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Article

Genesis of Conventional Reservoirs in Braided Fluvial Tight Sandstones: Evidence from the He 1 Member, Upper Paleozoic, Southern Ordos Basin, China

by
Xiaoqi Ding
1,
Yi Wang
1,*,
Jingyun Gao
1,
Feilan Lin
1,
Xiang Zhang
1,
Shujie Han
1 and
Ying Zhu
2
1
College of Energy, Chengdu University of Technology, Chengdu 610059, China
2
Chengdu Exploration and Development Research Institute of Daqing Oilfield Co., Ltd., Chengdu 610500, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(11), 1104; https://doi.org/10.3390/min15111104
Submission received: 30 August 2025 / Revised: 21 October 2025 / Accepted: 21 October 2025 / Published: 23 October 2025
(This article belongs to the Special Issue Natural and Induced Diagenesis in Clastic Rock)
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Abstract

The He 1 Member of the Xiashihezi Formation (Upper Paleozoic) in the Ordos Basin represents typical tight sandstones (Φ < 10%, k < 0.5 mD). However, against the extensive tight sandstone background of the He 1 Member in the southern basin, conventional reservoirs (Φ > 12%, K > 1 mD) occur locally. Elucidating the genetic mechanism of these conventional reservoirs is critical for evaluating gas reservoirs in this region. Based on core descriptions and systematic sampling from cored wells, reservoir types are classified according to pore types and porosity in sandstones. Depositional microfacies, petrology, and diagenesis of each reservoir type are then investigated to ultimately elucidate the genetic mechanism of conventional reservoirs. Results demonstrate that intense compaction and quartz overgrowths are the primary controls on the development of the He 1 Member tight sandstones. Alteration of volcanic lithic fragments and volcanic ash matrix generated abundant intragranular dissolution pores and micropores within the matrix, while simultaneously producing substantial illite–smectite mixed-layer clays and chlorite clays. Additionally, this process supplied silica for quartz overgrowths. Moderate amounts of chlorite coatings can inhibit quartz overgrowths, thereby preserving residual intergranular porosity. Conventional reservoirs exhibit low lithic fragment content (<20 vol.%) and are characterized by a porosity assemblage of both intergranular (avg. 2.3%) and intragranular dissolution pores (avg. 6.5%). Their formation requires weak compaction, intense dissolution, and weak quartz overgrowths. These reservoirs develop within high-energy transverse bars that are sealed by overlying and underlying mudstones. Such transverse bars constitute closed intrastratal-diagenetic systems with restricted mass transfer during burial. This study provides a compelling example of diagenetic heterogeneity induced by variations in sandstone architecture within fluvial successions.

1. Introduction

Tight gas is one of the key areas of unconventional natural gas exploration worldwide, and large-scale tight gas exploration and development has been achieved in many basins in China [1,2,3]. Within the Upper Paleozoic successions of the Ordos Basin, tight gas resources are 13.32 × 1012 m3, accounting for approximately 61% of China’s total tight gas resources [4]. Major gas fields include the Sulige, Mizhi, Zizhou, Shenmu, Daniudi, Yan’an, and Dongsheng, with annual production exceeding 400 × 108 m3. These fields constitute China’s major tight gas province, exhibiting the largest proven reserves and highest gas production in China [5]. Due to the low permeability of the reservoirs, production wells exhibit negligible natural gas flow capacity. After hydraulic fracturing stimulation, vertical wells yield 1 × 104–2 × 104 m3/d, whereas horizontal wells achieve up to 10.7 × 104 m3/d. The Permian He 1 Member gas reservoir exhibits extensive distribution in the Ordos Basin. Vertically and laterally amalgamated channel sandbodies form sheet-like geometries with remarkable regional continuity [6,7,8]. Reservoir lithologies predominantly comprise coarse lithic sandstones and sublithic sandstones [1,9]. Severe mechanical compaction and intensive cementation during diagenesis have resulted in pore systems dominated by intercrystalline micropores and intragranular dissolution pores, with minor intergranular pores and microfractures. Porosity ranges primarily between 4–8%, while permeability falls within 0.01–1 mD, defining typical low-porosity, low-permeability tight sandstone reservoirs [10].
The formation of sandstone reservoirs is governed by multiple factors including depositional environment, diagenesis, and tectonic activity [11,12,13,14,15,16,17], with variations in reservoir quality primarily resulting from the superimposed effects of primary sedimentary characteristics and subsequent diagenetic modifications [18,19,20,21,22,23]. The sedimentary environment determines the initial pore structure and mineral composition of sandstones. For instance, the Abu Madi Formation (Miocene) in the Nile Delta of Egypt, characterized by braided river and point bar deposits, exhibits primary porosity up to 26% [24]. Diagenesis exerts a dual influence on reservoir quality: constructive processes, such as feldspar dissolution generating kaolinite (up to 46%) in the Kerri-Kerri Formation (Paleocene-Eocene) of the Upper Benue Trough, Nigeria, significantly enhance secondary porosity [25], while destructive processes, exemplified by early calcite cementation occluding pores in Miocene sandstones of Southern California, necessitate acid stimulation to restore connectivity [26]. Tectonic activities, including fracturing and basin inversion, further modify reservoirs, as demonstrated by the Nubian Formation (Cretaceous) in Libya’s Sirt Basin, where fracture networks markedly improve permeability in low-porosity sandstones [27]. Analogously, the Permian sandstone reservoirs in the Ordos Basin result from the effects of favorable depositional facies, constructive diagenesis, and tectonic fractures. However, during exploration in the southern Ordos Basin, a set of conventional reservoirs with permeability >1 mD and natural gas productivity was encountered, which cannot be adequately explained by existing models for high-quality reservoirs. This has led to uncertainties in understanding the distribution and scale of these reservoirs, constraining target selection for further gas exploration.
The Upper Paleozoic clastic reservoirs in the Ordos Basin are predominantly composed of channel sand bodies, yet exhibit rapid lateral variations, significant heterogeneity in physical properties, and complex spatial stacking patterns [28,29,30,31]. To identify exploration “sweet spots”, previous studies adopted a progressive approach to fluvial reservoirs by systematically analyzing sedimentary facies, subfacies, and microfacies, achieving notable successes during early exploration phases [32,33,34,35]. However, as gas field exploration and development progressed, the limitations of traditional microfacies-based methods in characterizing reservoir heterogeneity became apparent. To address this, this study introduces the concept of architectural element analysis to refine the classification of braided channel sand bodies, investigating their petrophysical properties from an architectural perspective, thereby providing new insights into heterogeneity for reservoir stimulation and production enhancement.
Through lithofacies description of cored wells and systematic sampling, this study classifies reservoirs based on fundamental pore types and physical properties, investigating sedimentary microfacies, petrology, and diagenesis of different reservoir types. The research aims to address two key issues: (1) the formation mechanism of conventional reservoirs (k > 1 mD) in the He 1 Member of southern Ordos Basin; and (2) how architectural element variations control reservoir quality differentiation by regulating diagenetic fluid activities. These findings not only provide a case for understanding diagenetic and pore evolution differences in fluvial sandstones but also offer novel perspectives for ‘sweet spot’ prediction in braided channel tight sandstones.

