鄂尔多斯盆地西部奥陶系乌拉力克组页岩微观孔隙结构特征

Microscopic pore structure characteristics of shale of Ordovician Wulalike Formation in western Ordos Basin

采用氩离子抛光场发射扫描电镜观察、X射线衍射分析、氦气孔隙度测定、低温氮气吸附-脱附等实验手段,联合FHH分形理论模型,从多角度表征了鄂尔多斯盆地西部奥陶系乌拉力克组不同类型页岩的微观孔隙结构特征。研究结果表明:①研究区矿物成分复杂,黏土矿物含量较稳定,脆性矿物含量高、变化范围较大,可分为3类岩相类型,自下而上依次为硅质页岩岩相、混合页岩岩相和钙质页岩岩相。②研究区孔隙度整体较低,主要为0.16%~1.50%,平均1.20%,微裂缝发育造成少量孔隙度大于4.00%,硅质页岩孔隙度最大,钙质页岩孔隙度最小,混合页岩孔隙度介于二者之间;孔隙类型复杂且与岩相密切相关,钙质页岩整体致密,多为晶间孔、溶蚀孔,以狭缝状孔隙为主;硅质页岩孔隙相对发育,多为粒间孔、黏土矿物层间缝和微裂缝,以平板状开放孔隙为主,偶见“细颈广体”的墨水瓶式的无定形孔隙。③研究区孔隙结构可划分为3类,Ⅰ类以2~4 nm的介孔为主,中孔、宏孔均较发育,孔隙体积大,在硅质页岩中常见;Ⅱ类以0~4 nm的微孔、介孔为主,宏孔发育较少,在硅质页岩和混合页岩中常见;Ⅲ类以50~100 nm的宏孔为主,但体积小,在钙质页岩中常见。④研究区页岩微观孔隙结构具有明显的分形特征,内部结构复杂,非均质性强;TOC、黏土矿物和石英的含量越高,孔隙结构和孔隙表面越复杂。⑤研究区硅质页岩储层孔隙结构最好且有机质富集,是最有利的勘探目标。

By means of argon ion polishing field emission scanning electron microscope observation,X-ray diffraction analysis,helium porosity measurement,cryogenic nitrogen adsorption-desorption and other experimental methods,combined with FHH fractal theory model,the microscopic pore structure characteristics of different types of shale of Ordovician Wulalike Formation in western Ordos Basin were characterized. The results show that:(1)The study area has complex mineral composition,relatively stable clay mineral content,high brittle mineral content and wide variation range. It can be divided into three types of lithofacies,including siliceous shale lithofacies,mixed shale lithofacies and calcareous shale lithofacies from bottom to top.(2)The overall porosity of the study area is low,mainly ranging from 0.16% to 1.50%,with an average of 1.20%. Microfracture development results in a small amount of porosity greater than 4.00%. Siliceous shale has the highest porosity,calcareous shale has the lowest porosity,and mixed shale has the porosity between them. The pore types are complex and closely related to lithofacies. The calcareous shale is compact as a whole,dominated by intercrystalline pores and dissolved pores,and mainly slit pores. The pores of siliceous shale are relatively developed,which are mostly intergranular pores,interlayer fractures and microfractures of clay minerals. The open pores are mainly flat,and inkbottle shaped amorphous pores are occasionally seen.(3)The pore structure in the study area can be divided into three types. TypeⅠis mainly composed of 2-4 nm mesopores,with large pore volume and well-developed mesopores,which are common in siliceous shale. Type Ⅱ is dominated by 0-4 nm micropores and mesopores,with less macro-pores,which are common in siliceous shales and mixed rocks. Type Ⅲ is dominated by 50-100 nm macropores,but small in size,which are common in calcareous shale.(4)The microscopic pore structure of shale in the study area has obvious fractal characteristics,complex internal structure and strong heterogeneity. The higher the TOC,clay minerals and quartz content,the more complex the pore structure and pore surface.(5)The siliceous shale reservoir in the study area has the best pore structure and is rich in organic matter,which is the most favorable exploration target.