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ABSTRACT

A combined approach of sedimentology, carbon- and strontium-isotope chemostratigraphy, biostratigraphy, and seismic stratigraphy was used to constrain the sequence-stratigraphic correlation of the Shu’aiba reservoir at an oil field in northwest onshore Abu Dhabi. Sedimentologic and petrographic core description identified 14 reservoir and three non-reservoir lithofacies types, ranging from shallow-marine, rudist rudstone to basinal, planktonic foraminifera wackestone and shale.

Carbon-isotope curves from individual wells allowed for correlation between depositional sequences and numerical ages were obtained for selected levels using strontium-isotope stratigraphy (SIS). This has helped to develop a more precise chronostratigraphy for the Hawar Member of the Kharaib Formation and the overlying Shu’aiba Formation in the studied region. The importance of suitable sample material for SIS is discussed and different results obtained from skeletal calcite and bulk rock samples are evaluated. The numerical ages derived from SIS are consistent with the results of carbon-isotope stratigraphy and proved to be particularly helpful in the identification of the Lower/Upper Aptian boundary, which is difficult to draw based on the carbon-isotope curve alone. Distinct taxa of rudist bivalves such as Offneria spp. are characteristic for Early Aptian high-energy environments, but are missing in the Late Aptian due to a pronounced extinction event affecting rudists at the Early – Late Aptian transition.

The sequence stratigraphy of the studied field is discussed. The Hawar Member and the Shu’aiba Formation are interpreted in terms of seven third-order sequences. Early Aptian Hawar Sequence 1 (Hawar Member), together with Early Aptian Shu’aiba sequences 2, 3a, and 3b form the transgressive and early highstand systems tracts of the Arabian Plate Aptian Supersequence. These four sequences are seismically distinct; 3-D seismic data was used to map their areal extent over the interior platform and platform margin. Shu’aiba Sequence 2 contains the maximum flooding surface of the supersequence (MFS K80). Late Aptian Shu’aiba sequences 4a and 4b cannot be seismically separated and together represent the late highstand of the supersequence. Above the upper boundary of the supersequence, the youngest Shu’aiba Sequence 5 was deposited during a Late Aptian lowstand; it is evident in 3-D seismic data in the northern part of the field, which extends into the Bab Basin.

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