## 5 Discussion  
### Modern Sedimentary Analysis and Comparison in Arid Climates  
To clarify the sedimentary characteristics of shallow-water deltas in arid climates, this study integrated investigations of modern depositional systems with subsurface reservoir analysis. The shallow-water deltas in arid climates differ significantly from their humid counterparts, characterized by intermittent water-level oscillations, frequent lake-level fluctuations, prolonged subaerial exposure, and extreme precipitation events under hot climatic conditions [26; 27; 35; 94; 95]. The shallow-water delta in arid climates of Xiaoertala Lake, Xinjiang, serves as a modern analogue for the Qigu Formation shallow-water delta in the Yongjin area (Figure 13).  
Figure 13: Plane distribution of the shallow-water delta in arid climates of Xiaoertala Lake. (**A**,**B**) Distribution characteristics of the channel-mouth bar complexes in the Xiaoertala Lake delta; (**C**,**D**) characteristics of the crevasse channels-splays; and (**E**,**F**) characteristics of the distributary channel-mouth bar.  
The programation and retrogradation of the Xiaoertala Lake delta are controlled by seasonal fluvial discharge [26; 96]. During flood seasons, rising water levels trigger delta retrogradation, while dry seasons cause rapid water-level decline, leading to the incision of existing distributary channels and sediment transport to basin centers, forming new delta fronts [94]. The Xiaoertala Lake delta exhibited strong fluvial dominance, with each upstream main channel extending from the delta plain into the lake, forming a coherent main channel-distributary channel-mouth bar complex (Figure 13A,B). The upstream channels were well-developed with larger dimensions, while the associated mouth bars exhibited relatively limited growth. Additionally, crevasse channel-splay complexes were present within the system (Figure 13A,B). In contrast, downstream main channels typically bifurcated into multiple distributary channels, with both the channels and associated mouth bars showing greater abundance and larger dimensions (Figure 13C-F). When main channels bifurcate at single or multiple nodes to form distributary systems, significant differences occur in the developmental stage, morphology, hydrodynamic conditions, and dimensions of both the distributary channels and their associated mouth bars. Satellite imagery analysis revealed two distinct morphological types of terminal distributary systems: distributary channel-crevasse spays formed by lateral breaching of channel margins (Figure 13C,D) and distributary channel-mouth bars formed by the direct lake-entry of channels (Figure 13E,F).  
When main channels enter the lake to form distributary-mouth bar systems, they are typically controlled by multiple distributary channels. The lateral amalgamation and vertical stacking of these channel-bar complexes can generate larger-scale sand bodies with enhanced connectivity (Figure 13B,E,F). Comparative analysis demonstrated that the shallow-water delta in the study area shares fundamental architectural elements with the modern Xiaoertala Lake delta analogue. Specifically, a single main channel extends across the delta plain into the lake, successively forming a main channel, crevasse channel-splay, distributary channel, and mouth bar in the direction of the source. However, due to limitations in subsurface data and the similarities between sedimentary microfacies, a detailed distinction and characterization of microfacies (e.g., crevasse spays and crevasse channels) was not feasible.