Assessing thermal maturity beyond the reaches of vitrinite reflectance and Rock-Eval pyrolysis: A case study from the Silurian Qusaiba formation

Thermal maturity assessment in pre-Devonian shales is challenging due to the absence of vitrinite macerals that form the basis for vitrinite reflectance petrography, the most-widely used technique for organic maturity assessment. This paper presents an integrated analysis of thermal maturity on the basis of alternative spectroscopic and geochemical techniques in lieu of conventional organic petrography, applied in four drilled wells in the pre-Devonian (Silurian) Qusaiba Member of the Qalibah Formation in northwestern Saudi Arabia. The techniques comprise both bulk sample (Rock-Eval pyrolysis, infrared and Raman spectroscopy), and kerogen isolate analysis (elemental, density, surface area, and X-ray absorption near edge structure), with each method calibrated to the vitrinite reflectance scale. Rock-Eval pyrolysis, a common alternative to vitrinite reflectance measurements, provided unreliable maturity estimates in the majority of samples because of low S2 signal. In contrast, the other techniques defined a consistent and narrow maturity range within each well, and revealed a wide range of maturity between the wells. On the basis of the spectroscopic and geochemical results, equivalent vitrinite reflectance for the Qusaiba Member in northwestern Saudi Arabia ranges from at least 0.9 ± 0.1 to 2.1 ± 0.2%Ro, demonstrating significant variation in its maturation history. The integrated assessment of maturity across multiple wells provides data that can be used to construct maturity maps for oilfield exploration and appraisal. More generally, the methods and calibrations for thermal maturity presented here can be used to establish vitrinite-reflectance-equivalent maturities in shales as a complement to or especially in the absence of conventional maturity estimates.