Geochemical characteristics of source rocks and their contribution to petroleum accumulation of Chepaizi area in Sikeshu depression, Junggar Basin

Abstract：According to geochemical characteristics in combination with thermal evolution, there exist mainly three sets of source rocks in Sikeshu depression of Junggar basin, namely Jurassic (Badaowan Formation, Sangonghe Formation and Xishanyao Formation), Cretaceous and Paleogene. The Badaowan Formation source rocks which contain high abundance of organic matter of primarily Ⅲ and Ⅱ2 type have fairly good hydrocarbon-generation potential. The Sangonghe Formation source rocks have poor condition of hydrocarbon generation in that they have low abundance of organic matter and inferior type of kerogen. The organic matter abundance in source rocks of Xishanyao Formation is relatively high, but its hydrocarbon-generation potential is not ideal owing to relatively small thickness and poorer type of organic matter. The Cretaceous source rocks in Sikeshu depression have maximum thickness of 300m, but the condition of hydrocarbon generation is not favarable due to the lower maturity. Paleogene source rocks with high abundance of organic matter and Type Ⅱ organic matter have poor hydrocarbon-generation potential because of lower maturity. Through oil-source rocks correlation and quantitative identification of mixed oil, it is concluded that the crude oil in Chepaizi area was mainly derived from the Jurassic source rock in Sikeshu depression and mixed with the immature crude oil from the Cretaceous source, but the hydrocarbon generation of the Paleogene source rock is very little due to no distribution of the source kitchen. It is inferred that coal-formed oil dissolved the biomarker-rich Cretaceous bitumen of relatively low thermal maturity, and overprinting occurred during oil migration.