Abstract: This paper aims at ascertaining the geochemical environment of reservoirs, calculating the oil/gas prospects of reservoirs and establishing the identification indexes of hydrocarbon by the research of modern hydrogeologic characteristics and trackways. The Ordovician Majiagou formation water in the middle part gasfield of the Ordos Basin has high concentration and deep orthometamorphic degree, and is all CaCl₂-typed. This indicates that it enters into the complete hydrologic exchanging-stagnant zone and has good sealing conditions. It experienced the evolutionary process of the hydrochemical field, from carbonate rock synsedimentary sea-water to the leached water in the weathering and denuding period, and from the condensed metamorphic water with high mineralization degree in the burial period to the gasfield associated water in the pool-forming period. Compared with sea-water in-house evaporating and condensing curves, M5₁ gasfield water enriches Cl^- and Ca²⁺ and expends Na⁺ and Mg²⁺, showing that the completely isolated setting of reduction is favorable to the accumulation and preservation of natural gas. In this paper, the relationship between hydrochemical characteristic parameters and natural gas accumulation is discussed, and the index system to identify gas prospects by modern hydrochemical characteristics in M5₁ reservoir is set up.