f Regional Depositional History and Stratigraphy of the Shu’aiba Formation in the Middle East (Best of Shu'aiba)

The carbonates of the Aptian Shu'aiba Formation form major oil reservoirs in the Middle East. The formation shows a complex internal stratigraphic architecture and significant lateral variations in reservoir character. During a 3-day workshop held in Oman, which was attended by Shu’aiba experts all over the world, a common understanding of the stratigraphic framework was reached, which led to a consistent regional model at the scale of the southern Arabian Plate. The Shu'aiba Formation represents a depositional cycle within a Cretaceous epeiric carbonate platform system. It is built by three composite sequence sets, each with different facies associations and internal stacking patterns and, consequently, also different reservoir architecture. The lowermost unit is early Aptian in age and includes the Hawar Member of the underlying Kharaib Formation and the basal Shu'aiba Formation. It forms a Transgressive Sequence Set with laterally extensive shallow-marine to intertidal sediments followed by shallow-marine algal limestones that transgressed over exposed limestones of the Kharaib Formation. The development of algal-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. The second unit is early to late Aptian in age, and consists of a Highstand Sequence Set (HST) with progradational geometries. The "carbonate factory" was then dominated by rudist-biostrome complexes, which colonized the mounds. Intermound depressions were progressively filled leading to the formation of a large platform that surrounded an intra-platform basin. The third unit is formed by a Lowstand Sequence Set (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 LST contains off-lapping wedges of argillaceous carbonates and carbonate-rich claystones along the margins of the intra-platform basin. Clays deposited during the following transgression provide a regional seal to the Shu'aiba reservoirs. The advantage of this sequence stratigraphic framework is that it can be used as a template which: 1) Explains the regional variation in reservoir/flow properties. 2) Allows grouping of fields according to stratigraphic setting for comparison / analogues in reservoir studies.