Sedimentary units-layering system and depositional model of the carbonate Mishrif reservoir in Rumaila oilfield, Southern Iraq

The Mishrif Formation (Cenomanian–Lower Campanian) is the main productive oilfield at Basrah in southern Iraq. It mainly comprises of shales and carbonates characterized by vertical phase transitions, including seals and oil reservoirs. This study establishes the Cenomanian–Campanian sequence stratigraphic framework through the integration of geochemical and geophysical data in order to distinguish between the stratigraphic intervals of shale and non-shale along oil wells in the Rumaila Oil Field that penetrates the Mishrif, Khasib and Tanuma formations. The geochemical profile has shown that carbonate and shale are the main lithologies along the 315 m stratigraphic interval from the top of Tanuma to the base of Mishrif, in the oil well WQ2-139. The stratigraphic column accordingly was divided into twenty stratigraphic intervals (zones); four zones in Tanuma, five zones in Khasib and eleven zones in the Mishrif Formation. The lower part of the Mishrif Formation is the main reservoir in the stratigraphic column as already known, yet the problem lies in the controversy over the number of productive units and their stratigraphic intervals. The porosity, lithology and fluid characters were discriminated based on distribution temperature (DT), Distribution Temperature Sensing (DTS) and RHOB logs as well as the compressional-wave velocity (Vp), shear-wave velocity (Vs) and density logs. The shale, gas limestone, oil limestone and brine limestone are well estimated and presented where the chi angle is about 0° and fluid-fill (i.e. oil from brine) is 30°–50°. This study provided headway in establishing milestones in the stratigraphic framework that have an important effect in determining reservoir storage quality.