Pore Structure and Multi-Scale Fractal Characteristics of Adsorbed Pores in Marine Shale:A Case Study of the Lower Silurian Longmaxi Shale in the Sichuan Basin,China

Exploration practice indicates that free gas is the key to the large-scale development of shale gas,while adsorbed gas is also of great significance to the sustainable development of shale gas,and thus systematic researches on absorbed pores are needed.To date,researches on pore structure and multi-scale fractal characteristics of absorbed pores in marine shale are obviously insufficient,limited the understanding of gas production behavior from shale reservoir.In this study,total organic carbon(TOC),X-ray diffraction(XRD),CH_(4) adsorption,field emission electron microscopy(FE-SEM),and low temperature gas(i.e.,CO_(2)and N_(2))adsorption/desorption analyses were conducted on 10 continuously core samples from the Lower Silurian Longmaxi shale in the Fuling region of Sichuan Basin,China.The results indicate that the TOC content of marine shale samples changes from 0.95%to 4.55%with an average of 2.62%,showing an increasing trend with the increase of burial depth;moreover,quartz and clay are the dominated mineral compositions in marine shale,and they show a certain negative correlation.FE-SEM analysis indicates that almost all pore types in marine shale are related to organic matter(OM).Hysteresis loops of marine shale samples mainly belong to Type H2,further indicating that the pores in marine shale are mainly ink-bottle pores(i.e.,OM pores);moreover,adsorption isotherms obtained from CO_(2)adsorption data all belong to typeⅠ,indicating microporous properties for all shale samples.Comprehensive analysis indicates that pore volume and pore surface area of adsorbed pores(<300 nm)is mainly provided by the pores within the pore range of 0.6–0.7,0.80–0.85,and 1.7–5.0 nm.Based on the micropore filling model and the Frenkel-Halsey-Hill(FHH)model,multiscale fractal dimensions(D1,D2,and D3)are calculated from gas adsorption data(i.e.,CO_(2)and N_(2)),corresponding to part of micropore(0.6–1.1 nm),small-mesopore(1.7–5.0 nm),big-mespore and part of macropore(5.0–300 nm),respectively.Relationships between shale compositions,pore structure,and fractal dimensions(D1,D2,and D3)indicate that pore structure and multi-scale fractal characteristics of absorbed pores in marine shale are obviously influenced by the contents of TOC and quartz,while clay minerals have little effect on them.Comprehensive analysis indicates that the complexity of marine shale pores within the range of 0.6–1.1 and 1.7–5.0 nm has significant effects on CH_(4) adsorbability,while the larger pores(5.0–300 nm)almost have no effect.