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.

Fractures have significantly impacted the preservation, enrichment, and productivity of shale gas in the Longmaxi Formation in the complex structural area of the southern Sichuan Basin. Based on outcrop, core, imaging logging, inclusion analysis, acoustic emission experiments, and burial and thermal evolution history data, the formation stage, evolution and preservation conditions of structural fractures were comprehensively studied. Shear fractures are the main features of tectonic origin and are characterized by small widths, wide spacing, and high filling degrees. The structural fractures of the Longmaxi Formation can be divided into three groups in terms of their formation stage. The first-stage fractures were formed in the middle-late period of the Yanshanian tectonic movement (86.2–68.5 Ma), when the palaeotectonic stress was 68.33–71.82 MPa; and the homogenization temperature of the fluid inclusions in the corresponding fracture fillings is 150–178°C. The second-stage fractures were formed at the end of the Yanshanian -early Himalayan tectonic movement (68.5–35.2 Ma), when the palaeotectonic stress was 85.2–100.5 MPa; the homogenization temperature of the corresponding fluid inclusions is 123–148°C. The third-stage tectonic fractures formed during the middle-late period of the Himalayan tectonic movement (35.2–0 Ma), when the palaeotectonic stress was 55.6–63.8 MPa; fillings are scarce in these fractures, but the measured homogenization temperature of the corresponding fluid inclusions is 90–110°C. Shale gas preservation conditions were affected by the fracture formation stage, maximum horizontal crustal stress, fault scale, structural burial depth, formation pressure coefficient, etc. Three types of favourable preservation conditions were identified. The Type I target area is the low slope area in the middle of the syncline, which occurs more than 2.0 km from the fault. The research results have guiding significance for the evaluation of the preservation conditions of deep shale gas in the study area and similar areas.

The Lower Silurian Longmaxi Formation in the southern Sichuan Basin is composed of a series of dark carbonaceous shales deposited in a hydrostatic shelf reduction environment. In this study, the ratio of uranium to thorium (U/Th), the total organic carbon (TOC), and the biological silicon content (SiBio) were selected as the characteristic parameters to precisely analyze the sedimentary environment and its impact on reservoir quality. The results show that the Weiyuan area in the Early Silurian Longmaxi period experienced two transgression-regression cycles, forming two third-class sequences, SSQ1 and SSQ2, which can be divided into six sedimentary microfacies: organic-rich siliceous argillaceous shelf, organic-rich silicon-containing argillaceous shelf, organic-rich silty argillaceous shelf, deep-water silty argillaceous shelf, shallow-water silty argillaceous shelf, and shallow-water argillaceous silty shelf microfacies. The organic-rich siliceous argillaceous shelf and organic-rich silicon-containing argillaceous shelf microfacies developed in the deepest transgressive system tract (TST1), with high U/Th, high TOC, and high SiBio, which are identified as the main control facies for reservoir development. These two microfacies are located in the middle of the study area, while a transition occurs in the east affected by the Neijiang Uplift. According to the classification criteria proposed in this article, the favourable shale gas reservoirs in Weiyuan area are characterized with high U/Th (>1.25), high TOC (>3%), and high SiBio (>15%). This paper proposed an evaluation method for shale sedimentary facies based on elemental and electrical logging characteristics, avoiding the limitations of core samples, which makes the quantitative division of shale sediments and the efficient recognition of high-quality reservoirs available. It is of great significance for delineating the potential production areas in the study area and beneficial for the scaled development of shale gas reservoirs.