Abstract: The South Yellow Sea Basin is located in the eastern part of the lower Yangtze Block. The Mesozoic and Paleozoic and the sedimentary facies of the Longtan and Dalong Formations have not been well studied so far. As the result, oil and gas exploration in this area is constrained. Combining the new research findings recently achieved in the land area of the Lower Yangtze together with the drilling and seismic data offshore, we studied the Longtan-Dalong Formations in southern depression of South Yellow Sea Basin for their sedimentary facies. Results showed that the Longtan and Dalong Formations consist mainly of marine clastic rocks and depositional systems distributed in a north-east direction in general. During the period of deposition, water became deeper from the northwest to southeast. The northeastern part was dominated by fluvial facies, which changed gradually to delta, swamp-lagoon and foreshore-shoreface facies along southeast direction. Continental shelf was developed in the southeastern part. Shale and coal were well developed in the two formations and both are good source rocks. The research has founded a good geological basis for oil and gas exploration of the Longtan-Dalong Formations.