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 The Cretaceous (Lower Aptian) Shu’aiba Formation in Shaybah Field, Saudi Arabia, is a giant carbonate reservoir that formed on a regional carbonate ramp bordering an intra-shelf basin. The main objective of this study was to construct a high-resolution facies-based, sequence-stratigraphic framework using cores, logs and available isotope data for reservoir characterization and development. This study will also help refine our understanding of global climate and sea-level history in the Early Cretaceous Aptian Stage. The succession consists of a composite sequence of seven high-frequency sequences. Sequences 1 and 2 formed a deeper open platform of Palorbitolina-Lithocodium wackestone, with maximum flooding marked by planktic foraminifera mudstone. Sequence 2 built relief over the northern and southern blocks, separated by an intra-platform depression. Sequence 1 and part of Sequence 2 form the transgressive systems tract for the composite sequence. The remaining sequences developed a platform rimmed by rudist rudstone, backed by rudist floatstone back-bank and lagoonal fine skeletal peloidal packstone; slope facies are fine skeletal fragmented packstone. Aggradational sequences 3 to 5 make-up the composite sequence early highstand. Progradational sequences 6 and 7 are the composite sequence late highstand marking the deterioration of the Offneria rudist barrier and deposition of widespread lagoonal deposits, where accommodation may have been created by syn-depositional growth faulting that moved the northern block down. Shu’aiba deposition on the platform was terminated by long-term sea-level fall and karsting. The presence of fourth-order sequences and ca. 100 ky parasequences, which are driven by long- and short-term eccentricity, suggests that the Early Cretaceous climate may have been cooler than generally believed and was not an ice-free greenhouse World. Small ice sheets may have been present. This is pertinent to the debate concerning whether the Aptian was a time of green-house climate typified by small precessionally-driven sea-level fluctuations, or whether small ice sheets at the poles generated moderate amplitude fourth-order fluctuations, perhaps driven by eccentricity.

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.

ABSTRACT This study evaluates the sequence architecture, facies and reservoir quality of Shu’aiba carbonates deposited along the Bab Basin margin during the Late Aptian lowstand (Sequence 5 of Yose et al., 2010). Seismic data display a number of thin prograding and down-stepping high-frequency sequences within this second-order lowstand systems tract that are seismically imaged by one or occasionally two clinoforms. These thin sequences vary in thickness from approximately 50–80 m but the distance of progradation is remarkably constant in each pulse of progradation. Sixteen cores through various portions of this prograding system document the facies distribution within the clinoforms. Each prograding sequence has a narrow belt of grainstones and rudist biostromes at the shelf break, whereas the rest of the clinoform topset and foreset is dominated by mud-rich orbitolinid mud- to packstones. Argillaceous mudstone occurs in two places within the clinoforms; it dominates the bottomset but is also present in several topsets where argillaceous mudstones alternate with limestone. The near constant thickness of the argillaceous mudstone along the investigated segment of the margin indicates that argillaceous material is far-sourced and transported by longshore currents. The observed facies distribution determines the reservoir and seal potential of the prograding clinoforms. Porosity and permeability data from core plugs and diagenetic patterns observed in thin sections indicate that the best reservoir quality is associated with high-energy sediments at the topset, whereas both lagoonal and slope deposits might act as low-permeability flow barriers. The expected reservoir bodies in the Upper Shu’aiba seem vertically confined to a few tens of feet but laterally continuous over several tens of kilometres along the clinoform strike. Possible hydrocarbon accumulations in these clinoforms might therefore reach economically significant volumes.

ABSTRACT Compelling evidence has been found for major fluvial incisions at the top of the Aptian Shu’aiba Formation in Block 5, offshore Qatar. Log correlation, horizontal well results and seismic mapping reveal a major east-west trending, meandering, terraced, incised valley running across Block 5, with widely distributed subsidiary valleys. Locally, the main valley reaches 8 km in width and has a depth of at least 30 m. Additional evidence for exposure comes from numerous clay-filled and, occasionally, sand-filled karst fissures that have been encountered at the top of the Shu’aiba Formation, penetrating at least 25 m below the top Shu’aiba surface. At one location the complete channel-fill was cored and found to consists of tidally influenced estuarine sandstone deposits, rich in dispersed plant material, coal and amber at the base, passing up into shoreface deposits at the top. The overlying succession consists of a thin, sandy interval, rich in glauconite and ferruginous ooids, that blankets the entire Shu’aiba Formation. These deposits are interpreted as a transgressive, back-fill systems tract of a major valley system. The overlying oolitic ironstones and glauconitic sandstones are interpreted as highly condensed facies. The valley-fill sandstones are of late Late Aptian age and the overlying condensed sheet sandstones are of latest Aptian to Early Albian age. This entire siliciclastic package is interpreted as an uppermost Aptian – Lower Albian depositional sequence that, in both lithological composition and age, is distinct from the overlaying Nahr Umr Formation. This is the first reported case of Upper Aptian siliciclastics in Qatar. The importance of these observations is that they provide proof for (1) subaerial exposure of the Shu’aiba Formation in Block 5, (2) an early Late Aptian relative drop in sea-level of at least 30 m, (3) a latest Aptian sea-level rise of a similar magnitude and (4) a condensed phase, probably a sea-level stillstand, during the Early Albian. Since relative sea-level fluctuations of this magnitude are likely to have a regional expression, the here established Late Aptian sea-level curve provides a reference for the Arabian Plate. The palaeogeographic implication of these observations is that during the Late Aptian most of Qatar and adjacent offshore areas were subaerially exposed and the Bab Basin was connected with the siliciclastic sources in the northwestern part of the Arabian Plate by large river systems, which cut into the exposed carbonate platforms.

