Application of 4Methyl Steranes in OilSource Correlation:A case study from northern steep slope zone of Bonan Sag

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Abstract: Because of the samilar paleoclimate and palosedimentary environment, regular biomarkers and isotope distinction of the source rocks and generated petroleum in the subsag belong to the same sag is tiny, which bring difficulties to oilsource correlation. By extracting molecular fossils information using GCMS technology, according to the relative content of 4methyl steranes and distribution characteristics of 4methyl steranes isomers, oilsource correlation work are performed ,which provides a new method for oilsource correlation in similar area. Es4s and Es3x two sets of source rocks are developed in the study area, they are characterized by high organic matter abundance, mature to over mature thermal evolution degrees. Organic matter of Es4s are formed in brackishsaline water with reductive environment, organic matter of Es3x are formed in freshsaline water with reductive environment , organic matter input of both source rocks are dominated by lower aquatic organisms and terrigenous higher plants,  while terrigenous higher plants are more abundant near provenance. Relative content of 4methyl steranes of Es3x source rocks is higher than that of Es4s source rocks, indicating the better organic matter input of Es3x source rocks.The value of 4methyl steranes/C29 steranes of Es3x source rocks in the middle, west and deep subsag is 4.51,2.79 and 1.27, respectively, which is caused by the different proportion of dinophyceae in total organic matter. The distribution characteristics of C20, C21, C23 tricyclic terpanes and ααα20RC27,C
28,C29regular steranes of oils with alike maturity in different region is quite similar, 4methyl steranes/C29steranes value of crude oils in the middle, west and east region is 2.89, 2.39, 2.06, respectively, which make a big difference, while the carbon isotope characteristics of crude oils are opposite, distribution characteristics of 4methyl steranes isomers of each region is also different. Oilsource correlation demonstrate that Es4s and Es3x crude oils of northern steep slope zone mainly originate from Es3x source rocks in nearby sag, crude oils of the western zone are provided by Es3x source rocks of the western sag, crude oils of the middle zone are provided by Es3x source rocks of the middle sag, crude oils of the eastern zone are provided by Es3x source rocks of deep sag in the eastern zone, and no contribution from source rocks of the Gubei Sag.