As an important part of carbon dioxide capture, utilization and storage (CCUS), the progress of injecting CO₂ into oil reservoirs could increase the recovery rate and achieve large-scale carbon storage. It has become one of the most important carbon storage methods around the world. This paper selected the tight sandstone of the fourth member of the Quantou Formation in the southern Songliao Basin to carry out a CO₂ storage physical simulation experiment. Representative samples were collected at 24 h, 72 h, 192 h and 432 h to study the CO₂ water-rock interaction and to analyze the mineral composition, pore structure and the evolutionary characteristics of physical reservoir properties over time. Physical property analysis, Ion analysis, X-ray diffraction mineral analysis, QEMSCAN mineral analysis, scanning electron microscopy and high-resolution CT scanning techniques were adopted. The main points of understanding were: (i) It shows a differential evolution of different minerals following the storage time of CO₂, and carbonate minerals are mainly dissolved with ankerite as a typical representation; a small amount of calcite is formed in 24 h, and dissolved in the later period; feldspar and quartz were partially dissolved; clay mineral precipitation blocked the pores and gaps; (ii) The evolution in mineral variation leads to the complexity of pore structure evolution, following a trend of “small pores decreasing and large pores increasing” with extending storage time. The final porosity and permeability ratios gradually increase from 4.07% to 21.31% and from 2.97% to 70.06% respectively; (iii) There is a negative correlation between the increasing ratio and the original physical properties of the tight stones due to the dissolution of ankerite. Relevant research could provide scientific guidance and technical support for the geological storage of CO₂ in lacustrine tight continental sandstones and the development of CCUS technology.

This paper investigates high-maturity organic matter-rich shales with high clay mineral contents in the Qingshankou Formation, in the Gulong Depression of the Songliao Basin, at a sub-millimeter scale, using a new laminar division method based on XRF data. The influence of laminar structure on reservoir quality is examined using a combination of geochemistry, mineralogy, and pore structures. Explanatory models are established. Three types of laminar units are distinguished in the study area based on differences in pore structure. These are clay mineral laminae (UA), clay mineral-Ostracod laminae (UB), and clay mineral-felsic laminae (UC). UA has illite intergranular pores, micro-fractures, and organic pores, with diameters of 0.5~2 μm. UB primarily contains Ostracod shell margin fractures, pyrite intergranular pores, and chlorite intragranular pores. UC contains albite and illite intergranular pores. Nitrogen adsorption tests show that UA has the highest clay content and the best specific pore volume and specific surface area, indicating that clay minerals are the main contributors to the pores in this type of unit. 2D–3D models of different laminae reveal that carbonate cement is widely developed in UB and UC, but dissolution pores are less developed, with the result that the porosity of UA is two to three times greater than that of UB or UC. It appears that intergranular pores and fractures, formed during the transformation of clay minerals during the advanced thermal evolution stage, are the main contributors to storage space and flow channels. Thermal evolution, clay mineral transformation, and carbonate cementation are the key factors causing differences between laminar units. In addition, clay mineral laminae (UA) are the most important laminar units for shale oil enrichment in the study area. This finding is of great significance for accurately predicting the distribution of shale “sweet spots” and guiding shale oil and gas exploration.

The exploration and development of Gulong shale oil in the Songliao Basin has achieved a major breakthrough, with the result that a national shale oil demonstration area is being built. The shale in the Sanzhao Sag is abundant in organic matter and is an important replacement area. For this reason, it has recently become a focus for research and exploration. Compared with the Gulong Sag, the Qingshankou Formation in the Sanzhao Sag is relatively immature, and comparatively little is known about its pore structure. For this study, well ZY1, a key well in the Sanzhao Sag, was selected as the research object. Thin section observation, high-resolution field emission scanning electron microscopy, Modular Automated Processing Systemimaging, X-ray diffraction of whole rock and clay, TOC pyrolysis, and Low-pressure N₂ adsorption were used to investigate the pore structure in the sag to determine the factors influencing pore development. The principal understandings are as follows: (1) The Qingshankou Formation in the Sanzhao Sag is predominantly clay shale, with a high content of clay minerals (32.8 to 70%) and TOC contents of 0.7 to 11%. These values indicate good hydrocarbon generation potential. (2) Interparticle pores, intraparticle pores, and organic matter pores are developed. Intergranular pores are the main type, mostly observed between granular minerals or between granular minerals and clays. Intraparticle pores are observed in pyrite mass and dissolved unstable minerals. Organic matter pores (OMP) occur in primary organic matter and migrate into solid bitumen. (3) The shale pores in the Sanzhao Sag are mainly H3-type slit pores, with specific surface areas of 5.4~22.9 m²/g and pore volumes of 0.03~0.07 cm³/g. Mesopores make the largest contribution to pore volume and specific surface area. Scanning electron microscope (MAPS) imaging shows that the pore size distribution of shale is mainly 20~30 nm and 100~200 nm and that large pores, especially pores of 100~200 nm and 1~2 μm, make the largest contribution to the facies. (4) The pore structure in the Sanzhao Sag is affected by mineral composition, abundance of organic matter, thermal evolution degree, and diagenesis. Organic-inorganic interactions influence and determine the pore structure characteristics of the Qingshankou Formation shale. This paper is intended to provide scientific guidance and technical support for evaluating the effectiveness of shale oil reservoirs and selecting sweet spots in the Sanzhao Sag.

Shale has a unique laminar structure that is currently the focus of considerable interest and a great deal of research in the academic community and industry. Studies have focused primarily on the differences among laminar combinations, whereas only a few have investigated the structure of individual lamina. In this study, we use typical organic-rich shale from the Cretaceous Nenjiang Formation in the Songliao Basin as an example and analyze each individual lamina of the shale on a millimeter scale. We have investigated heterogeneities among the different laminae by using X-ray fluorescence analysis, rock slicing, laser scanning confocal microscopy, field emission scanning electron microscopy, total organic carbon (TOC), rock pyrolysis, X-ray diffraction mineral analysis, electronic computer tomography (nano-CT), nitrogen adsorption, and other experimental methods. Four laminar units — composed of clay minerals, feldspar-quartz, and calcite — are distinguished based on their different levels of Ca, K, and Fe. We designate these units as UA, UB, UC, and UD (from the top to bottom in the formation). The palaeoenvironment, organic geochemical parameters, and mineral compositions of the different laminar structures are “two-stage” in character. From UD to UA, TOC values indicate a slightly decreasing trend, whereas the calcite content indicates a substantial increase, which is related to gradual reduction in the paleodepth, increasing aridity of the climate, increase of salinity, and decrease of reducibility during the sedimentary period. Different laminae correspond to different pore structures. The pore types in units UC and UD are mainly clay mineral-related pores and pyrite intergranular pores. In contrast, calcite dissolution pores are common in units UA and UB. Nitrogen adsorption and nano-CT data indicate that the pore development degree and pore size of organic matter in units UC and UD are better than those of units UA and UB. The porosity of UD is 2.6 times higher than that of UA. Laminae have an important influence on shale quality. The traditional approach to shale evaluation takes laminar combinations in shale strata as the research unit. Switching the focus to the heterogeneity of individual lamina could help in the selection of “sweet spots” and identification of optimal locations for shale oil exploration and development.