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The AAPG/Datapages Combined Publications Database
Through much of Dimmit and northern Webb Counties, the Buda and Georgetown limestones are remarkably homogeneous; both consist of very dense algal calcisphere, Globigerina, Inoceramus, and echinoderm wackestones and packstones; calcispheres constitute the predominant biotal component in both.
In central and western Dimmit County dolomitization produced secondary intercrystalline porosity in several Georgetown intervals; these voids are now filled with solid hydrocarbon. In this area gas is produced in the Georgetown from tertiary voids which were formed when fresh groundwaters dissolved replacement anhydrite after hydrocarbons had accumulated in secondary intercrystalline voids. The Buda has no reservoir potential in this area.
Westward from eastern Dimmit County, the McKnight and West Nueces change facies from oobiograinstones and packstones to biopelgrapestone grainstones and packstones to biopelwackestones in western Dimmit County. The McKnight exhibits well-developed depositional and diagenetic cycles. These cycles record interaction of the following: (1) eustatic fluctuations in sea level, (2) regional progradation of supratidal, intertidal, and subtidal facies during stillstands of sea level, (3) changes in climate from arid to semiarid or subhumid, (4) continuous subsidence. Consequently, the McKnight has been subjected to highly complex multicycle diageneses that include freshwater diagenesis, dolomitization, anhydritization, silicification, and dedolomitization. Anhydrite layers of the upper and low r "anhydrites" were formed by replacement of carbonates. Secondary intercrystalline porosity in dolostone layers has been filled by what is now solid hydrocarbon which accumulated at shallow depths. Gas production in the McKnight, throughout the area, is from tertiary anhydrite molds which were created after solid hydrocarbons had accumulated in secondary voids. Much dickite cement also is present in secondary voids in the McKnight.
The West Nueces apparently contains no anhydrite, but tertiary anhydrite molds were abundantly formed and then largely filled by carbonate cements, as were primary and secondary voids. Reservoir potential of the West Nueces probably has not been properly evaluated.
Because mechanisms of anhydrite emplacement are so poorly understood, the distribution of porosity, formed by its dissolution, is unpredictable.
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