Depositional system and reservoir potential of the Middle Cretaceous Wasia Formation in central—eastern Arabia

Abstract

Stratigraphy and depositional facies of the Middle Cretaceous Wasia Formation in central—eastern Arabia were studied in detail using subsurface data. We also investigated the effects of depositional environmental changes and other geological parameters on the petroleum and groundwater reservoir potentials of the formation. In the subsurface, the Wasia Formation is divided into seven members composed mostly of siliciclastic deposits in the lower part and fine-grained siliciclastic and carbonate sediments in the upper part. Five different facies which depict vertical and horizontal variations have been recognized. Changes in facies may be related partly to changing depositional environment from non-marine (fluvial) and transitional in the lower part to very shallow and open marine shelf in the upper part. Interpretation of directions of increasing thickness and clastic percentage and of areas for potential stratigraphic traps shows that the lower siliciclastic part seems to have the highest probability to become petroleum and water reservoirs. Further, the calculated average porosity of different members indicates that both sandstones and limestones of the Wasia members are porous enough to be potential petroleum reservoirs, and that the shales would have sufficient sealing capacity. However, the chances that large structural traps exist are low. Mature source rocks are not present nearby, and unless a long distance migration of petroleum took place in the past, the petroleum potential of the Wasia in the study area appears very remote. The Wasia Formation is a potential groundwater aquifer as far east as the Arabian coastal area. High yields with excellent hydraulic properties as well as large volumes of abstractable water in storage have been reported from the study area. The regional differences of the parameters (such as thickness, porosity and permeability) of the Wasia aquifer in this area are mainly due to changes in depositional system. The presence of fresh groundwater in the Wasia Formation of central Arabia could have a damaging effect on the quality of petroleum, if it is present. Any hydrocarbons which remained would be heavy, viscous and immobile and could not be easily produced.