Role of Anhydrite Diagenesis in the Creation of Porosity in Middle Permian Dolostone Reservoirs: ABSTRACT

Abstract

In middle Permian dolostone reservoirs, the most abundant pore type is secondary intercrystalline porosity. Generally ranking second in abundance, but first in many instances, are pores created by dissolution of anhydrite that occurred in the Permian. Much anhydrite was emplaced diagenetically during and following early dolomitization, as nodules, blocky porphyroblasts, and cement, and as combination cement and replacement. Generally, anhydritization was followed by an influx of a low-salinity fluid, which dissolved anhydrite - creating tertiary porosity - or altered it to gypsum, hemihydrate, silica, calcite, or fluorite. Dissolution of anhydrite nodules and porphyroblasts creates distinctive molds with angular and stair-step outlines. Dissolution of anhydrite, which was emplaced as cement and replacement, enhances the original primary and secondary pores by the increment that was replaced. This dissolution also produces voids with angular and stair-step outlines. Replacive anhydrite tends to nucleate within organic-rich components, such as burrows, stromatolites, skeletal grains, ooids, and peloids. Many dolomite crystals have cloudy organic-rich centers, which represent the original replacive crystals, and clear rims of epitaxial dolomite cement. Anhydrite preferentially nucleates within the organic-rich cores, and subsequent dissolution produces hollowed dolomite crystals or intracrystalline porosity. Intracrystalline pores constitute the predominant pore types within many intervals. Anhydrite commonlymore » replaces skeletal grains or ooids, and the replacement of such anhydrite produces a mold that replicates the original shell (biomold). It is possible to determine how many biomolds actually record dissolution of replacive anhydrite. The best Permian reservoirs contain intercrystalline porosity in conjunction with pores created by dissolution of anhydrite and grains.« less