The study discussed the origin and accumulation characteristics of hydrocarbon in buried hills in Shaxibei area

Bohai Sea

through geological and geochemical research methods.Analysis of biomarker parameters reveals that the proved buried hill oil samples in Shaxibei area originate from the Shahejie Formation source rocks.Oil samples from the CFD2-1 buried hill carbonate reservoir have a high G/H index and relatively low 4MSI

and are characterized by depleted

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C for n-alkanes(less than -29‰ as a whole).Such characteristics may be an indicator of the oil origin from the Es

1

source rocks.Oil samples

taken from the granite reservoir of the CFD1-6 oil fields

are typified by relatively higher 4MSI (more than 0.4) and heavier carbon isotope of n-alkanes (more than -29‰)

which may indicate that CFD1-6 buried hill oil is dominated by the Es

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-derived oil. The CFD2-1 buried hill is overlain by the Es

1

formation unit

and the mature Es

1

source rock may supply adequate petroleum for charging the carbonate reservoirs without long migration distance.This accumulation model can be referred to as the assemblage of generation from the draped source rock and vertically charging to the underlying carbonate reservoir.CFD1-6 granite buried hill is covered by the Ed shale

being characterized by low organic matter abundance and thermal maturity

which can be served as a seal for oil accumulation.Oil generated from the source rock in the Qikou Sag can be transported through the laterally distributed sand body and charge the CFD1-6 buried hill traps

exemplifying a hydrocarbon accumulation model of migration through lateral sand body.On the basis of core observation and casting thin section experiments

we analyzed the textural characteristics of weathered crust from Well CFD2-1-2.The results show that the weathered crust structure in Shaxibei area may not be able to receive the laterally transporting petroleum from sags to buried hill traps

while the overlying effective source rocks on the surface of buried hills or the occurrence of sand bodies communicating source and reservoir are fundamental for hydrocarbon accumulation in buried hills.