Typical Reservoir Architecture Models, Thief-zone Identification and Distribution of the Mishrif Carbonates for a Super-giant Cretaceous Oilfield in the Middle East

Abstract The Upper Cretaceous Mishrif Formation is widely distributed in the Middle East, and it is one of main reservoirs in the super-giant Rm Oilfield. However, since development in 1954, its Mishrif carbonates, accounted for 45% oil-reserves, are not largely developed with only 5.5% oil-recovery due to complicated reservoir architectures and elusive thief zones, the abnormal high-permeable strips that can cause water-injection breakthrough. Therefore, it is critical for the successful development of the Mishrif to make certain the different reservoir architectures and thief-zone distributions. Based on the investigation of lithofacies sensibility and cluster analysis of logs, petrophysical interpretation models of lithofacies associations have been acquired calibrated by core facies. After studying rock characteristics, genesis and petrophysical-logging responses on high permeable layers, 2 kinds of petrophysical models have been extracted for thief-zone identification. In the framework of isochronal stratigraphy, the relationships between the depositional cycles, palaeogeomorphology, sedimentary differentiation, lithofacies associations, and thief-zone distributions were studied, and the reservoir architecture models of the Mishrif carbonates have been revealed in a typical section. Finally, the spacial distributions of coarse lithofacies associations and thief zones in the critical period were predicted integrating petrophysical interpretations and geology genesis. Results reveal that, the Mishrif Formation, generally associated with two upward-shallowing sequences, MA and MB, can be further divided into 6 intervals, from the bottom up: Z1, Z2, Z3, Z4 of MB, and Z5, Z6 of MA. 9 lithofacies associations in depositional units can be determined by petrophysical interpretations, and the Mishrif reservoirs have varied structures of lithofacies associations in different periods: Z1 has upward-shallowing structures as from distal-mid ramp, DMR, bioclast-shoal complex, BShC, to rudist-reef-shoal complex, RRfShC, vertically, and has progradation laterally; Z2 and Z3 have distributions of lagoon-back-shoal complex, RRfShC, BShC and DMR; Z4 and Z6 are mostly barriers or buffers; Z5 could have permeable patch rudist/coral biostromes in palaeohighs. Thief zones,formed by marine waves erosion and leaching on tops of reef-flat buildups, being less-than 0.5m single thickness and distributed in clustering areas laterally, are often developed in RRfShC and BShC. Considering different structures of lithofacies associations and thief-zones, and their influences on injectors and producers, the Mishrif reservoirs architectures can be divided into 5 types: Type A, Type B, Type C, Type D and Type E, in which, Type A has thief-zones in whole intervals of injection-production wells, and Type B has parts of thief-zones in either injection or production wells, and they are very important in oilfield development. The methodology and results in this paper are of great references for the Mishrif and similar carbonates.