--> Abstract: Climatic Sequence Stratigraphy, Permian post-Glacial Unayzah Reservoir, Central Saudi Arabia, by Osama Mahmoud Soliman, Saad A. Aba Al-Hassan, and Bader A. Al-Mutairi; #90039 (2005)

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Climatic Sequence Stratigraphy, Permian post-Glacial Unayzah Reservoir, Central Saudi Arabia

Osama Mahmoud Soliman, Saad A. Aba Al-Hassan, and Bader A. Al-Mutairi
Saudi Arabian Oil Company, Dhahran, Saudi Arabia

Facies analysis of a major Permian gas reservoir revealed a mixed fluvial-aeolian system. Low and high-order wet-dry-wet climatic cycles occur therein. A new three-fold sequence model is proposed to explain regional stratigraphic relationships between low order cycles. In an ascending succession, a sequence involves Wetting-up, Drying-up and Arid Systems Tracts; respectively WST, DST and AST. A distinct sequence boundary SB, and its correlative conformity, separates DST and overlying AST.

WST involves terminal fans prograding into expanding playa lakes. Avulsion processes allowed wind reworking of abandoned fluvial sediments and deposition of minor aeolian sands. Widespread playa-lake deposits mark top of WST. They serve as a local correlation marker. Basal boundary of WST is a regional correlation marker. It coincides with SB in proximal, low accommodation areas and top of AST in distal, high-accommodation areas.

DST records diminishing fluvial sedimentation, shrinking playa lakes and increasing deposition of aeolian sediments. It terminates at top with SB in proximal and low-accommodation areas.

AST includes large aeolian dunes, providing high quality reservoirs, in distal and high accommodation areas. Massive, slumped and contorted aeolian dune sands mark top of AST. They reflect resedimentation processes induced by increasing precipitation. AST correlates with SB in proximal and low-accommodation areas.

The Unayzah reservoir involves three sequences separated by two unconformities. They record a progressive shift from arid cold to semi-arid warm climates and span 20-25 Ma. The duration of each sequence and magnitude of hiatuses are poorly constrained. Syndepositional tectonics controlled distribution and development of these sequences. Low frequency climatic cycles, associated with transition from Icehouse to Greenhouse conditions, are inferred to have modulated their sedimentation.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005