Organic-rich shales from the Triassic Yanchang Formation in the Ordos Basin in China are mainly derived from aquatic organisms with type II kerogen.A reverse maturity trend,derived from the commonly used biomarker mat...Organic-rich shales from the Triassic Yanchang Formation in the Ordos Basin in China are mainly derived from aquatic organisms with type II kerogen.A reverse maturity trend,derived from the commonly used biomarker maturity parameter Ts/(Ts+Tm)occurs in the depth profiles of the Chang 7_(3) submember and the Chang 8 member.In contrast,maturity proxies derived from aromatic compounds show a normal sequence for more deeply buried Chang 8 samples exhibiting higher maturity levels,as expected.To explain the abnormal phenomenon,multiple controlling factors—including the paleoredox condition,water salinity,the clay mineral content and composition,biodegradation,the primary migration fractionation effect,and organofacies—are considered.It was found that organofacies BC(HIo=400-250)appear in the Chang 8 samples,while organofacies B(HIo=400-650)appear in the Chang 7_(3) submember.The results suggest that variation in organofacies has a significant impact on Ts/(Ts+Tm)values,even for samples with the same kerogen type and similar source input,and is primarily responsible for the reverse maturity trend in the depth profiles.展开更多
基金supported by The National Natural Science Foundation of China(Grant.No 41873049 and 42062011)the Doctoral Scientific Research Foundation of Yulin University,China(Grant No.2024GK12 and 2023GK62)+1 种基金the Youth Innovation Team of Shaanxi Universities,China(Grant.No 23JP200)the technical service project(Grant.No H2024060142).
文摘Organic-rich shales from the Triassic Yanchang Formation in the Ordos Basin in China are mainly derived from aquatic organisms with type II kerogen.A reverse maturity trend,derived from the commonly used biomarker maturity parameter Ts/(Ts+Tm)occurs in the depth profiles of the Chang 7_(3) submember and the Chang 8 member.In contrast,maturity proxies derived from aromatic compounds show a normal sequence for more deeply buried Chang 8 samples exhibiting higher maturity levels,as expected.To explain the abnormal phenomenon,multiple controlling factors—including the paleoredox condition,water salinity,the clay mineral content and composition,biodegradation,the primary migration fractionation effect,and organofacies—are considered.It was found that organofacies BC(HIo=400-250)appear in the Chang 8 samples,while organofacies B(HIo=400-650)appear in the Chang 7_(3) submember.The results suggest that variation in organofacies has a significant impact on Ts/(Ts+Tm)values,even for samples with the same kerogen type and similar source input,and is primarily responsible for the reverse maturity trend in the depth profiles.
