# Impacts of Thermal Maturity on the Carbon Isotopes of
## 1 Introduction  
Biomarkers play an irreplaceable role in elucidating the composition of petroleum sources, depositional environments, and thermal evolution [1, 2, 3, 4]. However, since sedimentary organic matter comprises a mixture of various biological precursors, relying solely on the chemical structure and composition of biomarkers can introduce ambiguity in source identification and depositional environment research. The stable carbon isotopes of biomarker compounds contain rich biological, geological, and geochemical information. By examining the carbon isotopic composition of organic compounds and the mechanisms of carbon isotope fractionation, complex geochemical processes can be elucidated. Hopane compounds are mainly from bacterial sources. They are commonly found pentacyclic triterpenoids in geological formations and are the most widely used biomarkers. Typically, propane compounds are utilized for oil-oil correlation [5], oil-source correlation [6], source-source correlation [7], and assessments of organic matter maturity [8; 9]. Moreover, propane compounds have been extensively studied in environmental geochemistry (especially in lacustrine environment) [10; 11; 12; 13; 14; 15; 16; 17; 18; 19], biogeochemistry [20], and marine geochemistry [21; 22], demonstrating their substantial potential for application. Although carbon isotope studies of propane monomeric hydrocarbons remain limited due to complex sample pretreatment requirements, advancements in chromatography-isotope ratio mass spectrometry (GC-IRMS) have enabled exploration of the carbon isotope characteristics of monomeric propane hydrocarbons, revealing valuable insights into source composition and depositional environments. For instance, the carbon isotopes of C\({}_{29}\) propane and C\({}_{30}\) propane from lacustrine source rocks in the Permian Lucaogou Formation in Jungggar Basin reflect the variations in their biogenic source and sedimentary environments [23; 24]. Additionally, the propane compositions and their monomer carbon isotopes in peat samples from Guangdong suggest that there may be diverse microorganisms with varying carbon isotope compositions of patchouli-like precursors [6]. Previous studies have indicated that thermal maturity significantly affects the carbon isotope values of hopanes, which can be utilized for oil-source correlation [5; 9]. Compared to other biomarkers, the variation of carbon isotope values during petroleum generation can be significantly larger, reaching up to 40% or more. For example, the carbon isotope values of propane monomers in lacustrine source rocks range from \(-\)68.7% to \(-\)32% in the Songliao Basin [25]. It remains unclear whether this substantial variation results from thermal evolution or from the distinct composition of their biogenic precursors. The evolution of carbon isotopes in propane compounds during thermal maturity is an area that has seen limited investigation. In order to overcome the constraints of other influencing factors such as biogenic derivation and sedimentary environment, thermal simulation experiments are usually used to exclude the influence of other factors and are closer to geological conditions. However, due to the limitations of detection for chromatography-isotope ratio mass spectrometry (GC-IRMS), most studies conducted direct measurements without adequate sample enrichment [25; 26; 27]. Consequently, these studies primarily focused on the carbon isotope variations in a limited number of propane compounds that are highly abundant, such as C\({}_{30}\) propane [28; 29]. However, the carbon isotopes of these compounds can be affected by the presence of numerous n-alkanes. So far, [PERSON] et al. [30] presented a small-scale column chromatography method for separating hopanes in crude oil or rock extracts using neutral alumina as a solid phase adsorbent and a Pasteur pipette as a separation device. Mixed reagents of hexane and petroleum ether (V:V = 8:2) were used to elute the oil samples.