Abstract: The free carbon and hydrogen isotopes in n-alkane from the Paleozoic source rocks in Huanghua depression are determined using techniques of GC-IRMS and GC-TC-IRMS to reveal the isotope compositions in different sedimentary environments. The results indicate that the δ¹³C and δD from n-alkane of Lower Paleozoic (marine facies) source rocks range from -29‰ to -33‰ and from -110‰ to -140%; while the δ¹³C and δD from n-alkane of Upper Paleozoic (transitional facies) source rocks range from -27‰ to-29‰ and from -140‰ to-170‰, respectively. From the marine facies to the transitional facies, there obviously exist the trend of rich-δ¹³C and poor-δD for the n-alkane. These also show that sedimentary environment could be the main factor controlling the isotopic compositions. It is concluded that in complex petroliferous systems, free C and H isotopic compositions and distributions from n-alkane are of significance in oil source correlations, and could be applied to investigation of related kerogens and sedimentary environments.

Key words: Huanghua depression, Paleozoic, n-alkane, carbon isotope, hydrogen isotope