Characterization and Appraisal of Reservoir Heterogeneity of the Lower Mishrif Formation in AD Oilfield, Central Mesopotamian Basin, Iraq

The results of a multiproxy study on Mishrif carbonates from the margins of the Central Mesopotamian Basin of Iraq display a wide range of rock fabrics and diagenetic features, all affecting reservoir heterogeneity and flow properties in a complex manner. Based on petrographic and facies analyses of the Mishrif Formation in the AD oilfield, twelve characteristic microfacies types are proposed and are set in six sedimentary facies belts: (i) restricted inner ramp, (ii) back-shoal, (iii) shallow shoal, (iv) fore-shoal, (v) storm-agitated bioclastic shell layer, and (vi) outer ramp. There is a good correlation between microfacies types, depositional components and reservoir quality. Twelve microfacies types are classified into five classes (i.e., Type I – V) with distinct reservoir quality. Besides, the content of typical components comprising green alga, planktonic foraminifera, micrite, and cement plays important roles in reservoir quality. The Mishrif reservoir properties are shaped by distinctive patchy cementation occluded by multiple-stages calcitic cements. These patchy carbonates are mainly developed in the Reservoir Type I lithology and result in distinctive cm-to dm-scale heterogeneity. The contrast between small, poorly-connected pore systems in nodules and medium-sized, well-connected pore systems in the host rock is best described as an inconspicuous, dual permeability system. Four types of reservoir heterogeneity (i.e., Type I – IV) are established by four coefficients and show a good logging response from NPHI and GR logs. Sedimentation and diagenesis are two main reasons for the heterogeneity generation: i) Sedimentology is a foundation for reservoir heterogeneity development. The spatial distribution of microfacies controls the meter-scaled inter- and intra-stratigraphic reservoir heterogeneity. ii) Subsequently, diagenesis further enhances the decimeter to centimeter scaled intra-stratigraphic reservoir heterogeneity in two aspects: diagenetic overprints below the 4th sequence boundary and intensity of patchy cementation. Data shown here and evidence from other studies dealing with the Mishrif in the Central Mesopotamian Basin are important to understand and predict reservoir properties in other carbonate fields with similar properties.