Abstract: Routine chemical analysis was performed for the gasfield water of Dongfang block and Ledong block in the Yinggehai Basin. Together with other analytical data, such as the water type, Na/Cl ratio(metamorphic coefficient), desulfurization coefficient, and Cl/Mg ratio, we studied the geochemical and hydrochemical characteristics of two blocks and their relation with the closure and preservation conditions of the gas reservoir. Based on the research work mentioned above, with the consideration of the geological conditions for accumulation of natural gas in the Yinggehai Basin,we paid more attention to the distribution pattern of gasfield water and its controlling factors in the two blocks, and the genetic relationship between the gasfield water and process of gas migration and accumulation.
It is found that (1) the gasfield water of the two blocks is similar in origin and evolution. The preservation of natural gas is basically in good condition. However, the water chemical composition parameters indicate that the preservation conditions in the Ledong block are little worse than those in the Dongfang block.(2) The Dongfang gasfield is dominated by NaHCO₃ type of gasfield water. The distribution pattern of the water has close relationship with the fault system. The Dongfang gasfield is low in opening degree and sealing property and the caprock of gas reservoir is good, as the gasfield water indicates. Therefore, it is favorable for the accumulation and preservation of hydrocarbons. The Ledong gasfield water is diverse with complex origin, the preservation condition is little worse than that in the Dongfang block.(3)Both the gasfield water and natural gas reservoirs of the two blocks have strong variations in spatial distribution.(4)The mud diapir and the accompanied thermal fluid in the Yinggehai Basin play an important role in controlling and affecting the origin and distribution pattern of the gasfield water as well as the gas migration and accumulation.