1. Introduction
Tight sandstone gas, as a critical type of unconventional natural gas, holds significant research importance due to its vast global resources estimated at approximately 114 × 108 m3. Notable tight sandstone gas reservoirs include the Bakken Formation (USA), Cooper Basin (Australia), and Neuquén Basin (Argentina). These reservoirs are typically characterized by low porosity and permeability (porosity < 10%, permeability < 0.1 × 10−3 μm2) and strong heterogeneity, with gas saturation ranging from 55% to 65%.
Current development technologies prioritize hydraulic fracturing as the core method, supported by petrological analysis, well logging, and well testing. While countries such as the United States and Canada have achieved technological maturity in reservoir evaluation and “sweet spot” identification, critical challenges persist in reservoir characterization accuracy. The determination of physical property thresholds (e.g., porosity and permeability cutoffs) relies on multi-method integrated assessments, which introduces statistical uncertainties that compromise the accuracy of reserve calculations. Additionally, the high-precision characterization of sandbody distribution and gas-enriched zones remains unresolved, particularly in predicting the morphology and spatial continuity of lenticular sandstone bodies. These limitations highlight the need for advanced methodologies to enhance the reliability of reservoir models and optimize exploitation strategies.
2. The Overview of the Research Area
The Ordos Basin is rich in oil and gas resources and has great potential for development [1,2]. At present, detailed geological work has been carried out in the south of the Tianhuan area [3,4,5,6,7,8], while the research in the north is relatively weak [3,9,10,11] and there is still room for oil and gas exploration. The He 8 member (He is the abbreviation of Shihezi) of the Permian Lower Shihezi Formation in the northern part of Tianhuan is an important gas-producing horizon in the Upper Paleozoic, and has good, favorable reservoir-forming conditions [12]. In this paper, using comprehensive drilling, core, logging and analysis of laboratory data, and the use of sand body well profiles, clay mineral X-ray diffraction, rock mechanics experiments, stress field reconstruction and other means, the reservoir characteristics of the He 8 member of Shihezi Formation in the northern part of Tianhuan are systematically studied, which will provide effective geological scientific basis for the further exploration and development of low porosity and permeability tight gas in the lower Shihezi Formation in this area.
The research region is situated in the northwest section of the Ordos Basin, structurally positioned within the northern Tianhuan Depression. It is bordered by the Yishan Slope to the east, the Weibei Uplift to the south, the Western Margin Thrust Belt to the west, and the Yimeng Uplift to the north, encompassing approximately 11,000 square kilometers (Figure 1) [3,4]. The study area is controlled by the broad and gentle tectonic background of the Ordos Basin, and the faults are not developed. In the geological history period, it has experienced five large-scale uplift and subsidence movements, and now it is a west-dipping monoclinic structure with a high northwest and a low southeast [11]. It borders the Sulige gas field to the east and has a similar accumulation background in natural gas geological conditions [13,14,15,16].
The northern Tianhuan section exhibits a well-preserved Upper Paleozoic succession. The stratigraphic sequence from base to top consists of the Carboniferous Benxi Formation, Taiyuan Formation, Permian Shanxi Formation, Lower Shihezi Formation, Upper Shihezi Formation, and Shiqianfeng Formation, in ascending order. This sedimentary series contains the principal rock type and gas-bearing intervals, with their spatial distribution demonstrated in Figure 1. The lithological assemblage primarily comprises a marine–continental transitional sedimentary system, predominantly characterized by shallow-water deltaic depositional environments. Notably, the He 8 member within the Shihezi Formation displays well-developed braided river delta facies characteristics [3,17,18].
The Tianhuan Depression has experienced multistage tectonic evolution, including the Indosinian, Yanshanian, and Himalayan movements, with the Yanshanian Movement (Jurassic–Cretaceous) serving as the dominant phase for fracture generation. Tectonic activation triggered stratigraphic uplift and fault proliferation, resulting in widespread high-angle tensile fractures. XMAC logging data reveals pronounced heterogeneity in Upper Paleozoic paleostress orientations across the Depression: the northern sector exhibits a predominant northwest-southeast principal stress alignment, while the southern domain displays an east-northeast trending stress configuration, reflecting differential stress partitioning controlled by structural inheritance and regional geodynamic regimes.