2. Sampling and Methods

Detailed core descriptions were conducted on nine cored wells within the study area, with 93 representative samples collected for integrated analysis including conventional petrophysical measurements, resin-impregnated thin-section petrography, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI) capillary pressure analysis, and X-ray diffraction (XRD) mineralogy. During thin-section analysis, 200–300 counts per section were made for composition analysis, and 400–500 counts per section were made for grain size and sorting examination.
Reservoir types were classified based on conventional petrophysical properties and pore-types analysis. Subsequent investigation delineated petrology, pore structure, and diagenesis of the different reservoir types. Finally, the study synthesized diagenetic sequences for each reservoir type and elucidated controlling factors of favorable reservoirs.

3. Geological Setting

Ordos Basin, occupying approximately 370,000 km2, is one of China’s major sedimentary basins. It features a large west-dipping monoclinal structure and ranks among China’s most hydrocarbon-rich basins (Figure 1).
Following deposition of the Middle Ordovician Majiagou Formation carbonates, the Caledonian Orogeny triggered regional uplift of the North China Platform (including the Ordos Basin). The Ordos Basin was subjected to 140 million years of subaerial exposure with intense weathering, leaching, and erosion. During the Late Carboniferous, persistent southward subduction of the Paleo-Asian Ocean (Solon Ocean) along the northern margin of the North China Platform and rapid expansion of the Mianlue Ocean to the south induced gradual subsidence of the North China Platform, initiating renewed sedimentation [36]. The Late Paleozoic stratigraphic successions in the Ordos Basin comprise Late Carboniferous Benxi Formation and Early Permian Taiyuan Formation mixed carbonate-siliciclastic sequences with coal seams. This progression continues into the Early Permian Shanxi Formation lacustrine-deltaic siliciclastics interbedded with coals, succeeded by Middle Permian Shihezi Formation and Late Permian Shiqianfeng Formation fluvial siliciclastic successions.
During He 1 Member deposition, persistent southward subduction and compression of the Amur Block along the northern margin of the North China Platform induced further crustal uplift [37]. The northern uplift generated abundant detrital supply and enhanced surface runoff activity, developing long-transported fluvial depositional systems. Concurrently, uplift of the Qinling Orogenic Belt in the southern basin intensified volcanic activity, evidenced by tuff intervals encountered in boreholes and pervasive volcanic ash matrix within sandstones. The He 1 Member consists of approximately 60 m of braided fluvial deposits. Sandbodies (1–3 layers, each 2–20 m thick) are composed of mid-channel bars (longitudinal and transverse bars) and channel fills.

4. Results

4.1. Sedimentary Microfacies and Characteristics

The He 1 Member is a braided fluvial system, within which four distinct sedimentary microfacies are identified based on sedimentary structures and sandstone stacking patterns: channel fills, longitudinal bars, transverse bars, and laminated sandstones (Figure 2).
Channel fills form through fluvial incising and rapid sedimentary infilling, depositing dominantly granule conglomerates and coarse sandstones with abundant floating granule clasts. These deposits feature basal gravel lags (Figure 3a) that fine upward into sand-dominated intervals. Bedding structures are poorly defined, though faint cross-bedding is discernible. The composition of sandstone is lithic to sublithic. They exhibit moderate rounding but poor sorting. The sandstones contain 59–83 vol.% quartz (average 62.5 vol.%), 15–38 vol.% rock fragments (average 28.34 vol.%), and 1–6 vol.% matrix (average 3.2 vol.%), with clay dominated by illite and illite–smectite mixed-layer clay minerals.
Longitudinal bars, characteristic of gravel-bed braided river systems, are sandbars with their long axes parallel to the flow direction, formed through the progressive downstream accretion of a series of foreset packages. During flood events, shallow flows across the bar surface deposit horizontally bedded layers, while the accretion at the downstream bar margins generates low- to moderate-angle cross-bedding, reflecting the dynamic interplay between flow energy and sediment depositional processes. These deposits consist predominantly of fine- to coarse-grained sandstones, occasionally intercalated with gravel-dominated bars (Figure 3b). Vertical grain-size trends are poorly defined. Sedimentary structures are dominated by low-angle planar cross-bedding, with subordinate trough cross-bedding. The sandstones are primarily lithic sandstones, containing 55–69 vol.% quartz (average 60.38 vol.%), 22–39 vol.% lithic fragments (average 32.69 vol.%), and 2–5 vol.% matrix (average 3.03 vol.%), with significant altered volcanic rock fragment material and illite–smectite mixed-layer clay.
Transverse bars, characterized by their orientation perpendicular to flow direction in sandy braided river systems, represent downstream-migrating sandbars whose vertical accretion is manifested by horizontal bedding surfaces and high-angle cross-bedding (Figure 3c). During peak flood events, sediments are transported along the upstream bar slope and subsequently avalanched on the lee side, forming slump foresets or tabular cross-bedding within the bar interior, while the bar tops typically develop parallel lamination or trough cross-bedding, reflecting the dynamic interplay between high-energy flow regimes and sediment depositional processes. These deposits consist predominantly of medium-to-coarse sandstones exhibiting faint vertical grain-size trends. The sandstones are primarily sublithic sandstones, containing 58–91 vol.% quartz (average 65.1 vol.%), 9–31 vol.% lithic fragments (average 20.2 vol.%), and 1–3 vol.% matrix (average 2.89 vol.%), with well-developed authigenic chlorite clay.
Laminated sandstones form through overbank deposition during flood events, typically accumulating atop bar surfaces or along channel margins. Individual beds range from several to tens of centimeters in thickness, exhibiting planar or slightly scoured basal contacts. These deposits display a typical vertical sequence: parallel-laminated or low-angle cross-bedded fine sandstones (Figure 3d) grading upward into ripple-laminated siltstones with mica-rich horizons. This succession records waning flow energy during individual flood episodes. Laminated sandstones are lithic sandstones, containing 52–68 vol.% quartz (average 57 vol.%), 27–42 vol.% lithic fragments (average 34.5 vol.%), and 4–6 vol.% matrix (average 5.33 vol.%). The clay is dominated by illite.

