An extensive play review was carried out of the Mishrif Formation in Abu Dhabi. The database existed of almost 3000 2D seismic lines and 28 3D seismic surveys, complemented by 450 wells, out of which 260 had reliable time-depth data. The study resulted in Abu Dhabi-wide depth and thickness maps and a good overview of the Shilaif basin development and Mishrif reservoir distribution. The Mishrif Formation in Abu Dhabi comprises the progradational shelf margin facies which is a time equivalent of the corresponding Shilaif basinal facies. The Mishrif development in the UAE is strongly asymmetric. The western Mishrif platform sediments are characterized by stacked clinoforms comprising clean, high-energy carbonates. Rudist reefs and grainstone shoals along the successive shelf edges, which form those clinoforms, generally have good reservoir properties. Similar favorable reservoir characteristics can also be found in isolated platforms and grainstone beds which are locally well developed in lagoonal settings found behind the shelf edges. In the east, the Mishrif is generally much thinner and is characterized by more differentiated, coarsening and shoaling-upwards sequences, each grading from fine basinal wackestone to medium/coarse-grained shallow marine packstone. This indicates sedimentation on a shallowing depositional slope. This slope lithofacies is made up of dark-colored, very fine- to fine-grained, well sorted, bioclastic packstones, grading into lighter colored, medium-grained, unsorted, bioclastic packstones to grainstones. High-quality reservoir facies occur only near the prograding shelf edges and over salt domes from which build-ups may have nucleated. Kerogen carbon isotope values and GC-MS fingerprints confirm that the Shilaif Formation is the source of the Mishrif oils. The Shilaif Formation is immature over large parts of Abu Dhabi, notably the western offshore part of the Emirate. In the deeper salt-withdrawal basins such as the West Bu Hasa and Falaha synclines the source rock is oil-mature and also in the Oman Mountain foreland basin, grading to gas mature further towards the east. In most of the mature Shilaif kitchens, oil generation commenced during Miocene times, in the southern offshore part of Dubai it may have started as early as the Early Oligocene. The Mishrif Play prospectivity evaluation will guide future appraisal drilling and will serve as a basis for further exploration activities related to conventional and unconventional trapping styles in the Formation.

A diagenetic trapping style is proposed for an oil accumulation found in the Mishrif Formation in the southwestern part of onshore Abu Dhabi. A diagenetic trapping scenario, which assumes updip sealing by cementation is much riskier than a structural trapping scenario, but has large up-side potential. Oil accumulations in diagenetic traps represent an equilibrium state between buoyancy forces trying to move oil through rocks and capillary pressures in the low permeable layer that resists this movement. The only requirement for a diagenetic trap to exist is that the capillary pressure in the sealing formation is greater than the migration (buoyancy) force in the reservoir rock. In addition to that, smaller pore throat sizes in the sealing formation and larger oil molecules, for instance the product of an early oil migration stage, in the reservoir, can enhance the sealing capacity of the stratigraphic trap and result in bigger oil columns. The potential of this particular Mishrif prospect was assessed on the basis of regional analogs and a geological model was developed. The prospect was matured by the acquisition of a 3D seismic survey in 2004 (Fig. 1), with the aim to delineate the limits of the stratigraphic trap. A detailed seismic reservoir characterization study was carried out and porosity maps were prepared on the basis of a calibrated acoustic impedance cube. Seismic attributes were generated with the aim to detect the fault/fracture patterns in the area. The Mishrif stratigraphic trap in southwestern onshore Abu Dhabi is highly prospective since it is located adjacent to the mature Shilaif kitchen in the Ghurab Syncline (Rub’ al Khali Basin). The Shilaif source rock, which is time equivalent to the Mishrif shelf facies (Fig. 2), was deposited under restricted water circulation and anoxic conditions. Basin modeling suggested that the timing of hydrocarbon charge from this kitchen into the Mishrif reservoir started as early as Late Miocene. The Mishrif play prospectivity evaluation, including the detailed and integrated geological model and 3D seismic interpretation will guide future appraisal drilling. In addition, the knowledge gained from this study can serve as a basis for further exploration activities related to unconventional trapping styles in the Mishrif Formation.