3. Methods
3.1. Core Observations
Core observation provides the most direct approach for fracture identification. Based on the examination of core samples from drilling wells in the northern Tianhuan Depression, the Upper Paleozoic He 8 member lithology predominantly consists of sandstone. These strata exhibit medium-to-coarse grain sizes with uniform gray coloration. Grain morphology is characterized by sub-circular particles demonstrating good sorting, typically associated with argillaceous cementation and displaying compact fabric. Sealed core testing revealed mist-like anhydrous bead formations, while immersion experiments indicated intermittent bubble release patterns (Figure 2).
3.2. Microstructure Observing
Sedimentary sequences undergo multiple diagenetic processes during subsurface evolution, including compaction, cementation, metasomatism, and dissolution [4,5,6], driven by the combined effects of evolving thermal-pressure regimes, fluid chemistry variations, and electrochemical potential fluctuations. The resultant primary and secondary micro-scale pore-throat architectures exert significant control on reservoir petrophysical properties, ultimately governing hydrocarbon accumulation patterns [7,8]. This investigation systematically evaluates reservoir quality through integrated petrographic analysis of thin sections combined with porosity–permeability characterization, establishing critical relationships between diagenetic modifications and pore-network configurations within the target interval (Figure 3).
3.3. Whole Rock/Clay Mineral X-Ray Diffraction Experiments
The X-ray diffraction experiment of whole rock and clay minerals was carried out according to the China Petroleum Industry Standard SY/T 5163-2010 [19]. The XRD equipment used was the German Bruker AXS D8-Focus X-ray diffractometer (Bruker, Karlsruhe, German). The sample state was solid powder, the environmental conditions were a temperature of 20 degrees Celsius and an air humidity of 32%, and the detection category was semi-quantitative analysis of the phase. The software used is XROCK XRD® (Ver 1.0). By identifying the characteristic peaks in the diffraction patterns, the author estimated the mineral content and clay mineral content in the samples.
3.4. Uniaxial Compression Stress Sensitivity Experiment
In order to further explore the variation law of rock physical properties under a stress–strain environment, this study selected the full-diameter core samples of the upper and lower section of the He 8 member for uniaxial compression stress sensitivity experiments. The experiments of keeping the axial pressure unchanged, changing the radial pressure, and keeping the radial pressure unchanged and changing the axial pressure were carried out.
In this paper, Terratek’s three-axis rock mechanics test system is selected for testing (Figure 4). The instrument can simulate different environmental temperature and pressure conditions, record the change of the target parameters of the test sample rock, and then generate the stress–strain curve by Excel. By comparing the curves and comparing the results with each other, it is easy to obtain the deformation characteristics and relative size of different types of rocks. The following Table 1 contains the main environmental indicators of the experiment.
3.5. Identifying Reservoir Fractures
Reservoir fracture networks exert significant control on hydrocarbon migration and accumulation processes. Contemporary fracture characterization methodologies primarily encompass core analysis, petrographic microscopy, standard well logging interpretation, and image logging evaluation. Core-based petrographic examination provides the most fundamental diagnostic approach, whereas logging techniques offer distinct advantages in spatial resolution and operational efficiency. Conventional logging systems, characterized by cost-effective data acquisition and comprehensive parameter coverage (including acoustic interval transit time (AC), bulk density (DEN), thermal neutron porosity (CNL), natural gamma radiation (GR), and spontaneous potential (SP) [18]), enable effective fracture identification through specific log response patterns.
Within China’s petroleum engineering practice, the “Qt” parameter in well log interpretation occasionally represents gas concentration indices, corresponding to real-time formation gas measurements during drilling. These metrics are critical for reservoir evaluation, providing a quantitative assessment of gas occurrence states, physical-chemical characteristics, and accumulation potential [20,21].
Shallow dual lateral resistivity (Rs) and deep dual lateral resistivity (Rd) curves in conventional logging curves can well reflect natural fractures. The identification principle of imaging logging is to convert the resistivity or acoustic impedance difference caused by various geological structures such as cracks, holes, and bedding in the formation into images [22]. Generally, bright colors are used to indicate high resistance anomalies, and dark colors are used to indicate low resistance anomalies (Figure 5).
