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<p>Aliphatic and aromatic biomarkers derived from higher land plants, conventional biomarkers, as well as the carbon stable isotope ratios of kerogens in late Eocene source rocks from the western margin of the Xihu Sag are presented here. Using these geochemical data, the organic matter source and depositional environment of the source rocks, and the correspondence between aliphatic and aromatic palaeovegetation indicators is evaluated. This allows for a reconstruction of the palaeovegetation and palaeoclimate, and reveals the influence of the organic matter source on thermal maturity proxies. Abnormally abundant aliphatic and aromatic diterpenoids from gymnosperms and trace amounts of oleanane-, ursane- and lupane-type triterpenoids from angiosperms were detected. Various source-related proxies, abundant diterpenoids, and non-hopanoid triterpenoids, indicate that the predominant organic matter source was higher land plants. Detection of microbe-derived biomarkers suggest that abundant microbial activity resulted in an additional source of organic matter input. Palaeovegetation proxies, based on aromatic terpenoids from higher plants, are in agreement with those based on aliphatic terpenoid biomarkers. Collectively, they indicate that gymnosperms, rather than angiosperms, contributed the majority of the organic matter. Palaeovegetation composition indicates that during late Eocene, the region surrounding the Xihu Sag was characterized by a relatively cool climate. Elevated Pr/Ph ratios, along with other biomarker indices, further indicate that the source rocks were deposited in a relatively oxic and sulphate-poor, fluvio-deltaic environment. A comprehensive evaluation of maturity indices demonstrates that samples are relatively immature and remain in the early-oil generation window. Despite this, some alkylnaphthalene- and alkylphenanthrene-based thermal maturity parameters were affected by the generation of 1, 2, 5-trimethylnaphthalene, 1, 2, 5, 6-tetramethylnaphthalene and 1-methylphenanthrene, and organic facies that are related to resins. Additionally, the methyldibenzothiophene ratio could be not applied to assess maturity for low mature samples. These indicate that caution must be exercised when viewing alkylnaphthalene-, alkylphenanthrene- and alkyldibenzothiophene-based thermal maturity parameters. Biomarker distributions suggest the Pinghu Formation could be the source of light oils discovered in the Xihu Sag.</p>

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Aliphatic and aromatic biomarkers from land plants in late Eocene source rocks of the Xihu Sag, East China Sea Shelf Basin

  • 期刊论文

  • 发表日期：

    2025年11月1日

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  • Xiong Cheng,
  • Dujie Hou,
  • Hui Diao,
  • Qian Huang

摘要

Aliphatic and aromatic biomarkers derived from higher land plants, conventional biomarkers, as well as the carbon stable isotope ratios of kerogens in late Eocene source rocks from the western margin of the Xihu Sag are presented here. Using these geochemical data, the organic matter source and depositional environment of the source rocks, and the correspondence between aliphatic and aromatic palaeovegetation indicators is evaluated. This allows for a reconstruction of the palaeovegetation and palaeoclimate, and reveals the influence of the organic matter source on thermal maturity proxies. Abnormally abundant aliphatic and aromatic diterpenoids from gymnosperms and trace amounts of oleanane-, ursane- and lupane-type triterpenoids from angiosperms were detected. Various source-related proxies, abundant diterpenoids, and non-hopanoid triterpenoids, indicate that the predominant organic matter source was higher land plants. Detection of microbe-derived biomarkers suggest that abundant microbial activity resulted in an additional source of organic matter input. Palaeovegetation proxies, based on aromatic terpenoids from higher plants, are in agreement with those based on aliphatic terpenoid biomarkers. Collectively, they indicate that gymnosperms, rather than angiosperms, contributed the majority of the organic matter. Palaeovegetation composition indicates that during late Eocene, the region surrounding the Xihu Sag was characterized by a relatively cool climate. Elevated Pr/Ph ratios, along with other biomarker indices, further indicate that the source rocks were deposited in a relatively oxic and sulphate-poor, fluvio-deltaic environment. A comprehensive evaluation of maturity indices demonstrates that samples are relatively immature and remain in the early-oil generation window. Despite this, some alkylnaphthalene- and alkylphenanthrene-based thermal maturity parameters were affected by the generation of 1, 2, 5-trimethylnaphthalene, 1, 2, 5, 6-tetramethylnaphthalene and 1-methylphenanthrene, and organic facies that are related to resins. Additionally, the methyldibenzothiophene ratio could be not applied to assess maturity for low mature samples. These indicate that caution must be exercised when viewing alkylnaphthalene-, alkylphenanthrene- and alkyldibenzothiophene-based thermal maturity parameters. Biomarker distributions suggest the Pinghu Formation could be the source of light oils discovered in the Xihu Sag.