The distribution of two formation pathways of biogenic methane, acetate fermentation and reduction of CO2, has been extensively studied. In general, CO2 reduction is the dominate pathway in marine envi- ronment where acetate is relatively depleted because of SRB consuming. While in terrestrial freshwater or brackish environment, acetate fermentation is initially significant, but decreases with increasing buried depth. In this paper, character of biogenic gases is profiled in the XS3-4 well of the Sebei 1 gas field in the Sanhu depression, Qaidam Basin. It indicates that those two pathways do not change strictly with increasing buried depth. CO2 reduction is important near the surface (between 50 m and 160 m), and at the mesozone (between 400 and 1650 m). While acetate fermentation is the primary pathway at two zones, from 160 to 400 m and from 1650 to 1700 m. δ 13C of methane generated in those two acetate fermentation zones varies greatly, owing to different sediment circumstances. At the sec- ond zone (160-400 m), δ 13C1 ranges from ?65‰ to ?30‰ (PDB), because the main deposit is mudstone and makes the circumstance confined. At the fourth zone of the well bottom (1650-1700 m), δ 13C1 is lighter than ?65‰ (PDB). Because the deposit is mainly composed of siltstone, it well connects with outer fertile groundwater and abundant nutrition has supplied into this open system. The high con- centration of acetate is a forceful proof. δ 13C of methane would not turn heavier during fermentation, owing to enough nutrition supply. In spite of multi-occurrence of acetate fermentation, the commercial gas accumulation is dominated by methane of CO2-reduction pathway. A certain content of alkene gases in the biogenic gases suggests that methanogensis is still active at present. The distribution of two formation pathways of biogenic methane, acetate fermentation and reduction of CO2, has been extensively studied. In general, CO2 reduction is the dominate pathway in marine envi- ronment where acetate is relatively depleted because of SRB consuming. While in terrestrial freshwater In this paper, character of biogenic gases is profiled in the XS3-4 well of the Sebei 1 gas field in the Sanhu depression, Qaidam Basin. It indicates that those two pathways do not change strictly with increasing buried depth. While acetate fermentation is the primary pathway at both zones (between 50 m and 160 m), and at the mesozone (between 400 and 1650 m) , from 160 to 400 m and from 1650 to 1700 m. δ 13C of methane generated in those two acetate fermentation zones vary greatly, owing to different sediment circumstances. A Since the main deposit is mudstone and makes the circumstance confined. At the fourth zone of the well bottom (1650), the area of ​​the 13C1 ranges from 65 ‰ to 30 ‰ (PDB) -1700 m), δ 13C1 is lighter than ~ 65 ‰ (PDB). Because the deposit is mainly composed of siltstone, it well connects with outer fertile groundwater and abundant nutrition has been supplied into this open system. The high con- centration of acetate δ 13C of methane would not turn heavier during fermentation, due to enough nutrition supply. In some cases, the commercial gas accumulation is dominated by methane of CO2-reduction pathway. A certain content of alkene gases in the biogenic stocks suggests that methanogensis is still active at present.