Wufeng Formation-Longmaxi Formation is an important sequence in Sichuan and Chongqing area for shale gas exploration. Based on the analysis of chemical component and stable carbon isotope composition of the shale gas acquired by desoption experiments on Wufeng-Longmaxi Formation shale cores from the Souteastern Chongqing, it is discussed for the genetic type of shale gas and the reason for alkane carbon isotopic reversal. The results showed that the methane content is high with a value from 94.33, to 98.96, in Longmaxi Formation shale gas, and the non-hydrocarbon content is low, mainly including N2 and CO2. The drying coefficient is more than 0.98, indicating typical dry gas. The values of carbon isotopes of methane and ethane are -49℅~-24.47℅ and -39.4℅~-29℅ respectively, and the carbon isotopes of C1 to C3 alkane in Well Y1 with good gas-bearing presents complete reversal, but Well Y2 presents normal. According to the statistical criteria,natural gas in the Southeastern Chongqing area is derived from the sapropelic source rocks, which is a result of high-temperature pyrolysis. It is a mixing product of pyrolysis of kerogens and secondary pyrolysis of crude oil,with obvious features of secondary pyrolysis of oil. The reason for complete reversal of carbon isotopic in Well Y1 is the mixing of two gases. Perhaps, it has some relationships with high production of shale gas.