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ABSTRACT

This paper presents a regional well and seismic based synthesis of the Shu’aiba Formation and Hawar Member of the southern Arabian Plate with an emphasis on the United Arab Emirates (UAE) and Oman. The focus of the study is on the upper part of the Shu’aiba Formation, which has been studied in detail, combining seismic geometries, well logs and core material to propose a depositional model and a sequence-stratigraphic interpretation, and evaluate the reservoir potential of this unit.

The Shu’aiba Formation was deposited in Aptian times on the southern part of the Arabian Plate, in and around an intra-cratonic basin, the Bab Basin, over a period of 11–12 million years. The Shu’aiba Formation consists of five third-order sequences. The first four, referred to as Shu’aiba sequences 1 to 4 in this paper, make up an Early to early Late Aptian second-order sequence, while the fifth (Shu’aiba Sequence 5) represents the Late Aptian lowstand system tract of the succeeding second-order sequence. Sequence 5 was deposited within the Bab Basin and comprises several higher-frequency (fourth-order) sequences or cycles prograding out from the basin margins into basin-centre facies. It is commonly referred to as “Upper Shu’aiba” or “Sequence 5” in this paper. The sequence boundary at the base of Sequence 5 is characterised by a major exposure across the Shu’aiba Platform and down-stepped deposition in the Bab Basin. The onset of clay deposition marks the beginning of this sequence.

New 3-D seismic is key in unravelling the Upper Shu’aiba depositional history and regional stratigraphy. Shu’aiba seismic facies, spectacularly displayed on the new 3-D seismic, allows detailed mapping of the Shu’aiba sequences. The many calibration points available in this study confer a high degree of reliability to the proposed depositional model of the Shu’aiba and the palaeogeographic maps presented in this paper.

The Upper Shu’aiba Sequence 5 is the main focus of this study. It consists of nine high-frequency sequences that prograded into the Bab Basin as a series of clinoforms over more than 30 km. The most striking observation is the extremely regular and seaward progradation of the clinoforms, sub-linear along the southern margin and concentric around the eastern isolated platforms, resulting in a series of almost perfectly parallel clinoforms. The prograding clinoforms visible on seismic attribute maps of a 3-D seismic survey in southeast Abu Dhabi can be grouped into three sets. The four clinoforms in the first set prograded about 18 km basinward. The top surface of these first clinoforms lies 30–40 m (98−131 ft) below the top of the previous shelf break. The three clinoforms of the second set prograded 8–10 km into the Bab Basin. The last two clinoforms (third set) prograded 8–10 km basinward. The almost perfect parallel geometry of the clinoforms may be the result of a regular distribution of clay and carbonate mud along the margins by long-shore currents.

The lithologic content of the clinoforms slightly varies within the three sets. The topsets of the first and second sets of clinoforms are occupied by rudist-bearing floatstone–rudstone, peloidal packstone and miliolid grainstone facies, the foresets are composed of burrowed argillaceous wackestone, and the bottomsets of calcareous shale. In the topsets of the second set of clinoforms interbedded muddy limestone and marls are widespread, and the third set is mud-dominated. The topsets rich in rudstone and grainstone form the main reservoir in the Shu’aiba Sequence 5, whereas the clay-rich intervals may provide seals for a stratigraphic play.

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