## 4 Sequence Framework  
The establishment of a robust sequence stratigraphic framework hinges on the integrated interpretation of geological and geophysical datasets to precisely delineate sequence boundaries and system tract interfaces. Specifically, three diagnostic markers: (1) seismic reflection termination patterns, (2) sedimentary lithofacies associations, and (3) well log facies signatures--have demonstrated superior reliability and operational feasibility in stratigraphic subdivision within continental sedimentary basins [1].  
In this investigation of the Dainan Formation within the Gaoyou Sag, we systematically analyzed the third-order sequence boundaries and system tract interfaces through a detailed characterization of seismic event termination patterns (including truncation, onlap, downlap, and toplap configurations). A critical stratigraphic discovery reveals that along the Hanliu Fault Terrace Zone: (1) the initial coastal onlap point corresponds to the first flooding surface (FFS) controlled by the Hanliu Fault, while (2) the most distal coastal onlap point marks the maximum flooding surface (MFS) (Figure 2).  
Based on the constraints of seismic sequence boundaries, this study integrates core data characteristics including paleosols, basal conglomerates, and abrupt sedimentary facies changes. Through a comprehensive analysis of the lithological properties and electrofacies curve mutation interfaces of the Dainan Formation, combined with the identification of parasequence set stacking patterns (including retrogradational, progradational, aggradational, symmetrical retrogradation-progradation, and asymmetrical retrogradation-progradation types), we systematically determined sequence and system tract boundaries. The stratigraphic framework was ultimately established through well-seismic integration using synthetic seismogram calibration from 20 boreholes, achieving high-resolution sequence stratigraphic division. This integrated approach successfully constructed a robust sequence stratigraphic framework for the Dainan Formation, demonstrating remarkable consistency between well log responses and seismic reflection configurations.  
Through a comprehensive integration of core analysis, well logging, and seismic interpretation, three third-order sequences (SQ1-SQ3) and eight system tracts were identified within the Paleogene Dainan Formation of the Gaoyou Sag, specifically encompassing the E\({}_{2}\)d\({}_{1}\)\({}^{1}\), E\({}_{2}\)d\({}_{1}\)\({}^{2}\), and E\({}_{2}\)d\({}_{1}\)\({}^{3}\) sub-members and the lower E\({}_{2}\)d\({}_{2}\)\({}^{5}\) interval. The stratigraphic architecture demonstrates a systematic progression (Figure 2). SQ1 (corresponding to E\({}_{2}\)d\({}_{1}\)\({}^{3}\) and lower E\({}_{2}\)d\({}_{1}\)\({}^{2}\)) comprises lowstand (LST), transgressive (TST), and highstand (HST) system tracts. SQ2 (upper E\({}_{2}\)d\({}_{1}\)\({}^{2}\) to E\({}_{2}\)d\({}_{1}\)\({}^{1}\)) contains LST and TST assemblages. SQ3 (equivalent to the Dainan Formation's second member) exhibits a complete LST-TST-HST cyclicity (Figure 2). Spatio-temporal analysis reveals distinct lithofacies distributions controlled by structural dynamics. The LST deposits, constrained by dual tectonic controls from the Hanliu Fault-induced slope break zone and the southern Zhenwu Fault system, manifest as  
Figure 2: Sequence stratigraphic framework and their seismic reflections in the Dainan Formation of the Gaoyou Sag (line 250).  
heterolithic success of light gray sandstone/mudstone of variable thickness. In contrast, the TST and HST units demonstrate basin-wide continuity, characterized by dark gray mudstone interbedded with light gray sandstone, reflecting stable depositional conditions during maximum flooding.