## 3 Materials and Methods
### Sedimentary Environment Analysis  
In terms of biological activity, the core samples from the X51-X42 layers lack both biotic and trace fossils, indicating that the sedimentary environment of these two layers belongs to a terrestrial setting. In contrast, the X41 layer core contains numerous burrows, with vertical and oblique holes distributed chaotically. These are frequently developed in the mudstone section, suggesting that this layer was deposited in a short-lived aquatic environment, specifically in a shallow water area on the fan edge. The sedimentation process of this layer is controlled by flood events (Figure 2a). The sediment color mainly consists of brownish-red and reddish-brown, indicating a high content of iron oxides or hydroxides, which confirms that the sedimentary medium was oxidizing or under strongly oxidizing conditions. Red sediments are typically found in terrestrial or coastal transition sediments under hot and humid climatic conditions, and the color of the sediment is a typical feature of exposed sediment (Figure 2b). Regarding sediment grain size, fine sandstone and siltstone are dominant, with coarse sandstone being rare. Large, angular to subrounded mud clasts can be observed at the bottom and middle of the thin sandstone layers, indicating that the sediment is fine-grained and derived from distant sources. These characteristics are typical of fan-edge deposits (Figure 2c,d). Sedimentary structures include low-angle to near-horizontal cross-bedding and ripple lamination in the sandstone-siltstone section, suggesting a lack of large river deposits during the sedimentation process. In the mudstone-conglomerate section, blocky and graded bedding are dominant, with individual mudstone-conglomerate layers generally less than 30 cm thick, indicating the presence of minor shallow channels in the strata (Figure 2e,f). The rhythmical features of the sand bodies are primarily anti-rhythmic and homogenously rhythmic, which suggests that the sedimentary environment was not underwater but was the product of progradational body deposition. Figure 2: Core photographs of the Xihepu Formation, Kekeya Gas-Condensate Field. (**a**) Bioturbation structures in X41 member core; (**b**) Brown-red to reddish-brown core; (**c,d**) Fine sandstone and siltstone cores; (**e**) Low-angle cross-bedding; (**f**) Horizontal bedding. The X4 section is primarily composed of fine sandstone, with lithology consisting of thick to very thick layers of light brown, light brownish-yellow muddy sandstone, siltstone, fine sandstone, brown sandy mudstone, and mudstone. The division of this section is based on the 25m thick noticeable cap layer at the top, with multiple sand lithologic and electrical cyclic characteristics below. The sorting and rounding are good, with high compositional maturity and structural layer maturity. The bedding is well-developed, primarily parallel bedding and inclined bedding, with the sandstone layers generally thin at the top and thicker at the bottom. There is strong heterogeneity. The interbedded fine sandstone, muddy siltstone, and sandy mudstone are characteristic of fan-edge deposits in alluvial fan systems. The natural gamma ray curve exhibits box-shaped, funnel-shaped, and bell-shaped composite rhythmic sedimentation with both positive and negative rhythms. The X5 section is mainly composed of thick, homogeneous sand bodies, with lithology consisting of thick layers of brownish-brown and brown fine sandstone, siltstone, muddy siltstone, gray-green conglomerate, and medium-thick layers of brown sandy mudstone interbedded with thin mudstone layers. The division of this section is based on the 20-m-thick cap layer at the top. The section primarily consists of fine sandstone, with good sorting and rounding and high compositional and structural maturity. Bedding is well-developed, with parallel bedding and inclined bedding as the main types, and the thickness is greater, with weaker heterogeneity. This section is divided into two cycles.