Objective Unclear mechanical properties and fracture development laws of sand−mud interbedded rock masses in ultra−deep clastic reservoirs limit fracture prediction accuracy and pose challenges to reservoir research and drilling−completion engineering.

Methods This study focuses on the Cretaceous reservoir rocks in the Kuqa Depression. Rock compression experiments were conducted by manually making sand mud interbedded rock samples, and combined with discrete element numerical simulation, the mechanical characteristics and crack development evolution laws of sand mud interbedded composite rock masses were analyzed from the aspects of loading curve, particle deformation, stress chain changes, and crack propagation.

Results The results show that: (1) the crack propagation mode during rock compression failure is mainly compression shear cracks, accounting for more than 70%; (2) the strength of rocks with a single lithology is high, while the strength of interlayered composite rocks is lower than that of rocks with a single lithology. The strength of sand mud interlayered rocks may even be lower than that of pure mudstone; (3) The failure of composite materials with sand and mud interlayers mainly occurs in mudstone with lower strength, while sandstone with higher strength often maintains good integrity; (4) the strength and fracture mode of interlayered rock masses are jointly influenced by the mineral composition of the rock and the thickness of the rock layers (interlayered ratio). When the sand and mud thickness (ratio) is close, the strength of the rock mass is low and the failure is most significant.

Conclusions The research results can provide support for reservoir prediction, wellbore trajectory optimization, and wellbore stability analysis in oil and gas exploration and development.