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Article

Characteristics and Control Factors of a High-Quality Deeply Buried Calcareous Sandstone Reservoir, the Fourth Member of the Upper Xujiahe Formation in the Western Sichuan Basin, China

by
Dong Wu
1,2,3,
Yu Yu
4,*,
Liangbiao Lin
4,
Hongde Chen
4 and
Sibing Liu
3
1
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 102206, China
2
SINOPEC Key Laboratory of Geology and Resources in Deep Stratum, Beijing 102206, China
3
College of Energy, Chengdu University of Technology, Chengdu 610059, China
4
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(9), 872; https://doi.org/10.3390/min14090872
Submission received: 18 July 2024 / Revised: 16 August 2024 / Accepted: 25 August 2024 / Published: 27 August 2024
(This article belongs to the Section Mineral Deposits)
Browse Figures
Versions Notes

Abstract

:
A special type of sandstone in which carbonate rock fragments (CRFs) dominate the composition developed in the Upper Triassic Xujiahe Formation’s fourth member (Xu4) in the western Sichuan Basin, known as calcareous sandstone. Calcareous sandstones are widely distributed in the western Sichuan and is the main production target of tight sandstone gas in the Sichuan Basin. In this study, thin sections, porosity–permeability testing, scanning electron microscopy, and X-ray diffraction are applied to examine the characteristics and control factors for high-quality reservoirs in the calcareous sandstones, with a view to providing guidance for natural gas exploration and development in calcareous sandstones. The results show that the calcareous sandstone belongs to litharenite, with an average framework grain composition of 30% quartz, 1% feldspar, and 69% rock fragments, while the Xu4 sandstone has a high quartz content (average content of 71%). Primary intergranular pores are the main storage space, and the reservoir quality is quite poor. Under the influence of different parent rock properties of sandstones, there are obvious differences in the composition of framework grains between the calcareous sandstone and the ordinary Xu4 sandstone, which in turn affects the reservoir storage space, diagenesis, and reservoir quality. High-energy depositional conditions, low content of late cements, and the development of fractures are the main controlling factors for the formation of high-quality reservoirs in Xu 4 calcareous sandstones.

1. Introduction

In sedimentary basins with low geothermal gradients in western China, including the Sichuan, Tarim, and Qaidam Basins, deep burial reservoirs are defined as those with burial depths between 4500 m and 6000 m [1]. Deep burial processes have resulted in complex and intense diagenetic alteration, leading to reservoirs characterized by high density and poor reservoir quality. In recent years, deep burial tight gas sandstone has become a significant and new fast-growing area for onshore oil and gas resource exploration in China [2,3,4]. The porosity of sandstone shows a gradual decrease with increasing burial depth, making it difficult for reservoirs with relatively good physical properties to exist under deep burial conditions [5,6,7,8]. However, many geological explorations have confirmed that, under certain conditions, high-quality reservoirs can also be developed in deeply buried sandstones [5,9,10,11]. For example, the development of fluid overpressure, grain coating, and hydrocarbon charging can positively impact the preservation of primary pores by limiting cementation and increasing the resistance of sandstones to compaction [5,6,12,13,14,15,16,17,18,19,20]. In addition, the development of secondary pores formed by dissolution in deeply buried sandstone reservoirs has a positive impact on reservoir quality and constitutes the main reservoir space for most deeply buried tight gas sandstones [6,17,21,22]. Therefore, clarifying the main types of reservoir space and pore development factors of deeply buried sandstone reservoirs can help to predict the development characteristics and distribution patterns of relatively high-quality reservoirs.
A special type of sandstone in which carbonate rock fragments (CRFs) dominate the composition has developed in the fourth member (Xu4) of the Upper Triassic Xujiahe Formation in the western Sichuan Basin, known as calcareous sandstone [23,24,25,26]. Calcareous sandstone refers to sandstone in which the rock fragment component is mainly CRFs of extra-basinal origin, with the proportion of CRFs in the sandstone exceeding 50% [23]. Although the calcareous sandstone is mainly composed of carbonate minerals, it is genetically classified as a terrestrial clastic rock, which differs from ordinary sandstone reservoirs in terms of diagenetic alteration and reservoir quality [25]. Calcareous sandstones have been reported only in the western and northern parts of the Sichuan Basin, where they are mainly developed in the Upper Triassic Xujiahe Formation and are the main production target of tight sandstone gas in the Sichuan Basin. Calcareous sandstones are widely distributed in western Sichuan and can be seen in almost every well in the Xu4 sandstone in the western Sichuan Basin [23]. They are among the most important reservoir rocks.
In recent years, industrial gas flow has been drilled in several calcareous sandstone drilling wells in the western and northern parts of the Sichuan Basin [23,25,27,28,29,30], demonstrating the good hydrocarbon exploration potential of this set of calcareous sandstones. In this paper, a combination of techniques, including thin sections, porosity–permeability testing, scanning electron microscopy, and X-ray diffraction, was applied to examine the characteristics and controlling factors of high-quality reservoirs in the calcareous sandstones, providing guidance for natural gas exploration and development in calcareous sandstones.

