Classification of pore structures in shale gas reservoir at the Longmaxi Formation in the south of Sichuan Basin

Taking the Longmaxi Formation in the south of Sichuan Basin as an example,this paper discussed the main factors influencing the pore structures,and studied the characteristics and classification of pore structures using the multivariate regression analysis which was determined by mercury injection method,total organic content(TOC) and mineral composition.The findings are as follows:the mean porosity is 4.71%;the reservoir space is a composite of five types of pores including ultra-large hole,macropore,mesopore,pore and micropore.The pore size less than 1 000 nm is mainly mesopore,pore and micropore,especially in 6-120 nm regions for large composition.Multiple regression analysis shows that TOC and fragile mineral content are in favor of the pore formation,while the clay mineral is opposite.The influence of clay minerals is far less than that of TOC and fragile mineral content,and the TOC is the most significant factor for the shale pore.Based on the mercury withdrawal curve shape and the TOC,pore structure types are divided into three categories:Type Ⅰ(the convex part in the mercury withdrawal curve and with high TOC),Type Ⅱ(the concave part in the mercury withdrawal curve and with low TOC),and Type Ⅲ(the concave part in the mercury withdrawal curve and with moderate TOC).TypeⅠpore structure is in most favor of shale gas reservoir.