Facies, Depositional Environments, Reservoir Potential and Palaeogeography of the Cambro-Ordovician Dibsiyah Formation, Wajid Outcrop Belt, Saudi Arabia

Abstract

This paper investigates the facies, palaeoenvironments, controls and sequence development of the Cambro-Ordovician Dibsiyah formation of the Wajid (Sandstone) Group in southwest Saudi Arabia. The facies analyses revealed that the lower member of the Dibsiyah formation is predominantly composed of coarse- to very coarse-grained, trough cross-bedded sandstone deposited in a high-energy braided stream channel and bar sub-environments. The facies form in vertically and laterally stacked fining-upward sequences, indicating a braided channel and bar aggradations and migrations. However, the upper member is composed of mature fine- to coarse-grained sandstone deposited in the middle- to upper shoreface to foreshore parts of a shallow marine environment. The lower member represents a low stand system tract overlain by marine shale and siltstone units of the Bukayriyah Member, representing the regional Ordovician transgression. This sequence is overlain by the high stand system tract of the shallow marine sandstone of the upper member. Several factors may have contributed to the sequence development, including periods of deformation, uplift and erosion, regional variations in the rate of subsidence around the plate margins, sediment supply and relative sea level changes. The Wajid Outcrop Belt, however, may have been subjected to a different sedimentation and palaeogeographic development than that in the central and northern Arabia basins. The Dibsiyah formation is the same as that of Risha, Bukayriyah and Sajir Members of the Saq formation in central and northern Saudi Arabia. Understanding the depositional and post-depositional heterogeneity of the Dibsiyah formation at the outcrop scales may provide a better understanding and prediction of reservoir quality and architecture in its subsurface equivalent.