构造变形作用下页岩孔裂隙结构演化特征及其模式——以四川盆地及其周缘下古生界海相页岩为例

Evolution characteristics and models of shale pores and fractures under tectonic deformation:A case study of the Lower Paleozoic marine shale in the Sichuan Basin and its periphery

构造应力能够使页岩发生变形或破坏,从而不同程度地影响页岩的宏微观结构。采用川东南下志留统龙马溪组和川东北下寒武统鲁家坪组海相页岩样品,运用聚焦离子束扫描电镜(FIB-SEM)、气体吸附和压汞法等手段,分析了构造类型和变形机制对页岩孔裂隙结构的改造和控制作用。结果表明:单斜岩层页岩中以有机质孔隙结构为主,而褶皱或断层等强烈构造部位页岩矿物粒间孔、溶蚀孔和裂隙结构占主导;相对于单斜岩层页岩,褶皱部位页岩的微孔略占优势,而断层部位页岩的大孔占绝对优势;不同构造类型页岩孔裂隙结构特征的差异与局部构造应力分布的不均一性相关;脆性变形页岩以发育微米级孔隙和裂隙结构为主,有利于页岩气的运移和聚集;韧性变形页岩以发育纳米级孔隙结构为主,有利于页岩气的吸附和赋存;总孔体积随脆性变形作用的增强而增加,随韧性变形作用的增强而减少;孔隙总比表面积与脆性变形作用的相关性不明显,而随韧性变形作用的增强而增加。在此基础上,探讨了构造变形过程中页岩孔裂隙结构的控制因素和演化模式,认为构造应力对页岩孔裂隙结构的影响首先体现在对页岩组分的改造,进而控制孔裂隙结构的演化,最终制约着页岩气的微观赋存和运移。

Tectonic stress may deform or damage the shale rocks,thus affecting their macro or micro structures to varying degrees.The marine shale samples from the Lower Silurian Longmaxi Formation and the Lower Cambrian Lujiaping Formation respectively in the southeast and northeast parts of the Sichuan Basin,were studied through focused ion beam scanning electron microscopy(FIB-SEM),gas adsorption,and mercury intrusion,to evaluate the effect of structural type and deformation mechanisms on the reconstruction of shale pore and fracture structures.Results indicate that shale in monoclines contains mostly organic matter pores,while shale in folds or faults resulted from strong tectonic activities contains largely mineral interparticle pores,dissolution pores and fractures.Comparatively,fold-related shale has more micro pores and fault-related shale has more macro pores than monoclinic shale.The pore difference in shale of different structural parts can be attributed to an uneven distribution of local structural stress.Brittle shale contains mainly micrometer-sized pores and fractures,which are conducive to gas migration and accumulation;while ductile shale is dominated by nanometer-sized pores,which are conducive to gas adsorption and occurrence.The total pore volume of the shale increases with brittle deformation and decreases with ductile deformation.The total specific surface area of the pores is loosely connected with brittle deformation,but has a positive relationship with ductile deformation.Based on these results,the paper discusses the controlling factors and evolution models of micro-nano shale pores and fractures under structural deformation,and concludes that the structural stress on shale results in the modification of shale compositions,the evolution of pores and fractures in shale,and the storage and migration of shale gas.