The methane content accounts for over 97% of natural gas compositions in the 4 pay zones in Xushen 1 well.The degree of hydrocarbon evolution increases with depth

which is accompanied by demethylation of alkanes

ring opening of cycloalkanes

aromatization and heightening alkylating degree.For the gas in Yingcheng Fm

the carbon isotope values of methane are heavier than that of ethane.For the gas in Huoshiling Fm

the carbon isotope value of methane is-29.20‰

but the carbon isotope value of ethane is not detectable.The source rocks in Yingcheng Fm in Xushen 1 well are characterized by their humic-type organic matters

and is in high-to over-mature evolution stage;While the source rocks in Huoshiling Fm are characterized by their sapropelic-type organic matters

and generate high-to over-mature natural gas.A quantitative model for natural gas generation is developed by using the hydrocarbon fingerprint chromatograms of source rocks and natural gas and mathematic simulation software.Calculation results show that the source rocks in Shahezi Fm would have contributed over 54% of gas to the reservoir in Yingcheng Fm in Xushen 1 well

while the source rocks in Huoshiling and Shahezi Fms would have contributed 50% and 43.08% of gas to the reservoir in Huoshiling Fm

respectively.Only very little gas in the Yingcheng and Huoshiling Fms have been derived from the Yingcheng Fm and Permo-Carboniferous source rocks.