Print this pageSilurian Slope Turbidites: A Predictive Sequence Stratigraphic Model for the Exploration for New Accumulations of Mid-Qusaiba Sand Reservoir, Saudi Arabia
By
(
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.