4.2. Reservoir Quality

4.2.1. Porosity-Permeability Characteristics

The He 1 Member sandstones exhibit predominantly tight, low-permeability characteristics, though reservoir quality varies significantly among depositional microfacies. Transverse bar facies demonstrate superior physical properties on average, followed by channel fills and longitudinal bars, with laminated sandstones exhibiting the poorest reservoir quality (Figure 2 and Figure 4).

4.2.2. Pore Type

The He 1 Member sandstones are mainly composed of intragranular dissolution pores and micropores within clay, with intergranular pores being subordinate. Pore type distribution exhibits significant variation across depositional microfacies (Figure 5). Transverse bar sandstones are dominated by dissolution pores and micropores, with highly variable amounts of intergranular pores. Channel fills and longitudinal bar sandstones primarily contain dissolution pores and micropores, with rare occurrence of intergranular pores. Laminated sandstones are principally characterized by micropores, occasionally displaying minor intragranular dissolution pores, and completely lack intergranular pores.
Based on pore types, the characteristics of porosity and permeability and gas production test data, sandstone reservoirs are classified into three categories:
Type I Reservoir: Dominated by dissolution pores and intergranular pores with porosity >10% and permeability >0.5 mD. These conventional reservoirs can get commercial gas flow rates without hydraulic fracturing stimulation.
Type II Reservoir: Primarily contain dissolution pores and micropores with sporadic intergranular pores. Porosity ranges from 8% to 10% and permeability 0.1–0.5 mD. Gas production ranges between 0.6 × 104–1.3 × 104 m3/d after fracturing stimulation.
Type III Reservoir: Characterized by micropores with subordinate dissolution pores. Porosity is 4–8% and permeability 0.07–0.1 mD. Gas production ranges from 0.1 × 104 to 0.5 × 104 m3/d after fracturing stimulation.

4.2.3. Pore Structure

Mercury injection capillary pressure (MICP) data indicate reservoirs with very fine to fine throats, poor throat sorting, and complex pore-throat structure. Different sandstone reservoirs have different pore-throat structures (Figure 6):
Type I Reservoir exhibits entry pressures <0.5 MPa and median pressures of 1.2–8.6 MPa (average 5.5 MPa). Pore volume accessible through throats >1 μm constitutes 15.9–51.1% of total porosity (average 25.7%), with mercury withdrawal efficiency exceeding 38%.
Type II Reservoir exhibits moderate entry pressures (0.5–1 MPa) and median pressures of 3.4–26.2 MPa (average 9.9 MPa). Throat-connected porosity (>1 μm) represents 9.9–31.8% of total pores (average 11.5%), with mercury withdrawal efficiency >30%.
Type III Reservoir features elevated entry pressures (typically >1 MPa) and median pressures of 10.3–79.5 MPa (average 27.1 MPa). Pores linked to >1 μm throats account for 2.7–8.2% of total porosity (average 4.8%), with mercury withdrawal efficiency exceeding 20%.

4.3. Petrological Characteristics

The three types of reservoir display marked differences in detrital grain texture, composition, and interstitial materials (Table 1 and Figure 7):
Type I Reservoirs comprise coarse-grained sublithic sandstones. Rigid quartz grains form frameworks with sparse, uniformly distributed lithic fragments. Minor volcanic rock fragments transform to illite–smectite mixed-layer clay. Well-developed pore-lining chlorite occurs with sporadic quartz overgrowths and heterogeneously distributed minor ferroan calcite.
Type II Reservoirs consist of medium-to-coarse-grained lithic sandstones to sublithic sandstones. Compared to Type I, they show elevated lithic content and significantly reduced pore-lining chlorite cementation. Clay mineral assemblages diverge markedly, with increased proportions of illite–smectite mixed-layer clay. Quartz overgrowths are pervasive.
Type III Reservoirs are medium-grained lithic sandstones with diminished intergranular chlorite clay relative to other types. Kaolinite and illite–smectite mixed-layer contents are similar to those in Type II Reservoirs, but the proportion of illite increases markedly. Quartz overgrowths are insignificant.

4.4. Diagenesis

4.4.1. Compaction

Despite similar burial depths (2600–3000 m) in the He 1 Member sandstones, compaction intensity varies substantially. Detrital grain contacts range from point contacts to concave-convex contacts (Figure 8a,b). Under comparable vertical stress conditions, variations in grain contacts are predominantly controlled by ductile grain content [39]. Within the He 1 Member, ductile grain content exhibits a significant negative correlation with grain size, a typical feature throughout the Ordos Basin [40]. The intergranular volume (IGV)–cement volume plot demonstrates that fine-grained sandstones undergo the most intense compaction, which may lead to porosity loss exceeding 75% (Figure 9). Typically, fine-grained sandstones exhibit porosity values below 10%. In contrast, medium-grained sandstones still experience significant compaction, yet their porosity generally reaches approximately 12%. When sandstones are coarse-grained with minimal cement content, their porosity often remains above 15%.

