Facies, petrography, reservoir heterogeneity and quality of the late Carboniferous-Permian Juwayl Member, Wajid Sandstone, SW Saudi Arabia

摘要

The Paleozoic Wajid Sandstone in southwestern Saudi Arabia comprises four members known as Dibsiyah, Sanamah, Khusayyayn and Juwayl. The study characterizes the sedimentary facies of the Juwayl Member from five outcrop localities in Wadi Ad-Dawasir area. The objective is to reveal the influence of facies heterogeneity on the reservoir distribution and quality. The field investigations involve facies description, sampling and architectural elements identification. The second step involves the interpretation of facies associations to predict the depositional environments and controls. The petrographic analysis, of thin section, SEM and XRD, helped to define the diagenetic controls that affected the quality and heterogeneity of rock properties such as porosity and permeability. The facies analysis revealed ten lithofacies corresponding to glacio-fluvial and braided-fluvial depositional environments. The glacio-fluvial deposits consist of pebbly to conglomeratic sandstone lithofacies that suggest high to medium energy conditions within a proximal fluvial setting. The braided-fluvial deposits are of medium to coarse-grained, trough to planner cross-bedded sandstone lithofacies. These lithofacies indicate a lower energy distal condition of laterally switching channels. The remaining lithofacies include overbank deposits of fine-grained argillaceous-ferruginous sandstone lithofacies, observed at the top of fining upward sequences. The study classified Juwayl sandstone as mineralogically mature quartz arenite with good to very good porosity and permeability. It dominates by (90%) quartz, and other components of calcite, clay and iron oxides with subordinate K-feldspars. The diagenetic controls including cementation, grain-size reduction, quartz overgrowth and clay minerals are affecting the Juwayl sandstone porosity and permeability. The depositional environments of Juwayl sandstone indicate a system switching from high energy glacio-fluvial to lower energy braided-fluvial setting. The thickness and width of the sandstone bodies tend to vary in this model implying vertical and lateral sandstone heterogeneity. Clay, hematite and calcite cements contributed to porosity reduction, while grain dissolution, clay coats and rims contributed to porosity enhancement. Macro-scale outcrop heterogeneity in terms of facies, paleonvironments and stratigraphic architecture and microscopic scale in terms of texture, composition and diagenesis, all might provide guides for subsurface reservoir assessment, comparison and correlations.