2. Geological Setting

The western SB is located in southwestern China and is surrounded by mountains (Figure 1) [31,32]. Several tectonic orogenies took place after the formation of the SB’s crystallized basement at 800 Ma. [32,33]. The current diamond shape of the basin was formed during the Yanshanian orogeny during the Jurassic, when the Himalayan orogeny shaped its current geomorphological characteristics [33]. The basin can be divided into six tectonic units (Figure 1A). The western Sichuan Basin is bordered by the Longmenshan mountain range to the west, where the Xinchang Tectonic Belt is the sub-tectonic unit with the highest number of drilling wells and the highest natural gas production.
During the Late Triassic, the uplift of the Longmenshan mountain in the western part of the Sichuan Basin ended the marine evolution of the basin and caused the basin to enter the stage of terrestrial lake basin evolution [31,33]. The Xujiahe Formation is a set of siliciclastic rocks deposited in the marine–continental depositional environment, which included braided river delta, shallow lacustrine, and shoreline depositional environments [31]. The Upper Triassic Xujiahe Formation has an unconformable contact with both the overlying Ziliujing Formation of the Lower Jurassic and the underlying Leikoupo Formation of the Middle Triassic (Figure 1C). The Xujiahe Formation can be divided into six members within the basin based on lithology, but their characteristics and thicknesses vary greatly from region to region. In the western Sichuan Basin, the Xu 1 to Xu 5 members are developed with total thicknesses between 500 m and 2500 m, with the Xu 2 and Xu 4 members being dominated by fine- to medium-grained sandstones. These members constitute the main tight gas sandstone reservoir systems in the western Sichuan Basin [24]. The Xu4 member reached its maximum depth in the Late Cretaceous and was then uplifted to its present depth of approximately 3000–4000 m by the Himalayan movement (Figure 2B).

3. Materials and Methods

In this study, core samples of the Xu4 sandstone were collected from 15 wells. The locations of the gas fields and wells are shown in Figure 1B. A total of 138 calcareous sandstone samples were prepared as thin sections (all were impregnated with blue epoxy and half were stained with Alizarin Red S dye to distinguish between calcite and dolomite) and observed by point-counting at least 300 points using a Leica Leica polarizing microscope (paired with 2×, 4×, and 10× lenses) to determine the compositions, texture, pore space, and diagenesis of the sandstones. Eight thin sections without covers were examined by optical microscope cathodoluminescence (CL) observation on a Leica microscope using a Cambridge Image Technology CL8200 MK5-2, with the operating conditions of a high voltage of 15 kV and a gun current of 280 uA. Additionally, 340 porosity and permeability test data (all sandstone samples) were collected from the Sinopec Southwest Oil & Gas Company to assess the reservoir quality of the calcareous sandstone.
Twelve samples were selected for bulk and clay fraction mineralogy, respectively, with an X-ray powder diffractometer (XRD) using a Bruker D8 at 45 kV, 35 mA, and a scan rate of 2°/min over a range of 2°–60° at the Wuxi Research Institute of Petroleum Geology, Wuxi. For the bulk XRD analysis, 10 g of a sample was pulverized to 200 mesh and, then, the sample powder was placed into the sample mold of the instrument for analysis. To separate the clay fraction, carbonate minerals and organic matter were removed using 2% HCl and dilute H2O2, respectively. The clay fraction with a particle size less than 2 μm was extracted by centrifugation. After air-drying, the sample racks were saturated with ethylene glycol vapor treatment for not less than 8 h at a constant temperature of 60 °C and then heated at 550 °C for 2 h. Fifteen core samples with small fraction (approx. 1 cm3) were examined for the morphology of the minerals and pores via scanning electron microscopy (SEM) at the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology. Firstly, gold was plated onto the naturally exposed surface of the samples to increase the electrical conductivity of SEM. Then, the coated samples were scanned with a field emission environmental scanning electron microscope (FSEM) (FEI Quanta 250 FEG) equipped with an Oxford INCAx-max20 energy dispersive spectrometer (EDS) with an accelerating voltage scanning of 20 kV.

4. Results

4.1. Sandstone Petrology

The thin-section results showed that all the calcareous sandstones fell into the litharenite category (Figure 2A), according to the sandstone classification scheme by Folk et al. [34]. The quartz content ranged from 1% to 62%, the feldspar content ranged from 0% to 12%, and the rock fragments ranged from 37% to 99%, with an average framework grain composition of Q30F1L69, indicating the characteristics of an extremely low content of feldspar.
Thin-section and SEM images (Figure 3) revealed that carbonate grains (including calcite and dolomite grains) are the most abundant framework grains in the calcareous sandstone. Under the conditions of Alizarin Red staining, the calcite grains were indistinguishable from the intergranular calcite cements. Compared to the cements, the carbonate rock fragments were mainly composed of minerals with much finer crystals, such as the micritic and microcrystalline forms. Quartz was the framework grain with the highest content, except for the carbonate rock fragments. The calcareous sandstones were fine- to medium-grained, generally moderately well sorted, and predominantly sub-angular, with mainly point and line contacts.

