GEOCHEMICAL CHARACTERISTICS OF HYDROCARBONS IN CRUDE OILS FROM THE QAIDAM BASIN

The Qaidam Basin is an important Mesozoic-Cenozoic petroliferous basin in west China.18 oil fields have been found in the western and northern Qaidam Basin since oil was first discovered in the late 1950s,and the western Qaidam Basin contains more oil fields compared to the northern Qaidam Basin.Geneses of crude oil in these 2 areas are significantly different.Hydrocarbon biomarkers in 16 crude oil samples from 13 oil fields of the 2 areas are analyzed and their geochemical characteristics are studied.The distribution and composition of biomarkers indicate that,crude oils in the western area originated from source-rocks under weak oxidation and freshwater-marsh lacustrine environment.Crude oils in the western area derived mainly from bacteria and algae;however,those in the northern area from terrestrine high plants.According to the characteristics of biomarkers and the compositions of carbon isotope,crude oils in the Qaidam Basin can be divided into 2 genetic groups and 7 genetic subgroups.