# Re-Understanding the Sedimentary Environment of the Wufeng-Longmaxi Shales in the Sichuan Basin
## 4 Conclusions  
1. A shallow-water environment is a crucial condition for the enrichment of marine shale gas. Closed lagoons or semi-closed bays represent stagnant low-energy environments that facilitate the accumulation of organic matter. 2. The abundance of organic matter in shales exhibits a negative correlation with their distance from land. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Sample** & **Small-Layer Name** & **V** & **Cr** & **Ni** & **Sr** & **Ba** & **Th** & **U** & **Sr/Ba** & **V/Cr** & **U/Th** &
\begin{tabular}{c} **Salinity** \\ **Level** \\ \end{tabular} \\ \hline S1 & Longmaxi 5 & 288.10 & 82.27 & 117.96 & 160.16 & 1326 & 16.34 & 10.44 & 0.12 & 3.50 & 0.64 & Low \\ S2 & Longmaxi 5 & 306.28 & 68.28 & 112.20 & 206.36 & 1401 & 12.95 & 13.50 & 0.15 & 4.49 & 1.04 & Low \\ S3 & Longmaxi 4 & 585.89 & 57.54 & 57.61 & 56.97 & 1248 & 6.59 & 7.41 & 0.05 & 10.18 & 1.13 & Low \\ S4 & Longmaxi 3 & 584.49 & 57.23 & 76.50 & 65.70 & 1312 & 6.33 & 8.47 & 0.05 & 10.21 & 1.34 & Low \\ S5 & Longmaxi 3 & 497.57 & 46.35 & 74.87 & 42.98 & 1153 & 5.42 & 7.06 & 0.04 & 10.74 & 1.30 & Low \\ S6 & Longmaxi 2 & 815.97 & 66.27 & 82.89 & 55.10 & 1197 & 9.93 & 15.71 & 0.05 & 12.31 & 1.58 & Low \\ G2 & Guanyingiao & 487.21 & 28.95 & 41.75 & 432.58 & 1368 & 5.75 & 7.34 & 0.32 & 16.83 & 1.28 & Medium \\ G1 & Guanyingiao & 69.77 & 56.80 & 39.05 & 616.19 & 2735 & 7.46 & 1.93 & 0.23 & 1.23 & 0.26 & Medium \\ W7 & Wurfeng 1 & 220.18 & 43.63 & 30.14 & 26.35 & 999 & 4.25 & 6.62 & 0.03 & 5.05 & 1.56 & Low \\ W8 & Wufeng 1 & 240.03 & 128.94 & 45.21 & 41.48 & 1221 & 15.00 & 7.13 & 0.03 & 1.86 & 0.48 & Low \\ L10 & Baota 1 & 48.51 & 26.98 & 21.55 & 442.95 & 686 & 10.58 & 0.67 & 0.65 & 1.80 & 0.06 & Medium \\ L11 & Baota 2 & 33.54 & 17.36 & 19.14 & 324.26 & 334 & 4.39 & 0.64 & 0.97 & 1.93 & 0.15 & Medium \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Trace elements in the Wufeng–Longmaxi shale profile of Huangying, Wulong, and Chongqing (\(\mu\)g/g). Figure 8: Vertical changes in lithology and salinity of the Middle Upper Ordovician–Lower Silurian. The period during which the WLS developed is characterized by a lower salinity environment in comparison to Late Ordovician limestone formations. This suggests a certain degree of isolation of the Sichuan Basin from the open sea during this time, coupled with the influence of freshwater input from surrounding ancient uplifts [84]. 3. Numerous shale oil and gas fields, including those in the Weiyuan and Changning areas, exhibit characteristics of organic matter enrichment that occur closer to land and farther from the water center. These fields share a common metallogenic model, known as the horsetail model. Additionally, the condensation period of maximum flooding is not conducive to the enrichment of organic matter. 4. The dissolved oxygen levels in water do not consistently decrease with an increasing depth. The assertion that \"the deeper the water body, the stronger the reducibility\" is incorrect. 5. The marine shales of the Wufeng-Longmaxi Formation in the middle to upper Yangtze region developed during a marine regression phase, characterized by a relatively tranquil, enclosed to semi-enclosed, highly compartmentalized, and shallow lagoon environment. 6. The depositional environment of organic matter-rich shales is characterized by three primary features: closure, shallow depth, and continental conditions. These features indicate strong closure, the presence of a relatively shallow water body, and proximity to land and terrestrial sources. Writing--original draft preparation, X.M. ; writing--review and editing, F.Y. ; visualization, X.C. ; supervision, S.L. ; project administration, Z.L. ; funding acquisition, Y.Y. All authors have read and agreed to the published version of the manuscript. The funder is \"State Energy Key Laboratory for Carbonate Oil and Gas\"; funding number is 33550000-24-ZC0613-0105. Not applicable.