ABSTRACT The Lower Cretaceous (Barremian–Aptian) Main Pay Member of the Zubair Formation is the main producing reservoir in the supergiant Rumaila oil field in southeast Iraq, one of the world’s largest oil fields. While the field has been on production for approximately 60 years, significant resources remain. Key to their economic development is an improved understanding of the geological controls on reservoir performance and how this impacts reservoir management decisions. This has been achieved through an integrated study of numerous static and dynamic datasets. The Zubair Formation was deposited by fluvial-dominated, tide-influenced deltas during a period of enhanced sediment supply from the hinterland. High sediment supply and low accommodation space led to very high net to gross deposits on the alluvial plain and into the proximal delta front. Nevertheless, several scales of geological heterogeneity exist, each exerting a strong control on reservoir performance. At the coarsest scale, the reservoir is layered, with layers defined by mudstones deposited above widespread flooding surfaces. These mudstones cause multiple moved oil–water contacts and hold back significant pressure. Their identification and correlation enables the development of a perforation strategy where each reservoir flow unit is completed separately, avoiding cross-flow and lost production. Between the flooding mudstones, an array of paralic depositional elements was deposited as the delta system repeatedly advanced and retreated. The type, geometry, and connectivity of these depositional elements define the reservoir architecture, which controls large-scale sweep efficiency and the habitat of remaining hydrocarbons. An improved understanding of these elements and their control on sweep has facilitated a successful infill drilling campaign. Finally, many of the reservoir sandstone types contain numerous fine-scale geological heterogeneities, which exert a control on small-scale sweep efficiency. An improved understanding at this detailed level is important for determining the expected recovery factors and future water-handling capacity requirements.

Abstract: An integrated sedimentological and petrophysical approach was implemented to define the role of facies diversity and cyclicity on the reservoir quality of the Mishrif Formation in several oil fields in southern Iraq. The reservoir quality in most regressive cycles was enhanced upwards from deep-marine facies towards the shallower shelf-margin facies. The change in reservoir quality could be detected in the facies stacked systematically within the regressive cycles, which was also easily recognized using the porosity logs. The impact of early diagenetic overprints was quite obvious in developing both reservoir and non-reservoir rock types within the Mishrif Formation in the study area. A simple rock-typing nomenclature was proposed based on the available data in order to classify the existing reservoir (R) and non-reservoir (S) rock types. The best-recognized reservoir rock types were rudistid microfacies with grain-dominated fabrics (including both grainstone (R1) and grain-dominated packstone (R2)), which were subjected to an early diagenetic dissolution process, usually located beneath discontinuity surfaces. Such reservoir units or rock types have a regional extent within the southern Mesopotamian Basin, as they have often developed during the Mishrif shelf-margin progradation. In addition, the other important reservoir rock type was a microbialite (i.e. peloidal mud-dominated packstone (R3)), which was additionally characterized by micropores within the mud-dominated portion of the facies. However, owing to the variable intensity of the diagenetic effects and differences in the depositional texture components, the reservoir quality in this rock type could vary regionally. The regional distribution of the rudistid grainstone and grain-dominated packstone reservoir rock types (R1 and R2) was mostly related to the palaeogeographical highs that existed during deposition. However, such reservoir rock types could pinch out within the depositional sequences, showing their potential to become stratigraphic traps outside the structural crest of the field. The delineation of the reservoir rock types within a sequence-stratigraphic framework can be quite beneficial for reservoir prediction and exploration within and outside of the field.