黔西北地区石炭系祥摆组页岩微观孔隙结构及分形特征

为研究海相含煤地层页岩的微观孔隙特征,选取黔西北地区石炭系祥摆组页岩作为研究对象,利用扫描电镜(SEM)和液氮吸附实验研究孔隙特征,同时研究其分形特征,并探讨孔隙结构的影响因素。研究结果表明:研究区石炭系祥摆组页岩在扫描电镜下可观察到4类微观孔隙(粒内孔、粒间孔、有机质孔、微裂缝),其中微裂缝、有机质孔发育较丰富,具有较强的生成烃类气体能力和良好的储集性能;液氮吸附等温线在形态上呈反“S”形,表明中孔在微观孔隙中最为发育,滞后回线类型主要为H2型的细颈广体的墨水瓶孔;BJH总孔体积和BET比表面积值均较大,平均值分别为0.0155 cm³/g和13.20 m²/g,平均孔径为6.22 nm,纳米级微观孔隙大量发育,为烃类气体提供丰富的储集空间;页岩样品微观孔隙结构分形维数D较大,主体大于2.723 2,反映出孔隙结构复杂、非均质性较强;BET比表面积与TOC、石英含量呈一定的负相关性,与分形维数呈一定的正相关性;平均孔径值与石英含量正相关性较好,与分形维数负相关性较好,与黏土矿物含量呈一定的负相关性。石炭系祥摆组页岩微观孔隙的BET比表面积较大、平均孔径较小,微观孔隙结构较为复杂。

Microscopic Pore Structure and Fractal Characteristics From the Carboniferous Xiangbai Formation Shale in Northwestern Guizhou

To study the microscopic pore characteristics of shale in marine coal-bearing strata, we studied the Carboniferous Xiangbai Formation shale in northwestern Guizhou, and used SEM and N₂ adsorption analyses and fractal theory to study its fractal characteristics. We then explore the influencing factors of microscopic pore structure. The results show that four types of pores can be identified (intragranular, intergranular, organic matter and micro-fracture pores), a large number of microfractures and organic matter pores are developed, which have strong hydrocarbon generating ability and good reservoir performance. The adsorption isotherms of N₂ show an inverse S-type, indicating that the adsorption isotherms were mainly mesopores. The hysteresis loops formed by adsorption isotherms and desorption isotherms were mainly H2-type thin-necked wide-body ink bottle pores. The specific surface area is large (average 13.20 m²/g), and the total pore volume is large (average 0.015 5 cm³/g). The average pore size is 6.22 nm and the massive development of nano-scale micropores would provide rich reservoir space for hydrocarbon gases. The fractal dimension D of microscopic pore structure of shale samples is larger (average 2.723 2), reflecting strong heterogeneity of pore structure. BET specific surface area shows negative correlation with TOC and quartz content, and positive correlation with fractal dimension. The average pore size has good positive correlation with the quartz content, strong negative correlation with fractal dimension, and moderate negative correlation with clay mineral content. Larger BET specific surface area and smaller average pore size correspond to larger fractal dimension and more complex pore structure.