## 5 Discussion  
### Oil Source Input  
The TOC/TN ratio in sedimentary rocks is an effective indicator for analyzing the source of organic matter [14]. The original protein content of microalgae in lakes is higher than that of terrestrial higher plants, so the TOC/TN ratio of aquatic phytoplankton is relatively low, usually from 4 to 10, whereas the TOC/TN ratio of terrestrial plants is often greater than 10 [36]. Therefore, in the study of lake environments, the TOC/TN ratio is often used to determine whether the organic matter in the sediment originates from microorganisms or higher plants [36].  
In this study, through systematic testing and calculation of the TOC/TN value of the source rock in the Hengtongshan Formation, it was found that the TOC/TN value of samples between 107.84 and 184.3 m and between 264.2 and 292.16 m is greater than 10, with a maximum of 21.00. In contrast, the values in the other sections are less than 10 (Table 2). Therefore, higher plants may have made a greater contribution to the organic matter in the sediments of the first two sections. However, in terrestrial formations, organic nitrogen is easily mineralized or oxidized, and the presence of large amounts of exchangeable fixed ammonium in organic deposits in terrestrial sediments accounts for 10% of organic ammonia [37], which can result in a low TOC/TN ratio. Therefore, using this value to determine the source of organic matter is very uncertain, so in this study, biomarker compounds were used to further comprehensively determine the source of the organic matter.  
Previous studies have shown that lower aquatic organisms are rich in lipid compounds, and n-alkanes are mainly composed of low carbon number components, while higher plants are often rich in wax and mainly composed of high carbon number components [16]. Therefore, the n-alkane distribution characteristics of saturated hydrocarbons can reflect the type of organic matter in source rocks. It is generally believed that the main peak carbon number is generally less than that of n-alkane C\({}_{23}\) and is unimodal. Additionally, the main carbon peak of higher plants is usually greater than that of n-alkane C\({}_{25}\), and the main carbon peak of mixed organic matter is between the peaks of n-alkane C\({}_{23}\) and n-alkane C\({}_{25}\)[16]. In this study, it was found that the main carbon peaks of the n-alkanes in both the extracts and the shale oil in the m/Z 85 diagram are less than the peak of n-alkane C\({}_{23}\), which indicates that the contribution of lower aquatic organisms in organic matter is larger than that of higher plants. Steroids and hopanoids represent eukaryotic and prokaryotic contributions to the source material of sediments, respectively [27], and the average values of the steroid/hopanoid ratio in this study are 0.12-0.51 (average 0.26). Low steroid/hopanoid ratios are characteristic of lacustrine or special bacterial-influenced facies [18; 38], indicating strong microbial activity [16].  
According to the results of Pr/n-C\({}_{17}\) and Ph/n-C\({}_{18}\), the source of the organic matter in the sediments of the Hengtongshan Formation may have been a mixed source (Figure 7). This is further confirmed by the relative ratio of C\({}_{27}\)C\({}_{28}\)C\({}_{29}\) sterane to C\({}_{27}\)/C\({}_{29}\), which indicates that the source of the organic matter in the source rock in the Hengtongshan Formation was a mixed source and that plankton and higher plants were the main contributors in many samples. Combined with the trend between the sample depth and the TOC/TN value, the relative proportions of C\({}_{29}\) sterane in the 107.84-184.3 m and 235.69-292.16 m depth ranges are greater than 50%, indicating that plankton and higher plants are dominant in the oil-related organic matter, which is consistent with the conclusion based on the TOC/TN value. As a whole, the source material of the source rock of the Hengtongshan Formation is a mixture of aquatic organisms (algae and bacteria) and higher plants and may have marine biological components.