As an unconventional oil and gas reservoir, the diagenesis and evolution of continental shale controls the formation and occurrence of inorganic and organic pores. In order to quantitatively characterize the pore characteristics of a continental shale reservoir and their influence on the evolution of the diagenesis stage, the characteristics of organic and inorganic pore types of continental shale in the Da’anzhai section of the lower Jurassic Ziliujing Formation were identified by means of X-ray diffraction mineral composition analysis and argon ion polishing scanning electron microscope measurements and observations, and the influence control of the diagenesis stage on the pore development of the continental shale reservoir and its control were clarified. The results show the following: ① The organic matter pores in continental shale are developed in large quantities, including organic matter pores in the mineral asphalt matrix and organic matter pores in the kerogen; the pore types of inorganic minerals are very rich, the main pore types are linear pores between clay minerals, intergranular (intergranular) pores, and intragranular corrosion pores, and microcracks are also developed. ② When affected by compaction diagenesis, the inorganic pores of continental shale decrease with an increase in the burial depth and diagenesis degree. ③ The burial depth of continental shale is 2000–3000 m in the middle of diagenetic stage A, and a large number of organic matter pores and dissolved inorganic pores develop at this depth, meaning that the total porosity of shale increases significantly. The burial depth of continental shale is 3000–4000 m at diagenetic stage B, where kaolinite and other clay minerals are dehydrated and converted into illite, the brittleness of shale is increased, and the interior of the shale is subject to external stress, causing microcracks to form. In the late diagenetic stage, when the buried depth of the continental shale is more than 4000 m, the organic matter is subject to secondary cracking and hydrocarbon generation, the organic pores of shale increase in number again, and the inorganic pores decrease in number due to compaction. In conclusion, we found that the burial depth is the main control factor for the development of pores and microfractures in continental shale reservoirs; diagenesis caused by burial depth is the main factor affecting the development of pores and microfractures in continental shale reservoirs; and the shale burial depth in this case is more than 3500 m, which is in the middle of diagenetic stage B. Inorganic porosity in shale is reduced, and the number of microfractures is increased. When the shale is buried more than 4000 m deep in the late diagenetic stage, the thermal evolution of organic matter in shale is high, and methane gas is generated in large quantities, which is conducive to the formation and development of organic matter pores in continental shale.

Characterization of pore structure and heterogeneity is crucial for exploring and evaluating shale oil and gas resources. Existing methodologies exhibit various limitations, and intuitive, quantitative description techniques are lacking. To address these issues, shales from the second member of the Funing Formation (E₁f₂) in the Subei Basin were studied in this study. A comprehensive research methodology was employed that combined petrological analysis, physical structure testing, and SEM images supplemented with multifractal analysis and partial least squares regression (PLSR) data processing to achieve nuanced qualitative and quantitative characterization of the heterogeneity in lacustrine shale pore structures. This study revealed that E₁f₂ shale pores predominantly consist of mesopores (2–50 nm) and macropores (>50 nm). A positive correlation was observed between the development of mesopores and calcite content, while a negative correlation exists with orthoclase content. Conversely, a positive correlation with plagioclase content was found in macropore development. The growth in total pore volume is inhibited by the total organic carbon (TOC) content. Employing multifractal methods to analyze pore morphology data extracted from scanning electron microscopy (SEM) images facilitated a quantitative characterization of the heterogeneity within the pore structures. The results showed that organic-medium mixed shale (OMMS) has the strongest heterogeneity. The weakest heterogeneity is exhibited by the organic-medium calcareous shale (OMCS). PLSR analysis indicates that the structural heterogeneity of the E₁f₂ shales is positively correlated with TOC content and negatively correlated with orthoclase content. Additionally, the type of pore influences the degree of heterogeneity. Increasing the total and macropore volume reduces the heterogeneity, while increasing the micropore (<2 nm) volume enhances it in the E₁f₂ shales.