ORIGIN OF HYDROCARBONS IN OIL SAND IN THE SOUTH SLOPE OF NIUZHUANG SAG, THE BOHAIWAN BASIN, EAST CHINA

Correlation of the hydrocarbons in the oil sand and the adjacent Es⁴ shale extracts from the south slope of Niuzhuang Sag,Bohai Bay Basin shows obvious differences between the oil sand with buried depth less than 2 700 m and the immediately adjacent Es⁴ shales which was originally considered as the main oil source rocks in the south slope,suggesting the hydrocarbons in shallow oil sand samples are hydrocarbons migrating from the deep seated source rocks.Hydrocarbons in the oil sand samples have normal GC fingerprints,while the adjacent shale extracts with buried depth less than 2 700 m are characteristic of high odd-even predominance of normal alkane,and a distinct bulge composed of undistinguished compounds of steroids and terpanoids was observed in the behind section of the GC fingerprint with high retain time.The low-matured Es⁴ shales with buried depth less than 2700m also have distinct features including low concentration of geological stereochemical configuration(i.e.20R20S and ααααββ) and such the high content of compounds with low thermal stability as 5-β steranes,13α(H)14α (H)-tricycle terpanes.In addition,little amount of dinosteranes were discovered in all of the oil sand samples while dinosteranes are the main constitution in Es⁴ shale above normal oil window,further indicating no possibility of the algae-rich Es⁴ interval with buried depth less than 2 700 m to be the source rocks of the oils in the area.With the buried depth decreasing,the maturity of hydrocarbons in oil sand samples and the level of C₃₅-/C₃₄-hopane increased and the concentration of Gamacerance decreased.These can not only indicate the direction of oil migration,but also reveal the different chemical compositions of hydrocarbons generated by the source rocks with different thermal stages and/or mixing of immature oils or bitumen with normal oils.