Comparative experimental investigation on pore structure and fractal characteristic of marine and continental shales

Abstract

Clarifying the pore structure characteristics of shale reservoirs, which are low porosity, low permeability and high heterogeneity, is an essential prerequisite for the efficient development of shale oil and gas. Fractal theory is especially suited for characterizing the complex pore structures of shales. This work compares the pore structure characteristics between marine shales from the Longmaxi Formation and continental shales from the Shahejie Formation through low-temperature nitrogen adsorption, nuclear magnetic resonance, and scanning electron microscopy. Different fractal scaling models are adopted to determine the fractal dimensions and lacunarities of shales by low-temperature nitrogen adsorption data and scanning electron microscopy images. In addition, the mineral compositions from X-ray diffraction are analyzed to elucidate the mechanisms by which mineral content influences fractal dimensions. Finally, the correlations between total organic carbon content and microscopic structure are discussed. These results indicate that the pore size of marine shale is smaller than that of continental shale. Additionally, the fractal dimensions of marine shales are greater than that of continental shales, suggesting a more complex pore structure. The more quartz and clay content lead to greater complexity in pore space, resulting in higher fractal dimensions. The illite/smectite mixed layer shows a strong positive correlation with fractal dimensions for marine shales, whereas this correlation is less pronounced for continental shales. The presence of microfractures in organic matter leads to a reduction for the pore surface fractal dimension in continental shales.

Graphical abstract

Keywords

shale reservoirs / lithofacies / pore structure / fractal dimension / lacunarity

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