Three-dimensional Seismic- and Sequence-stratigraphic Framework of a Lower Cretaceous, Aptian, Supergiant Carbonate Field in Onshore Abu Dhabi (Best of Shu'aiba)

A three-dimensional (3-D) dataset over a carbonate field in onshore Abu Dhabi was evaluated to develop integrated sequence-stratigraphic and reservoir models for the Lower Cretaceous (Aptian) Shu’aiba Formation. The study area is positioned over the Shu’aiba platform-to-basin transition and provides a type area for linking the stratigraphic framework across the shelf, slope and basin environments (Yose et al., 2006). The more than 50 kilometer 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 reservoir interval (Shu’aiba Formation) records a second-order supersequence that is divided into five (5) depositional sequences. Rudists, carbon and strontium isotopes and nanno-fossils provide internally consistent age constraints on Upper versus Lower Aptian sequences. The age dating also allows for tying sequences to the higher resolution Aptian (AP) chronostratigraphic zonation in Hardenbol (1998). Sequence 1 (Lower Aptian; possible AP-1 and AP-2 age) and Sequence 2 (Lower Aptian; dated as AP-3, lower part) were deposited during the transgressive phase of the supersequence. These sequences 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 (Lower Aptian; dated as AP-3, upper part) was deposited during the highstand phase of the supersequence, is mainly aggradational, and records the proliferation or rudists across the platform top. Grain-dominated platform interior and margin facies of Sequence 3 are the highest quality reservoir facies within the Shu’aiba reservoir. Sequence 4 (Upper Aptian; dated as AP-4) and Sequence 5 (Upper Aptian; dated as AP-5), were deposited during the late highstand phase of the supersequence. These sequences are progradational and record the progressive downstepping (forced regression) of the platform margin 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 6 (probable AP 6 time; Upper Aptian to Albian?) was deposited during the second-order lowstand, records the influx of fine-grained siliciclastics, and forms the base of the next overlying supersequence. 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 evolved into a series of isolated platforms during the transgressive to highstand phases in the Early Aptian. Some of these isolated platforms are interpreted to have coalesced via progradation as second-order sea level began to fall in the Late Aptian. The platform interior and margin facies show a marked asymmetry from north to south that are interpreted to be related to ocean currents or prevailing winds. These results provide new insights on reservoir quality and connectivity. In the platform interior area of Sequence 3, 3-D seismic data reveal a complex mosaic of tidal channels, high-energy rudist shoals, and inter-shoal ponds 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.