## 2 Geological Setting  
The Junggar Basin, located in the northwest of Xinjiang, China (Figure 1A), is a multi-cycle, superimposed basin surrounded by folded mountains. It evolved from the Carboniferous to the Quaternary and spans an area of 130,000 km\({}^{2}\) (Figure 1B) [38; 39]. The Junggar Basin is bounded by the Algarti and Zaire Mountains to the northwest, the Klameli and Qinggelidi Mountains to the northeast, and the Bogda and Irinhebilgen Mountains to the south [40; 41]. Based on the current structural morphology and characteristics of the underlying basement, the basin can be divided into six structural units. The study area is located in the southeastern part of the Shawan Sag, within the central depression of the basin. Structurally, it is positioned between the southern Mosowan Uplift, the northern Monan Uplift, and the southeastern Zhongguai and Dabasong Uplifts. The gently dipping strata form a southward-inclined monocline structure, creating favorable structural conditions for hydrocarbon accumulation in the region (Figure 1B-D).  
Figure 1: (**A**) The location of the Junggar Basin in China; (**B,C**) tectonic units and the study area therein in the Junggar Basin (modified after [36]). (**D**) The location of the wells in the Yongjin area.  
During the Late Jurassic, the Yanshan Movement triggered the gradual formation and subsequent emergence of the Che-Mo paleouplift, which reached its peak development by the end of the period. This tectonic event led to extensive erosion of Middle-Upper Jurassic strata in the Yongjin area, resulting in the absence of the Kalazha and Toutunhe Formations [37]. The Qigu Formation is 60-110 m thick and exhibits an unconformable contact with both the overlying Cretaceous Qingshuihe Formation and the underlying Xishanyao Formation (Figure 2A,B). The Qigu Formation is subdivided into two members, from bottom to top, based on the lithological cyclicity, well-log characteristics, and INPEFA curve patterns. Member 1, the main oil-producing zone, is primarily composed of brownish-red and gray pebbly coarse-to-medium sandstones, medium-to-fine sandstones, and fine-grained sandstones (Figure 2B). Member 2 consists of thick sequences of brownish-red mudstone interbedded with thin layers of siltstone (Figure 2B). During the sedimentary period of the Qigu Formation, the alternating appearance of purplish-red and gray rocks may record fluctuations in the paleoclimate between humid and arid conditions. Paleontological evidence reveals abundant fern and spore-pollen fossils in the Xishanyao Formation, whereas the Qigu Formation contains scarce spore-pollen fossils [33]. This distribution suggests reduced weathering and leaching in the Qigu Formation, indicating that an arid paleoclimate prevailed during its deposition [33; 35]. The widespread development of purplish-red mudstones further indicates the persistence of arid conditions. Against this combined tectonic and climatic backdrop, the sediment provenance was primarily influenced by proximal sources from the Che-Mo paleouplift and distal sources from the Zaire Mountains to the north [42], collectively forming a shallow-water braided delta system shaped by the interplay of sediment supply, climate, and paleotopography [36].  
Figure 2: (**A**) Development characteristics of the Jurassic-Cretaceous strata in the Yongjin area of the Junggar Basin (Modified after [31; 43; 44]). (**B**) Stratigraphy of the Qigu Formation of well Y12.