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ABSTRACT

The Shu’aiba Formation is the most important carbonate reservoir for oil in the Sultanate of Oman. A seismo-stratigraphic review revealed a complex internal stratigraphic architecture that explains major local and regional variations in reservoir character that occurs within this interval. The seismic data allowed significant refinement and improvement of the previous sequence-stratigraphic models, which were based on well data only.

The Shu’aiba Formation, which is up to 130 m thick, is part of a Cretaceous epeiric carbonate platform system. It conformably overlies the dense carbonates of the Hawar Member (upper part of Kharaib Formation) and is covered by the shales of the Nahr Umr Formation, in south and east Oman with an unconformable contact. The Shu’aiba Formation is built by three major genetic units (systems tracts) each with a different distribution both in time and space, in response to different trends in relative sea-level change. Each of these contains different facies associations and internal stacking patterns and, consequently, also a different reservoir architecture.

The lowermost unit is Early Aptian in age and includes the upper part of the Hawar Member and the basal Shu’aiba Formation. It forms a transgressive systems tract (TST) with laterally extensive shallow-marine to intertidal sediments followed by shallow-marine algal limestones that transgressed over exposed limestones of the upper Kharaib Formation. The development of algal-stromatoporoid mound complexes led to a differentiation on the platform. Between these mounds fine-grained, in places organic-rich, deeper-water deposits represent the maximum flooding interval (MFI).

The second unit is Early to Late Aptian in age, and consists of a highstand systems tract (HST) with progradational geometries. The “carbonate factory” was then dominated by rudist-biostrome complexes, which colonised the mounds. Inter-mound depressions were progressively filled leading to the formation of a large platform that surrounded an intra-platform basin.

The third unit was formed by a lowstand systems tract (LST) deposited in the Late Aptian during and following a regional drop in sea level. The drop caused subaerial exposure of the HST platform and the influx of fine-grained clastics. The early LST contained off-lapping wedges of argillaceous carbonates and carbonate-rich claystones along the margins of the intra-platform basin. Ultimately the whole platform interior was exposed and a late LST wedge was deposited along the ocean margin of the platform. The basal clays of the Nahr Umr Formation formed the TST, which is capped by an MFI, the Marker Limestone Bed, located a few metres above the Shu’aiba Formation.

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