# Impacts of Thermal Maturity on the Carbon Isotopes of
## 3 Results and Discussion
### Hopane Isotope
#### 3.3.3 Variation in propane Composition and Carbon Isotopes of Their Monomers with Temperature
The variation in carbon isotopes of monomeric hopane hydrocarbons can provide valuable insights into this process. During hydrocarbon generation, organic matter has undergone a series of biochemical and thermochemical changes, which will lead to the fractionation of carbon isotopes. Notably, there is a strong correlation between the carbon isotopes of monomeric hopane hydrocarbons and the hydrocarbon generation process. Generally speaking, the carbon isotopes of monomeric hopane hydrocarbons undergo a significant change around the peak of oil generation (T = 350 \({}^{\circ}\)C, \(R_{0}\) = 1.05%). With the increase in maturity, the carbon isotopes of the hopane compounds such as C\({}_{29}\alpha\beta\) and C\({}_{29}\)Ts become lighter and then heavier when they approach the peak of oil generation. At the peak of oil generation, the carbon isotopes of the hopane compounds such as Ts, Tm, 22S-homohopane, and 22R-homohopane are heaviest and subsequently become lighter. Previous studies have pointed out that thermal effects significantly influence the formation  
Figure 7: Distribution characteristics of carbon isotopes of monomeric hopane hydrocarbons at different thermal maturity levels. (**a**) The first category. (**b**) The second category. (**c**) The third category. of rearranged hopanes in source rocks. [PERSON] et al. [45] pointed out that the formation of rearranged hopanes primarily involves its precursor substances during low mature stage and both the pre-existing hopanes and rearranged hopanes in the source rock undergo thermal cracking during the maturation stage. [PERSON] et al. [46] conducted a study on the carbon isotopic analysis of kerogen pyrolysis, finding that the experimental products exhibited progressively heavier carbon isotopes with increasing thermal alteration. The authors attribute this phenomenon to special, unknown precursors present in immature kerogen. The entire process becomes complex due to the superposition of primary and secondary isotope fractionation. However, previous studies have proposed that during the thermal evolution of organic matter, the cleavage of chemical bonds may exhibit isotope-specific selectivity [5]. These studies indicate that with the increase in thermal evolution level of organic matter, \({}^{12}\)C-\({}^{12}\)C bonds break preferentially, causing the carbon isotopes of biomarkers to become heavier. As the temperature rises further, kerogen is cracked, potentially causing a thermodynamic fractionation effect. This may lead to the redistribution of light and heavy isotope molecules between compounds or phases in the system, resulting in the difference in isotope composition in each compound, so that the \(\delta^{13}\)C value becomes lighter. The noticeable fluctuation of carbon isotopes of hopane compounds may be related to the cracking and mutual transformation of preceding compounds during the hydrocarbon generation. In general, there are differences in the carbon isotope compositions of monomeric hopane hydrocarbons from various sources. However, it is important to note that while the observed trends in our data seem to imply a strong relationship between thermal maturity and the carbon isotopes of monomeric hopane hydrocarbons, this conclusion should be regarded as preliminary.