## 5 Discussion
### Migration and Accumulation Process  
As mentioned above, oil and gas in the eastern Kuqa Depression were derived from Triassic and Jurassic source rocks. However, how oil and gas were expelled and migrated to the present position has not yet been researched. In this study, hydrocarbon migration history was modeled to help interpret this issue (Figure 13).  
Figure 12: Hydrocarbon generation of different lithologic source rocks at different geological times in the eastern Kuqa Depression. The histogram is formed by the cumulative hydrocarbon generation of different lithologic source rocks.  
Figure 13: Hydrocarbon migration conduits and accumulation saturation of the AA\({}^{\prime}\) section at different geological times in the eastern Kuqa Depression.  
From the Jurassic to the Cretaceous, the Triassic and Jurassic source rocks began to expel a little liquid hydrocarbon into the adjacent layers over short distances.  
At around 23.03 Ma, some Triassic oil migrated into the central anticline zone via short vertical migration and long lateral migration through the J1a layer. As no traps were formed at that time, no oil accumulated in the central anticline zone. A small amount of gas was expelled into the adjacent layers over short distances (Figure 13d).  
At around 12 Ma, the Jurassic source rocks began to expel a large amount of oil. In the central anticline zone and the deep sag zone, oil was expelled upward, and some of this oil migrated through faults and lithological carrier beds. Some oil accumulated in faulted traps in the J1a formation. In the northern slope zone, a little oil generated by J1y coal was expelled, and most of the oil accumulated within the source or near the source. During that time, the J1y coal expelled gas into the adjacent layers, and some gas migrated into the central anticline zone and the deep sag zone via long lateral migration through the J1y layers (Figure 13c).  
At around 5.3 Ma, a large amount of oil migrated from source rocks to the adjacent reservoirs through the sandstone transport layer. However, in the northern slope zone, gas was expelled upward, and some of this gas migrated through faults and lithological carrier beds. Some oil and gas accumulated in faulted traps in the J1a and J1y formations in the northern slope zone and the central anticline zone. Some oil generated by source rocks in the northern slope zone migrated southward to the deep sag zone (Figure 13b).  
During the late Pliocene (3-1.8 Ma), uplift occurred in the Kuqa Depression related to the late Himalayan orogen, with more than 1000 m of striatal erosion occurring in most parts of the depression. Therefore, hydrocarbons could not be generated and migrated during that time [4].  
At present, oil and gas accumulations in the Jurassic, the Cretaceous and the Paleogene formations in the depression with the maximum petroleum saturation attaining about 90%. Petroleum saturation of the faulted reservoir in the Jurassic formation is approximately 10-40% (Figure 13a).  
Hydrocarbon saturation in the reservoir accounts for 55.13% of the total, and both the deep sag zone and the northern slope zone account for more than 20%. In the deep sag zone, a large number of hydrocarbons have accumulated in the source, especially in the mudstone, and an amount in the northern slope zone has obviously accumulated in the coal (Figure 14a). In order to reflect the resource abundance of different zones, we normalized the hydrocarbon saturation. It can be seen that, except for the deep sag zone, the hydrocarbon abundance of the other zones is high and has mainly accumulated in reservoirs, especially in the central anticline zone (Figure 14b). In recent years of exploration, oil and gas have also been found in the TG-6 well in the northern slope zone and the YT-1 well in the deep sag zone, but there is no oil and gas reservoir available for exploitation. A large-scale gas field was found in the TD-2 well area of the central anticline zone.