## 2 Regional Geological Background  
The Hailar Basin is situated in southwestern Hulunbuir, Inner Mongolia, extending from the western shore of Hulun Lake and Bayan Hushu to the east as far as the Yimin River, and from Chenbarhu banner in the north to Beier Lake in the south, extending into Mongolia. It forms part of the Central Asia-Mongolia extension-driven basin system, withthe Derbugan Fault marking its boundary. To the west, it belongs to the Erguna fold belt, while to the east, it aligns with the Inner Mongolia-Daxing'anling fold belt. The basin is a superimposed Mesozoic-Cenozoic fault depression developed on a Hercynian fold basement [30; 31], experiencing two major evolutionary stages: rifting and sagging [5; 32]. During the Jurassic, mantle uplift transformed the tectonic regime from a compressional state to an extensional state, accompanied by extensive volcanic activity [8]. In the Early Cretaceous, the basin reached its peak rifting stage, experiencing distinct evolutionary stages: intense extension during the Tongbomiao Formation deposition, rapid extension during the Nantun Formation deposition, stable extension during the Damoguaihe Formation deposition, and overall uplift and contraction during the Yimin Formation deposition. This period was characterized by active faulting, frequent volcanism, an expanding depositional area, and progressively deepening water bodies, leading to the accumulation of thick sedimentary sequences in the rift center. From the Late Cretaceous to the present, the basin entered a sagging stage, undergoing regional compressional uplift [8]. Under nearly east-west compressional stress, the fault basin experienced structural inversion, resulting in the deposition of a fluvial-swamp facies sequence consisting of conglomerates, sandstones, and mudstones.  
The Hailar Basin currently exhibits a structural pattern characterized by two lifted up domains and three depression domains, namely, the Zalainuoeer Depression, Cuogang Uplift, Beier Lake Depression, Bayanshan Uplift, and Huhenuo Depression. The primary structural units are further subdivided into 16 sags and 4 uplifts [5; 32]. The Hongqi Depression is located in the northern part of the Hailar Basin and is classified as a secondary structural unit within the basin. It is an NE-trending half-graben fault depression, with a steep faulted boundary to the north and a gentle slope to the south, forming a \"two sags and one uplift\" structural pattern, with developed southern and northern sub-sags (Figure 1a). The total area of the Hongqi Depression is approximately 840 km\({}^{2}\). Stratigraphically, from bottom to top, the formations present are the Jurassic Tamulangou Formation (J\({}_{3}\)tm), Cretaceous Tongbomiao Formation (K\({}_{1}\)t), Nantun Formation (K\({}_{1}\)n), Damoguaihe Formation (K\({}_{1}\)d), Yimin Formation (K\({}_{1}\)y), Qingyuangang Formation (K\({}_{2}\)q), Paleogene, Neogene, and Quaternary strata (Figure 1b,c). The effective Jurassic hydrocarbon source layer is the Tamulangou Formation.  
Figure 1: (**a**) Structural unit division and well distribution in the Hongqi Depression; (**b**) stratigraphic column; (**c**) structural profile map.