## 5 Discussion
### Relationship between REDOX Conditions and Total Organic Carbon Content  
In the oxidation environment, U element is generally dissolved in marine water with +6 valence, and in the reduction environment, U6+ is reduced to U4+, which is absorbed by asphalt in the form of oxide or forms a complex and enriched in sediments [23; 24; 25; 26; 27]. In the oxidation environment, V element is dissolved in water with +5 valence in the states of \(\mathrm{HVO_{4}}^{2-}\) and \(\mathrm{H_{2}VO_{4-}}\). In the reduction environment, it will be reduced to +4 or +3 valence, and it is often enriched in sediments in the form of insoluble hydroxides, organometallic extraction coordination, or adsorption by groups [28]. Mo is mainly dissolved in marine water in the form of \(\mathrm{MoO_{4}}^{2-}\)[29], and it can be enriched in sediments in the form of sulfide under the reduction condition of sulfate-reducing bacteria. The REDOX environment controls the enrichment of REDOX sensitive elements such as U, V and Mo, and the content of REDOX sensitive elements in sediments can reflect the REDOX conditions during deposition [9]. However, due to their own chemical characteristics, the elements tend to attract each other and combine into compounds. Therefore, in this study, the element ratio and multiple indexes are used to judge the REDOX characteristics during deposition. Some scholars have proposed to indicate the REDOX environment during deposition by U/Th, Ni/Co, V/Sc, \(\delta\)U and DOP (Table 1). The REDOX index of the Wufeng Formation  
Figure 2: Standardization diagram of trace elements and rare earth elements in Wufeng Formation–Submember Long-1\({}_{1}\). of the seven test wells in the research area is shown as (Figure 3). The U/Th value ranges from 0.21 to 3.82, with an average of 1.29. The Ni/Co value ranges from 5.20 to 15.98, with an average of 9.37. The V/Sc value ranges from 8.06 to 33.54, with an average of 21.13. The \(\delta\)U value ranges from 0.76 to 1.84, with an average of 1.40. The DOP\({}_{T}\) value ranges from 0.19 to 0.77, with an average of 0.46. In Bed Long-11\({}^{1}\), the U/Th value ranges from 1.79 to 4.63, with an average of 3.1. The Ni/Co value ranges from 10.85 to 15.69, with an average of 13.27. The V/Sc value ranges from 30.28 to 49.78, with an average of 40.53. The \(\delta\)U value ranges from 1.69 to 1.87, with an average of 1.79. The DOP\({}_{T}\) value ranges from 0.55 to 0.62, with an average of 0.59. Combining all the indexes, it reflects that the whole Bed Long-11\({}^{1}\) is an anaerobic environment. In Bed Long-11\({}^{2}\), the U/Th value ranges from 1.06 to 3.78, with an average of 1.87. The Ni/Co value ranges from 8.15 to 18.57, with an average of 11.95. The V/Sc value ranges from 18.53 to 77.08, with an average of 34.44. The \(\delta\)U value ranges from 4.52 to 1.84, with an average of 1.67. The DOP\({}_{T}\) value ranges from 0.43 to 0.71, with an average of 0.62. Combining all the indexes, it reflects that the whole Bed Long-11\({}^{2}\) is an anaerobic environment. In Bed Long-11\({}^{3}\), the U/Th value ranges from 0.24 to 1.79, with an average of 1.15. The Ni/Co value ranges from 4.00 to 7.85, with an average value of 6.29. The V/Sc value ranges from 6.67 to 21.28, with an average of 16.37. The \(\delta\)U value ranges from 0.84 to 1.69, with an average of 1.50. The DOP\({}_{T}\) value ranges from 0.22 to 0.70, with an average of 0.53. Combining all the indexes, it reflects that the whole Bed Long-11\({}^{3}\) is an oxygen-poor environment. In Bed Long-11\({}^{4}\), the U/Th value ranges from 0.30 to 1.47, with an average of 0.56. The Ni/Co value ranges from 3.61 to 8.26, with an average value of 5.29. The V/Sc value ranges from 8.90 to 19.53, with an average of 12.80. The \(\delta\)U value ranges from 0.95 to 1.63, with an average of 1.2. The DOP\({}_{T}\) value ranges from 0.24 to 0.52, with an average of 0.35. Combined with all the indexes, it reflects that Bed Long-11\({}^{4}\) is changed from the oxygen-poor environment at the bottom to the oxygen-rich environment at the top.