Dolomitization and its relationship with sedimentary facies and reservoir quality of the upper Dalan and Kangan formations in southeast district of the Persian Gulf

The Upper Dalan and Kangan formations with the late Permian- early Triassic age are considered as the main reservoir rocks of some fields in the Persian Gulf. These formations with carbonate-evaporate interval are composed of shoal, lagoon and tidal flat facies deposited in shallow parts of a carbonate ramp under the warm and arid climate condition. In these facies, dolomitization is a common diagenetic process which has been discussed in terms of the formation, distribution and its relationship with the reservoir quality. This process with regard to the original sedimentary properties, and also the effect of such diagenetic processes as dissolution and anhydrite cementation, has different effect on the reservoir characteristics. Dolomite has been mainly distributed as replacement with an insignificant part as cement. Fine crystalline replacive dolomites in mud dominated facies of tidal flats, have been formed by surface evaporation and pore water concentration in early diagenesis. In contrast, replacive dolomites in grain dominated facies, as fabric selective and non-fabric selective, have been developed by the effect of evaporate brines penetration during the burial diagenesis. In these facies, anhydrite has been formed as patchy and pervasive cement. Dolomitic facies with pervasive anhydrite, similar to fine crystalline facies of tidal flats, show low reservoir quality. Dolomitic facies with patchy anhydrite and also coarse crystalline replacive dolomites have higher reservoir quality. The results show that dolomitization when is associated with development of fabric destructive and coarse replacement dolomites, especially within the grain dominated facies, has an effective role on reservoir quality improvement.