四川盆地超深层泥页岩纳米孔隙特征及其地质意义

利用SEM-PCAS孔隙定量表征技术与低压N_2等温吸附实验研究X井深度为6 875~8 042m超深层泥页岩的纳米孔隙特征,并选取四川盆地及周缘地区从地表到5km左右的样品组作为对比,探索深埋藏作用对泥页岩孔隙系统的影响。研究发现,X井志留系龙马溪组、奥陶系五峰组及下寒武统筇竹寺组超深层泥页岩32个样品的孔隙特征相似,孔隙类型以有机质孔、粒间孔为主,孔隙形态以狭长-裂缝型为主。N_2等温吸附线类型为IV-H3型,QSDFT孔径分布显示其纳米孔隙主要分布于4~16nm段,BET比表面积为8.63~16.13m2/g。与对照组样品相比,X井超深层泥页岩的孔径分布更加分散,微孔体积和微孔比表面积更小,介孔/微孔的体积比值及介孔/微孔的比表面积比值比非超深层泥页岩均具有数量级的优势。X井超深层泥页岩的孔隙特征主要受埋藏深度控制,深埋藏作用会使泥页岩孔径缩小并改变孔隙的形态。

Nanopore characteristics of super-deep buried mudstones in Sichuan Basin and its geological implication

SEM-PCAS method and N2 gas adsorption are used to study the nanopore characteristics of super-deep buried mudstones from the drilling well X at the depth of 6 875 ～8 042 m,so as to understand the influence of buried depth on shale pore system.A group of mudstones buried at the depth of 0～5 km in Sichuan Basin and its periphery area are selected as parallel samples.It shows that the nanopore characteristics of super-deep buried mudstones of 32 samples from Longmaxi Formation of Silurian,Wufeng Formation of Ordovician and Qiongzhusi Formation of Cambrian are fundamentally similar.Pore types of super-deep buried mudstones are mainly organic matter pores and intergranular pores,while intragranular pores constitute only a small part,with the narrowly shaped fissures as the major pore shape.N2 adsorption experiment shows that the isotherms types of all 32 samples are Ⅳ-H3 and the QSDFT pore size distribution focus on the range of 4～16 nm with the BET surface areas of being 8.63～16.13 m2/g.Compared to parallel samples,super-deep buried mudstones in Well X have more dispersed pore size distribution,smaller micropore volume and micropore surface area,possess advantage on the ratios of mesopore/micropore volume and mesopore/micropore surface area.It reveals that the nanopore characteristics of super-deep buried mudstones in Sichuan Basin are controlled by depth and the compaction related to depth also play a role in variation of pore sizes and pore shapes.