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页岩气是一种蕴藏于页岩层系中具有自生自储特征的非常规天然气资源,页岩储层特征不仅影响页岩气的富集程度,而且对于后期勘探开发工作也具有重要影响[1⁃6]。页岩储层特征同时受岩石有机质和矿物组分,以及后期成岩作用（如溶蚀作用、胶结作用和压实作用等）的控制[7⁃15]。有机质作为页岩储层孔隙的主要载体之一,特别是腐泥型干酪根及固体沥青常含有丰富的纳米级有机质孔隙,早期一些研究普遍认为TOC是影响页岩孔隙度的重要因素[16⁃17]。但近年来进一步的研究表明,TOC与孔隙度的关系是复杂的,二者之间并非一定存在相关关系,这可能与页岩存在显著的无机孔隙（也称基质孔隙）有关[12,18⁃20]。页岩矿物组成不仅是影响储层孔隙发育和保存的重要因素,也是影响页岩气富集的重要因素[21⁃22]。页岩中的脆性矿物能够形成稳定的框架,可以减少有机质颗粒受到的有效应力,有助于减少对有机质孔隙的破坏,进而有利于孔隙的保存[23⁃27]。
石英具有高弹性模量、低泊松比和低韧性的特点,是页岩中最重要的脆性矿物,具有比其他矿物更高的脆性,对岩石强度和储层质量起着至关重要的积极作用[2,12,24,28]。页岩中的脆性矿物（包括石英、长石及碳酸盐矿物等）被广泛用于脆性指数（BI值）的计算[2,29⁃31],但目前关于脆性矿物的形式,尤其是石英中的生物成因微晶石英对于页岩脆性等岩石力学方面影响的研究较少。石英是海相页岩中最重要的矿物组分之一,以多种形式存在,并存在多种硅质来源和成因[32⁃33]。在海洋环境中,硅质来源和石英的形成机制是多样且复杂的,按硅质来源的不同,可分为碎屑硅、生物硅和热液硅三类[32⁃37]；此外,次生石英还可在成岩过程中通过多种机制形成,如硅质生物碎片的溶解、碎屑石英和硅酸盐颗粒的溶解或压溶作用,以及黏土矿物的转化等[33,38]。二氧化硅有多种来源,包括初级来源和次级来源,不同来源的石英在形态和大小等方面存在差异。近年来,基于硅质来源和石英晶体形态,并借助偏光显微镜、场发射扫描电镜（FE-SEM）、能谱分析（EDS）和阴极发光（SEM-CL）等识别方法,目前已在美国得克萨斯州上白垩统鹰滩组（Eagle Ford）、白垩系莫里组（Mowry）、米德兰盆地宾夕法尼亚系克莱恩组（Cline）、北达科他州威利斯顿盆地上泥盆统—下密西西比统巴肯组（Bakken）、我国上扬子地区上奥陶统五峰组—下志留统龙马溪组、上中扬子地区下寒武统牛蹄塘组和塔里木盆地下寒武统玉尔吐斯组等海相页岩中发现了不同类型的石英[22,25,28,33,39⁃47]。最新研究显示,生物硅含量与页岩孔隙度间可存在较好的正相关关系[12,48],表明生物成因石英对页岩孔隙的保存具有积极作用。
扬子地块广泛发育的古生界富有机质海相页岩是我国页岩气勘探开采的重点目标,近年来在上、中扬子地区已陆续有古生界海相页岩多套层系页岩气的重大突破,并相继实现试采和商业性开采[49⁃56]；而下扬子地区页岩气研究相对滞后,目前在古生界勘探突破较少。尽管目前对上、中扬子地区古生界海相页岩的初步研究表明微晶石英有利于页岩孔隙的发育与保存[12,28,32⁃33,37,57],但总体而言,生物成因微晶石英对于页岩孔隙演化和储层力学性质的研究还不够完善,尤其缺少对下扬子古生界海相页岩的相关研究。本文梳理了近年来国内外学者对海相页岩石英分类方面的最新认识,并结合中国地质调查局青岛海洋地质研究所于2017年在下扬子巢湖地区实施的全取心钻井——鼓地1井所揭示的厚层上奥陶统五峰组—下志留统高家边组海相页岩,发现前期未引起足够关注的生物成因微晶石英在该套页岩中广泛发育,并进一步讨论了生物成因微晶石英特征及其对海相页岩储层孔隙发育的影响。以期从新的研究视角进一步揭示下扬子地区古生界海相页岩孔隙发育规律,并为页岩气储集和赋存机理提供较为可靠的地质依据。
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不同类型的石英对于页岩孔隙发育和演化的贡献是不同的,因此识别和分析页岩石英类型至关重要。根据硅质来源和石英晶体形态,并借助偏光显微镜、场发射扫描电镜观察分析、能谱分析和阴极发光等方法手段,近年来在美国上白垩统鹰滩组页岩[40]、中国上扬子下寒武统牛蹄塘组页岩[42,46]和上扬子五峰组—龙马溪组页岩[25,33,58]等多套海相页岩层系中发现了不同的石英类型,详细石英分类如表1所示。本文借助偏光显微镜、场发射扫描电镜（FE-SEM）、能谱（EDS）和阴极发光（SEM-CL）等手段,选取下扬子地区鼓地1井五峰组—高家边组底部黑色富含笔石页岩层段（1 208.0~1 234.0 m）,对应晚奥陶世凯迪阶—早志留世鲁丹阶,涵盖Dicellograptus complexus⁃Paraorthograptus pacificus带（WF2~WF3）、Akidograp tus ascensus带（LM2）等笔石带序列[60⁃61],发现该套页岩中广泛存在前期未引起足够重视的多种石英类型,主要包括碎屑石英、微晶石英和生物骨架石英。总体而言,海相页岩中碎屑石英主要通过河流搬运和沉积,主要为陆源输入,由于长距离的搬运,碎屑石英多呈粉砂状和次圆状,在SEM-CL下基本为明亮的颗粒[33,41],鼓地1井五峰组—高家边组页岩中的碎屑石英表现出类似的特征,且粒径多介于10~30 μm（图1）。
