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Abstract: The Lower Cambrian shales in the Sichuan Basin are considered one of the most promising shale gas resources in China. However, large-scale commercial development has not been achieved due to the relatively low and significantly variable gas contents of the drilled shales. Excitingly, the first major breakthrough in deep and ultra-deep Lower Cambrian shale gas was made recently in the well Z201 in the southern Sichuan Basin, with a gas yield exceeding 73 × 104 m3/d. The success of well Z201 provides a favorable geological case to reveal the distinct enrichment mechanism of deep and ultra-deep Lower Cambrian shale gas. In this study, at drilling site of well Z201, fresh shale core samples with different gas-in-place contents were collected, and their geochemical, pore development and water-bearing characteristics were analyzed systematically. The results showed that the Z201 organic-rich shales reached an overmature stage, with an average Raman maturity of 3.70%. The Z201 shales with high gas-in-place contents are mainly located in the Qiongzhusi 12 section and the upper Qiongzhusi 11 section, with an average gas-in-place content of 10.08 cm3/g. Compared to the shales with low gas-in-place contents, the shales with high gas-in-place contents exhibit higher total organic carbon contents, greater porosities, and lower water saturations, providing more effective pore spaces for shale gas enrichment. The effective pore structures of the deep and ultra-deep Lower Cambrian shales are the primary factors affecting their gas-in-place contents. Similar to the shales with high gas-in-place contents of well Z201, the deep and ultra-deep Lower Cambrian shales in the Mianyang-Changning intracratonic sag, especially in the Ziyang area, generally developed in deep-water shelf facies with high total organic carbon contents and thick sedimentary thickness, providing favorable conditions for the development and preservation of effective pores. Therefore, they are the most promising targets for Lower Cambrian shale gas exploration.