Abstract: Analysis on the testing results of conventional components,trace elements and δD、δ¹⁸O and δ³⁴S isotopes in the formation water of Jiannan gasfield showed that the longitudinal changes of hydrochemical components in this area were different from those in other oilfields.Several sets of independent and enclosed hydrochemical environments existed instead of positive or reverse-typed hydrochemical sections.They were not congenetic.T₃x formation water has good enclosed hydrochemical environments,and the alternation degree of surface water downward is not great.Below T₁j²,there are several independent and enclosed hydrochemical environments.Among them,hydrochemical alternation of T₁f² formation water with C₂h formation water happened in the Late Yanshanina Himalayan period by fault systems.