4.4.2. Dissolution

Dissolution primarily occurs in volcanic rock fragments and volcanic ash matrix, generating intragranular dissolution pores, moldic pores, and matrix-hosted micropores. Intragranular dissolution pores form through selective dissolution of unstable minerals such as volcanic rock fragments or feldspars in acidic fluids (Figure 8d), preserving the original mineral outlines. Moldic pores result from complete dissolution of grains, leaving voids that retain the morphology of the precursor particles (e.g., dissolution of volcanic ash matrix). Matrix-hosted micropores originate from dehydration-induced shrinkage of volcanic ash matrix or clay minerals (Figure 8c), creating intercrystalline microcracks and micropores. This process develops predominantly in sandstones with moderate ductile rock fragments content. When the content of ductile rock fragments is high, early mechanical compaction eliminates intergranular pores, inhibiting pore-water circulation and trapping the dissolution by-products within the reaction system. Conversely, sandstones lacking volcanic components exhibit underdeveloped dissolution due to the absence of soluble minerals.
Dissolution of volcanic rock fragments predominantly yields illite–smectite (I/S) mixed-layer clay and microquartz (Figure 8e), with partial in situ precipitation within dissolution pores. Dissolution of volcanic ash matrix primarily generates illite–smectite mixed-layer clays and illite (Figure 8c,d). Meanwhile, dissolution of volcanic materials releases substantial iron ions, magnesium ions and silica, supplying essential constituents for chlorite cements and quartz overgrowth [41].

4.4.3. Cementation

(1)
Chlorite cementation
Chlorite cements mainly occur as pore-lining with a thickness of 2–28 μm, exhibiting pronounced spatial heterogeneity in abundance. Pore-lining chlorite is abundant (up to 3%) in sandstones with well-developed intergranular pores (Figure 8b). In contrast, they are virtually absent in sandstones characterized by pronounced quartz overgrowths, abundant matrix, and severe compaction caused by ductile rock fragment deformation (Figure 8a,c,f).
(2)
Silica cementation
Silica cements occur in two forms: quartz overgrowths and microquartz within micropores (Figure 8e,f). Quartz overgrowths are widespread but highly variable in abundance. In quartz-rich sandstones, they may completely occlude intergranular pores, forming tightly cemented, non-porous sandstones. Maximum quartz overgrowth content reaches 8.3%, with hand specimens resembling quartzite. Conversely, sandstones with well-developed pore-lining chlorite or those containing high ductile lithic content under severe compaction lack quartz overgrowths (Figure 8a,b). Microquartz primarily occurs within intragranular dissolution pores associated with dissolution products: illite–smectite mixed-layer clays, illite, and kaolinite (Figure 8e). These crystals range from several μm to 30 μm in size and are genetically linked to volcanic lithic dissolution. Following alteration of volcanic materials, microquartz precipitates to varying degrees within the micropores [41].

4.4.4. Other Diagenetic Alterations

(1)
Ferroan calcite replacement and cementation
Ferroan calcite locally replaces volcanic rock fragments or matrix (Figure 8g), while rarely occurring as intergranular pore-filling cement. Its content ranges from 0–8% (average 1.3%), exerting negligible impact on reservoir quality.
(2)
Fracturing
A set of shear fractures is observed in the sandstone, with planar fracture surfaces. The average width of the fractures is 3 μm. The fractures cut across all diagenetic cements and remain unfilled (Figure 8h). This type of shear fracture shows no apparent correlation with reservoir lithology. Being unfilled, it is inferred to have formed relatively late, most likely during the Himalayan period, although conclusive evidence is currently lacking.

5. Discussion

5.1. Reservoir Pore Evolution

The He 1 Member sandstones have undergone intense diagenetic alteration including mechanical compaction, dissolution, and multiple cementation. Diagenetic sequences vary significantly across different types of sandstone reservoirs, ultimately controlling reservoir pore evolution (Figure 10).

5.1.1. Type I Reservoir

At the onset of diagenesis, mechanical compaction induced minimal intergranular pores loss due to limited ductile grain deformation, preserving substantial primary intergranular pores. Localized pore loss occurred only where ductile grains were abundant. In eogenetic freshwater environments, the volcanic ash matrix was altered to form mixed-layer illite/smectite (I/S) and illite, generating abundant dissolution pores. Concurrently, minor amounts of kaolinite, illite, and authigenic quartz precipitated as cements, partially filling intergranular pores, while chlorite coatings developed as continuous rims around quartz grains, exhibiting a coverage of 78 ± 12% (Table 2). In the late diagenetic stage, the chlorite coatings effectively inhibited quartz overgrowth (overgrowth coverage: 15.2 ± 1.8%). Concurrently, clay mineral transformations occurred, including the illitization of kaolinite and chlorite. With increasing burial depth and temperature, mechanical compaction further reduced intergranular porosity. During this phase, clay minerals were dominated by late-stage chlorite and illite. Consequently, Type I reservoir exhibits coexisting intergranular and intragranular dissolution porosity with superior pore-throat connectivity.

5.1.2. Type II Reservoir

Early compaction induced ductile grain deformation that partially occluded intergranular pores, yet substantial primary intergranular pores persisted. During eogenesis, acidic freshwater fluids selectively dissolved detrital components. Volcanic lithics and volcanic ash matrix were altered to I/S mixed-layer clay and kaolinite. This dissolution generated intragranular pores and micropores. Due to abundant matrix content, pore-lining chlorite was poorly developed. During this stage, authigenic cements were predominantly composed of chlorite, kaolinite, and illite. In the late diagenetic phase, the alteration of volcanic materials released abundant silica, leading to extensive quartz overgrowth (quartz overgrowth coverage: 42.7 ± 8.9%) under alkaline conditions. This process occluded intergranular volume, further densifying the sandstone. Clay minerals during this stage were dominated by kaolinite and illite, with illite primarily derived from authigenic precipitation from pore fluids and transformation of other clay minerals. Consequently, Type II reservoirs are dominated by isolated intragranular dissolution pores, resulting in poor pore connectivity.

5.1.3. Type III Reservoir

Throughout early diagenesis, severe mechanical compaction induced pervasive plastic deformation of ductile rock fragments, substantially reducing intergranular pores. With increasing burial depth and temperature, the volcanic ash matrix between grains undergoes alteration, forming micropores within the I/S mixed clay. This alteration process is accompanied by the precipitation of minor amounts of authigenic cements (e.g., kaolinite, illite, and quartz), which partially fill intergranular pores. In mesogenesis, minor quartz overgrowths occluded residual intergranular pores. Syneresis of altered volcanic ash matrix generated microfractures. Ultimately, Type III reservoir exhibits pore systems dominated by micropores within clay matrix and volcanic rock fragments, resulting in poor pore connectivity.

