MICROFACIES, DEPOSITIONAL ENVIRONMENTS AND DIAGENETIC PROCESSES OF THE MISHRIF AND YAMAMA FORMATIONS AT FAIHA AND SINDIBAD OILFIELDS, SOUTH IRAQ
The Faiha and Sindibad oilfields are located in the northeastern part of the Basrah Governorate, south Iraq. More than 50 thin sections of limestone samples, collected from wells penetrating the Mishrif and Yamama formations in these oilfields, are examined in this work. Several microfacies are identified in the formations studied; four main facies and seven subfacies are found in the Yamama Formation and three main Formation. The main microfacies include lime mudstone, wackestone, packstone and grainstone. They were deposited in various marine sedimentary systems of open marine, foreslope, reef and lagoonal environments. Benthonic foraminifera, Mollusk shells, echinoids, and algae, are the fossils identified in the Mishrif and Yamama formations. Dolomite and calcite are the main minerals in the studied samples. Petrographic examinations revealed that diagenesis varied in intensity from microfacies to another. Dissolution and neomorphism (recrystallization) have created and controlled the development of porosity. The dominant pore types are vuggy, interparticles, intraparticles, channel and moldic. Whereas, cementation, micritization, and compaction processes have had destructive effects, by reducing porosity and permeability that led to reducing reservoir quality. Other processes, such as dolomitization and authigenic minerals (pyrite) do not have strong effects on the reservoir quality. The depositional basin model shows that the Yamama Formation was deposited in the ramp carbonate environment, whereas, the Mishrif Formation was deposited in the rimmed carbonate shelf environment.