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Facies, Depositional Environment and Evolution of the Al-Wajh Formation (Late Miocene), Red-Sea Region, Saudi Arabia

M.M. Nazrul Islam1, Osman Abdullatif1, Khalid Al-Ramadan1, and Arun Kumar2
1Earth Sciences, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia
2Research Institute, Center for Petroleum & Minerals, King Kahd University of Petroleum & Minerals, Dhahran, Saudi Arabia

The late Miocene Al Wajh Formation is one of the less explored formations in the Midyan Peninsula, Red Sea region, Saudi Arabia. The Formation is exposed at many localities in the northern part of the Ifal Basin as well as in the subsurface. Detailed facies analysis enables us to document successive sedimentary environment, to establish a depositional model and to understand depositional evolution of the Formation. Outcrop based facies analysis, vertical and horizontal sedimentological profile construction, and laboratory based petrographic study was the main concern of the study. The formation is a red to reddish-brown, poorly sorted and friable siliciclast wedge. Graded clast-supported conglomerate, planner coarse sandstone and planner fine sandstone are the main facies type identified in the field. Petrographic study reveals that the formation is highly porous and mainly consists of sandy conglomerate, litharenite to sublitharenite sandstone and siltstone. The vertical profile portrays the repetitive cycle of mainly fining upward sequences and few coarsening upward sequences. Horizontally the succession is not always continuous because of the small tectonic pulses of the major active tectonic phage of the Red Sea rifting. Other sedimentary control such as climate, sediment supply and intra basinal process can be inferred from its facies and mineralogy. It is obvious from the study that distal facies is missing or covered by recent sediment and proximal to medial facies is distinct. An alluvial fan model is proposed for the Al Wajh Formation in this research. Mineralogical composition and paleocurrent direction suggest that sediment was derived from the adjacent Proterozoic Basement rock. The formation holds a good prospect to serve as petroleum reservoir because of its highly porous nature and overlying evaporates.

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