The Benxi Formation of the Upper Carboniferous in the Yanchang field of the Ordos Basin is one of the important natural gas production layers, with reservoirs mainly composed of barrier, tidal channel, and shoal microfacies sandstones. Most of the drilled sandstones produce gas, but some of the drilled Benxi Formation sandstones have become dry layers due to densification. Identifying the origin and distribution of tight sandstone and effectively avoiding dry layers has become one of the important issues in the deployment of production wells. This article combines sandstone cores, sedimentary microfacies, microscopic characteristics, and logging responses to study the genesis and prediction methods of tight dry sandstone layers. The results indicate that there are three types of dry sandstone: tuffaceous basal cemented sandstone, carbonate basal cemented sandstone, and tightly compacted sandstone. The first type is concentrated and limited along the ancient volcanic crater near the ancient tidal channel of southern Jingbian; The second type is distributed in the relatively thickened area of the overlying Taiyuan Formation limestone; The third type is distributed in isolated small shoal sandstones, which may result in the lack of dissolution pores during the reservoir formation period which needs acidic fluids and oil and gas to enter the reservoir. This understanding is of great significance for the formation mechanism of effective reservoirs and the prediction of "sweet spot" distribution.

Quantitative characterization and dynamic evolution study of full-scale structure provides an important basis for understanding the forming mechanism of shale reservoirs and clarifying the relationship between pores and hydrocarbon-rock interactions. Using field emission scanning electron microscopy integrated with high-pressure mercury intrusion as well as N₂ and CO₂ adsorption experiments, representative core plugs of different maturity and corresponding samples subjected to hydrocarbon generation and expulsion simulation experiments from Chang 7 oil layer group in the Yanchang area, Ordos Basin, were quantitively characterized for their full-scale structure. Furthermore, combined with hydrocarbon generation and expulsion simulation and X-ray diffraction test results of whole rock/clay mineral content, the interaction of organic hydrocarbon generation and expulsion, mineral diagenesis and pore structure evolution was studied, and the dynamic evolution of reservoir pore structure and fractal characteristics was quantitatively analyzed. The results show that the micropores, small pores and mesopores in shale reservoirs of the Chang 7 oil layer group contribute more to the pore volume, while the micropores and small pores provide most pore specific surface area. With hydrocarbon generation and expulsion of organic matter, dissolution of inorganic minerals and transformation between minerals, the total pore volume first decreases and then increases, and the proportion of meso-macropores decreases first and then increases, and the heterogeneity of micro-small pores is generally enhanced, while the heterogeneity of mesomacropores has a trend of increasing first and then decreasing. The correlation analysis reveals that the pore size and mineral composition are coupled with the heterogeneity of the reservoir, in which the fractal dimension D₁ value of microsmall pores is positively correlated with the proportion of micropores to micro-small pores and the content of clay minerals, while the fractal dimension D₂ value of meso-macropores is negatively correlated with the proportion of macropores to meso-macropores, and positively correlated with the content of brittle minerals. This study provides an accurate determination of the full-scale structure and evolution of the shale in Chang 7 oil layer group, which is of great significance for identifying sweet spots in shale reservoirs of terrestrial facies.