Silurian Slope Turbidites: A Predictive Sequence Stratigraphic Model for the Exploration for New Accumulations of Mid-Qusaiba Sand Reservoir, Saudi Arabia

By

 Riyadh A. Rahmani¹

(1) Saudi Aramco, Dhahran, Saudi Arabia

 In central and eastern Saudi Arabia evidence suggests that deposition of Silurian Qusaiba Member was dominated by slope mudstones and episodes characterized by introduction of slope and basin floor fan sandy turbidites (Mid-Qusaiba Sand) during and following periods of sea level lowstands. These slope clinoforms prograded by downlapping on the underlying Qusaiba “Hot Shale”, a rich hydrocarbon source rock. It is believed that at periods of lowstands, the Qusaiba deltas, to the west, must have reached the shelf margin to form shelf-margin deltas. Sediments from these deltas spilled past the shelf break to the slope, in the form of gravity flows, to deposit the Mid-Qusaiba Sand (MQS) amongst Qusaiba mudstones. In northern and northwestern Saudi Arabia MQS turbidites have not been reported, therefore suggesting the absence of a shelf-slope break in these areas. The Qusaiba-Sharawra unconformity, or any of the disconformities reported within the Qusaiba Member, is here interpreted to have been caused by the same relative sea-level drop that resulted in the progradation of Qusaiba deltas to shelf margin and lead to the delivery to the slope of MQS turbidites. Thus it is believed that basins with shelf-slope breaks formed locally in areas that experienced faster subsidence, perhaps through reactivation of older basement faults.

This shelf to basin slope relationship becomes an important tool in prediction of new MQS accumulations into unknown areas. It implies that locations of Qusaiba thick deltaic lobes in periods of sea level lowstand should lead in the deeper offshore to discovery of MQS turbidite accumulations.