Abstract: The natural gas of Tianchipu in the east Sichuan Basin is dry gas,with the characteristic of thermal cracking petroleum gas,meanwhile it consists of a little higher evolution kerogen cracking gas.It is a kind of higher carbon isotope,condensed oil,made up by light hydrocarbons almostly.This kind of oil hasn′t been biodegraded.By researching carbon isotope,light hydrocarbon,petroleum and source rocks biomarkers.It is pointed out that the gas and oil of carboniferous are sapropeltype with a higher maturity index,all are from lower Silurian source rocks.The study shows that natural gas is different from the coal gas from late Permian epoch.The formation of higher carbon isotope is petroleum thermal cracking and retrograde condensation.The geochemistry analysis of oil reservoir demonstrates that in the Carboniferous beds in Tianchipu,there exist the ancient reservoirs of Indo Chinese epoch.According to bury,thermal cracking history and geothermal gradient,it is known that the oil accumulated in Carboniferous changed to gasoil reservoir at late Cretaceous period.This research systematically describes that gas reservoirs of Carboniferous in the eastern Sichuan basin belong to paleoreservoir cracking origin,and provides a scientific basis for exploring oil cracked gas in the east Sichuan Basin.

Key words: hydrothermal alteration, light hydrocarbon, carbon isotope, biomarker, Carboniferous