Gas and water are simultaneously produced from the reservoirs in the 5

th

Member of the Upper Triassic Xujiahe Formation (T

3

x

5

) in the Xinchang gas field in the Western Sichuan Depression

and there is no consensus on whether the gas is free or dissolved.For this purpose

methane solubility experiments were conducted under different temperatures and pressures on the T

3

x

5

formation water

and the results were compared with the methane solubilities in 1 mol/kg NaCl solution.The fluid state in T

3

x

5

was demonstrated in combination with the gas/water ratio of the produced fluid and ideal gas state equation.The results indicate that methane solubilities in T

3

x

5

formation water under present average depth and historical largest depth are 2.260 m

3

/m

3

and 3.194 m

3

/m

3

respectively.The water-soluble gas in T

3

x

5

formation water was mainly released by reduction of pressure and temperature during uplifting since the end of Late Cretaceous.The volumetric proportions of free gas and formation water in the T

3

x

5

pores were generally lower and higher than 50%

respectively.The volumetric proportion of free gas increased slightly due to the volumetric expansion caused by reduction of pressure and temperature since the end of Late Cretaceous

whereas that of formation water decreased as the water was displaced.Due to the partial degasification of water-soluble gas by reduction of pressure and temperature

the proportions of free gas in the total natural gas increased while that of water-soluble gas decreased.The T

3

x

5

natural gas was dominated by free gas

with water-soluble gas accounting for less than 5%.Moreover

about 29.3% of the T

3

x

5

water-soluble gas was released and turned into free gas by reduction of pressure and temperature during uplifting

and the total amount of gas released from the T

3

x

5

formation water reached 1.8×10

9

m

3

.