页岩中的石英类型、成因及意义

Types,genesis and significance of quartz in shales

石英作为页岩中重要组成部分,厘清其成因机制对重建页岩成岩过程及页岩油气储集层形成至关重要。页岩中石英成因复杂,利用普通薄片观察、扫描电镜、能谱分析、阴极发光等手段,可以识别出页岩中石英主要类型包括陆源碎屑石英、生物成因石英和成岩自生石英,其中成岩自生石英硅质来源主要有黏土矿物转化、长英质溶蚀和生物硅溶解、火山—热液活动。陆源碎屑石英多呈粉砂级,阴极发光下强发光,光谱一般为2个峰值,微米级生物成因石英不发光,黏土矿物成因石英多在伊利石等黏土矿物周围或嵌入其中。不同类型石英除形貌及赋存特征外,地球化学元素也存在差异。受热液活动影响的石英多富集Fe、Mn元素;陆源碎屑石英多富Ti、Al元素,SiO_(2)与Zr呈正相关;生物成因石英常与TOC含量呈正相关,Fe/Ti值介于0.4~20之间。成岩自生石英为多阶段形成,与页岩埋藏成岩过程有着密切关联,影响着页岩储集层性质,主要形成于早、中成岩阶段。陆源碎屑石英与生物成因石英有利于抑制压实作用,保护原生孔隙,黏土矿物转化、长英质溶蚀等成岩自生石英过程发育溶蚀孔、有机孔等孔隙,石英充填部分孔隙降低孔隙度。明确页岩中石英成因,对于页岩沉积环境分析及有利储集层的分布预测都有重要意义。

Quartz is an important part of shale,making it crucial to clarify its genetic mechanism for the reconstruction of shale diagenetic process and the formation of shale oil and gas reservoir.The genesis of quartz in shale is complex,with the main types,including terrigenous detrital quartz,biogenic quartz and diagenetic authigenic quartz,identifiable through ordinary thin section observation,scanning electron microscopy,energy spectrum analysis and cathodoluminescence.The main sources of diagenetic authigenic quartz silica are clay mineral transformation,felsic dissolution and biosilicon dissolution,and volcano-hydrothermal activity.Terrigenous detrital quartz is mostly silt-grade and strongly emits light under cathodoluminescence,typically displaying a two-peak spectrum.Micron-sized biogenic quartz does not emit light,and clay mineral-derived quartz is predoninantly found around or embedded within clay minerals,such as illite.In addition to the morphology and occurrence characteristics of different types of quartz,there are also differences in geochemical elements.Quartz affected by hydrothermal activities is rich in Fe and Mn,while terrigenous detrital quartz is rich in Ti and Al elements.Detrital SiO_(2) and Zr are positive correlation,as does biogenic quartz with TOC content.The biogenic quartz Fe/Ti ranges between 0.4 and 20.Diagenetic authigenic quartz is formed in multiple stages,which is closely related to the burial diagenesis process of shale and affects the properties of shale reservoirs.Diagenetic authigenic quartz is mainly formed in early and middle diagenetic stages.Terrigenous detrital quartz and biogenic quartz are conducive to inhibiting compaction and protecting primary pores.Processes involving diagenetic authigenic quartz,like clay mineral transformation and felsic dissolution create dissolution pores and organic pores,with quartz filling some pores,thereby reducing porosity.Identifying the origin of quartz in shale is of great significance to the analysis of shale sedimentary environment and the prediction of favorable reservoir distribution.