# Re-Understanding the Sedimentary Environment of the Wufeng-Longmaxi Shales in the Sichuan Basin
## 2 Evidence That Marine Source Rocks Developed in a Shallow-Water Restricted Environment
### An Analogy with the Mineralization Model of Shale Oil and Coal  
Shale gas, shale oil, and coal share analogous metallogenic models, indicating that the organic matter in question possesses the characteristics of being \"near land and far from water\". Coal forms in stagnant, low-energy, low-lying swamp environments [55]. The developmental morphology of coal seams exhibits a lenticular shape, extending from the margins of a lake basin toward its center. The thickness of these coal seams increases as they approach the basin margins. In most basins, coal forms in a horsetail-like pattern, as illustrated in Figure 3, characterized by multilayered, extremely thick coal seams at the landward end, which taper to pinch-outs toward the center of the lake basin [56]. This pattern is exemplified by the Shaerhu Coalfield, located on the southern margin of the Turpan-Hami Basin, where a single coal seam reaches a thickness of 276 m [57]. During periods of water regression, peat swamps migrate toward the water body, leading to the formation of coal seams. Conversely, during water transgressions, peat swamps shift in the direction of the landmass, resulting in a \"near-land, far-water\" horsetail pattern of coal seams. Therefore, the development of a set of coal seams or coal lines indicates that the basin exists in a local low-lying, low-energy, and low-water environment. During this period, coal seams serve as environmental indicators; their presence signifies a corresponding swampy environment characterized by shallow water bodies. A similar phenomenon is observed with shale oil. Extensive exploration has confirmed that shale oil is predominantly distributed in shallow water bodies rather than in basin depocenters. Furthermore, the oil content tends to be higher near the edges of a basin [58; 59]. The enrichment of shale gas exhibits a similar horsetail pattern. Figure 4 illustrates the developmental profile of marine shales in the Cretaceous basin of the Western United States [60]. This figure demonstrates that, throughout the evolution of the entire basin, organic-rich shales constituted only a small fraction, while the majority of the shales rapidly filled with terrigenous debris devoid of organic matter. Furthermore, the primary area of shale gas enrichment is not located in the sedimentary center of the basin but rather at its edges. The total organic carbon (TOC) and biogenic quartz increase toward the continental margin [61; 62], while clay and debris accumulate toward the center of the basin, following the same metallogenic model as coal, namely, the horsetail model. Therefore, the presence of high-quality hydrocarbon source rocks and coal serves as a reliable indicator of depositional facies, suggesting their formation in stagnant, low-energy shallow-water environments at the margins of sedimentary basins, such as lagoons, rather than in open water environments like shallow-water shelves or deep-water shelves. A contradiction arises between high sea levels and the enclosed nature of a water body. As the water level rises, previously restricted embayments or entirely enclosed lagoons become fully submerged, achieving unobstructed connectivity with the open ocean, which leads to water bodies trending towards normal salinity. Furthermore, the condensed sections or intensive intervals during the maximum flooding surface period are notably deficient in organic matter. Figure 4: Developmental profile of Cretaceous shales in the Western United States (modified from [PERSON] et al. (2017) [59]). The distributional characteristics of thickness and TOC in black shales exhibit similarities to those of coal. As one approaches the basin edge, both thickness and TOC values increase. In contrast, as one moves toward the depositional center of the basin, thickness gradually decreases, accompanied by a decline in TOC values. Figure 3: Characteristics of the coal seam profile in the eastern part of the Shahu coalfield in the southern margin of the Tuha Basin (modified from [PERSON] et al. (2011) [57]).