表 1 国内外典型海相页岩石英分类
Table 1. Classification of quartz in typical marine shales
样品来源 识别方法 划分依据 石英类型 文献来源 美国德克萨斯上白垩统鹰滩组 （Eagle Ford）页岩 FE-SEM、EDS、SEM-CL 石英来源、形态和大小、 SEM-CL下的颜色和强度 碎屑石英（盆外碎屑石英、盆内碎屑石英）、自生石英（石英胶结物、交代的石英颗粒） 据文献[40] 美国下白垩统莫里组（Mowry）页岩 FE-SEM、EDS、SEM-CL 石英形态、大小和分布特征、 SEM-CL下的颜色和强度 碎屑石英、生物骨架石英、石英次生加大、异化颗粒粒内孔隙填充的石英颗粒、 分散在黏土基质内的微晶石英 据文献[41] 美国侏罗系海恩斯维尔—波西尔（Haynesville-Bossier）页岩 偏光显微镜、FE-SEM、 EDS、SEM-CL 石英形态、大小和分布特征、 SEM-CL下的强度 碎屑石英、交代的石英颗粒、 石英次生加大 据文献[59] 美国得克萨斯州米德兰盆地 宾夕法尼亚系克莱恩（Cline）页岩 FE-SEM、EDS、SEM-CL 石英形态、大小、 SEM-CL下的颜色和强度 碎屑石英、石英次生加大、生物骨架石英、交代的石英颗粒、石英胶结物和黏土大小的微晶石英 据文献[44] 美国北达科他州威利斯顿盆地 上泥盆统—下密西西比统巴肯组 （Bakken）页岩 偏光显微镜、SEM、EDS 石英晶体形态、 大小和分布方式 碎屑石英、生物骨架石英、微晶石英 据文献[39] 上扬子上奥陶统五峰组—下志留统龙马溪组页岩 FE-SEM、EDS、SEM-CL 石英形态和大小、SEM-CL强度 碎屑石英、微晶石英、石英次生加大 据文献[58] 偏光显微镜、SEM、EDS 石英形态、结构、 大小和分布特征 碎屑石英、生物成因石英、 黏土转化形成的石英 据文献[43] 偏光显微镜、FE-SEM、EDS、SEM-CL 石英形态和大小、 SEM-CL强度 碎屑石英、微晶石英、生物骨架石英 据文献[22] FE-SEM、EDS、SEM-CL 石英形态和大小、SEM-CL强度 碎屑石英、生物骨架石英、石英次生加大、分散在黏土基质内的微晶石英、 自形石英聚集体 据文献[25] 偏光显微镜、FE-SEM、SEM-CL 石英晶体形态、分布特征、 SEM-CL下的颜色和强度 碎屑石英、生物骨架石英、微晶石英、 石英次生加大、石英脉 据文献[28] 偏光显微镜、FE-SEM、SEM-CL 石英晶体形态、来源和形成时间、 SEM-CL强度 碎屑石英、生物骨架石英、石英次生加大、微晶石英、石英脉 据文献[33] 上、中扬子下寒武统牛蹄塘组页岩 FE-SEM、EDS、SEM-CL 石英形态和大小、SEM-CL强度 碎屑石英、石英聚集体、 石英次生加大、微晶石英 据文献[42] 偏光显微镜、FE-SEM、SEM-CL 石英大小、形态和分布特征、 SEM-CL强度 碎屑石英、生物骨架石英、 微晶石英、自形石英聚集体 据文献[46] 下扬子五峰组—高家边组页岩 偏光显微镜、FE-SEM、SEM-CL 石英大小、形态和分布特征、 SEM-CL强度 碎屑石英、微晶石英、生物骨架石英 本次研究 
图 1 下扬子地区鼓地1井高家边组页岩碎屑石英与微晶石英镜下特征
Figure 1. Microscopic characteristics of detrital quartz and microcrystalline quartz in the Gaojiabian shale from well Gudi⁃1 of the Lower Yangtze Platform
页岩中绝大多数石英可能并非碎屑成因,而主要为自生成因[62]。微晶石英为最常见的自生石英之一,在SEM-CL下不发光,为灰暗的形式[12,25,32],可见于多套海相页岩层系（图2）。根据微晶石英的晶体形态和分布特征,鼓地1井中的微晶石英可进一步细分为3种类型：Ⅰ型,分散于黏土基质中的微晶石英,这类石英在富黏土页岩中较为常见,在黏土矿物附近呈片状或颗粒状分布,多与蒙脱石的伊利石化有关（图2a,g）；Ⅱ型,自形微晶石英（图2b,h）,具有独特的晶体形态,在抛光样中多呈六边形,形貌样中为六方棱柱状,发育程度好,直径多介于1~2 μm；Ⅲ型,无定形微晶石英（图2c,d）,直径从数百纳米到几微米不等,没有特定的形状,发育丰富的粒间孔隙[33]。其中大部分的Ⅱ型和Ⅲ型微晶石英可能来源于放射虫等硅质生物的溶解,即生物成因来源[62⁃63]。
图 2 海相页岩微晶石英的扫描电镜镜下特征
