This porosity type refers to the voids formed between mutually supportive grains during sedimentation, characterized by their cleanliness and regular polygonal shapes. Thin-section data analysis indicated that the average porosity of the intergranular pores within the different strata of the Upper Paleozoic in the Ordos Basin was 0.44% for the Yanghugou Formation, 0.37% for the Taiyuan Formation, 0.16% for the Shanxi Formation, and 0.35% for the He 8 member. The Yanghugou and Taiyuan Formations exhibit the highest frequencies of intergranular pore samples at 29.59% and 27.49%, respectively (Figure 5). Intergranular pores primarily comprise two categories: residual pores enlarged by secondary quartz and infilled with microcrystalline quartz (Figure 6A) and original residual intergranular pores enveloped by clay minerals such as kaolinite and illite (Figure 6B).

Influenced by organic acids from widespread hydrocarbon generation from Permian coal-bearing source rocks in the Ordos Basin, detrital minerals, the clay matrix, and soluble components, such as tuff, directly dissolve to create secondary porosity. Alternatively, secondary dissolution porosity is indirectly formed by the early diagenetic replacement of less soluble siliceous minerals by other minerals, followed by dissolution. Based on their genesis, the dissolution pores include honeycomb-like erosional pores within or between clastic particles (Figure 6E), embayment-shaped erosional pores in quartz particles and authigenically enlarged quartz (Figure 6C), feldspar dissolution pores (Figure 6D), matrix dissolution pores (Figures 6D, I), and a limited number of indirect erosional pores resulting from the later dissolution of quartz replaced by early carbonates (Figure 6F). Lithic dissolution pores were developed throughout the Upper Paleozoic stratigraphic units, with sample frequencies of 21.11%, 40.8%, 30.10%, and 25.96%, respectively. Feldspar dissolution pores primarily contributed to reservoir construction within the Shanxi Formation, with an observable sample percentage of 15.13% (Figure 5). Intergranular dissolution pores developed more extensively in the Yanghugou Formation, with a sampling frequency of 19.03%.

Intercrystalline pores are primarily formed during diagenesis and develop between authigenic minerals within the pore spaces. The larger the crystal volume, the greater the development of intercrystalline porosity (Han et al., 2019). There has been widespread development of kaolinite and illite intercrystalline pores in the study area. Authigenic kaolinite crystals are pure and coarse, forming hexagonal book-like aggregates under scanning electron microscopy, with larger pore sizes ranging from 5 to 25 μm (Figures 6G, 9K). Illite crystals are fine-grained and polymorphic, exhibiting hair-like, curled lamellar, fibrous forms, with particle sizes typically below 2 μm (Figures 6G, 9J). Compared with other strata, the Shanxi Formation exhibited a higher frequency of intercrystalline pore samples (26.62%), but this did not significantly increase the porosity (Wei et al., 2022).

Microfractures have apertures less than 0.1 mm in size and are primarily caused by compaction, shrinkage, and various tectonic stresses. Fractures induced by tectonic stress tend to be relatively fine and straight (Figure 6H). They often penetrate plastic rock debris and the matrix and are relatively clean inside, with a minority filled with argillaceous and siliceous minerals. Shrinkage fractures are widely present in sandstones and are predominantly located within pore-filling clay minerals, that is, montmorillonite and illite. These clay minerals lost bound or structural water during diagenesis, resulting in the contraction toward detrital grains and the formation of linear shrinkage fractures (Figure 6I).

Owing to protracted burial, the Paleozoic strata have undergone complex diagenetic evolution, leading to various types of diagenetic processes. Among them, compaction, cementation, and dissolution exerted the most significant influence on reservoir properties. Comparing the results with other studies on sandstone porosities due to compaction, which generally decrease below 20% at depths exceeding 3,000 m (Lee et al., 2020).

