Characteristics of Lamination in Deep Marine Shale and Its Influence on Mechanical Properties: A Case Study on the Wufeng-Longmaxi Formation in Sichuan Basin

The extensive development of lamination structures in shale significantly influences its mechanical properties. However, a systematic analysis of how laminae affect the macroscopic mechanical behavior of rocks remains absent. In this study, field emission scanning electron microscopy (FE-SEM), thin section observation, X-ray diffraction (XRD), triaxial compression and Brazilian tests were carried out on the deep marine shale of the Wufeng-Longmaxi Formation in Sichuan Basin. The results reveal four distinct laminasets: grading thin silt-thick mud (GSM1), grading medium thick silt-mud (GSM2), grading thick silt-thin mud (GSM3) and alternating thick silt-thin mud (ASM). GSM3 and ASM laminasets exhibit the weakest mechanical properties and the simplest fracture patterns, while GSM2 demonstrates moderate mechanical properties and more complex fracture patterns. GSM1 shows the highest mechanical strength and the most intricate fracture patterns. Mechanical properties are positively correlated with siliceous mineral content and negatively correlated with clay mineral content and scale of laminae development (average density and thickness), revealing that lamination plays a key role in fracture behavior, with more intensively developed laminasets leading to the concentrated distribution of brittle silty minerals, facilitating microcrack propagation. Moreover, microstructure has an important effect on both mechanical properties and fracture pattern. In grain-supported structures, closely packed silty brittle mineral grains reduce the energy required for crack extension. In matrix-supported structures, widespread silty brittle mineral grains increase energy requirements for crack extension, leading to more irregular and complex fracture networks. This study enhances the understanding of the effects of lamination on the rock mechanical behavior of shales, optimizing hydraulic fracturing design in shale reservoirs.