The Wufeng-Longmaxi Formations shale gas in Pengshui area

southeastern Chongqing

was studied through several pore characterization experiments including low-temperature carbon adsorption (LTCA)

low-temperature nitrogen adsorption (LTNA)

nuclear magnetic resonance (NMR)

high pressure mercury intrusion porosimetry (MIP)

scanning electron microscopy (SEM)

and Helium porosity measurement.The shale pore structure was comprehensively described

and the characterization and classification methods of full range of pore size were established to study their individual contributions to shale gas occurrence and migration.The results show that the pore volume by Helium porosity measurement is the largest

followed by LTNA and NMR.The latter two have obvious advantages in characterizing smaller (<10 nm) and larger pores

and thus the combination of LTNA and NMR can be applied to the characterization of full range pore size distribution of shale.The distribution reveals a wide range of shale gas pore distribution

but 70% of the pore volume is concentrated in pores less than 25 nm in diameter.Combined with fractal features

the pores can be divided into micropores

small pores

mesopores and large pores in diameter with a lower boundary of 5 nm

25 nm and 100 nm.The pore throat size is mainly a result of organic matter and clay contents.The larger pores are composed of intergranular pores and inter-clay-layer fractures.The mesopores are also affected by intragranular dissolved pores.The micropores and small pores are the main sites where the shale adsorbed gas and free gas occur.The small pores and mesopores are inter-connected

providing channels for the shale gas to migrate in the matrix.The research results are significant as it may guide the classification of shale gas reservoirs and the understanding of infiltration mechanism.