Upper Trassic in center Shichuan area is a reservoir forming system with self-g eneration and self-storage.Natural gas is mainly composed of hydrocarb on gas with methane between 88% to 96%.the heavy hydrocarbon has content betwee n 6% to 9%.The non-hydrocarbon gas is mainly composed of  H₂S and CO₂ that  co ntent are lower.The drying coefficient (C₁/C_1-5) is between 5 to 15 an d increased with depth.Methylcyclohexane，heptane and isoheptane of Light hydro carbon in nature gas indicated that nature gas are characterized by their humic-type organic matters.The carbon isotope value of methane is between -39.00‰～-41.00‰ and ethane between-25.00‰～-27.00‰（PDB）, which indicated that nat ural gas is typical  sapropelic gas.The auther consider that during the upper T rassic continusly subsidence and buried in Yanshan movement,the source rock has  entered in maturity and early stage of high maturity.Eroded Strata Thickness w as about 2500m,so the stratum temperature and pressure were dramatically red uced .Exsolution of waterUpper Trassic in center Shichuan area is a reservoir forming system with self-g eneration and self-storage.Natural gas is mainly composed of hydrocarb on gas with methane between 88% to 96%.the heavy hydrocarbon has content betwee n 6% to 9%.The non-hydrocarbon gas is mainly composed of  H₂S and CO₂ that  co ntent are lower.The drying coefficient (C₁/C_1-5 ) is between 5 to 15 an d increased with depth.Methylcyclohexane，heptane and isoheptane of Light hydro carbon in nature gas indicated that nature gas are characterized by their humic-type organic matters.The carbon isotope value of methane is between -39.00‰～-41.00‰ and ethane between -25.00‰～-27.00‰（PDB）, which indicated that nat ural gas is typical  sapropelic gas.The auther consider that during the upper T rassic continusly subsidence and buried in Yanshan movement,the source rock has  entered in maturity and early stage of high maturity.Eroded Strata Thickness w as about 2500m,so the stratum temperature and pressure were dramatically red uced .Exsolution of water-soluble gas and pressure drop and expansion of residual g as in reservoir that was low matured,provides important gas resource for gas ac cumulated in Late Stage and lightens the methane carbon isotope of gas reservoir .