Pore Structure and Fractal Analysis of Low-Resistivity Longmaxi Shale in the Southern Sichuan Basin Combining SEM, Gas Adsorption, and NMR.

Abstract

Pore structure can affect the reservoir property, petrophysics, and fluid migration/adsorption, which is critical for shale evaluation and development. In this paper, the pore structure, fractal characteristics, and their influencing factors on low-resistivity shale (LRS) from the Longmaxi Formation in the Southern Sichuan Basin were analyzed by combining geochemistry experiments, physical property analysis, X-ray diffraction, scanning electron microscopy (SEM), N2/CO2 gas adsorption experiments, and nuclear magnetic resonance (NMR). The results indicate that in LRS, the layered clay mineral/pyrite distribution and more developed pores with a larger size and better connectivity can build a complex and superior conductive network. In gas adsorption tests, the pore volume (PV) is primarily contributed by mesopores in sizes of 2-4, 10-30, and 40-50 nm; the specific surface area (SSA) is mainly controlled by mesopores of 2-4 nm and micropores of 0.5-0.7 nm. The pore structures characterized by NMR, gas adsorption experiments, and SEM are consistent. In addition, gas adsorption is more suitable than NMR for describing the fractal dimension, where the development of micropores enhances the heterogeneity and complexity of the pore surface and pore structure. The gas-producing LRS has larger D1 and D2 than water-producing LRS. Moreover, TOC contributes to the development of micropores to some degree. Quartz and illite are negatively correlated with the PV and SSA of mesopores and total pores, while pyrite, clay mineral, and illite/smectite (I/S) are converse with a positive relationship. There exists only one negative relationship between chlorite and D2, and chlorite is weakly positively correlated with the large pore volume and negatively correlated with the micropore SSA.