4. Results
4.1. Petrological Characteristics
In Section 3.1, the authors conducted detailed observations of core samples from eight drilling wells in the northern Tianhuan Depression. Core samples from Wells Li-20, Li-39, Li-59, and Li-23 exhibited a grayish-white coloration, while those from other wells displayed lighter gray hues. This chromatic variation is interpreted to be primarily attributed to two factors: (1) differential coring depths influencing diagenetic alteration and mineralogical composition, and (2) the spatial distribution of the sampled wells within the extensively developed delta-front sedimentary facies, where localized hydrodynamic conditions and redox environments (e.g., Shihezi Formationic vs. dysoxic zones) variably controlled iron oxide precipitation and organic matter preservation during deposition.
In order to further clarify the petrological characteristics of the reservoir, according to the three-terminal element classification of sandstone, the rock composition of the sandstone samples of the He 8 member of the well in the north of Tianhuan was plotted (Figure 6). The results show that the main components of the target layer in the study area are quartz sandstone and lithic quartz sandstone (Table 2). Among them, the content of quartz is the highest, accounting for more than 75%, up to 90%, and the content in the lower section of the He 8 member is slightly higher than that in the upper section; the content of rock debris is about 15%. The content of feldspar in most samples is less than 5%, but some of them can reach more than 20%. The type of debris is mainly rigid debris, mainly metamorphic rock debris, followed by sedimentary rock and magmatic rock debris. The combination of a high volume fraction of quartz and a low volume fraction of cuttings gives the reservoir strong pressure resistance, which is conducive to the formation and preservation of pore structure, provides reservoir space for oil and gas molecules, and is an important condition for the formation of high-quality reservoirs [3,12,17].
Stratigraphic analysis of 12 cored intervals within the study area enabled quantitative evaluation of sand-shale distribution patterns in the Upper Paleozoic He 8 member in the northern Tianhuan Depression. Analytical results revealed a predominant sandstone thickness exceeding mudstone deposits by a factor of 5.2:1. This lithological configuration demonstrates favorable geological conditions for tight gas accumulation within the He 8 reservoir unit (Figure 7).
4.2. Microstructure Characteristics
The main secondary pores are developed in the He 8 member of Shihezi Formation in the northern section of the Tianhuan Depression, such as dissolved pores, intercrystalline pores, intergranular pores, debris dissolved pores, etc., and the fractures are filled with clay minerals to varying degrees. (Figure 8 and Figure 9). The red arrows refer to the clay minerals filling in the cracks, which are white or grayish white under a single polarizing microscope and black under an orthogonal microscope.
The interstitial materials are mainly clay minerals, of which kaolinite content is high, which is helpful to improve the physical properties of the reservoir [2,3,4]. Through X-ray diffraction analysis of clay minerals in rock samples, it is concluded that the upper and lower limits of clay mineral content in the He 8 samples in the northern Tianhuan Depression are 26.0% and 2.2%, respectively, mainly distributed in the range of 8.7% to 12.3%. Kaolinite is the most important mineral, accounting for more than 59% of the total clay minerals, with an average content of about 62.7%. It is followed by illite group minerals such as hydromica, accounting for about 17.2% on average; siliceous cement, mainly composed of montmorillonite and containing about 12%; chlorite content, which is less or does not contain any; the mixed layer ratio of illite and montmorillonite which is >15%, followed by hydromica and a small amount of siliceous cementation (Figure 9).
4.3. Pore Network Distribution
According to the results of porosity and permeability measurements, the minimum porosity of the He 8 reservoir in the study area is 1.70%, the maximum porosity is 17.71%, the average porosity is about 7.01%, and the main distribution interval is 6–10%. The minimum permeability is 0.01 × 10−3 μm3, the maximum permeability is 12.72 × 10−3 μm3, the average permeability is about 0.5 × 10−3 μm3, and the main distribution range is 0.01–0.1 × 10−3 μm3. The lower sub-member of the He 8 is better than the upper sub-member. The reservoir has relatively low physical properties, relatively dense structure and strong heterogeneity. It is a typical low-porosity, low-permeability, and ultra-low-permeability tight reservoir [6,7]. The porosity, permeability, and water saturation data were linearly fitted, and both showed a good positive correlation (Figure 10).
