Unayzah Group in Western Central Saudi Arabia: Utilization of a Chemostratigraphic Analysis

Summary “The Permo-Carboniferous Unayzah Group of central Saudi Arabia is generally composed of clastic deposits forming four members: in ascending order the Ghazal, Jawb, Wudayhi and Tinat. At its top, the Unayzah Group is truncated by an unconformity overlying which is the Middle Permian Basal Khuff Clastics Member (BKC), comprising an alternating succession of sandstones, shales, and thin carbonate lithofacies. Correlation of the intra-Unayzah sand bodies is problematic due to the lithological similarities between the sandstones encountered within the members as most of them are barren. Chemostratigraphy can offer a potential high-resolution stratigraphic tool, through use of the geochemical signature of the sediments, determined from their elemental composition and mineralogical content. For this reason, a workflow was developed to identify the chemostratigraphic characteristics of the Unayzah Group in the study area based on specific changes in elemental ratios. The correlation scheme includes Zr/Nb, Zr/Th (Zr*Hf)/(Nb*Ta), Zr/(Nb*Ta), Al/(Ca+Mg+K+Na) and (Rb+Cs)/La. They reflect changes in sediment provenance and weathering with strong variation components. The chemozones and their inherent sub-zones identified in this study are found to broadly approximate to Unayzah litostratigraphic members and the technique offers considerable potential for high-resolution correlation in the subsurface, especially when integrated with sedimentological and palynological data.”