# Re-Understanding the Sedimentary Environment of the Wufeng-Longmaxi Shales in the Sichuan Basin
## 1 Introduction  
During the Late Ordovician to Early Silurian period in the Sichuan Basin, Wufeng-Longmaxi shales (WLSs) were extensively developed [1]. These shale formations are not only significant for marine shale gas exploration and development in China but are also widely developed globally [1, 2]. Consequently, understanding their sedimentary environment is a fundamental geological issue that has garnered considerable attention, as this holds significant implications for guiding the exploration of marine shale oil and gas. The depositional environment of these two sets of black shales has been the subject of debate for over 50 years, with multiple viewpoints regarding their formation environment. This includes, for example, the black shales developed in epicontemital seas [3], semi-closed shallow-water basins [4, 5, 6], deep oceans [7, 8], semi-deep to relatively deep seas [9], among others. However, there is a general consensus that black shales were formed during periods of warm climates and sea-level rise/transgressions.  
In sequence stratigraphy, the concept that organic matter is readily enriched during periods of maximum marine flooding significantly influences the understanding of organic matter enrichment in marine shales [10, 11]. [PERSON] (1995) [12] was the first to utilize a maximum sea flooding surface to elucidate the phenomenon of organic-matter-rich Wufeng-period shales. Graptolites, radiolarians, and siliceous shales are regarded as fossils typically found in deep-water reducing environments; consequently, many subsequent studies haveposited that Wufeng-Longmaxi shales represent deep-water shelf facies [13; 14; 15; 16; 17]. A sulfidation anoxic event transpired from the late Katian stage to the early Hirnantian stage of the Late Ordovician, during which rising sea levels (transgressions) prompted increased burial of organic carbon [18; 19; 20].  
Based on this logic, organic-rich black shales can be continuously developed within deep-water shelf environments. However, actual shale gas exploration results indicate that, although the majority of the Sichuan Basin is classified as a deep-water shelf environment, only a few confined and relatively small areas at the basin's edge demonstrate high shale-gas production [1]. Consequently, conducting thorough research into the developmental environments of organic-rich shales is of significant scientific importance.  
This study integrates findings from ecology; water salinity; and the paragenetic relationships between shales, coal, and evaporite salt rocks while examining the depositional environments of black shales, oil shales, and coal. Building on this foundation, research was conducted from two aspects: the degree of water body confinement (horizontal dimension) and water depth (vertical dimension). Consequently, this led to the proposition that the conditions of \"Shallow, Terrestrial, Enclosed, and Cold\" are both sufficient and necessary for organic matter enrichment during shale deposition. Specifically, \"Shallow\" refers to environments characterized by shallow water bodies, \"Terrestrial\" implies proximity to land and the influx of terrestrial materials, \"Enclosed\" signifies a relatively confined water body, and \"Cold\" describes a cool and wet climatic condition.