## 4 Result and Discussion
### Distribution and Evolution of Fan Deltas of S3 Sand Group
#### 4.4.2 Distribution and Evolution of Single-Stage Fan Delta  
(1) Fan formed in the first stage  
During the sedimentary period of S32, the initial stage of the fan body was formed. The typical cross-section (Figure 10b) shows that the fan body is distributed in the NW-SE direction, with a divergent \"high-value strip\" reflection on the plane. The reflection toward the outer edge of the fan body gradually weakens. The front fan-delta sedimentary areas in the eastern and southeastern parts of the study area show a large range of weak reflections. The frequency decomposition RGB fusion results (Figure 10d) show that the study area contains two major fan bodies with thick sand bodies. The southern fan body is extensively developed and is divided into two lobes near well 1392, which is located in the middle of the study area. The continuous branch of the partial flow channel within the fan body is clearly visible. The northern channel gradually spreads eastward in an arched shape, causing deterioration in the continuity of the high-value area on the conventional stratigraphic section, and the seismic reflection gradually weakens toward the east. The river gradually bifurcates into the lake, with the maximum extension distance of the distributary channel being 3.4 km. The main channels of the two major lobes in the central and southern parts of the study area have extension distances of 3.3 and 4.6 km, respectively. On the plane, the S32 fan is divided into the S32-1 fan, controlled by Channel A, and the S32-2 single fan, controlled by Channels B and C (Figure 10d). Channel A exhibits medium-strong brightness with medium thickness, uniform distribution, and minimal  
Figure 10: Seismic characteristics and sedimentary facies map of sublayer. (**a**) Seismic attribute map of S31 sublayer; (**b**) Seismic attribute map of S32 sublayer; (**c**) RGB fusion map of S31 sublayer; (**d**) RGB fusion map of S32 sublayer; (**e**) Sedimentary facies map of S31 sublayer; (**f**) Sedimentary facies map of S32 sublayer. variation; it is scattered in flakes at the end. Channel B has a similar extension distance to that of Channel A, but its thickness changes evenly. Alternatively, Channel C experiences a decrease in thickness after passing through the boundary fault, showing medium-weak brightness characteristics. This information cannot be reflected by a single stratum slice. (2) Fan formed in the second stage  
The second fan body was developed in the S31 area. A typical stral slice shows that two large single fans were developed in each of the two fan bodies. However, compared to the first fan body, the seismic weak-amplitude reflection area of the second fan body decreased, while the development range of the main fan body increased. Additionally, the extension direction of the second fan body and the degree of bifurcation of the distributary channel differ from those of S31 (Figure 10a). The water channel reflection intensity of the southward extension of the S31 fan weakened, with an extension distance of 3.9 km. Along the NNW-SSE direction of the southern channel extension, the near-source area exhibits abnormally strong reflection; the continuity and reflection intensity gradually decreased. The distributary channel bifurcates into three secondary channels, with a maximum extension distance of 4.3 km. The end of the channel is a medium reflection intensity area with continuous distribution. The frequency-divided RGB fusion slice demonstrates an increase in the overall distribution of bright colors, an enhanced development degree of the fan body, and a decrease in the area of dark regions between various fans. This suggests a reduction in the deposition of fine-grained sediments between the fans. Between well 1230 and well 1399, there exists a small transition zone with uneven brightness. Moving outward toward the vicinity of well 1270, there is a region of medium-bright pink intensity followed by a strip of dark sedimentary area. This indicates the thinning of sand body development between different fan bodies (Figure 10c).