Relationship Between Paleo-Depositional Environment Evolution and Organic Matter Enrichment in Shales Based on Mineralogical–Petrographic and Geochemical Analyses: A Case Study of the Lower Cambrian Qiongzhusi Formation in the Western Hunan–Hubei and Northern Guizhou Region

The Lower Paleozoic Qiongzhusi Formation is an important next target for shale gas exploration and development in South China, following the Wufeng–Longmaxi Formation. The total organic carbon (TOC) content is a key indicator of shale gas enrichment; however, systematic comparative studies of the factors controlling the organic matter enrichment of the Qiongzhusi Formation shales across the western Hunan–Hubei and northern Guizhou region are lacking. In this study, we compared the sedimentological and geochemical characteristics of the Qiongzhusi Formation shale deposited at the passive continental margin versus within the cratonic depression in the western Hunan–Hubei and northern Guizhou region and explored the main controlling factors and formation models of the differential organic matter enrichment. The results show that the shale intervals with high TOC values are located at the base of the Qiongzhusi Formation and were formed in environments characterized by low terrigenous input, high paleoproductivity, and favorable preservation conditions. Subsequently, the basin rifting gradually weakened, and the sea level continuously fell, which manifested as increased terrigenous input and deteriorating preservation conditions. In addition, the ocean current activity weakened as the sea level fell, resulting in a corresponding decline in the paleoproductivity. Therefore, the vertical decrease in the TOC content of the Qiongzhusi Formation was jointly controlled by the reduced paleoproductivity, poorer preservation conditions, and increased terrigenous input. Laterally, from the passive continental margin toward the interior of the cratonic depression, both the sea level and the intensity of the oceanic circulation exhibited decreasing trends, accompanied by simultaneous deterioration of the preservation conditions and paleoproductivity, resulting in progressively lower TOC contents.