Abstract: Microscopic pore structures of shale are an important factor influencing the storage capacity of shale gas reservoirs. To
evaluate the reservoir properties of the Lower Cambrian Qiongzhusi Formation shales in the southern Sichuan Basin, we investigate
genetic types of microscopic pores, characteristics of pore structures and its influencing factors in this paper, based on the data from
core and drilling samples. Multiple methods were applied, including scanning electron microscopy, field emission scanning electronmicroscopy with argon ion polishing, image J2x software analysis, low-temperature CO2 and N2 adsorption, and high-pressure mercury
injection experiments. Results show that the porosity of the Qiongzhusi Formation shales ranges from 0.25% to 5.80% with an average
of 2.49%. Multiple genetic types of pores are present in the shales, with the inter-granular pores being dominant and the intra-granular
pores, organic matter pores and micro-cracks being subdominant. The total surface porosity of the microscopic pores in the shales
ranges from 3.58% to 5.92%. The total pore volume of the shales ranges from 2.86×10-3 to 12.55×10-3 mL/g, and the total specific
surface area ranges from 2.727 to 21.992 m2/g. Their pore-size distribution mainly occurs in three ranges: 0.30~1.00 nm, 2.5~4.7 nm
and 55~70 nm. Micropores (<2 nm) and mesopores (2~50 nm) are the main reservoir storage space for shale gas. Types of pore structure
are mainly circular pores, wedge-shaped pores, parallel-plate pores and mixed pores. The porosity and total specific surface area of the
shales show positive correlations with TOC and abundance of brittle minerals. The micropore volume and specific surface area of shales
show positive correlations with TOC. The porosity, total pore volume and specific surface area of shales show negative correlations with
Ro and abundance of clay minerals.

Key words: shale gas reservoir, pore type, pore structure, the Qiongzhusi Formation shale, Southern Sichuan Basin