oa Facies, depositional environment and reservoir properties of the Devonian- Carboniferous Khusayyan Member, Wajid Sandstone, Saudi Arabia

The Devonian-Carboniferous Khusayyayan Member represents an important part of the Wajid Sandstone aquifer of southwest Saudi Arabia, and also a potential hydrocarbon reservoir in Rub’ Al-Khali Basin in the Kingdom. This study attempted to characterize the facies, depositional environments and petrophysical properties of the member using integrated outcrop facies analysis, petrographic and petrophysical analysis. The facies analysis revealed that the Khusayyan Member consists of channel and bar sequences that were deposited in a proximal to medial fluvial settings within shallow lowsinuosity braided streams. The facies predominantly consist of planar and trough cross-bedded sandstones, with minor horizontally bedded sandstone and scourfill 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 <br>sub-rounded, moderately well-sorted to poorly sorted. The mature quartz arenite suggested shallow burial conditions. Under such conditions, alterations by eudiagenesis may have played a role in the digenetic history that is manifested by 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. The porosity of the 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, were the main factors that controlled the porosity and permeability evolution and the reservoir heterogeneity of the Khusayyayan Member.