Abstract The United Arab Emirates is located on the stable Arabian foreland of the Arabian plate and is separated from the unstable Iranian fold belt by the Arabian Gulf. During the Late Paleozoic (Upper Permian) to the Cenozoic (Tertiary) Eras the vast Arabian platform lay along the southern margin of the Tethys Ocean. During this period epeiric shelf carbonates associated with only minor clastics and evaporites were deposited. Sedimentation patterns were controlled by many factors such as epeirogenic vertical movements due to basement tectonism, halokinesis, climatic variations, and, most importantly, sea-level variations. The Late Paleozoic to Cenozoic stratigraphic sequence shows lateral variations in formation thicknesses as well as in the distribution and continuity of lithofacies characteristics. Abundant giant oil and gas reservoirs have been found in Jurassic (Araej and Arab) and Cretaceous (Habshan, Lekhwair, Kharaib, Shuaiba, Mishrif, and Simsima) formations. Gas was discovered in the Upper Permian (Khuff Formation) carbonates in offshore and Upper Jurassic (Arab Formation) carabonates in onshore Abu Dhabi. Most hydrocarbon accumulations are related to structural traps, although combined stratigraphic-structural or stratigraphic traps exist in some areas. In western Abu Dhabi, most of the oil and gas are in Jurassic reservoirs, whereas in the central areas most of the oil is in Lower Cretaceous reservoirs. In the eastern offshore areas (Abu Dhabi and Dubai), oil and gas reservoirs are of Permian and Middle Cretaceous with minor production from Lower Cretaceous intervals. In western Abu Dhabi, structural traps started developing in the Upper Jurassic, getting progressively younger toward the east. The main (peak) tectonic event that shaped most of the structures occurred at the end of the Middle Cretaceous and was related to the obduction of the Oman Ophiolite and the formation of the Oman Mountains. The Tithonian Hith anhydrite and the Albian Nahr Umr shale are the two principal sealing formations of the oil and gas accumulations in the Jurassic and Cretaceous reservoirs. However, secondary seals and barriers also exist throughout the stratigraphic sequence. The Silurian Qusaiba Formation is the main source rock for the Permian gas reservoirs. The Upper Jurassic Diyab/Hanifa Formation and the Middle Cretaceous Shilaif/Khatiyah Formation are the main source rocks for the giant Jurassic and Cretaceous carbonate reservoirs, respectively. Other potential source rocks are also identified within the Lekhwair and Shuaiba (Bab Member) formations.

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.

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.

Abstract Important hydrocarbon accumulations occur in platform carbonates of the Lower Cretaceous Kharaib (Barremian and early Aptian) and Shuaiba (Aptian) formations (upper Thamama Group) of Abu Dhabi. The Kharaib and Lower Shuaiba formations contain three reservoir units separated by three low-porosity and low-permeability dense zones. From base to top, the thickness of the reservoir intervals range from approximately 80, 170, to 55 ft (24, 51, to 16 m), respectively, for the Lower Kharaib, Upper Kharaib, and Lower Shuaiba Reservoir Units. Core and well-log data of a giant oil field of Abu Dhabi, as well as outcrop data from Wadi Rahabah in the Emirate of Ras Al-Khaimah were used to establish a sequence-stratigraphic framework and a lithofacies scheme, applicable to all three reservoir units and the three dense zones. The Lower and Upper Kharaib Reservoir Units, as well as the lower, middle, and upper dense zones are part of the late transgressive sequence set of a second-order supersequence, made up of two third-order composite sequences. The overlying Lower Shuaiba Reservoir Unit belongs to the late transgressive sequence set and the early highstand sequence set of this second-order supersequence and is made up of one third-order composite sequence. The three third-order composite sequences are composed of 19 fourth-order parasequence sets that show predominantly aggradational and progradational stacking patterns, typical of greenhouse cycles. Conventionally, composite sequence boundaries are placed at or near the base of the three dense zones. As an alternative scenario, the possibility that the major composite sequence boundaries actually occur on top of these dense zones is discussed. On the basis of faunal content, texture, sedimentary structures, and litho-logic composition, 13 reservoir lithofacies and 8 nonreservoir (dense) lithofacies are identified from core. Similar lithofacies are identified in time-equivalent rock exposures studied in Wadi Rahabah. Depositional environments of reservoir units range from lower ramp to shoal crest to near-back shoal open-platform deposits. Dense zones were deposited in an inner-ramp, restricted shallow-lagoonal setting. Intensively bioturbated wackestone and packstone, and interbedded organic- and siliciclastic-rich limestone, characterize the dense zones. Locally, mud cracks, blackened grains, and rootlets are observed. Outcrop analogs of subsurface reservoirs allow for a detailed investigation of facies architecture and structure of carbonate bodies. Integration of subsurface and outcrop data (e.g., low-angle clinoforms that cannot be seen in core data) leads to more insightful and realistic geological models of subsurface stratigraphy. Geological model realizations based on core, outcrop, well-log, and seismic data constrain fluid flow-simulation models. Results mimic known behavior in analogous producing fields, and the process of going from rock data to simulation provides a useful training tool for reservoir characterization methods and techniques.

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.

Abstract A new sequence-stratigraphic framework is proposed for the Burgan and Mauddud formations (Albian) of Kuwait. This framework is based on the integration of core, well-log, and biostratigraphic data, as well as seismic interpretation from giant oil fields of Kuwait. The Lower Cretaceous Burgan and Mauddud formations form two third-order composite sequences, the older of which constitutes the lowstand, trans-gressive, and highstand sequence sets of the Burgan Formation. This composite sequence is subdivided into 14 high-frequency, depositional sequences that are characterized by tidal-influenced, marginal-marine deposits in northeast Kuwait that grade into fluvial-dominated, continental deposits to the southwest. The younger composite sequence consists of the lowstand sequence set of the uppermost Burgan Formation and transgressive and highstand sequence sets of the overlying Mauddud Formation. This composite sequence is sand prone and mud prone in southern and southwestern Kuwait and is carbonate prone in northern and northeastern Kuwait. The lowstand sequence set deposits of the Burgan Formation are subdivided into five high-frequency depositional sequences, which are composed of tidal-influenced, marginal-marine deposits in northeastern Kuwait that change facies to fluvial-dominated deposits in southwestern Kuwait. The transgressive and highstand sequence sets of the Mauddud Formation are subdivided into eight high-frequency, depositional sequences. The Mauddud transgressive sequence set displays a lateral change in lithology from limestone in northern Kuwait to siliciclastic deposits in southern and southwestern Kuwait. The traditional lithostratigraphic Burgan-Mauddud contact is time transgressive. The Mauddud highstand sequence set is carbonate prone and thins south- and southwestward because of depositional thinning. Significant postdepositional erosion occurs at the contact with the overlying Cenomanian Wara Shale. The proposed sequence-stratigraphic framework and the incorporation of a depositional facies scheme tied to the sequence-stratigraphic architecture allow for an improved prediction of reservoir and seal distribution, as well as reservoir quality away from well control.