5.2. The Relationship Between Volcanic Material and Dissolution

5.2.1. Dissolution Time

Within the diagenetic sequence, dissolution primarily preceded or was synchronous with quartz overgrowth cementation. Diagnostic evidence includes: (1) pervasive ductile grain deformation contemporaneous with dissolution and alteration; (2) partial occlusion of dissolution pores by quartz cement; (3) widespread quartz overgrowths enveloping dissolution residues (Figure 8f); (4) late-diagenetic ferroan calcite infilling of dissolution pores.

5.2.2. Dissolution Material Sources

The He 1 Member sandstones contain volcanic constituents comprising: (1) volcanic rock fragments and (2) volcanic ash matrix. Volcanic rocks originated from provenance weathering. Southern basin provenance analysis identifies Paleoproterozoic Qinling Group and Mesoproterozoic-Neoproterozoic Kuanping Group as primary provenances. The Qinling Group comprises high-grade metamorphic rocks, whereas the Kuanping Group consists of low-grade metasediments derived from terrigenous clastics and basaltic volcanic rocks [42,43,44]. This provenance is the primary reason for the ubiquitous occurrence of volcanic and low-grade metasedimentary lithics in He 1 Member sandstones. Moreover, varying proportions of synsedimentary intermediate- to felsic- volcanic ash matrix are present, and are altered to I/S mixed-layer clay during diagenesis.

5.2.3. Differential Dissolution Patterns

Certain volcanic and low-grade metasedimentary rock fragments exhibit ductile properties, undergoing pronounced plastic deformation during mechanical compaction that destroys intergranular pores and throats. When substantial intergranular porosity diminishes and throats close, diagenetic fluid mobility becomes restricted, thereby reducing dissolution efficiency. Even where dissolution occurred, the cessation of dissolution reactions was inevitable as reaction products could not be efficiently transported out of the reaction system [45,46,47].
Hence, moderate lithic content is a critical control on dissolution. Here, “moderate” denotes that rigid quartz grains form a framework, with lithic fragments dispersed among quartz grains. Under such conditions, even when ductile lithics undergo plastic deformation, they will not cause multiple throats to close simultaneously. Statistics confirm that when ductile grain content remains below 10%, deformation-induced pore-throat occlusion rates are maintained below 20%, thereby enhancing subsequent dissolution efficiency.
Volcanic ash matrix exhibits an analogous influence. When intergranular volcanic ash content is low, significant dissolution generates abundant secondary pores. Conversely, if volcanic ash matrix content is sufficiently high to pervasively infill intergranular pores, alteration yields micropores with negligible macropores observed.
Effective dissolution of volcanic materials requires sustained pore-water flux during diagenesis [48]. This requires limited mechanical compaction to preserve intergranular pores and moderate clay matrix content within pores. Flume experiments demonstrate that platy lithic fragments are deposited under lower flow velocities [49]. A considerable proportion of the platy rock fragments or minerals are ductile. As a result, lithic content increases with decreasing grain size. He 1 Member sandstones exhibit 14.6 vol.% lithic content in coarse sandstones versus 26.3 vol.% in fine sandstones on average. When the lithic content exceeds 20%, early mechanical compaction dramatically reduces intergranular porosity, hindering further dissolution. Petrographic studies show that coarse-grained sublithic sandstones formed in high-energy settings satisfy this requirement.

5.3. Controls on Quartz Overgrowth

5.3.1. Silica Sources for Quartz Overgrowths

The content of quartz overgrowths in the sandstones of the He 1 Member ranges from 0.2% to 8.3 vol.%, exhibiting significant variability. High contents of quartz cement require pore fluids that are rich in silica. In fluvial facies strata, the primary sources of silica include feldspar dissolution, pressure solution of quartz grains, and dissolution of volcanic materials [39,50]. The He 1 Member sandstones are characteristically feldspar-poor, so feldspar dissolution is not the dominant silica source. Although mechanical compaction has induced significant ductile grain deformation, pressure dissolution of quartz grains remains limited, indicating that pressure dissolution is also not a major silica source. Both volcanic rock fragments and volcanic ash matrix in the sandstones are subjected to varying degrees of dissolution (Figure 8d,e), generating substantial silica. The most pronounced silica cementation in the He 1 Member sandstones occurs in the southern basin, demonstrating a strong genetic linkage to a provenance enriched in volcanic lithics and abundant synsedimentary volcanic ash.

5.3.2. Chlorite Coatings Inhibiting Quartz Overgrowths

The inhibitory effect of pore-lining chlorite on quartz overgrowths has been extensively documented [51,52,53]. Pore-lining chlorite preserves primary porosity by mitigating compaction, inhibiting quartz overgrowth, and suppressing carbonate cementation [20,54,55,56]. Correspondingly, pore-lining chlorite provides an effective barrier for quartz cementation from the detrital surface into the pore space due to the high crystal interconnection in the He 1 Member sandstones (Figure 8b). In sandstones with high primary intergranular pores, the absence of pore-lining chlorite would permit quartz overgrowth to completely occlude intergranular pores (Figure 8f,h).
In braided river systems, the formation of chlorite coatings exhibits distinct sedimentary facies controls [55,57]: The coarse-grained sublithic sandstones deposited in high-energy transverse bar environments exhibit a rigid grain-supported framework that effectively preserves primary intergranular porosity, while the dissolution of lithic fragments and volcanic ash matrix releases substantial amounts of iron, magnesium, and silicon ions, promoting the widespread development of chlorite coatings along grain margins. In contrast, longitudinal bars and laminated sandstones, formed under lower hydrodynamic energy conditions, contain higher proportions of ductile grains, where early compaction induces significant grain deformation and pore occlusion, thereby inhibiting the formation of chlorite coatings due to reduced intergranular pores and restricted fluid flow.
Chlorite formation requires substantial supplies of iron and magnesium, sourced primarily from the early diagenetic hydrolysis or dissolution of biotite, volcanic lithic fragments, and volcanic ash matrix within sandstones, which liberates these critical cations [50,51]. During early diagenesis, clay mineral precursors on grain surfaces react with iron- and magnesium-enriched pore waters, initiating chlorite grain coatings [53]. With ongoing diagenesis, chlorite progressively precipitates onto these coatings, ultimately developing into pore-lining chlorite, with crystal morphologies evolving from amorphous at grain contacts to euhedral terminations toward pore centers, demonstrating multi-stage growth. To form well-developed pore-lining chlorite, two essential conditions must be met. First, the intergranular porosity must be preserved during diagenesis to provide the necessary space for the continuous growth of the chlorite. Second, a sustained supply of iron ions and magnesium ions is required [58,59].

