鄂西地区下寒武统牛蹄塘组页岩元素地球化学特征及沉积古环境恢复

目前关于牛蹄塘组页岩沉积古环境及其对有机质富集影响的认识尚显不足。为此，基于总有机碳（TOC）含量测定、XRD矿物组分分析、沉积地球化学分析等方法，以鄂西地区X井和Y井牛蹄塘组页岩为研究对象，揭示了页岩的沉积古环境，并探究了其对页岩有机质富集的影响。研究结果表明牛蹄塘组牛二段下部为富有机质层段，石英含量最高。牛一段页岩的Ca、P富集显著，牛二段以及牛三段页岩的Ca、Fe富集显著，其中牛二段页岩中的氧化还原敏感元素Mo、U富集程度最高。牛蹄塘组页岩无明显的Eu异常，轻稀土元素（LREE）比重稀土元素（HREE）更加富集。硅质页岩沉积于高的古生产力、较低的碎屑输入以及缺氧的底水环境；钙质页岩沉积于较高的古生产力、相对较低的碎屑输入以及贫氧—氧化的底水环境；而黏土质页岩和硅质—黏土质页岩沉积于较低的古生产力、较高的碎屑输入以及氧化的底水环境。此外，Ba_bio、Si_bio、U/Th、C_org/P_tot、Al、Ti/Al等古环境指标在垂向上的变化表明牛蹄塘组从沉积的早期到晚期，水深逐渐变浅，水动力条件变强，沉积底水的氧含量逐渐增加，表层水体的古生产力逐渐降低。缺氧的底水条件是控制牛蹄塘组富有机质页岩发育的关键因素。

Elemental Geochemistry and Paleo-environmental Conditions of the Lower Cambrian Niutitang Shale in Western Hubei Province

The Lower Cambrian Niutitang Formation is a target layer for marine shale gas exploration and development in southern China. However， little is known about the paleo-environment of shale deposition and its influence on organic matter accumulation in the Niutitang Formation. Using TOC content， XRD mineralogical analysis and sedimentary geochemical analysis， this study revealed the sedimentary paleo-environment of the region as part of an investigation into the accumulation mechanisms of organic matter. It was found that the lower part of the shale in the Niu 2 member is rich in organic matter， and has the highest quartz content. The major elements Ca and P are significantly enriched in the Niu 1 member. The major elements Ca and Fe are significantly enriched in the Niu 2 and Niu 3 members. The redox-sensitive trace elements Mo and U are most highly enriched in the Niu 2 member. No clear Eu anomaly occurs in any of these shales. Light rare earth elements （LREE） are more abundant than heavy REEs （HREE）. Siliceous shale was deposited as high-paleoproductivity and low-input terrigenous debris in anoxic bottom water conditions. Calcareous shale was deposited as high-paleoproductivity but relatively low-input terrigenous debris in dysoxic–oxic bottom water conditions. Argillaceous shale and siliceous-clay mixed shale were deposited as low-paleoproductivity， high-input terrigenous debris in oxic bottom water conditions. The stratigraphic distribution of the paleoenvironment proxies Ba_bio， Si_bio， U/Th， C_org/P_tot， Al and Ti/Al indicate decreasing paleoproductivity， increasing oxic conditions and increasing detrital flux during deposition from the lower to upper members of the Niutitang Formation. The anoxic bottom water condition was a key factor in the development of organic-rich shales in the Niutitang Formation.