Fractal and Multifractal Characteristics of Nanopores and their Controlling Factors in Marine–Continental Transitional Shales and their Kerogens from Qinshui Basin, Northern China

Abstract

Nanopores in organic matter (OM) and minerals in shales and their heterogeneity constrain the occurrence, enrichment and flow behavior of shale gas, for which marine shales have been intensively investigated. Marine–continental transitional (MCT) shales have significantly different kerogen types and mineral compositions from marine shales, and so it is essential to characterize their pore structure and heterogeneity. In this study, the single fractal (Frankel–Halsey–Hill and Sierpinski) and multifractal were jointly applied to investigate the heterogeneity of micropores and non-micropores as well as their influencing factors for the MCT shales collected from the Qinshui Basin, China and their isolated kerogens. The results show that the fractal dimension of micropores (D_S), that of < 8 nm non-micropores (D_F1) and that of > 8 nm non-micropores (D_F2) of shales have certain correlations with their pore structure, while D_S and D_F1 of the kerogens are correlated only with their pore structure. The total organic carbon content of shales controls their D_S and D_F1, while the mineral compositions, especially different clay minerals, not only restrict their D_F2, but also affect their non-micropore heterogeneity (Δα_non) and connectivity (Hurst_non). With increasing maturity from 1.25 to 3.9% Ro, the D_S, D_F1, Δα_mic and Hurst_non of the kerogens increase significantly, indicating that the structure of OM pores of < 8 nm becomes more complex, and the micropore heterogeneity and non-micropore connectivity are enhanced. The nanopore structure of MCT shales differs from that of marine shales and has some unique fractal characteristics, to which attention should be paid in shale gas exploration and development.

Appendices

Appendix

Appendix A: Experimental Methodology

The measuring instruments for Ro were the Leica DMR XP microphotometer. The standard samples were NR1149 (Ro = 1.24%) and cubic zirconia (Ro = 3.11%).

The chemical method was used to isolate kerogen from the shale samples (Suleimenova et al., 2014). Samples were crushed to less than 100 mesh, reacted with a sufficient amount of hydrochloric acid (HCl) (molarity: 6 mol/L) in a 70 °C water bath for 6 h to remove carbonate minerals, and rinsed three times with deionized water to remove Ca2 + etc. Subsequently, the samples were treated with a mixture of HCl (6 mol/L) and 40% hydrofluoric acid (HF), reacted in a 70 °C water bath for 6 h to remove silicate minerals, and then washed three times with HCl (1 mol/L). Pyrite was removed with HCl and zinc in a water bath at 70 °C. Finally, the obtained products were rinsed with deionized water and dried in an oven at 80 °C to obtain the kerogen (Tables 5, 6).

Table 5 Basic geological and geochemical information of shale samples

Table 6 Pore structure parameters of shale samples and their kerogens

For the TOC test, the carbonate minerals in samples were first removed with 5% hydrochloric acid. After that, the samples were dried at 120 °C for 4 h, then mixed with an accelerant of iron filings and instantly heated to 3000 °C with a LECO CS-200 analyzer. The TOC content was calculated from the peak area of carbon dioxide produced by the combustion. The standard samples used were German IVA33802180 (C = 0.83%) and IVA33802176 (C = 15.50%).

A Bruker D8 Advance X-ray diffractometer was used for the mineral composition analysis. Shale powder of less than 200 mesh was mixed with glycol and smeared on a slide for whole-rock mineral analysis. 200 mesh shale powder was placed in water, dispersed using ultrasonic waves, and the suspension was centrifuged to produce purified clay. The purified clay was dried and mixed with glycol for clay mineral analysis. Various types of clay content were measured at 3–30° (2/min) intervals, while whole rock mineral content was measured at 5–70° (2/min) intervals.

LPNA and LPCA were performed using a Micromeritics ASAP 2020 analyzer to characterize their pore structure. Samples were dried in a 110 °C oven for 12 h to remove their moisture and volatile components. LPNA and LPCA were used to characterize the non-micropores and micropores in the range of the method. LPNA experiments were carried out at a relative pressure of 0.001–0.999 at 77.35 K. The Barrett–Joyner–Halenda model was used to explain the pore size distribution from 2 nm to about 200 nm. For LPCA experiments, a relative pressure range of 0.0001–0.035 was set, and the temperature was 273.15 K. The density functional theory was used to explain the 0.36–1.1 nm pore size distribution.

Appendix B: Basic Data of Samples

Among the 12 shale samples and their corresponding kerogens in this study, the data of nine samples were from Lu et al. (2023b), and the data of the other three samples were supplementally tested in this study (with the same methods and/or instruments). It should be noted that clay mineral species were obtained after separating clays from shale powder by X-ray diffraction.