5.4. Development Patterns of Conventional Reservoirs

The above analysis demonstrates that conventional reservoirs (partly Type I reservoirs) in tight sandstone exhibit four diagnostic diagenetic features: limited compaction, well-developed pore-lining chlorite, restricted quartz overgrowths, and enhanced dissolution. Sandstones simultaneously satisfying these criteria must be quartz-rich coarse- to granule-grained sublithic sandstones. Nevertheless, such quartz-rich sandstones do not invariably form conventional reservoirs. This needs to be discussed under two distinct scenarios.
Scenario 1: The coarse-grained sublithic sandstones formed in transverse bar microfacies occur as isolated sandbodies bounded by mudstone seals above and below (Figure 11a). These sandstones exhibit low lithic content (<15 vol.%), with volcanic lithics averaging 6% and matrix content <3%. Because ductile rock fragments are scarce, rigid grains preserve a significant portion of intergranular pores during compaction. Volcanic lithic dissolution generates both intragranular dissolution pores and moldic pores, while simultaneously releasing ferroan ions that precipitate pervasive pore-lining chlorite. The mudstone-sealed sandbodies function as a closed diagenetic system, restricting fluid exchange with external sources. Consequently, all dissolution and precipitation processes occur internally within the indigenous fluids. The relatively low volcanic lithic content yields limited dissolved silica, while pore-lining chlorite inhibits quartz overgrowth. This configuration represents a closed intrastratal-diagenetic system.
Scenario 2: The coarse-grained sublithic sandstones formed in transverse bar and lithic-rich sandstones of other genetic types occur as amalgamated sandbodies, lacking stable intervening mudstone seals (Figure 11b). Within this configuration, extensive dissolution of volcanic lithics releases SiO2−, Fe2+, and Mg2+ enriched fluids that undergo vertical migration and diffusion between sandbodies. Lithic-rich laminated sandstones and longitudinal bar deposits are subjected to severe porosity losses due to ductile grain deformation during mechanical compaction. In contrast, sublithic sandstones formed in transverse bars maintain preserved intergranular pores through rigid grain support. Diagenetic alteration of volcanic material liberates substantial silica, which precipitates as quartz overgrowths where pore-lining chlorite is underdeveloped. During cementation, quartz overgrowths envelop clay, forming embayed grain boundaries. Consequently, sublithic sandstones formed in transverse bar develop only isolated intragranular dissolution pores with limited connectivity, while intergranular pores become occluded by quartz overgrowths. This configuration represents an open interstratal-diagenetic system.

6. Conclusions

He 1 Member is dominated by braided channel deposits, within which three distinct types of reservoirs are identified. Type I reservoirs, primarily developed in transverse bar microfacies, consist of coarse-grained sublithic sandstones formed under high-energy hydrodynamic conditions, characterized by low lithic fragment and volcanic ash matrix contents. These reservoirs exhibit well-connected intergranular and intragranular dissolution pores, with porosity greater than 10% and permeability greater than 0.5 mD. Type II reservoirs, deposited in channel fill and longitudinal bar microfacies, were formed under unstable hydrodynamic regimes. They show slightly higher lithic content and significantly elevated volcanic ash matrix content, resulting in medium- to coarse-grained lithic sandstones with isolated intragranular dissolution pores and poor pore connectivity. Porosities range from 8% to 10%, and permeabilities range from 0.2 to 0.5 mD. Type III reservoirs, associated with laminated sandstone microfacies, formed under weak hydrodynamic conditions. They are marked by substantially increased lithic and volcanic ash matrix contents, composed mainly of medium-grained lithic sandstones, and dominated by intercrystalline micropores with poor connectivity. Porosities range from 4% to 8%, and permeabilities range from 0.08 to 0.2 mD.
The development of conventional reservoirs (permeability >1 mD) in the braided fluvial tight sandstones of the He 1 Member requires a specific diagenetic assemblage: weak compaction, enhanced dissolution, and restricted quartz overgrowth. Weak compaction is facilitated by a rigid framework of quartz grains within coarse-grained sublithic sandstones, which preserves substantial primary intergranular porosity. Enhanced dissolution mainly affects volcanic lithic fragments and the volcanic ash matrix, creating abundant intragranular dissolution pores and micropores. Pore-lining chlorite continuously inhibits quartz overgrowth, thereby preventing the occlusion of intergranular pores by silica cement.
Conventional reservoirs are predominantly developed in transverse bars of the braided channel system. When such bar sandbodies are vertically sealed by mudstones, they undergo limited early compaction and restrict the influx of external silica-rich fluids. Under such closed intrastratal-diagenetic conditions, transverse bars typically form high-quality conventional reservoirs. The combined effect of microfacies and diagenetic assemblage provides the essential foundation for the formation of conventional reservoirs in tight sandstone setting.

Author Contributions

Conceptualization, Methodology, Supervision, Writing—Review and Editing, X.D.; Conceptualization, Writing—Original Draft, Formal analysis, Y.W.; Visualization, Data Curation, Project administration, J.G.; Investigation, F.L.; Project administration, X.Z.; Software, S.H.; Data curation, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Financially supported by the Sinopec Science and Technology Research project (No. 34550000-24-ZC0611-0012).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

We sincerely appreciate the constructive comments and valuable suggestions provided by the editors and reviewers, which have significantly improved the quality of this manuscript. We also acknowledge the North China Oil & Gas Company, SINOPEC, for providing core samples and logging data. Special thanks to Han Meimei (Southwest Petroleum University) for her technical assistance in sample analysis. The authors declare no competing financial interests.

