By measuring chemical and carbon isotopic compositions of 74 gas samples from the Northeast Sichuan basin, all of the natural gases are dominated by hydrocarbon gases, with absolutely high content of methane and minor heavy hydrocarbons (i.e C2H6, C3H8) . All gases are very dry, the gas dryness of C1/C1+ ranging from 0.989 to 1.0. The vary concentration of H2S and CO2 occurs, with coarsely positive correlation while the concentration of them is more than 5.0%. CO2 in the Northeast Sichuan basin is derived from thermal decompostion of carboante and thermochemical sulfate reduction to hydrocarbons, where the CO2 deocmposed from carboante is characterized as low content (CO2 generally less than 5.0%), 13CCO2 < -2‰ and existence of positive correlation between CO2content and 13CCO2 value, in contrast the CO2 generated from TSR has high content (CO2 generally more than 5.0%), more enrichment in 13C carbon isotope (13CCO2 > -2‰) and weakly negative correlation between CO2 and 13CCO2. The TSR level can be well described by the ratios of CH4/CO2 and (H2S+CO2）/（H2S+CO2+∑C1-3）. After the rato of CH4/CO2 and (H2S+CO2）/（H2S+CO2+∑C1-3）is less and more than 10 and 0.1, respectively, the decrease of CH4/CO2 ratios and exponential increase of (H2S+CO2）/（H2S+CO2+∑C1-3）is observed with the TSR intensity. The TSR resulted from the high content of CO2 and much less negative of 13CCO2 values. The discrepancy of 13CCO2 values between natural and pyrolysis gases would be related to the CO2 with much less negative of 13CCO2 values, which are composed of the remnant CO2 after partially precipitated as carbonate with metal ions (i.e. Mg2+、Fe2+ and Ca2+) and resolved CO2 eroded by sour gas to carbonate reservoir.