Application of C5–C13 light hydrocarbons in depositional environment diagnosis

Based on the geological background and Gammacerane/C31H(S + R) ratios, source rock depositional environments of the studied oil samples (194) from 13 oilfields were classified into three groups according to salinity:saline-lacustrine facies, fresh to brackish lacustrine facies (including limnetic facies), and semi-saline to saline facies (including marine facies of the Tarim Basin).C5-C13 compound groups in the crude oils were separated by GC, and about 286 compounds were qualitatively analyzed.The geochemical application of the C6-C13 compound groups and their ratios between each group with individual carbon number was investigated.Our studies show that (1) C6-C13 compound groups and their ratios in the crude oils could serve as new reliable parameters for oil-oil correlation because the C6-C13 light fractions are the major oil component, especially in light oil or condensate, in which they account for almost 90% of the whole oil;(2) the compound groups of C7 light hydrocarbons in oils derived from difierent depositional environments with difierent salinity have difierent characteristics;(3) C6-C13 compound groups and ratios may be afiected by other factors such as maturity, but they are mainly controlled by salinity of depositional waters.