## 4 Results
### Characteristics of Delta Sedimentary Facies
#### 4.1.1 Delta Front Subfacies  
The delta front subfacies represents the seaward extension of the delta plain, located in the lower region of the wave base. Depositional activities are frequent here. It is the frontal zone where river-borne sediments vigorously interact with the marine environment, forming the main area of sandy deposits in deltas. The notable features of this subfacies include mudstones with a grayish or grayish-green hue, reduced oxidation traces, and abundant pyrite and other deep-water authigenic reducing minerals. Compared to the inland distributary plain subfacies, it exhibits more developed and distinct bedding structures and more prominent lacustrine bedding; the lithology is mainly siltstone, dominated by an interlayered structure of very thin sandstone and mudstone. There is a variety and large number of fossils, particularly freshwater biogenic traces such as mussel shells, ostracod fossils, and abundant plant fragments. Subaqueous distributary channels are thin and narrow, with relatively weak scouring capacity, sandwiched between the delta distributary plain and the pro-delta [29, 30]. This subfacies can be subdivided into five sedimentary microfacies: subaqueous distributary channels, estuary dams, estuary dam margins, sheet sands, and interdistributary bars (Table 1, Figure 7a-e). The following list details the features of different subfacies:  
1. The subaqueous distributary channel microfacies is the continuation of the river system underwater, inheriting characteristics such as positive rhythm deposition and bottom scour surfaces from inland distributary channels. However, due to the influence of multiple factors such as water flow, waves and tides in the depositional environment, it exhibits distinct features: the sediments are generally finer and of higher purity, dominated by siltstones with scattered fine sandstones and very few mudstones, while depositional mutations are mostly located at the top. Additionally, the electrical logging curves show high similarity. The natural potential and natural gamma present either medium-to-high-amplitude bell-shaped or box-shaped patternsor bell-shaped or box-shaped patterns with tooth-like features, and the thickness is reduced. GR, 50-100 API; RLLD/RLLS, 20-60 \(\Omega\)\(\cdot\)m; and AC, 200-230 \(\upmu\)s/m for the subaqueous distributary channel. Core analysis of the Qing 1 Member in Well Hei 60 reveals that the bottom rock strata consist of alternating fine siltstones rich in light brown oil patches and gray siltstones. A typical scour mutation surface exists at the bottom of this interval, with prominent interface characteristics. It is an indicator of the scour surface formed during the deposition process of subaqueous distributary channels. 2. The estuary dam microfacies serves as a distinctive sedimentary body on both sides of subaqueous distributary channels. And it is the zone of the most intense interaction between rivers and marine waters, with the fastest deposition rates. Its sedimentary characteristics are prominent, mainly consisting of well-sorted, sand-pure silt and fine sand, exhibiting a reverse cyclic sequence structure and widely developed cross-bedding. In logging responses, natural gamma and natural potential show medium-to-high-amplitude funnel shapes, which are typical features of estuary dam deposits. GR, 50-100 API; RLLD/RLLS, 20-70 \(\Omega\)\(\cdot\)m; and AC, 200-230 \(\upmu\)s/m for the estuary dam microfacies. Core analysis of the Qing 1 Member in Well Hei 60 indicates that depositional mutations are mostly at the top, with the lithology dominated by siltstone interspersed with sandy mudstone and calcareous argillaceous siltstone. 3. The estuary dam margin microfacies typically exhibits a gradual transition from sandy to muddy sediments. The depositional characteristics are similar to those of the estuary dam, but the thickness is thinner and the lithology is finer than that of the estuary dam.