oa Organic Geochemical Evolution of the Silurian Qalibah Formation, Saudi Arabia: Insights from Kerogen Characteristics and Thermal Maturity

The Silurian Qalibah Formation in Saudi Arabia is evaluated for its unconventional hydrocarbon potential through a detailed organic geochemical study of its lower mud-rich Qusaiba Member and upper sand-rich Sharawra Member. Nearly 200 outcrop samples from the Qassim (central) and Tabuk (northwestern) regions were analyzed using Rock-Eval pyrolysis to assess total organic carbon (TOC), kerogen type, thermal maturity, and hydrocarbon generation potential. In central Saudi Arabia, the Qusaiba Member averaged about 0.5 wt.% TOC, while in Tabuk it averaged 2 wt.% with peaks up to 6.5 wt.%, indicating significant regional variability. In contrast, the Sharawra Member consistently exhibited low TOC (<0.1 wt.%). Kerogen analyses revealed a predominantly mixed Type III composition—with some mixed Type II/III in Tabuk—suggesting contributions from both marine and terrestrial sources, which points to a gas-prone system that could generate liquid hydrocarbons upon further maturation. Additionally, variations in organic geochemical parameters reflect early diagenetic processes, such as stratified, low-oxygen conditions and rapid sedimentation, that enhanced organic matter preservation. These insights refine our understanding of source rock quality and maturation in siliciclastic systems and have broader implications for deep-time sedimentary basins.