Nuclear magnetic resonance (NMR)T₂cutoff value is an important parameter for pore structure evaluation. It is complicated and uneconomical to obtainT₂cutoff value by an experimental method; therefore, it is necessary to explore a prediction method ofT₂cutoff value. In this paper, 10 samples of tight gas reservoirs in the eastern Ordos Basin were selected, and then saturation and centrifugal experiments of nuclear magnetic resonance were carried out. On this basis, multifractal theory was introduced to calculate the multifractal characteristics of the NMRT₂spectrum of each sample, and the relationship between multifractal parameters andT₂value was analyzed. The influencing factors of theT₂cutoff value were clarified, and the prediction model of theT₂cutoff value was constructed accordingly. The results show that theT₂spectra of sandstones in the study area can be divided into three types: single steeple peak, double steeple peak, and irregular double peak. The pore diameter of the three types is 1 nm ~ 3×10⁴ nm, 1 nm ~ 10⁴ nm and 1 nm ~ 4×10³ nm, respectively. TheT₂cutoff value ranges from 9.72 to 35.16 ms. The correlation analysis suggests that the symmetrical fractal dimension difference and symmetrical multifractal dimension ratio (D_min−D_max, D_min/D_max) shows a positive linear correlation with theT₂cutoff value. The value ofT₂cutoff gradually decreases with the increase of the flow zone indicator (FZI). Therefore, three parameters, including symmetrical fractal dimension difference, symmetrical multifractal number ratio, andFZIare optimized, and the prediction model for the NMRT₂cutoff value of sandstone samples in the study area is proposed. The introduction of porosity‐related parameters compensates for the shortcomings of previousT₂cutoff value prediction models. At the same time, the prediction model is proven to be accurate and reliable by testing the measured data of the samples near the study area. The results of this paper can be used for further study of the NMRT₂cutoff value prediction of tight sandstone reservoirs in different areas.

The Upper Carboniferous sandstone reservoir is a vital replacement area for natural gas exploration in Ordos Basin. In this study, 157 Jinci sandstone samples were selected to conduct a series of experiments and analyses. The reservoir material composition and pore structure analysis shows that the lithology of the reservoir is mainly quartz arenite, followed by sublithic arenite. The detrital particles are mainly quartz (69-97.5%), followed by rock fragments (0.1-24.5%), and the content of feldspar is less than 0.01%. The cement consists of siliceous material, clay minerals, and carbonate, with averages of 2.34, 5.96, and 1.81%, respectively. Three types of pore-throat structures (HPMI curve: types 1, 2, and 3) are identified in the Jinci sandstone reservoir, corresponding to different pore-throat radius distributions (RCP curves: types A, B, and C). The study of the factors affecting reservoir pore structure and its internal mechanism shows that the reservoir pore-throat combination, affecting the reservoir performance, is mainly controlled by deposit composition and the subsquent diagenetic modification. A higher rigid particle content and an appropriate amount of siliceous cementation (2-10%) would lead to resistance of the compaction, in favor of the preservation of primary intergranular pores. When the content of ductile particles is more than 3%, the original intergranular pores tend to be substantially reduced. The deposit composition of sandstone controls the preservation of residual intergranular pores by affecting the intensity of compaction and dissolution controlling the amount and type of cementation. Compared with dissolution-subjected quartz arenites, the sublithic arenites are characterized by a common occurrence of altered kaolinite and recrystallized illite, which would destroy the reservoir property. The early diagenetic carbonate cementation, as well as the strong siliceous cementation in "sedimentary quartz arenite", are unfavorable to the formation of high-quality reservoirs. Then, on the basis of the characteristics of various diagenesis and their interaction and internal relationship, the diagenetic sequence and diagenetic-pore evolution patterns of different types of reservoirs were established. Finally, according to the lithological characteristics and the diagenetic-controlled pore-throat evolution patterns of different types of reservoirs, the reservoir quality in the study area was predicted.

