Sedimentary Environment and Organic Matter Enrichment of Shale under Tectonic-Depositional Differentiation from the Lower Cambrian Niutitang Formation Guizhou, South China

Abstract

To investigate variations in depositional environments and organic matter (OM) enrichment across different tectonic deformation zones, this study analyzed samples from three wells with contrasting deformation intensities: QinX1 (strongly deformed), YF1 (moderately deformed), and QiX1 (weakly deformed). Elemental geochemistry was used to characterize paleosedimentary environments and mechanisms of OM enrichment. Results show that paleoenvironmental assemblages under different tectonic deformation regimes significantly influence OM enrichment. Total organic carbon (TOC) is highest in well QinX1 (mean 4.83%), followed by QiX1 (3.26%) and YF1 (1.28%). Ga/Rb–K2O/Al2O3 cross-plots suggest warm-humid deposition for QiX1, arid-cold for QinX1, and transitional climate for YF1. Paleowater depth proxies (Fe/Co and Zr/Al) indicate a decreasing water depth gradient from YF1 through QinX1 to QiX1. Redox proxies (U/Th and Ni/Co) indicate predominantly anoxic conditions for QinX1 and YF1, and dysoxic to anoxic conditions for QiX1. Mo/TOC ratios reflect moderate restriction in QinX1 (mean 16.94), weak restriction in YF1 (68.48), and moderately strong restriction in QiX1 (8.41). Paleoproductivity proxy (Babio) shows the highest value in well YF1 (mean 5616.57 μg/g), whereas wells QinX1 (2019.39 μg/g) and QiX1 (2410.13 μg/g) exhibit relatively lower productivity. Terrigenous input proxy (Ti/Al) shows a decreasing trend from YF1 (mean 0.082) through QiX1 (0.061) to QinX1 (0.058). Hydrothermal activity proxy (Co/Zn) exhibits a decreasing trend from YF1 (mean 0.091) through QinX1 (0.107) to QiX1 (0.136). Comprehensive analysis reveals that the strongly deformed zone (Well QinX1) features low productivity, low dilution and strong preservation; the moderately deformed zone (Well YF1) shows low TOC value characteristics of high productivity, strong dilution and weak preservation; and the weakly deformed zone (Well QiX1) exhibits balanced enrichment with moderate productivity, moderate dilution and relatively strong preservation.