Compaction: The Paleozoic strata in the western Ordos Basin have been buried at depths exceeding 3,000 m since the mid-Jurassic period and are subject to intensive compaction processes. This has resulted in a substantial reduction in primary porosity, as evidenced by the deformation and twisting of plastic particles. Particles exhibit concavo-convex and suture line contacts (Figure 6A), with rigid particles embedded in plastic particles, consequently diminishing the pore space (Figures 9E, F). The mechanical fragmentation of particles, such as lithic fragments and feldspars, is visible under a microscope, indicative of compaction effects. Compaction has a more significant impact on lithic sandstone and lithic quartz sandstone than on quartz sandstone, with quartz sandstone showing superior resistance to compaction. Therefore, the compaction and pressure solution effects are more prominent in the Shanxi Formation and He 8 member.

Cementation: The primary types of cementation are siliceous and carbonate cementation, with occasional cementation by clay minerals.

Siliceous cement primarily manifests as a substantial secondary enlargement of quartz and a small amount of authigenic microcrystalline quartz aggregates. The development of secondary enlargement of quartz occurs in three stages (Chen, 1990; Li et al., 2015; Su et al., 2021; Zhang et al., 2022). Stage I: During hydrocarbon generation by source rocks, accompanied by the release of abundant organic acids, volcano-like embryonic crystals regrow in a core style on basal crystals, with a limited width of less than 10 μm. The enlargement edges are discontinuous with signs of dissolution, occupying minimal porosity space and formed during the early diagenetic stage B (Figure 9A). Stage II: Quartz growth unevenly encloses quartz grains, forming idiomorphic crystals, with growth margins approximately 15–30 μm wide, partially obstructing porosity. Stage III: In an acidic pore water environment, lithic fragments and feldspars are A (Figure 9B). Stage III: In an acidic pore water environment, lithic fragments and feldspars are dissolved, providing sufficient siliceous material. If there is sufficient porosity, the secondary growth enlargement of quartz from Stage II expands, forming completely polyhedral quartz. Dust lines from the first two stages can be observed under the microscope (Figure 9C), with widths of approximately 15–55 μm, occurring during mesogenetic stage B. In the Yanghugou Formation–He 8 member, the secondary enlargement of quartz and the increase from siliceous influence were especially prevalent, including stage III quartz enlargement in the Shanxi Formation–He 8 member.

Carbonate cement mainly consists of (ferroan) calcite and (ferroan) dolomite, with minor amounts of siderite and glauconite. Cementation can be divided into three stages based on the petrological characteristics of carbonates. Stage I consists of early poikilitic bright crystalline calcite exhibiting ground-mass-style cementation, in which feldspar remains largely undissolved, and compaction is weak. It primarily developed during syngenetic diagenetic stage B (Figure 9D) (Liu et al., 2014). It is predominantly found in the Yanghugou Formation, with limited development in the Shanxi Formation and He 8 member. Stage II is characterized by the formation of organic acids from coal-measure source rocks that dissolve feldspar and lithic fragments. These acids are buffered to a weakly alkaline state, under which small-scale ferroan calcite forms within the tight sandstones. Isotopic analyses (Song et al., 2020) confirmed that Stage II primarily occurred during mesogenetic stage A, with a widespread presence in the Shanxi Formation (Figure 9E). Stage III involves tectonic uplift, where bright crystalline calcite develops along dissolution-induced fractures in grains, sometimes crosscutting mineral particles (Figure 9F), related to late-stage basin uplift and atmospheric freshwater inorganic carbon sources. Siderite mainly occurs in nodular forms, with specific feldspar grains replaced by siderite (Figures 6D, E). These are predominantly distributed in the Yanghugou Formation with limited occurrence in the Shanxi Formation, formed during the syngenetic–early diagenetic stage A. Present in trace amounts and distributed as detrital grains, glauconite is indicative of syngenetic diagenesis.

The primary clay mineral cement includes kaolinite and illite. One source of kaolinite is the alteration of volcanic ash, as indicated by simulation experiments using tuffaceous interstitial materials (Liu et al., 2017; Wang et al., 2005; Zhou et al., 2001). Under the influence of permeable formation water leaching, metallic elements such as Al³⁺, Fe³⁺, and Ca²⁺ can separate, forming impure intergranular silicate clusters with the concurrent formation of dissolution pore spaces. Some tuffaceous interstitial materials are altered to kaolinite clusters. Kaolinite formed by alteration typically has a poor crystalline form, dirty surfaces, and fine grain size. It often retains fine-grained volcanic ash remnants, making it difficult to provide intercrystalline porosity (Figure 9G). This commonly occurs during early diagenetic stage B. Another form occurs during diagenesis, where acidic fluids continuously leach feldspar grains and silica–alumina-rich minerals. When the silicoaluminates reach a certain concentration, kaolinite can precipitate directly, appearing clean under the microscope with a suitable crystalline form and worm-like shape (Figure 9H). This type of kaolinite is mainly distributed in the Yanghugou–Shanxi formations, associated with well-developed intercrystalline pores.

