This paper describes The experimental methods of gas desorption of igneous rocks in vacuum and gas-genetic simulation of rocks at high temperatures. The quantitative assay of gas from thermal simulation is made on chromatograph and the C-isotope compositions of CO2 is analyzed as well. The isotope fractionation rules of CO2 from thermal simulation and nature gas, and their relations of CO2 abundance and 5 13C values are studied in Sanshui basin, Guangdong. It can be concluded that there are two genetic sources of CO2, in nature gas of Sanshui basin. One is the inorganic genesis of the mixture from the crust and The maple and the other is the cracking of organic matters.
