost interval (110 ft thick) consists of crinoid-brachiopod grainstones and packstones. These sands are flat-lying to low-angle crossbedded, and are interpreted as shelf sheet sands. They may represent a reef flank facies but lack significant depositional dip characteristic of flank deposits elsewhere. The buildup or reef lithofacies is 98 ft thick and composed of stromatoporoid-coral boundstone, packstone, and floatstones. This interval contains significant vuggy moldic porosity associated with stromatoporoids. Overlying the reef lithofacies is about 38 ft of crossbedded crinoidal grainstone and lesser amounts of thin stromatoporoid boundstone. This interval is interpreted as a high-energy shoal facies, deposited on top of the buildup. This interval is unconformably overlain by a darker dolomite with chert and shale cobbles, possibly a remnant of Devonian Jeffersonville Limestone (no black shale was cored). The Lewis reef lies outside the Muldraugh storage reservoir. Its relationship to the larger structure and anomalous dolomite section in its center is not fully understood. Work is continuing to interpret controls on reef development in the broader context of the Muldraugh structure.

AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019