The Upper Triassic Chang-7 Member of the Ordos Basin contains typical tight sandstone reservoirs. Reservoir quality is affected by diagenesis, which is a critical factor in tight oil exploration. In this study, the Chang-7 Member tight sandstones were studied by a variety of experimental methods, including thin sections, scanning electron microscopy (SEM), and X-ray diffraction (XRD), to determine the reservoir characteristics and diagenesis and discuss their influences on the reservoir quality. The Chang-7 Member sandstones are mainly lithic arkose and feldspathic litharenite with an average porosity and permeability of 7.17% and 0.13 mD, respectively. The pore type is mainly primary intergranular pores, and the secondary pores are feldspar dissolved pores, which are relatively developed, with pore radii of 2–20 µm. Diagenesis of Chang-7 Member tight reservoirs mainly includes compaction, quartz cementation, carbonate cementation, clay mineral cementation, and dissolution. The diagenetic stage develops into Mesodiagenesis A. The average porosity loss from compaction and cementation of sandstone in the Chang-7 Member is 15.93% and 18.67%, respectively. With the increase in burial depth, the porosity and permeability of the reservoir gradually decrease. In mesodiagenesis, the authigenic illite and carbonate cementation compacts the reservoir. The acid fluid carried by the two stages of oil and gas filling during diagenesis dissolved feldspar and carbonate cement, which plays a certain role in transforming the tight reservoir.

This study explores the sedimentary environment and tectonic implications of the Upper Carboniferous Yanghugou Formation on the western margin of the Ordos Basin through the analysis of major elements, trace elements, and rare earth elements in 23 mudstone samples. The results indicate moderate chemical weathering conditions, as reflected in the Chemical Alteration Index (CIA) and Component Variability Index (ICV) values. Warm and humid paleoclimatic conditions are suggested by Rb/Sr, Sr/Cu, spore, and pollen fossil samples. The paleosalinity of the water is identified as a transitional phase of a brackish water environment based on Sr/Ba and Th/U ratios. Additionally, V/(V + Ni) and δU values imply a transitional phase of a weak oxidation–reduction environment for the water. In accordance with the La-Th-Sc, Th-Sc-Zr, Th-Co-Zr, (La/Yb)-∑REE, and (La/Th)-Hf discrimination diagrams, it is inferred that the study area is part of a continental marginal tectonic setting. The sediments are primarily derived from an upper-crust felsic source area. The sedimentary period of the Yanghugou Formation in the western margin of the basin is considered a marginal rift basin characterized by north–south compression and western rift extension. This setting is influenced by the active continental margin of the Alxa block and the passive continental margin of the Qilian–north Qinling orogenic belt.

Diagenetic anomalies within the Upper Paleozoic coal-bearing strata of the Longdong area, Ordos Basin, represent a complex interplay between thermal maturation and fluid evolution, yet their governing mechanisms remain poorly understood. This study integrates petrographic analysis, X-ray diffraction, vitrinite reflectance (Ro) measurements, and fluid inclusion microthermometry to evaluate the discrepancy between organic thermal maturity and mineralogical diagenetic records. The results indicate that the mudstones achieved high thermal maturity, with mean Ro and Tmax values of 2.3% and 555.1 °C, respectively. However, the associated sandstones exhibit anomalous mineral assemblages, characterized by persistent high levels of illite/smectite (I/S) mixed-layer minerals and authigenic kaolinite, which are inconsistent with the anticipated advanced diagenetic stage. Furthermore, homogenization temperatures (Th) of fluid inclusions are significantly lower than expected, implying a localized suppression of illitization. We propose that this atypical diagenetic trajectory is governed by sluggish fluid–rock interactions in a confined diagenetic environment. Specifically, the dissolution of feldspars during acidic diagenesis provided a localized Al³⁺ supply, favoring kaolinite precipitation, while the limited availability of reactive feldspar precursors and pore-fluid retention effectively stalled the progression of illitization. These findings demonstrate that reactant availability and reaction kinetics can decouple mineralogical evolution from organic thermal maturation in coal-bearing sequences. This study provides a novel mechanistic framework for interpreting anomalous diagenetic signatures in heterogeneous sedimentary basins, offering significant implications for reservoir quality prediction in deep-seated, thermally mature strata.