Geological Overview of the Middle Cretaceous Mishrif Formation in Abu Dhabi

Abstract 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.