AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Advance and Retreat of the Safaniya Delta, Offshore Northeast Saudi Arabia
(1) Exploration, Saudi Aramco, Dhahran, Saudi Arabia.
An Albian siliciclastic wedge, spanning several million years, records the development of the Safaniya delta. This wedge is confined between two transgressive carbonate units, upper Mauddud and lower Dair members. The delta advanced into a carbonate platform at the Gotnia basin margin. Its evolution and depositional controls are analyzed using cores.
Core logging allowed the recognition of seven facies associations that were deposited in fluvial to shallow marine environments. These are stacked into four sequences (S1-S4) that are bound by maximum flooding surfaces and arranged in an ascending regressive to transgressive pattern. The lower two sequences (S1 & S2) include NW-SE trending shoreface deposits that change laterally into river mouth bar facies. They mark episodic progradation of a tide-influenced and wave-dominated delta during an overall rise of sea level. The next sequence (S3) is partitioned by a major unconformity that defines incised valleys. The lower interval (S3L), below the unconformity, consists mainly of NE-SW trending tidal bars changing laterally into tidal flat and tide-influenced fluvial facies. It marks the development of a regressive tide-dominated delta. The upper interval (S3U) comprises fluvial-estuarine valley-fill and transgressive shoreline deposits. It includes fluvial sandstones changing distally into tidal flat, bayhead delta, bay, flood tidal delta, tidal inlet and shoreface deposits. These facies record the formation of a wave-dominated estuarine system. The upper sequence (S4) steps landwards, to the SW, relative to S3. Similarly, it consists of lower progradational deltaic and upper retrogradational estuarine facies. The delta was tide-influenced, whereas the estuarine system was wave-dominated.
The advance of the Safaniya delta, throughout S1; S2 and S3L, is marked by a process-regime change from tide influence and wave dominance into tide dominance. In contrast, its retreat; during S3U and S4; is accompanied by a switch into tide influence and wave dominance. These conversions are controlled by the evolving basin morphology and the constantly altering accommodation/sediment supply ratio (A/S). Differential subsidence along NE-SW trending faults increased the A/S and created shoreline embayments dominated by tidal currents. A declining rate of subsidence decreased the A/S and caused gradual infilling of the embayments and a switch into wave dominance.