Cenomanian Stratigraphic Traps in Western Abu Dhabi, U.A.E.

Abstract

ABSTRACT Cenomanian rocks in western Abu Dhabi comprise a sequence of shallow and deep marine carbonates with terrigenous shales and sandstones. Four rock units, in ascending order; the Shilaif, the Tuwayil, the Ruwaydha and the Mishrif are recognized within the sequence. The Shilaif Formation is represented by pelagic carbonates, deposited in anoxic intrashelf basin, developed in central Abu Dhabi during the Cenomanian. It proved to be the source rock of the oil reservoired in the overlying Mishrif. The Tuwayil, consists of shales and sandstones, and represents a period of terrigenous clastic influx, which has formed a large tongue protruded into the Shilaif basin. It includes four sandstone units, which are found oil bearing in some localities. The Ruwaydha represents the last basinal depositional phase of the Cenomanian in the area. The Mishrif Formation is a thick shallow marine carbonate sequence, exhibiting minor cyclic deposition reflecting periodic oscillation of sea level. It represents deposition on epicontinental shelf, with its margin prograding eastward into the Shilaif basin. It is a good reservoir, known to be oil productive in east Abu Dhabi and offshore Dubai from rudist reefal build-ups and/or accumulations. Large oil and gas accumulations have been found in the area, mainly in structural traps of the Middle and Lower Jurassic, with moderate oil accumulations in Middle and Lower Cretaceous rocks. A transitional zone between the Mishrif and Ruwaydha/Tuwayil exists in the area. The width of this transitional zone appears to extend along few kilometres. However, interfingering of the Tuwayil, the Ruwaydha and possibly the Shilaif appear to extend into the Mishrif reflecting possible occurrence of localised paleo-embayments at the Mishrif shelf margin. Seismic-stratigraphic investigation, helped to define facies limits, patterns and occurrence of prospective stratigraphic traps in both the Mishrif and the Tuwayil Formations. The Tuwayil sandstone and Mishrif carbonate clastics pinching out against structural highs, are prospective localities for stratigraphic hydrocarbon entrapment. In addition, areas where the Mishrif develops possible carbonate build ups and/or rudist reefal accumulations are also of interest and could have hydrocarbon potential. The sequence of seismo-stratigraphic/facies definition procedure, presented in this paper, were applied to define stratigraphic traps in the Mishrif and Tuwayil Formations.