A syngenetic classification of anhydrite textures in carbonate reservoirs and its relationship with reservoir quality: a case study from the Permo-Triassic Dalan and Kangan formations

The presence of anhydrite in various forms and textures is a common diagenetic feature in many carbonate reservoirs. In this study, focusing on carbonate reservoirs of the upper Dalan and Kangan formations in the Persian Gulf, a syngenetic classification of anhydrite based on petrographic evidence and core description is proposed. As a result, early diagenetic evaporite textures (layered, nodular, and sparse crystals) related to the depositional environment conditions and and burial textures (pervasive, patchy and fracture-filling) in the form of cements are identified. Among these textures, pervasive and patchy cements have the main effect on the destruction of reservoir quality, mostly in grain-dominated facies. Study of facies categories based on dominant diagenetic features and pore types in mud-dominated carbonates (MF-1 and MF-2) and grain-dominated ones (GF-1 to GF-5), and investigation of the introduced reservoir rock types (RT-1 to RT-3) indicate that the distribution of anhydrite in the reservoir and its impact on reservoir quality is predominantly dependent on diagenetic history of the studied formations, before and after anhydrite mineralization. As, the extensive flow of sulfate-rich brines and thus pervasive occurrence of anhydrite in the pore system of reservoir rocks have occurred during shallow burial and before significant compaction and widespread calcite cementation. On the other hand, the essential effect of dissolution on anhydrite cement during the burial has created reservoir facies with large dissolution vugs and high reservoir quality.