4.2. Mineral Compositions

The XRD results (Figure 2B; Table 1) showed calcite as the most abundant mineral in the calcareous sandstone, ranging from 26.3% to 56.7%, with an average of 37.3%. This was followed by quartz (ranging from 9.1% to 54.8%, with an average of 30.4%), dolomite (ranging from 7.2% to 49.5%, with an average of 23.8%), and clay minerals (ranging from 2.1% to 17.5%, with an average of 8.2%). Other minerals were less than 1%. Clay mineralogy by XRD (Table 1) showed that illite is the most abundant mineral in the calcareous sandstone, ranging from 12% to 73%, with an average of 43.6%, followed by kaolinite (ranging from 0 to 88%, with an average of 30.4%), chlorite (ranging from 0 to 27%, with an average of 16%), and illite/smectite mixed-layer clays (ranging from 0 to 24%, with an average of 10%).

4.3. Diagenetic Alterations

4.3.1. Compaction

Compaction is one of the most significant diagenetic alterations in the calcareous sandstones, as evidenced by the disappearance of a large number of intergranular pores due to compaction. However, the predominantly point-to-line contact in the calcareous sandstone indicates that the effect of compaction on this sandstone is weaker than that on other ordinary sandstone reservoirs at the same depth. As the depth increases, the quartz-dominated sandstone reservoirs experience significant quartz cementation due to chemical compaction, which adheres to the surface of the grains and results in predominantly linear or even concave contact of the grain contacts.

4.3.2. Cementation

There are various types of cements in the calcareous sandstones. Microscopic thin-section observation showed that carbonate cementation is the most dominant type in the calcareous sandstone, including both calcite and dolomite cements (primarily calcite), which mainly fill the intergranular pores. These cements often exhibit a bicrystalline texture (Figure 4a,b) and strong orange luminescence under CL (Figure 4c). In terms of crystal size, carbonate cement includes microcrystals (Figure 4g), fine crystals (Figure 4h), and sparry crystals. Microcrystalline calcite fills intergranular pores and develops intergranular micropores (Figure 4g). Quartz cement was also present in the calcareous sandstone, with most occurring as authigenic quartz filling in intergranular pores (Figure 4d,e). This can be distinguished by the contact between quartz cement and quartz grains and the non-luminescence of quartz cement under CL (Figure 4f). Quartz cement is more prevalent in areas where quartz grains are more concentrated, indicating that quartz grains are the main source of quartz cement. Clay minerals in the calcareous sandstones mainly included illite and kaolinite, with illite primarily appearing as flakes attached to the surface of grains (Figure 4i).

4.3.3. Dissolution

Dissolution is one of the most significant processes during diagenesis that improves the porosity of sandstone by creating secondary pores [21,35,36]. In general, feldspar and aluminosilicate minerals in sandstone are the most commonly dissolved minerals, with dissolution pores within feldspar grains constituting the main type of secondary dissolution pores in sandstone reservoirs. However, an extremely low content of feldspar in calcareous sandstone prevents the development of dissolution pores with feldspar grains.
In the Xu4 calcareous sandstone, dissolved carbonate grains can be observed forming dissolution intragranular pores (Figure 5a,d). Previous studies have shown that, under deep burial conditions, dolomite is more prone to dissolution than calcite, leading to the formation of secondary dissolution pores [37,38]. As a result, a greater amount of dolomite rock fragments has been dissolved. The reaction equation for dolomite with acid is as follows:
M g C a ( C O 3 ) 2 + 4 H + → C a 2 + + M g 2 + + 2 H 2 O + 2 C O 2 ( R 1 )

4.4. Pore Space

The thin-section observations (Figure 5) showed primary intergranular pores as the most dominant type of pore space in the calcareous sandstones. A considerable volume of primary pores could be observed in some samples, which is related to weak cementation (especially carbonate cementation). The primary intergranular pores were predominantly irregular, with small amounts of authigenic calcite cement visible along the pore margins. These pores were moderately connected in plane, mainly occurring in isolated distributions, with pore diameters generally ranging from 20 µm to 100 µm. Dissolution intragranular pores also contribute to pore space (Figure 5a,b,d), and a small number of carbonate rock fragments could be observed, which dissolved to form dissolution intragranular pores. However, the dissolved rock fragments were mainly dolomite, not calcite. Compared to the primary pores, the dissolution pores of carbonate rock fragments had relatively low pore diameters (Figure 5).

4.5. Reservoir Quality

The 340 porosity and permeability data (Figure 6) showed significant differences in the reservoir quality of the calcareous sandstone. The porosity ranged from 0.62% to 13.6%, with an average value of 3.63%, and nearly half of the data had a porosity of less than 2%. The permeability ranged from 0.001 to 54.6 mD, with a median value of 0.026 mD, and more than half of the data had a permeability of less than 0.04 mD. The porosity–permeability plot (Figure 6A) indicates two different features of the reservoir quality characteristics of the calcareous sandstones, with some of the low-porosity samples having high permeability, which is related to the development of fractures and belongs to a fractured reservoir (a reservoir that is significantly influenced by fractures). The remaining samples had obvious linear positive correlations between porosity and permeability, indicating that the high or low permeability is mainly related to porosity without the influence of fractures.

