Multi-scale pore structure heterogeneity characteristics and its controlling factors in hybrid shale reservoirs of the Shahejie Formation, Dongpu Depression

Shale oil and gas represent a research focus in petroleum exploration and development. Shale reservoirs exhibit strong heterogeneity, and their microscopic pore structure serves as an important basis for evaluating hydrocarbon storage performance. Taking the shale of the Shahejie Formation in the Dongpu Depression as the primary research object, this study employed various analytical techniques, including whole-rock X-ray diffraction, total organic carbon measurement, argon ion polishing, field emission scanning electron microscopy, low-temperature nitrogen adsorption, and high-pressure mercury intrusion, to analyze the heterogeneity of mixed shale reservoirs and identify the controlling factors. The research results indicate: The shale in the Shahejie Formation of the study area primarily consists of carbonate and clay minerals, with three lithofacies types: calcareous shale, argillaceous shale, and mixed shale. The mixed shale mainly develops intercrystalline pores in clay minerals, bedding fractures in clay minerals, intercrystalline pores in pyrite, intercrystalline pores in calcite, intragranular dissolution pores, organic matter shrinkage fractures, and round or elliptical organic matter pores. The mixed shale in the study area primarily develops slit-shaped micropores and small pores, which exhibit good connectivity. The average fractal dimension of macropores is 2.9924, and that of mesopores is 2.9630, both higher than those of micropores and small pores (2.4290 and 2.6361, respectively). Larger pores exhibit more complex internal structures and stronger heterogeneity. The distribution of micropores and small pores is relatively concentrated, while mesopores and macropores are more widely distributed, enhancing the heterogeneity of the reservoir structure. In the study area, carbonate and clay minerals are the main factors influencing reservoir heterogeneity. The dehydration of clay minerals forms bedding fractures, while carbonate minerals are prone to dissolution, forming intragranular dissolution pores. Due to the abundance of carbonate minerals in the study area, dissolution pores serve as connections between various types of pores, thereby reducing the heterogeneity of the mixed shale to some extent. A comparative analysis of the pore structure heterogeneity between mixed shale and calcareous/argillaceous shales revealed that argillaceous shale exhibits the strongest heterogeneity, mixed shale ranks second, and calcareous shale shows the weakest heterogeneity. • Through combined characterization by nitrogen adsorption and high-pressure mercury intrusion, the pore structure and fractal dimensions of mixed shale in the Shahejie Formation of Dongpu Depression were analyzed. Results indicate the predominant development of micro-nano scale pores, where smaller pore sizes correspond to lower fractal dimensions, reflecting weaker heterogeneity. • The influence of mineral composition on pore structure and fractal dimensions was investigated, revealing that carbonate minerals in the study area exhibit significant effects on both parameters. • Comparative analysis of heterogeneity differences among mixed, calcareous, and argillaceous shale lithofacies in the study area demonstrates that calcareous shale shows the weakest heterogeneity, followed by mixed shale, while argillaceous shale displays the strongest heterogeneity.