## 5 Discussion
### Deposition Environment  
Commonly used biomarker parameters include isoprene alkanes such as pristane and phytane; chlorophylls produced through photosynthesis in green plants; and purple sulphur compounds found in bacterial chlorophylls that can also generate pristane and phytane [41]. Phytols formed in chlorophyll and phytoliths can be transformed into prismates and phytanes under specific redox conditions, thereby demonstrating their potential for evolution. This ratio is frequently employed to indicate the parent material of the hydrocarbon source rock and depositional environment. Abundant pristine and phytane were detected in the samples collected from the Huangkekou Depression of the Shahejie Formation. The former was utilised to classify the sedimentary environment of source rocks using Ph/nC\({}_{18}\)-Pr/nC\({}_{17}\)[41] and the geochemical parameters of the source rocks within the Shahejie Formation in Huangkekou Depression, and the Ph/nC\({}_{18}\) versus Pr/nC\({}_{17}\) cross-plot suggested that the Shahejie Formation was deposited in varied and blended settings, featuring partially Type II and Type III organic matter in reducing and oxidising environments, respectively, as well as a mixture of different types of OM in transitional environments (Figure 6). The geochemistry of elements is also responsive to changes in the palaeenvironment, and alterations in the redox conditions of sedimentary water resulted in the fractionation of variable-valence elements, thereby causing element redistribution. Significant advancements have been made in the use of elemental geochemistry for sedimentary environment analyses. The commonly used indicators for identifying oxygen-poor and anoxic environments include V/(V + Ni), V/Cr, U/Th, Ni/Co, and V/Sc [45]; for example, an environment is considered anaerobic when V/(V + Ni) is less than 0.6, while a reducing environment is indicated by a w(V)/w(V + Ni) ranging between 0.6 and 0.84, and a (V)/w(V + Ni) value greater than 0.84 indicates a strongly reducing environment. Further, V/Cr values less than 2 indicate an oxygen-abundant environment, whereas V/Cr values between 2 and 4.25 suggest oxygen scarcity. Conversely, V/Cr values exceeding 4.25 indicate an anoxic environment [45; 46; 47; 48]. According to the cross-diagram of Co and V/(V + Ni), as well as those illustrating V/(V + Ni) and V/Cr (Figure 7a,b),  
Figure 5: Ternary diagram of the C\({}_{27}\)-C\({}_{28}\)-C\({}_{29}\)\(\alpha\alpha\alpha\) 20R steranes for the source rock in the study area, showing the source of the OM. the depositional environment of E\({}_{3}\)s\({}_{1}\) was characterised by an oxygen-poor and anoxic sulphide environment, and that of E\({}_{2}\)s\({}_{3}\) was a sub-oxygen-poor environment. Generally, E\({}_{3}\)s\({}_{1}\) displayed higher reducibility through the geochemical indices of elements. Ga is a C\({}_{30}\)-banded naphthene derived from tetrahymenolol and serves as a significant molecular marker for water stratification [49]. The stratification of water became more pronounced with increasing Ga/C\({}_{30}\)Hop levels. This can be observed through the Gammac-erane index (Ga/C\({}_{30}\)Hop) (Table 3). The Ga/C\({}_{30}\)Hop values in E\({}_{3}\)s\({}_{1}\), E\({}_{2}\)s\({}_{3}\), and E\({}_{2}\)s\({}_{3}\) ranged from 0.05 to 0.73 (average: 0.24), 0.50 to 1.62 (average: 0.23), and 0.02 to 0.19 (average: 0.15), respectively. The higher Ga/C\({}_{30}\)Hop of E\({}_{3}\)s\({}_{1}\) suggested that organic matter was deposited in water during a period characterised by salinity stratification, as indicated by the mean values provided above. A cross-map (Figure 8) utilising the Pr/ph and Ga indices revealed that the salinity of the water body during the deposition period of E\({}_{3}\)s\({}_{1-2}\) was relatively elevated, ranging from saline to brackish conditions. Conversely, the salinity of the water body during the deposition period of E\({}_{2}\)s\({}_{3}\) was comparatively low, ranging from freshwater to brackish conditions.