The pore structure is an important factor affecting reservoir capacity and shale gas production. The shale reservoir of the Longmaxi Formation in the Changning area, Southern Sichuan Basin, is highly heterogeneous and has a complex pore structure. To quantitatively characterize the shale’s pore structure and influencing factors, based on whole rock X-ray diffraction, argon ion polishing electron microscopy observations, and low-temperature nitrogen adsorption-desorption experiments, the characteristics of the shale pore structure are studied by using the Frenkel-Halsey-Hill (FHH) model. The research reveals the following: 1) The pores of the Longmaxi Formation shale mainly include organic pores, intergranular pores, dissolution pores and microfractures. The pore size is mainly micro-mesoporous. Both ink bottle-type pores and semiclosed slit-type pores with good openness exist, but mainly ink bottle-type pores are observed. 2) The pore structure of the Longmaxi Formation shale has self-similarity, conforms to the fractal law, and shows double fractal characteristics. Taking the relative pressure of 0.45 (P/P₀ = 0.45) as the boundary, the surface fractal dimension D_sf and the structural fractal dimension D_st are defined. D_sf is between 2.3215 and 2.6117, and the structural fractal dimension D_st is between 2.8424 and 2.9016. The pore structure of micropores and mesopores is more complex. 3) The mineral components and organic matter have obvious control over the fractal dimension of shale, and samples from different wells show certain differences. The fractal dimension has a good positive correlation with the quartz content but an obvious negative correlation with clay minerals. The higher the total organic carbon content is, the higher the degree of thermal evolution, the more complex the pore structure of shale, and the larger the fractal dimension. The results have guiding significance for the characterization of pore structure of tight rocks.

To identify the factors controlling high-quality deep shale gas reservoirs and the exploration and development potential of the Lower Paleozoic marine shale in the Sichuan Basin, the sedimentary environment of deep shale was comprehensively analysed using core thin sections, scanning electron microscopy, gamma ray spectrometry logging, and elemental logging data. In addition, the geological conditions of deep shale gas accumulation and the effect of tectonic processes on the preservation conditions are discussed based on the experimental data of mineral composition analysis, geochemical features, and reservoir spatial characteristics. (1) The sedimentary environment changes from an anoxic water environment to an oxygen-rich oxidizing environment from bottom to top in the Wufeng-Longmaxi Formation in southern Sichuan. The deep shale gas reservoir shows overpressure and rich gas characteristics, namely, high formation pressure (2.0~2.2), high porosity (20%~55%), and high gas content (4.0~5.0 m³/t). (2) The favourable sedimentary environment has a higher hydrocarbon generation potential and deposits of rich organic matter and siliceous particles. During the hydrocarbon generation process, the rich organic matter generates a large number of organic pores and a large specific surface area, which provides the main reservoir and adsorption space for free and adsorbed shale gas. A large number of biogenic siliceous particles provide a solid rock support framework for the shale reservoir, thereby maintaining excellent reservoir physical properties. (3) Late and small stratigraphic uplifts result in a short shale gas escape time and favourable preservation conditions. Additionally, the small-scale faults and a high-angle intersection between the fracture strike and the geostress direction are conducive to the preservation of shale gas. (4) A high formation pressure coefficient, a sedimentary environment rich in organic siliceous deep-water continental shelf microfacies, and a relatively stable tectonic structure are conducive to the accumulation of deep shale gas.