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Taking the Lower Silurian Longmaxi Formation in the Changning area of the southern Sichuan Basin as an example, the characteristics and formation stages of tectonic fractures are comprehensively studied by regional scale geological analysis and microscopic scale experimental tests. Regional scale geological analysis shows that there are mainly high-angle and vertical structural fractures, which have the characteristics of small opening, large spacing, small fracture density, and high filling degree. The fracture systems can be divided into three stages and six groups, which correspond to the structural compression in the near south−north, northwest, and northeast directions. The microscopic scale experimental test analysis confirmed that the formation of tectonic fractures mainly experienced three stages of tectonic movements, namely, the Middle−Late Yanshanian tectonic movement (136− 94 million years ago (Ma)), the Late Yanshanian tectonic movement−Early Himalayan tectonic movement (94−67 Ma), and the Middle−Late Himalayan tectonic movement (67−0 Ma). The corresponding uniform temperatures of the fracture filling inclusions are 118.5−140.2, 91.6−108.5, and 73.2−82.2 °C, respectively. Based on the tectonic analysis and geomechanical principles, an evolutionary model of structural fractures was established. Fractures with early formation and high filling degree and fractures with late formation but large angle between the fracture orientation and current geostress direction are beneficial to shale gas enrichment.

Tectonic fractures are the key factors affecting hydrocarbon migration and accumulation in ultradeep marine carbonate gas reservoirs. Taking the Maokou Formation in the Jiulongshan Gas Field as an example, tectonic fracture formation and distribution are quantitatively characterized by the outcrops, cores, Fullbore Formation MicroImager (FMI) imaging logging, acoustic emission experiments, fluid inclusion experiments, and burial–thermal evolution history analysis. The formation stage of the tectonic fractures in the study area can generally be divided into three stages: the Indosinian stage, the early middle Yanshanian stage, and the late Yanshanian–Himalayan stage. The key stages are the early middle Yanshanian stage and the late Yanshanian–Himalayan stage. According to the theory of tectonic geomechanics, the evolution pattern of different stages of tectonic fractures and faults in the Maokou Formation is established. The finite element method was used to simulate the three-dimensional paleotectonic stress field during the key stages of fracture formation, and a rock failure criterion (η) was used to quantitatively predict the development and distribution of the tectonic fracture. In the early middle Yanshanian stage, the fracture degree was relatively small, and the highly fractured areas were mainly concentrated in the areas near the northern fault zone and the high part of the anticline, with the highest rock failure proximity of 1.118. In the late Yanshanian–early Himalayan stage, the highly fractured areas are distributed in the northeast and northwest, near the E–W fault rupture zone, the high parts of the Jiulongshan and Tadongping areas, and the local tectonic high parts. The degree of rock failure mainly concentrated between 0.890 and 1.127. There is a good positive correlation between the fracture density and the degree of rock failure.

Influences of the characteristics of organic matters and mineral compositions on the development of shale microscopic pores were discussed through performing low-temperature nitrogen adsorption, whole-rock "X" diffraction, and field emission scanning electron microscope on black organic-rich shale samples of Longmaxi Formation in the Dingshan area, southeastern Sichuan Basin, in combination with the characteristics of shale organic matters. The shale in the Dingshan area has complex mineral composition, which is mainly quartz and clay minerals. Both contents range from 23% to 72% and 36% to 70%, respectively. The vertical variation is obvious, and there are few feldspar and carbonate rocks; the pyrite content is more than 2%. The bottom shale has high brittleness and gradually decreases in the vertical direction. The brittleness index is between 0.481 and 0.627 and is concentrated above 0.5. It shows strong compressibility and is easy to form a complex network system in hydraulic fracturing. Quartz and feldspar mainly provide secondary dissolved pores, intercrystalline mineral pores, and nanoedge gaps in contact with organic matter. They have no obvious correlation with the specific surface area of shale but have a weak correlation with pore volume. They mainly control the development of macropores. Organic matter develops many hydrocarbon-generating pores, which strongly correlate with the specific surface area and a weak correlation with pore volume. It mainly controls the development of micromesopores. Clay minerals mainly provide a large number of interlaminar pores and interlayer fractures in the clay. The intergranular pores of clay and clay have a weak correlation with pore volume and specific surfaces. They contribute to the development of shale micropores, mesopores, and macropores. Pyrite mainly provides intercrystalline pores and mold pores. By restricting the interaction with organic matter, the development of shale pores is promoted within a certain content range. When the content exceeds this range, the development of micropores is inhibited. The conversion threshold in the Dingshan area is 5.0%.

