Evolution of pore structure in gas shale related to structural deformation

Abstract

The pore characterization in shale depends on the shale composition, affected by the tectonic deformation. To obtain information about the influence of tectonic deformation on shale pore characteristics, the geochemical, mineralogical, structural and textural properties analysis, porosity and pore structure feature investigations are performed using two sets of shale (deformed shale and undeformed shale) collected from the same shale bed of the Wufeng-Longmaxi Formations (Upper Ordovician-Lower Silurian) of southeast of Sichuan Basin, China. The diagenetic environment and shale composition controls on total porosity and pore structure. Both undeformed and deformed samples have a similar porosity (>1.3%). All-scale pore structure analysis reveals that the pore size distribution of all shale samples is mainly from 0.35nm to 1.E+5nm. The surface area of both undeformed and deformed samples were mainly contribution from the micropores (>82%). The geochemical and mineralogical analysis suggested that there was a good correlation between the organic maturity, quartz content and shale deformation. Tectonic stress and structural deformation have an effect on the pore surface area and adsorption capacity of shale. Undeformed shale has a higher surface area and nitrogen adsorption capacity, and the strong deformed shale samples have the lowest surface area and nitrogen adsorption capacity. The total porosity was positively correlated with the micro- and mesopores pore volume for undeformed shale (R2>0.85), while the porosity of deformed shale was only related to the macropores pore volume (R2=0.77). Experimental analysis showed that all shale samples from the same bed had similar total porosity and pore volume, while the deformed shale had notable higher macropores percentages than undeformed shale. The results indicated that accompanied by tectonic deformation, the porosity, total pore volume and micropores content did not change significantly, while the percentages of pores were changed as macropores increased and mesopores decreased. The proportions of micro-, meso- and macropores were changed by structural deformation. Part of mesopores was disappeared due to compression of the tectonic stress, and macropores were generated due to the development of microcracks.