Illite cementation and filling action were more pronounced in the western areas (Figure 4C), influenced by the rates of dissolution, precipitation, and the degree of system openness. The direct formation of illite from feldspar seldom occurs. Kaolinite minerals are the most authigenic clay minerals associated with the dissolution of feldspar and other aluminosilicates in clastic rock formations. However, in the western Upper Paleozoic, the illite content was higher than that of kaolinite. SEM observations with scanning electron microscopy indicate two primary modes of illite formation (Tian et al., 2022; Zhou et al., 2016). 1) Transformation from montmorillonite, including I/M mixed layers in volcanic materials (temperature <120°C–140 °C), the morphology under the electron microscope mainly presents a bilayer structure, bent flaky, with the aggregates appearing honeycomb-like (Figure 9I). This was most evident in the He 8 member, in which typical I/S layers were visible (Figure 9L). Such illite is almost nonexistent in the Taiyuan Formation due to the lack of volcanic material and is also relatively low in the Yanghugou Formation. 2) Reaction of kaolinite with potassium feldspar under high-temperature burial conditions (temperature >120°C–140°C), which is rapid until the feldspar or kaolinite is exhausted. These are often present as hair-like, fibrous, or ribbon-like structures (Figure 9J). The alteration of kaolinite into illite was evident, with hair-like illite visible on the edges of the kaolinite (Figure 9K). This type of illite is present throughout the Upper Paleozoic, especially in the Yanghugou Formation–Shanxi Formation. Sandstones with high original detrital content and complete pore filling by volcanic ash have low initial permeability, preventing hydrolysis reactions from occurring and hindering the infiltration of acidic fluids later (Chen et al., 2015). Until mesogenetic stage B, kaolinite was transformed into illite under alkaline conditions. The transformation in the older strata was more thorough. Therefore, the illite content was higher in the Yanghugou Formation than in the Shanxi Formation.

Corrosion: The study area has extensive dissolution of potassium feldspar, lithic dissolution, and intergranular dissolution. During the Late Paleozoic, frequent volcanic activity supplied the western basin margin with abundant volcanic and felsic minerals (Lv et al., 2017). The dissolution provided a matrix for forming secondary porosity in the reservoirs, indirectly creating various related secondary pores, such as lithic dissolution pores (Figure 6E), intercrystalline porosity (Figures 9G, I), dissolution fractures (Figure 6I), and tuffaceous dissolution (Figure 6E). The Early Cretaceous thermal anomaly acidic fluid leaching event led to extensive dissolution and disappearance of feldspar within the Upper Paleozoic coal-bearing strata (Liu et al., 2017; Wei et al., 2017). This left only residual plagioclase and minor microcline (Figure 9O), enhancing the storage space in tight sandstone reservoirs. The degree of feldspar dissolution is most profound in acidic environments, resulting in a lower feldspar content in the Yanghugou and Taiyuan Formations than in the Shanxi Formation and the He 8 member. Other dissolution processes primarily involved intergranular spaces of quartz and within lithic fragments (Figures 6C, E), with quartz silicate dissolution commonly occurring in alkaline environments. The dissolution of quartz grain edges forms embayed shapes. The contact edges are irregularly concave–convex and expand to form fractures, as seen in the Taiyuan Formation and He 8 member.

