Study of the Pore Structure and Fractal Characteristics of the Leping Formation Shale in the Junlian Block, Southern Sichuan Basin

The pore structure of shale is a key factor affecting the occurrence and flow of shale gas, and fractal dimensions can be used to quantitatively describe the complexity of the shale pore structure. In this study, the Leping Formation shale in the Junlian block of the southern Sichuan Basin was investigated. The pore structure characteristics of this shale were examined via low-pressure CO2 adsorption (LP-CO2A) and low-temperature N2 adsorption (LT-N2A) methods via field emission scanning electron microscopy (FE-SEM), shale geochemistry, and mineral composition analysis. Pore fractal dimensions were calculated via the Frenkel-Halsey-Hill (FHH) model, and the relationships among the fractal dimensions, shale composition (total organic carbon (TOC), quartz, and clay mineral contents), and pore structure were discussed. The results revealed that the TOC contents of the Leping Formation shale in the study area were high and ranged from 0.9% to 4.48%, with an average of 2.25%. The quartz contents were 17.2% to 60.1%, and the clay mineral contents were 33.8% to 67.2%. On the basis of the FE-SEM and N2 adsorption-desorption curve analyses, the pore types of the Leping Formation shale were complex and significantly variable in terms of the scale and development of organic pores, intragranular pores, and microfractures. The pore morphologies were mostly narrow slit-type flat pores and four-sided open or cone-type flat pores. The pore size distribution exhibited a multimodal pattern. The pore type was mainly mesopores, followed by micropores and minimal macropores. The specific surface area (SSA) of micropores accounted for more than 78% of the total SSA. The fractal dimension D 1 of the shale ranged from 2.262 to 2.618 (with a mean of 2.519), and the fractal dimension D 2 ranged from 2.662 to 2.843 (with a mean of 2.739). D 2 was greater than D 1, indicating that the internal structure of the pores was significantly more complex than that of the surface. The TOC and clay mineral contents were positively correlated with the Brunauer-Emmett-Teller (BET) SSA and the Barret-Joyner-Halenda (BJH) PV, whereas the quartz content was negatively correlated with the BET SSA and BJH PV. The considered fractal dimensions were positively correlated with the TOC content, clay mineral content, BET SSA, and BJH PV but negatively correlated with the quartz content and average pore diameter. The complexity and heterogeneity of the pore structure of the studied shale were quantitatively evaluated through fractal dimension analysis; thus, this approach can be applied in studies of the characteristics of the shale pore structure distribution and reservoir evaluation.