For the second member of the Permian Lucaogou Formation in Malang Sag, Santanghu Basin, we used field emission scanning electron microscopy (SEM), cryogenic nitrogen gas adsorption, and the micro/nano CT method, combined with the fractal theory, to depict the dense reservoir space types of the reservoir and the microcosmic pore structure characteristics, perform the quantitative evaluation of aperture size, pore shape, and connectivity, and to analyze the mineral composition of the micropore structure of the reservoir. The results show that the area is dominated by sandy/argillaceous dolomite, and the reservoir space types mainly develop dissolved intergranular pores and intergranular pores, a few microfractures, and parallel plate and slit nanoscale pores. There is a positive correlation between pore volume and specific surface area, with micropore volume accounting for 14.95%, mesopore volume at 82.47%, and macropore volume at 2.58%. The mesoporous volume provides the main pore storage space. The combined specific surface area of micropores and mesoporous pores accounts for more than 99% of the total specific surface area, providing almost all the pore surface area, which is the main site for shale oil and gas adsorption. The fractal dimension D value of the samples is between 2.39 and 2.49, and the pore distribution of shale is relatively uniform, mainly developing mesoporous pores. The specific surface area and average radius are positively correlated with the content of dolomite in mineral components. The results of the CT experiment also confirm that the pore throat of samples with high dolomite content is mostly a coarse tubular and banded distribution in three-dimensional space, with good connectivity.

The research on pore structure and heterogeneity of shale reservoirs has always been a hotspot in the study of unconventional reservoir characteristics. China is a country dominated by continental shale. Compared with marine shale, continental shale has lower maturity and stronger reservoir heterogeneity. In this study, Sha-4 shale in the Liaohe Western Depression was selected for low-temperature nitrogen adsorption, scanning electron microscopy and other experiments revealing the pore structure and fractal characteristics of continental low mature organic-rich shale. The fractal dimension was calculated by the FHH model and the effects of TOC and mineral composition on pore structure and fractal characteristics were discussed. The results show that the Sha-4 shale in the study area is mainly mesoporous and the main pore types are inorganic pores with relatively large pore diameters, such as intergranular pores and inter-crystalline pores. The pore morphology is very complex, mainly narrow slit and flat pore, and the pore is often filled with organic matter. The fractal dimensions D₁ range from 2.58 to 2.87 and D₂ range from 2.18 to 2.55, and the pore structure shows obvious dual fractal characteristics. The pore structure and fractal characteristics of shale are mainly affected by TOC and quartz due to the low degree of the thermal evolution of shale and their effects are different from those of marine shale reservoirs. The increase in TOC reduces the heterogeneity of the shale reservoir. In addition, mineral particles with strong weathering resistance and stability such as quartz can protect the pore structure of shale, improve the pore structure and reduce the reservoir heterogeneity. This study can provide support for the study of low maturity continental shale reservoir heterogeneity in the Sha-4 member of the Liaohe Western Depression.

Oil content and the movability of shale oil are important indicators for the evaluation of continental shale oil. In recent years, the sandwiched shale oil in the fourth member of the Shahejie Formation in the Liaohe Western Depression area of the Bohai Bay Basin has shown great exploration potential, while the understanding of shale oil content and the movability of shale oil is weak. In this study, through a combination of core observations and experiments, we clarified the shale lithofacies types in the fourth member of the Shahejie Formation in the Liaohe Western Depression and explored the influencing factors of the characteristics in the oil-bearing and movability of shales in different lithofacies. The results of the study show that the organic matter content of the shale is high (TOC = 2.2–4.3%), but the maturity of thermal evolution is low (R_o = 0.38–0.55%), and the mineral component is dominated by clay minerals (30.3–72.7%), with quartz, feldspar, and other feldspar minerals developing secondarily. According to the content of organic matter, the mineral component, and the sedimentary structure, five types of lithofacies can be classified: organic-rich laminated clay-bearing felsic shale lithofacies (LS1), organic-rich laminated clay felsic mixed shale lithofacies (LS2), organic-rich layered clay felsic mixed shale lithofacies (LS3), organic-containing massive felsic-bearing clay shale lithofacies (LS4), and organic-containing massive clay felsic mixed shale lithofacies (LS5). The oil content of shale is mainly affected by the organic matter. The rate of increase in oil content of shale is fastest when the organic matter content is between 2 and 4%. The movability of shale oil is mainly controlled by the sedimentary structure, mineral component, and microscopic pore structure; the more the shale laminae is developed, the lower the clay content is, and the more the pore space is developed, the better the movability of shale oil is. Combined with the results of the shale oil content and mobility analysis in the study area, LS2 and LS3 are the dominant lithofacies in the fourth member of the Shahejie Formation in the study area, followed by LS1 >LS5 >LS4, so shale oil exploration should focus on the development of LS2 and LS3.