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Seismic Sequence Stratigraphy and Reservoir Distribution of the Mid-Cretaceous Mishrif Formation, United Arab Emirates


Hydrocarbon-bearing rudist buildup is one of the most important onshore and offshore oil and gas exploration targets within the Cretaceous strata of the Arabian Basin and is common in the mid-Cretaceous Mishrif Formation. An integrated reprocessing 3D seismic evaluation of the Mishrif Formation in the both offshore and onshore of the UAE provides new insights into regional stratigraphic framework, sedimentation patterns and reservoir distribution. The mid-Cretaceous Mishrif Formation is divided into two parts by a widely distributed unconformity within Mishrif Formation from shelf down to the basin. The upper part of Mishrif Formation consists of a 3rd-order transgressive – regressive cycle characterized by a slope fan located in the narrow slope area beyond the shelf edge and a basin floor fan distributed in the wide area in the basin ward. The rudist buildups presented along the shelf edge and suffered severe erosion during falling base level. The lower part of Mishrif Formation consisted of four high frequency carbonate prograding wedges from north west to south east during base level fall. Regional uplift resulted in eroded surface which represent the possible productive reservoir zones in the lower part of Mishrif Formation. Direct inversion of post stack seismic impedance data reveals that the low acoustic impedance anomalies are consistent with the porous segments and three possible porous zones as described below: Firstly, the porosity increases beneath sequence boundaries with the meteoric dissolution and karstification, and the mainly corresponds to the top of Mishrif Formation in the whole seismic survey area, and the top of carbonate prograding wedges in the lower part of Mishrif formation. Secondly, the early TST interval which includes the slope fan and basin floor fan is relatively porous due to the rising sea level induced dolomitization which caused porosity enhancement. Thirdly, the thick, off-platform rudist buildup facies may be porous due to meteoric dissolution and karstification. Mishrif reservoirs are HC-bearing in several onshore wells. Based on the differentiated porous zones, the exploration targets could be focused on the different units: the slope fan, the basin floor fan, the off-platform rudist buildups, the eroded rudist buildups along the shelf edge in the top Mishrif Formation, and the eroded top carbonate prograding wedges in the lower part of Mishrif Formation.