Based on burial history studies of the research area (Cao et al., 2015) and according to the clastic rock diagenetic stage classification scheme (Zhou J. S. et al., 2022), diagenesis is categorized into three phases (Figure 10). Considering the diagenetic features, the Yanghugou Formation to the He 8 member is within the syngenetic–mesogenetic diagenetic B phase, with heterogeneity at various stratigraphic levels. In the sandstones of the Yanghugou Formation to the He 8 member, quartz growth was consistently associated with secondary enlargement. The primary carbonate minerals were bright crystalline spar calcites with widespread development of ferroan calcite. The dominant clay minerals are kaolinite and illite, with intergranular contacts predominantly of the point-line type and locally observed suture line contacts. All of these indicate that the stratigraphic levels have generally entered the mesogenetic B phase of diagenesis (Mesodiagenesis refers to the continuous burial process under surface diagenesis, with long time intervals and rather slow pore changes, mainly related to the pressure dissolution of sediments or rocks and the redistribution of materials, recrystallization or metasomatism.). Various diagenetic actions differentially influence the reservoir and may enhance or reduce its petrophysical properties.

Syngenetic diagenetic stage: Freshly deposited and not yet lithified clastic materials steadily subside within the strata and are influenced by weakly acidic groundwater from volcanic detritus and coal-bearing strata. Diagenesis was weak and primarily involved the hydrolytic alteration of volcanic materials (Figure 6E). Mobile elements readily migrated to form mineral assemblages with the local development of siderite rhombs (Figure 9E) and glauconite precipitation (Figure 9N), particularly within the Yanghugou Formation strata. The grains were primarily in point contact or not in contact with the well-developed original intergranular pores.

Early diagenetic stage A was dominated by mechanical compaction, characterized by point contacts between detrital grains and the deformation of pliable lithic fragments. This included slate and schist (Figure 9O), with local fracturing of rigid minerals also occurring (Figure 9F). Coal-bearing strata are subjected to a strongly acidic fluid environment, initiating the early dissolution of alkali feldspar and lithic fragments, with indications of a pressure solution becoming apparent. An alkaline diagenetic environment prevailed in the He 8 member, with early carbonate cementation commencing (Figure 9D) (Sun et al., 2017). During the early diagenetic stage B, some volcanic materials served as pore-filling substances. In strata rich in potassium feldspar, they facilitated the production of early illite and kaolinite, whereas siliceous cement began to precipitate, further reducing the porosity between grains (Figure 9A).

Mesogenetic stage A: As the basin fluctuated and subsided, coal-bearing strata continued to release organic acids, making dissolution the most important diagenetic process that dissolved feldspars, rock fragments, and early carbonate minerals. With increasing compaction, the grains exhibited linear contacts, leaving behind a small amount of primary residual porosity (Figures 6A, 9H). Feldspars are often replaced by other clay minerals, and intercrystalline pores develop within the new minerals. As vitric alteration releases alkali metal ions and acidic fluids that are consumed over time, the diagenetic environment shifts toward alkalinity. This leads to the secondary enlargement of quartz (Stage II) and the appearance of ferroan calcite (Stage II). Although compaction and cementation continued to destroy the primary porosity during this stage, dissolution and alteration generated many secondary and intercrystalline pores.

Mesogenetic stage B: The iron-rich diagenetic fluids gradually approached alkaline equilibrium, while kaolinite was transformed into illite. This resulted in a relative increase in the illite content. Owing to regional uplift, many microfractures were formed (Figure 9P), and quartz and rock fragments were dissolved (Figure 9M). However, as the burial depth increased, cementation by Fe-rich carbonate minerals and quartz intensified (Figure 9F), filling most of the residual porosity (Zhang et al., 2022).

