Integrated sedimentological and geochemical characterization of the Silurian Qalibah Formation in central Saudi Arabia: Implications to paleoenvironmental reconstruction and organic richness evaluation

Unconventional hydrocarbon reservoirs are challenging due to their vertical and lateral heterogeneities. The Silurian Qalibah Formation in Saudi Arabia comprising of the Qusaiba and Sharawra members, serves as a prime example. This study investigates reservoir heterogeneity and variations in depositional conditions to produce a high-resolution chemozonation of the Qalibah Formation. Mineralogical and elemental compositions were determined using X-ray fluorescence (XRF) and X-ray diffraction (XRD), while organic matter characterization was performed using Rock-Eval pyrolysis. Three facies were identified: laminated grey-red mudstones in the Qusaiba Member, heterolithic sandstone-mudstone alternation in the lower Sharawra Member, and thickening upward fine sandstones in the upper Sharawra Member. Clay minerals dominate the Qusaiba Member (50 %), while quartz, mica, and K-feldspars prevail in the Sharawra Member. Organic matter is in early maturation stage with poor-to-fair total organic carbon (TOC) content, averaging 0.2 wt% in the Qusaiba Member and 0.05 wt% in the Sharawra Member. Detrital influx proxies indicate higher sedimentation rates in the Sharawra Member compared to the Qusaiba Member. Paleoclimatic proxies suggest humid to semi-arid conditions in the Qusaiba Member and semi-arid to arid conditions in the Sharawra Member. Paleomarine proxies consistently reveal brackish to saline and suboxic to oxic conditions. Five chemozones were established: three in the Qusaiba Member (Q-1: Ti/P depletion; Q-2: elevated TOC; Q-3: spiking Sr/Ba) and two in the Sharawra Member (S-1: lower Zr/Al and Sr/Cu than S-2). This chemozonation highlights spatial-temporal geochemical variability, offering insights into depositional dynamics and reservoir heterogeneity. The framework supports chemo-steering applications to optimize drilling, enhance reservoir targeting, and improve hydrocarbon recovery in analogous unconventional systems.