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A set of nodular limestones was developed in the first member of the Middle Permian Maokou Formation (denoted as the Mao‐1 member) in the south‐eastern Sichuan Basin, south‐west China. Borehole coring has shown excellent signals of oil and gas from the Mao‐1 member carbonates, and many drilling wells have obtained industrial‐level gas flow. To assess the natural gas exploration potential of the Mao‐1 member in this area, sedimentary characteristics of drilling cores are described, and the formation mechanism of natural gas reservoir and its key controlling factors are also evaluated using data obtained from field geological survey, core observation, and laboratory experimental analysis. On this basis, the process of natural gas accumulation and evolution is examined. The results show that the Mao‐1 member nodular limestone comprises abundant marls and packstone to wackestone, and they were deposited in a lower part of the carbonate ramp setting. The marls exhibit rather high organic‐matter abundance and hydrocarbon generation capacity. Natural gas is mostly enriched in the marls developed talc diagenetic shrinkage pores and a certain number of organic‐matter pores, and their reservoir properties are much better than that of packstone‐wackestone. The natural gas in the Mao‐1 member is mainly self‐generated oil pyrolysis gas. The gas reservoir is characterized by the co‐existence of source rocks and reservoir, a lithology‐controlled reservoir, and a structure‐fracture‐controlled enrichment mechanism. Organic‐matter‐rich marl constitutes the material basis for natural gas accumulation. Talc diagenetic shrinkage pores provide storage space for natural gas. Preservation conditions are the key to natural gas accumulation, and natural fractures are beneficial to high gas production. The gas reservoir is characterized by early intraformational enrichment and late interformational adjustment. The finding of the nodular limestone gas reservoir of the Mao‐1 member provides new insights into unconventional gas exploration in the Sichuan Basin, and shows great potential for increasing natural gas reserves and production in China.

The oil expulsion efficiency and retention efficiency of shale affect the enrichment and preservation of shale oil. Two series of semi-closed hydrous pyrolysis experiments were performed under in situ geological conditions on a Paleogene shale sample as a comparable analog to evaluate the generation and preservation potential of shale oil in the Funing Formation shale in the Subei Basin. The results show that 1) the oil-generation capacity evolution of different lithological combination source rocks in the Funing Formation of the Subei Basin can be roughly divided into four stages: a) relatively slow oil-generating and slow gas-generating, b) relatively fast oil-generating and slow gas-generating, c) oil cracking into gas, and d) kerogen cracking into gas; 2) different lithological combinations have different hydrocarbon generation, expulsion, and retention efficiencies. The total oil generation rate and gas generation rate of pure shale are higher than those of shale with a silty interlayer, and the exchange point between the oil expulsion rate and retention rate of pure shale is earlier than that of shale with the silty interlayer, which indicates that the pure shale experienced the expulsion and retention process earlier. Oil retention mainly occurs at an EqVRo of 0.84%–1.12%, while oil is mainly discharged to the adjacent siltstone at an EqVRo of 1.12%–1.28%. Based on the simulation under geological conditions, it is recognized that for shale oil exploration in the Subei Basin, the favorable thermal maturity is at an EqVRo of 0.84%–1.12%, and the favorable lithology is the shale with the silty interlayer. On one hand, the siltstone interlayer can provide pore space for the early generated oil, and the concentration difference of hydrocarbons between the shale and the interlayer can be formed so that the generated shale can continuously enter the interlayer. On the other hand, the shale above the interlayer can be used as a cap rock to preserve shale oil. The favorable area for shale oil exploration in the Subei Basin is the area with relatively high maturity (at a VR value of about 1.1%)