Keywords: 
• Paleogene /
• Es₃-Es₄ stage /
• Bohaiwan Basin /
• ancient salinity /
• imbalance property

Abstract: From the drilling core data in different sags within Bohaiwan Basin, the rhythmic layered mudstone, limestone, dolomite, gypsum which had deposited at the Paleogene Es₃-Es₄ stage, was inter-layer distribution for each other. The rock thickness, whether or not the missing one, or the prosodic integrity, is inconsistent in the different sags or dips. Through the analysis of pollen data, the weak degree change between drought and wetting can't result in the huge difference of the deposition profile under a condition of the stable ancient climate. According to the deposition record and ancient salinity period of Xialiaohe, Dongying, Zhanhua and Dongpu sags, the result was founded that the different lithological combination is due to the different ancient salinity from each basin. Comprehensive analysis and inference, during the deposition of the Es₃-Es₄ stage, dynamic fresh water is injected into the basin, from Northeast Xialiaohe sag which is upstream region clamped by uplift and rift valley. Finally, water is collected in the southwest of the low-lying areas, the downstream region, Dongpu sag. The salty environment in the basin is not balanced. In the upstream region, salty environment is diluted by fresh water, single rock type and low thickness. In the downstream region, the high salinity environment is stable, and salt rock and gypsum rock strata are well developed when it comes to a dry climate. In the middle reaches of Dongying and Zhanhua sags, water is sometimes too fresh, sometimes too salty, and sometimes in transient state. Salty environment of high salinity is favorable for the preservation of organic matter, and it is capable of contributing to the early discharge of hydrocarbons. The salty environment is favorable for the formation of the oil and gas resources.