Factors controlling distribution of different textures of anhydrite cement and its relation to reservoir quality in the Upper Dalan and Kangan carbonate reservoirs, South Parsfield

Reservoir quality of Upper Dalan and Kangan carbonate reservoirs in the South Pars field has been affected by various diagenetic processes. The formation of anhydrite cement is one of the most important of these processes, developed in layered, poikilotopic, pore-filling and pervasive, nodular and sparse crystals, fracture filling and veinlet forms. These textures during different diagenetic stages, by replacement, displacement and pore occluding and in different degrees, have been affected various facies in the reservoir. The results of this study indicate anhydrite cement has mostly affected the dolomite facis than the lime facies. In addition, this also shows the effect of sulfate- rich brines in dolomitization. In the view of reservoir quality, Pore-filling and pervasive anhydrite texture is more effective than the others. As, occluding of whole pore spaces in dolograinstone and grain-dominated dolopackstone facies by this anhydrite and during shallow burial diagenesis, has resulted in extreme damaging of the reservoir quality. In contrast, the reservoir quality of these facies has been improved due to dissolution of this cement and creation of the secondary vuggy pore spaces in subsequent diagenetic stages. It can be concluded that the development and effect of the anhydrite in different textures in the reservoir, controlled by three main factors including the presence of sulfate-rich brines, initial sedimentary texture and the effect of the later diagnetic processes (i.e.dissolution and fracture).