Abstract: The vitrinite reflectance of 29 source rock samples selected from three strata-Carboniferous, Middle-Upper Ordocician and Middle-Lower Cambrian in 9 wild wells of the study area were measured, layering data of 40 artificial boreholes were obtained by using 3 seismic profiles, and analysis of hydrocarbon generating history in the source rock was made in detail integrated with data of stratigraphic denudation thickness and geothermal gradient available. Study shows that the source rock in north slope of southwest depression in Tarim experienced fast burial with hydrocarbon generating, quick growth of organic maturity associated with tremendous gas generation in Neogene. In contrast, the source rock in Bachu arch was subjected to and ended up with early evolution and no hydrocarbon generation took place after 250Ma. In its largest gas accumulation, Hetian river gas field, the carbon isotope of ethane (δ ¹³C₂) ranges from 37.8‰ to -34.9‰, indicating that the gas was generated from type- II kerogen, the natural gas maturity is within 1.67%~2.34% based on δ ¹³C₁-R₀ reduction formula. It is given that the source rock should be that of Middle-I ower Cambrian compared with its organic matter type and maturity (measured and simulated values). The study shows that natural gas from Hetian river gas field was mostly generated by the way that north slope of southwest depression in Tarim was fast buried by Tertiary strata hence resulted in thermal pyrolysis of kerogen in Middle-Lower Cambrian source rocks and migrated all the way into the joint area between the north slope and Bachu arch.

Key words: Tarim basin, source rock, maturity, hydrocarbon generating history