4.4. Rock Mechanics Characteristics
The results show that when the axial pressure is a fixture value, the gas permeability of the core decreases with the increase of the radial pressure, and reaches the minimum value at the fracture point. Subsequently, as the pressure gradually decreases, the gas permeability gradually increases but cannot reach the initial value. When the radial pressure is constant, the change trend of core gas permeability with axial stress is the same (Figure 11). Therefore, it is believed that the existence of fractures affects the size of permeability and plays an important role in improving reservoir physical properties [20,21,22].
4.5. Reservoir Features Characteristics
In this study, natural fractures were identified by drilling coring data and logging interpretation in the Tianhuan Depression, and the occurrence data of fractures were obtained. The polar stereographic projection software Stereonet (Ver 5.9.2) was used to perform linear projection and equal-area projection on the dip angle and the trend of fractures, and the result interpretation map (Figure 12) was obtained. The results show that the natural fractures in the He 8 member of the Tianhuan Depression are mainly high-angle structural fractures (dip angle = 75–90°), accounting for more than 72% of the total number of fractures. The overall trend of the fractures is NW-SE.
5. Discussion
5.1. Reasons for the Development of Dissolution Pores in Sandstone
The genesis of dissolution pores in sandstones is governed by multifactorial coupling mechanisms, dominantly driven by acidic fluids (e.g., organic acids and CO2), and selectively dissolving metastable minerals such as feldspars and carbonate cements. This process necessitates synergistic contributions from organic acids generated during source rock thermal maturation, acidic conditions induced by deep hydrothermal activity, and optimal thermal-temporal regimes (80–120 °C over geologic timescales). Additionally, sandstones with elevated feldspar content (>15%) and medium-to-coarse grain textures exhibit preferential secondary porosity development due to enhanced mineral reactivity and permeability, collectively optimizing reservoir quality through heterogeneous pore-network evolution.
5.2. Causes of Rock Mechanics Experimental Results
This study attributes the observed phenomenon to two primary factors. First, the increase in overburden pressure and confining stress with greater burial depth (simulated by dual-stress conditions in this experiment) induces the plastic deformation of rocks prior to reaching a critical depth, resulting in pore structure compaction and permeability reduction. Beyond this threshold, elastic deformation and brittle failure dominate, generating fracture networks that enhance permeability through secondary porosity systems.
Second, lithological variations play a decisive role. Upper He 8 Formation samples consist of fine-grained sandy mudstone, while lower intervals are dominated by coarse-grained sandstone. Fine-grained lithologies typically exhibit higher strain accumulation during stress loading, leading to more efficient fracturing and consequently achieving higher peak post-fracture permeability compared to coarser-grained counterparts.
5.3. Distribution Characteristics of Sandstone Bodies
The distribution of sedimentary relative dominant reservoirs has an important influence. In the late Early Permian, the surface water system was widely distributed in the Lower Shihezi period, and fluvial facies sedimentation was generally developed. The core color of the eighth member of the Shihezi Formation was mainly gray and variegated, with more sandy components, indicating that it was in a shallow-water sedimentary environment with alternating oxidation-reduction.
In this study, four wells located in the EW direction of the dominant distribution of sand bodies in the northern part of the Tianhuan in the northwest of the Ordos Basin were selected, and the sand bodies of the Upper Paleozoic were completed. The drawing of the well profile (Figure 13) shows that the gas-bearing layer does not have the characteristics of contiguous distribution, and has a complex gas–water relationship in the vertical zoning and plane zoning, indicating the radiation effect of the deep source rock on natural gas accumulation [14]. A large amount of free water is developed in the He 8 member of the Shihezi Formation; the gas–water mixed layer accounts for a large proportion, and the sand body is moderately sealed. The physical properties of the sandstone reservoir are general, but the thickness is thick, and it also has the exploration potential to become a high-quality reservoir [13,16].