Figure 2. Scanning electron microscope characteristics of microcrystalline quartz in marine shales
生物骨架石英,主要指硅质生物骨骼、碎片及其分泌物[33],在页岩中最为常见的是放射虫和海绵骨针。放射虫等硅质生物生长发育需要大量的硅,页岩中大量硅质生物的存在表明沉积时的水体富含硅[23,64⁃66]。以晚奥陶世—早志留世时期扬子地区为例,扬子地区大致表现为一种隆凹相间的古地理格局,这种格局导致古扬子海与外海隔离,形成半封闭局限滞留海盆,伴随冰期后海侵事件而导致海平面上升,上升流提供了丰富的营养物质,海洋初级生产力高,硅质生物较为繁盛[67⁃70]。鼓地1井五峰组—高家边组页岩放射虫和海绵骨针分布较为广泛（图3）,这些微体生物化石多为硅质、有机质所填充,或被溶蚀而产生孔洞。
图 3 下扬子地区鼓地1井五峰组—高家边组页岩硅质生物化石
Figure 3. Siliceous fossil images of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
此外,页岩中石英类型还包括石英次生加大和石英脉,其中石英次生加大在阴极发光图像下多为暗发光或弱发光,可与碎屑石英相区分（图4a,b）；石英脉常与方解石和黏土矿物等矿物相伴生,这些石英脉宽度多为几微米到几千微米不等（图4c,d）[25,28,33],但这两种石英类型在鼓地1井中很少见。
图 4 海相页岩石英次生加大和石英脉
Figure 4. Quartz overgrowths and veins in marine shales
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页岩自生微晶石英的硅质来源较为广泛,包括火山玻璃转化、黏土矿物转化、硅酸盐矿物溶解,以及硅质生物骨架溶解与再沉淀等[33,71⁃73],可通过镜下观察、主微量元素和生物硅含量等多种指标和方法手段来综合判定样品中的硅质来源。
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页岩中存在放射虫和海绵骨针等硅质生物骨架,可通过镜下观察来识别,这些硅质生物可为成岩作用早期自生石英的沉淀提供较为丰富的硅质来源[12,28,66,74]。鼓地1井五峰组—高家边组页岩镜下可见放射虫（图3a,b）和海绵骨针（图3c,d）,其中放射虫多呈纺锤形、椭圆形和圆形,直径大多在100 µm左右,显微镜下部分样品可见放射状结构。
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Al/ (Fe+Al+Mn)比值通常用于评估热液活动对于海洋沉积物的影响,且比值随着热液输入的减少而增加[75],可以作为确定硅质成因的一项关键指标。其中,纯热液的Al/(Fe+Al+Mn)比值小于0.01,而日本半深海Kamiaso生物燧石的Al/(Fe+Al+Mn)比值为0.60[35,66,75⁃76]。鼓地1井五峰组—高家边组页岩的Al/(Fe+Al+Mn)比值为0.65~0.76,平均为0.70（表2）,表明硅质为非热液成因。
表 2 下扬子地区鼓地1井五峰组—高家边组页岩主量元素特征
Table 2. Major elements analysis of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
样品深度/m SiO2/% Al2O3/% Fe2O3/% MnO/% TiO2/% SiO2bi/% SiO2bio/SiO2/% Al/(Fe+Al+Mn) 1 208.0 69.84 12.10 5.39 0.05 0.55 32.21 46.12 0.65 1 210.2 63.31 16.63 5.33 0.03 0.70 11.59 18.31 0.72 1 213.5 62.03 14.48 6.14 0.04 0.64 16.98 27.38 0.66 1 222.0 68.12 11.98 5.38 0.03 0.54 30.86 45.30 0.65 1 227.0 80.27 6.70 1.91 0.03 0.31 59.43 74.04 0.74 1 228.1 72.76 10.27 3.49 0.03 0.45 40.83 56.12 0.71 1 229.1 75.23 9.46 3.58 0.02 0.41 45.80 60.88 0.69 1 230.0 77.04 9.69 2.55 0.02 0.49 46.91 60.89 0.76 1 231.0 71.20 11.26 3.80 0.04 0.51 36.17 50.80 0.71 1 232.8 68.71 13.21 3.85 0.04 0.55 27.63 40.21 0.74 1 233.4 61.06 13.80 5.32 0.06 0.65 18.15 29.73 0.68 1 234.0 67.26 14.67 3.93 0.03 0.73 21.64 32.18 0.75 主微量元素含量对于判别硅质来源具有重要意义,其中Fe、Mn元素的富集主要与热液有关,而Al元素富集则与陆源碎屑相关[66,77],因而可通过Al-Fe-Mn三角图来判别页岩是否为热液成因[35,75]。