摘要： 华南上扬子地区下奥陶统湄潭组是一套厚度较大的泥页岩沉积,黑色页岩十分发育,但对其沉积环境的认识争议较大。文中以贵州遵义板桥剖面为例,通过对黑色页岩及其共生岩石的沉积特征及地球化学特征的综合研究,着重分析湄潭组黑色页岩沉积期的相对水深、沉积水动力条件以及古海水物理化学性质等沉积环境条件。结果表明: (1)板桥剖面湄潭组黑色页岩以颜色暗黑和水平纹理发育良好为显著特征,不产出宏体生物化石,主要以与生屑灰岩或钙质粉砂岩不等厚频繁互层的共生形式产出,说明湄潭组黑色页岩应该不是深水环境的沉积产物,而是在混积台地内低能的浅水泥质潟湖环境中沉积的;(2)与黑色页岩互层的生屑灰岩层段多发育下细上粗的逆粒序结构,顶部可见铁质氧化物,钙质粉砂岩则多发育浪成交错层理和生物钻孔,均呈现出浅水沉积特征;(3)黑色页岩的V/(V+Ni)、Th/U、V/Cr以及Ni/Co的平均值分别为0.71、5.52、1.07和2.04,均指示次氧化—氧化环境;Sr/Ba值介于0.11~2.20之间,平均值为0.55,指示淡水—半咸水环境;(4)生屑灰岩的部分层段可见明显的侵蚀面,镜下可见岩溶发育,同时其REE配分形态趋于水平,∑REE值较高、Y/Ho值较低,具暴露岩溶特征;(5)黑色页岩TOC值介于0.09%~1.05%之间,平均值为0.39%,恢复的原始TOC含量介于0.28%~3.16%之间,平均值为1.25;R_O平均值为2.095,有机质类型主要为腐泥型。综合上述分析结果,认为遵义板桥剖面湄潭组黑色页岩为一套形成于浅水、次氧化-氧化环境的有效烃源岩,高的古生产力是有机质富集的主要原因。研究结果为判识川南地区湄潭组黑色岩系沉积环境和生烃潜力提供有力的证据,也为浅水成因黑色页岩沉积环境条件判识研究提供了一个新的实例。

关键词: 黑色页岩, 烃源岩, 浅水沉积环境, 微量元素, 稀土元素, 湄潭组

Abstract: The Lower Ordovician Meitan Formation in the Upper Yangtze area of South China is characterized as the thick mud shale deposits,in which the black shale is well developed. However,there is a great controversy on its sedimentary environment. Taking the Banqiao section of Zunyi as an example,this paper focuses on the comprehensive study on the sedimentary characteristics and geochemistry of the black shale and the associated rocks,especially the relative water depth,sedimentary hydrodynamics and physical and chemical properties of palaeo-seawater during the deposition of black shale of Meitan Formation. The results show that: (1)The black shale of Meitan Formation is characterized as the dark color and well-developed horizontal bedding,without macroscopically paleontological fossils. The interlayer of bioclastic limestone and calcareous siltstone with different thickness developed,indicating that the black shale of Meitan Formation was not the sedimentary product of deep-water environment,but deposited in the low-energy shallow muddy lagoon environment in the mixed platform. (2)The bioclastic limestone that is interbedded with the black shale section mostly develops the inverse grain structure with the fine grain at the bottom and the coarse grain at the top,and iron oxides can be seen at the top,while the calcareous siltstone,interbedded with black shale,mostly develops wave cross bedding and biological drilling,showing shallow water deposition characteristics. (3)The average values of V/(V+Ni),Th/U,V/Cr and Ni/Co of black shale are 0.71,5.52,1.07 and 2.04,respectively,indicating the hypooxidation-oxidation environment. The Sr/Ba ratio ranges from 0.11 to 2.20,with an average value of 0.55,indicating the fresh water-brackish water environment;(4)Macroscopically,obvious erosion surface can be seen in some sections of bioclastic limestone,and karst development can be seen through microscopic observation. At the same time,the REE distribution pattern tends to be horizontal,and the ∑ REE value is higher and Y/Ho is lower,showing the exposed karst environment. (5)TOC values of black shale ranged from 0.09% to 1.05%,with an average value of 0.39%. TOC values of original black shale ranged from 0.28% to 3.16%,with an average value of 1.25. The average value of R_O is 2.095,and the organic matter is mainly sapropelic. Based on the above analysis,it shows that the black shale of Meitan Formation in Banqiao section of Zunyi is a set of effective hydrocarbon source rocks formed in a shallow water and suboxidation-oxidation environment and high paleoproductivity is the main reason for the organic matter enrichment. The research results provides strong evidence for identifying the sedimentary environment and hydrocarbon generation potential of black shale rocks series in Meitan formation in southern Sichuan,and also provides a new example for the identification of shallow-water sedimentary environment conditions of black shale.

Key words: black shale, source rocks, shallow water sedimentary environment, trace elements, rare earth elements, Meitan Formation

中图分类号: 

• P595