Characteristics and Oil-source Investigation of the Oils in the Nanpu Oilfield,Bohai Bay Basin

Nanpu oilfield is a recently discovered field with substantial petroleum reserves.Totally 43 oils and 31 rock samples were collected for performing detailed geochemical analyses to unravel the characteristics and oil source for the oils.Three oil families have been recognized according to biomarkers and compound specific carbon isotope of n-alkanes,namely Tertiary oils in No.1 and 2 structural belts(OF-Ⅰ),Tertiary oils in No.3,4 and 5 structural belts(OF-Ⅱ) and Ordovician oils(OF-Ⅲ).The OF-Ⅰ is characterized by relatively low ratios of diasteranes to regular steranes but high ratios of steranes to hopanes,4-methyl steranes to C29 regular steranes and gammacerane to C30 hopane with a little heavier δ13C values of the compound specific isotope of n-alkanes relative to the OF-Ⅱ.The OF-Ⅲ is distinguished from both the OF-Ⅰ and OF-Ⅱ with relatively higher thermal maturity and heavier δ13C values of n-alkanes.Detailed oil-source rock correlation showed that the OF-Ⅱ and OF-Ⅲ were mainly derived from the source rocks of Ed3-Es1 and Es2+3 respectively,and the OF-Ⅰ was derived from both the Es2+3 and Ed3-Es1 source rocks.We suggested that the Es2+3 is one of the most important source rocks of the Tertiary oils in the Nanpu oilfield,which is different from the previous idea that only the Ed3-Es1 is responsible for the Tertiary oils.This study should be helpful to petroleum resources evaluation and further petroleum exploration of the Nanpu oilfield.