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Facies Independent Porosity, Permeability, and Production Trends in the Micropore-Dominated Word Field (Edwards Formation), Lavaca County, TX


Accurate reservoir characterization and economic production in oil- and gas-bearing, micropore-dominated carbonates remains challenging. Conventional geologic models rely on relationships between depositional textures and petrophysical properties to distribute reservoir properties and inform performance predictions. Relationships between porosity, permeability and depositional texture become tenuous with increased burial as the space within interparticle macropores becomes cemented and micropores become an increasing percentage of total pore volume. Understanding the distribution of pore systems and predicting the fluid flow behavior of micropore-dominated reservoirs is important, as micropores house a significant percentage of the remaining global hydrocarbon reserves. The the gas producing, micropore-dominated Word Field in Lavaca County, Texas, is characterized by a facies independent, diagenetically controlled pore system that approaches 100% microporosity. Four cored wells through the Albian Edwards Formation were described and correlated using interpreted stacking patterns and vertical facies trends. Pore type characterization was done by integrating thin section petrography, scanning electron microscopy, routine core analyses, and mercury injection capillary pressure (MICP) data. A systematic relationship exists between porosity, permeability, median pore-throat size and microcrystalline textures that is independent of facies and depositional fabrics. Regardless of depositional texture, macropores have been cemented during burial diagenesis, while micropores have remained open to depths greater than 13,000 ft. Cumulative gas production data correlates with total porosity, which is predictable based on the structural position of the wells. Wells near the crest of the structure have relatively high porosity and permeability, while both reservoir quality and cumulative gas production are relatively low in flank wells drilled in similar facies. In the Word Field, an equally well- connected micropore network exists in mud-dominated rocks via the matrix when compared to that observed in grain-dominated rocks via microporous grain-to-grain contacts. The well-connected intragrain micropore network in cemented grainstones is a new carbonate flow path that will likely become important as more unconventional carbonate reservoirs are explored and re-developed.