Abstract: Based on 319 measured analysis data of formation water in Chang 6 pay zone of Yanchang Formation in the Jiyuan area of the Ordos Basin, the features (e.g. type, chemical composition, ion ratio, and salinity) and planar distribution rule of formation water were investigated, and the implications of such features for hydrocarbon accumulation were revealed. Salinity of formation water of Chang 6 pay zone is overall higher in Mesozoic strata, where the formation water shows higher salinity, recording as CaCl₂-V and IV type with major ions of Cl^- and K⁺+Na⁺, and the average values of Na⁺/Cl^- coefficient, desulphidation coefficient and metamorphism coefficient range in 0.49-0.53, 5.8-7.2, and 40-51, respectively. The overall stratigraphic enclosed environment is fine. The current formation water in Chang 6 pay zone is in a closed fluid dynamic environment, with the features varying vertically and horizontally. In Chang 6₁ at the top of Chang 6 pay zone, the salinity and metamorphism coefficient of formation water in the western and eastern parts are generally higher than that in the central part, and the Na⁺/Cl^- coefficient and desulphidation coefficient of formation water in the western and eastern parts are generally lower than that in the central part. This indicates that the sealing capacity of the fluid dynamic environment in the western and eastern parts are better than that in the central part. It coincides with the fact that there are more oil reservoirs discovered in the western and eastern parts than that in the central part. The present fluid dynamic environment reflected by the formation water can effectively represent the accumulation and preservation degree of oil reservoirs, and the regions with weaker present fluid dynamic environment are more favorable for oil reservoir preservation.

Key words: salinity, fluid dynamic environment, chemical feature of formation water, Chang 6 pay zone, Jiyuan area