Conflicts of Interest

Author Ying Zhu was employed by the company Chengdu Exploration and Development Research Institute of Daqing Oilfield Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
VRFVolcanic rock fragment
MRFMetamorphic rock fragment
I/SIllite/smectite mixed layers
ChlChlorite
FFractures
QoQuartz overgrowth
AqAuthigenic quartz
FcFerroan calcite
ScSyneresis cracks
KaoKaolinite
MpMicropores
QQuartz grain
DpDissolution pore
MoMoldic pore
SEMScanning electron microscopy
HPMIHigh-pressure mercury injection
XRDX-ray diffraction
MICPMercury injection capillary pressure

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Minerals 15 01104 g001
Figure 1. (A) Location of the study area in the Ordos Basin and sedimentary facies map of He 1 Member. The braided rivers from the south and north converge in the central part of the study area and then flow out eastward. (B) Stratigraphic column of He 1 Member, sandbodies include a series of transverse bars, longitudinal bars and gravel bars.
Figure 1. (A) Location of the study area in the Ordos Basin and sedimentary facies map of He 1 Member. The braided rivers from the south and north converge in the central part of the study area and then flow out eastward. (B) Stratigraphic column of He 1 Member, sandbodies include a series of transverse bars, longitudinal bars and gravel bars.
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Figure 2. Sedimentary microfacies and physical properties of He 1 Member of Well R 6.
Figure 2. Sedimentary microfacies and physical properties of He 1 Member of Well R 6.
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Figure 3. Core photographs showing representative lithofacies of the He 1 Member: (a) Light gray, massive-bedded, normally graded pebbly coarse sandstone with poorly sorted but well-rounded pebbles, Well XF8, 2897.23 m; (b) Light gray, massive-bedded, fine-to-medium pebble conglomerate containing poorly sorted, well-rounded pebbles (3 mm to 2 cm diameter), Well R6, 2970.03 m; (c) Light gray, planar cross-bedded, coarse sandstone containing granules with poor sorting and rounding, Well R7, 3155.43 m; (d) Gray, horizontally laminated fine sandstone exhibiting abundant argillaceous laminae, Well R602, 3017.82 m.
Figure 3. Core photographs showing representative lithofacies of the He 1 Member: (a) Light gray, massive-bedded, normally graded pebbly coarse sandstone with poorly sorted but well-rounded pebbles, Well XF8, 2897.23 m; (b) Light gray, massive-bedded, fine-to-medium pebble conglomerate containing poorly sorted, well-rounded pebbles (3 mm to 2 cm diameter), Well R6, 2970.03 m; (c) Light gray, planar cross-bedded, coarse sandstone containing granules with poor sorting and rounding, Well R7, 3155.43 m; (d) Gray, horizontally laminated fine sandstone exhibiting abundant argillaceous laminae, Well R602, 3017.82 m.
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Figure 4. Plot of porosity versus permeability for the four types of sedimentary microfacies.
Figure 4. Plot of porosity versus permeability for the four types of sedimentary microfacies.
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Figure 5. Pore types of the four types of sedimentary microfacies of the He 1 Member.
Figure 5. Pore types of the four types of sedimentary microfacies of the He 1 Member.
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Figure 6. Throat radius distribution of the three types of reservoirs.
Figure 6. Throat radius distribution of the three types of reservoirs.
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Figure 7. Quartz-feldspar-lithic fragment (Q-F-L) ternary diagram showing composition of the three types of reservoirs from the He 1 Member. Classification after Folk [38].
Figure 7. Quartz-feldspar-lithic fragment (Q-F-L) ternary diagram showing composition of the three types of reservoirs from the He 1 Member. Classification after Folk [38].
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Figure 8. Optical photomicrographs of He 1 Member sandstone: (a) Ductile metamorphic rock fragments deformed during compaction showing concave-convex contacts, and negligible porosity, Well XF15-P4, 3180.14 m (SEM); (b) Point and long grain contacts with pore-lining chlorite and intergranular porosity; quartz overgrowths occur where pore-lining chlorite is absent, featuring a dissolution pore at right, Well R7, 3158.62 m (PPL); (c) Micropores within volcanic ash matrix, exhibiting desiccation cracks formed by diagenetic dehydration, Well XF5, 2695.51 m (PPL); (d) Intragranular dissolution pores in volcanic lithics containing flaky illite clays, Well XF15-P1, 3087.54 m (PPL); (e) Authigenic illite–smectite mixed-layer clay, kaolinite, and microquartz precipitated from volcanic lithic dissolution, Well XF5, 2695.51 m (SEM); (f) Quartz overgrowths with embayed boundaries containing clay inclusions and dust rims at crystal interfaces, Well XF3, 2696.08 m (PPL); (g) Ferroan calcite cements replacing lithics, volcanic ash matrix infilling pores, and authigenic quartz occurring in residual spaces, Well XF8, 2898.82 m (PPL); (h) Subparallel shear fractures transecting grains and quartz overgrowths with no mineral infilling, and intragranular dissolution pores and moldic pores in volcanic rock fragments. Well XF1201, 2776.55 m (PPL). VRF: volcanic rock fragments; MRF: metamorphic rock fragments; I/S: illite/smectite mixed layers; Chl: chlorite; F: fractures; Qo: quartz overgrowths; Aq: authigenic quartz; Fc: ferroan calcite; Sc: syneresis cracks; Kao: kaolinite; Mp: moldic pore; Dp: Dissolution pore.
