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Micritized ancient ooids are important to hydrocarbon exploration because they may be reservoirs for gas or bound water. Porosity within micritized ooids may exceed 15%. Porosity and permeability in micritized ooids are greatest where micrite crystals are euhedral, relatively large, and uniform in size. A study of the massive Cotton Valley limestone in east Texas indicated that intra-ooid porosity, with permeabilities up to about 1 md, may constitute a significant reservoir for gas. Accordingly, a survey of ancient ooids was conducted to assess the nature and variability of their crystal fabric. Micritized ooids of Mesozoic and Paleozoic age were collected from outcrops and well cuttings. Most are composed of euhedral to subhedral 1-5 µm calcite rhombs. Crystal size nd shape are most uniform within a single ooid and most variable between localities. Subsequent diagenesis (excluding leaching or replacement) produced either coarser anhedral crystals or cemented the rhombs with micropoikilotopic overgrowths. Micritized ooids from 13 samples exhibit a common fabric and may have been altered by a common process. Nearly all recent ooids are aragonitic, and most ancient ooids, including those examined in this study, were probably aragonitic as well. Mineralogical stabilization from aragonite to calcite is suggested as the micritizing process.
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