Abstract: Nanoscale organic pores are the dominant pores in the marine organic-rich shales of the Wufeng Fomration and the Longmaxi Formation's first Member in the Jiaoshiba Block,Sichuan Basin,which provide important reservoir space for shale gas.In this study,a combination of field emission scanning electron microscopy(FE-SEM),statistical analysis based on ImageJ software and gas adsorption test,was conducted to investigate structural characteristics of organic pores,such as pore shape,pore quantity,pore-size distribution.The surface pore rate of organic matter(OM)was calculated,based on 20 sample rocks from the three sets of shales in the Wufeng Formation and the lower and upper Longmaxi Formation's first Member.FE-SEM images show that the pore sizes are within a range of 2-900 nm,and significant variations exist for the nanoscale organic pore structures with the sizes of 10-900 nm among the chosen shale samples.Most of the organic pores in the Wufeng Formation shales are irregular polygons in shape,while elliptical and sub-rounded organic pores are observed in the Longmaxi's first Member.Organic pores with width of 10-50 nm are most popular in the Wufeng Formation shale samples,followed by the upper part of the first Member of the Longmaxi Formation,while the lower part of the first Member of the Longmaxi Formation mostly develop organic pores with the width of 50-900 nm,with the upper part of the Member ranked the second.Based on the statistic results over 61 400 organic pores with the size ranging from 10 nm to 900 nm in the shale samples,we observe that the average surface pore rate of OM for a single grain has a weak positive correlation with the total organic carbon(TOC)content,but the average surface pore rate of OM in the sample shales of the Wufeng Formation is relatively smaller than that in the Longmaxi Formation.The gas adsorption test and pore size measurement show that a good positive correlation between TOC content and nanometer pores with pore-sizes of 0.3-1.5 nm and 2-10 nm exists.Thus we may conclude that the structural differences of organic pores in the sampled shales are derived from the TOC content variation and the special location of the Wufeng Formation,the shales of which are located at the bottom of compressive decollement formations and had experienced layered decollement reconstruction which in turn results in partial gas release under overpressure.

Key words: organic pore, structure difference, surface pore rate, shale gas, Wufeng Formation, Longmaxi Formation, Jiaoshiba Block, Sichun Basin