5.4. The Influence of Tectonic Stress Fields on Reservoir Fracture Development
In Section 4.5, the author obtains the fracture trend of the reservoir in the He 8 member of the study area in the northwest through drilling and logging data. The author believes that the result is the superposition of two tectonic movements. During the Indosinian movement in the late Triassic, the Ordos Basin was controlled by the NS-trending compressive stress field as a whole [20]. At the same time, due to the closure of the Paleo-Tethys Ocean on the south side, the basin underwent dextral shear, in which the development of NS-trending fractures was inhibited. The northern part of the western margin of the basin is also squeezed by the Alashan platform at the same time, so that the direction of the principal stress formed during this period should be NW-SE, and most of the natural fractures are NW-SE. In addition, there are a small number of NE-SW trending fractures, which may be due to the NNE-SSW horizontal compression during the Cenozoic Himalayan movement and the previous superposition (Figure 14) [21]. According to the above, the author speculates that the difference of rock anisotropy caused by the different development degree of two groups of fractures formed in different periods leads to the fracture strike pattern which is dominated by NW-SE direction and a small amount of NS direction and NE-SW direction [22].
6. Conclusions
Extensive exploration and development have shown that the He 8 member of Shihezi Formation in the northern section of the Tianhuan Depression has favorable conditions for tight gas enrichment. Consequently, precise delineation of reservoir petrophysical properties constitutes a critical prerequisite for identifying prospective hydrocarbon-bearing zones and optimizing field development strategies within the study area.
The He 8 member reservoir in the northern Tianhuan Depression predominantly comprises quartz sandstone lithofacies, with subordinate occurrences of lithic quartz sandstone. Stratigraphic analysis reveals substantial sandstone thickness development, exhibiting a sand-to-shale ratio of approximately 5.2:1. Primary porosity development is dominated by intergranular pore systems, with surface porosity values exceeding 3.35%. Interstitial constituents predominantly consist of clay minerals, particularly kaolinite-dominated assemblages. Petrophysical characterization indicates average porosity and permeability values of 7.01% and 0.5 × 10−3 μm2, respectively, defining a characteristic low-porosity, ultra-low-permeability tight reservoir system.
Sandbody geometries exhibit preferential NW-SE orientation within the He 8 interval, characterized by substantial thickness and moderate containment capacity. Uniaxial compression experiments demonstrate significant fracture-induced permeability enhancement, with dominant NW-SE trending high-angle structural fractures. Contemporary in situ stress analysis identifies principal compressive stresses oriented N10° W–N20° W, with secondary stress components trending approximately E15° N. Tectonic evolution records show predominant controls from Indosinian and Himalayan orogenic events.
Revision and reconstruction of the framework, Z.D.; conceptualization, J.Z. and Z.C.; methodology, Z.C.; validation, X.Y.; formal analysis, J.Z. and Z.C.; investigation, Z.C.; data curation, X.Y.; writing—original draft preparation, Z.C.; writing—review and editing, Z.D. and Z.C.; visualization, X.Y.; supervision, J.Z. and X.Y.; project administration, J.Z.; software provided, H.M. All authors have read and agreed to the published version of the manuscript.
The data that support the findings of this study are available from the corresponding author, upon reasonable request.
This study was supported by technology and data provided by CNPC Changqing Exploration Institute.
Author Xinzhi Yan was employed by the company Shaanxi Yanchang Petroleum (Group) Corp. Ltd. Author Hongxing Ma was employed by the company Geological Survey Engineering Institute, PowerChina Xinjiang Survey, Design and Research Institute 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.
Footnotes
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Figure 1 Zoning of tectonic units in Ordos Basin and sedimentary strata in the northern Tianhuan Depression (the study area and study stratum have been marked in yellow).
