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ABSTRACT

A three-dimensional (3-D) dataset over a carbonate field in onshore Abu Dhabi, United Arab Emirates, was evaluated to develop an integrated sequence-stratigraphic model for the Lower Cretaceous (Aptian) Shu’aiba Formation. The study area is positioned over the Shu’aiba platform-to-basin transition and provides a key area for linking the stratigraphic framework across the shelf, slope and basin environments. The more than 50 kilometer (31 mile) long shelf-to-basin transect records a diverse range of depositional facies and stratal geometries that have a large impact on reservoir quality and connectivity across the study area.

The Aptian section in the study area records a second-order supersequence that is divided into four (4) depositional sequences (sequences 1-4). An additional sequence is recognized (Sequence 5) that is interpreted as the Lowstand Phase of the overlying Upper Aptian to Albian Supersequence. Results from biostratigraphic and isotopic age dating provide improved age control on the Upper and Lower Aptian sequences. There is good correlation of sequences recognized in this study with those recognized in the Aptian from other localities on the southern Arabian Plate. An attempt was made to correlate sequences recognized on the Arabian Plate with the Hardenbol et al. (1998) sequences derived from European basins. While there is remaining uncertainty in the age model, current results indicate some differences in second- and third-order sea-level trends between the two areas.

The Aptian section on the Arabian Plate records a second-order rise (Early Aptian) and fall (Late Aptian) of sea level on which the third-order sea-level trends are superimposed. In response, third-order sequences display a predictable stacking pattern and have characteristic stratal geometries and depositional facies. Sequences 1 and 2 are Lower Aptian and were deposited during the Transgressive Phase of the Supersequence. Sequence 2 is age equivalent to the global oceanic anoxic event (OAE1a). Sequences 1 and 2 are retrogradational, record the initial formation of a low-relief ramp, and are dominated by algal-microbial facies. Ramp interior and margin facies of the Transgressive Phase are characterized by high porosity and low permeability due to mud-dominated textures and development of microporosity. Sequence 3 is Lower Aptian and was deposited during the Highstand Phase of the Supersequence. Sequence 3 is aggradational to progradational and records the proliferation of rudists across the platform top. High-energy platform interior and margin facies of Sequence 3 are the highest quality facies within the reservoir. Sequences 4a and 4b are Upper Aptian and were deposited during the Late Highstand Phase of the Aptian Supersequence. These sequences are progradational and record the initial downstepping of the carbonate factory onto a low-angle (1-2 degree) slope. Clinoforms of the Late Highstand Phase are characterized by alternations of high (grainy) and low (muddy) reservoir quality developed in response to relative sea-level changes. Sequence 5 is Late Aptian to Early Albian? and was deposited during the second-order lowstand. Sequence 5 records the influx of fine-grained siliciclastics, and forms the prograding lowstand wedge of the overlying Albian Supersequence. Sequence 5 prograded a significant distance into the Bab Basin, outside of the study area.

Results from the study have led to an improved understanding of platform evolution and reservoir architecture. A new platform margin was recognized to the south, indicating the development of an intra-platform seaway. With margins now recognized to the north and south, it is possible that the Shu’aiba may have evolved into an isolated platform in the study area and that other isolated platforms may have developed along the Shu’aiba Platform margin during the long-term sea-level rise. Hydrodynamics played an important role in controlling stratal geometries and depositional facies distributions across the carbonate platform. Platform interior and margin facies show a marked asymmetry from north to south that is interpreted to be related to cross-bank currents. Also, long-shore currents, developed along the northern margin, influenced sediment redistribution in the slope and basin environments and impacted clinoform geometries. As second-order sea level gradually fell during the Late Aptian, isolated platforms are interpreted to have coalesced and currents in the southern seaway were diminished. Progradation continued into the Bab Basin during the second-order lowstand.

Results from this study have exploration- to production-scale implications. The recognition of Early Aptian isolated platforms along the margin of the Bab Basin and the basinward shift of carbonate sequences during the Late Aptian indicate that stratigraphic or combination traps may exist in these areas. The 3-D seismic data and updated sequence framework also provide new insights on reservoir quality and connectivity at the field scale. In the platform interior area of Sequence 3, seismic data reveal a complex mosaic of rudist shoals and mounds (high reservoir quality), inter-mound channels (variable reservoir quality), and inter-mound ponds (low reservoir quality) that impact reservoir sweep and conformance. At the basin margin, the seismic data provide high-definition images of platform margin clinoforms that impact reservoir architecture and well-pair connectivity.

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