Mishrif Formation is one of the main oil reservoirs in southern Iraq. This study analyzes the microfacies and depositional environment for Mishrif Formation in North Rumaila oilfield. The study was based on the analysis of 17 wells core. The Mishrif Formation represents deposition in a carbonate platform ramp system, with scattered patch reefs and shoals developed across the ramp margin and the platform top. It is characterized by skeletal grains (bioclasts) which are dominated such as foraminifera, rudist, calcareous Algae and other skeletal grains included mollusks shell fragments with Chondrodonta sometimes, and Echinoderms while non-skeletal grains are less abundant which are represented by Peloids and Ooids. According to the petrography analysis of Mishrif Formation, the fossils are dominated in the formation, four groups of these fossils are diagnosed, and these are Oligosteginid, Alveolinids, Dicyclina and Miliolids. Can be identified and build the sedimentary model with microfacies which apply to the Mishrif Formation. The sedimentological and stratigraphic analysis of Mishrif Formation core led to identification of 16 facies association, seven of which are found in the mB unit (lower part of the Formation) whereas they are deposited in outer ramp, mid ramp, ramp margin and lagoon, while nine in the mA unit (upper part of the formation) which are deposited in intraplate basin, mid ramp, inner ramp, ramp margin and lagoonal supra tidal.

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Marine environment is considered one of the richest environments in fauna of benthic marine micropaleontology because of the suitable conditions for living requirements like food, oxygen, sediment texture and salinity. In the current study, a fauna and their distribution were studied according to the depths of subsurface marine sediments from two regions at the NW Arabian Gulf, the first region at Khor Shytanah (Core 1) at the north of the study area representing a sever environment with high currents, and contain more than 95% sand grains, it is of light color, large size, rounded to sub-rounded grains and clean due to the continuous washing and water movement, whereas the second region was near the oil Basrah Port (Core 2) at the south of the study area, where the quite environment with weak currents and fine sand of dark color due to the activity of non-anaerobic bacteria , the region has angular to sub-angular grains. The type of sediment texture was determined at each depth and consisted of silt, sand silt, sand and silty sand. On the other hand, the percentage of calcium carbonate was determined in the sediments to correlates that with the size and density of fauna that live in this area.