Genesis of High Permeable Thief-zones and Integrated Reservoir Modeling of Intense Heterogeneous Reef-flat Carbonates: A Case Study for the Mishrif Formation of the Rumaila Oilfield, Iraq

Abstract The super-giant Rumaila oilfield, with siliciclastic and carbonate reservoirs, has been developed for nearly 70 years. However, the Mishrif carbonates, which account for nearly half of Rumaila's reserves, could not be mass developed, as the current recovery rate is only 6%. One of the complexities is due to the presence of thin layers with abnormally high permeability, called thief-zones, which make reservoirs extremely heterogeneous and injection water breakthrough very early. Therefore, it is necessary to decipher the reservoir genesis and complicated architectures by genetic analysis, reservoir characterization and modeling for Mishrif's efficient development. Based on key flooding and erosion surfaces, sedimentary cycles in lithofacies and petrophysical responses, stratigraphy correlations and seismic strata stacking configurations, the fine isochronal stratigraphy framework of the Mishrif Formation has been constructed. Then, the genetic processes of lithofacies differentiation, reservoir distributions and properties, and thief-zones in response to palaeotopography and relative sea-level changes are analyzed synthetically in lithofacies, well-log sections, dynamic data and layer models from geological seismic units interpreted. According to petrophysics detection and reservoir genesis, the thief-zone distribution model is acquired. Subsequently, constrained by genetic conditions, 3D-gridding porosity model and double medium permeability model are constructed by simulation and integrated understanding of intricate architectures and permeability from cores and dynamic data. This paper demonstrates: 1) The Mishrif Formation can be divided genetically into six zones, from Zone Z1 to Z6; 2) The palaeotopography and relative sea-level changes control reef-flat deposition and differentiation so that the thick, coarse and clean lithofacies are usually distributed on upper parts of reef-flat build-ups but become finer in lower-side places. Furthermore, the Z1 thickness has important influences on Z1 and subsequent depositions; 3) Thief zones are not stress fractures, but rather erosion and leaching surfaces developed on top of reef-flat buildups in coarse lithofacies due to relative sea-level fluctuations; 4) The porosity models demonstrate high porosity (30-18%) in relative coarse lithofacies of reef-flat buildups, and reflect genesis of depositional energy environments by porosity distributions; 5)The permeability model has revealed reservoir intense heterogeneity (0.01- 4000mD), in which, the extremely high (>1000 mD) reservoirs, the thin thief-zones, are distributed mostly with lateral extension of 250-2300 m. The extremely-high, high and low reservoirs comprise complicated reservoir architectures which can at least be divided into three scenarios for reservoir development, in which the development of thief-zones in injectors should be paid more attention to designing perforation intervals because of possibly influences of extremely high permeable thief-zones. The methodology and results in this paper provide crucial references for the development of Rumaila oilfield and similar heterogeneous carbonate reservoirs.