5. Discussion

5.1. Comparison of the Reservoir Characteristics between Calcareous Sandstone and Ordinary Xujiahe Tight Gas Sandstone

Calcareous sandstones are mainly composed of carbonate minerals but are genetically classified as terrestrial clastic rocks, which differ from ordinary sandstone reservoirs (with high quartz and silicate mineral contents) in terms of diagenetic changes and reservoir properties [25,27]. Based on previous studies [24,26], a comparative analysis of the differences in reservoir characteristics was carried out between the calcareous sandstone and the ordinary Xu4 tight gas sandstone reservoirs (Xu4 sandstone), and the results are shown in Table 2. In terms of framework grain composition, the calcareous sandstone is quite distinct from the Xu4 sandstone in that the former is characterized by being rich in rock fragments, poor in quartz, and almost free of feldspar; meanwhile, the Xu4 sandstone has a high quartz content and a low feldspar content. Similar conclusions were obtained from the mineral composition using XRD.
As for the storage space and quality of the reservoirs, the reservoir pore space in the calcareous sandstone is predominantly composed of intergranular pores, while that of the reservoir pore in the Xu4 sandstone is dominated by feldspar intragranular dissolution pores. The preservation of primary pores and the development of secondary dissolution pores are essential for the development of relatively high-quality reservoirs under deep burial conditions. However, differences in mineral compositions create different reservoir pore spaces between calcareous sandstone and Xu4 sandstone. Although these two types of sandstone reservoirs have similar ranges of physical properties, the mean porosity and median permeability values of the calcareous sandstone are less favorable than those of the Xu4 sandstone, indicating that the calcareous sandstone reservoirs are more heterogenous.
The parent rock properties of sandstone influence the type, structure, and components of the sandstone, which further controls the diagenetic alterations, pore structure, and reservoir properties [9,24,39,40,41,42]. In calcareous sandstone reservoirs, the extreme abundance of carbonate rock fragments means that there are not enough minerals to dissolve even if acidic fluids enter the reservoir. Additionally, the lack of feldspar minerals limits the formation of quartz cementation and clay cement such as kaolinite and illite, resulting in very low cement contents in calcareous sandstone reservoirs, except for carbonate cement. As for Xu4 sandstone reservoirs, the presence of feldspar minerals not only provides secondary pores as a storage space after dissolution but also provides sources for quartz and clay cementation, leading to a relatively complex cementation pattern.

5.2. Diagenetic Evolution Sequence of Calcareous Sandstone

The diagenetic evolution sequence of calcareous sandstone was constructed based on the petrologic characteristics, the textural relationships between framework grains and cements, and previous studies (Figure 7). Previous studies showed that the paleo-temperature of Xu4 member peaked around 170~180 °C, reaching a late stage of mesodiagenesis [24,26]. During the eodiagenesis stage (T < 70 °C), the diagenetic alterations mainly included mechanical compaction, feldspar dissolution, carbonate cementation, quartz cementation, and kaolinite. After entering the buried stage, the strong mechanical compaction of the Xu4 calcareous sandstone led to a significant decrease in primary porosity. Previous studies have shown the existence of early atmospheric freshwater dissolution in the Xu4 member [24]. Weakly acidic atmospheric freshwater entered the calcareous sandstone, which decreased the K/H value of the pore fluid and caused the dissolution of feldspar minerals and the precipitation of early kaolinite and quartz cements. In addition, the formation of early carbonate cement comes from the precipitation of alkaline fluids [26]. During the mesodiagenesis stage (T > 70 °C), with increasing burial depth, mechanical compaction is converted to pressure dissolution. The second phase of dissolution that occurred during the mesodiagenesis stage is associated with organic matter, which matures and forms organic acids at temperatures between 80 °C and 120 °C [21,35,39,40]. The dissolution acted on carbonate rock fragments and redidual feldspar, and the dissolved carbonate rock fragments are mainly dolomite, not calcite, which may be related to selective dissolution. The transformation of clay minerals (smectite to illite) involved the dissolution of feldspar, formed kaolinite, and quartz cements.

5.3. Main Control Factors of Calcareous Sandstone Reservoirs

5.3.1. Influence of Texture on Calcareous Sandstone Reservoirs

The texture of sandstone influences the initial porosity and the evolution of primary porosity during the burial period due to mechanical compaction [43]. Previous studies have shown that poorly sorted sandstone has much lower initial porosity than well-sorted sandstone [44]. By examining the porosity of different grain sizes in the calcareous sandstones (Figure 8A), the median porosity values of medium-grained calcareous sandstones and fine-grained calcareous sandstones were comparable, with the mean value of the medium-grained sandstones being higher. Coarse-grained calcareous sandstones had the lowest median and mean porosity values. Investigating the porosity distribution of sandstones with different levels of sorting (Figure 8B) showed that the very well-sorted and well-sorted calcareous sandstones had the highest porosity, followed by the moderately well-sorted and moderately sorted sandstones, while the poorly sorted and very poorly sorted sandstones (which also made up the lowest percentage of the total) had the lowest porosity.

