# 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
### The Relationship Between Paleoclimate and Organic Matter Enrichment  
The deposition and enrichment of unconventional oil and gas resources are closely linked to significant geological environmental changes. This phenomenon results from the depositional coupling of global or regional structures with various geological events, including fluctuations in sea (or lake) levels, volcanic activity, climate changes, and biological extinctions [67]. The sedimentary period of the Wufeng-Longmaxi Formation coincided with a time of considerable climate change (Figure 6), occurring during the major glacial period at the end of the Ordovician [68; 69], which corresponds to the third major glacial period and the first mass extinction event in Earth's history. The transition from the Ordovician to the Silurian marked the onset of the Great Ice Age [70,71]. It was previously believed that prior to and following the deposition of the Guanyinqiao section, the water depth resulting from marine transgressions surpassed that of the carbonate platform, leading to the deposition of organic-rich shales. Consequently, it was concluded that these sedimentary rock layers represent deep-water shelf facies, with depths reaching up to 200 m [15,72], as illustrated by the light blue and dark blue blocks in Figure 5. However, this interpretation appears to be inconsistent with the overall uplift trend of the region, making it unlikely that this area could have developed into a water body deeper than a shallow-water carbonate platform. The continuous reduction in sea area indicates that the upper Yangtze region underwent a significant regression phenomenon during the Late Ordovician, a finding corroborated through paleogeographic analyses [73,74]. [PERSON] (1984) [75] observed that the Lingxiang Formation, Wufeng Formation, and Guanyinqiao Bed of the Upper Ordovician illustrate a gradual process of shallowing water bodies (see the right side of Figure 6). It is evident that when the shales of the Wufeng-Longmaxi Formation were deposited, the sea level was low due to the ice age conditions. [PERSON] (2021) [76] even posited that the temperature at the end of the Ordovician was lower than that during the Carboniferous-Permian. Figure 5: Upper Yangtze and Paleogeographic Plan at the end of the Ordovician (modified from [PERSON] et al. (2017) [15]). The area of shale gas fields (filled with red) with a high gas-production capacity is quite limited, although most of the Sichuan Basin is classified as a deep-water shelf environment (deep blue area). The climatic characteristics from the Proterozoic to the late Paleozoic exhibit a strong correlation with the enrichment of organic matter. For instance, the end of the Ordovician glacial period corresponds to Wufeng-Longmaxi shales, which represent the only large-scale commercial marine-shale development in China. Furthermore, the significant glacial period at the end of the Carboniferous correlates with the most substantial reserves of the Tianyuan-Shanxi Formation, encompassing marine and continental transitional coal measures and shales from the Paleozoic era. Notably, the largest area for mining these coal measures and shales is found in the Appalachian coal field, the largest in the world. Through an analysis of these correlations, it can be inferred that the primary factors include a decrease in sea level, an increase in land area, an expansion of marshes and closed aquatic environments, and the preservation of organic matter. It is estimated that a 100 m decline in sea level would result in an approximate 13.38% increase in the global land area. [PERSON]'s team utilized results from ocean drilling and concluded that the rate of global seafloor organic carbon burial during the Neocene was lower in the Middle Miocene [77], suggesting a significant negative correlation between carbon burial in the Miocene and global temperature.