Abstract: The relationship between elements in oilfield water and hydrocarbon generation,migration,accumulation and preservation is discussed based on the basic features of oilfield water and the vertical and lateral changing pat-tern in Yanqi basin.According to the inorganic compo-nents,chemical factors and gross salinity of oilfield water,Yanqi basin is divided into four hydrogeological zones.The detention zone is located in the Sangonghe Formation in Baozhong and Baobei blocks where sealing potential is high and preservation conditions are good while the slow interchanging zone is located in Benbutu and Zhong-machang structural zones and Baonan block where oil and gas reservoirs are damaged to different degree.