During the Late Paleozoic era, the Alxa ancient land in the northwest, the Yinshan–Yanshan orogenic belt in the northeast, and the Northern Qilian–Qinling orogenic belt in the south emerged as the primary source areas (Han et al., 2014; Wang, 2017). The parent rocks of the Yinshan–Yanshan orogenic belt are predominantly medium to high-grade metamorphic rocks. The Alxa ancient land is characterized mainly by clastic rocks, low-grade metamorphic rocks, and granite. The Northern Qilian orogenic belt is characterized by greenschist schists, slate, and quartzite. The Northern Qinling Orogenic Belt has high-grade metamorphic rocks, magmatic rocks, and a minor amount of sedimentary rocks. Clastic reservoirs based on lithological characteristics can be categorized into two types (Qiu et al., 2019): porosity-increasing clastics, including high-grade metamorphic clastics, quartz clastics, metamorphic sandstone, carbonate clastics, and granite clastics, and porosity-reducing clastics, including volcanic clastics, cryptocrystalline clastics, slate clastics, schist, phyllite, mica clastics, siltstone, mudstone clastics, and chlorite debris. Porosity-increasing clastics underwent extensive internal dissolution along contact fractures under the influence of alkaline fluids, leading to a discernible positive correlation with reservoir porosity (Figure 11A). Porosity-reducing clastics are resistant to dissolution and deformation, occupy space under compaction (Figure 6E), and exhibit a negative correlation with reservoir properties (Figure 11B). According to the nature of the source area parent rocks (Wei, 2002), the Yinshan orogenic belt primarily contributes to porosity-increasing clastics such as gneiss, granulite, and migmatite. The Alxa ancient land mainly yields porosity-reducing clastics such as quartzite and metamorphic sandstone. The Qilian–Qinling orogenic belt is chiefly the source of gneiss, slate, and basalt clastics. This has resulted in distinct reservoir quality variations attributed to the north–south source areas, especially the intense contributions from the north in the Shanxi Formation and the He 8 member. Taking the Shanxi stage as an example, the study area received abundant terrestrial supplies with high sand body thicknesses (Figure 11C). Alxa and Yinshan served as northern material suppliers, whereas the Qilian–Qinling orogenic belt rarely provided materials. Porosity statistics for the west and east parts showed average porosities of 6.39% and 7.05%, respectively, with permeabilities of 0.20 × 10⁻³ μm² and 0.39 × 10⁻³ μm². This suggests that the type of clastic material controlled by the material source governs the evolution of reservoir porosity and final petrophysical properties. Porosity-increasing and porosity-reducing clastics are supplied by the southern material source area, with developed sedimentary clastics, having porosity and permeability of 5.05% and 0.15 × 10⁻³ μm², respectively. These values are lower than those of the northern region, leading to differences in reservoir quality between the north and south. The reservoir quality during other periods was affected by the time and intensity of material supply.

The variations in the physical properties of reservoirs across different depositional facies are primarily governed by the sedimentary environment (Aminul Islam, 2009). Hydrodynamic conditions, as controlled by the sedimentary setting, affect the size, sorting, and rounding of clastic particles, composition and matrix content. This leads to differences in the original porosity and permeability of the sediments (Dong et al., 2014; Weedman et al., 1992).

In the study area, the Yanghugou Formation sand bodies are primarily developed as barrier islands, subaqueous delta distributary channels, river mouth bars, tidal flats, tidal sandbars, tidal channel deposits, and tidal sand flat depositional environments. The Taiyuan Formation sand bodies are deposited in delta plain distributary channels, delta front subaqueous distributary channels, and barrier sandbars, with a more confined sandbar system development compared to the Yanghugou Formation. The Shanxi Formation sand bodies mainly occur in high-gradient alluvial plain meandering river channels, point bars, natural levees, delta plain distributary channels, and delta front subaqueous distributary channel depositional environments. During the Shihezi stage, active tectonic movements and substantial uplift of the paleo-land area resulted in a more robust progradation of the delta into the lake. Compared to the Shanxi Formation, the delta front facies were more developed, with repeated and overlapping multi-stage river channels promoting the development of thick layers of coarse-grained sand bodies (Tian et al., 2011).

The sedimentary environment controls the sandstone thickness, composition, and depositional extent (Bloch et al., 2002). Sandstone thickness is a critical indicator of the depositional environment. The frequent lateral migration of channels and seasonal variations in river flow have led to the formation of sand bodies of varying types and thicknesses (Tian et al., 2011). Sandstone with a thickness of less than 5 m has petrophysical properties with an average porosity of 4.27% and permeability of 0.17 × 10⁻³ μm² that are inferior to those of sandstones with a thickness of 5–10 m, which has a porosity of 5.66% and permeability of 0.31 × 10⁻³ μm². These values are significantly lower than those of sandstones with a thickness of 10–20 m with a porosity of 6.76% and permeability of 1.49 × 10⁻³ μm². The correlation between rock porosity, permeability, and sand body thickness (Figure 12A) suggests that thicker sand bodies can enhance reservoir development, especially within the Yanghugou Formation. Such thick sand body layers indicate a high-energy depositional environment. They represent the intercrossing, stacking, and compounding of various sand body facies controlled by their respective depositional environments (Yang et al., 2014). The thickness of the sand bodies in the Yanghugou Formation ranges from 2 to 10 m and in the Taiyuan Formation from 5 to 20 m. In the Shanxi Formation, they have shown a gradual increase, concentrated in the 10–20 m range, with the He 8 member centering at 15–25 m. This indicates that the petrophysical properties of the continental depositional system are generally more suitable to those of the marine–continental transitional depositional system (Shen et al., 2022).

