Abstract 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.

中文翻译：

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沙特阿拉伯西南部瓦吉德砂岩晚石炭纪-二叠纪 Juwayl 段的相、岩相、储层非均质性和质量

摘要 沙特阿拉伯西南部古生代瓦吉德砂岩由 Dibsiyah、Sanamah、Khusayyayn 和 Juwayl 四个成员组成。该研究描述了来自 Wadi Ad-Dawasir 地区五个露头地点的 Juwayl 段的沉积相特征。目的是揭示相非均质性对储层分布和质量的影响。实地调查涉及相描述、取样和建筑元素识别。第二步涉及解释相关联以预测沉积环境和控制。薄片、SEM 和 XRD 的岩相分析有助于确定影响岩石性质（例如孔隙度和渗透率）的质量和非均质性的成岩控制。相分析揭示了与冰川河流和辫状河流沉积环境相对应的十种岩相。冰川河流沉积物由卵石到砾岩砂岩岩相组成，表明在近端河流环境中具有高至中等能量条件。辫状河流沉积物为中至粗粒、槽至规划交错层状砂岩岩相。这些岩相表明横向切换通道的低能量远端条件。其余岩相包括细粒泥质-铁质砂岩岩相的堤岸沉积，在向上细化层序的顶部观察到。该研究将 Juwayl 砂岩归类为矿物学上成熟的石英砂岩，具有良好到非常好的孔隙度和渗透率。它以 (90%) 石英和方解石的其他成分为主，粘土和氧化铁与次要的钾长石。包括胶结作用、粒度减小、石英过度生长和粘土矿物在内的成岩控制正在影响 Juwayl 砂岩的孔隙度和渗透率。Juwayl 砂岩的沉积环境表明系统从高能冰川河流环境转变为低能量辫状河流环境。在该模型中砂岩体的厚度和宽度趋于变化，这意味着垂直和侧向砂岩的非均质性。粘土、赤铁矿和方解石胶结物有助于孔隙度降低，而颗粒溶解、粘土层和边缘有助于孔隙度增加。相、古环境和地层结构的宏观尺度露头非均质性，以及质地、组成和成岩作用的微观尺度，