--> --> Chemostratigraphy, Lithostratigraphy, and Sedimentology of the Unayzah Group, Eastern Saudi Arabia

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Chemostratigraphy, Lithostratigraphy, and Sedimentology of the Unayzah Group, Eastern Saudi Arabia


The Permo-Carboniferous Unayzah Group comprises numerous hydrocarbon-bearing reservoirs in eastern Saudi Arabia. The aims of the following study were to use chemostratigraphy in conjunction with lithostratigraphy and sedimentology to place formation/member boundaries within the group, and to define correlative zones within each member. In addition to examining core and wireline log data for 8 wells, 1130 core and cuttings samples were analysed by XRF (X-Ray Fluorescence), with data acquired for 43 elements in the range Na-U in the periodic table. The Unayzah Group is subdivided into the Juwayl and Nuayyim formations in stratigraphic order. The Ghazal Member occurs in the lower part of the Juwayl Formation, which is dominated by fluvio-glacial sandstones, and is succeeded by diamictites, fluvial and lacustrine sediments of the Jawb Member deposited during glacial retreat. This is overlain by the Wudayhi Member of the Nuayyim Formation, comprising red siltstones and fine grained sandstones deposited in marginal marine-continental environments. The Tinat Member exists in the upper part of this formation, mainly taking the form of continental sediments. Placement of the boundaries between the various members is often challenging, and recovery of palynomorphs is limited due to the generally unfavourable lithologies. For these reasons, chemostratigraphy was employed to produce a robust scheme of high resolution. The scheme relates to variations in the following ‘key’ elements and ratios: Zr/Ti, Zr, Zr/U, Zr/Yb, Zr/Gd and Th/Y. These parameters are largely linked with changes in source/provenance, and the resulting framework comprises a hierarchical zonation of six zones, eight subzones and two divisions. Based on a detailed comparison between chemostratigraphic, lithostratigraphic and sedimentological data, specific chemozones could be associated with particular members of the Unayzah Group, though only confidently over cored intervals, and in sections where members were clearly identified based on GR log trends. After the associations were established between chemozone and member boundaries, chemostratigraphy was employed to define member boundaries in sections where this could not be achieved using lithostratigraphy/sedimentology. Chemostratigraphy was also utilized to recognize correlative boundaries within each member, thus adding an additional level of resolution.