Characterization of microscopic pore structure and its influence on gas content of shale gas reservoirs from the Lower Paleozoic in southern Sichuan Basin

The genetic type of microscopic pore,the characteristics of pore structure and its influence on gas content from the Lower Cambrian Qiongzhusi and Lower Silurian Longmaxi Formations shale gas reservoirs in southern Sichuan Basin were studied by using scanning electron microscope,argon ion polishing-field emission scanning electron microscope,Image J2 x software analysis,high pressure mercury injection,low temperature N2 and CO2 adsorption experi- ments. The results show that the multiple genetic types of microscopic pores,including inter-granular pore,intragranu- lar pore,organic matter pore and microcrack,are developed in the Lower Paleozoic shale in southern Sichuan Basin. The total surface porosity of microscopic pores is ranged from 3. 95% to 7. 48% . The total surface porosity and organic pore surface porosity of the Qiongzhusi Formation shale are lower than those of the Longmaxi Formation shale. The total pore volume of the Lower Paleozoic shale is (3. 93-24. 96) ×10-3 mL / g,total specific surface area is 2． 727-29． 399 m2 / g in range. These pores whose pore sizes are 0. 35-1. 00,2. 5-4. 7 and 55-75 nm in range are the main contribu- tion of total pore volume,and those pores whose pore sizes are 0. 3-1. 0 nm and 2. 5-5. 5 nm in range account for the main total specific surface area. Total pore volume and total specific surface area of the Qiongzhusi Formation shale are lower than those of the Longmaxi Formation shale. All the surface porosity,organic pore,pore volume,specific surface area and pore size distribution of microscopic pores will affect the gas content of shale. The above differences of pore structure characteristics in the Lower Paleozoic Qiongzhusi and Longmaxi Formations shale provide a basis for revea- ling the difference of gas content between the Qiongzhusi Formation shale and the Longmaxi Formation shale in south- ern Sichuan Basin.