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Methodology for research on the provenance of black shales and its problems and prospects: A case study of the Wufeng-Longmaxi formations, southern Sichuan Basin
Oil & Gas Geology 2025, 46(1): 62-77
Published: 28 February 2025
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The provenance of black shales is primarily investigated using elemental geochemistry and mineralogy. Specifically, elemental geochemical approaches are employed to analyze the process of sediment transport and deposition, as well as ascertain the nature of parent rocks and tectonic setting through the analysis of major and trace elements, and rare earth elements (REEs). Mineralogical techniques, on the other hand, serve to identify the locations of provenance areas and pathways of sediment transport by examining variations in mineral composition. The main limitation of this methodology is that it can only determine potential provenance areas, leaving their specific locations and extents, as well as sediment transport pathways unclear. This study focuses on the provenance of black shales in the Wufeng-Longmaxi formations in the southern Sichuan Basin. The results indicate that the active elements and compounds in the shales include Si, Al, Ca, Fe, Ba, Ta, Th, and Zr and that the inactive elements and compounds encompass Ti, P, Na, Mn, Yb, Lu, and Tm. The parent rocks exhibit low compositional maturity overall, having undergone low to moderate degrees of chemical weathering and transformation under warm and humid climatic conditions. In contrast, sedimentary sorting and recycling produce minimal impacts on variations in the elemental characteristics of the parent rocks. The parent rocks consist mainly of felsic igneous rocks, with tectonic setting dominated by a collision environment, followed by a rift environment locally. The sediment originates primarily from the Qianzhong-Xuefeng paleo-uplift, succeeded by the Leshan-Longnvsi paleo-uplift. Specifically, the sediment in well block Y203, the Changning and Luzhou areas, and the northeastern part of the southern Sichuan Basin is derived principally from the Qianzhong-Xuefeng paleo-uplift, while that near well W207 in the Weiyuan area originate primarily from the Leshan-Longnvsi paleo-uplift. Provenance analysis reveals multiple factors influencing the contents of major and trace elements, and mineral components in the sediment. Therefore, to accurately determine the provenance areas of shales and their distribution, it is necessary to integrate the stable isotope analysis, high-resolution geochemical techniques, multi-index, multidisciplinary study, and big data analytics.

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