5.3.2. Influence of Diagenesis on Calcareous Sandstone Reservoirs

Cementation is the most dominant diagenetic alteration affecting porosity reduction in calcareous sandstones, besides compaction. To quantify the effects of compaction and cementation on reservoir pore reduction, Houseknecht [45] proposed the intergranular volume–cementation plot (Figure 9A). The results showed that compaction is the main factor in porosity loss for the vast majority of calcareous sandstones, while cementation is the main cause of porosity loss in less than 15% of the total samples. It is noteworthy that some of the samples still exhibited good porosity despite a high carbonate cement content. The visual compaction rate method was also devised by Houseknecht [45]. The visual compaction rate (VCR) can measure the percentage of porosity reduction due to compaction and can be calculated as VCR = (original porosity—cement volume percentages—thin-section porosity)/original porosity [45,46]. The results (Figure 9B) showed that while VCR values show an overall negative correlation with reservoir porosity, the sample data points are extremely scattered. This suggests that while compaction is the main cause of porosity loss, other factors are also involved.
The relationship between cements and reservoir physical properties in the calcareous sandstones is more complex. Calcite cement, which is the most abundant in the calcareous sandstone, affects reservoir porosity (Figure 10A). When the calcite cement content was less than 20%, it generally showed a negative effect on the porosity in most samples; however, some of the samples still exhibited high reservoir porosity, and only after the calcite cement content became higher than 20% did the reservoir porosity decrease to lower values. The relationship between calcite cement and reservoir permeability showed similar results (Figure 10B). In general, a calcite cement content between 10% and 20% is significantly destructive to reservoir physical properties, but this is not very evident in calcareous sandstones, which may be related to the formation of most of these calcite cements in the early diagenetic period. Although the early formation of calcite cements blocked the pore space and caused a significant decrease in reservoir porosity, it also prevented further pore loss in the reservoir due to compaction and functioned as a resistance.
Dolomite and quartz cements, and reservoir quality showed strong negative correlations (Figure 10C–F). Both dolomite and quartz cements exhibited negative correlations with calcite cement (Figure 11), and the calcite cement levels tended to be lower in the sandstone samples with higher dolomite cement contents (Figure 12). Thus, the source and formation periods of calcite cement in calcareous sandstones differ from those of dolomite and quartz cements. Previous studies have shown that there are at least two diagenetic generations of calcite cementation in the Xu4 sandstone of the west Sichuan Basin, in which the content of early cementation is much higher, while dolomite cementation is mainly formed in the late diagenetic stage [26]. The early-stage calcite cement, while filling the pores in large quantities, also helped support the pores to some extent. In contrast, late-stage dolomite cement fills the residual pores and can destroy the existing pores. Moreover, because calcareous sandstones as a whole are dominated by carbonate minerals and poor in framework grains, such as feldspar and igneous rock fragments, there are few substances in the diagenetic system that provide a source for quartz cement. This results in a very low quartz cement content in calcareous sandstones. It is assumed that the quartz cement in calcareous sandstones is related to the transformation of clay minerals (mainly illite from montmorillonite), a process that requires temperatures higher than 70 °C. Therefore, quartz cement forms relatively late and also disrupts the pore space during the later stages of diagenesis (Figure 10E,F).

5.3.3. Influence of Fracture on Calcareous Sandstone Reservoirs

Fractures contribute to a significant improvement in reservoir permeability and are one of the key elements in the formation of industrial gas reservoirs [47]. The degree of fracture development is poor in the calcareous sandstones in the Xu4 sandstone, which is only found in a small number of sandstones, but it is extremely important for an improvement in permeability because it is possible for independent pores to spread and increase connectivity (Figure 13a). As shown in Figure 5a, a number of samples with porosity values lower than 2% have permeability values higher than 0.1 mD. In addition, organic matter is observed to fill the fracture (Figure 13b), suggesting that the fractures play an important role in the transportation of hydrocarbons in the calcareous sandstone.

