The biomarker characteristics and sedimentary environment of Lower Cambrian Niutitang Formation source rock in northern Guizhou

In order to investigate source material and sedimentary environment of Lower Cambrian Niutitang Formation source rock in northern Guizhou, the authors studied the biomarkers. The results show that they have the following characteristics: (1) OEP values of the n-alkanes are close to 1.0, showing no odd-even carbon dominance; N-alkanes mostly have bimodal performance(C18/C25), and the ratios of (nC21+nC22)/(nC28+nC29)are 0.54-1.01, indicating a dominant role of heavy hydrocarbon components; the average ratios of Pr/Ph vary from 0.51 to 0.82, showing phytane dominance.(2) According to the relative abundance, tricyclic terpane>pentacyclic triterpane>quartcyclic terpane. The tricyclic terpane contents of C21, C23 and C24 show inverted V-shaped distribution, and the ratios of Ts/ (Tm+Ts) are between 0.45 and 0.50. (3) The average ratio of C27 and C29 of the sterane is slightly smaller than or close to 1. Aromatic compounds DBT/P are between 0.05 and 0.09. Based on these features combined with the isotope of kerogen and the isotope of the monomer hydrocarbon composition and variation, the authors believe that the black rock of Niutitang Formation was formed in a reduction environment in shallow water in the Early Cambrian, with high degree of thermal evolution of organic matter, and the main sources were the bacteria, algae and other aquatic organisms. The constant index of MnO/TiO2, the trace indicators of Ba, Sr/Ba, U/Th, V/(V+Ni), and the combination of organic geochemical indicators depict the aquatic environment from Upper Sinian Dengying Formation to Lower Cambrian Niutitang Formation and Mingxinsi Formation, and the source rocks of Niutitang Formation hydrocarbon are divided into three stages, the first stage being the initial rapid transgression stage in Early Cambrian , the second stage being a stage of deep-water reducing environment with high organic matter productivity, and the third stage being a stage of shallow waters with high organic matter productivity. The deposition period of Mingxinsi Formation nonhydrocarbon source rocks was a shallow water oxidation stage.