The main geological factors controlling the accumulation and yield of marine‐facies shale gas reservoirs are the focus of the current shale gas exploration and development research. In this study, the Wufeng‐Longmaxi Formation in the Dingshan area of southeast Sichuan was investigated. Shale cores underwent laboratory testing, which included the evaluation of total organic carbon (TOC), vitrinite reflectance (Ro), whole‐rock X‐ray diffraction (XRD), pore permeability, and imaging through field emission scanning electron microscopy (FE‐SEM). Based on the results of natural gamma ray spectrum logging, conventional logging, imaging logging, and seismic coherence properties, the exploration and development potential of shale gas in the Dingshan area have been discussed comprehensively. The results showed that (1) layer No. 4 (WF2‐LM4) of the Wufeng‐Longmaxi Formation has a Th/U ratio <2 and a Th/K ratio of 3.5‐12. Graptolites and pyrite are relatively abundant in the shale core, indicating sub‐high‐energy and low‐energy marine‐facies anoxic reducing environments. (2) The organic matter is mainly I‐type kerogen with a small amount of II1‐type kerogen. There is a good correlation among TOC, Ro, gas content, and brittle minerals; the fracturing property (brittleness) is 57.3%. Organic and inorganic pores are moderately developed. A higher pressure coefficient is correlated with the increase in porosity and the decrease in permeability. (3) The DY1 well of the shale gas reservoir was affected by natural defects and important late‐stage double destructive effects, and it is poorly preserved. The DY2 well is located far from the Qiyueshan Fault. Large faults are absent, and upward fractures in the Longmaxi Formation are poorly developed. The well is affected by low tectonic deformation intensity, and it is well preserved. (4) The Dingshan area is located at the junction of the two sedimentary centers of Jiaoshiba and Changning. The thickness of the high‐quality shale interval (WF2‐LM4) is relatively small, which may be an important reason for the unstable production of shale gas thus far. Based on the systematic analysis of the geological factors controlling high‐yield shale gas enrichment in the Dingshan area, and the comparative analysis with the surrounding typical exploration areas, the geological understanding of marine shale gas enrichment in southern China has been improved. Therefore, this study can provide a useful reference for shale gas exploration and further development.

The primary gas layer of the Maokou Formation of the JLS structure in the Sichuan Basin is a typical pore-fracture carbonate gas reservoir. Natural fractures (mainly tectonic fractures) serve as the critical factor in improving the reservoir's physical properties. The characteristics, formation mechanisms of natural fractures, and their impact on the reservoir are analyzed using the core, thin slice, full microresistivity imager imaging logging, production testing, and the Monte Carlo method multiple approximations. The results show that the fractures in the Maokou Formation reservoir generally refer to shear fractures of a tectonic origin, with the characteristics of a medium scale (10−20 cm), a large inclination angle (45− 90), a high density (greater than 5 m −1 ), a medium opening (1−3 mm), and a low filling degree. There are three primary formation stages of natural fractures in the area, the Indosinian stage (232−210 Ma), the early-middle Yanshanian stage (145−90 Ma), and the late Yanshanian−Himalayan stage (90−0 Ma). The corresponding principal stress directions are NW, near-SN, and NNW, respectively. Natural fractures have a positive effect on the improvement of reservoir physical properties. The fracture's physical parameters show a good positive correlation with the physical properties of the reservoirs. The fracture parameters influence the reservoir's physical properties ranked from biggest to smallest: opening, length, and density. The greater the development degree of fractures, the greater the corresponding natural gas production capacity. The research results provide a new technical method for quantifying the influence of fracture parameters on reservoir's physical properties.

To investigate the geological characteristics and exploration potential of shale gas in the southern Sichuan Basin, we analyze the coupling relationship between the hydrocarbon generation and storage conditions of the Longmaxi Formation and discuss the preservation conditions from the lateral and vertical migration mechanisms of shale gas. According to the results, the organic-rich shale at the bottom was formed in a strongly euxinic and anoxic reducing environment, the oxygen content increased in the upper water body in which the Longmaxi Formation was deposited, and the water body became oxidized. The organic matter type in the shale is dominantly type I kerogen and a small amount of type II1 kerogen. The organic matter content is more than 3.0% and is in the high-to postmature stage. The enrichment of siliceous organisms increases the organic matter and enhances the brittleness of shale, resulting in “superior hydrocarbons and a favorable reservoir”. Pyrolysis of organic matter promotes the formation of organic matter pores and dissolution pores, resulting in a coupled “source-reservoir” accumulation control system. The high vertical formation pressure guarantees the sealing of shale and restrains the lateral escape of shale gas. The high-angle intersection of the highly filled fractures and the current crustal stress can effectively enhance the fracture sealing and inhibit the vertical escape of shale gas, forming a three-dimensional effective closure. Hence, the area featuring a short tectonic uplift time, small amplitudes, large-scale underdeveloped fractures, and a high formation pressure coefficient is a favorable area for shale gas exploration, according to the analysis of three-dimensional preservation conditions.