The difference in quartz types in shales not only affects the porosity and permeability of the rocks, but also reflects the difference in the sedimentary environments. We established the formation mechanism and numerical model of quartz in shales of Wufeng and Longmaxi formations in the Wangjiawan Section, South China, based on thin-section studies using SEM (scanning electron microscope), SEM-CL (cathodoluminescence), XRD (X-ray diffraction) and geochemical analyses. There are two types of quartz in the shales: detrital quartz and authigenic quartz. Detrital quartz is mostly silt-size, typically ranging from 10 to 60 μm in size and subangular to angular monocrystal in shape, and brighter than authigenic quartz by CL intensity; authigenic quartz is present in two phases in shape: grain overgrowths and crystallite grains. Overgrowth surfaces are subhedral. Crystallite grains are typically less than 10 μm in size, euhedral or subhedral monocrystal in shape. Authigenic quartz can be subdivided into biogenic quartz and clay mineral transformed quartz according to the source of silicon. In the numerical model, the content of detrital quartz is relatively consistent (20%); the content of biogenic quartz ranges from 40% to 70%, with a sharp fall (0–30%) in the Guanyinqiao mudstone. During the Katian, a lower anoxic and dense water column make the dissolution of biogenic silica well preserved. Biogenic quartz is the major contributor to the sediment. During the early Hirnantian interval, due to the drop of sea level and the oxygenation of seafloor, the sediment is mainly composed of clay transformed quartz and detrital quartz. During the latest Hirnatian and Rhuddanian, rapid sea level rise and anoxic ocean enhance the preservation of the biogenic silica, thereby biogenic quartz re-emerges as the major contributors to the sediment. Authigenic crystallite grains and grain overgrowths have filled in primary pore space and have decreased the interparticle porosity, however, as a rigid framework, they can suppress compaction and maintain the internal pore structure. The formation of authigenic quartz results in the increase of total quartz, which fortifies the brittleness of rocks and is beneficial to the development of shale gas.

中文翻译：

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细粒有机质丰富页岩中石英的形成机理和数值模型-以湖北省西部的五峰组和龙马溪组为例

页岩中石英类型的差异不仅影响岩石的孔隙度和渗透率，而且反映了沉积环境的差异。基于SEM（扫描电镜），SEM-CL（阴极发光），XRD（X-X-）的薄层研究，我们建立了华南王家湾段五峰组和龙马溪组页岩中石英的形成机理和数值模型。射线衍射）和地球化学分析。页岩中的石英有两种：碎屑石英和自生石英。碎屑石英大部分为粉砂状，大小通常为10至60μm，形状为亚角至角单晶，并且在CL强度方面比自生石英更亮；自生石英的形状分为两个阶段：晶粒过度生长和微晶晶粒。过度生长的表面是近角的。晶粒的尺寸通常小于10μm，形状为单面或亚面单晶。根据硅的来源，自生石英可以细分为生物石英和粘土矿物转化石英。在数值模型中，碎屑石英的含量相对一致（20％）。生物石英的含量在40％至70％之间，在观音桥泥岩中急剧下降（0–30％）。在Katian期间，较低的缺氧和浓水柱使生物硅的溶解得到很好的保存。生物石英是造成沉积物的主要因素。在Hirnantian早间期，由于海平面下降和海底氧化作用，沉积物主要由粘土转化石英和碎屑石英组成。在最近的赫尔纳提人和罗丹那人期间，快速的海平面上升和缺氧的海洋增强了生物硅的保存，从而使生物石英重新成为沉积物的主要贡献者。自生的微晶颗粒和晶粒过长填充了初级孔隙空间，降低了颗粒间的孔隙度，但是，作为刚性骨架，它们可以抑制压实并保持内部孔隙结构。自生石英的形成导致总石英的增加，这增强了岩石的脆性，并有利于页岩气的开发。自生的微晶颗粒和晶粒过长填充了初级孔隙空间，降低了颗粒间的孔隙度，但是，作为刚性骨架，它们可以抑制压实并保持内部孔隙结构。自生石英的形成导致总石英的增加，这增强了岩石的脆性，并有利于页岩气的开发。自生的微晶颗粒和晶粒过长填充了初级孔隙空间，降低了颗粒间的孔隙度，但是，作为刚性骨架，它们可以抑制压实并保持内部孔隙结构。自生石英的形成导致总石英的增加，这增强了岩石的脆性，并有利于页岩气的开发。