According to the analysis of water quality of more than 100 wells

the formation water of the Carboniferous in east Sichuan possesses high density and high direct metamorphic degree.The formation water in most parts of the study region is CaCl

2

type

and the water in local places is NaHCO

3

and Na

2

SO

4

-types.The formation water underwent the process of contemporary sea water→erosion leaching water→condensed metamorphic water with high mineralization in buried stage→associated water in gasfield.The places containing high mineralized formation water are favourable for gas accumulation;the places containing CaCl

2

-type water are favourable for gas accumulation and preservation.When Nal/Cl ratio of formation water is ≤0.65

natural gas could be found;when desulphuration index is less(except gypsum and rock salt distributed area)than one

gas preservation condition is favourable.