Characterization on micropore structure of epimetamorphic shale from Lujiaping Formation in northern Dabashan thrust belt

A large amount of very low-grade epimetamorphic organic-rich shale of Lujiaping Formation developed in northern Dabashan thrust belt. However,the study on nature gas generating conditions such as hydrocarbon generation potential and reservoir conditions of it is very poor. In this study,some experiment technical means such as low-temperature nitrogen gas adsorption,high-pressure mercury injection，argon ion figuring-field emission scanning electron microscope(Ar-FESEM) and X-ray dimaction(XRD) were carried out to analyze the pore structure of low-grade epimetamorphic organic-rich shale and development factors. The results show that the low-grade epimetamorphic shale of Lujiaping Formation is intensity heterogeneity with multiple-porosity systems. The range of pore size is wide and the combination is diverse. The development of macropore,mesopore and micropore is comparable or more inclined to macropore and mesopore in Ziyang Area. That in Langao Area,the micropore is dominant and macropore and mesopore are complementary. The pore type is closely related to the rock mineral composition. The pore types related to the brittle minerals as quartz,feldspar and carbonate are dissolution pores in grains,gaps between grains and intergranular pore. However,those pores are usually filled by clay minerals and organic matter and the pores in clay and organic matter would be reformed and even closed by the mutual squeezing of brittle minerals that restrained the pore volume development. The pores related clay minerals are interlayer pores which are mainly mesoporous that increase the pore volume of Lujiaping formation. Organic pores are main types of micropores in Lujiaping formation,which provide a large amount of specific surface area for gas adsorption. The micropore structure of low-grade epimetamorphic organic-rich shale is good for nature gas adsorption and preservation but not for migration and accumulation of free gas because of small pore throat and poor pore connectivity.