Microfacies, Sedimentary Environment and Diagenetic Processes Analysis of Ilam Formation in Gachsaran Oil Field, Zagross Basin Gachsaran Oil Field, Zagross Basin

The study of sedimentary characteristics of carbonate formations as one of the main hydrocarbon reservoirs is important. In this research paper, we used thin section, and petrophysical logs data to analysis of facies, sedimentary environment and diagenetic variation of the Ilam Formation (Bangestan group). Thin sections petrography revealed that the Ilam Formation which was deposited as a full series in western part of the Gachsaran oil field and characterized by a neritic facies indicating a shallow marine. The sedimentary facies of Ilam Formation are mainly limestone, and sometimes shaley limestone (in restricted part of the basin). The lack of Coniancian-Turonian sediments in wells drilled in eastern part of the field can be related to erosional phase after Cenomanian-Turonian and developement of paleoheight parallel to Khark-Mish fault. The determined facies are deposited over a carbonate platform (shelf type) in three sedimentary environments: shoal, lagoon and semi restricted lagoon. The petrographic characteristics of detected facies of Ilam Formation revealed that they were experienced different diagenetic realms such as marine, meteoric, and burial and during uplift periods. These sediments then influenced by diagenetic processes such as micritization, compaction, cementation, neomorphism, solution, fracturing and replacement (dolomitization, hematitization, and pyritization). The dominant porosity types are interconnected vugs, fracture and channel which are played a profound effects on the variation of reservoir quality. The measurements of petrophysical parameters present that the averages of porosity, water saturation and net to gross ratio are varied in the Ilam reservoir, but these parameters are generally 2.3%, 88% and 0.016, respectively. In view of low values of porosity, the role of fractures and diagenetic processes are important in the reservoir quality. The paleohigh presence in eastern section is not only affected on facies distribution, but it also has an important role in subsequent evolution. Therefore, the quality of the Ilam reservoir is a function of sedimentation, diagenesis, and tectonic activity; and therefore, it will be variable in different parts.