# 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 composition of propane compounds and their monomer carbon isotopes are closely related to temperature, which has a significant influence on the carbon isotope composition of monomeric propane hydrocarbons. It can be seen from the relationship between stable carbon isotope composition of a series of propane compounds and temperature (Figure 7) that temperature has different effects on carbon isotopes of different propane series compounds, and its variation law with temperature can be obviously divided into three categories. In the first category, overall, the carbon isotopes of monomeric propane hydrocarbons exhibit noticeable fluctuations with the increasing pyrolysis temperature. The primary compounds in this category are C\({}_{29}\alpha\beta\) and C\({}_{29}\) Ts. Before 350 \({}^{\circ}\)C, the carbon isotopes of these compounds became slightly heavier and then lighter with the increase in pyrolysis temperature, reaching the lightest value around 350 \({}^{\circ}\)C, then becoming heavier around 350-365 \({}^{\circ}\)C, and finally, when the temperature was greater than 365 \({}^{\circ}\)C, the carbon isotopes became heavier. The overall range of change can be as much as 17.31%. It is hypothesized that the significant fluctuations in carbon isotopes for these compounds may result from thermal fractionation during diagenesis and epigenesis. In the second category, on the whole the carbon isotopes of monomeric propane hydrocarbons also exhibit noticeable fluctuations with increasing temperature. The primary compounds include Ts, Tm, 22S-homophone, and 22R-homophone. Before 350 \({}^{\circ}\)C, with the increase in temperature, the carbon isotopes of these compounds first became slightly lighter, then became heavier, reaching the maximum value at about 350 \({}^{\circ}\)C, then became lighter at about 350-365 \({}^{\circ}\)C, and finally gradually  
Figure 6: Variation range of carbon isotopes of monomeric propane hydrocarbons in lacustrine shale during compaction pyrolysis experiments. became heavier when the temperature was higher than 365 \({}^{\circ}\)C with the continuous increase in temperature, and the overall change range could reach 21.31%. Our results preliminarily indicate consistency with previously reported studies that the isotopic fractionations of C\({}_{29}\alpha\beta\), 22S-homohopane and 22R-homohopane are quite evident [24; 25]. For the third category, the carbon isotopes of C\({}_{29}\beta\alpha\), C\({}_{30}\alpha\beta\), C\({}_{30}\beta\alpha\), and non-hopane gammacerane almost keep subtle changes at different thermal stages. The relationship between the carbon isotopes of these compounds and the maturity is not clearly defined, indicating that the overall changes in carbon isotopes are within 6%, with gammacerane showing changes within 3.3% as maturity increases. It is generally believed that relative to biological precursors, the carbon isotope of kerogen, which has undergone diagenesis and thermal maturation, will not change significantly [44], while the carbon isotope composition of the monomeric compound depends on the carbon source utilized by the precursor organism and the carbon isotope fractionation effect during their biosynthesis and assimilation. Isoalkanes originating from different sources exhibit distinct carbon isotope compositions [8]. The variance in carbon isotope values among different hopane compounds in this sample, coupled with their varying responses to temperature, suggests that it is a potential explanation that these hopanes likely have distinct biological sources and exhibit different thermal response characteristics. However, the specific mechanisms require further validation. 3.4 Relationship Between Carbon Isotopes of Monomeric Hopane Hydrocarbons and Hydrocarbon Generation  
The hydrocarbon generation process, specifically the process of transforming organic matter into hydrocarbon compounds, is a crucial stage in oil and gas formation.