Abstract: Based on the extraction and the GC?MS analysis of saturated hydrocarbons from oil sand samples of the Palaeogene Lower Youshashan formation and Lower Ganchaigou formation in western Qaidam basin, the geochemical characteristic of biomarkers in these hydrocarbons are studied. The results show that the hydrocarbons of oil sands and crude oils formed in a strong reducing saline lacustrine depositional environment. The type of organic matter is mainlyⅡkerogen, in which contains abundant C27 regular sterane, hopance and long?chain tricyclene, reflecting the contribution of the algae of lower aquatic plants and bacteria to the hydrocarbons from organic matter. The ratios of m(ααα C29 sterane 20S)/m[αααC29 sterane (20S+20R)] and m(ααα C29 steraneββ)/m[ααα C29 sterane (αα+ββ) ] reveal that the oil sands of Lower Youshashan and Lower Ganchaigou formations are generated after low mature? mature crude oil migrating into the reservoir. The loss of relative low n?alkane and Pr and Ph suggests that hydrocarbons in the oil sands once endured heavy biodegradation and some other secondary alterations. It is related to the geological background that the oil sands were uplifted and exposed to the surface due to tectonic movement