如图5所示,选取的扬子地区下古生界海相页岩样品具有高Al值和极低的Mn值,为非热液成因；而中扬子新元古界埃迪卡拉系留茶坡组页岩则基本落在高Fe值一侧[78],反映为热液成因。Zr可表征与重矿物相关的碎屑输入[79],在判别页岩样品为非热液成因的基础上,可通过SiO2与Zr的二元图解来进一步判断其是否为生物成因。若SiO2与Zr呈正相关关系,反映为碎屑成因；若SiO2与Zr呈负相关关系,则表明为生物成因[80]。鼓地1井五峰组—高家边组页岩SiO2与Zr呈较好的负相关关系,且相关系数（R2）与已证实硅质为生物成因的上扬子地区牛蹄塘组和龙马溪组页岩类似（图6）,因此,鼓地1井五峰组—高家边组页岩生物成因构成了硅质的重要来源。
图 6 扬子地区下古生界海相页岩石英含量与锆（Zr）含量的关系图
Figure 6. Relationship between SiO2 and Zr in the Lower Paleozoic marine shale of the Yangtze Platform
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陆壳中SiO2/Al2O3值约为3.6[81⁃82],即若SiO2/Al2O3值位于3.6附近,则表明页岩中的硅质均为陆源输入。鼓地1井五峰组—高家边组页岩SiO2/Al2O3值介于3.81~11.98,平均为6.25,反映明显存在其他硅质来源。此外,在Si含量与Al含量交汇图中,位于伊利石Si/Al线之上的样品表明其存在过量硅[83]。鼓地1井页岩样品均位于伊利石Si/Al线的上方区域（图7）,由于前文已排除硅质的热液来源,故过量硅可视为生物硅。因此,生物硅可通过总硅含量减去碎屑硅含量来估计[21],其含量可通过公式（1）进行计算：
(1) 式中：Sisample和Alsample分别为样品中的SiO2含量和Al2O3含量,（Si/Al）background取3.11[35,57,77]。一般而言,生物硅含量越高,表明样品中硅质生物来源越高[12,33,37]。鼓地1井五峰组—高家边组页岩生物硅含量介于11.59%~59.43%,平均为32.35%,约占总硅含量的45.17%（表2）,反映了较高的硅质生物来源。
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页岩组分与结构是控制其力学性能的重要因素,高脆性的矿物（包括石英、长石、黄铁矿和碳酸盐矿物）对岩石的力学强度具有积极的贡献[84⁃85]。脆性指数（BI值）,已被广泛用于表征页岩的脆性,主要包括基于岩石力学弹性系数（杨氏模量和泊松比）的力学BI值和脆性矿物含量的矿物BI值两种。力学BI值需要大量样品的岩石力学分析测试和昂贵的成本,而矿物BI值往往导致岩石脆性的人为优化,为此,本文借助矿物组成和力学性能相结合的方法来计算鼓地1井五峰组—高家边组页岩的BI值（表3）,计算方法见公式2[31]：
表 3 下扬子地区鼓地1井五峰组—高家边组页岩矿物组成、有机质丰度、脆性指数和孔隙度特征
Table 3. X⁃ray diffraction (XRD), total organic carbon (TOC), brittleness index (BI), and nuclear magnetic resonance (NMR) porosity results of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
样品深度/m 石英/% 黏土矿物/% 斜长石/% 铁白云石/% 黄铁矿/% 菱铁矿/% TOC/% BI 核磁共振孔隙度/% 1 208.0 47.60 26.60 11.40 7.90 0 6.50 0.47 55.84 — 1 210.2 51.40 19.50 14.90 4.50 0 9.70 0.58 59.69 0.53 1 213.5 48.60 26.60 10.40 5.20 4.00 5.10 1.40 61.19 — 1 222.0 61.60 20.90 8.20 0 5.60 3.70 1.49 73.56 — 1 227.0 51.10 5.30 1.70 38.90 3.10 0 1.16 70.31 1.45 1 228.1 70.20 19.40 5.00 1.50 2.20 1.60 2.13 76.52 1.21 1 229.1 77.40 7.40 2.20 8.70 3.90 0.40 2.27 87.34 — 1 230.0 79.20 6.40 2.10 8.40 3.90 0 1.32 88.74 1.28 1 231.0 71.80 17.80 5.40 1.20 3.80 0 1.72 79.78 0.81 1 232.8 63.50 23.40 6.90 1.60 0 4.60 0.78 67.97 0.67 1 233.4 64.30 23.10 5.60 1.60 0 5.50 0.92 68.84 — 1 234.0 65.50 22.40 6.90 0 0 5.20 1.51 69.62 1.18 (2) 式中：M石英、M白云石、M黄铁矿、M斜长石、M黏土矿物、M菱铁矿、M方解石和M有机碳分别为石英、白云石、黄铁矿、斜长石、黏土矿物、菱铁矿、方解石和有机碳的含量。