# Re-Understanding the Sedimentary Environment of the Wufeng-Longmaxi Shales in the Sichuan Basin
## 3 The Sedimentary Environment of the WLS in the Sichuan Basin
### Characteristics of Organic Matter Enrichment near Land and Far from Water  
The Wufeng-Longmaxi shales in the Weiyuan area of the middle and upper Yangtze region exhibit characteristics similar to American marine shales, displaying a pattern of being closer to land and farther from the water center. Wells W202, W10-2, and W204, located near the semi-closed bay of the central Sichuan ancient land, demonstrate high gas production, each exceeding 16 \(\times\) 10\({}^{4}\) m\({}^{3}\)/d. From the middle Sichuan bulge to the southeast, the thickness of the shales decreases as the water depth increases; for instance, well W205, situated east of Weiyuan, recorded a gas production of only 2.6 \(\times\) 10\({}^{4}\) m\({}^{3}\)/d. On a broader scale, Figure 7 illustrates the shale thickness and TOC distribution of the Wufeng-Longmaxi Formation in Southern Sichuan [78]). This Figure indicates that the most organic-rich areas (TOC > 3.4%) are located closer to the middle Sichuan ancient land  
Figure 6: Schematic diagram of Paleozoic climate, sea level, and fossil development. The temperature is from the literature [68], and the sea level rise and fall is from the literature [69]; the fossil assemblage is from the literature [75]. The Late Ordovician to Early Silurian period represents the third major ice age on Earth. The sea level during the deposition of WLS was notably lower compared to that during the deposition of its overlying and underlying strata, equivalent to a relative marine regression environment during glacial periods.  
(W202, Luxian L203) and the middle Guizhou ancient land (Changing Gas Field), rather than in the depositional center of Luzhou (dark blue area). Based on field observations and seismic profiles, [PERSON] et al. (2023) [79] demonstrated that the graptolite-bearing shales in this region were formed in a shallow and restricted sedimentary environment characterized by relatively shallow water. These shales are interbedded with silt and exhibit typical shallow-water sedimentary structural features such as bioturbation and cross-lamination.  
The characteristics of near-shore and shallow-water environments can be inferred from fossil evidence and specific rock types. In the Wufeng Formation section of the Ordovician Shanwangmiao, located in Hongyuanzi, Tongzi, Southern Guizhou, [PERSON] et al. (2012) [80] observed a distinct directional arrangement of graptolites. They speculated that the hydrodynamic force in the sedimentary environment of the Wufeng Formation was likely influenced by coastal currents [80]. The presence of multiple layers of bentonite and volcaniclastic rocks indicates a shallow-water environment. [PERSON] et al. (2009) [81] suggest that the formation of bentonite is associated with glacial periods. Additionally, we identified bentonite occurrences in various locations, including the Wulong section in Chongqing and the Wangjiawan section in Yichang. The dilution of bentonite, organic  
Figure 7: Thickness and total organic carbon (TOC) plan of sub-segment 1 of the Wufeng–Longmaxi Formation. During the deposition period of WLS, the areas with a high TOC content (>3.4%) were not located near Luzhou, which is the depositional center of the basin, but rather in proximity to ancient uplifts, namely, the Chuanzhong Uplift and the Qianzhong Uplift.  
matter, phosphate, and other materials upon entering the open sea, where they cannot be effectively preserved, further supports the notion of a closed environmental condition during that time.