Crude oils produced from the oilfields in western Qaidam Basin are sourced from saline lacustrine faˉcies，thus they share certain geochem ical characteristics such as lower Pr/Ph ratio and predom inance of even carbon in normal alkanes.However，a comparison of the samples from the south and north parts of the western basin reveals that crude samples from the south part feature in an obvious nC 37 predom inance in normal alkanes，lower Pr/Ph ratio and high gammarcerane index，while those from the north part has no nC 37 predom inance in normal alkanes，but higher Pr/Ph ratio and lower gammacerane index，indicating differences in geochem ical characteristics of the source rocks between the two parts of the basin.It is noted that a negative correlation beˉtween nC 37 predom inance and Pr/Ph ratio，and a positive correlation between nC 37 predom inance and gammaceˉrance index couldmean that the nC 37 predom inance be related to the nature ofdepositional environment and that a strongly reduced and highly saline environment is themain factor causing an obvious nC 37 predom inance in the crude samples.