计算结果显示,鼓地1井五峰组—高家边组页岩的BI值介于55.84~88.74,平均为71.61,具有良好的脆性。
石英与其他脆性矿物相比,具有更高的脆性[2,31],是页岩中最重要的脆性矿物之一。不同来源的石英是影响岩石脆性的主要因素,会表现出不同的岩石力学性能[30,57,85]。近期的研究表明,与以碎屑石英和蒙脱石伊利石化形成的分散在黏土基质中的微晶石英为主的页岩相比,以生物成因微晶石英为主的页岩往往具有更高的杨氏模量与脆性[57]。鼓地1井五峰组—高家边组页岩的生物硅含量与BI值具有较好的正相关性（图8）,表明生物成因微晶石英的发育在一定程度上提高了页岩的脆性,这也与前人在上扬子四川盆地牛蹄塘组页岩[57]、上扬子四川盆地龙马溪组页岩[33]和中、上扬子五峰组—龙马溪组页岩[85]等研究较为一致。上述结果进一步表明,存在于页岩基质中大量的生物成因微晶石英可以相互连接,构成刚性框架[17,33,48],形成有效的支撑,进而提高页岩的力学性能。此外,近期对上扬子东南缘下寒武统牛蹄塘组页岩的研究发现,当石英含量介于55%~70%时,碎屑石英和生物成因石英的比例更适合裂缝的产生,页岩脆性相对更高；而当石英含量高于70%,且其类型主要为生物成因石英时,脆性反而会有所降低[57]。
图 8 下扬子地区鼓地1井五峰组—高家边组页岩生物硅含量与脆性指数的关系图
Figure 8. Relationship between SiO2bio and BI of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
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核磁共振（NMR）是一种快速、无创、无损的技术,近年来已被初步应用于测定页岩的孔隙类型、孔径分布以及孔隙度等[48,86]。通过分析鼓地1井五峰组—高家边组页岩核磁共振孔隙度与岩石组成的关系可知,孔隙度与TOC具有一定的正相关性,与生物硅具有较好的正相关性,而与伊利石之间具有较好的负相关性（图9）,表明TOC对孔隙度具有一定的促进作用,而生物硅的富集有利于孔隙的发育与保存。鼓地1井页岩样品中的黏土矿物主要为伊利石,相对含量占97.75%,填充粒间孔隙而导致孔隙度降低。此外,孔隙度分量与孔径曲线显示,鼓地1井页岩中低生物硅含量的样品以直径为10 nm（绿色条带）为主,而较高生物硅含量的样品在10~100 nm的范围（蓝色条带）内孔隙度有所增加且孔径分布更为均匀。这可能是随着生物硅含量的增加,处于10~100 nm的微晶石英粒间孔隙和有机质孔隙得以更好的发育与保存。
图 9 下扬子地区鼓地1井五峰组—高家边组页岩岩石组成与核磁共振孔隙度的关系
Figure 9. Rock composition with NMR porosity of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
生物成因微晶石英来源于硅质生物的溶解和再沉淀,硅质生物的原始成分一般为蛋白石-A,蛋白石-A是一种高度无序的非晶态硅质物质且性质不稳定,在40 ℃~50 ℃时会发生快速溶解—脱水—再沉淀反应,生成蛋白石-CT,并在60 ℃~75 ℃时会进一步发生溶解—再沉淀反应,逐渐形成高硬度结构的隐晶质和微晶石英集合体[32,48,87⁃88]。由于蛋白石-A和蛋白石-CT的稳定性均不高,其成岩的温度和压力相对较低,在成岩作用早期便完成向更为稳定的生物成因微晶石英的转变[63,89]。蛋白石-A向蛋白石-CT转化阶段,孔隙度损失率高；而蛋白石-CT向微晶石英转化阶段,孔隙度损失率低,且损失幅度显著减小[89]。因而这些形成于成岩作用早期的生物成因微晶石英便构成了刚性框架,提高了页岩的抗压实能力,并有效抑制原生孔隙在埋藏压实过程中的进一步减小,从而使原生孔隙得以良好保存,且在发生显著的孔隙度损失之前就已开始保持孔隙度[48,89]。而在成岩作用晚期,压实作用对页岩原生孔隙的破坏程度较为有限,孔隙度损失极为缓慢[89]。在一些生物硅对总硅贡献较大的海相页岩中,常见次生有机质填充生物成因微晶石英粒间孔隙空间,且这些有机质内部有机质孔隙较为发育[12,25,33,37,48]。