Genesis and maturity identification of oil and gas in the Xihu Sag, East China Sea Basin

Based on the composition and stable carbon isotope ratios of condensate and natural gas in the Xihu Sag, East China Sea Basin, origin and maturity of the hydrocarbon gas, CO2, N2 and condensate are determined using multiple methods. The CO2, low content of N2, condensate and hydrocarbon gas are mainly derived from thermal pyrolysis of coal-bearing measures. The relatively high content of N2 is collectively controlled by the thermal degradation of kerogen, vertical migration of gas and microbiological deterioration. Considering “evaporation fractionation” effect, a modified relationship between heptane and isoheptane has been used to calculate maturity of the condensate, which is in consistence with the results of the Mango method, i.e., the condensate is from mature source rock. The “two-segments of methane stable carbon isotopic fractionation model” has been used to calculate the hydrocarbon gas maturities. There are two mature stages of gas: one is mature gas which is equivalent to the condensate in maturity; the other is high maturity gas.