Abstract:

To better understand the development characteristic of nanopores and its geological controlling factors,we collected 11 shale samples from the Taiyuan,Shanxi,and Yanchang formations in the Weibei,southeastern Ordos Basin,and performed low-temperature nitrogen adsorption,Argon-ion milled and field emission scanning electron microscopy(FE-SEM),vitrinite reflectivity(R_o)and maceral composition,total organic carbon(TOC)content and X-ray diffraction(XRD)analyses.The selected shale samples are in the transition period from low mature to mature,and are quite different in TOC.The nanopores in the shales are dominated by pores within the size of 2~50 nm,which account for 63.5% of the total pore volume.The BET specific surface area is commonly higher than 10 m²/g,indicating a favorable condition for methane adsorption.The pore types are mainly of narrow-slit pores and ink-bottle shaped pores.The dominant mineral compositions are different from area to area:clay minerals being dominated in the Hancheng area,while brittle minerals being dominated in the Tongchuan area.Clay mineral content is positively correlated with the total content of nanopores smaller than 2 nm and that within 2-50 nm,while the brittle mineral content is positively correlated with the content of nanopores larger than 50 nm.This means that the difference in mineral types is the main control factor on the development of nanopores.Moreover,R_o and TOC also have some influences on the development of nanopores.The higher the R_o is,the higher content of large nanopores is,but the lower content of medium to small nanopores is.In addition,the higher the TOC,the higher content of medium to small nanopores is.

Key words: low-temperature nitrogen adsorption, nanopore, shale gas, Weibei area, Ordos Basin