A series of studies were carried out on 11 tight sandstone samples of Upper Carboniferous in Ordos Basin. Firstly, the deposit composition and pore structure characteristics are investigated based on analysis and experiments including cast thin section scanning electron microscope high-pressure mercury intrusion and nuclear magnetic resonance Then, combined with DP-P test, the stress-dependent permeability change and pore compressibility characteristics of sandstone reservoirs were studied to reveal the influencing factors and mechanism of reservoir pore compressibility. The detrital particles of the sandstone reservoir in the study area are mainly quartz (75.8%–89%), followed by fragments (3%–16.1%), and almost no feldspar. The content of interstitial materials is 6.5%–11.2%. The type I reservoirs mainly consist of mesopores and macropores, accounting for 60.57% and 32.84% respectively. Mesopores are dominated in Type II reservoirs, accounting for 78.98% of the total pore volume. There are almost no macropores, while a similar proportion of mesopores, micro mesopores and micropores in the type Ⅲ reservoirs. The study of pore compressibility shows that the pore compressibility coefficient decreases with the increase of effective stress, and the reduction rate shows the two-stage characteristics of rapid in the early stage and slow in the later stage. The pressure turning point is between 3 and 10 MPa. The average pore compressibility coefficient increases from type I to type Ⅲ reservoirs. The compressibility coefficient is directly proportional to the changing rate of the pore volume. The higher the content of rigid detrital particles, quartz and carbonate cement in sandstone, the smaller the pore compressibility coefficient, while the higher the content of ductile components such as soft rock fragments and clay minerals, the greater the pore compression coefficient. The pore-throat structure is closely related to the pore compressibility, reservoirs with low displacement pressure, T_2glm value, and large average pore-throat radius show lower compressibility coefficient. In addition, the compressibility coefficient of the reservoir is positively correlated with D_L (dimension of large pores such as mesopores and macropores), and negatively correlated with D_S (the fractal dimension of micropores and micro mesopores). It is considered the pore compression of sandstone including two stages, viscoplastic destructive deformation of ductile components for the first and then the small-scale non-ideal elastic deformation on rigid particles.

The provenance and sedimentary environment of peat swamps control the material composition of coal and represent one of the key factors contributing to coal heterogeneity. To clarify the sedimentary environment, provenance characteristics, and coal-controlling mechanisms of the No. 5 Coal Seam in the north-central Ordos Basin, this study investigated the coal petrological, coal quality, geochemical, and coal-accumulating environmental characteristics of the seam via systematic drill core observation and sampling, maceral identification combined with proximate analysis, determination of total sulfur and sulfur forms, vitrinite reflectance measurement, and major-trace element testing. The results show that the macroscopic coal types of the No. 5 coal seam are dominated by bright and semibright coal with primary texture, belonging to low-volatile bituminous coal. Overall, it is characterized by low ash content, low sulfur content, and ultralow total moisture. Among the macerals in the coal, vitrinite is dominant, followed by inertinite, while the exinite content is extremely low; inorganic components mainly include clay minerals and pyrite. During the coal-accumulating period, the paleoclimate was mainly warm and humid, and the water medium environment of the peat swamp was significantly affected by seawater, characterized by a brackish-saline, dysoxic–anoxic reducing environment. The coal seam was formed in a humid, water-covered peat swamp environment dominated by herbaceous plants, featuring weak groundwater dynamics, poor fluidity, strong gelification, intense plant degradation, and high reducibility. The provenance supply of the No. 5 coal seam was stable and diverse, mainly derived from intermediate-felsic materials stably supplied by the Yinshan Paleoland in the northern basin. The coal-accumulation process of the No. 5 coal seam began with the transgression following the regression. The stable tectonic setting, steady provenance supply, and suitable paleoclimate and sedimentary environment all provided favorable conditions for organic matter accumulation in the peat swamp. During the coal-accumulating period of the No. 5 coal seam, short-term fluctuations in the sedimentary environment occurred due to periodic marine transgressions. Based on the North–South differentiation of seawater and paleotopographic relief characteristics, the peat swamp depositional environment was classified into four types: ombrotrophic moist interdistributary depression bog, minerotrophic moist interdistributary depression fen, ombrotrophic water-covered interdistributary bay bog, and minerotrophic water-covered interdistributary bay fen, each with unique coal petrological, coal quality, and sedimentary parameter characteristics. This study is of great significance for revealing the coal-forming mechanism and paleoenvironmental evolution and can provide theoretical support for the efficient and clean utilization of coal resources and the exploration and development of deep coalbed methane (CBM).