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Objective The Huayingshan fault zone, the largest fault zone within Sichuan Basin, exhibits notable differences in geological structures on both sides. Historically, earthquakes with a magnitude of ≤5 have occurred frequently along this fault zone, which remains relatively active to this day, disturbing the crustal stress field of the basin. Clarifying the crustal stress state of the Huayingshan fault zone can enhance our understanding of its active deformation and its tectonic and seismic activities and serve as a reference for subsequent research regarding this fault zone. Currently, geostress studies in this area rely mainly on analyzing seismic data, whereas the investigation of borehole data remains relatively scarce. Methods We collected and collated imaging data regarding borehole collapse and borehole-induced tensile fractures from eight borehole logging sites in the southern segment of the Huayingshan fault zone; the geostress directions of these eight boreholes were determined by analyzing these data. Subsequently, a comprehensive analysis of the geostress characteristics was performed by combining the data regarding China’s modern stress field and the earthquake focal mechanism solutions in the southeastern margin of Sichuan Basin. Results The maximum horizontal principal stress in four boreholes located in the southern region of the southern segment of the Huayingshan fault zone was oriented in the NWW-SEE direction, which aligns with the regional stress field direction in Sichuan Basin; only one borehole in the southern region exhibited a maximum horizontal principal stress in the NEE—SWW direction, representing a counterclockwise deviation relative to the regional stress field of Sichuan Basin. Meanwhile, the maximum horizontal principal stress in all three boreholes in the central region of the southern segment of the Huayingshan fault zone was also oriented in the NEE—SWW direction, representing a counterclockwise deviation relative to the regional stress field of Sichuan Basin. Conclusion Through the comprehensive analysis of the above results and the tectonic features and basement properties of the study area, the following conclusions are drawn: (1) The deviations in stress direction in the southern segment of the fault zone are primarily caused by the combined effects of changes in the basement properties of the region, tectonic stress near the boreholes, and the regional stress field. (2) The geostress characteristics obtained in the present study, along with the focal mechanism solutions of earthquakes in the southern segment of the Huayingshan fault zone, indicate that the stress direction in the southeastern margin of Sichuan Basin is relatively divergent, resulting from the combined effects of complex local structures and the regional stress field in this region. Significance The present study aims to supplement the geostress data regarding the southern segment of the Huayingshan fault zone based on borehole data, analyze the current stress field state of the rock mass, and determine the direction of the maximum horizontal principal stress. These findings will provide supporting geostress data for subsequent research on the frequent seismic activities in this region.

The marine shale within the Sichuan Basin constitutes China's significant shale gas production, featuring old formation age, high degree of thermal evolution, multiple tectonic movements, and complex structural conditions. However, there are significant differences in the shale gas preservation conditions and reservoir quality in different areas, limiting future large-scale exploration and development. Pore structure significantly influences shale reservoir quality, gas content, and exploration of shale gas occurrence, migration, and enrichment mechanisms. The influence of structural-dominated preservation conditions on shale pore structures is essential to comprehend for effective shale gas exploitation. This study employs field-emission scanning electron microscopy in conjunction with other techniques (low-temperature N2 adsorption, low-temperature CO2 adsorption, and nuclear magnetic resonance) for detailed analyses of the pore structure across varied structural zones, revealing the influence of structural attributes, fault systems, depth of burial, and formation pressure on pore architecture, and examining the relationship between pore structure and shale gas preservation conditions. The results show that stable structural condition is conducive to the development and preservation of shale pores. Structural compression causes inorganic and organic pores to become narrow and elongated due to shrinkage, with a significant increase in microfractures. The porosity of shale with stable structural conditions exhibits markedly increased porosity compared to samples under structural compressions. Under conditions of similar TOC and mineral composition, the pore size distribution (PSD), pore volume (PV), and specific surface area (SSA) of shale after structural compression are significantly lower than those of samples with stable structural conditions. As the burial depth increases, the shale porosity shows a decreasing trend, but the decrease is limited. Burial depth significantly impacts the SSA and PV of high-TOC samples (3%-6%). As the burial depth increases, both SSA and PV show a significant decreasing trend. When the burial depth reaches 4000 m, SSA and PV tend to concentrate. The formation pressure coefficient is an important factor for the development and preservation of shale pores, and porosity is positively correlated with the formation pressure coefficient. Increased formation pressure coefficient indicates superior preservation conditions and enhanced pore development. [ABSTRACT FROM AUTHOR]

