The Unayzah Formation is a Lower Permian non-marine siliciclastic unit immediately underlying the Permian-Triassic Khuff Formation. The Unayzah is bounded by unconformities and comprises lithologies ranging from quartzose sand, argillaceous sandstone, silty sandstone, mudstone and minor carbonaceous shale. Gas was discovered in Bahrain in the Unayzah Formation during 1940–1950 at about 11,000 ft. The formation is typically 350–400 thick at Awali field. There is a characteristic vertical succession of two major fining upward cycles; the lower cycle (130–250 ft) consists of lower and middle Unayzah and the upper cycle (135–220 ft) corresponds to upper Unayzah. The upper Unayzah is upward-fining with massive sand at the base and inter-bedded sand at the top. The stacked sands in the lower interval were possibly deposited in a braided channel system. The inter-bedded interval represents channel margin and flood-plain facies. The upper Unayzah reservoir sands are characterized by lower acoustic impedance based on evidences from the well logs. The presentation elaborates on a seismic inversion study carried-out for the delineation of the upper Unayzah reservoir sands based on seismic-impedance characteristics. The steps involved in the study included well-to-seismic correlation utilizing statistical and well-derived wavelets, structural modelbuilding and model-based inversion of the Awali 3-D seismic data. From the evaluation of the seismic inversion results, it was inferred that the upper Unayzah reservoir sands are better-developed in the southwestern part of the Awali structure.


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