## 5 Discussion
### The Origin and Possible Migration Paths of Hydrocarbons  
Geochemical correlation analyses of crude oils from the study area contribute to a better understanding of the origin of these reservoirs. In general, ratios including Pr/n-C\({}_{17}\), Ph/n-C\({}_{18}\), and Pr/Ph are frequently used to study the parent material types, depositional environments, and the maturity of oils and source rocks [34; 35]. As shown in Figure 11, the cross-plot of Pr/n-C\({}_{17}\) and Ph/n-C\({}_{18}\) indicates that the studied oil samples are sourced from type-II organic matter in a marine reduction environment, similar to the oil of the Luntan1 (LT1) well, which is derived from the Lower Cambrian source rocks [74; 75; 76]. Similarly, Pr/Ph is an important parameter by which to classify the organic matter deposition environment, but it is also influenced by the parent source and the maturity, with a high Pr/Ph (>3) representing the input of organic matter from terrestrial sources under oxidizing conditions and a low Pr/Ph (<0.8) representing an anoxic environment [45]. Here, the average value of Pr/Ph of the studied oil samples is 1.02, which is slightly higher than that of the crude oils from south-central Tabei (mean value 0.79) [77], is due to the influence of maturity but likewise indicates a stronger reducing environment. The plot of Pr/Ph versus DBT/P has also been used to reflect the depositional environment of the source rocks [68]. As displayed in Figure 12, all the oil samples fall at the intersection of zone 1b, zone 2, and zone 3, all indicating a marine reducing environment. Moreover, the ternary diagram of n-heptane (nC\({}_{7}\)), dimethyl cyclopentanes (\(\Sigma\)DMCP), and methylcyclohexane (MCH) has frequently been used to determine the parent material source of oil and gas [54]. In this study, the predominance of the nC\({}_{7}\) content among C\({}_{7}\) LHs (Figure 3), together with the  
Figure 10: Cross-plots showing carbon number versus the molar fraction plot of n-alkanes for the studied oils: (**a**) no/limited evaporative fractionation; (**b**) obvious evaporative fractionation. Figure 9: The plot of \(\delta\)\({}^{13}\)C versus 1/Cn of the Ordovician gas from the F1 zone in the Shunbei region. Data on the carbon isotopes of natural gas were collected from the SINOPEC Northwest Company. cyclohexane index ICH, ranging from 16.85% to 22.53%, and the methylcyclohexane index, varying from 29.21% to 34.35%, in the studied oil samples (Table 4) all equally support the idea that they originate from a similar marine algal source rock. In addition, numerous studies have shown that the carbon isotopic compositions of hydrocarbons are mainly controlled by the carbon isotopic compositions of the source rocks. The effect of thermal evolution on the carbon isotopic composition of crude oil does not generally exceed 2%. [78; 79; 80]. Therefore, the carbon isotopic composition of crude oil can be used as an effective means by which to identify the source of oil. Previous studies [78] on the carbon isotopes of source rocks and crude oils in the Tarim Basin indicated that the \(\delta^{13}\)C\({}_{\text{Kerogen}}\) values below \(-34\)% are attributable to source rocks dominated by benthic algae, while those above \(-30\)% were attributable to source rocks dominated by planktonic algae. [PERSON] et al. [81] argued that the \(\delta^{13}\)C\({}_{\text{Kerogen}}\) values of crude oils were below \(-34\)% for the Lower Cambrian source rocks, above \(-29\)% for the Middle-Upper Cambrian source rocks, and in-between values were for the Cambrian source rocks. The \(\delta^{13}\)C values of oils in this study vary from \(-31.34\)% to \(-32.66\)%, with a deviation of 1.32% (Table 1), which may support the idea of a common Cambrian oil source for these oils, with a mixed biogenic  
Figure 11: The cross-plot of Ph/nC\({}_{18}\) versus Pr/nC\({}_{17}\) for the studied oils and the well, Luntan1 (LTI) (modified from Reference [74]; data for the LTI well, as used for comparison, are from Deng [76]).