Figure 2 The coring of the He 8 member in the northern section of the Tianhuan Depression, Ordos Basin. (a) Light gray coarse-grained quartz sandstone, well Li 11, lower section of the He 8 member, 2850.43 m; (b) Gray-white medium-grained lithic quartz sandstone, well Li 20, lower section of the He 8 member, 4391.09 m; (c) Gray medium-coarse quartz sandstone, well Li 2, lower section of the He 8 member, 3774.3 m; (d) Gray medium-coarse grained lithic quartz sandstone, well Li 57, lower section of the He 8 member, 3786.38 m; (e) Gray-white medium sandstone, well Li 39, lower section of the He 8 member, 3989.20 m; (f) Gray-white medium-coarse grained sandstone, well Li 59, lower section of the He 8 member, 3824.49 m; (g) Shallow gray coarse grained lithic quartz sandstone, well Li 7, lower section of the He 8 member, 4054.63 m; (h) Gray-white gravelly coarse-grained quartz sandstone, well Li 23, lower section of the He 8 member, 4481.74 m; (i) Gray-white gravelly coarse-grained quartz sandstone, well Li 23, lower section of the He 8 member, 4482.77 m.
Figure 3 Microscopic structure characteristics of the He 8 member in the northern section of the Tianhuan Depression. (a) Dissolved pores, intercrystalline pores, well Li 20, He 8 member, 4390.98 m; (b) Debris dissolution pore, kaolinite intercrystalline pore, well Li 2, He 8 member, 3776.5 m; (c) Intergranular pore, kaolinite intergranular pore, well Li 2, He 8 member, 3787 m; (d) Dissolved pores, kaolinite intercrystalline pores, well Li 59, He 8 member, 3827.56 m; (e) Intercrystalline pores, dissolved pores and intergranular pore, well Li 59, He 8 member, 3827.56 m; (f) Dissolved pore, intercrystalline pore, well Su 87, He 8 member, 3629.92 m; (g) Dissolved pore, intergranular pore, Su 40 well, He 8 member, 3767.0 m; (h) Dissolved pores, intercrystalline pores, Su 40 well, He 8 member, 3716.75 m; (i) Kaolinite intercrystalline pore, Su 40 well, He 8 member, 3736.38 m.
Figure 4 Triaxial rock mechanics test system device.
Figure 5 Logging curves and electrical imaging logging results of the He 8 section of well Li 7 in the northern section of the Tianhuan Depression.
Figure 6 Rock composition classification map in some wells of the He 8 member in the northern part of the Tianhuan Depression.
Figure 7 Sand–mud thickness ratio of the Upper Paleozoic He 8 reservoir in the northern section of the Tianhuan Depression.
Figure 8 Clay minerals fill fractures.
Figure 9 Statistics of reservoir pores and interstitial material types in the He 8 member of the northern section of the Tianhuan Depression. (a) Pore type; (b) Interstitial material type.
Figure 10 Correlation diagram of reservoir physical properties of the He 8 member in the northern section of the Tianhuan Depression. (a) Correlation between porosity and water saturation; (b) Correlation between permeability and water saturation.
Figure 11 The experimental results of rock mechanics uniaxial compression in the He 8 member of the northern section of the Tianhuan Depression. (a) Sample of the upper section of the He 8 member; (b) Sample of the lower section of the He 8 member.
Figure 12 Statistics of reservoir fracture occurrence in the northern section of the Tianhuan Depression. (a) Linear projection of fracture dip angle; (b) Isometric projection of fracture dip angle; (c) Linear projection of fracture strike; (d) Isometric projection of fracture strike.
Figure 13 Li 11-Li 7-Li 57-Li 59 Upper Paleozoic sand body well section in the northern area of Tianhuan, Ordos Basin.
Figure 14 Reconstruction map of paleostress field in the He 8 period of the northern Tianhuan Depression. (a) Schmidt network positioning point data distribution; (b) Stress field reconstruction.
The main environmental indicators of the experiment.
| Sample Source | Number of Sets | Index | Parameter |
|---|---|---|---|
| Li 57 | 12 | Axial force | 1.46 × 103 kN |
| Li 59 | 3 | Confining pressure | 140 Mpa |
| Li 11 | 1 | Temperature | 392 ℉ |
| Li 7 | 2 | Sample size | 25.4 mm × 50.8 mm |
| Su 40 | 1 | Lithologic characters | Sandstone |
Statistical table of sandstone type and corresponding proportion.
| Quartz Sandstone | Lithic Quartz Sandstones | Feldspathic Quartz Sandstones | Lithic Sandstone |
|---|---|---|---|
| 82% | 15% | 3% | <1% |
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