MICROFACIES AND DEPOSITIONAL ANALYSIS OF THE MISHRIF FORMATION IN SELECTED WELLS OF RATAWI OILFIELD, SOUTHERN IRAQ
This paper includes studying the distribution of microfacies, diagenetic processes and depositional basin of the Mishrif Formation in selected wells of Ratawi oilfield (Ratawi-2 and Ratawi-5), southern Iraq. More than 200 thin sections of samples were examined of the above-mentioned wells. The Mishrif Formation is divided into three major microfacies: lime mudstone, wackestone, and packstone, which are deposited in sub-basinal, lagoonal open shelf margin, and, shoal environments. Benthonic foraminifera, planktonic, Echinodermata and algae represent the fossils in the Mishrif Formation. Dolomite and calcite are the main minerals components of formation. Seven diagenetic processes were recognized in the Mishrif Formation, showed positive and destructive effects on the reservoir quality; dissolution and neomorphism (recrystallization) had highly positive effects through creating and improving porosity and permeability, which led to improving reservoir quality. Cementation, micritization, and compaction have destructive effects, through reducing porosity and permeability and led to reducing reservoir quality. Other processes such as, dolomitization, authigenic minerals (pyrite) did not have strong effects on reservoir quality. Based on the genetic classification of the porosity, most of the porosity within the Mishrif Formation in this field was formed by diagenesis processes, in which the predominant pore types are vuggy, intraparticle, fractured, and moldic. Analysis of microfacies showed that there are three main facies and eight secondary facies in the Mishrif Formation. The longitudinal section of the depositional environments showed, that Mishrif Formation deposited in the sub-basinal, lagoonal, open shelf margin, and shoal environments depending on fossils observed in a number of microfacies.