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.

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The Middle Cretaceous "Upper Wasia Group" in West Abu Dhabi is divided into four Formations: the Shilaif, Mishrif, Tuwayil and Ruwaydha. In this paper, a new stratigraphic subdivisions based on sequence stratigraphy, for the "Upper Wasia Group" is introduced. It consists of three distinct Sequences: I (Mishrif/Shilait), II (Tuwayil) and III (Ruwaydha) that were deposited in the area from Late Albian to Turonian. Mathematical modeling applied on the Shilaif Formation (Sequence I) study· area, has indicated However,.the volume of hydrocarbon accumulations discovered in the Middle Cretaceous Upper Wasia reservoirs is much less than volumetric calculations of predicted oil-in-place. This demonstrates the generation of adequate volumes of oilfrom the Shilaif, which justifies further exploration activity.

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.