Crude oil source in the western slope zone of Xihu sag
Crude oil source analysis is a crucial basis for determining the direction of oil and gas exploration.Utilizing organic geochemical techniques,the characteristics of crude oil and source rocks in the western slope zone of Xihu sag were investigated.Subsequently,employing the comparisons of biomarker spectra and biomarker parameters,and combined with the distribution features of crude oil and source rocks,the origin of crude oil in different zones of the western slope of Xihu sag was ascertained.The results indicate that:①The crude oil in the study area is generally abundant in biomarkers indicative of terrestrial higher plant origins.However,in Pingbei area,the crude oil exhibits a heavier carbon isotope composition,enriched diterpenoids,and predominance of C29 sterane.In Pinghu area,the crude oil displays a lighter carbon isotope composition,higher diterpenoids content,and lower gammacerane.In Huangyan-Tiantai area,the crude oil is characterized by a lighter carbon isotope composition,diminished diterpenoids content,a dominance of C29 sterane in regular sterane,and a higher content of C27 sterane.②The study area develops two types of source rocks including coal measure source rocks(coal and carbonaceous mudstone)and dark mudstone.The coal measure source rocks are enriched in isopimarane derived from gymnosperms and 16β(H)-phyllocladane from ferns.From Pingbei to Huangyan-Tiantai,the content of 16β(H)-phyllocladane displays an increasing trend in the coal measure source rocks,the diterpenoids content is decreasing in the dark mudstones,and 16β(H)-phyllocladane dominates over norisopimarane.Moreover,the content of C27 sterane and 16β(H)-phyllocladane increases in dark mudstone.③In Pingbei area,the crude oil is mainly contributed by coal measure source rocks,while in Pinghu area,the crude oil is contributed by a combination of coal measure source rocks and dark mudstone,with a significant contribution from coal measure source rocks.In Huangyan-Tiantai area,the crude oil is primarily contributed by dark mudstone.These research findings significantly enhance understanding of the oil source in Xihu sag and provide valuable insights for the deployment of oil and gas exploration.
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