Currently, Luzhou in the Sichuan Basin is a focal point for shale-gas exploration and development in China. However, a lack of detailed research on the mineral composition of the Wufeng Formation-Longmaxi Formation (WF-LF) shale is hindering the extraction of deep-buried shale gas in the Luzhou shale play. Herein, a field emission scanning electron microscope (FESEM) equipped with the Advanced Mineral Identification and Characterization System (AMICS) software was employed to analyze the mineral composition of the WF-LF shale from six wells in Luzhou. Quartz was the dominant mineral type, (16.9–87.21%, average 51.33%), followed by illite, calcite, dolomite, and pyrite. Our study revealed that (1) quartz content showed a moderate positive correlation with the total organic carbon (TOC) content, indicating that the quartz found in the shale is mostly of biological origin; and (2) the sum content of siliceous minerals and carbonaceous minerals was moderately positively correlated with the brittleness index (BRIT) in well SS1H2-7 and in the well group of RS8 and RS5, indicating that the siliceous minerals and carbonaceous minerals had an active effect on reservoir compressibility. Finally, according to the mineralogical features of each sublayer, we identified four types of reservoirs to determine their scope for exploration. [ABSTRACT FROM AUTHOR]

[Display omitted] • The shale gas resource potential of the Longmaxi Formation was evaluated by using mud gas isotope logging technology. • The reservoir properties of shale were evaluated based on gas isotope fractionation effects. • The shale gas in the Changning area belongs to late kerogen cracking gas. • The lower part of the Longmaxi Formation has a better shale gas exploration potential. The Longmaxi Formation in the Sichuan Basin is one of the most important shale gas producing formations in China. Previous studies of shale gas were mostly carried out on based on laboratory analyses and did not provide effective guidelines for shale gas exploration. In this paper, we present a large amount of carbon isotopic data from mud gas isotope logging and rock-cutting headspace gas in wellsites. The δ13C 1 values showed a significant decrease at the bottom of the Longmaxi Formation. The δ13C 1 and δ13C 2 values in the Changning area were observed to be heavier than those in the Weiyuan and Jiaoshiba areas. The second half of the N216H27-3 horizontal well had obvious outgassing and carbon isotope fractionation effects. This indicates that the bottom of the Longmaxi Formation has better shale gas exploration potential. The shale gas of the Longmaxi Formation in the Changning area belongs to late kerogen cracking gas, where exploration prospects are poorer than those in the Weiyuan and Jiaoshiba areas. There were more oil cracking gas resources and better reservoir conditions at the end of the N216H27-3 horizontal well; therefore, drilling should be continued at this site. [ABSTRACT FROM AUTHOR]

Based on low-temperature thermochronological data (i.e., apatite fission-track (AFT) and (U–Th)/He (AHe)), structural evolution, burial and thermal history, this paper examines the relationship of multi-stage evolution between the upper Ordovician Wufeng and lower Silurian Longmaxi formations (i.e., the WL formation) and rapid exhumation at the Changning shale gas field in the southern Sichuan basin. AFT ages are generally young from southeast to northwest, decreasing from ca. 40 to 15 Ma while AHe single-grain ages range from ~35 to ~5 Ma. Inverse thermal histories show that a multi-stage cooling history with initial cooling began in the Late Cretaceous to early Cenozoic (middle Eocene), followed by accelerated rapid cooling in the late Cenozoic, with cooling rates of 2–4 °C/Myr across the Changning shale gas field. In particular, thermal models of samples in the western Changning area show a substantial increase in the cooling rates, i.e., from less than 1 °C/Myr to 3–5 °C/Myr in the Miocene. Integrated with inverse T-t models of low-temperature thermochronological data, the burial history indicates a four-stage thermal evolution of the WL Formation in the Changning shale-gas field: Early Mature from the middle Silurian to middle Permian, Middle Mature from the late Permian to Late Triassic, Late Mature from the Early Jurassic to Early Cretaceous, and Over mature since the middle Cretaceous, reaching a maximum burial depth of ~6000 m and maximum temperature of 190–200 °C at ca. 70–80 Ma. This evolution further suggests that a normal hydrostatic pressure index dominated the WL Formations in the Paleozoic. The gas generation and pressure index, however, have substantially increased since the Late Jurassic, particularly from the Early to middle Cretaceous, with a maximum pressure index of 2.1, which is indicative of the main shale gas accumulation and enrichment period in the WL Formation. Post-gas-generation structural deformation, uplift, and exhumation had a significant impact on shale gas enrichment, mainly through their influence on shale gas preservation. This suggests some relationships among the higher pressure coefficient of the WL Formation, the higher productivity and total gas content of the WL Formation, a weaker post-gas-generation structural deformation, and a weaker uplift, as indicated by the erosion and burial depth across the Changning shale gas field. Due to a significantly stronger cooling and uplift that occurred the in Late Cenozoic across the western Changning area, the pressure index decreased rapidly from ~2.0 to 1.0 with a normal pressure condition, indicating the destruction of the reservation condition in the WL Formation. • New low-temperature thermochronological data was from the SE Sichuan basin. • An accelerated cooling in the late Cenozoic was found. • Post-gas-generation deformation and exhumation controlled the shale gas enrichment. [ABSTRACT FROM AUTHOR]