Using Biofacies and Lithofacies to Determine Palaeoenvironments and Depositional Cyclicity of the Sulaiy and Yamama Formations of Subsurface Saudi Arabia

Geraint W. Hughes¹*, Nassir Naji¹, and Osman Varol^2
1Saudi Aramco
²Varol Research, UK
*[email protected]

The Sulaiy and Yamama formations of Saudi Arabia consist of Late Jurassic (Tithonian) to Early Cretaceous (Berriasian) carbonates. Although exposed in Saudi Arabia, the Sulaiy is difficult to access and the Yamama is very poorly exposed. The Sulaiy Formation lies unconformably on evaporites of the Hith Formation at outcrop, but overlies carbonates of the Manifa Member of the Hith Formation in the subsurface. The Manifa is currently being evaluated as being genetically linked with the Sulaiy rather than its traditionally assigned Hith Formation. Micropalaeontology and sedimentology of the Sulaiy and Yamama formations in subsurface have revealed a succession of clearly defined shallowing upwards depositional cycles, of 50 ft average thickness. These typically commence with a deep-marine biofacies within wackestones and packstones, capped with a mudstone-wackestone maximum flooding interval and an upper unit of packstone to grainstones containing shallow-marine biofacies. The upper part of the Sulaiy Formation is highstand-dominated with common grainstones that host the Lower Ratawi reservoir and is capped by karst that defines the sequence boundary. The Yamama Formation, in contrast, contains fewer grainstones, and is predominantly transgressive. Although smaller grainstone units host the Upper Ratawi reservoir, it is considered that the highstand-associated, main reservoir facies equivalent to the Lower Ratawi reservoir must have been deposited but was removed by the very extensive episode of erosion that accompanied the major sea-level fall during the Valanginian. It is tantalising to contemplate the destination of the transported highstand grainstones as they would provide excellent stratigraphically trapped pre-Buwaib reservoirs elsewhere within the basin.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain