A suite of 16 crude oil samples from 13 oilfields in the Qaidam Basin were analyzed using techniques including gas chromatography and gas chromatography - mass spectrometry. Biomarker compositions and parameters were used to investigate the polaeoenvironmental and depositional conditions and to correlate the oils with eachother. Oils from the western Qaidam Basin have pristane/phytane (Pr/Ph) ratios of less than 0.7, and contain abundant gammacerane, C-27 steranes, 4-methyl steranes and long-chain tricyclic terpanes. C-29 sterane 20S/(20S+20R) and beta beta/(beta beta+alpha alpha) ratios show that the western Qaidam oils have variable maturities ranging from immature to mature. Oils from the northern Qaidam Basin, by contrast; have Pr/Ph ratios greater than 3, low gammacerane contents, and relatively abundant C-29 steranes, bicyclic terpanes and alkylcyclohexanes. C-29 sterane 20S/(20S+20R) and beta beta/(beta beta+alpha alpha) ratios indicate that the northern Qaidam oils are mature. delta C-13 values, which range from -25.4 parts per thousand to -28.3 parts per thousand. with the exception of one oil from the north (-31.6 parts per thousand), are similar for oils from both the northern and western parts of the Qaidam Basin. The oils' carbon isotope compositions are similar to those of the organic matter in potential source rocks. The western Qaidam oils are inferred to have originated from Tertiary source rocks deposited under anoxic and saline-hypersaline lacustrine conditions with dominant algal organic matter. The northern Qaidam oils are interpreted to be derived from Jurassic source rocks which were deposited in a freshwater lacustrine environment and which are dominated by terrigenous organic matter.