川东南龙马溪组页岩微观孔隙结构特征

MICRO-PORE STRUCTURE OF LONGMAXI SHALE FROM SOUTHEAST SICHUAN BASIN

以川东南龙马溪组页岩为研究对象,借助扫描电镜及压汞-吸附联合技术对其微观孔隙结构进行表征,利用吸-脱附曲线特性及FHH分形理论对孔隙形态和分形维数进行研究,结果表明,龙马溪组页岩储集空间中约80%为微-介孔,其中贫有机质页岩以平板状孔隙为主,分形维数一般小于2.75,在扫描电镜下呈现颗粒矿物粒缘缝及伊利石片层晶间孔;富有机质页岩则以"墨水瓶"状孔隙为主,分形维数一般大于2.85,主要为有机质孔。TOC含量与总孔容、吸-脱附滞回面积及分形维数均呈显著正相关关系,表明随着TOC含量增加,页岩孔隙由平行板状为主的无机孔隙网络过渡至"墨水瓶"效应显著的有机孔隙网络,有机质孔隙为富有机质页岩主要储集空间,同时多重毛管力叠加及扩散能力降低,使得页岩微观封闭能力提高,有利于页岩气富集与保存。

This paper is devoted to the micro-pores of the Longmaxi Shale from the southeast Sichuan basin using SEM and mercury injection combined with nitrogen adsorption method.Based on the derived adsorption-desorption hysteresis curves and FHH fractal model,pore shape and fractal property were studied.The results show that micro-and meso-pores contribute more than 80% of the pore space.Organic-poor shale of the Longmaxi Formation is dominated by plate-like pores with fractal dimension values less than2.75 as micro-fractures along silt borders and between illite layers.On the contrary,organic-rich shale exhibits ink-bottle like characteristic with fractal dimension values larger than 2.85 which is contributed by organic pores.TOC content is positively related to pore volume,hysteresis capacity and fractal dimension values.It is revealed that with the increase in TOC content,shale pore network changes from plate-like pores dominated inorganic pores to ink-bottle dominated organic pores.This change is not only beneficial for gas storage but also for gas retention and enrichment because of multi-capillary seal and diffusion reduction.