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The formation water characteristics are closely related to hydrocarbon migration and accumulation. The main factors affecting differences of the formation water from the Yan'an and Fuxian Formations are analyzed, based on the vertical and plane distribution characteristics of water type and hydrochemical composition of the formation water. The relationship between formation water characteristics and reservoir distribution and formation is discussed, and the factors that are important to in the process of finding Jurassic reservoirs are clarified in this paper. Study of formation water of the lower Jurassic Yan'an and Fuxian Formations in the Ordos basin reveal the complexity and regularity of the hydrochemical characteristics. The complexity lies in the diversity of formation water and the hydrochemical parameters, such as water salinity, sodium chloride coefficient, metamorphism coefficient, etc., which vary significantly vertically or laterally. The regularity is reflected in that the varying formation hydrochemistry forms zonation, vertically, east-westward, or geomorphologically. It is pointed out that: 1) The mixing and alternation of formation water in different hydrological cycles lead to the vertical differentiation of formation water chemical characteristics. From Fuxian to Y10 to Y9 strata, the salinity of the formation water gradually decreases, the proportion of CaCl2 type water gradually decreases, and the proportion of NaHCO3 type and Na2SO4 type formation water gradually increases. The coefficient of sodium chloride and desulfurization gradually increase, while the coefficient of metamorphism decreases. 2) Structural evolution determines the east-west zonation of the hydrochemical characteristics. The Jurassic strata at the east or west margin of the basin are buried shallowly or exposed where infiltration water supply alternates easily with a relative lower salinity, and the strata at or close to the axis of the Tianhuan depression are buried deeply far away from the supply area, imprisoned in a relatively stagnant environment, making the salinity relatively high. 3) Regionalization of the formation hydrochemical feature changes with geomorphology of the Fuxian Formation-Yan10 member and is strengthened by the water-rock reaction during the diagenesis process. In the river developed area, formation water salinity is low. In the direction of the slope, terrace, and highland, it increases gradually, while the coefficient of the sodium chloride and desulfurization decreases, and the coefficient of metamorphism increases by degrees. Formation water type changes from Na2SO4 or NaHCO3 to CaCl2. 4) The terraces, slopes, and channel bars have the advantage of reservoir formation in terms of the infiltration water alternating extent, hydrodynamics, lithologic association, and low-amplitude structural trap development conditions, and they are the favorable targets prediction method of Jurassic Palaeogeomorphic reservoir is summarized. [ABSTRACT FROM AUTHOR]