Impact of Bioturbation on Sandstone Brittleness: Insights from the Qassim Formation, Northwestern Saudi Arabia

Understanding how biological processes affect the mechanical behavior of sedimentary rocks is vital for predicting reservoir performance. Brittleness, which controls fracture propagation and fluid flow, is crucial for hydrocarbon recovery, CO₂ storage, ground water management, and geoenergy applications. Yet, the impact of bioturbation on sandstone integrity remains poorly understood. This study examines its influence on brittleness in the Ordovician Kahfah Member, Qasim Formation, Saudi Arabia, offering insights relevant to bioturbated sandstones globally. A composite stratigraphic section revealed two sandstone lithofacies: bioturbated and non-bioturbated. Field observations, petrography, and geomechanical testing demonstrated distinct contrasts in fracture density, mineralogy, and elastic moduli between the two facies. Bioturbated sandstones, dominated by vertical trace fossils such as Skolithos and Taenidium (bioturbation index 2–4), exhibit lower fracture density and reduced stiffness. Laboratory measurements confirm this, with Young’s modulus ranging from 8.16 to 15.88 GPa and shear modulus from 3.26 to 6.72 GPa. In contrast, non-bioturbated sandstones (bioturbation index 0–1) show higher fracture density and significantly greater stiffness, with Young’s modulus of 22.37–34.39 GPa and shear modulus of 9.07–14.96 GPa. Brittleness indices likewise differ, with lower values (0.05–0.55) in bioturbated samples compared to higher values (0.38–0.90) in non-bioturbated samples. These results demonstrate that bioturbation modifies sandstone mineralogy by incorporating clay through physical and biochemical processes, thereby reducing brittleness. The findings reveal that bioturbation is an important control on geomechanical behavior in heterogeneous strata. Accounting for its effects improves predictions of rock response to stress in subsurface systems, with broad implications for energy recovery, and sustainable reservoir management.