6. Conclusions

(1)
The calcareous sandstone is classified as litharenite, with an average framework grain composition of Q30F1L69, where carbonate grains are the most abundant framework grain. Primary intergranular pores were predominant in the calcareous sandstone, and dissolution intragranular pores were rare. The reservoir quality of the calcareous sandstone was quite poor, with an average porosity of 3.63% and a median permeability of 0.026 mD. Compaction is one of the most significant diagenetic alterations in the calcareous sandstone, while dissolution has had little effect, affecting only a small number of dolomitic rock fragments. Carbonate cementation was the most dominant cement type in the calcareous sandstone, including both calcite and dolomite cements (primarily calcite), whereas quartz cement and clay minerals (mainly illite and kaolinite) were relatively rare.
(2)
Under the influence of different parent rock properties of sandstones, there were obvious differences in the composition of framework grains between the calcareous sandstone and the ordinary Xu4 sandstone, which in turn affected the reservoir storage space, diagenesis, and reservoir quality. The Xu4 sandstone was characterized by high quartz and low feldspar contents, with the storage space primarily consisting of feldspar intragranular dissolved pores. As a result, the porosity and permeability of the Xu4 sandstone were generally better than those of the calcareous sandstone. While the diagenesis processes of the Xu4 and calcareous sandstones were similar, the degree of dissolution, quartz cementation, and clay cementation was higher in the Xu4 sandstone than in the calcareous sandstone, while the carbonate cementation was weaker.
(3)
Texture influences the reservoir quality of sandstone. Medium-grained calcareous sandstones, as well as very well-sorted and well-sorted calcareous sandstones, have the highest porosity values. Compaction is the main factor contributing to porosity loss in the vast majority of calcareous sandstones. The relationship between cement and reservoir physical properties in the calcareous sandstones is more complex. When the calcite cement content was lower than 20%, it generally had a negative effect on the porosity of most of the samples; however, some samples still exhibited high reservoir porosity. It was only when the calcite cement content was higher than 20% that the reservoir porosity decreased. Dolomite and quartz cements are strongly negatively correlated with reservoir quality. Additionally, the source and formation periods of calcite cement differed from those of dolomite and quartz cements in the calcareous sandstones. Fracture development plays an important role in high-quality reservoirs in calcareous sandstones and also affects hydrocarbon transportation.

Author Contributions

Conceptualization, D.W.; methodology, Y.Y.; software, D.W. and L.L.; validation, Y.Y.; formal analysis, D.W.; investigation, Y.Y.; resources, H.C.; data curation, S.L.; writing—original draft preparation, Y.Y.; writing—review and editing, D.W.; visualization, L.L.; supervision, L.L.; project administration, H.C.; funding acquisition, D.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Open Fund (Different diagenetic evolution of tight sandstone reservoirs in Xujiahe Formation, western Sichuan Basin and its significance for hydrocarbon accumulation) of the SINOPEC Key Laboratory of Geology and Resources in Deep Stratum.

