Factors controlling different types of anhydrite textures and their relation to reservoir quality in the Asmari reservoir in Ahvaz Oil Field

Various diagenetic processes have affected reservoir quality of the Asmari Formation in Ahvaz Oil Field in well No. 19 with a thickness of 357 meters. It is composed of limestone dolomite, dolomitic limestone, sandy dolomitic limestone, sandstone, siltstone and shale. Study of 1100 thin sections from available cores at this well as well as the well’s numeral data, porosity and permeability of cores, indicates that the most important diagenetic processes is anhydritic cement with different types of textures. It appears as poikilotopic, porefilling and pervasive, nodular, fracture filling, sparce crystals, and evaporite veins. During different steps of diagenesis, these textures show replacement and pore filling porosity that have affected different facies to some degrees. Results of this study show that anhydrite cement occurs mainly in dolomitic and sandstone facies and to some extent, affected the reservoir quality. This is due to the effect of sulphate rich brines during dolomitization. Where as, the presence of anhydrite cements in limestone facies is sparse and rare, it has little effect on reservoir quality. However, pore filling and pervasive anhydrite texture filling all pore-space in dolograinstone and dolopackstone grain-supported facies during shallow burial intensively reduced reservoir quality. Dissolution of cements at later stage of diagenetic processes (creating secondary porosity) improved reservoir quality. In addition, dissolution of poikilotopic anhydrite texture in sandstone facies and reprecipitated as patchy anhydrite with poikilotopic texture, results in reduction of porosity but it doesn’t make any changes in throat pores. Hence, patchy anhydrite with poikilotopic texture that reduces porosity converts the sample from Lucias class 2 to class 1.