The study of extraordinarily high organic matter content (EHOMC) is beneficial to promote the fine evaluation of shale oil and gas, but so far, there have been few studies on its mechanism. This paper carried out a comprehensive lithological and geochemical analysis of the black shales of the Wujiaping Formation in the Kaijiang-Liangping Trough. The results showed that the black shales of the Wujiaping Formation can be divided into two units (the upper and lower parts), and EHOMC occurs in its upper part. The redox-sensitive trace elements (RSTEs), Mo_EF-U_EF covariation, and Ni/Co correlation showed that the lower part of Wujiaping Formation was a weakly restricted oxidation environment, while the upper part evolved into a sulfidation reducing environment. The productivity indicator elements (Ba_XS, Ni_XS, Cu_XS, P, and Mo) indicated that the upper Wujiaping Formation had higher primary productivity than the lower part. However, rare earth elements ((La/Yb)_N) indicated that the deposition rate in the lower part of the Wujiaping Formation was higher than that in the upper part. In general, although the oxygenated water in the lower part of the Wujiaping Formation is not conducive to the preservation of organic matter, the high input of nutrients from land sources and the high deposition rate inhibit the decomposition of organic matter, so the lower part has a certain degree of organic matter accumulation. The sulfidation reducing environment and high paleoproductivity are the main reasons for the enrichment of organic matter in the upper part of the Wujiaping Formation. In addition, the sulfidation reducing environment and high paleoproductivity occurred during the violent upwelling at the end of Guadeloupe, a period of high sea level, these factors have jointly contributed to the enrichment of OM in the upper the Wujiaping Formation.

The origin of the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation is complex and controversial. This paper reports the geochemical data of Wufeng-Longmaxi Formations in the Upper Yangtze region to restore the paleoenvironment and explore the accumulation mechanism of organic matter. The total organic carbon (TOC) content of the Wufeng Formation was relatively high, with an average of 2.86%. The Lower Longmaxi Formation showed the highest TOC content, with an average of 3.99%, and the upper part was a continuously low value with an average of 1.22%. The paleoproductivity proxies (Ba_bio, Cu/Al, Ni/Al, Si_excess) showed that in the Katian and Rhuddanian-Aeronian Stages, the Upper Yangtze Sea had high primary productivity, indicating that organic matter accumulation was more affected by terrigenous influx and redox conditions. Al, Zr, and Zr/Al indicated that terrigenous influx was relatively high in the Kaitian-Hirnantian Stages, it was at a constant low in the Rhuddanian Stage, and increased again in the Aeronian Stage. The correlations between redox-sensitive trace elements (Mo, U, V) and TOC indicate that the organic-rich shale of the Wufeng Formation was deposited in the anoxic–euxinic environment. In the Longmaxi Formation, organic-rich shales formed in a more hypoxic environment, and overlying organic-lean shales formed in a suboxic environment. Therefore, the anoxic–euxinic conditions of the Late Ordovician Yangtze Sea were the main reason for the organic matter accumulation, but the high terrigenous influx caused by regression and/or structural controls diluted the organic matter to some extent. For the Early Silurian, a complete transgression–regression cycle changed terrigenous influx and redox conditions, resulting in significant differences in organic matter accumulation.

The enrichment of organic matter in high-quality marine shale is generally controlled by factors such as the redox conditions of sedimentary environments, productivity levels, terrigenous input, and ancient productivity. However, the controlling effect of the sedimentary environment on organic matter enrichment in intracratonic sag is still unclear. This study takes samples from the Qiongzhusi formation shale in southern Sichuan Basin as the research object, focusing on trace elements as well as rare earth elements in different stratigraphic intervals. The provenance of the Qiongzhusi formation shale is mainly terrigenous, with sediment sources mainly consisting of sedimentary rocks and granites. The primary sedimentary environment transitions from a continental margin setting, influenced by rift-related tectonic activity and sediment influx from adjacent landmasses, to an open oceanic environment characterized by mid-ocean ridge processes and oceanic plate subduction zones. During sedimentation, saline water was present, with predominant sedimentary environments ranging from shallow water to deep water continental shelves. The shale in the study area is characterized by a higher content of silicates and a lower content of carbonate minerals. Its siliceous sources are mainly influenced by biogenic and terrigenous debris, indicating higher ancient primary productivity and representing a favorable target for shale gas exploration.

This destruction mechanism of organic matter enrichment plays an important role in improving the mudstone sedimentary structure system. For the lower Silurian marine mudstone in the western South China Block, the destruction mechanism of organic matter enrichment is unclear, with previous studies ignoring the control from paleogeography and spatial differences. Lamination is a typical sedimentary structure in mudstone and different types of lamination in mudstones can reflect a range of depositional environments. Lenticular lamination, composed of arranged lenses of variable composition, is a common type of lamination in marine mudstone and mainly occurs in organic-lean mudstone. The origin of lenticular lamination remains controversial, and a multidisciplinary approach is needed that integrates sedimentology, petrography, and paleogeography. The main objectives of this approach are to (1) interpret the formation mechanism of lenticular laminations in the Longmaxi marine mudstone, and (2) discuss the significance of lenticular lamination to organic matter destruction. The results not only allow for a complete understanding of the destruction mechanisms of the Silurian marine mudstones but also provide new insights into the sedimentary processes of marine mudstone.