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<p>The lacustrine shale of the Jurassic Lianggaoshan Formation in the Sichuan Basin is a key shale oil exploration target. Reconstructing redox conditions of depositional processes is essential for understanding the distribution of organic-rich shale intervals. Geochemical and redox-sensitive elemental thresholds are effective indicators, however, due to different geological settings, commonly used proxies and previously established thresholds are difficult to apply to the Lianggaoshan shale. To identify suitable redox proxies for the Lianggaoshan shale, this study investigated and evaluated these commonly used indicators, including redox-sensitive element enrichment factor, bi-metal ratio, C-S-Fe-P system, etc. To accurately characterize the redox conditions of the study area, this study relied on the co-variation theory of above proxies within the redox framework according to three key redox thresholds (T<sub>1</sub>, T<sub>2</sub>, T<sub>3</sub>), quantitatively re-evaluated and calibrated their thresholds separately. The results indicate that the fixed thresholds among widely used proxies are not fully applicable to the studied succession. Reassessment of C<sub>org</sub>/P and bi-metal ratios based on co-variation theory suggests that the C<sub>org</sub>/P ratio is the most useful for redox analysis to the studied succession, while V/(V + Ni), (Cu + Mo)/Zn, Ni/Co, and V/Cr are not fully applicable, and the Fe/Al ratio is completely inapplicable. Among trace element enrichment factors, Cu<sub>EF</sub>, V<sub>EF</sub>, and Ni<sub>EF</sub> also have good effects on the recovery of redox conditions. It is essential to calibrate the thresholds of proxies that have provided useful results based on the proxy covariation theory prior to their application. By selecting the most effective proxies, the redox thresholds of elemental indicators can be accurately quantified and calibrated based on the “simple” or “compound” covariation patterns. A comparison of the recalibrated thresholds shows that T<sub>2</sub> and T<sub>3</sub> are similar, whereas T<sub>1</sub> and T<sub>2</sub> differ significantly. Based on the threshold range established from the C<sub>org</sub>/P ratio, this study reveals that the Lianggaoshan Formation shale was predominantly deposited under persistent euxinic conditions, which were occasionally interrupted by oxic conditions induced by seasonal events. Our study provides a method and process for threshold value correction of lacustrine shale in other basins.</p>

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Re-evaluation and calibration of the redox threshold for lacustrine shale: a case study from Jurassic Lianggaoshan Formation in the Northern Sichuan Basin, SW China

  • 期刊论文

  • 发表日期：

    2026年4月8日

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  • Shaomin Zhang,
  • Han Jiang,
  • Chunyu Qin,
  • Xinyu Zhang,
  • Haitao Hong,
  • Luyuan Han,
  • Hua Bai

摘要

The lacustrine shale of the Jurassic Lianggaoshan Formation in the Sichuan Basin is a key shale oil exploration target. Reconstructing redox conditions of depositional processes is essential for understanding the distribution of organic-rich shale intervals. Geochemical and redox-sensitive elemental thresholds are effective indicators, however, due to different geological settings, commonly used proxies and previously established thresholds are difficult to apply to the Lianggaoshan shale. To identify suitable redox proxies for the Lianggaoshan shale, this study investigated and evaluated these commonly used indicators, including redox-sensitive element enrichment factor, bi-metal ratio, C-S-Fe-P system, etc. To accurately characterize the redox conditions of the study area, this study relied on the co-variation theory of above proxies within the redox framework according to three key redox thresholds (T1, T2, T3), quantitatively re-evaluated and calibrated their thresholds separately. The results indicate that the fixed thresholds among widely used proxies are not fully applicable to the studied succession. Reassessment of Corg/P and bi-metal ratios based on co-variation theory suggests that the Corg/P ratio is the most useful for redox analysis to the studied succession, while V/(V + Ni), (Cu + Mo)/Zn, Ni/Co, and V/Cr are not fully applicable, and the Fe/Al ratio is completely inapplicable. Among trace element enrichment factors, CuEF, VEF, and NiEF also have good effects on the recovery of redox conditions. It is essential to calibrate the thresholds of proxies that have provided useful results based on the proxy covariation theory prior to their application. By selecting the most effective proxies, the redox thresholds of elemental indicators can be accurately quantified and calibrated based on the “simple” or “compound” covariation patterns. A comparison of the recalibrated thresholds shows that T2 and T3 are similar, whereas T1 and T2 differ significantly. Based on the threshold range established from the Corg/P ratio, this study reveals that the Lianggaoshan Formation shale was predominantly deposited under persistent euxinic conditions, which were occasionally interrupted by oxic conditions induced by seasonal events. Our study provides a method and process for threshold value correction of lacustrine shale in other basins.