The pre-Khuff principal hydrocarbon reservoir, Unayzah Formation, consists mainly of distal braid plain sandstones characterized by aeolian and sabkha facies with minor fluvial units. It extends between the pre-Khuff and the Hercynian unconformities. In Abu Dhabi, the Unayzah-A is further subdivided into three members, Members 1 and 2 are comprised of sandstone reservoirs and Member 3 consists of siltstone and shale sediments. Facies controls on reservoir quality are weak. The main controls on porosity reduction of the reservoir are mechanical compaction and silica cementation. Quartz cementation tends to be the most severe in the cleanest, coarsest sandstones and near certain fractures. The presence of clay mineral grain coatings, although reducing the permeability, but locally protects the rock from secondary quartz overgrowth and preserve the porosity to great depths of burial. Without the grain coating, porosity will decrease with depth until the reservoir rock is completely tight. Unayzah reservoir seals are provided by the Basal Khuff Clastics, tight Basal Khuff Carbonate and Middle Khuff Anhydrite. The Basal Khuff Carbonate seal does not appear to be regionally extensive but localized and potentially prospect specific. However, there are insufficient data to accurately define the seal for the Unayzah hydrocarbon accumulations. Due to lack of deep penetrations in Abu Dhabi, basin modeling for Silurian hot shale source rock is challenging. Therefore, much of the unknown source and tectonic information were derived from the surrounding countries. This comes from understanding the regional tectonics and depositional trends of the southeastern Arabian plate, which helped to extrapolate the source trends into the Abu Dhabi area. The basin model shows that oil from Silurian source rock was generated early in the basin history and was widespread by the Late Triassic (220 Ma). Significant gas generation occurred during Lower Cretaceous (140 Ma) and dominated the hydrocarbon system by Middle Cretaceous (110 Ma). During the Early Tertiary (50 Ma), the source rock was highly mature for gas generation and at present-day, the charge is still active in the north offshore of Abu Dhabi. The pre-Khuff charge history showed that the southern offshore and onshore structures are underfilled. The filling of these structures ranges between 50% and 80%, but in some onshore structures the filling is less than 50%. The middle and northern offshore structures are expected to be filled to spill point.

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Previously, the deep reservoirs of Middle Jurassic (Araej Formation), Upper Permian (Khuff Formation) and pre-Khuff (Berwath Formation) age in a salt-related field offshore Abu Dhabi were interpreted to be uneconomical due to their small GIIP volume which was based on limited data from only one exploration well. However, new geological studies including basin modeling suggest a much higher potential for deep gas in this particular field. In view of the increasing demand for gas in Abu Dhabi, ADNOC has initiated the re-evaluation of a number of deep gas reservoirs using updated geological models, integrating reprocessed 3D OBC seismic and all available well data. The major challenge in modeling these deep reservoirs is lack of well control. It is challenging to calculate statistically meaningful averages for the lateral variations in reservoir characteristics with high confidence, only based on a few wells. On the contrary, reservoir property-related seismic attributes such as RMS amplitude and acoustic impedance from post-stack inversion of the 3D OBC seismic are laterally continuous datasets, providing the database for lateral statistics. In this study, geostatistics are combined with horizontal variogram modeling based on seismic and vertical variogram modeling based on wells, which bridges the vertical-lateral gap of resolution and continuity between wells and seismic. Variogram maps rather than a single variogram model are calculated from the average seismic attribute maps, which extend statistic control of lateral continuity to 360 degrees. In addition, variograms are mapped for each fault segment separately, which honors the reservoir behavior to the maximum. Statistically, seismic attributes show a similar trend in their probability curves and vertical distribution as reservoir properties calculated from well logs. Therefore the final reservoir property models are built using co-krigging sequential Gaussian simulation with seismic-well combined variograms. As a result, the new geological model enlarges the area of deep-gas prospectivity and locates the primary deep gas target in the Upper Khuff and pre-Khuff around the crest of this field. The re-calculated GIIP with low, average and high cases now shows an economical gas volume in these deep reservoirs. This integrated statistics and modeling approach, combining seismic and well data reduces uncertainty in reservoir characterization with limited well data. Considering the good coverage of seismic in Abu Dhabi, this approach can be applied to the other fields with lack of deep well penetrations.