下扬子地区鼓地1井五峰组—高家边组页岩Ro值介于1.67%~2.11%,平均为1.83%,大多处于高成熟阶段,同样可见有机质填充于生物成因微晶石英粒间孔隙空间,且有机质孔隙发育较为广泛（图10）。页岩中有机质和黏土矿物受压实作用影响易发生塑性变形,而生物成因微晶石英则可形成刚性框架,增强其抗压实能力,使其内部的有机质孔隙得以保存[33]。近年来越来越多的研究表明,在刚性框架存在的情况下,页岩原生粒间孔隙和有机质孔隙往往得以较好的保存[17⁃18,24⁃25]。对鼓地1井五峰组—高家边组底部页岩的研究进一步表明,生物成因微晶石英可在下扬子地区古生界海相页岩中广泛发育,有利于孔隙的保存,有助于进一步揭示下扬子地区古生界海相页岩孔隙发育与演化规律,为寻找页岩气有利赋存区提供进一步的地质依据。
图 10 下扬子地区鼓地1井高家边组页岩微晶石英及其内部有机质孔隙的扫描电镜镜下特征
Figure 10. Scanning electron microscope characteristics of microcrystalline quartz and its internal organic pores in the Gaojiabian shale from well Gudi⁃1 of the Lower Yangtze Platform
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（1） 海相页岩石英类型可大致划分为碎屑石英、自生微晶石英、石英次生加大、生物骨架石英和石英脉五种类型；其中微晶石英作为最常见的一种自生石英类型,基于其晶体形态和分布特征的差异,可细分为分散于黏土基质中的微晶石英、自形微晶石英和无定形微晶石英共3种类型。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,其中碎屑石英在阴极发光下显示为明亮的颗粒,而微晶石英呈现为在阴极发光下不发光或暗发光的自生来源特征,生物骨架石英则多为放射虫和海绵骨针的生物碎片。
（2） 海相页岩中微晶石英可存在多种硅质来源,镜下及地球化学指标指示,生物成因构成了鼓地1井五峰组—高家边组页岩微晶石英重要的硅质来源。
（3） 生物成因微晶石英对页岩储层发育具有重要的影响,能够增强页岩的脆性并形成刚性框架,提高了页岩的力学性质,使其抗压实能力增强,有利于页岩内部孔隙空间特别是粒间孔隙和有机质孔隙的保存。
Biogenic Microcrystalline Quartz and Its Influence on Pore Development in Marine Shale Reservoirs
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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 microcrystalline quartz on geomechanical properties and pore evolution, marine shale samples were collected from the Upper Ordovician Wufeng Formation and the Lower Silurian Gaojiabian Formation, specifically well Gudi-1 of the Lower Yangtze 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 cathodoluminescence. The aim was to further reveal the pore evolution pattern in the Lower Paleozoic marine shales of the Lower 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 elements, 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 relatively 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 content and porosity indicates that the enrichment of biogenic silica is conducive to pore evolution and preservation. Organic matter and clay minerals in shale are susceptible to ductile deformation by compaction, while biogenic microcrystalline 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.
