Monsoonal upwelling and organic matter preservation in the Eocene Rashrashiyah Formation: Implications for hydrocarbon potential in the Turayf-Sirhan Basin, northwestern Arabia

Source rocks are recognized for their ability to capture past seawater conditions while also holding significant economic value. This study evaluates the hydrocarbon potential and depositional environment of the Eocene Rashrashiyah Formation within the Turayf-Sirhan Basin, northwestern Arabia, through an integrated analysis combining organic and inorganic geochemistry, petrography, and paleoenvironmental reconstruction based on field observations. The lower bituminous marl unit demonstrates the highest total organic carbon (TOC: up to 2.11%) and Type II/IIS kerogen, indicative of oil and gas potential. Vertical TOC variability is linked to monsoonal upwelling of nutrient-rich Tethyan waters, which enhanced primary productivity, coupled with dysoxic preservation below the storm wave base. In contrast, deposition under oxic conditions in the overlying chalky marl and marly limestone units led to stronger organic degradation and lower TOC contents (<0.5%). Trace elements serve as valuable paleoproxies, with Ni and Zn indicating paleoproductivity, while Mo and V act as anoxia proxies. Additionally, redox indicators such as Fe and S confirm reducing conditions during periods of high TOC deposition. The benthic foraminifera oxygen index (BFOI) was applied to estimate dissolved oxygen levels, allowing interpolation of bottom-water oxygen levels. Regional comparisons with coeval strata in the Arabia, suggest that the Rashrashiyah Formation may possess greater hydrocarbon potential in deeper, less weathered intervals. Surface oxidation and tectonic uplift likely contributed to TOC depletion in outcrop samples, underscoring the importance of targeted subsurface assessments. This study underscores the interplay of monsoonal upwelling, productivity, and preservation in shaping carbonate source rocks and provides critical insights into unconventional resource exploration in analogous paleo-upwelling systems.