Anisotropic velocity properties and lithofacies identification of shahejie lacustrine shales

Lacustrine shales have gained increasing attention in the global energy transition. However, their more complex lithofacies variations compared to marine shales pose significant challenges for precise reservoir characterization. This study investigates the anisotropic elastic properties of Shahejie lacustrine shales to identify key parameters for lithofacies discrimination. Anisotropic ultrasonic velocity measurements were performed on 59 sample pairs, classified into four lithofacies, as argillaceous, mixed, siliceous, and calcareous. The results demonstrate distinct velocity and anisotropy behaviors across lithofacies, primarily governed by the alignment of clay particles, laminated kerogen, bedding planes, and associated micro-cracks. Although global linear P- and S-wave velocity trends and exponential relationships between bedding-normal velocities and Thomsen's anisotropy parameters are observed, the bedding-normal P-wave velocity and P-to-S wave velocity ratio exhibit systematic, lithofacies-dependent variations. Consequently, a cross-plot template based on bedding-normal P-wave impedance and velocity ratio is established to effectively distinguish the four lithofacies. These findings provide critical rock-physics constraints for lithofacies prediction in Shahejie shales, supporting the optimization of exploration targets and development strategies in analogous lacustrine shale reservoirs.