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Marine shale gas in the southern Sichuan Basin is the most successful area of shale gas exploration and development in China. In order to open up new shale gas fields and search for new shale gas reserves and production replacement blocks, it is necessary to continuously establish and complete the standards on shale gas reservoir evaluation and area selection under different structural settings. The early exploration practice shows that shale in the mountainous complex structural area along the southwestern margin of the Sichuan Basin varies greatly in the gas bearing property, so systematical analysis and study on the shale gas enrichment mechanisms in this area is of great significance to searching for new shale gas exploration areas. Based on drilling data of 5 wells in the mountainous complex structural area along the southwestern margin of the Sichuan Basin, the main factors controlling the shale gas bearing property and the shale gas enrichment model were discussed based on the systematical experiments and analysis of the outcrops, cores and cuttings of Longmaxi Formation shale from the aspects of organic geochemistry, physical property, gas content, hydrocarbon generation history and pore evolution history, so as to lay a solid geological basis for the birth of a new shale gas area. And the following research results were obtained. First, the shale of the first submember of first Member of Longmaxi Formation (S1l11) in the mountainous complex structural area along the southwestern margin of the Sichuan Basin has an average TOCof 3.02%–4.97% and Roof 2.38%–3.37%, and the average total gas content in local enrichment zones is up to 4.62 m3/t, so it is classified as quality shale. Second, the detailed studies on hydrocarbon generation history indicate that the shale has the characteristics of “low thermal evolution rate and low maturity”. It is characterized by late hydrocarbon generation, low thermal evolution rate, low current maturity and short late diffusion time, which are favorable for shale gas enrichment. Third, the pore evolution history reveals that shale pore evolution can be divided into six stages, i.e., sharp pore reduction, pore reduction, organic pore formation, pore preservation, organic pore dissipation and karstification, and organic pore and TOCare the most direct control factors of shale gas content. Fourth, favorable shelf facies belt is conducive to the formation of large-scale reservoir space and effective pores, and diversities of preservation conditions under different structural styles and at different structural positions control different pore evolution stages. The shale gas in the mountainous complex structural area is characterized by horizontal zoning and differential enrichment. “Low thermal evolution rate and low maturity” and “slow structural uplifting” are beneficial to the long-term enrichment and preservation of shale gas. In conclusion, the mountainous complex structural area along the southwestern margin of the Sichuan Basin has a shale gas enrichment model of “sedimentation controlling source rocks, diagenesis controlling reservoirs and structure controlling preservation”. This research result provides idea and reference for searching for new shale gas areas and fields.

The Neoproterozoic Kangdian Rift Basin is a continental rift basin in the western Yangtze Continent. Determining its time of opening and subsequent filling pattern is an important aspect of understanding the formation and evolution of the Yangtze Continent, South China. The Luliang Formation is the early filling in the eastern part of this rift basin, and its deposition age and filling sequence are significant for studies of the regional stratigraphic correlation, opening time, and filling pattern of this basin. Having been correlated to the upper part of the Mesoproterozoic Kunyang Group or to the Neoproterozoic Chengjiang Formation, the age and regional stratigraphic correlation of the Luliang Formation have long been uncertain. This is due to a lack of reliable absolute age constraints. To address this issue, the first zircon SHRIMP U-Pb geochronology has been established for this formation, yielding two high-precision isotopic ages of 818.6±9.2 and 805±14 Ma for the tuff layers at the bottoms of the lower and upper members of the Luliang Formation, respectively. Given the error factor, the bottom age of the lower member of the Luliang Formation can be interpreted as ca. 820 Ma, corresponding to the bottom age of the lower part of the Banxi Group, which is the early filling of the Neoproterozoic Xianggui Rift Basin, a representative basin of the Neoproterozoic rift basins in the Yangtze Continent, South China. The bottom age of the upper member of the Luliang Formation can be interpreted as ca. 800±5 Ma, corresponding to the bottom age of the Chengjiang Formation in the western part of the Neoproterozoic Kangdian Rift Basin and also corresponding to the bottom age of the upper part of the Banxi Group in the Neoproterozoic Xianggui Rift Basin. These ages indicate that the Neoproterozoic Kangdian Rift Basin shares the same opening time and filling sequences as those of the other Neoproterozoic rift basins in South China. Basin analysis shows that the Neoproterozoic Kangdian Rift Basin is a typical half-graben basin, with its main boundary fault in the west and basin center in the east. This basin consisted of mini unidirectional half-graben basins in its juvenile stage and simplified to become a large united half-graben basin in its mature stage, i.e., a supradetachment basin.

Abstract: The Chengjiang Formation is the earliest continental clastic deposit after the Jinning Orogeny in central Yunnan Province, and therefore its accurate depositional age is significant for understanding the formation and evolution of the Neoproterozoic rift basins in southern China. However, hampered by accuracy of the dating technique, the existing age data for the Chengjiang Formation are not very reliable. A large number of magmatic zircons were obtained from the tuff interbed in the lower part of the Chengjiang Formation in Jinyang area and the bottom part of the Chengjiang Formation in Dongchuan area, central Yunnan Province, and high-precision SHRIMP U-Pb dating was carried out on these zircons. The results show that the weighted mean 206Pb/238U ages are 797.8±8.2 and 803.1±8.7 Ma respectively. In combination with related geological evidence, it has been demonstrated that the bottom boundary age of the Chengjiang Formation should be 800±5 Ma, whereas the top boundary age could be ca 725 Ma. By synthetically analyzing the latest age data for the relevant strata, it has been confirmed that the Chengjiang Formation should be correlated with the Suxiong Formation and the Kaijianqiao Formation in western Sichuan Province, the Liantuo Formation in the middle and lower Yangtze, the Hongchicun Formation and the Shangshu Formation in northern Zhejiang Province, the Puling Formation in southern Anhui Province, the Luokedong Formation and the Majianqiao Formation in northwestern Jiangxi Province, the Wuqiangxi Formation in northwestern Hunan Province, the Fanzhao Formation and the Qingshuijiang Formation in southeastern Guizhou Province, and the Sanmenjie Formation and the Gongdong Formation in northern Guangxi Province. Sedimentary cycle analysis shows that the sedimentary filling sequence of the Neoproterozoic rift basins in southern China can be divided into four cycles. Among them, Cycle II began at ca. 800 Ma, accompanied by intensive tectonic-thermal events. The zircon U-Pb ages from the bottom of the Chengjiang Formation reported in this paper indicate that the Neoproterozoic Kangdian rift subbasin probably started to develop at ca. 800 Ma and therefore missed Cycle I of the Neoproterozoic sedimentary filling sequence in southern China.