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ABSTRACT

This study evaluates the sequence architecture, facies and reservoir quality of Shu’aiba carbonates deposited along the Bab Basin margin during the Late Aptian lowstand (Sequence 5 of Yose et al., 2010). Seismic data display a number of thin prograding and down-stepping high-frequency sequences within this second-order lowstand systems tract that are seismically imaged by one or occasionally two clinoforms. These thin sequences vary in thickness from approximately 50–80 m but the distance of progradation is remarkably constant in each pulse of progradation. Sixteen cores through various portions of this prograding system document the facies distribution within the clinoforms. Each prograding sequence has a narrow belt of grainstones and rudist biostromes at the shelf break, whereas the rest of the clinoform topset and foreset is dominated by mud-rich orbitolinid mud- to packstones. Argillaceous mudstone occurs in two places within the clinoforms; it dominates the bottomset but is also present in several topsets where argillaceous mudstones alternate with limestone. The near constant thickness of the argillaceous mudstone along the investigated segment of the margin indicates that argillaceous material is far-sourced and transported by longshore currents.

The observed facies distribution determines the reservoir and seal potential of the prograding clinoforms. Porosity and permeability data from core plugs and diagenetic patterns observed in thin sections indicate that the best reservoir quality is associated with high-energy sediments at the topset, whereas both lagoonal and slope deposits might act as low-permeability flow barriers. The expected reservoir bodies in the Upper Shu’aiba seem vertically confined to a few tens of feet but laterally continuous over several tens of kilometres along the clinoform strike. Possible hydrocarbon accumulations in these clinoforms might therefore reach economically significant volumes.

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