## 5 Discussion
### Thickness and Planar Map of the Caprock  
The thickness and planar distribution of the caprock have a significant impact on its sealing ability. The thicker the caprock and the larger and more stable the planar distribution, the better the sealing performance of the caprock; conversely, the sealing ability of the caprock is poorer [5]. Taking the Well ZS 5 as an example, the lithology of the Cambrian can be divided into four categories: gypsum rock, salt rock, gypsiferous dolomite and gypsiferous mudstone. Among them, gypsum rock can be further divided into dolomitic gypsum and argillaceous gypsum (Figure 6). The logging-response characteristics of each lithology are obvious, with salt rocks having high acoustic waves, low density, and high resistivity, while pure gypsum rocks have low natural-gamma values and high density. Among these four rock types, salt rocks have the strongest sealing ability, due to their extremely low porosity and permeability, as well as strong plasticity. The burial depth of the Middle Cambrian in well ZS 5 exceeds 3300 m, and gypsum is in a plastic state with fewer internal pores, and also has good sealing properties. Due to the interlayer relationship among these four lithologies, this stratum has excellent sealing property for oil and gas.  
The gypsiferous salt lake formed by the deposition of gypsum-salt rocks is located in the central and western part of the basin, with the maximum depth in the middle of the Awat Depression, and the thickness can reach more than 300 m. Through the statistical analysis of the caprock thickness and the caprock-to-stratum ratio of each well exposed in the Cambrian strata, it is known that the layer that can serve as the caprock has the thickest thickness, of up to 376.5 m; the maximum single-layer thickness of the caprock can reach 47.2 m, and, at the same time, the ratio of the caprock-to-stratum thickness is 21.2-86.2%.  
Figure 9: Lithofacies paleogeography map of the Middle Cambrian series in Tarim Basin (modified after [PERSON] et al., 2024 [46]).  
The gypsum-salt rock exhibits strong amplitude, medium-high continuity, and parallel-sub-parallel seismic facies characteristics in seismic profiles (Figures 8 and 9). The distribution of the Middle Cambrian caprock in the western-platform facies area of the platform-margin belt in the Tarim Basin is controlled by the evaporation-platform facies in the sedimentary period. Therefore, tracking the seismic facies of the evaporation platform is helpful for studying the planar distribution of the caprock. The boundary is determined based on the characteristics of high continuity and strong amplitude in the reflection of gypsum-salt rocks, and drilling data are used as constraints in the study area (Figure 7). Then, combined with the lithofacies paleogeographic characteristics of the Middle Cambrian, the thickness distribution of gypsum-salt rock in the study area is obtained (Figure 10). It can be seen that the distribution range of gypsum-salt rock in the study area is relatively wide, mainly concentrated in the northern part of Awati Sag, Tazhong Uplift, the southern part of the Manxi Low Uplift, and the central northern part of the Bachu Uplift. The thickest gypsum-salt rock is located in the central northern part of the Bachu Uplift, reaching over 300 m. The thickness of the gypsum-salt rocks decreases irregularly around the center, which is the part with the highest thickness, and is distributed in a circular pattern. Meanwhile, as shown in Figure 10, the Middle Cambrian gypsum-salt rock has good continuity and stability. It has high breakthrough pressure and low permeability, and is interbedded with gypsum-sulforous dolomite and gypsum-mudstone. The large-area distribution of gypsum-salt rock provides good sealing conditions for the subsalt oil and gas accumulation, making it a high-quality regional caprock.