ABSTRACT A regional sequence-stratigraphic model is presented for the Barremian to Lower Albian sedimentary systems of southwest Iran, based on seismic-scale outcrop sections, and constrained by new biostratigraphic and chemostratigraphic age dating (ammonites, planktonic forams, orbitolinids and carbon and oxygen isotope curves). This study proposes a fundamental revision of the geometrical relationships between classically used lithostratigraphic units, and demonstrates the relative influence of both eustatic sea-level fluctuations and regional tectonic control on sedimentation. These new insights have significant implications for both the understanding of the sedimentation patterns, as well as the architecture of the mid-Cretaceous petroleum systems in the studied region. The following depositional sequences are defined: (1) two third-order sequences of Barremian age, which have a flat-bedded stratigraphic architecure and consist of low-angle, mixed carbonate-siliciclastic ramp systems (Gadvan Formation). The maximum flooding surface of the Arabian Plate (AP) Bar 2 Sequence has been dated with the occurrence of the short-range index fossil Montseciella arabica . (2) an Aptian second-order Supersequence which shows a geometrical evolution from a flat-bedded architecture to a carbonate platform to organic-rich intra-shelf basin topography and corresponds to part of the Dariyan Formation and the Kazhdumi Formation. This evolution can be subdivided in three phases which are related to the Aptian third-order sequences defined in the UAE and Oman: the early transgression (Apt 1 sequence), the late transgression (Apt 2) and the highstand (Apt 3 and 4). The maximum flooding occurred in, or just after, the D. deshayesi ammonite zone. (3) an Upper Aptian – Lower Albian second-order Supersequence, represents the continuation of the carbonate platform to organic-rich intra-shelf basin system established in the previous supersequence. It corresponds to the Upper Dariyan and Kazhdumi formations and the ‘Burgan equivalent’ informal unit. During the initial lowstand sedimentation (Apt 5) was confined to the intra-shelf basin, and during the transgression and highstand (Apt 6, Alb 1-2) only part of the exposed carbonate platforms were flooded, with facies varying from orbitolinid limestones along the northern margin to condensed siliciclastics along the southern margin. Sedimentation was controlled by the eustatic sea-level variations, the influx of siliciclastics and the activation of regional fault systems such as the Kazerun Fault and the Hendijan Fault. The fault activiation caused the differentiation between the southern coastal Fars area, with relatively less accommodation, and the northern Khuzestan and Lurestan area, which subsided more rapidly. Due to the high-resolution biostratigraphic control the sedimentation patterns in these two tectonically different-behaving areas could be compared and depositional sequences defined. The time control also allowed comparison with the sequences defined in Oman, Qatar and the UAE, which are for the Barremian and Aptian very similar. The Upper Aptian – Lower Albian Supersequence is well documented in Iran and in Qatar, whereas in the UAE and Oman only the lowstand of this system is documented.

