Abstract: The total salinity of formation water of Upper Paleozoic in Hangjinqi area in northern Ordos basin ranges from 2 333.55 mg/L to 89 445 mg/L, averaging 47 565.81 mg/L. It belongs to the category of brine water and bittern, with pH value of 4.37~7.3 and mild acid property. The type of formation water is of CaCl2 one. The geothermal gradient in Boerjianghaizi fault belt is the highest, in the south part of it is the next place, and in the north part is the lowest. The formation water salinity, pH value and geothermal gradient near the belt are abnormal due to mixing process according to this study. The interaction between the fluids and the rocks assumes the process of albitization in plagioclase.

Key words: Ordos basin, Upper Paleozoic, formation water, origin, process