Abstract: The exploration potential of clay-rich limestone is the frontier in oil and gas exploration. Recently, the discovery of marine unconventional oil and gas in the Lei 3-2 member, the Well CT-1, has indicated a new exploration field and potential for the Leikoupo Formation. However, the organic matter enrichment mechanism and exploration potential of the lacustrine marlstone in the sub-salt lagoon are still unclear, which has restricted further oil and gas exploration. This paper takes the CT-1 well in the central Sichuan area as the key research member. Through the study of rock and mineral characteristics and palea-ocean geochemical environment proxies, the organic matter enrichment mechanism of the marlstone is revealed. Based on reservoir characterization and sedimentary facies analysis, the performance and distribution of the unconventional reservoir are clarified to reveal its exploration potential. The results show that the sedimentary background of the Leikoupo Formation's marlstone is anoxic and reducing conditions in the deep lagoon of the platform, and it has high paleo-productivity conditions, which is controlled by both reducing conditions and productivity for organic matter enrichment. The CT and SEM experimental results of the marlstone reservoir reveal that the storage space is mainly composed of nano-micropores and microfractures, with a porosity of more than 3%. The development background is the deep-water lagoon with mudstone and gypsum in the carbonate platform of the epicontinental sea. The extensive distribution of deep-water lagoons within the platform during the Lei 3-2 member is related to the maximum transgression period. At the same time, the monsoon climate at that time greatly promoted enhanced weathering and nutrient element input, and the evaporite formed by the regression of the overlying sea provided a guarantee for long-term and efficient preservation. Comprehensive analysis suggests that the widespread distribution of organic-rich deep-water lagoon marlstone during the transgression period in the Lei 3-2 member within the Sichuan Basin's depression is not only an unconventional exploration target but also a source of hydrocarbons for the conventional reservoirs of the Leikoupo Formation.