ABSTRACT The Shu’aiba reservoir that is the focus of this study is one of a family of reservoirs in the Middle East that produces from a carbonate ramp-margin rudist complex along the margin of the Lower Aptian Bab Basin (field referred to here as North Oman Ramp Margin Field or RMF). This margin depositional setting imparts a level of petrophysical heterogeneity that adds greatly to the production complexities of the field. Accordingly, a series of detailed reservoir studies have been undertaken to better understand the stratigraphic and petrophysical architecture of the reservoir. As part of this field development study for the RMF, regional and outcrop data along with field analogues were reviewed and integrated to refine geological models and concepts for the field. Newly acquired well data (core, high-resolution resistivity image logs, dipmeter and conventional electric logs) and seismic data were integrated with the available production performance data for input into 3-D static reservoir models. Of the six stratigraphic sequences defined for North Oman, three third-order composite sequences of the Lower Shu’aiba Formation (LSh1, LSh2, LSh3) and one third-order composite sequence of the Upper Shu’aiba Formation (USh1) are recognised in the field. The Lower Shu’aiba 1 Sequence (LSh1) is the initial transgressive sequence comprised of laterally extensive shallow-marine to intertidal deposits, including the argillaceous limestones of the Hawar Member. The Lower Shu’aiba 2 Sequence (LSh2) is typically more mud-rich and characterised by Bacinella-Lithocodium wackestones alternating with Planolites -burrowed skeletal wackestones and less common mollusk-skeletal mud-dominated packstones. This sequence represents continued deepening following the initial transgression recorded by the Lower Shu’aiba 1 Sequence. The Lower Shu’aiba 3 Sequence (LSh3) is comprised of aggradational to slightly progradational rudist buildup complexes with a multifaceted lateral and vertical arrangement of high-energy caprinid rudist banks and interbank channels and shoals. The LSh3 Sequence is by far the most complex in terms of facies distribution, stratal geometry and flow-unit architecture within the Shu’aiba of Oman. Towards the northwest of the Lower Shu’aiba ramp margin, a wedge of Upper Shu’aiba rocks (USh1) is preserved. Comprised of carbonate grainstone/packstones wedges, these strata reflect a shift towards lower accommodation within the Bab Basin, where individual clinoforms pinch out into tight outer-ramp/basin carbonate mudstones. Reservoir properties are strongly related to depositional facies and facies-related diagenesis. Grain-dominated lithofacies of the buildup/flank/channel facies tract in the LSh3 Sequence constitute the main reservoir rocks with porosity of about 18% and permeability of 100s to 1,000s of millidarcies. Carbonate mud-dominated lithofacies of the buildup core and lower transgressive sequences have higher porosity but only a few to 10s of millidarcy permeability. In general, the vertical permeability profile is one of decreasing permeability with depth, with significant flow baffles (Kh < 1mD) present in the LSh3 reservoir units enhancing the permeability contrast. Based on field and simulation experience, the permeability contrast has a large impact on field performance and ultimate recovery.

ABSTRACT When building static reservoir models for fields with limited well data, the use of analogue data can help constrain uncertainty ranges for property modelling. Several Shu’aiba fields in the Ghaba Salt Basin (northern Sultanate of Oman) with good well coverage were used as analogues to model a nearby field with very limited well coverage. The analogue fields are located within a 50 km radius, roughly along strike of the Bab Basin and in a similar stratigraphic position and tectonic setting. Correlations between the modelled and analogue fields were set within a regional sequence-stratigraphic framework and are based on gamma-ray and porosity logs, carbon-isotope stratigraphy, as well as facies successions. Systems tract thicknesses and vertical facies successions were found to follow predictive trends. This allowed their interpretation in three dimensions away from well-data points with more confidence. Porosity histograms were established for each systems tract and porosity-permeability relationships, as well as vertical permeabilities, were determined for three facies from all analogue fields in the area. Diagenetic overprint led to very similar porosity-depth trends and improvement of reservoir properties below the seal in all fields. The established relationships and data ranges considerably helped constrain the rock property uncertainties of the field with limited well data. Furthermore, a common modelling approach for all the genetically related fields was established, which reduced model-building and cycling times for all fields.

ABSTRACT Characterizing fractures is essential in effectively developing carbonate reservoirs, especially when they have high porosity but very low matrix permeability. It is important to know and locate fractured areas and define the intensity and orientation of open fractures because they provide the main conduits of fluid flow. Open fractures cause anisotropy in reservoir rocks that may be reflected as a measurable anomaly in azimuthal variation of amplitude or amplitude-versus-offset (AVO), in velocity or travel-time, and in shear-wave splitting. All these anomalies, usually referred to as azimuthal anisotropy, can be extracted from multi-component, wide-azimuth seismic data and after integration with image-log information, can be used for fractured reservoir characterization. Recently, Occidental Petroleum of Qatar Ltd., who operates under a development production sharing agreement (DPSA) from Qatar Petroleum, acquired over Idd El Shargi fields one of the largest wide-azimuth 3D4C surveys in the region. In this paper, we show results of integration of seismic anisotropy and other fracture-related seismic attributes with image-log information to map fracture intensity and orientation in the Shu’aiba carbonate reservoir. We found that the mean dip-azimuth of open fractures is 137° and the mean dip 81°. The calculated fracture density for the fracture swarms vary from 0.5 to 1 fracture/meter. The results have been instrumental in day-to-day drilling operations and are crucial data in reservoir fracture characterization and discrete fracture network (DFN) model building.