The pore-throat structure is a critical factor in the study of unconventional oil and gas reservoirs, drawing particular attention from petroleum geologists, and it is of paramount significance to analyze to enhance oil and gas production. In tight sandstone, which serves as a significant hydrocarbon reservoir, the internal pore-throat structure plays a decisive role in the storage and migration of fluids such as water, gases, and hydrocarbons. This paper employs casting thin section (CTS), field emission scanning electron microscope (FE-SEM), high-pressure mercury injection (HPMI), and low-temperature nitrogen gas adsorption (LT−N₂−GA) experimental tests to qualitatively and quantitatively analyze the characteristics of the pore-throat structure in tight sandstone. The results indicate that the pore types in tight sandstone include intergranular residual pores, dissolution pores, intercrystalline pores, and microfractures, while the throat types encompass sheet-shaped, curved-sheet-shaped, and tubular throats. Analysis of the physical and structural parameters from 13 HPMI and 5 LT−N₂−GA samples reveals a bimodal distribution of pore-throat radii. The complexity of the pore-throat structure is identified as a primary controlling factor for reservoir permeability. The fractal dimension (D) exhibits an average value of 2.45, displaying a negative correlation with porosity (R² = 0.22), permeability (R² = 0.65), the pore-throat diameter (R² = 0.58), and maximum mercury saturation (R² = 0.86) and a positive correlation with threshold pressure (R² = 0.56), median saturation pressure (R² = 0.49), BET specific surface area (R² = 0.51), and BJH total pore volume (R² = 0.14). As D increases, reservoir pores tend to decrease in size, leading to reduced flow and deteriorated physical properties, indicative of a more complex pore-throat structure.

Utilizing well logging data, outcrop profiles, and previous research, this study analyzes the sedimentary and tectonic evolution of the Yan’an Formation in the Ordos Basin, correlating the resulting sedimentary facies with hydrocarbon reservoirs to establish the necessary connections. The study reveals that: (1) Vertically, the sediment grain size shows a pattern of coarser grains at the bottom and top, with finer grains in the middle. Horizontally, the grain size tends to become finer from the northern, western, and southern parts of the basin toward the central-western region. (2) Tectonic movements during the Yan’an period controlled the sedimentary environment. These tectonic activities, through uplift and subsidence, caused the Yan’an Formation to experience four stages of sedimentary environments: braided river, lake, delta, and meandering river. (3) The Yan’an Formation exhibits four types of reservoir sandbody stacking patterns—continuous superposition, intermittent superposition, interbedded sand-mud, and single sandbody types—with continuous and intermittent stacking being the most common. (4) The hydrocarbons in the Yan’an Formation originated from the Chang 7 Member of the Yanchang Formation and migrated into the Yan’an reservoirs. The oil is characterized by its low density, low viscosity, and low pour point, indicating it is a high-maturity, high-quality crude oil.

The Longdong region is a recently discovered area for exploring natural gas in the Upper Paleozoic era within the Ordos Basin. The primary layer that produces gas in this area is the Shan1 member of the Permian Shanxi Formation. The research focused on analyzing the petrological characteristics, physical properties, pore structure, and diagenesis characteristics of the reservoir in the 13th member of the Permian Shanxi Formation in the Longdong area of the southwest Ordos Basin. The research was conducted by combining core observation, cast-thin members, physical properties, and other relevant data. Additionally, the study also discussed the primary factors that influence reservoir performance. The analysis indicates that the dominant sandstone types found in the 13th member of the mountain reservoir are primarily quartz sandstone and lithic quartz sandstone. These sandstone types originate from high-energy environments and are located far from the source. The predominant types of pores are intragranular pores and cutting-karst pores, with the pore size primarily falling within the microporous to mesoporous range. The sedimentary facies and diagenesis exert control over the physical qualities of the reservoir. Specifically, the sedimentary facies determines the fundamental physical conditions of the reservoir, while the structure of the sand body significantly influences its physical properties. Compaction is the primary cause of the increased density of the layer's physical properties. The presence of illite is the main factor contributing to the densification of the reservoir through cementation. The limited extent of reservoir dissolution has a minor effect on enhancing reservoir quality.