The Lower Jurassic Daanzhai Member in Sichuan Basin can be divided into Dayi, Dayisan and Dasan Submembers from top to bottom, which is constituted by dark clay shale, crystalline shell limestone and muddy shell limestone. The dark clay shale that formed in deep to semideep lacustrine environment is the main source rock. The crystalline shell limestones (CSL) that formed in high-energy beach to shallow lacustrine environment has been deemed to be the major hydrocarbon reservoir for fractures are dominant in it. Through the geological survey of outcrops, the observation of cores and thin sections, the analysis of physical parameters and formation tests, it is indicated that the shallow to semideep lacustrine organic muddy shell limestones(OMSL) that formed in lower-energy beach are the main reservoirs or payzones. They are of thin single layers but large accumulative thickness with a wide range of distribution. These limestones are usually intercalated within the dark clay shales which comprise the integrative source/reservoir rocks to be beneficial to hydrocarbon accumulation. In this type of muddy shell limestone, intershell pores make up the main reservoir space and the intershell pores are commonly filled with dark organic matter. It is suggested that the exploration target may be extended from the CSL to the OMSL in Daanzhai Member. If the OMSL formed in lower-energy beach zone becomes a new prospecting domain, a preliminary prediction shows that the favorable exploration area will increase 6100km2 and 6600 km2 respectively in Dayi and Daysan Submembers.