# Impacts of Thermal Maturity on the Carbon Isotopes of
## 3 Results and Discussion
### Hopane Isotope
#### 3.3.2 Overall Variation Range of Carbon Isotopes of Hopane Monomers  
The carbon isotopes of monomeric hopane hydrocarbons at different pyrolysis temperatures are shown in Table 2. Overall (Figure 6), at various pyrolytic temperature stages, the \(\xi^{13}\)C values of monomeric hopane hydrocarbons in the residual oil of the E\(k_{2}\) Kongdian Formation are generally lighter, and the carbon isotopes of these monomeric hopane hydrocarbons range from \(-46.22\)% to \(-73.31\)%, which are significantly lighter than the \(\xi^{13}\)C values of hopane compounds found in several other major domestic and international hydrocarbon source rocks. For comparison, the \(\xi^{13}\)C values of the monomeric hopane hydrocarbons from the Permian Lucaogou Formation in the Santanghu Basin, Northwest China, range from \(-33\)% to \(-52\)% [24], while those from the Nenjiang Formation, Songliao Basin, are predominantly between \(-32\)% and \(-50\)%. [25]. In the Green River Formation of the Uinta Basin, the \(\xi^{13}\)C values span from \(-29\)%\(\sim-59\)%. [5], and the values from the Eocene Messel Shale range from \(-35\)% to \(-65\)% [39]. These findings highlight the distinct isotopic characteristics of the monomeric hopane hydrocarbons in the study area. It is indicated that the \(\xi^{13}\)C values of the monomeric hopane hydrocarbons exhibit distinct multi-source characteristics across different geological formations. Generally, the \(\xi^{13}\)C values first become lighter and then heavier with the increasing carbon number in hopanes. Specifically, the carbon isotopes of C\({}_{29}\)Ts and C\({}_{30}\)\(\beta\)\(\alpha\) are the lightest, while those of non-hopane gammacerane are the heaviest. The \(\xi^{13}\)C values for C\({}_{29}\) Ts range from \(-54.93\)% to \(-72.24\)%, and for C\({}_{30}\)\(\beta\)\(\alpha\) from \(-64.75\)% to \(-70.54\)%. In contrast, the \(\xi^{13}\)C values for gammacerane are the highest and exhibit less variation, ranging from \(-29.53\)% to \(-26.16\)%, with fluctuations generally within \(3\)%. The carbon isotopes of monomeric hopane hydrocarbons are influenced by several factors, including secondary action, bacterial processes, and the mixtures formed by primary producers [40]. It has been suggested that hopane with a light carbon isotope originates from methanotrophic bacteria. In the reduction zone of paleosedimentary environment, methanotrophic bacteria reduce acetic acid and carbon dioxide to methane enriched in \({}^{12}\)C, which is used as a carbon source by methane autotrophic bacteria to synthesize hopane with a very light carbon isotope composition [39]. Judging by the extremely light carbon isotope of the sample, it may be related to the intense temperature stratification in the water body during the extremely hot climate conditions of that time and the shallow redox interface [41], with highly active bacterial action occurring in the bottom water body. However, gammacerane exhibits a heavier carbon isotope value, exceeding \(-25\)% compared to the carbon isotopes of hopane compounds. It is considered that gammacerane is derived from tetrahymenol produced by certain protozoa and photosynthetic bacteria [42], with tetrahymenol itself derived from tetraethylene glycol, which originates from