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图 1 下扬子地区鼓地1井高家边组页岩碎屑石英与微晶石英镜下特征
Figure 1. Microscopic characteristics of detrital quartz and microcrystalline quartz in the Gaojiabian shale from well Gudi⁃1 of the Lower Yangtze Platform
图 2 海相页岩微晶石英的扫描电镜镜下特征
Figure 2. Scanning electron microscope characteristics of microcrystalline quartz in marine shales
图 3 下扬子地区鼓地1井五峰组—高家边组页岩硅质生物化石
Figure 3. Siliceous fossil images of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
图 6 扬子地区下古生界海相页岩石英含量与锆（Zr）含量的关系图
Figure 6. Relationship between SiO2 and Zr in the Lower Paleozoic marine shale of the Yangtze Platform
图 8 下扬子地区鼓地1井五峰组—高家边组页岩生物硅含量与脆性指数的关系图
Figure 8. Relationship between SiO2bio and BI of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
图 9 下扬子地区鼓地1井五峰组—高家边组页岩岩石组成与核磁共振孔隙度的关系
Figure 9. Rock composition with NMR porosity of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
图 10 下扬子地区鼓地1井高家边组页岩微晶石英及其内部有机质孔隙的扫描电镜镜下特征
Figure 10. Scanning electron microscope characteristics of microcrystalline quartz and its internal organic pores in the Gaojiabian shale from well Gudi⁃1 of the Lower Yangtze Platform
表 1 国内外典型海相页岩石英分类
Table 1. Classification of quartz in typical marine shales
样品来源 识别方法 划分依据 石英类型 文献来源 美国德克萨斯上白垩统鹰滩组 （Eagle Ford）页岩 FE-SEM、EDS、SEM-CL 石英来源、形态和大小、 SEM-CL下的颜色和强度 碎屑石英（盆外碎屑石英、盆内碎屑石英）、自生石英（石英胶结物、交代的石英颗粒） 据文献[40] 美国下白垩统莫里组（Mowry）页岩 FE-SEM、EDS、SEM-CL 石英形态、大小和分布特征、 SEM-CL下的颜色和强度 碎屑石英、生物骨架石英、石英次生加大、异化颗粒粒内孔隙填充的石英颗粒、 分散在黏土基质内的微晶石英 据文献[41] 美国侏罗系海恩斯维尔—波西尔（Haynesville-Bossier）页岩 偏光显微镜、FE-SEM、 EDS、SEM-CL 石英形态、大小和分布特征、 SEM-CL下的强度 碎屑石英、交代的石英颗粒、 石英次生加大 据文献[59] 美国得克萨斯州米德兰盆地 宾夕法尼亚系克莱恩（Cline）页岩 FE-SEM、EDS、SEM-CL 石英形态、大小、 SEM-CL下的颜色和强度 碎屑石英、石英次生加大、生物骨架石英、交代的石英颗粒、石英胶结物和黏土大小的微晶石英 据文献[44] 美国北达科他州威利斯顿盆地 上泥盆统—下密西西比统巴肯组 （Bakken）页岩 偏光显微镜、SEM、EDS 石英晶体形态、 大小和分布方式 碎屑石英、生物骨架石英、微晶石英 据文献[39] 上扬子上奥陶统五峰组—下志留统龙马溪组页岩 FE-SEM、EDS、SEM-CL 石英形态和大小、SEM-CL强度 碎屑石英、微晶石英、石英次生加大 据文献[58] 偏光显微镜、SEM、EDS 石英形态、结构、 大小和分布特征 碎屑石英、生物成因石英、 黏土转化形成的石英 据文献[43] 偏光显微镜、FE-SEM、EDS、SEM-CL 石英形态和大小、 SEM-CL强度 碎屑石英、微晶石英、生物骨架石英 据文献[22] FE-SEM、EDS、SEM-CL 石英形态和大小、SEM-CL强度 碎屑石英、生物骨架石英、石英次生加大、分散在黏土基质内的微晶石英、 自形石英聚集体 据文献[25] 偏光显微镜、FE-SEM、SEM-CL 石英晶体形态、分布特征、 SEM-CL下的颜色和强度 碎屑石英、生物骨架石英、微晶石英、 石英次生加大、石英脉 据文献[28] 偏光显微镜、FE-SEM、SEM-CL 石英晶体形态、来源和形成时间、 SEM-CL强度 碎屑石英、生物骨架石英、石英次生加大、微晶石英、石英脉 据文献[33] 上、中扬子下寒武统牛蹄塘组页岩 FE-SEM、EDS、SEM-CL 石英形态和大小、SEM-CL强度 碎屑石英、石英聚集体、 石英次生加大、微晶石英 据文献[42] 偏光显微镜、FE-SEM、SEM-CL 石英大小、形态和分布特征、 SEM-CL强度 碎屑石英、生物骨架石英、 微晶石英、自形石英聚集体 据文献[46] 下扬子五峰组—高家边组页岩 偏光显微镜、FE-SEM、SEM-CL 石英大小、形态和分布特征、 SEM-CL强度 碎屑石英、微晶石英、生物骨架石英 本次研究 
