Comprehensive characterization and analysis of the microstructure characteristics of shale are significant for understanding its transport mechanisms and determining highly efficient production programs. In this study, composition analysis, scanning electron microscopy imaging, and gas adsorption−desorption measurements are first conducted on 29 marine shale samples from the Longmaxi Formation, Sichuan Basin, China, to obtain their petrophysical properties and microstructure characteristics. Then, surface fractal dimension, pore fractal dimension, lacunarity, and succolarity are introduced to analyze the microstructure of these shale samples. Finally, the correlations between total organic carbon (TOC) content and microstructures and the relationships among the microstructures are analyzed systematically. The results show that the complex pore network in these shales is constituted by microfractures, intergranular pores, intragranular pores, dissolved pores, and organic pores. The typical type IV adsorption isotherm and type H3 hysteresis loop are identified from nitrogen adsorption−desorption data. TOC content has positive influences on the adsorption capacity and nanopore distribution. The surface fractal dimensions under low and high relative pressure are similar in these marine shale samples, which is different from the two-section characteristic of continental and marinecontinental shale. The statistical self-similarity characteristic of the pore-size distribution in these shales is further confirmed based on the analytical fractal dimension equation. The normalized lacunarity can quantitatively characterize the heterogeneity of the poresize distribution of these shales in two-dimensional space. The succolarity analysis in two-dimensional space explains the low fluid transport capacity of these shales. Universal and logically explained correlations are found between TOC content and microstructures. Moreover, logically explained correlations also exist among the microstructures of the shale samples evaluated.

As the commercial development of shallow shale gas reservoirs has been successfully achieved in Sichuan Basin, the deep shale gas reservoirs become the key development target in the next step. The accurate quantitative characterization of the fluid storage and structural properties of deep shale is of great significance in resource evaluation and gas exploitation. In this study, fifteen deep Longmaxi Formation shale samples belonging to three sublayers are collected from the L208 well area, Luzhou Block, southern Sichuan Basin. Nuclear magnetic resonance and nitrogen adsorption are conducted on these samples to obtain the fluid storage capacity and microstructural information. Pore fractal dimension and surface fractal dimension are interpreted from these experimental data for quantitative evaluation of the pore distribution complexity and pore surface irregularity, respectively. The controlling factors of storage and microstructural properties of these deep shales are analyzed. From the perspective of stratigraphic division, the shales belonging to the lower sublayers possess a larger transverse relaxation time of the main peak, a weaker nitrogen adsorption amount, a lower pore volume, a lower specific surface area, and a higher surface fractal dimension as compared to those of the shales from the upper sublayer. From the perspective of shale composition, clay mineral content and total organic carbon content are the main and minor controlling factors of the storage capacity, pore fractal dimension, and surface fractal dimension of deep Longmaxi shale, respectively. Moreover, the pore surface irregularity is relevant to the average pore diameter and specific surface area.

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