## 5 Discussion
### Alteration Processes of Hydrocarbons  
In general, alterations in reservoirs after hydrocarbons enter the trap, including thermal cracking, biodegradation, evaporative fractionation, and thermochemical sulfate reduction (TSR), among others, can differentially affect the composition and distribution of hydrocarbons [53,55]. In recent years, multiple deep reservoirs influenced by these alteration processes have been discovered in both the northern and central Tarim Basin areas [56, 57, 58]. Therefore, determining the extent of the influence of alteration processes on hydrocarbons is particularly essential. As shown in Figure 2a, the chromatograms of the samples showed no obvious unresolved complex mixture (UCM) \"hump\", indicating that no significant biodegradation processes prevailed. The 17\(\alpha\)(H) 25-norphelanes are frequently observed in degraded oils and have been used to indicate the level of biodegradation in crude oils [59]. In this study, the 17\(\alpha\)(H) 25-norphelanes are undetected because of their low concentrations. Additionally, in Figure 7, it is also evident that the oil samples in the study area did not undergo significant biodegradation, based on the nC\({}_{7}\)/MCH versus ToL/nC\({}_{7}\) plot proposed by [PERSON] [48]. Collectively, these results suggest that crude oils around the F1 zone are well-preserved and do not suffer from significant biodegradation. Conventionally, it is commonly believed that the thermal cracking of liquid oil typically increases with reservoir depth until it completely reaches the so-called oil deadline, which is typically near 5 km and at temperatures of 150-175 \({}^{\circ}\)C [60]. However, as the exploration of ultra-deep reservoirs progresses, the concept of the oil deadline has been challenged. Based on the empirical equation proposed by [PERSON] and [PERSON] [61], C(%) = GOR \(\times\) 100/(22428.8 + GOR), where C is the oil conversion and GOR is the gas-to-oil ratio, the calculated oil cracking degree in this study ranges from 0.87% to 2.0%, indicating that the oil samples in the F1 zone have basically not undergone thermal crack  
Figure 6: Plot of the correlation between the aromatic maturity parameters, DNR (DNR = (2,6-DMN + 2,7-DMN)/1,5-DMN) and MDR (MDR = 4-/1-MDBT), for the studied oils. ing. Additionally, the gas composition and carbon isotopes in this study also support this view. Based on the plot of Ln(C\({}_{1}\)/C\({}_{2}\)) versus Ln(C\({}_{2}\)/C\({}_{3}\)), Figure 8 shows that the natural gas in this study is dominated by kerogen cracking gas, with trace amounts of wet gas from the early stages of oil cracking [62; 63; 64]. The carbon isotopes of natural gas are relatively light, with \(\delta^{13}\)C\({}_{1}\) being less than \(-45\%\) and \(\delta^{13}\)C\({}_{2}\) ranging from \(-37\%\) to \(-38\%\) (Figure 9), as is similar to the gas characteristics of the ultra-deep Ordovician reservoirs in the Fuyuan well area in the southwestern part of the Tabei region [60] and distinctly different from those of previous studies on oil cracked gas (ranging from \(-35\%\) to \(-45\%\)) in the Tarim Basin [65]. Moreover, previous studies on diamondoids (with relatively low methyl-diamantane content (\(<100\) ppm)) in the study area also indicated that the Shunbei oils had cracked but had not yet reached the stage of oil cracking gas, corresponding to the stage of light oil generation [20]. Therefore, it is reasonable to infer that no significant thermal degradation of the ultra-deep Ordovician hydrocarbon occurred, which may be related to the low geothermal gradient in the study area and the depositional pattern of \"early rapid burial and subsequent slow burial\" [12]. Thermochemical sulfate reduction (TSR) is a process in which petroleum hydrocarbons react with inorganic sulfate to form CO\({}_{2}\), H\({}_{2}\)S, and solid bitumen in gypsum- and paste-mud-bearing formations at higher reservoir temperatures [66]. It is generally considered that a hydrogen sulfide content above 3% has the possibility of triggering TSR alteration [67].