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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.

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