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表 2 下扬子地区鼓地1井五峰组—高家边组页岩主量元素特征
Table 2. Major elements analysis of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
样品深度/m SiO2/% Al2O3/% Fe2O3/% MnO/% TiO2/% SiO2bi/% SiO2bio/SiO2/% Al/(Fe+Al+Mn) 1 208.0 69.84 12.10 5.39 0.05 0.55 32.21 46.12 0.65 1 210.2 63.31 16.63 5.33 0.03 0.70 11.59 18.31 0.72 1 213.5 62.03 14.48 6.14 0.04 0.64 16.98 27.38 0.66 1 222.0 68.12 11.98 5.38 0.03 0.54 30.86 45.30 0.65 1 227.0 80.27 6.70 1.91 0.03 0.31 59.43 74.04 0.74 1 228.1 72.76 10.27 3.49 0.03 0.45 40.83 56.12 0.71 1 229.1 75.23 9.46 3.58 0.02 0.41 45.80 60.88 0.69 1 230.0 77.04 9.69 2.55 0.02 0.49 46.91 60.89 0.76 1 231.0 71.20 11.26 3.80 0.04 0.51 36.17 50.80 0.71 1 232.8 68.71 13.21 3.85 0.04 0.55 27.63 40.21 0.74 1 233.4 61.06 13.80 5.32 0.06 0.65 18.15 29.73 0.68 1 234.0 67.26 14.67 3.93 0.03 0.73 21.64 32.18 0.75
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表 3 下扬子地区鼓地1井五峰组—高家边组页岩矿物组成、有机质丰度、脆性指数和孔隙度特征
Table 3. X⁃ray diffraction (XRD), total organic carbon (TOC), brittleness index (BI), and nuclear magnetic resonance (NMR) porosity results of shale in the Wufeng Formation⁃Gaojiabian Formation from well Gudi⁃1 of the Lower Yangtze Platform
样品深度/m 石英/% 黏土矿物/% 斜长石/% 铁白云石/% 黄铁矿/% 菱铁矿/% TOC/% BI 核磁共振孔隙度/% 1 208.0 47.60 26.60 11.40 7.90 0 6.50 0.47 55.84 — 1 210.2 51.40 19.50 14.90 4.50 0 9.70 0.58 59.69 0.53 1 213.5 48.60 26.60 10.40 5.20 4.00 5.10 1.40 61.19 — 1 222.0 61.60 20.90 8.20 0 5.60 3.70 1.49 73.56 — 1 227.0 51.10 5.30 1.70 38.90 3.10 0 1.16 70.31 1.45 1 228.1 70.20 19.40 5.00 1.50 2.20 1.60 2.13 76.52 1.21 1 229.1 77.40 7.40 2.20 8.70 3.90 0.40 2.27 87.34 — 1 230.0 79.20 6.40 2.10 8.40 3.90 0 1.32 88.74 1.28 1 231.0 71.80 17.80 5.40 1.20 3.80 0 1.72 79.78 0.81 1 232.8 63.50 23.40 6.90 1.60 0 4.60 0.78 67.97 0.67 1 233.4 64.30 23.10 5.60 1.60 0 5.50 0.92 68.84 — 1 234.0 65.50 22.40 6.90 0 0 5.20 1.51 69.62 1.18
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