Triassic Facies Evolution of the Lower Jilh Formation: Integrated Sedimentological, Ichnological, and Geochemical Insights from Central Saudi Arabia

The Middle to Late Triassic Jilh Formation is a mixed siliciclastic-carbonate succession with complex stratigraphy. It is recognized across the Arabian Peninsula as hosting complete petroleum systems, including source rocks, reservoirs, and seals, as well as aquifers. Despite its economic significance, research on the Jilh Formation remains limited, largely due to its internal complexity and production challenges, such as overpressure zones. Furthermore, its depositional and ichnological characteristics remain poorly constrained.

This study investigates a 45-meter-thick outcrop of the Lower Jilh Formation in the Riyadh Region through an integrated sedimentological and ichnological approach. Field-based analyses were complemented by laboratory investigations, including thin section petrography and geochemical characterization.

Six siliciclastic facies were identified: (1) green, massive, fissile shale with sparse bioturbation; (2) yellow to reddish, massive to faintly laminated siltstone with limited Skolithos and Arenicolites; (3) horizontally laminated very fine-grained sandstone with sparse bioturbation and diverse ichnofauna; (4) heavily bioturbated, very fine- to fine-grained sandstone dominated by monospecific ichnogenera (e.g., Taenidium, Skolithos, and Rhyzoliths); (5) current-rippled, very fine- to fine-grained sandstone with moderate bioturbation (e.g. Skolithos and Lockeia); (6) low- to high-angle cross-bedded, fine- to medium-grained sandstone devoid of bioturbation.

These facies represent an estuarine to fluvial system, including channels, point bars, proximal estuarine settings, and a central estuarine basin. The dominance of monospecific ichnofacies, low bioturbation intensities, and absence of fully marine trace fossils suggest environmentally stressed conditions, likely related to salinity fluctuations, rapid sedimentation, and intermittent subaerial exposure.

Geochemical analysis of the outcrop reveals distinct vertical variations in elemental concentrations, delineating three distinct geochemical zones: from bottom to top (1 moderate Si and Al concentrations, indicative of mixed siliciclastic input, with minor peaks in Ca and Mg suggesting intermittent carbonate influence; (2) high Si content accompanied by a sharp decline in Al, Ca, and Mg, reflecting a dominance of coarse, quartz-rich terrigenous sediment; and (3) moderate Si and Al values with localized increases in Ca and Mg, pointing to a return of mixed siliciclastic–carbonate deposition. These geochemical trends can further be utilized to interpret paleoproxies, such as weathering intensity, paleoprovenance and fluctuating depositional conditions within the Jilh Formation.

Despite evidence supporting a fine-grained fluvial system, lateral facies correlation remains challenging due to the erosional nature of thick channel deposits and frequent pinch-outs of sedimentary units. Future studies should include multiple outcrop descriptions to address these limitations. Nonetheless, this study contributes to a deeper understanding of the heterogeneity and paleoenvironmental characteristics of the Lower Jilh Formation, offering valuable insights for improved reservoir characterization.