Pore Types, Origins and Control on Reservoir Heterogeneity of Carbonate Rocks in Middle Cretaceous Mishrif Formation of West Qurna Oilfield, Iraq

Abstract

Carbonate rocks in Middle Cretaceous Mishrif Formation in West Qurna oilfield of Iraq are characterized by complicated pore types and complicated pore structure with strong heterogeneity which result in the challenges in reservoir evaluation. Cores, thin sections, physical properties and mercury injection testing involving 421 samples of three wells were conducted to classify the pores of carbonates, analyze pore origins and identify the relationships between pore types and reservoir characteristics. Six types of pores are classified, and they are intraparticle pores, moldic pores, residual moldic pores, interparticle pores, micropores and intercrystal pores. The pore origins were controlled by the sedimentary environment and diagenesis. Pore types were controlled by lime mud contents, and bioclastic textures, types and abundance that were controlled by water dynamics and biological living conditions of sedimentary environments. Pore occurrence was determined by various diagenesis, while it was mainly attributed to dissolution, which was influenced by the mineral composition of bioclastics. In addition, pores were altered by micritization, cementation and compaction. According to the dominant pore types and assemblages of pores, the samples were classified as 6 classes, and they are intraparticle pore classes, residual moldic pore classes, micropore classes, moldic pore classes, mixing pore classes and interparticle pore classes. The dominant pore types are closely related to reservoir characteristics, and each class corresponds to two or more rock texture types, while the physical properties and pore structure are similar, indicating that pore type is a crucial cause for strong reservoir heterogeneity of carbonates. This study revealed the pore origins of various pore types of bioclastic carbonates, presented the method to evaluate carbonate reservoirs with complex pore types which could be applicable to porous bioclastic carbonate reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018