摘要
[目的]石英是海相页岩中最重要的矿物之一,以多种形式存在,并具有多种的硅质来源和成因,而不同类型的石英对于岩石力学性能和孔隙演化的贡献是不同的,且相关研究目前仍较薄弱,制约了对页岩储层特征的深入认识.[方法]简述了近年来海相页岩石英分类的最新进展,并在此基础上,在前期研究较为薄弱的下扬子地区,利用鼓地1井上奥陶统五峰组—下志留统高家边组海相页岩样品,综合运用薄片分析、X射线衍射分析、地球化学分析、场发射扫描电镜、核磁共振、能谱分析和阴极发光等多种方法手段,探究海相页岩石英类型和硅质来源,并进一步讨论生物成因微晶石英对页岩力学性质和孔隙发育等储层性质的影响.[结果]下扬子地区鼓地1井五峰组—高家边组页岩石英类型主要为碎屑石英、微晶石英和生物骨架石英,其中碎屑石英为陆源输入,而微晶石英则为自生来源.硅质生物骨架镜下证据、生物硅含量、主微量元素特征等指标综合分析表明硅质生物可为微晶石英提供重要的硅质来源.[结论]海相页岩中的生物成因微晶石英增强了页岩的脆性,并且相互连接,形成刚性的硅质基质框架,很大程度上提高了页岩的力学性能.此外,这一刚性框架能够有效地保护微晶石英内部的有机质孔隙和粒间孔隙不被压实,有利于孔隙的保存.
Abstract
[Objective]Quartz is one of the most important minerals in marine shale and can be found in several forms with multiple origins. In addition,different types of quartz may contribute differently to the geomechanical pro-perties and pore evolution of marine shale,and related research is lacking. It restricts a deep understanding of shale reservoir characteristics.[Methods]This study briefly reviews recent progress in the classification of quartz in typical marine shales. In order to better understand quartz types,silica sources,and the influence of biogenic microcrystal-line quartz on geomechanical properties and pore evolution,marine shale samples were collected from the Upper Or-dovician Wufeng Formation and the Lower Silurian Gaojiabian Formation,specifically well Gudi-1 of the Lower Yang-tze Platform. These samples were analyzed using thin section-,X-ray diffraction-,and geochemical analysis,optical microscopy,nuclear magnetic resonance,and field-emission scanning electron microscopy combined with cathodolu-minescence. The aim was to further reveal the pore evolution pattern in the Lower Paleozoic marine shales of the Low-er Yangtze Platform from a new perspective and to provide a more reliable geological basis for the storage capacity of shale gas reservoirs.Optical microscope evidence was found of siliceous organism fragments,including biogenic silica accounting for approximately 45.17％ of the total silica content,as well as characteristics of major and trace ele-ments,such as a non-hydrothermal area located in the Al-Fe-Mn ternary diagram,negative correlations of Zr and SiO2,an Al/(Al+Fe+Mn) ratio of 0.65 to 0.76,and a cross-plot of Si versus Al. Overall,these indicate that siliceous organisms provided an important silica source for microcrystalline quartz. These siliceous organisms provide a rela-tively rich silica source for the precipitation of authigenic quartz during early diagenesis.[Results and Discussions]The quartz in well Gudi-1 is mainly detrital,microcrystalline,and organism skeletal quartz. Detrital quartz is of terrigenous origin and is characterized by bright luminescence. Most detrital quartz particles are silt-sized and range from 10-30 μm. Microcrystalline quartz is one of the most common authigenic quartz types and can be found in several marine shale formations. Microcrystalline quartz has no or low luminescence,indicative of authigenic origin. Furthermore,microcrystalline quartz can be divided into three categories:clay matrix-dispersed microcrystalline quartz,euhedral microcrystalline quartz,and amorphous microcrystalline quartz cements. Organism skeletal quartz is widely distributed in the studied shale samples and was mostly found in fragments of siliceous organisms,such as radiolarians and sponge spicules. Most radiolarians are oval and round with diameters of approximately 100 μm.[Conclusions]This study indicates that biogenic microcrystalline quartz may have implications for rock mechanics and the evolution of porosity. The positive correlation between the silica content and brittleness index of the studied shale reveals that the development of biogenic microcrystalline quartz enhances the brittleness of the shale to a certain extent. Biogenic microcrystalline quartz can be interconnected to form a rigid siliceous matrix framework,which largely promotes the geomechanical properties of the studied shale. Moreover,the positive correlation between the silica con-tent and porosity indicates that the enrichment of biogenic silica is conducive to pore evolution and preservation. Or-ganic matter and clay minerals in shale are susceptible to ductile deformation by compaction,while biogenic micro-crystalline quartz can form a rigid framework to enhance its resistance to compaction,which is conducive to the preser-vation of the internal pore space within shale,especially intergranular-and organic matter pores.
基金项目
国家自然科学基金项目(41702162)
国家自然科学基金项目(42076220)
国家自然科学基金项目(42206234)
中国科学院海洋地质与环境重点实验室开放基金课题(MGE2021KG16)
崂山实验室"十四五"重大项目(2021QNLM020001-1)
中国地质调查局项目(DD20190818)
中国地质调查局项目(DD20160152)
中国地质调查局项目(DD20221723)
中国地质调查局项目(DD20230317)
中国地质调查局项目(DD20230410)
山东省自然科学基金项目(ZR2020MD071)
山东省自然科学基金项目(ZR2020QD038)
维普
万方数据