Depositional Framework and Reservoir Potential of an Upper Cotton Valley (Knowles Limestone) Patch Reef, Milam County, Texas: ABSTRACT

ABSTRACT The Knowles Limestone is an upper unit of the Cotton Valley Group, and in Milam County, Texas, is approximately 350 feet thick and consists of shales, terrigenous dolomitic limestones, grainy limestones, and algal boundstones with stromatoporoids and corals. The boundstones represent an elongate, wave resistant, encrusted skeletal patch reef which probably developed on a subtle salt-generated topographic high. The reef appears to be slightly more than 1 mile across in its narrowest lateral dimension as determined by facies correlations of three cored wells in the study area. Principal reef framebuilders included massive and columnar stromatoporoids, solitary or dendroid corals, and encrusting Tubiphytes and Lithocodium algae. Dendroid stromatoporoids, digitate corals, Tubiphytes oncoids, echinoids and various mollusks were the principal contributors to the detrital infill of the reef. Increased dip and dip reversal of draping beds suggest that the Knowles reef achieved relief above the sea floor. Evaluation of dipmeter logs indicates that the beds dip 5° northwest on the updip reef flank and 10° southeast on the downdip reef flank. Reef core boundstones and reef talus were consistently present downdip, and lagoonal to tidal flat facies were common updip throughout Knowles deposition. The reef organisms eventually became overwhelmed with terrigenous sediment transported downdip as the tidal flat environment prograded over the lagoonal, reef talus and reef core facies at the end of Knowles deposition. Early dolomitization of tidal flat and lagoonal facies has created local porous zones in some of these rocks. However, the reef, per se, is cemented by sparry calcite and is not a potential reservoir facies.