## 5 Discussion  
### Organic Matter Sources  
Biomarkers have been extensively utilised to identify organic matter sources. Saturated hydrocarbons and \(n\)-alkanes serve as the most commonly used indices [31; 32; 33; 34; 35; 36; 37]. \(n\)-alkanes, being the simplest compounds found in sedimentary organic matter, play a crucial role in investigating hydrocarbon source rocks and are widely employed as indicators to determine the depositional palaeoenvironments and parent material types associated with source rock formations [35]. For instance, \(n\)-alkanes exhibit front-peak distribution characteristics with a low main peak carbon number, suggesting that the organic matter originates primarily from aquatic organisms and algae. Conversely, \(n\)-alkanes displaying post-peak distribution characteristics with a high main peak carbon number indicate that the organic matter is predominantly derived from terrigenous higher plants [31; 32; 33; 34; 35; 36; 37; 38]. If \(n\)-alkanes exhibit a bimodal distribution, organic matter is suggested to be derived from a blend of various resources, encompassing both lesser aquatic organisms and higher plants from terrestrial environments. Additionally, the carbon preference index (CPI), average chain length (ACL), and terrigenous/aquatic ratio (TAR) have been extensively employed to discern the sources of organic matter [34; 38; 39; 40]. Generally, C\({}_{27}\)-C\({}_{29}\) regular stereanes are the most prevalent and widely used compounds, and their relative content serves as an indicator of the input information from the organic source [41]. The presence of C\({}_{27}\) stereanes implies that the origins of the parent material are lower aquatic organisms and algae, whereas C\({}_{29}\) stereanes indicate a higher plant source [41; 42; 43; 44]. The organic matter in E\({}_{3}\)s\({}_{1-2}\) and E\({}_{2}\)s\({}_{3}\) in the study area exhibited characteristics of mixed-source input, indicating the existence of both lower aquatic organisms and terrigenous higher plants. However, the contributions from these two sources differed significantly. The \(m/z\) 85 mass chromatograms (Figure 3) demonstrated that E\({}_{3}\)s\({}_{1-2}\) exhibited a front-peak distribution, with some displaying bimodal features. Similar traits were observed in E\({}_{2}\)s\({}_{3}\), suggesting that the organic matter present in both E\({}_{3}\)s\({}_{1-2}\) and E\({}_{2}\)s\({}_{3}\) originated primarily from lower aquatic organisms. A cross-map of Ph/nC\({}_{18}\) and Pr/nC\({}_{17}\)[28] revealed that the organic matter in the Shahejie Formation originated from a mixture of sources, with some contributions derived from terrigenous higher plants. The \(m/z\) 217 mass chromatograms (Figure 4) demonstrated a distinctive distribution pattern of C\({}_{27}\)-C\({}_{28}\)-C\({}_{29}\) regular stereanes, forming an asymmetric \"V\" shape with the order of abundance as C\({}_{27}\) > C\({}_{29}\) > C\({}_{28}\). The dominance of C\({}_{27}\) regular stereanes suggested a significant contribution from lower aquatic organisms in the E\({}_{3}\)s\({}_{1-2}\) and E\({}_{2}\)s\({}_{3}\) samples. A ternary diagram (Figure 5) revealed that all available samples were a concentrated cluster, indicating a mixture of terrestrial and \begin{table}
\begin{tabular}{c c c c c c} \hline
**Shahejie** & **SiO2 (\%)** & **Al2O3 (\%)** & **Al2O3 (\%)** & **Co (\(\mu\)g/g)** & **V/(V + Ni) (\(\mu\)g/g)** & **V/Cr (\(\mu\)g/g)** \\
**Formation** & **Average** & **Average** & **Average** & **Average** & **Average** \\ \hline E\({}_{3}\)s\({}_{1}\) & 50.02 & 19.42 & 15.68 & 0.74 & 0.69 \\ E\({}_{2}\)s\({}_{3}\) & 50.37 & 16.92 & 23.5 & 0.71 & 1.12 \\ \hline  & MgO/CaO (\%) & C Value & K\({}_{2}\)O/Al\({}_{2}\)O\({}_{3}\) (\%) & V/Ni (\(\mu\)g/g) & \\ \hline E\({}_{3}\)s\({}_{1}\) & 1.92 & n.d. & n.d. & 3.01 & n.d. \\ E\({}_{2}\)s\({}_{3}\) & 0.42 & 0.51 & 0.24 & 2.41 & n.d. \\ \hline \end{tabular} Note: n.d.: no data.  
\end{table} 
Table 4: The concentration of major and trace elements in the studied area.