This paper is the result of oil and gas exploration engineering.

Objective The heterogeneity is the inherent nature of shale. Study of the microheterogeneity of shale is of great significance for determining the enrichment mechanism of shale gas and the selection of high−quality reservoirs.

Methods This study investigated the marine shale of the Wufeng−Longmaxi Formation in the northern Guizhou area. Through XRD mineralogy, low−temperature N₂ adsorption and high−pressure mercury intrusion (HPMI) analyses, we explored the macro and micro heterogeneity characteristics of pore structures of this formation.

Results The Wufeng−Longmaxi Formation shales in the northern Guizhou area are mainly of siliceous lithofacies, followed by mixed lithofacies and clayish lithofacies. The shales of different lithofacies exhibit large differences in total organic carbon (TOC) contents, mineral compositions, and pore structure characteristics. The FHH fractal model of N₂ adsorption curves, and the porous fractal theory of HPMI methods, were utilized to calculate the low pressure fractal dimension D₁ (0<P/P₀<0.45) of N₂ adsorption as 2.5351−2.6722, and the high pressure fractal dimension D₂ (0.45<P/P₀<1) as 2.8311−2.9113. Additionally, the fractal dimension D_Hg of HPMI was determined to be 2.0904−2.3736, indicating strong heterogeneity within the Wufeng−Longmaxi Formation shale pore structures. A larger fractal dimension corresponds to a more complex pore structure within the shale reservoir and stronger adsorption capacity for shale gas. Furthermore, there are notable differences between various types of pore fractal dimensions and TOC content, mineral composition, pore structure parameters, and other influencing factors. Specifically, it has been found that the fractal dimension D_Hg exhibits a strong correlation with different mineral contents, suggesting that macropore fractal characteristics are primarily influenced by mineral components. Moreover, there is a clear correlation between fractal dimensions D₁ and D₂ with TOC content and pore specific surface area parameters within the shale, indicating that micropore and mesopore heterogeneity are mainly influenced by organic carbon contents and development of organic pores.

Conclusions Generally, siliceous shale exhibits relatively high total organic carbon (TOC) contents, high proportions of brittle minerals, and high fractal dimensions. This confirms the siliceous shales are the primary high−quality lithofacies within the Wufeng−Longmaxi Formation in the northern Guizhou area, followed by the mixed lithofacies. Meanwhile, higher organic matter contents indicate not only more favorable conditions for hydrocarbon generation, but also better conditions for shale gas exploration and extraction. Our study offers theoretical and practical support for guiding the optimal selection of favorable reservoirs in marine shales in the northern Guizhou area.