A systematic analysis is conducted on compositions and geochem ical characteristics of carbon isotopes of natural gas in the Tazhong area，Tarim Basin.It is found that the gaseous hydrocarbon compositions of the naˉtural gas from the Tazhong area basically show a pattern ofδ 13 C 1 <δ 13 C 2 <δ 13 C 3 <δ 13 C 4 with their values ofδ 13 C 1 all below-30‰，indicating their organic origin.According to the relationship between ln（C 2 /C 3 ）and ln（C 1 /C 2 ）and that between C 2 /iC 4 and C 2 /C 3 ，natural gas in the Ordovician is believed to originate from pyrolyˉsis of crude oil.While natural gas in the Carboniferous is m ixed gas originating from pyrolysis of both kerogen and crude oil.In the eastern and western parts of the Tazhong area，the source rocks are different in age and thermal history，leading to the significant differences of hydrocarbon compositions and carbon isotopes of natural gas in these areas.In the western part，natural gas mainly originates from the MiddleˉUpper Ordovician source rocks.While in the eastern part，natural gas probably originates from the highˉto overˉmature Cambrian source rocks.Thermochem ical sulfate reduction in the eastern wellblock is stronger than that in the western wellblock.