Diagenesis processes impact on the carbonate Mishrif quality in Ratawi oilfield, southern Iraq

Abstract

This paper aims to identify the impact of the diagenesis processes on the carbonate reservoir’s characteristics (porosity and permeability) of the Mishrif Formation. A detailed investigation of available core samples and microscopic study of more than 200 thin sections of the core limestone samples were examined from two wells (Ratawi-2 and Ratawi-5) in Ratawi oilfield, which are located in the Basrah Governorate, southern Iraq which revealed that different diagenetic parameters influence the reservoir quality in Mishrif Formation. Diagenetic features and four identified diagenetic environments that have affected the carbonate rocks in the Ratawi oilfield were the meteoric vadose, meteoric phreatic, marine phreatic and mixing zone. Petrographic examinations revealed that diagenesis varies in intensity from microfacies to another, micritization, dissolution, cementation, compaction, neomorphism, dolomitization and authigenic minerals (pyrite) are the dominant diagenetic parameters which are identified in the current field of study. Among all the observed diagenetic features, dissolution and neomorphism (recrystallization) were formed as a result of subaerial exposure during the meteoric diagenesis and contribute to porosity enhancement and reservoir quality. The cementation (calcite and dolomite), compaction, dolomitization, and micritization have a negative impact on the reservoir behavior, through reducing porosity and permeability that led to reducing reservoir quality. Other processes such as authigenic minerals (pyrite) did not have strong effects on reservoir quality. Based on the genetic classification of porosity, most of the porosity within Mishrif Formation is a combination of depositional, diagenetic and fracturing. The diagenetic porosity is the far more dominant types of porosity such as vuggy, intraparticles, fractured and moldic, implying that carbonate Mishrif reservoir is the type of diagenetic reservoirs.