## 4 Results
### Lithofacies Analysis
#### 4.3.2 Lithofacies Associations  
Five lithofacies associations were identified, corresponding to shallow-water delta front deposits. These include the main distributary channel, subaqueous distributary channel, mouth bar, channel and mouth bar complexes, and subaqueous interdistributary mud lithofacies associations (Figures 7 and 8). LA1: Main Distributary Channel Deposits  
Description. This lithofacies association was mainly composed of light grey and brownish-red, vertically stacked, thick-bedded, fine-to coarse-grained sandstones with normally graded bedding. A representative lithofacies succession consisted of lithofacies Cs, Ss, and Spc or Spb from the base upward (Figures 7A and 8A). LA1 exhibited poor sorting with moderate roundness. The erosional basal surface was developed at the base of LA1, with chaotically arranged gravels (Figure 8A). The base of LA1 typically exhibited abrupt contact with gray-black and brown-red mudstones (Figure 8A). Parallel bedding and planar cross-bedding were commonly observed in LA1 (Figure 8A). The grain-size probability curve displayed a saltation-suspension transitional pattern (Figure 9A). The one bed thickness of LA1 ranged from 2.00 m to 8.00 m, and the overall thickness of LA1 was more than 10.00 m.  
Figure 6: Core photographs of the facies documented in the Qigu Formation. (**A**) Structureless conglomerate, Well Y1-1, 5837.90 m. (**B**) Parallel bedded sandstone, Well Y306, 5698.97 m. (**C**) Planar cross-bedded sandstone, Well Y306, 5703.78 m. (**D**) Normally graded sandstone, Well Y306, 5693.39 m. (**E**) Trough cross-bedded sandstone, Well Y306, 5692.59 m. (**F**) Sandstone with climbing ripples, Well Y306, 5700.42 m. (**G**) Structureless sandstone, Well Y306, 5688.74 m. (**H**) Massive siltstone, Well Y304, 5533.20 m. (**I**) Horizontal bedded siltstone, Well Y306, 5687.84 m. (**J**) Rippled cross-bedded siltstone, Well Y304, 5613.75 m. (**K**) Chaotic deposits, Well Y12, 5787.50 m. (**L**) Massive mudstone, Well Y304, 5614.60 m. (**M**) Horizontal bedded mudstone, Well Y304, 5633.90 m.  
Figure 8: Core, lithofacies, and sedimentary sequence characteristics of typical sedimentary facies in the Qigu Formation. (**A**) From 5832 to 5838 m of the Y1-1 well, showing vertical stacking of LA1. (**B**) From 5699 to 5705 m of the Y306 well, showing vertical stacking of LA2. (**C**) From 5624 to 5630 m of the Y304 well, showing vertical stacking of LA3 and LA5. (**D**) From 5642 to 5648 m of the Y302 well, showing vertical stacking of LA4. Figure 7: Lithofacies associations of the delta front depositional system in the Qigu Formation. (**A**) Lithofacies Association 1, main distributary channel deposits. (**B**) Lithofacies Association 2, subaqueous distributary channel deposits. (**C**) Lithofacies Association 3, mouth bar deposits. (**D**) Lithofacies Association 4, channel and mouth bar complexes deposits. (**E**) Lithofacies Association 5, subaqueous interdistributary mud deposits. Interpretation. LA1 was interpreted as the main distributary channel deposits of the delta front [71]. Basal erosional surfaces with poorly sorted, size-heterogeneous gravels characterized subaqueous distributary channel deposits. Cross-bedding and parallel bedding indicate high-energy hydrodynamic conditions and rapid sedimentation rates [53]. The basal erosional surfaces and upward-fining grain size trend indicate significant channel-fill deposits. The upward transition from conglomerate to finer lithologies recorded a complete channel-fill sequence and a systematic transition in flow regime [51,72,73]. Mudstone rip-up clasts at the formation base provide further evidence of the channel transport processes. LA2: Subaqueous Distributary Channel Deposits  
Description. This lithofacies association was mainly composed of light grey and brownish-red, vertically stacked, medium to thick-bedded, silt-to coarse-grained sandstones with normally graded bedding. This vertical succession was characterized by lithofacies Ss, Stc, Spc or Scr, Spb, and Shb from the base upward.