Distribution and Significance of Steroids in Bamianhe Oilfield, East China

samples of shales and mudstones developed in reducing hypersaline and freshwater setting and 20 oil samples of Bamianhe oilfield, Bohai Bay basin were collected for a detailed study of petroleum system. Composition and distribution of specific significance of steroids were discussed as a part of the study. Abundant steroidal biomarkers including C_ 27-29 regular steranes, C_ 28-30 4-methyl steranes, dinosteranes and aromatic steranes were detected. The composition and distribution of the steroids in the samples studied were a function of thermal maturity, organic source, paleoenvironment and lithology of potential source rocks. Alga-rich Es_4 shales (brackish water) developed in the south slope of depocenter (Niuzhuang sag) were found particularly rich in steroidal biomarkers including C_ 30 4-methyl steranes and dinosteranes. While Es_3 mudstones (fresh water) displayed devoid of dinosterane, abundant 4-β steranes were only found in argillaceous limestone and calcareous mudstone, clearly distinguished from the other rocks. Distribution patterns of regular steranes are different from methyl steranes completely indicating different origins of specific compound of the fraction. Isomerization of steroids were observed to be determined by paleoenvironment and lithology besides thermal maturity. Methyl steranes were proved much more useful in oil-source rock correlation than regular steranes. It was not possible that alga-rich Es_4 shales located on the south slope were the primary source rock responsible for the oil discovered based on composition and distribution of steroids.