Origins of oil seepages in the southern Guizhou Depression, SW China: Evidence from carbon isotopes, sulfur isotopes and biomarkers

An oil-source rock correlation method based on carbon isotopes, sulfur isotopes and biomarkers is described for potential source rocks and oil seepages in the southern Guizhou Depression to elucidate the origins of oil seepages. The results show that the oil seepages have bulk delta C-13 values from -33. 04%o to -31. 63%o, average -32. 26%0 (n =18), and have delta S-34 values from + 16. 06%o to + 23. 06%o, average + 18. 99%o (n = 4); the oil seepages are biodegraded with a varying degree, there are many differences in distribution of steranes and terpanes, but they are all relatively rich in tricyclic terpanes, gammacerane, triaromatic steroids, aryl isoprenoids, retene and dibenzothiophene; C-25 tricyclic terpane to C-24 tetracyclic terpane and the cross plot of dibenzothiophene to phenanthrene and pristane to phytane indicate that the oil seepages are marine origin petroleum; Ts/(Ts + Tm), C-29 sterane20S/(20S + 20R) and methylphenanthrene index all demonstrate that the oil seepages are mature to highly mature. The source rocks from the Lower Cambrian Niutitang Formation in the study area have bulk delta C-13(kerogen) values from -35. 79%o to -29. 88%o, average -32. 85%o (n =35), and have delta S-34(kerogen) values from + 14. 78%o to + 17. 60%o, average + 16. 32%o (n =4), they are well-correlated with those of oil seepages. However the source rocks from the Devonian, Carboniferous and Permian all show bulk delta C-13(kerogen), values of more than -29. 00%o, which are significantly heavier than those of oil seepages. The source rocks from the Lower Devonian, Middle Devonian and Permian show delta S-34(kerogen) values from -9. 10%o to -6. 78%o, + 0. 63%0 to + 7. 93%o, and -7. 40%o to + 4. 00%o, respectively, which are significantly lighter than those of oil seepages. Moreover, the source rocks from the Lower Cambrian are relatively rich in gammacerane, triaromatic steroids, aryl isoprenoids, retene and dibenzothiophene, whereas the other source rocks are absent or poor of triaromatic steroids and retene, and show lower abundance of gammacerane and dibenzothiophene. Thus it is demonstrated comprehensively that the oil seepages from the southern Guizhou Depression have the same origin, and are genetically linked to the source rocks of the Lower Cambrian.