Abstract: Chemical analysis of formation water in the 6^th member of Yanchang Formation(Chang 6 member)in the sou-thern Ordos Basin reveals that the formation water is high in total salinity;Cl^-,(Na⁺+K⁺)and Ca²⁺ are dominant,and thus the water is of CaCl₂ type.Vertically,the total salinity,mainly controlled by ions of Cl^- and(Na⁺+K⁺),tends to increase with increasing depth,demonstrating the ever-condensing feature of formation water with depth and the ever-growing sealing conditions subsurface.Laterally,the distribution of the total salinity is relatively consistent with the sand body thickness,and is controlled by sedimentary facies.Given that the formation water in Chang 6 member is characte- rized by low sodium-chloride ratio,magnesium-calcium ratio,coefficient of desulfurization and exchange coefficient of cations,and high metamorphism coefficient in chemistry,we are sure that the formation water therein is located in alternating stagnant zones,a closed anoxic and reducing environment,which is conducive for hydrocarbon accumulation and preservation.Besides,the formation water therein belongs to the enclosed deep water isolated from the surface water,and both its total salinity and Ca²⁺ concentration have a certain positive correlation with porosity of Chang 6 member.Accor-dingly we may conclude that the origin of formation water in the area is closely related with the diagenesis and thermal evolution of organic matters,based on the regional thermal evolution history.What’s more,it is original sedimentary water,having undergone strong condensation and metamorphism and been affected by organic fluids such as oil and gas.All in all,the formation water associated with oil and gas in the 6^th member of Yanchang Formation in the southern Ordos Basin manifests that the hydrocarbon reservoirs in the southern part of the basin is in a stable and closed environment,and the prospects for its oil-gas exploration are brighter.

Key words: geochemistry, formation water, Yanchang Formation, Upper Triassic, Ordos Basin