## 5 Discussion
### The Origin and Possible Migration Paths of Hydrocarbons
Figure 12: The cross-plot of the pristane/phytane (Pr/Ph) ratio versus the dibenzothiophene/phenanthrene (DBT/P) ratio of the studied oils (after [PERSON] et al. [68]). parent material type of benthic and planktonic algae. Furthermore, the plots of \(\delta^{13}\)C versus \(1/\)Cn of natural gases (Figure 9) in this study show a positive sequence distribution (i.e., \(\delta^{13}\)C\({}_{1}<\delta^{13}\)C\({}_{2}<\delta^{13}\)C\({}_{3}<\delta^{13}\)C\({}_{4}\)) and a similar distribution pattern, also indicating a similar organic origin. The recent discovery of the Lower Cambrian Yuertusi formation source rock from the Luntan1 well (LT1) in the northern Tarm Basin further confirms the view that the Lower Cambrian source rocks have become the dominant source rocks in the Tarim Basin [11]. Previous studies suggested that the light oil of the Cambrian Wusonggeer formation from the LT1 well was derived from the underlying Cambrian Yuertusi formation source rock, which was deposited in an anoxic-sulfur slow-slope terrestrial shelf environment, with the parent material type being mainly a mixture of benthic and planktonic algae and its maturity Ro ranging from 1.4% to 1.7% [11], which finding is generally consistent with the maturity Rc value of the studied oil samples (1.48-1.58%). The \(\delta^{13}\)C values (\(-\)31.3-32.4%) of the crude oils in this study are consistent with those of the crude oil (\(-\)32.0%) and the source rocks (\(-\)30.5-32.50%) from the LT1 well [82], indicating that the studied oil samples have a strong affinity with the Lower Cambrian Yuertusi formation source rocks. Moreover, it has been shown that the Lower Cambrian Yuertusi formation source rocks are widely distributed in the eastern and central Tarim Basin, with a thickness of about 10-25 m; they cover an area of more than 260,000 km\({}^{2}\), with a high TOC content of 2-6% [30]. The thermal evolution of hydrocarbon source rocks reveals that the lower Cambrian hydrocarbon source rocks in the SLU region area are still at the stage of producing highly mature liquid oil and condensate in a \"large burial depth and high pressure\" environment [83]. These findings all suggest that the studied oil samples may have been derived from the in situ Lower Cambrian Yuertusi formation source rocks. Furthermore, the trend of the maturity calculated from the oil samples in this study versus the burial depth of the Ordovician reservoir (Figure 13) may suggest that the crude oils in the southwestern part of the F1 area are slightly more mature than those in the northeastern part. Generally, higher-maturity oils are closer to the source kitchen or initial charging points than lower-maturity oils [77]; thus, the southwestern area of the F1 zone is closer to the source kitchen. Furthermore, according to the lower crude oil maturity and the lower gas drying coefficient of the well SL1, far from the No. 1 fault, it can be deduced that the oil and gas migration is dominated by vertical migration along the No. 1 Fault. Based on the migration fractionation effect, the content of low molecular hydrocarbons (such as methane) would increase with the increase in the migration distance when lateral migration occurs [84]. In this study, the drying coefficient of natural gas in the southwest region of the study area is lower than those in the northeast (Table 2), indicating the possibility of gas migration from the southwest to the northeast and the northeast-high and southwest-low shape of the Ordovician reservoir, providing conditions for the lateral migration of natural gas from southwest to northeast, also supports this view. In conclusion, a comprehensive analysis of the maturity of crude oils and the dryness coefficient of natural gas in the study area concludes that the hydrocarbons in the present study mainly migrate vertically from the in situ Lower Cambrian Yuertusi formation source rocks to the Ordovician reservoirs, followed by a certain degree of lateral migration from southwest to northeast.