Figure 8. Optical photomicrographs of He 1 Member sandstone: (a) Ductile metamorphic rock fragments deformed during compaction showing concave-convex contacts, and negligible porosity, Well XF15-P4, 3180.14 m (SEM); (b) Point and long grain contacts with pore-lining chlorite and intergranular porosity; quartz overgrowths occur where pore-lining chlorite is absent, featuring a dissolution pore at right, Well R7, 3158.62 m (PPL); (c) Micropores within volcanic ash matrix, exhibiting desiccation cracks formed by diagenetic dehydration, Well XF5, 2695.51 m (PPL); (d) Intragranular dissolution pores in volcanic lithics containing flaky illite clays, Well XF15-P1, 3087.54 m (PPL); (e) Authigenic illite–smectite mixed-layer clay, kaolinite, and microquartz precipitated from volcanic lithic dissolution, Well XF5, 2695.51 m (SEM); (f) Quartz overgrowths with embayed boundaries containing clay inclusions and dust rims at crystal interfaces, Well XF3, 2696.08 m (PPL); (g) Ferroan calcite cements replacing lithics, volcanic ash matrix infilling pores, and authigenic quartz occurring in residual spaces, Well XF8, 2898.82 m (PPL); (h) Subparallel shear fractures transecting grains and quartz overgrowths with no mineral infilling, and intragranular dissolution pores and moldic pores in volcanic rock fragments. Well XF1201, 2776.55 m (PPL). VRF: volcanic rock fragments; MRF: metamorphic rock fragments; I/S: illite/smectite mixed layers; Chl: chlorite; F: fractures; Qo: quartz overgrowths; Aq: authigenic quartz; Fc: ferroan calcite; Sc: syneresis cracks; Kao: kaolinite; Mp: moldic pore; Dp: Dissolution pore.
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Figure 9. Plot of intergranular volume (IGV) versus cementation and compaction of different types of sandstone.
Figure 9. Plot of intergranular volume (IGV) versus cementation and compaction of different types of sandstone.
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Figure 10. Sandstone pore evolution of He 1 sandstone reservoir. Q: Quartz grain; VRF: Volcanic rock fragment; MRF: Metamorphic rock fragment; Chl: Chlorite; Qo: Quartz overgrowth; Kao: Kaolinite; Dp: Dissolution pore; Mo: Moldic pore; Mip: Micropore; MP: Moldic pore.
Figure 10. Sandstone pore evolution of He 1 sandstone reservoir. Q: Quartz grain; VRF: Volcanic rock fragment; MRF: Metamorphic rock fragment; Chl: Chlorite; Qo: Quartz overgrowth; Kao: Kaolinite; Dp: Dissolution pore; Mo: Moldic pore; Mip: Micropore; MP: Moldic pore.
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Figure 11. Reservoir formation mechanism of the He 1 member sandstones. (a) Transverse bar sandbodies are sealed above and below by mudstones, so diagenesis is confined entirely within the sandbodies, resulting in Type I Reservoirs. (b) Where transverse bar, longitudinal bar, and laminated sandstones are vertically amalgamated, interstitial pore waters can carry ions among different sandbodies. The large amount of silica released by dissolution of volcanic material in longitudinal bars and laminated sandstones can migrate into the transverse bars, producing abundant quartz overgrowths. Under these conditions, Type I Reservoirs do not develop within the transverse bars.
Figure 11. Reservoir formation mechanism of the He 1 member sandstones. (a) Transverse bar sandbodies are sealed above and below by mudstones, so diagenesis is confined entirely within the sandbodies, resulting in Type I Reservoirs. (b) Where transverse bar, longitudinal bar, and laminated sandstones are vertically amalgamated, interstitial pore waters can carry ions among different sandbodies. The large amount of silica released by dissolution of volcanic material in longitudinal bars and laminated sandstones can migrate into the transverse bars, producing abundant quartz overgrowths. Under these conditions, Type I Reservoirs do not develop within the transverse bars.
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Table 1. Statistical summary of the petrographic parameters of He 1 Member sandstones.
Table 1. Statistical summary of the petrographic parameters of He 1 Member sandstones.
ComponentsType I Reservoir (n = 18)Type II Reservoir (n = 21)Type III Reservoir (n = 19)
Max, %Min, %Aver, %Max, %Min, %Aver, %Max, %Min, %Aver, %
FragmentsQuartz90.666.571.687.950.569.975.8646.363.6
Feldspar0.20.00.12.70.01.64.80.02.3
Volcanic rock fragment7.71.81.58.73.13.610.63.14.4
Metamorphic rock fragment16.27.912.524.65.616.835.212.217.8
Mica0.80.30.62.70.00.66.21.34.1
CementsCalcite4.80.01.25.60.42.210.30.42.3
Chlorite8.43.55.66.72.23.52.10.51.1
Illite1.80.30.74.20.32.45.30.23.5
Kaolinite1.80.00.22.10.00.53.10.01.6
Quartz overgrowth2.10.40.68.30.64.13.30.20.9
Table 2. Diagenetic parameters of He 1 Member sandstones.
Table 2. Diagenetic parameters of He 1 Member sandstones.
ParametersType I Reservoir
(n = 18)		Type II Reservoir
(n = 21)		Type III Reservoir
(n = 19)
Compaction Index (IGV) (%)28.7 ± 3.223.5 ± 2.818.9 ± 2.1
Dissolution-induced Porosity Enhancement Rate (%)8.5 ± 1.85.2 ± 1.32.1 ± 0.9
Quartz Overgrowth Coverage (%)15.2 ± 1.842.7 ± 8.928.4 ± 7.1
Chlorite Coating Coverage (%)78 ± 1242 ± 1523 ± 9
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Ding, X.; Wang, Y.; Gao, J.; Lin, F.; Zhang, X.; Han, S.; Zhu, Y. Genesis of Conventional Reservoirs in Braided Fluvial Tight Sandstones: Evidence from the He 1 Member, Upper Paleozoic, Southern Ordos Basin, China. Minerals 2025, 15, 1104. https://doi.org/10.3390/min15111104

AMA Style

Ding X, Wang Y, Gao J, Lin F, Zhang X, Han S, Zhu Y. Genesis of Conventional Reservoirs in Braided Fluvial Tight Sandstones: Evidence from the He 1 Member, Upper Paleozoic, Southern Ordos Basin, China. Minerals. 2025; 15(11):1104. https://doi.org/10.3390/min15111104

Chicago/Turabian Style

Ding, Xiaoqi, Yi Wang, Jingyun Gao, Feilan Lin, Xiang Zhang, Shujie Han, and Ying Zhu. 2025. "Genesis of Conventional Reservoirs in Braided Fluvial Tight Sandstones: Evidence from the He 1 Member, Upper Paleozoic, Southern Ordos Basin, China" Minerals 15, no. 11: 1104. https://doi.org/10.3390/min15111104

APA Style

Ding, X., Wang, Y., Gao, J., Lin, F., Zhang, X., Han, S., & Zhu, Y. (2025). Genesis of Conventional Reservoirs in Braided Fluvial Tight Sandstones: Evidence from the He 1 Member, Upper Paleozoic, Southern Ordos Basin, China. Minerals, 15(11), 1104. https://doi.org/10.3390/min15111104

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