## 3 Sample and Method  
The samples in this study were collected from the Upper Ordovician Wufeng Formation and Submember Long-1\({}_{1}\) of the Lower Silurian Longmaxi Formation in the Changing area, including 70 shale samples from seven core holes, including N203 (6 pieces), N208 (11 pieces), N209 (6 pieces), N210 (7 pieces), N211 (10 pieces), N215 (12 pieces) and N217 (18 pieces). The sampling location is shown in Figure 1. The analysis and test of major and trace elements of the samples were completed in Guangzhou ALS Minerals Laboratory. The test method of the major element was to add the lithium borate-lithium nitrate melting flux containing lithium nitrate into the sample and mix, and then melt at high temperature. After the molten material was poured into the platinum mold to form a flat glass sheet, it was analyzed with an X-ray fluorescence spectrometer. The test method of trace elements involved taking two test samples: one was decomposed with perchloric acid, nitric acid and hydrofluoric acid, which is then dissolved by dilute hydrochloric acid to constant volume after nearly drying, and then analyzed via inductively coupled plasma emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometer (ICP-MS); the other test sample was added to lithium metaborate/lithium tetraborate flux, mixed evenly, and melted in a furnace above 1025 \({}^{\circ}\)C. After cooling, the molten liquid was metered to constant volume with nitric acid, hydrochloric acid, and hydrofluoric acid, and  
Figure 1: Structural location of the research area. then analyzed via inductively coupled plasma mass spectrometer (ICP-MS). TOC data was derived from well logging interpretation. Enrichment coefficient (EF) is used to evaluate the enrichment degree of an element in this shale sample. The formula for calculating EF is as follows [8]:  
\[EF_{X}=\frac{(X/Al)_{\text{Sample}}}{(X/Al)_{PAAS}} \tag{1}\]  
where \((X/Al)_{\text{Sample}}\) refers to the percentage ratio of \(X\) element to aluminum element in the sample, \((X/Al)_{PAAS}\) refers to the percentage ratio of \(X\) element to aluminum element in the post-Archean Australian shale (PAAS) and \(EF_{X}\) is the enrichment coefficient of \(X\) element. When \(EF_{X}\)\(>\) 1, it means that \(X\) element is more enriched than post-Archean Australia shale (_PAAS_); otherwise, it is deficient. Degree of mineralization of pyrite (DOP\({}_{T}\)) is used to judge the REDOX environment of water body during deposition. It refers to the ratio of Fe element in pyrite to total active Fe (Fe element dissolved by hydrochloric acid and Fe element in pyrite), and the content of Fe element in pyrite is calculated through the content of S element [10; 18; 19]. Assuming that all S elements exist in rocks in the form of pyrite (FeS\({}_{2}\)), the degree of mineralization of pyrite (DOP\({}_{T}\)) can be calculated using the following formula:  
\[\text{DOP}_{T}=\frac{\omega(\text{S})\times 55.85/64.13}{\omega(\text{F})} \tag{2}\]  
[PERSON] believes that Ce anomaly degree can indicate the relative change in sea level, and the calculation formula of \(\delta\)Ce is as follows:  
\[\delta\text{Ce}=\lg\frac{5 Ce_{N}}{(4 La_{N}+Sm_{N})} \tag{3}\]  
\(Ce_{N}\), \(La_{N}\) and \(Sm_{N}\) refer to the values of Ce, \(La\) and \(Sm\) normalized by the Post-Archean Australian shale (_PAAS_). The \(\delta\)Ce value of the sample decreases with the rise in sea level, and it increases otherwise. [PERSON] [20] proposed that the authigenic uranium (\(\delta\)U) could reflect the REDOX condition of sedimentary environment of the sample, and its calculation formula is as follows:  
\[\delta\text{U}=\frac{2\text{U}}{\text{U}+\text{Th}/3} \tag{4}\]  
When \(\delta\)U \(>\) 1, it reflects the anaerobic environment when the sample was deposited; otherwise, it is the normal sedimentary environment.