## 2 Geological Background  
The Junggar Basin, China, is a crucial terrestrial oil and gas basin characterized by its vast size. The Mahu Oilfield lies within this basin, and it is the world's largest glutenite oilfield [40; 41; 42]. Throughout its formation, the basin has experienced multiple stages of tectonic movements, including the Hercynian, Indosinian, Yanshan, and Himalayan orogenies, resulting in a complex regional structure. The northwestern margin of the Junggar Basin exemplifies a typical compressional basin margin. During the early Triassic period, the basin inherited the impacts of intense collisions and compressions from the late Permian period. This gave rise to a substantial difference in structural elevation at the junction between the basin and the adjacent mountain range, ensuring an ample supply of sediment. Additionally, due to the prevalence of a subtropical arid climate, extensive alluvial fan groups have developed. In the Middle Triassic, the basin structure stabilized, resulting in a gradual reduction in terrain elevation differences. The climate in this period became more humid, which resulted in the formation of a gentle slope and wide basin with shallow water along the basin's edge, creating a favorable environment for the development of a fan delta-lake sedimentary system [43; 44; 45]. Due to the continuous supply of sediments, the Upper Karamay Formation witnessed the development of shallow-water fan-delta sedimentary bodies predominantly composed of coarse debris under frequent fluctuations in lake levels. The study area under investigation is the Bai 21 area of the Baikouquan Oilfield, situated in the northwest margin of the Jungeer Basin. It is approximately 70 km east of Karamay City and lies in the footwall of the Baikouquan section of the Ke-Wu fault zone. The Bai 21 area covers an extent of ~70 km\({}^{2}\); it is adjacent to Zaire Mountain and Hala'alate Mountain in the northwest and the Mahu Depression to the east. The study area spans approximately 9 km in the east-west direction and 8 km in the north-south direction. The updip direction of the study area is intersected and sealed off by two contemporaneous sedimentary overthrust faults, namely the Ke-Wu fault and the Bai 19 well crack, which controlled the deposition of the whole Triassic. The southeast boundary is defined by the presence of edge water, and the study area gradually transitions into a gentle monoclinic toward the basin's center. The dip angle of the stratum is approximately 3\({}^{\circ}\)-5\({}^{\circ}\), indicating a relatively mild structural inclination (Figure 1). The strata in the Bai 21 well area are well developed, with the Triassic system comprising the Baikouquan Formation, Karamay Formation, and Baijiantan Formationfrom bottom to top. The Triassic period in the study area is considered a comprehensive second-order sequence that can be further subdivided into three third-order sequences ([PERSON], 2020). One of these third-order sequences is situated in the middle of the Triassic period within the Upper Karamay Formation, which represents the upper portion of the Middle Triassic Karamay Formation [46]. It is in close contact with the overlying and underlying strata. The Upper Karamay Formation exhibits a complete transgressive-regressive cycle from bottom to top. Six sand groups can be distinguished from bottom to top--R5, S5, S4, S3, S2, and S1 (Figure 2) The Upper Karamay Formation witnessed the development of shallow-water fan-delta sedimentary bodies predominantly composed of coarse debris un der frequent fluctuations in lake levels [47; 48; 49; 50; 51]. R5-S5 represents deposits of the low-stand system tract characterized by shallow-water fan-delta plain debris flow dominated by an oxidation color. The mudstone of the S4 sand group is more pronounced than that of R5 and S5, indicating its affiliation with the lake transgressive system tract deposit.