Organic and Inorganic Geochemical Investigation of the Silurian Shale of Qusaiba Formation, Tayma Area, Northwestern Saudi Arabia: Implications for Oxidation and Leaching of Organic Matter

The current study investigated the effectiveness of the chemical weathering conditions of the outcrop sections of the Silurian Qusaiba Formation in the Tayma area of Northwestern Saudi Arabia, and their implications for the oxidation and leaching of organic matter within the shale units. The shale units’ organic matter characteristics, mineralogy, and elemental geochemistry were studied using total organic carbon (TOC) content, Rock–Eval pyrolysis, X-ray diffraction, and X-ray fluorescence spectrometer methods. The analyzed Qusaiba shale units have moderate to high TOC and sulfur contents in the range of 0.64–1.46 wt % and 0.59–4.64 wt %, respectively, resulting in S/TOC ratio between 0.56 and 3.94 and indicates the marine environment for the studied Qusaiba shale units. The analyzed Qusaiba shale samples have intensive chemical weathering in the source area, as demonstrated by the comparatively high kaolinite, illite, and chlorite clay minerals of up to 60 wt %. The strong weathering of silicates (primarily K-feldspars) and the degradation of the mica in warm, humid climate conditions are confirmed by the high values of the chemical index of alteration (CIA), plagioclase index of alteration, and chemical index of weathering in the range of 72.63–78.62%, 92.72–99.53%, and 95.15–99.69%, respectively. Therefore, the chemical weathering has strongly affected the concentration of organic matter content in the Qusaiba shale units and its petroleum generation potential, as implied by the high amounts of oxygenation organic matter (Type IV) and the low hydrogen index values of up to 50 mg of hydrocarbon (HC)/g of TOC. The oxygenation condition of the outcrop sections was confirmed by the high oxygen index values of up to 242 mg carbon dioxide (CO₂)/g TOC and low values of relative hydrocarbon index in the range of 0.09–0.58 mg HC/g TOC. Based on the results highlighted in this study, the weathering conditions of the outcrop sections during the mid-Carboniferous Hercynian deformation erosion event strongly affected the organic matter in the Silurian Qusaiba shale units, which were oxidized and leached. Thus, their characteristics are not suitable for the HC generation potential.