Mixed Oil Distribution and Source Rock Discrimination of the Napu Depression, Bohaibay Basin

The origin of the hydrocarbons in the Napu depression is still not clear. Compound specific carbon isotope of n-alkanes has been utilized, combined with biomarkers to carry out detail geochemical investigations for the oils and source rocks within the whole depression for the first time. The results show that the Tertiary Es₃ rocks (type-Ⅰ) in the Shichang sub-depression are characterized by abundant 4-methyl steranes and dibenzothiophenes but low concentration of gammacerane with a "tilted linear" shape of compound specific carbon isotope distribution. The type-Ⅰ genetic related oil has been discovered in the Tertiary Es₃ interval of Gaoshangpu and Liuzan oilfields. The Tertiary Es₂₊₃ rock (type-Ⅱ) in the south Nanpu depression is featured by a relatively high content of gammacerane while little 4-methyl sterane contained, and it has a similar carbon isotope curve and dibenzothiophene concentration to that of the type-Ⅰ rock. The type-Ⅱ genetic oil has been discovered in the Ordovician burial hills. The Tertiary Es₁-Ed₃ rock (Ⅲ) in the Nanpu depression is distinguished from the others by obviously high content of diasteranes and pregnant series with a relatively lowerδ¹³C of n-alkane and nearly a "W" shape of carbon isotopic pattern. The type-Ⅲ genetic related oils was discovered in the NP4, 5 tectonic belts. It is observed that the oils in the NP1, 2 tectonic belts, Laoyemiao oilfield and the Tertiary Nm-Es₁ of Gaoshangpu and Liuzan oilfield have intermediate biomarker and isotope characteristics between the type-Ⅱ and type-Ⅲ source rocks and the related oils, indicating mixed sources of the oils derived from both the type-Ⅱ and type-Ⅲ rocks. It is found that the related contribution of the Es₂₊₃ source rocks to the mixed oils is up to 82%. We suggest that the Es₂₊₃ is an important set of source rock in the Nanpu depression, responsible for most of the oils. This result is different from the previous ones that most of the oils in the Nanpu depression were derived from the Es₁-Ed₃ source rocks rather than the Es₂₊₃, which provides essential information for further petroleum exploration of the depression.