Characterization of Pore Structures of Jiulaodong Formation Shale in the Sichuan Basin by SEM with Ar-ion Milling
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摘要:
微观孔隙结构是页岩储层研究的重点, 而扫描电镜方法无法识别机械抛光中由于页岩硬度差异所造成的不规则形貌。本文利用氩离子抛光-扫描电镜方法对四川威远区块九老洞组页岩进行研究, 发现了三种孔隙类型:① 无机孔以粒间孔和黏土矿物层间孔为主, 同时发育晶间孔和生物孔, 孔径主体100~500 nm; ② 有机孔受控于热成熟度或有机黏土复合体, 孔径范围数十纳米至数微米; ③ 微裂缝包括成岩收缩裂缝、高压碎裂缝、构造裂缝和人为裂缝, 缝宽数微米, 缝长数微米至数十微米。研究表明无机孔和微裂缝是九老洞组页岩气的主要储集空间。
Abstract:Microscopic pore structure is the key to studying shale gas reservoirs. Traditional Scanning Electron Microscope cannot be used to identify topographic irregularities caused by differential hardness of components during mechanical polishing. Microscopic pore structures of Jiulaodong formation shale in the Weiyuan area of the Sichuan basin were investigated using scanning electron microscope coupled with an Ar-ion milling technique. Three types of pores were identified: (1) mineral matrix pores (pore diameters are mostly 100-500 nm), mainly including interparticle pores and intraplatelet pores within clay aggregates, and intercrystalline pores within pyrite/anatase and biological pores; (2) organic matter pores (pore diameters range from tens of nanometers to several micrometers), either controlled by thermal evolution of kerogen or clay minerals coated by organic matter; (3) micro fractures (with length of dozens of micrometers and width of a few micrometers), including shrinkage fracture, tectonic fracture, overpressure fracture and artificial fracture. In conclusion, micro-nano pores in Jiulaodong shale are mainly composed of mineral matrix pores and micro fractures which provide the main storage space and flow channel for shale gas.
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图 1 W201和W202页岩无机孔赋存形式及特征：a—黏土碎屑粒间孔；b—脆性矿物粒间孔；c—矿物粒间孔；d—矿物解理孔；e—黏土矿物粒间孔；f—草莓状黄铁矿晶间孔；g—他形黄铁矿晶间孔；h—狭缝型层间孔；i，j—无规则层间孔；k—网状层间孔；l—粒内孔；m，n—生物孔和相应的能谱；o，p，q，s，t—锐钛矿粒内孔和相应的能谱。图r是图q中黑色矩形框内矿物结构放大图。图m、图o、图s中的红色十字标记为元素能谱分析的位置
Figure 1.
图 2 W201和W202页岩中有机孔和有机质的赋存特征：a，b—W201页岩中微米级有机孔，呈凹坑状或近圆形；c—W201页岩中纳米级有机孔，呈片麻状；d—W201页岩中致密有机质；e，f，h—W202页岩中的有机孔，呈层状、狭缝状等；g—图f中红色十字标记区域能谱分析结果；i—有机质吸附于黏土表面；j—有机质与黏土呈粘附态；k—有机质与黄铁矿共生；l—有机质有方解石共生，呈结合态
Figure 2.
表 1 W201与W202九老洞组页岩孔隙类型及其特征
Table 1. Pore types and characteristics of Jiulaodong formation shale from W201 and W202
孔隙类型 孔隙形态 孔隙示意图 孔径范围 分布特征 无
机
孔粒间孔 不规则，三角形、
多边形、狭缝形
百纳米~微米 多见于石英、长石及黏土矿物碎屑颗粒之间 黄铁矿晶间孔 无规则 
50~750 nm 发育在黄铁矿微晶之间 粒内孔 椭圆形或近圆形 
100~1000 nm 多见于石英、长石、碳酸盐岩、磷灰石、锐钛矿等矿物内部 黏土矿物层间孔 狭缝形、不规则状、网状 
百纳米~微米 伊利石或绿泥石等层状黏土矿物层间 有机孔 近圆形，凹坑形、
狭缝形、层状等
数十纳米~微米 有机质中，其中W202页岩样品中，有机孔多见于与黏土矿物共生的有机质中 微裂缝 长条状、锯齿状 
百纳米~百微米 多见于黏土矿物、脆性矿物边缘或矿物基质内 
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