Mechanical properties of thinly interbedded sandstone-mudstone rocks in Lower Shihezi Formation from eastern Ordos Basin

Mechanical properties of thinly interbedded rocks are fundamental for investigating the crack propagations and designing hydraulic fracturing treatments in multi-layered formations. However, conventional geomechanical tests face challenges in precisely characterizing the mechanical properties of individual interlayers. For the issues mentioned above, the grid nanoindentation, high-resolution scanning electron microscopy assist with energy-dispersive spectroscopy (SEM-EDS) techniques are employed to finely characterize the differences in mineral features and mechanical properties between the sandstone layer and mudstone layer within the thinly interbedded sandstone-mudstone core rocks that collected from the Lower Shihezi Formation in the eastern Ordos Basin. Furthermore, the indentation features and cracking patterns of typical minerals are analyzed. On this basis, the impact of inclination angle and interlayer/weak interface on the tensile strength and fracture pattern of the thinly interbedded sandstone-mudstone rocks is investigated through Brazilian splitting tests. Results indicate that ① the sandstone layer exhibits the higher mechanical strength due to the higher content of brittle minerals such as quartz, feldspar, hematite, and calcite, with an average Young's modulus of 51.4 GPa, average hardness of 1.74 GPa, and average fracture toughness of 5.4 MPa∙m0.5, which are 1.10 times, 1.14 times, and 1.10 times than that of mudstone layer, respectively. ② When using a three-sided pyramid Berkovich diamond indenter, the indentation of a single mineral exhibits an approximate equilateral triangle shape. In the case of multiple minerals, the indentation is primarily influenced by the softer mineral phase, making it easier to press into the softer mineral phase. ③ The indentation sizes of quartz, hematite/calcite, and phyllosilicate minerals increase sequentially. Moreover, indentations on brittle minerals are prone to generate shear and radial cracks, while indentations on ductile minerals are more likely to produce chippings. ④ As the inclination angle increases (0°−90°), the Brazilian tensile strength of the thinly interbedded sandstone-mudstone rocks gradually decreases, and the failure is more likely to occur along the interlayer/weak interface. Besides, the variation in inclination angle from 0° to 90° represents a transition in the failure pattern of thinly interbedded sandstone-mudstone rocks from tensile-dominated to shear-dominated, and then back to tensile-dominated. In addition, the heterogeneity and anisotropy of thinly interbedded sandstone-mudstone rocks in the Lower Shihezi Formation of the study area are apparent. The differences in mineral compositions and mechanical properties are significant factors contributing to geomechanical variations between the thin interlayers.