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Facies of the Permian San Andres Formation, Howard-Glasscock field, Howard County, Texas, consist mainly of thick shelf carbonates topped by a carbonate-evaporite sabkha unit. Clastics are present as thin shale beds occurring sporadically throughout the sequence. Carbonate facies reflect a broad progradation across a promontory of the Eastern shelf bordering the Midland basin, punctuated by minor transgressions and onlap due to differential subsidence.
Core examination demonstrates that the carbonates have been pervasively dolomitized and plugged by sulfates, principally anhydrite. Dolomite crystal size shows a broad increase downcore,
while anhydrite content decreases. Anhydrite precipitation appears to increasingly postdate the onset of dolomitization with depth. Reflux processes are felt to be largely responsible for the ubiquitous dolomitization and sulfate precipitation.
Porosity developed as a result of a sulfate solution event, producing vuggy, moldic, and intercrystalline dolomitic porosity due to leaching of sulfates from the dolomite fabric. Lateral solution pathways developed, particularly through the leaching of sulfates from packstone allochem replacements and void fills. Insoluble residue content was a major inhibiting factor in solution, especially because of stylolite development in shaly dolomites, which created low-porosity horizons. Hydrocarbon shows are primarily intercrystalline.
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