SEM-Maps-based quantitative characterization of multi-scale organic and inorganic pore-fracture structures in shales: A case study of the Jurassic Lianggaoshan Formation, northeastern Sichuan Basin

Studies on shale reservoir spaces mostly focus on matrix pores, as manifested by a lack of studies on microfractures’ role and on the quantitative characterization of multi-scale pore-fracture structures as a whole. This study delves into the shales of varying lithofacies from the Jurassic Lianggaoshan Formation, northeastern Sichuan Basin, using field emission scanning electron microscopy (FE-SEM) aided by a random forest algorithm for image processing, and achieves the automatic identification and quantitative characterization of organic and inorganic pores and fractures in the shales. Furthermore, by combining the large field-of-view (FoV) image stitching for SEM (SEM-Maps), we comprehensively characterize the distribution characteristics of multi-scale pores and fractures in the shales. The results indicate that the random forest model can effectively distinguish organic and inorganic pores and fractures. The SEM-Maps images at the FoV scale of 300 μm × 300 μm are representative and can exclude the effects of heterogeneity. In addition, the shales with various lithofacies exhibit significantly different characteristics of pore-fracture structures. In detail, the medium-high organic matter lamellar felsic shales have a total pore-fracture areal porosity of up to 2. 66%, significantly higher than that of low-organic-matter laminated felsic shales (1. 66%) and low-organic-matter massive silty to fine-grained sandstones (0. 99%). In contrast, the shales with various lithofacies exhibit similar distributions of pore and micro-fracture scales, with the pore scales primarily falling in the range of 20 to 1000 nm, and the microfracture scales predominantly between 200 nm and 5000 nm. Calculations based on single-component pore-fracture development coefficients, indicate that clay minerals are easier to form pores and fractures compared with felsic minerals in the Lianggaoshan Formation. This study not only reveals the differences in the multi-scale pore-fracture structures of shales with various lithofacies in the Lianggaoshan Formation, but also identifies the shale lithofacies with a high pore-fracture areal porosity and favorable pore-fracture types. These findings provide an important scientific basis for the precise assessment of sweet spots and efficient shale oil development.