## 4 Results
### Distribution and Evolution of Sedimentary Facies  
Within a fourth-order sequence stratigraphic framework, the planar distribution characteristics of sedimentary microfacies for different depositional stages were constructed based on single and connected wells facies analyses, combined with the characteristics of sandstone thickness and sand-mud ratio data. During the sedimentary period of the Qigu Formation, the sediment provenance was primarily derived from the Zaire Mountain in the northwestern margin and the Che-Mo paleouplift in the north [35; 42]. During SQ1 to SQ6, the relative base level progressively rose, accompanied by increasing aridity and decreasing sediment supply. The increasing accommodation-to-sediment supply ratio (A/S) promoted the development of a retrograding shallow-water delta system (Figures 11 and 12) [35; 94]. The sedimentary period of SQI coincided with a rising base level, and its distribu-tion range was constrained by paleoscour. The sediments completely infilled and leveled the unconformity surface. At this stage, water depth was at its minimum, sediment supply was abundant, and the accommodation-to-supply ratio (A/S) was below 1. The study area was primarily influenced by northwest sediment sources, resulting in a relatively small-scale delta system. Only one main NW-SE trending main distributary channel developed, exhibiting strong fluvial dominance and considerable channel extension. Subaqueous distributary channels and associated mouth bars were predominantly developed along both sides of the main channel (Figures 11 and 12A). Sheet sands displayed tabular distribution  
Figure 11: Sedimentary microfacies profile map of well-tie in the Qigu Formation (distances between the wells are not to scale). The profile position is shown in Figure 1D. (**A**) Vertical provenance profile. (**B**) Longitudinal provenance profile. patterns. The lithology was coarse-grained, mainly consisting of conglomeratic and pebbly coarse sandstones with poor sorting. During the sedimentary period of SQ2, the base level began to rise, and the sediment supply remained relatively abundant (A/S ratio \(<\) 1). This resulted in a reduction in the delta front area, accompanied by decreased dimensions and a shortened extension of the main distributary channel (Figures 11 and 12B). The proximal delta front remained predominantly fluvial-dominated, with a band-like distribution, while the distal portion was influenced by both fluvial and lacustrine wave processes, resulting in a lobe-like distribution. During this period, fluvial influence gradually weakened, and channels began to bifurcate. Subaqueous distributary channels continuously scoured and incised pre-existing mouth bars, resulting in extensive, interconnected sand bodies. This process typically formed architectural patterns of going over or in the mouth bar, ultimately forming a framework of sand bodies dominated by subaqueous distributary channels and mouth bars (Figures 11 and 12B). The lithology consisted mainly of pebbly fine sandstone and medium to fine-grained sandstone, with moderate sorting. During the sedimentary period of SQ3, the base level rose continuously, accompanied by frequent lake-level fluctuations, with an A/S ratio \(\geq\) 1. The western branch's main distributary channel extended over considerable distances, whereas the eastern branch primarily consisted of subaqueous distributary channels and mouth bars with limited lateral extent (Figures 11 and 12C). Compared with the sedimentary period of SQ2, fluvial influence diminished during SQ3, leading to further reduction in the delta-front area but an increased lateral distribution of sand bodies. During this period, subaqueous distributary channels eroded pre-existing mouth bars, though the erosional intensity was weaker than during SQ2. The framework sandbodies remained dominated by distributary channels and mouth bar deposits (Figures 11 and 12C). The lithology was predominantly composed  
Figure 12: Planar distribution of sedimentary facies in the Qigu Formation.