Pore Structure and Fractal Characteristics of Marine-Continental Transitional Black Shales: A Case Study of the Permian Shanxi Formation in the Eastern Margin of the Ordos Basin

To study the microscopic pore characteristics of marine-continental transitional shale, we studied the Daning-Jixian block of the Shanxi Formation using low-pressure CO2 adsorption (LP-CO(2)A) and low-temperature N-2 adsorption (LT-N(2)A) methods in conjunction with field emission scanning electron microscopy (FE-SEM), geochemistry, and mineral composition analysis in order to obtain pore structure characteristic parameters. The fractal dimension of the pores was calculated using the Frankel-Halsey-Hill (FHH) model, and the study also discusses the factors that influence the pore structure. The study found that the marine-continental transitional phase shale of the Shanxi Formation has clay mineral contents ranging from 36.24% to 65.21%. The total organic carbon (TOC) contents range from 0.64% to 9.70%. Additionally, the organic matter maturity is high. The FE-SEM and gas adsorption experiments revealed that the transitional shale of the Shanxi Formation possesses a diverse range of pore types with relatively large pore sizes. The dominant pore types are organic and intragranular pores, with pore morphologies predominantly appearing as slit and parallel plate structures. According to the experimental data on gas adsorption, the total SSA values range from 11.126 to 47.220 m(2)/g. The total PV values range from 0.014 to 0.056 cm(3)/g. Micropores make up a greater proportion of the total SSA, whereas mesoporous pores make up a greater proportion of the total PV. The distribution of shale pore fractal dimensions D-1 and D-2 (D-1 is 2.470 to 2.557; D-2 is 2.531 to 2.755), obtained through LT-N(2)A data, is relatively concentrated. D1 and D2 have a positive correlation with the TOC content, clay mineral content, and BET-SSA, and D-1 and D-2 have a negative correlation with the quartz content. D-2 is positively correlated with the Langmuir volume, showing that D-2 can be used to evaluate the methane adsorption capacity.

Automated summary

This study uses low-pressure CO2 adsorption (LP-CO(2)A) and low-temperature N-2 adsorption (LT-N(2)A) methods, combined with field emission scanning electron microscopy (FE-SEM), geochemistry, and mineral composition analysis, to study the microscopic pore characteristics of the marine-continental transitional shale in the Daning-Jixian block of the Shanxi Formation. The study found that the shale has diverse pore types with relatively large pore sizes, and the dominant pore types are organic and intragranular pores. The distribution of shale pore fractal dimensions is relatively concentrated, and they are correlated with various factors such as TOC content and clay mineral content.