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 thank Zhengxiang Lü for his valuable work. We also thank Bo Pan, Xinglong Wang, and Zhikang Wang. We gratefully acknowledge the insightful reviews and constructive suggestions from the editors and anonymous reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (A) The location and geological map of the Sichuan Basin; (B) location map of drilling wells in the western Sichuan Basin; (C) generalized stratigraphic columns of the Xujiahe Formation in the western Sichuan Basin.
Figure 1. (A) The location and geological map of the Sichuan Basin; (B) location map of drilling wells in the western Sichuan Basin; (C) generalized stratigraphic columns of the Xujiahe Formation in the western Sichuan Basin.
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Figure 2. Sandstone classification (A) and bulk mineralogy by XRD (B) of the calcareous sandstone. Cal = calcite, Dol = dolomite.
Figure 2. Sandstone classification (A) and bulk mineralogy by XRD (B) of the calcareous sandstone. Cal = calcite, Dol = dolomite.
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Figure 3. Thin-section images showing the petrographic characteristics of the calcareous sandstone. (a,b) Fine-grained to medium-grained, moderately well sorted, sub-angular, and point-to-line contact, XC22 well, 3550.8 m, PPL and XPL; (c,d) fine-grained to medium-grained, moderately sorted, sub-angular, and line contact, XC31 well, 3745.62 m, PPL and XPL; (e) carbonate rock fragments composed of microcrystalline calcite and dolomite cement filling in the intergranular pores, X11 well, 3579.6 m, SEM; (f) microcrystalline calcite filling in the intergranular pores, MS1 well, 4312.74 m, SEM. Q = quartz, C-RF = carbonate rock fragment, D-RF = dolomite rock fragment, CC = calcite cement.
Figure 3. Thin-section images showing the petrographic characteristics of the calcareous sandstone. (a,b) Fine-grained to medium-grained, moderately well sorted, sub-angular, and point-to-line contact, XC22 well, 3550.8 m, PPL and XPL; (c,d) fine-grained to medium-grained, moderately sorted, sub-angular, and line contact, XC31 well, 3745.62 m, PPL and XPL; (e) carbonate rock fragments composed of microcrystalline calcite and dolomite cement filling in the intergranular pores, X11 well, 3579.6 m, SEM; (f) microcrystalline calcite filling in the intergranular pores, MS1 well, 4312.74 m, SEM. Q = quartz, C-RF = carbonate rock fragment, D-RF = dolomite rock fragment, CC = calcite cement.
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Figure 4. Thin-section images showing the cementation characteristics of the calcareous sandstone. (a–c) Calcite cements (red arrow) show strong orange luminescence, X11 well, 3576.95 m, PPL, XPL, and CL; (d–f) quartz cement (red arrow) has non-luminescence and can be clearly distinguished from quartz grains with blue luminescence, MS1 well, 4396.26 m, PPL, XPL, and CL; (g) intergranular micropores developed in microcrystalline calcite (yellow dash), CF563 well, 3746.3 m, SEM; (h) fine-crystalline calcite cement (red arrow) filled in the intergranular pores of microcrystalline calcite, MS1 well, 4308.1 m, SEM; (i) flake illite (red arrow) attached to the surface of microcrystalline calcite grain, MS1 well, 4312.74 m.
Figure 4. Thin-section images showing the cementation characteristics of the calcareous sandstone. (a–c) Calcite cements (red arrow) show strong orange luminescence, X11 well, 3576.95 m, PPL, XPL, and CL; (d–f) quartz cement (red arrow) has non-luminescence and can be clearly distinguished from quartz grains with blue luminescence, MS1 well, 4396.26 m, PPL, XPL, and CL; (g) intergranular micropores developed in microcrystalline calcite (yellow dash), CF563 well, 3746.3 m, SEM; (h) fine-crystalline calcite cement (red arrow) filled in the intergranular pores of microcrystalline calcite, MS1 well, 4308.1 m, SEM; (i) flake illite (red arrow) attached to the surface of microcrystalline calcite grain, MS1 well, 4312.74 m.
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Figure 5. Thin-section images showing the pore space characteristics of the calcareous sandstone. (a) Primary intergranular pore developed and the dolomite rock fragment dissolved to form intragranular pore, CF563 well, 3741.96 m, PPL; (b) primary intergranular pore and dissolution intragranular pore can be observed, CF563 well, 3741.96 m, PPL; (c) the grains are in point and line contact, and intergranular pore developed, CF563 well, 3743.63 m, PPL; (d) small amount of calcite cement filled in the intergranular pore and the dolomite rock fragment dissolved, CF563 well, 3743.63 m, PPL. Q = quartz, C-RF = carbonate rock fragment, D-RF = dolomite rock fragment, CC = calcite cement, PP = primary intergranular pore, DP = dissolution intragranular pore.
Figure 5. Thin-section images showing the pore space characteristics of the calcareous sandstone. (a) Primary intergranular pore developed and the dolomite rock fragment dissolved to form intragranular pore, CF563 well, 3741.96 m, PPL; (b) primary intergranular pore and dissolution intragranular pore can be observed, CF563 well, 3741.96 m, PPL; (c) the grains are in point and line contact, and intergranular pore developed, CF563 well, 3743.63 m, PPL; (d) small amount of calcite cement filled in the intergranular pore and the dolomite rock fragment dissolved, CF563 well, 3743.63 m, PPL. Q = quartz, C-RF = carbonate rock fragment, D-RF = dolomite rock fragment, CC = calcite cement, PP = primary intergranular pore, DP = dissolution intragranular pore.
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Figure 6. Reservoir quality of the calcareous sandstones. (A) Cross plot between porosity and permeability, the sample within the red circle influenced by fractures; (B,C) frequency histogram of porosity and permeability.
Figure 6. Reservoir quality of the calcareous sandstones. (A) Cross plot between porosity and permeability, the sample within the red circle influenced by fractures; (B,C) frequency histogram of porosity and permeability.
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Figure 7. Diagenetic evolution sequence of calcareous sandstone.
Figure 7. Diagenetic evolution sequence of calcareous sandstone.
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Figure 8. Porosity distribution of the calcareous sandstones with different grain sizes (A) and level of sorting (B).
