Organic matter rich gas- bearing shale in marine- continent transitional facies is an important field of shale gas exploration. Based on the data of seismic, drilling, well logging, analysis and testing, the Nanchuan area of Chongqing is selected as the research object. This paper systematically evaluates the shale gas exploration potential of the Longtan Formation in the southeastern margins of the Sichuan basin. The main controlling factors of shale gas enrichment are discussed from the aspects of sedimentary environment, lithological assemblage, organic geochemistry, reservoir physical properties, gas- bearing property, mineral composition, the desert area and desert section were selected. The results show that ① the Longtan Formation in the Nanchuan area is located in the coastal marsh facies where the black organic- rich mud shale with a thickness of 50~65 m is developed. The single layer thickness is 26~32 m and the burial depth is between 1000~4000 m. ② Geochemical indexes are moderate, TOC is 2. 0%~3. 0%, Ro is 2. 0%~2. 2%, the type of organic matter is type III~II2. ③ Gas- bearing property is good, gas logging total hydrocarbon content is 3%~25.5%, and the gas contentis 1. 5~3. 5 m3/t. ④ The content of brittle minerals such as siliceous and carbonate minerals is high indicating a great potential for shale gas exploration. ⑤ The enrichment and high yield of shale gas are mainly controlled by the “three conditions” of gas production condition i. e. , lagoon controlling hydrocarbon, preservation controlling enrichment, and pressure controlling production. The lagoon facies affect the thickness and geochemical index of shale and control the hydrocarbon accumulation base of shale gas. Preservation conditions affect formation energy and shale gas content, and control the degree of shale gas enrichment. The fracturing property affects the reconstruction volume and the complexity of fracture network, and controls the production of shale gas wells. ⑥ The target evaluation method and standard of shale gas in the Longtan Formation of the Nanchuan area were preliminarily established. The Yangchungou anticline and the Dongsheng anticline are selected as class- I area. ⑦ The Reservoir Comprehensive Evaluation Index (RCEI), which includes TOC, porosity, gas content and brittle mineral content as four main parameters is established, and the third Member of the Longtan Formationis selected as the sweet spot for shale gas exploration, and the upper part of the third Member in the Longtan Formation is the optimal target window of horizontal well.