## 2 Geological Setting  
The Subei Cenozoic Basin in eastern China constitutes a characteristic multi-convex and multi-concave faulted basin system, demonstrating significant tectonic differentiation. Located along the southern margin of the Subei Basin (32\({}^{\circ}\)30\({}^{\prime}\)-33\({}^{\circ}\)00\({}^{\prime}\) N, 119\({}^{\circ}\)20\({}^{\prime}\)-120\({}^{\circ}\)30\({}^{\prime}\)), the Gaoyou Sag study area is structurally bounded by four major basement uplift units: the Wubao High to the east, the Tianchang Uplift and Lingtangqiao Low High to the west, the Tongyang Uplift to the south, and the Zheduo Low Uplift to the north, collectively forming a NE-trending rhombic tectonic framework (Figure 1). This sag extends 100 km longitudinally with a 25-30 km latitudinal width, encompassing a total area of 2670 km\({}^{2}\). Its architecture exhibits a distinctive half-graben configuration characterized by southern fault-bounded subsidence and northern onlap deposition [13, 14]. The Gaoyou Sag represents a classic half-graben rift lacustrine basin, formed during the Late Cretaceous Yizheng Movement and the Late Paleocene Wubao Movement, primarily controlled by differential block subsidence. Within this sag, faulting activity was the principal manifestation of Meso-Cenozoic tectonism, exerting fundamental controls on the basin's configuration, stratigraphic deposition/erosion patterns, and hydrocarbon accumulation. The southern boundary is demarcated by the Zhenwu Fault Zone (Figure 1), comprising Nitric normal faults (Zhen 1, Zhen 2, etc.) with vertical throws exceeding 3500 m. Internally, the Hanliu Fault Zone (800-1200 m throw) subdivides the sag into three structural zones: the southern fault terrace zone, the central deep depression, and the northern gentle slope zone. Additionally, over 650 NE-striking subsidiary faults (density:2.8 faults/km\({}^{2}\)) mechanically associated with basin evolution govern the spatial distribution of depositional microfacies [15; 16]. The sedimentary stratigraphy of the Gaoyou Sag contains primarily the Paleogene to Upper Cretaceous Taizhou Formation, with an accumulated thickness exceeding 7000 m. This stratigraphic column includes, in ascending order: the Upper Cretaceous Taizhou Formation, the Paleocene Fung Formation, the Lower Eocene Dainan Formation, the Middle Eocene Sanduo Formation, and the Miocene-Pliocene Yancheng Formation. The Lower Eocene Dainan Formation is of particular significance due to its hydrocarbon enrichment, attaining maximum thicknesses approaching 2000 m and exhibiting pronounced lithologic cyclicity characterized by coarse-fine-coarse-fine alternations. The lower Dainan Formation (E2d1) consists primarily of deltaic and sublacustrine fan deposits, comprising interbedded gray-black mudstones, sandstones, and pebbly sandstones with a cumulative thickness of \(-\)1000 m, showing an unconformable contact with the underlying Funing Formation. This interval is subdivided into three electrofacies-defined subunits (E\({}_{2}\)d\({}_{1}\),\({}^{3}\), E\({}_{2}\)d\({}_{1}\)\({}^{2}\), and E\({}_{2}\)d\({}_{1}\)\({}^{1}\)) based on lithologic and geophysical log characteristics. The upper Dainan Formation (E\({}_{2}\)d\({}_{2}\)) is dominated by a braided delta with variable intercalations of light gray sandstones, siltstones, and brown to gray-black mudstones (100-900 m thick). This succession has been further partitioned into five subunits (E\({}_{2}\)d\({}_{2}\)\({}^{5}\) to E\({}_{2}\)d\({}_{2}\)\({}^{1}\)) through integrated lithost stratigraphic and electrofacies analysis. Hydrocarbon exploration in the Gaoyou Sag has revealed that the Paleogene Dainan Formation hosts relatively small-scale and thin-bedded sand bodies that frequently form composite hydrocarbon reservoirs through lithologic-fault configurations. Particularly promising exploration targets occur in the central deep depression, where source rocks interface with sandstone in different depositional system tracts.