Figure 8. Porosity distribution of the calcareous sandstones with different grain sizes (A) and level of sorting (B).
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Figure 9. (A) Plot of cements versus intergranular volume (modified from [45]; (B) plot of thin-section porosity and VCR.
Figure 9. (A) Plot of cements versus intergranular volume (modified from [45]; (B) plot of thin-section porosity and VCR.
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Figure 10. Plot of cements versus reservoir quality. (A) Calcite cement versus porosity (%); (B) calcite cement versus permeability (mD); (C) dolomite cement versus porosity (%); (D) dolomite cement versus permeability (mD); (E) quartz cement versus porosity (%); (F) quartz cement versus permeability (mD). Red dash line represents the distribution range of calcite cement content and the corresponding porosity or permeability.
Figure 10. Plot of cements versus reservoir quality. (A) Calcite cement versus porosity (%); (B) calcite cement versus permeability (mD); (C) dolomite cement versus porosity (%); (D) dolomite cement versus permeability (mD); (E) quartz cement versus porosity (%); (F) quartz cement versus permeability (mD). Red dash line represents the distribution range of calcite cement content and the corresponding porosity or permeability.
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Figure 11. Plot of calcite cement versus dolomite cement (A) and quartz cement (B). Green dash line represents the corresponding calcite cement at different quartz cement.
Figure 11. Plot of calcite cement versus dolomite cement (A) and quartz cement (B). Green dash line represents the corresponding calcite cement at different quartz cement.
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Figure 12. Calcite and dolomite cement contents in the calcareous sandstones of the FG21 well, the blue bars represents the sample of dolomite cements development.
Figure 12. Calcite and dolomite cement contents in the calcareous sandstones of the FG21 well, the blue bars represents the sample of dolomite cements development.
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Figure 13. Thin-section images showing the fracture characteristics of the calcareous sandstone. (a) Two fractures (red arrow) across the framework grain, CH139 well, 3779.32 m, PPL; (b) fracture filled with organic matter (red arrow), CF563 well, 3741.96 m, PPL. PP= primary intergranular pore.
Figure 13. Thin-section images showing the fracture characteristics of the calcareous sandstone. (a) Two fractures (red arrow) across the framework grain, CH139 well, 3779.32 m, PPL; (b) fracture filled with organic matter (red arrow), CF563 well, 3741.96 m, PPL. PP= primary intergranular pore.
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Table 1. The mineral composition (%) of the calcareous sandstone. (In this table, Q. = quartz, PF. = potash feldspar, PL. = plagioclase, Cal. = calcite, Dol. = dolomite, I = Illite, K = kaolinite, C = chlorite, I/S = illite/smectite mixed layer).
Table 1. The mineral composition (%) of the calcareous sandstone. (In this table, Q. = quartz, PF. = potash feldspar, PL. = plagioclase, Cal. = calcite, Dol. = dolomite, I = Illite, K = kaolinite, C = chlorite, I/S = illite/smectite mixed layer).
WellDepth (m)ClayQ.PF.PL.Cal.Dol.I.K.C.I/S.
CF5633743.652.1 9.10 0.3 41.4 46.4
CF5633741.903.1 14.30 0.4 33.3 49.3
CF5633742.4010.524.500 45.419.1128800
X113579.756.954.80 0 26.312.673 0270
X113577.6913.5 39.20.1 0.2 33.6 13.55232619
CH1393774.636.2 23.600.2 56.7 13.445151822
CH1393772.592.8 31.90.1 027.8 37.4
XC223555.427.9 34.20 0 32.1 25.143141924
XC223550.8714.8 35.70 0.2 42.3 7.2
FG213750.5917.530.500.335.415.9
FG213758.896.324.10047.322.65821165
FG213769.376.343.10026.423.4
Table 2. Comparison of the characteristics of calcareous sandstone and ordinary Xu4 tight gas sandstone.
Table 2. Comparison of the characteristics of calcareous sandstone and ordinary Xu4 tight gas sandstone.
CharacteristicsCalcareous SandstoneXu4 Sandstone
Framework grain compositionQuartz: 1%~62%, feldspar: 0%~12%, and rock fragment: 37%~99%, framework grain composition of Q30F1L69Quartz: 45%~89%, feldspar: 0%~18%, and rock fragment: 8%~55%, framework grain composition of Q71F5L24
Mineral composition by XRDCalcite: 37.3%, quartz: 30.4%, dolomite: 23.8%, and clay minerals: 8.2%Quartz: 69.2%, clay minerals: 16.5%, calcite: 4.9%, dolomite: 4.1%, and plagioclase: 3.7%
Storage spacePrimary intergranular poreSecondary pore: feldspar intragranular dissolved pore
Reservoir qualityPorosity: 0.62% to 13.6%, with average value of 3.63%; permeability: 0.001 mD to 54.6 mD, with the median value 0.026 mDPorosity: 0.58% to 12.71%, with average value of 6.13%; permeability: 0.002 mD to 9.93 mD, with the median value 0.142 mD
DiagenesisCompactionMain factor of porosity reduction
DissolutionWeak, a few dolomite rock fragments dissolvedStronger, formed feldspar intragranular dissolved pore constitute the main storage space
CementationCarbonate cements predominant, other cements minorCarbonate cements, quartz cement, and illite and kaolinite cements
Note: Data of ordinary Xujiahe tight gas sandstone from Yu et al. [24] and Lin et al. [26].
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Wu, D.; Yu, Y.; Lin, L.; Chen, H.; Liu, S. Characteristics and Control Factors of a High-Quality Deeply Buried Calcareous Sandstone Reservoir, the Fourth Member of the Upper Xujiahe Formation in the Western Sichuan Basin, China. Minerals 2024, 14, 872. https://doi.org/10.3390/min14090872

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Wu D, Yu Y, Lin L, Chen H, Liu S. Characteristics and Control Factors of a High-Quality Deeply Buried Calcareous Sandstone Reservoir, the Fourth Member of the Upper Xujiahe Formation in the Western Sichuan Basin, China. Minerals. 2024; 14(9):872. https://doi.org/10.3390/min14090872

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Wu, Dong, Yu Yu, Liangbiao Lin, Hongde Chen, and Sibing Liu. 2024. "Characteristics and Control Factors of a High-Quality Deeply Buried Calcareous Sandstone Reservoir, the Fourth Member of the Upper Xujiahe Formation in the Western Sichuan Basin, China" Minerals 14, no. 9: 872. https://doi.org/10.3390/min14090872

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