鄂西黄陵背斜南缘下寒武统牛蹄塘组一段古沉积环境演化特征——以秭地1井为例

下寒武统牛蹄塘组为鄂西地区重要的页岩气勘探目的层之一，针对牛蹄塘组一段(Ꞓ₁n¹，简称牛一段)古沉积环境研究较为薄弱的现状，对宜昌黄陵背斜南缘秭地1井牛一段的古气候、古氧化还原条件及古生产力等沉积环境演化特征进行了综合分析，并建立了沉积环境演化模式图。与贫有机质层段相比，富有机质层段TOC含量高，SiO₂含量更高，而CaO含量更低。U、V、Ni、Zn、Cu更为富集。化学蚀变指数(CIA)显示牛蹄塘组一段总体处于中等风化强度，富有机质层段气候温暖湿润，且地层往上气候变得炎热潮湿。由底到顶，牛一段主要经历了富有机质层段下部厌氧到富有机质层段上部缺氧再到贫有机质层段含氧3个阶段。总体上，研究区处于弱滞留—中等滞留盆地，富有机质层段由于Mo含量更高而滞留程度更高。富有机质层段具有更高的古生产力，且存在古生产力快速下降的2个时期，这与海平面下降导致水体由厌氧转化为氧化有直接关系。根据古气候、古氧化还原条件和古生产力特征可将研究区牛一段划分为3个演化阶段。

Paleo-environment of the first member of Niutitang Formation on the southern margin of Huangling anticline,western Hubei province: a case study of well ZD-1

The Lower Cambrian Niutitang Formation is one of the important targets for shale gas exploration in the western Hubei province. In view of the relatively weak research on the paleo-sedimentary environment of the 1st member of Niutitang Formation (C₁n1), the paleo-climate, paleo-redox conditions and paleo-productivity of the C₁n1 were comprehensively studied based on data collected from well ZD-1 on the southern margin of the Huangling anticline in Yichang area, and a model of the evolution of sedimentary environment was proposed. Compared with the organic-poor strata, the organic-rich strata have higher TOC content, much higher SiO₂, U, V, Ni, Zn and Cu contents, and lower CaO content. The chemical index of alteration (CIA) shows that the C₁n1 has experienced weathering effect of moderate intensity. The climate was warm and humid when the organic-rich layers deposited, and became hot and wet when the upper layers deposited. The redox conditions of C₁n1 have experienced 3 stages, including euxinic in the lower part of organic-rich strata, dysoxic in the upper part of organic-rich strata, and oxic in organic-poor strata. In general, the study area developed in a weak-medium retention basin. The ratio of Mo_EF/U_EF is usually 1-3 times than that of normal seawater due to the high content of Mo, showing much higher retention rate. The organic-rich strata have higher paleo-productivity, and there are 2 periods of rapid decline of paleo-productivity, which is directly related to the change of water from anaerobic to oxidized due to the decrease of sea level. According to paleo-climate, paleo-redox conditions and paleo-productivity characteristics, the C₁n1 can be divided into 3 evolution stages.