Abstract: To address the problem of poorly understood mechanical mechanism of stress-caused brittle failure in shale, the brittle fracture model of laminated shale with longitudinal mechanical inhomogeneity was established by applying the theory of rock fracture mechanics and combining it with generalized Hooke's law on the basis of analyzing the characteristics of laminated shale, and the main influencing factors of the fracture mode of laminated shale were analyzed. The study shows that the fracture modes of laminated shale are mainly divided into shear failure and tensile failure, hard rock formations are prone to tensile failure, while soft rock formations are prone to shear failure under external stress conditions; the larger the Young's modulus of the rock formation and the smaller the Poisson's ratio, the more prone the formation is to tensile failure and vice versa; the larger the minimum horizontal principal stress and the smaller the horizontal principal stress difference, the more prone the formation is to shear failure; the variability of mechanical properties between shale layers is the fundamental reason for the brittle fracture of shale; the greater the variability between shale layers, the more brittle the rock is, and the more favorable it is for fracturing. This study can provide theoretical guidance for shale brittleness evaluation and hydraulic fracturing scheme development.

Key words: laminated shale, brittle fracture, rock characteristics, fracture model, failure mechanism, network fracturing, microscopic characteristics