The successful development of shale oil in the United States has completely changed the global energy landscape. In recent years, shale oil in the Junggar Basin in China has attracted widespread attention. The Permian Lucaogou Formation in the Junggar Basin features a mixed sedimentary reservoir with a complex pore structure, which hinders the understanding of the micro- and nano-scale enrichment process of shale oil. In this paper, high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption (LTNA), and laser scanning confocal microscopy (LSCM) are conducted to quantitatively characterize the pore structure of the reservoir. The results show that the pore size distribution is bimodal, with the main peak appearing near 1.89–3.15 nm and the secondary peak appearing near 30.75–84.58 nm. The specific surface area (SSA) ranges from 3.155 m²/g to 20.681 m²/g, and the mesopores are the main contributor. The pore space is characterized by multiple fractals, with D_M ranging from 2.0366 to 2.9872 derived from high-pressure mercury injection. The D_N1 and D_N2 obtained from low-temperature nitrogen adsorption are 2.1662–2.6254 and 2.0768–2.7283. Oil content is strongly influenced by reservoir porosity. The fractal characteristics of pores (2.32–36.90 nm) have a more obvious effect on oil content. The maturity stage of organic matter determines the generation of light and heavy components in shale oil, while the coupling of charging force and pore capillary resistance controls the enrichment pattern of light components in the pore center and heavy components in the pore edge. The results are helpful for the optimization of favorable shale oil blocks and provide theoretical guidance for the exploration of oil and gas resources and the development of oil recovery technology.

Although many studies based on naturally deformed samples have been carried out to investigate the pore-crack characteristics of shales, studies based on high temperature (T) and high pressure (P) deformation experiments, which can exclude sample heterogeneity factors, simulate deep T-P conditions, and generate a continuous deformation sequence, are still rare. In this study, shales with different deformation levels are generated by triaxial compression experiments, and methods including scanning electron microscopy, mercury injection, and gas sorption are utilized to characterize their influence factors and pore-crack characteristics. Results indicate that T is the primary factor influencing shale deformation when P is low, while P is dominant under high P conditions. At T < 90 ℃ and P < 60 MPa, shales undergo brittle deformation and their macropores decrease due to the compaction of primary pores, while mesopores increase because of the interconnection of micropores. At 90 ℃ < T < 200 ℃ and 60 MPa < P < 110 MPa, shales experience brittle-ductile transitional deformation, and their macro- and micropores increase because of the extension of open cracks and the plastic deformation of clay flakes respectively, while mesopores decrease dramatically. At T > 200 ℃ and P > 110 MPa, shales are subjected to ductile deformation, and their micro- and mesopores drop significantly due to the intense compaction in the matrix while macropores continuously increase with crack expansion. The permeability of shale increases with the degree of deformation and ductile material contents are predicted to be a key factor determining whether open microcracks can be preserved after ductile deformation. To account for these experimental results, an ideal model of micro pore-crack system evolution in deformed shales is further proposed, which can provide guidance for the exploration of shale gas resources in the deep or structurally complex zones.

Geological storage of CO₂ in depleted oil and gas reservoirs is approved due to its advantages, such as strong storage capacity, good sealing performance, and complete infrastructure. This review clarified the existing projects, advantages, significances, influencing factors, mechanisms, and storage potential evaluation procedures of CO₂ storage in depleted oil and gas reservoirs. In this review, the storage capability of depleted oil and gas reservoirs has been confirmed, and factors affecting the CO₂ storage potential, including geological factors and engineering factors, are concluded. CO₂ trapping mechanisms of different storage processes in depleted oil and gas reservoirs are elaborated and divided into three stages. The evaluation stages of CO₂ storage potential of depleted oil and gas reservoirs are summarized as basin selection evaluation stage, oil and gas reservoir selection evaluation stage, storage security evaluation using the bowtie method, and storage capacity calculation stage. The calculation accuracy of CO₂ storage capacity in depleted oil and gas reservoirs can be optimized by determining the mineralization storage volume and the actual reservoir characteristics of the dissolution storage coefficient numerically. This work intends to provide support for the storage of CO₂ by analyzing and studying the geological theory and engineering achievements of CO₂ storage in depleted oil and gas reservoirs.

Studies on matrix-related pores from the nanometer to the micrometer scale in shales have made considerable progress in recent decades. However, nanoscale pores and cracks developed in the shear thin layers have not been systematically discussed. In this work, interlayer shear slip occurring in shales are observed through practical examples. The results show that the shear thin layer constructed by nanograin coating is widely distributed on superimposed shear slip planes. Usually, the development of the shear thin layer undergoes viscoelastic-rheological-embrittling deformation stages, and the nanograin texture assembled in the shear thin layer can demonstrate three pore and crack structure types. Based on the mechanical analysis concerning nanoscale cohesion force, it is identified that, as long as force remains a state, the shear thin layer must bear a nanoscale pore and crack character. Furthermore, the shale gas reservoir effect of the nanoscale pore and crack is simply discussed. Obviously, the adsorbed gas effect of the nanograin itself has a larger nanoscale size and surface functionality than those of kerogen and clay particles in the shales; three structure types of the nanoscale pore and crack can act as given controlling factors of storage and permeability for the free gas. Both the matrix-related pores and the three pore and crack structures have an intimate connection with respect to each other in the genetic mechanism and temporal-spatial evolution. This work has important theoretical implications for supplementing the pore and crack classification of shale. Moreover, it makes a significant contribution to shale gas exploration and development.