Distribution and Significance of Methyl Steranes in Bohai Bay Basin, East China

31 samples of shales and mudstones developed in reducing hypersaline andfreshwater setting and 20 oil samples of Bamianhe oilfield, Bohai Bay basin were collected for adetailed study of petroleum system. Composition and distribution, especially significance ofsteroids, are discussed as a part of those. Abundant steroidal biomarkers, including C_(27-29)regular Steranes, C_(28-30) 4-methyl steranes, di-nosteranes and aromatic steranes, were detected.Results show that the composition and distribution of the steroids in samples studied are functionsof 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 anddinosteranes. Es_3 mudstones (fresh water) were found devoid of dinosteranes. Distribution patternsof regular steranes are completely different from methyl steranes indicating different origins ofthe specific compounds of the fractioa Diagnostic distribution of steranes in rock extracts ofdiverse intervals makes the compounds to be essential indictors of source-rock tracing. Methylsteranes prove to be much more useful in oil-source rock correlation than regular steranes. Resultsalso show that alga-rich Es_4 shales located in the south slope are not likely the primary sourcerock responsible for the oils discovered based on the composition and distribution of steroids.