中上扬子地区在早寒武世沉积了一套富有机质页岩层系,是该地区页岩气勘探的优选层位之一,揭示该套富有机质页岩形成机制对于准确认识海洋生物环境系统与地球资源内在联系具有重要意义.本文以雪峰隆起西缘下寒武统牛蹄塘组斜坡相海相页岩为研究对象,分析了其沉积古环境特征,揭示了斜坡相海相页岩有机质富集主控因素.结果表明,研究区牛蹄塘组有机质丰度高,下段TOC整体要高于上段.沉积地球化学分析显示,研究区牛蹄塘组页岩整体具有高古生产力水平,处于贫氧—厌氧甚至硫化的强还原环境,沉积晚期较早期还原程度逐渐降低.牛蹄塘组页岩整体处于中等滞留环境,因处于斜坡带,陆源碎屑输入程度较低,且热液活动相对较弱.牛蹄塘组有机质富集主要受控于古氧化还原条件,但上升洋流作用为有机质富集奠定了基础.牛蹄塘组沉积早期,上升洋流带来的营养元素促进了初级生产力,造成藻类生物勃发并引发后续的细菌硫酸盐还原作用(BSR)和海水缺氧硫化;牛蹄塘组沉积晚期,水体逐渐变浅,还原程度下降导致有机质丰度较早期要偏低.成果对深化海相富有机质页岩形成机理认识具有重要意义.

The lower Cambrian organic-rich shales are widely distributed in the middle-upper Yangtze platform,which are also the main target layers for shale gas exploration and extraction.Revealing the enrichment mechanisms of organic matter in these shales is of great significance for understanding the relationship between ocean bio-environmental systems and Earth resources.Through systematical geochemical analyses,this study investigated the lower Cambrian Niutitang Formation slope facies shales at the western Xuefeng Uplift,to elucidate the paleoenvironmental characteristics and their main controlling factors.The results show that the Niutitang Formation shales generally have high TOC contents,with higher TOC contents in the lower part than those in the upper part.Sedimentological geochemistry analyses indicate that the Niutitang Formation formed under high pa-leo-productivity and anoxic-anaerobic even sulfurized conditions.The formation generally has a lower degree of reduction in the late period of sedimentation than at earlier times,and was deposited in a moderately hydrologically restricted setting,with weak terrestrial input and hydrothermal activities.The enrichment of organic matter in the Niutitang Formation slope facies shales was mainly controlled by the redox conditions,however upwelling currents also contributed to the primary input of organic matter.During the early deposition stage of the Niutitang Formation,the nutrients brought by upwelling currents promoted primary paleo-productivity,resulting in the booming of algal organisms and subsequent bacterial sulfate reduction(BSR)and anoxic even sulfurized environments.During the late deposition stage of the Niutitang Formation,a drop in sea level led to a weaker degree of reduction and lower contents of TOC.Our results are of great significance to the enrichment mechanisms of marine organic-rich shales in petroliferous basins.