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Collected from a Late Permian to Early Triassic sedimentary section in the ZhongliangMountain of Chongqing, Southwest China, sixty marine carbonate samples were measured for the 87 Sr/ 86 Sr ratios, and corresponding evolution curve was constructed. The concentrations of SiO 2 , CaO, MgO, Mn and Sr are used to evaluate reservation of strontium isotopic composition for original seawater and the credibility of the dissolution method for sample preparation. The results show that most of the samples (except seven samples with the Mn/Sr ratios higher than 2) contain the original geochemistry signatures of ancient seawater. Compared to the published 87 Sr/ 86 Sr ratios from the Late Permian to Early Triassic, our database reported here is the largest and the curve constructed is the most complete. The strontium isotopic curve from Late Permian to Early Triassic is consistent globally and exhibits a general trend of steady increase during this period. The minimum of 87 Sr/ 86 Sr ratios (0.707011) occurs in the Late Permian (30 m in thickness below the Permian-Triassic boundary), and the maximum (0.708281), near the Early-Middle Triassic boundary. The lack of land plants and the rapid continental weathering result in the increase of 87 Sr/ 86 Sr ratios during the interval. The Permian-Triassic boundary in Zhongliang Mountain Section has been accepted internationally. The 87 Sr/ 86 Sr ratios of six samples near the boundary vary from 0.70714 to 0.70715 with an average of 0.70714, which is consistent with the value of 0.70715 (samples are from articulate brachiopod shells) from Korte et al. published in 2006 (within the error range in experiment). Accordingly, the strontium isotope composition in the Permian-Triassic boundary in this paper is of global significance. It can be confirmed that the 87 Sr/ 86 Sr ratios of the seawater in the Permian-Triassic transition are in the range of 0.70714-0.70715.

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The Upper Ordovician Wufeng‐Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery, which is the key stratum for marine shale gas exploration and development (E&D) in China. Based on sedimentary environment, material basis, storage space, fracability and reservoir evolution data, the reservoir characteristics of the Wufeng‐Longmaxi shale and their significance for shale gas EE&D are systematically compared and analyzed in this paper. The results show that (1) the depocenter of the Wufeng (WF)‐Longmaxi (LM) shale gradually migrates from east to west. The high‐quality shale reservoirs in the eastern Sichuan Basin are mainly siliceous shales, which are primarily distributed in the graptolite shale interval of WF2‐LM5. The high‐quality reservoirs in the southern Sichuan Basin are mainly calcareous‐siliceous and organic‐rich argillaceous shales, which are distributed in the graptolite shale interval of WF2‐LM7. (2) Deep shale gas (the burial depth >3500 m) in the Sichuan Basin has high‐ultrahigh pressure and superior physical properties. The organic‐rich siliceous, calcareous‐siliceous and organic‐rich argillaceous shales have suitable reservoir properties. The marginal area of the Sichuan Basin has a higher degree of pressure relief, which leads to the argillaceous and silty shales evolving into direct cap rocks with poor reservoir/good sealing capacity. (3) Combining shale gas exploration practices and impacts of lithofacies, depth, pressure coefficient and brittle‐ductile transition on the reservoir properties, it is concluded that the favorable depth interval of the Wufeng‐Longmaxi shale gas is 2200∼4000 m under current technical conditions. (4) Aiming at the differential reservoir properties of the Wufeng‐Longmaxi shale in the Sichuan Basin and its periphery, several suggestions for future research directions and EE&D of shale gas are formulated.

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A systematic comparative analysis of shale reservoir characteristics of the Wufeng-Longmaxi (O3 w-S1 l) and Niutitang (Є1 n) Formations in southeastern Sichuan Basin and its neighboring areas was conducted with respect to mineralogy, organic geochemistry, pore structure, methane sorption, brittleness, and fractures. Results indicate that (1) organic matter (OM)-hosted pores that are hundreds of nanometers to micrometers in size in the Longmaxi shale are well-developed in migrated OM rather than in the in situ OM, and they are the dominant reservoir spaces. Furthermore, the total organic carbon (TOC), brittleness, organic pores, and bedding fractures have good synergistic development relationships. However, there are fewer OM-hosted pores in the Niutitang shale; they are smaller in size, usually less than 30 nm, and have a more complicated pore structure. The intergranular pores in cataclastic organic-inorganic mineral fragments are the dominant reservoir spaces in the Niutitang shale and are coupled with stronger methane sorption and desorption capacities. (2) The piecewise correlation between TOC and brittleness indicates the significant differences in pore and fracture characteristics. When the TOC [Formula: see text], the TOC, brittleness, organic/inorganic pores, and fractures synergistically develop; when the TOC [Formula: see text], even though the increase in ductility reduces the number of fractures, the lower cohesive strength, internal friction angle, and weaker surfaces of interlayer fractures and cataclastic minerals promote the development of slip fractures, which significantly improves the fracture effectiveness and reservoir spaces for free and absorbed shale gas. (3) The Longmaxi, Wufeng, and Niutitang shales formed and evolved in different evolutionary stages. With the evolution of hydrocarbon generation, diagenesis, tectonic deformation, and pressure, the size and proportion of OM-hosted pores gradually decrease. At the same time, the complexity of the pore-fracture structure, the methane adsorption/desorption capacity, and the proportion of inorganic pores and fractures increase.