## 4 Results
### Lithofacies Analysis
#### 4.3.2 Lithofacies Associations
LA2 exhibited moderate sorting with moderate roundness (Figures 7B and 8B). The erosional basal surface at the base of LA2 was relatively small and flat, with occasional gravel clasts observed (Figure 8B). Planar cross-bedding, massive bedding, and trough cross-bedding were commonly observed in medium- to coarse-grained sandstones at the base of the succession, whereas ripple bedding and horizontal bedding were occasionally found in siltstones or muddy siltstones at the top (Figure 8B). Mud stripes and mudstone rip-up clasts were also observed within this lithofacies association (Figure 8B). The grain-size probability curve exhibited a transitional pattern between saltation and suspension (Figure 9B). The one bed thickness of LA2 ranged from 0.50 m to 4.00 m and the overall thickness of LA2 was more than 5.00 m.  
Interpretation. LA2 was interpreted as subaqueous distributary channel deposits of the delta front [74,75]. The grain size of LA2 was generally finer than that of LA1. The brownish-red mudstones at both the top and base of LA2 indicate subaerial exposure under oxidizing conditions [76,77]. The moderately flat erosional base and normal grading further characterized the channel-fill deposits. Planar cross-bedding, trough cross-bedding, and parallel bedding are indicative of high-energy hydrodynamic environments [53]. Climbing ripples bedding is likely formed during rapid deposition in the transition from supercritical to subcritical flow and is commonly observed in subaqueous distributary channels [78]. The upward-fining trend reflects a progressive decrease in flow velocity and hydrodynamic energy. Figure 9: Characteristics of the grain-size cumulative probability curve in the Qigu Formation. (**A**) Saltation-suspension transitional pattern, Well Y306, 5686.65 m. (**B**) Saltation-suspension transitional pattern, Well Y306, 5699.74 m. (**C**) “two saltation-one suspension” pattern, Well Y1-1, 5838.1 m.  
L43: Mouth Bar Deposits  
Description. This lithofacies association primarily consisted of light grey and brownish-red, vertically stacked, thin to thick-bedded, silt-to fine-grained sandstones with inverse-graded bedding. This vertical succession was characterized by lithofacies Shb, Src, Spb, and Ss from the base upward (Figures 7C and 8C). LA3 exhibited moderately good sorting with moderate roundness. It showed a coarsening-upward succession, transitioning from muddy siltstone at the base to fine sandstone at the top, with an inverse grading pattern (Figures 7C and 8C). Lithofacies Ss, Src, and Spb dominated this association, with horizontal and ripple bedding commonly developed in the lower intervals (Figure 8C). The grain-size cumulative probability curve exhibited a \"two saltation-one suspension\" pattern (Figure 9C). The one bed thickness of LA3 ranged from 0.50 m to 2.00 m, and the overall thickness of LA3 was more than 4.00 m.  
Interpretation. LA3 was interpreted as mouth bar deposits of the delta front, formed by fluvial-derived sand deposition at river mouths due to flow velocity deceleration [79]. Coarsening-upward trends and inverse grading are typical features of mouth bar deposits [80,81]. Wave bedding reflects a reduction in hydrodynamic energy and flow velocity [82]. The coarse-grained top of mouth bar sandbodies in this study are often eroded by strong fluvial processes, resulting in indistinct inverse grading, and preserved as relict mouth bar deposits. L44: Channel and Mouth Bar Complexes Deposits  
Description. This lithofacies association primarily consisted of light grey and brownish-red, vertically stacked, medium-to thick-bedded, fine-to medium-grained sandstones showing a transition from inverse to normal grading. This vertical succession was characterized by lithofacies Sm, Src, Spc (or Stc), and Spb from the base upward (Figures 7D and 8D). Ripple and wave bedding were predominantly developed in the siltstone or argillaceous siltstone of the lower LA4 unit, similar to those observed in LA2 (Figure 8D).