页岩气以吸附态为主赋存于有机质和页岩矿物颗粒表面,为强化对龙马溪组一小层一亚段页岩纳米级微、介孔孔隙结构的认识,并对比2类孔隙结构对页岩气吸附能力的影响,采用氩离子抛光-场发射扫描电镜、低压N2/CO2吸附、CH4等温吸附实验,分析了不同页岩岩相微孔、介孔孔隙结构和孔径分布的差别,建立微孔、介孔孔隙结构和页岩气吸附能力的关系,对比了2类孔隙对页岩气吸附能力的贡献。研究表明:龙马溪组页岩主要发育微孔,优势孔径范围为0.4～0.6 nm、0.8～1.0 nm,介孔相对发育程度差;泥质页岩微孔孔隙比表面积最大,钙质页岩介孔比表面积最大,泥质页岩的微孔、介孔孔体积均为最大;泥质、硅质、碳酸盐质和混合质页岩孔隙结构均具有分形特征,硅质页岩微孔分形维数最大,钙质页岩介孔分形维数最大;随着TOC增大,页岩微孔的分形维数、比表面积、孔体积和介孔孔体积均相应增加;TOC、微孔和介孔的比表面积、分形维数、孔体积对页岩气吸附能力均为正贡献,且微孔对吸附能力影响作用更强;高TOC、最大微孔比表面积和孔体积的泥质页岩是页岩气吸附能力最强的岩相。研究结果对川南龙马溪组页岩气勘探与开发具有重要的指导意义。
Shale gas primarily exists in an adsorbed state on the surfaces of organic matter and shale mineral particles. To enhance the understanding of the nanoscale micro-and meso-porous structures within the first sub-bed, first sub-member of the Longmaxi Formation and to compare the differences in the impact of two types of pore structures on shale gas adsorption capacity, this study employed argon ion polishing-field emission scanning electron microscopy, low-pressure N2/CO2 adsorption, and CH4 isothermal adsorption experiments, analyzed the differences in micro porous structure and meso porous structure and pore size distributions among various shale lithofacies, established the relationship between micro porous structure and meso porous structure and shale gas adsorption capacity, and compared the contributions of the two types of pores to shale gas adsorption capacity. The study shows that: the Longmaxi Formation shale predominantly develops micropores, with dominant pore size ranging from 0.4 to 0.6 nm and 0.8 to 1.0 nm, and has a relatively underdeveloped meso porous structure. The argillaceous shale has the largest micropore specific surface area, while the calcareous shale has the largest meso pore specific surface area. The argillaceous shale has both the largest micro porous structure and meso pore volumes. Shales of argillaceous, siliceous, carbonate, and mixed lithofacies all exhibit fractal characteristics in their pore structures, with the siliceous shale having the largest micropore fractal dimension and the calcareous shale having the largest meso pore fractal dimension. As the Total Organic Carbon (TOC) content increases, the fractal dimension, specific surface area, pore volume, and meso pore volume of shale micropores correspondingly increase. The TOC content, micro pore and meso pore specific surface areas, fractal dimensions, and pore volumes all positively contribute to shale gas adsorption capacity, with micropores having a stronger influence. The argillaceous shale with high TOC content, maximum micropore specific surface area, and pore volume is the most favorable lithofacies for shale gas adsorption capacity. The findings of this study have significant implications for the exploration and development of shale gas in the Longmaxi Formation in southern Sichuan Basin.
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