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Reservoir Facies and their Distribution in Upper Leonardian Carbonate Rocks from Veterans Oil Field, Glasscock County (Eastern Midland Basin), Texas


The Veterans Oil Field located in Glasscock County, Texas, is along the eastern side of the Midland Basin very close to the Eastern Shelf boundary. The studied interval consists of Upper Leonardian shelf to basinal, matrix- to clast-supported conglomerate, Fusulinid packstone to wackestone, and skeletal packstone to mudstone, that are entirely encased in basinal shales. Oil production within this interval is highly variable as some wells produce oil at economic rates following cross-linked gel fracture stimulation whereas others do not produce any fluids. The distribution of reservoir facies rocks was interpreted from slabbed cores and thin-sections from wells in the field, whereas the petrophysical parameters that are favorable to the accumulation, storage, and flow of hydrocarbon were interpreted from wireline logs using Techlog software. Four main reservoir facies were identified in the Leonardian strata: (i) Polymict conglomerate facies–light grey to light brown clast-support polymict conglomerate consisting of slope-derived skeletal packstone to grainstone lithoclasts, and reef-derived sponge-archeolithoporella boundstone. Bioclasts consist of fusulinids, fragmented crinoids, brachiopods, sponges, and bryozoans (Fistilopora sp). Fusulinids have biomoldic porosity that is enhanced by leaching. The facies is characterized by vuggy porosity due to dissolution and removal of some of the grains; (ii) Fusulinid wackestone to packstone facies–light to dark grey, argillaceous fusulinid wackestone to packstone with bioclasts of fragmented thin-shelled brachiopod, fossil hash and crinoids. Some fusulinids were dissolved and replaced by sparry calcite. This facies has biomoldic porosity due to preserved fusulinid chambers; (iii) Skeletal mudstone to wackestone facies–light grey skeletal (non-fusulinid) mudstone to wackestone with fragments of thin-shelled brachiopods, crinoids, and hash fossils. It has a chalky texture with dispersed skeletal grains in lime mud matrix giving micro-intergranular porosity, and occasional vugs due to leaching; (iv) Fusulinid packstone to grainstone facies–light grey, moderately to poorly sorted, fusulinid packstone to grainstone with shale rip-up clasts. Fusulinids have biomoldic porosity that is enhanced by leaching. The entire Upper Leonardian carbonate interval is a shallowing upward shelf-to-basin transition of detrital carbonates derived from platform top, platform edge, and upper and lower slope environments.