The main lithology of the reservoir in the study area is quartz sandstone, with quartz content exceeding 60% in each stratum. Quartz grains usually demonstrate strong resistance to compaction, which helps to preserve primary porosity (Weedman et al., 1992). Higher quartz content is correlated with better reservoir quality (Figure 12B). Although feldspar generates secondary porosity during diagenesis, its relatively low content does not substantially enhance the reservoir quality. Other rock particles tend to reduce porosity and permeability during compaction by undergoing cementation and replacement processes that fill the primary pores. Rock fragments have weak resistance to compaction, easily fill primary pores, and lower reservoir quality, especially in the He 8 member (Figure 12C). However, the correlation coefficient was weak. The presence of abundant dissolved pores can increase reservoir porosity and enhance permeability, particularly where the feldspar content is the highest in the Shanxi Formation, making the correlation even weaker. Nevertheless, quartz grains are the most important factor affecting the petrophysical properties of Paleozoic reservoirs. The properties of the quartz sandstone were more suitable than those of the other sandstone types (Figure 7D). The matrix content is an indicator of the structural maturity of sandstones, restricting fluid flow within pores. Diagenetic fluids are less likely to improve pore quality, resulting in a negative correlation between the physical properties of the reservoir and the matrix content (Figure 12D). Characterized by relatively high quartz, low rock fragments, and low matrix content, the Taiyuan Formation and He 8 member form a superior petrophysical foundation.

The grain size of sandstones is also a primary factor affecting the sedimentary reservoir properties, with the average grain size of the principal Paleozoic sandstones gradually increasing. Grain size and petrophysical property statistics from the western regions indicate that the average porosities of conglomeratic coarse sandstone, conglomeratic medium sandstone, coarse sandstone, medium sandstone, fine sandstone, and siltstone are 8.06%, 5.99%, 5.76%, 4.62%, and 2.34%, respectively. The permeabilities are 0.54 × 10⁻³ μm², 0.23 × 10⁻³ μm², 0.15 × 10⁻³ μm², 0.08 × 10⁻³ μm², and 0.03 × 10⁻³ μm², correspondingly. NMR experiments showed that sandstones with larger grain sizes exhibited more suitable micropore structures (Figure 6E). As the grain size became finer, the parameters from the HPMI tests diminished, indicating a reduction and homogenization of pore throats. This suggests that sandstones with larger grain sizes are prone to more open diagenetic environments during diagenetic evolution, facilitating the expulsion of pore fluids and thereby forming more intergranular porosity and enhanced permeability.

In continental systems, the fluvial facies, distributary channels, and subaqueous distributary channels are primarily composed of medium to coarse sandstone. Owing to substantial variations in hydrodynamic conditions, better sorting, higher structural maturity, higher quartz content, and low clay and matrix contents, along with weak compaction and cementation or increased dissolution, these facies have the most suitable reservoir properties. Despite being well sorted from persistent and stable strong hydrodynamic action, mouth bars/natural levee deposits are primarily fine sandstones with a high matrix content, resulting in poorer reservoir properties. In the marine–continental transitional system, barrier sandbars develop abundant quartz sandstones due to repeated scouring by seawater, meaning that they have the highest reservoir quality. Although sand flats are mainly composed of quartz sandstone, they have a higher matrix content, finer grain size, and the lowest compositional maturity, resulting in the poorest reservoir quality. In tidal channels and sandbar deposits, the primary porosity is easily filled by the argillaceous matrix. Therefore, the quality is poor. The petrophysical properties of the Upper Paleozoic reservoirs on the western margin of the Ordos Basin are strongly controlled by depositional facies, with differences in sedimentary processes primarily reflecting variations in the hydrodynamic conditions of each depositional area. Different depositional facies determine the variations in the reservoir’s physical properties (Zhu et al., 2022).

