Qian Zhang

Drawing on extensive geological surveys, as well as systematic mineralogical, petrological, and geochemical analyses, the study evaluates the provenance, tectonic setting, paleo-redox conditions, and paleoclimate characteristics of the Wufeng Formation black shale in the Fenggang area. The analysis of mineral components reveals that quartz content is notably the highest, suggesting that the shale is primarily siliceous. The TOC content was highest in the YI Outcrop, ranging from 2.19 to 5.56, with an average of 3.23, followed by the SX Outcrop. Redox-sensitive indices including Mo_EF, U_EF, V/Cr, and U/Th exhibit considerable variability, indicating significant heterogeneity of the redox conditions, which is primarily characterized as a restricted marine shelf setting. The bottom water of YI Outcrop has the strongest reducing property, and mainly deposited in an anoxic environment. Organic-rich siliceous mudstone is widely distributed across the region. The provenance analysis indicates that the study area is predominantly sourced from felsic igneous rocks. Additionally, paleoclimate and paleo-weathering analyses suggest that the region underwent intense chemical weathering under warm climatic conditions. We found that the sedimentary environment exhibits pronounced spatial variability. In the northern part of the study area, water conditions were deeper, anoxic, and reducing. Toward the east, water depth gradually decreased, transitioning to weakly oxidizing and suboxic conditions. Furthermore, significant tectonic activity in the region led to the formation of multiple underwater highs, indicative of an active continental margin likely associated with the rapid uplift of paleo-uplifts. The formation of organic-rich shale was primarily influenced by two factors: favorable preservation conditions in reducing water bodies and high primary productivity driven by biological activity in weakly oxic environments.

To further study the sedimentary environment of the black mudstone in the early Carboniferous Dawuba Formation in the Middle and Upper Yangtze regions and support regional shale gas exploration and related research, the major and trace elements of the Dawuba Formation in Well CY1, located in deep water shelf facies, were tested and analyzed. The results show that the study area contains mainly continental margin deposits affected by hydrothermal deposition, and they are rich in organic matter and have high primary productivity. The parent rocks are mainly acidic rocks, such as felsic igneous rocks, granites and some sedimentary rocks. And the provenance is mainly provided by acidic igneous rocks of the Jiangnan Paleouplift. An ICV<1 and high CIA and Th/U values indicate a warm and humid climate and under stong chemical weathering conditions. The values of V/(V+Ni), Cu/Zn and Ce/La suggest that organic-rich intervals of the Dawuba Formation accumulated under predominantly dysoxic conditions. The warm and humid climate is conducive to the flourishing of micropaleontology, and the high primary paleoproductivity and weakly reducing environment are conducive to the formation of organic-rich shale, forming high-quality reservoir source rock in the Dawuba Formation.

This study investigates the provenance and tectonic background of the lower Silurian Longmaxi Formation black shale of the Yanyuan Basin in the western Kangdian ancient land and provides guidance for shale gas exploration and development in the area. The mineral petrological and geochemical characteristics of the Longmaxi Formation black shale in this area have been studied in detail. The study area is mainly a passive continental margin environment, but also has the attributes of an active continental margin and island arc environment due to the collision between the western oceanic crust and the Yangtze continental crust. The source rocks are mainly felsic igneous rocks, with a small contribution of intermediate–basic rocks. It is inferred that the Kangdian ancient land in the eastern part of the area could be the main provenance area, but with the contribution of sediments derived from oceanic island arc located in the west. During the whole Longmaxi period, the tectonic movement was intense, the climate was cold, the degree of chemical weathering was low, and it was a highly volatile geological sedimentary basin. Therefore, the highly siliceous organic-rich mudstone, which was different from the Sichuan Basin, was deposited.

The petrology, mineralogy, elemental geochemistry, biology and PGEs characteristics of the black shale of the Wufeng and Longmaxi Formations in the CN outcrop of the Sichuan Basin are studied in this paper. The distribution and change in PGEs at different horizons are summarized, and the reasons for PGEs enrichment are discussed. The results show that the PGEs in the organic-rich shale segment are obviously enriched, which is positively correlated with the Total Organic Carbon (TOC) content and has a Pt–Pd distribution characteristic. δEu_N, Pd/Pt, (Pt + Pd)/(Os + Ru + Ir), etc., and SiO₂–Al₂O₃ and It/Pt–Pd/Pt discrimination diagrams, indicate that they are not extraterrestrial sources, nor are they the origins of normal seawater deposition but they are related to hydrothermal deposition. The enrichment process of PGEs is relatively complicated and is controlled by the impact of the sedimentary environment in the restricted basin. The anoxic water environment and organic-rich environment are the main factors of PGEs enrichment. PGEs and TOC have a good positive correlation and can be used as index parameters for sedimentary environments and organic matter enrichment.