Depositional environments and facies reservoir properties of the Khusayyayan member, Wajid Sandstone, Saudi Arabia

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Saudi Digital Library
The Devonian-Carboniferous Khusayyayan member of the Wajid Sandstone is an important part of the Wajid Sandstone aquifer of southwest Saudi Arabia and also a potential reservoir in Rub Al Khali basin. This study tends to characterize the facies, depositional environments and petrophysical properties of the Khusayyayan member using integrated outcrop fades analysis, petrographic and petrophysical analysis. The facies analysis revealed that the Khusayyayan member is deposited in a proximal to medial fluvial depositional setting within shallow low sinuosity braided stream composed of channel and bar sequences. The facies are predominantly made of planar cross-bedded and trough cross-bedded sandstone facies with minor horizontally bedded sandstone and scour fill facies. The fluvial architecture shows vertically and laterally stacked sandstone bodies of channel and bar types. Petrographic studies revealed that the sandstone facies are composed of a mature quartz arenites, predominantly medium to coarse grained, rounded to sub-rounded, moderately well sorted to poorly sorted. The mature quartz arenite suggested shallow burial conditions. Under such conditions alterations by eudiagenesis which have played a role in the digenetic history that manifested in shallow compaction, cementation, grain alteration and dissolution and quartz overgrowth. Kaolinite is the main clay mineral and occurs both as grain coat and pore fill. Porosity of Khusayyayan member is moderate to very good whereas the permeability is good to very good. The depositional facies and their architecture, as well as the diagenetic history are the main factors controlling the porosity and permeability evolution and the reservoir heterogeneity of the Khusayyayan member.