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 different depositional environments with different salinity have different characteristics; (3) C6–C13 compound groups and ratios may be affected by other factors such as maturity, but they are mainly controlled by salinity of depositional waters.