Organic and inorganic pore networks in medium- to low-maturity shale: Insights from SEM analysis of Shahejie Formation in the Dongying Sag

The spatial distribution and structural characteristics of organic and inorganic pore networks are crucial for evaluating the storage capacity of medium- to low-maturity shale reservoirs. Based on scanning electron microscopy images of Shahejie Formation shales in the Dongying Sag, this study established an efficient, low cost, and reliable technical method for quantitatively analyzing the structural characteristics and geological control factors of organic and inorganic pores. Shales exhibit three main pore systems: (1) nanoscale intragranular pores in clay minerals, calcite, and pyrite; (2) nano- to micron-scale pores between mineral particles and organic-inorganic interface pores; and (3) micron-to millimeter-scale microfractures at the laminar interface. The number and surface porosity of organic and inorganic pores are highly correlated with the content of shale components. Organic pores in shales mostly exist in the form of organic-inorganic interface pores, with an average pore size of 600–800 nm, providing the surface porosity of 14.9%, which has an important influence on the development of submicron (200–1000 nm) scale pores. The number of inorganic pores contributes 91.45% of the total number of pores and accounts for 85.1% of the total surface porosity. The influence of clay mineral-related pores is the most significant in inorganic pores; it not only has a significant impact on the pores in the pore size range of 0–200 nm but also affects the development of micron-scale pores together with calcite. The fractal results show that the morphological complexity of inorganic pores is higher than that of organic pores. In addition, low-resolution images will greatly underestimate the contribution of nanoscale pores to the reservoir space. Therefore, the quantitative evaluation of dense porous media should comprehensively consider the weight of resolution and field of view. The results of this study provide important microstructural data for an in-depth understanding of the organic and inorganic pore structure characteristics of medium- and low-maturity shales, and for providing more accurate reservoir quality evaluation for shale resource exploration.