Three-dimensional Characterization of a Heterogeneous Carbonate Reservoir, Lower Cretaceous, Abu Dhabi (United Arab Emirates)

Abstract High-effort three-dimensional (3-D) seismic data collected by the Abu Dhabi Company for Onshore Oil Operations (ADCO) are some of the highest quality data ever collected for a carbonate field. The 3-D seismic data were integrated with core and log data to develop a new, volume-based framework for enhanced reservoir characterization. The Lower Cretaceous (Aptian) reservoir is positioned over a platform-to-basin transition and records a diverse range of depositional facies and stratal geometries. Reservoir properties vary predictably based on position along the platform-to-basin profile and position in the sequence-stratigraphic framework. The Aptian reservoir interval (Shuaiba Formation) records a second-order sequence set that is divided into five depositional sequences. Sequences 1 and 2 were deposited during the transgressive phase of the sequence set. These sequences are retrogradational, record the initial formation of a low-relief ramp, and are dominated by algal-prone facies. Ramp interior and margin facies of the transgressive phase are characterized by high porosity and low permeability because of mud-dominated textures and development of microporos-ity. Sequence 3 was deposited during the highstand phase of the sequence set, is mainly aggradational, and records the proliferation or rudists across the platform top. Grain-dominated platform interior and margin facies of the highstand phase are the highest quality reservoir facies in the Shuaiba reservoir. Sequences 4 and 5 were deposited during the late highstand phase of the sequence set. These sequences are progradational and record the progressive downstepping of the platform margin onto a low-angle (1-2°) slope. Clino-forms of the late highstand phase are characterized by alternations of high and low reservoir quality developed in response to relative sea level changes. Sequence 6 was deposited during the second-order lowstand and forms the base of the next overlying sequence set. Sequence 6 is composed primarily of finegrained siliciclastics and is a nonreservoir. Results from the study have led to an improved understanding of platform evolution and a volume-based framework for reservoir characterization. The integrated data set provides new insights on platform paleogeography, carbonate facies architecture, and the geometry and mechanisms of carbonate platform progradation. In the platform interior area, 3-D seismic data reveal a complex mosaic of tidal channels, high-energy rudist shoals, and intershoal 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. Business applications of the volume-based reservoir framework include (1) use of 3-D seismic visualization technology for optimizing well placement, identifying bypassed reservoirs, and evaluating reservoir connectivity; (2) integration of quantitative, volume-based seismic information into reservoir models; (3) maximizing recovery through full integration of all subsurface data; and (4) enhanced communication among geoscientists and engineers, leading to improved reservoir management practices.