Abstract: In order to break through the constraint of the earlier understandings of sedimentary facies on oil and gas exploration in the Neogene Guantao Formation in Liaodong Bay Depression,this paper studied the geological background and the evidences of the occurrence of lake and shallow-water delta through integration of sedimentary geology,geophysics and paleontology.The sedimentary patterns of the Guantao Formation and the Lower Minghua Formation in the Liaohe Depression,as well as the modern sedimentary patterns of the Zhuonai-Kusai Lake area were taken as analogies for comparison,and a semi-quantitative analysis method was used to determine paleohumidity and paleotemperature.It is believed that the Guantao Formation was deposited under subtropical climate conditions with increasing humidity,dominated by alluvial fan and braided river facies,and characterized by sedimentary pattern of shallow-water delta with a large plain and a small front.The delta plain consists of the upper plain dominated by alluvial fan and the lower plain dominated by braided river.The delta front features in small scale,instability,as well as underdeveloped mouth bar and distal bar.The area of closed lake was small during deposition of the lower Guantao Formation and delta plain deposits predominated.In contrast,the area of the open lake was enlarged during deposition of the upper Guantao Formation,delta front was well developed,and the subsags were connected by the braided river system.According to the new understandings on the shallow-water delta sedimentation system of the Neogene Guantao Formation in Liaodong Bay Depression,the front delta mudstone may act as a regional cap rock,providing an important geological guidance for future oil and gas reservoir exploration.

Key words: shallow-water delta, Guantao Formation, Neogene, Liaodong Bay Depression