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

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 (Delta alpha(non)) and connectivity (Hurst(non)). With increasing maturity from 1.25 to 3.9% Ro, the D-S, D-F1, Delta alpha(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.

Automated summary

The pore structure and heterogeneity of marine-continental transitional shales were investigated. The fractal dimension of micropores and non-micropores were found to be correlated with their pore structure, and the organic carbon content controls the fractal dimension of micropores and certain non-micropores. The mineral compositions, especially clay minerals, affect the heterogeneity and connectivity of non-micropores.