Pore types, origins and control on reservoir heterogeneity of carbonate rocks in Middle Cretaceous Mishrif Formation of the West Qurna oilfield, Iraq

Carbonates in Upper Cretaceous Mishrif Formation of the West Qurna oilfield in Iraq are characterized by complicated rocks and pores types, and strong heterogeneity. Based on the cores and cast thin sections of three wells, pore types were identified, and pore origins were analyzed from the perspective of sedimentary and diagenesis features. Porosity and permeability measurement, and high pressure mercury injection capillary pressure tests were conducted to determine the relationships between pores and reservoir heterogeneity. Six types of pores are identified, and they are intragranular pores, moldic pores, residual moldic pores, intergranular pores, micropores and intercrystal pores. The pore origins were controlled by the sedimentary environments and diagenesis. Pore types were controlled by lime mud contents, bioclastic textures, types and abundance that were controlled by sedimentary environments. Pore occurrence was determined by various diagenesis, while pore occurrence was mainly attributed to dissolution, which was influenced by the mineral composition of bioclastics. In addition, pores were altered by micritization, cementation and compaction. Results from the comparison of reservoir parameters show that the reservoir physical properties and pore structure of the each dominant pore type are similar. The study proves that reservoir characteristics are controlled by dominant pore types and reveals that pore type is a crucial cause for strong reservoir heterogeneity of carbonates.