Abstract: The Nanpu Sag is a hydrocarbon-rich sag in the north of Bohai Bay Basin.The Dongying Formation is an important oil and gas exploration target.Statistical analyses of chemical characteristics of the Dongying Formation water have indicated that the formation water in the sag belongs to NaHCO₃ type with medium-low mineralization degree.Preservation conditions in member Ⅰ and Ⅱ of the Dongying Formation as well as in the Beipu Oil Field are the best.Fault sealing tracing has been done with parameters of chemical characteristics of formation water.Combined with analyses of characteristics of regional cap rock,fault activity and hydrocarbon accumulation,it is found out that in the regional cap rock generation area,main faults and shallow branch faults are the important targets for shallow oil and gas exploration.The region around the main faults below member 2 of the Dongying Formation is the main target for middle-deep oil and gas exploration.