After the sediments are buried, a series of diagenetic processes related to interactions between sedimentary components, pore water, and diagenetic fluids are initiated due to increases in temperature and pressure. These processes affect the development of pore structures, which can degrade or enhance the quality of the reservoir (Baker, 1991). The cumulative effects of diagenesis substantially alter the composition and structure of sandstones and strongly affect their reservoir properties.

Compaction can be categorized into mechanical and chemical dissolution, which occurs at different burial depths (Figure 13A). The first phase occurs at an approximate burial depth of ∼2,500 m and is characterized by early mechanical compaction. Here, increasing pressure leads to the rearrangement of detrital particles into tighter point contacts and the precipitation of authigenic minerals from pore waters into interstitial spaces. This results in a substantial loss of original porosity (Figures 6E, 9O). The second phase occurs between 2,500 and 5,000 m. As the burial depth increased (2,500–4,000 m), the support from rigid particles and cement made further compaction of the detrital particles difficult (Figure 9F). The formation of secondary porosity at this depth slightly enhanced the rock properties. Beyond 4,000 m, the solubility of contact points between rigid particles increases with temperature and pressure, leading to dominant chemical dissolution. This primarily transformed contact modes into sutured and stylolitic contacts and substantially reduced porosity connectivity (Figure 9G). In the Yanghugou Formation, He 8 member, the burial depths are concentrated at 3,500–4,500 m, where chemical dissolution is exceptionally intense. The graphical representation of cement volume to grain volume (Figure 13B) indicates that compaction destroys more original porosity than cementation. Compaction reduced the porosity by a range of 1%–92.5%, averaging 66%, highlighting compaction as the most important factor in degrading the physical properties of the reservoir.

Cementation reduced the porosity within the range of 2.5%–97.5%, with an average reduction of 33%, indicating that cementation is an important factor in sandstone densification. Siliceous cement in tight sandstone reservoirs occurs primarily as secondary enlargements of quartz, with a dual effect on reservoir properties. When the cement content was below 4%, the compressive strength of the rock increased, and the primary pores were protected. At this point, the porosity and permeability are positively correlated. However, the pore throats contract, leading to a decrease in permeability. The Yanghugou Formation to the He 8 member exhibits this distribution. With an increase in the secondary enlargement rim width and contents ranging from 4% to 8%, under the dual diagenetic processes of dissolution in an alkaline environment and pore filling, the correlation between them was not significant. When the content exceeded 8%, pore filling became dominant, with further intensification of the Phase III secondary enlargement of quartz. This affected the densification process of the Yanghugou–Shanxi Formation sandstone reservoirs, with a weaker destructive effect on the He 8 member reservoir (Figure 14A). Commonly observed under microscopy and formed in various stages, carbonate cementation effectively counteracts compaction. Meanwhile, it occupies a considerable volume of reservoir space and blocks pore throats. This is detrimental to the later stages of fluid invasion, leading to reservoir densification. Consequently, sandstones with a high carbonate cement content exhibited lower porosity and permeability, with the primary impact observed in the Shanxi Formation and He 8 member (Figure 14B). The developmental forms and contents of different clay minerals vary, resulting in distinct controlling effects on the reservoir properties of each layer. The genesis, forms of occurrence, and pore-filling patterns of kaolinite contribute to its complex petrophysical relationships. Under a microscope, most kaolinite was derived from feldspar dissolution, fostering the development of dissolution pores. Given that crystals inherently possess intercrystalline pores and have limited filling capacity, the kaolinite content in the study area is positively correlated with porosity but has an indistinct relationship with permeability. The Yanghugou Formation contains less kaolinite with no significant impact on the reservoir, whereas the effect is most pronounced in the He 8 member (Figure 14C). Regardless of its origin, illite tends to form a network-like distribution within the pores, constraining them and resulting in lower petrophysical properties. This can result in manifestations in the petrophysical relationships of both the Yanghugou Formation and the He 8 member (Figure 14D). Chlorite suppresses compaction (Figure 14E) and prevents the destruction of pores by subsequent cementation processes such as quartz enlargement (Figure 9I). It may occupy pores and reduce pore connectivity, with the main expression in the Shanxi Formation–He 8 member. However, owing to its relatively low content (<0.6%), its impact on reservoirs is minimal.

