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.

In this work, the potential influences of grain-coating clays on water-CO₂-rock interactions in sandstones and subsequent ramifications for CO₂ storage were investigated using reactive transport simulations. The results indicated that, compared to pore-filling smectite, grain-coating smectite leads to significant pH decrease, increases in the CO₂-species concentrations, and decreases in smectite dissolution and the precipitation of secondary minerals. Moreover, it was revealed that smectite and chlorite coats dissolve preferentially over detrital K-feldspar being covered, while K-feldspar is dissolved preferentially over illite and kaolinite coats. While the mineral trapping mechanism is only important for smectite and chlorite coats, sandstone porosity is significantly reduced for chlorite coat but increased for the other three clay coats. The main causes of the differences between pore-filling and grain-coating scenarios for smectite and chlorite coats are ascribed to the inhibitory effect of clay coats on the growth of secondary quartz and the dissolution of clay. In addition to the above two factors, the decelerating effect of clay coats on the dissolution of K-feldspar is also important for illite coat; meanwhile, for the kaolinite coat, the dissolution of clay is less important and the other two factors are more critical. Furthermore, the coverage and thickness of clay coats, fluid flow rate, detrital grain size, detrital lithology, partial pressure of CO₂, and temperature may all impact the role of clay coats.

Utilizing small angle neutron scattering techniques on organic shales, this study presents an innovative approach for characterizing the status of oil occurrence, and new insights into pore scale assessment through scattering vector-pore size relationship. The results indicate the successful identification of different shale oil occurrence status, before and after solvent extraction of residual oil for four shale samples with different contents of total organic carbon. In addition, coupled with density distribution analyses, the work demonstrates that shale samples with lower total organic carbon contents typically signify a smaller radius of gyration with better oil mobility, which indicates a greater wave oscillation with a larger pore size to be estimated from the scattering vector. This work also elucidates the notable scenarios of an increasing pore size could correspond to a decreasing radius of gyration caused by mass density redistribution. For polydisperse systems, this research illustrates the variations in pore volumetric ratio impact the scattering intensity, whereas pore scale changes affect the oscillation pattern. This novel research of analyzing mass density distribution and pore scale information in real space is also suitable for other porous media systems.