Characteristics of Micro/Nano Pore in the Shale Gas Reservoir of Da’anzhai Section in Eastern Sichuan Basin

Argon ion polishing and FE-SEM techniques were used to study the Micro/nano pore of the shale Gas reservoir of Da'anzhai section in eastern Sichuan basin. Six types of pores were found to have developed in the reservoir: (1) Organic matter with nm-grade pores on the surface in dispersed form with poor connectivity; (2) nm-grade intergranular pores inside berry spherical pyrite aggregation with poor connectivity; (3) nm-grade grain pores in biological debris with poor connectivity; (4) alignment nm-grade pores between layers of clay minerals with good connectivity; (5) nm-mu m grade solvent hole in clay mineral layer with good connectivity; and (6) microcracks, also nm-um grade, most of which are developed in the organic matter and brittle mineral edge with good connectivity. Among them, types 4, 5, and 6 are the main reservoir spaces. With the use of the cryogenic liquid nitrogen adsorption and BET equation, the calculated pore size distribution of shale reservoir in Da'anzhai includes 10 nm-25 nm mesopores, 100 nm-150 nm large pores, which account for a greater proportion, and a few <5 nm small pores. The specific surface area of shale reservoir was positively correlated to the pore volume, confirming that the pore diameters were mainly nanoscale mesopores. Correlated analyses were conducted between the specific surface area and clay minerals, organic carbon content, and vitrinite reflectance. Results show that clay mineral content exerted the most obvious control over shale reservoirs relative to nanoscale pore development. Meanwhile, organic maturity also had a certain influence, whereas content exerted very limited control over pore development.