Under strong compaction and cementation, the sandstones were almost devoid of primary intergranular porosity. Thus, dissolution played a critical role in forming sweet spots within the reservoir. The secondary porosity in the reservoirs at various stratigraphic levels originated differently, with the Yanghugou and Taiyuan Formations mainly developing lithic and intergranular dissolution pores. Meanwhile, the Shanxi Formation and He 8 member primarily exhibited lithic and feldspar dissolution pores (Figure 5). The presence of abundant volcanic material in the Yanghugou Formation promoted the extensive dissolution of feldspar. Meanwhile, the depositional environment of the Taiyuan Formation sand bodies, predominantly transitional marine–continental barrier islands with high quartz content, did not favor the development of feldspar dissolution pores. The sand bodies of the Shanxi Formation and He 8 member reservoirs are mainly derived from the subaqueous distributary channels of the delta front, which contain abundant feldspar. Pervasive hydrocarbon generation in coal-bearing strata produces organic acids, leading to the extensive dissolution of unstable particles, such as lithic fragments and feldspar. Organic acids also accelerated the erosion rate of quartz (Guo et al., 2003), resulting in the partial intergranular dissolution of quartz (Figure 6C).

The composition, structure, and diagenetic processes of tight sandstone are crucial for reservoir quality. However, in the west, some tight sandstones with high permeability (>1 × 10⁻³ μm²) have a weak relationship with these factors and show a low correlation with the depositional and diagenetic processes. This indicates that relatively high-permeability reservoirs are controlled by other factors. The extractive values of low permeability and ultralow-permeability oil and gas reservoirs are closely related to the presence of various structural and nonstructural fractures within the reservoir (Fan et al., 2016).

The thrust fault belt along the western margin began to bulge in the Early Triassic, reached its peak during the Yanshanian period, and became more complex during the Himalayan orogeny. High-permeability areas in the Upper Paleozoic were mainly distributed in structural regions affected by post-depositional Yanshanian and Himalayan movements (Gao et al., 2019; Xia et al., 2005; Zhao et al., 2016). Structural fracture reservoirs have been discovered in structures, such as the Shigouyi and Liujiagou back anticlines (He et al., 2021). This indicates that these reservoirs were controlled by later tectonic movements (Fan et al., 2022). Image logging showed that multiple phases of fractures and faults developed in different layers of the structural belt. Image logging of Well L 61 shows blocky and stratified core structures with high-angle fractures of varying orientations, such as predominantly SW-oriented fractures in the Yanghugou Formation and NW–SE-oriented fractures in the Taiyuan Formation (Figure 15). The structural regions formed by compressive stresses in different directions suggest shallow burial depths and lower overlying stratum pressures that prevented extensive diagenetic damage in later periods. The folding and faulting action in the foreland basin thrust belt forms a network fracture system with infiltration and dissolution by atmospheric freshwater. This plays an essential role in enhancing the permeability of the reservoir (Li et al., 2024). Under the microscope, multiple phases of structural fractures can be observed (Figures 6H, 9P). They cut through debris particles, promoting late-stage dissolution modification and increasing porosity connectivity. In the 4,168.5–4,176 m interval of Well L 61, the rock is relatively dense, with an average porosity of 1.7% and a permeability of 0.3 × 10⁻³ μm². There are a total of 19 fractures, and at the fracture at 4,174.16 m, the permeability sharply increases to 5.1 × 10⁻³ μm². The permeability at the fractures was measured, with an average permeability of 3.45 × 10⁻³ μm² and a maximum of up to 53 × 10⁻³ μm². However, the correlation between porosity and permeability was not pronounced, indicating that this controlling effect is limited and that it is essential to ensure that the original porosity of tight reservoirs is not compromised. If the original reservoir lacks sufficient resistance to compaction and has substantial pore filling, such fractures will not contribute significantly to the formation of high-quality reservoirs.