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Influence of pore structure characteristics on the Permian Shan-1 reservoir in Longdong, Southwest Ordos Basin, China

  • Guangjun Xu , Hao Li , Lei Tang , Xiaoke Gong , Yuxin Xie , Danni Zhao , Jiangmeng Chen and Qinlian Wei EMAIL logo
Published/Copyright: December 27, 2024
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Open Geosciences
From the journal Open Geosciences Volume 16 Issue 1

Abstract

This study aims to characterize the pore structures and controlling factors of the Shan-1 Member reservoir in the southwestern Ordos Basin, a geological formation known for its significant gas production. Thin section analysis, scanning electron microscopy, high-pressure mercury intrusion (MICP), and reservoir physical property analysis were employed to investigate the pore structures within the reservoir. The Shan-1 Member reservoir is primarily characterized by lithic dissolution and intergranular pores, with generally small throat radii. Reservoirs with high quartz content (>70%) are associated with the development and preservation of primary pores, resulting in superior pore structures (Types I and II) with larger pore throats. Similarly, reservoirs with low lithic content (<10%) exhibit Type I and II pore structures, also with larger median pore throat radii. In contrast, Type III and IV pore structures, characterized by smaller median pore throat radii, are linked to higher lithic content. Compaction significantly degrades pore structures, while cementation and dissolution play key roles in controlling their variability. These processes underscore the complexity of the Shan-1 reservoir, with important implications for optimizing exploration strategies. This study provides novel insights into the interplay between mineral composition and diagenetic processes in shaping pore structures, offering valuable guidance for the exploration of tight gas reservoirs within the Permian strata of the southwestern Ordos Basin.

Keywords: tight reservoir; pore structure; Shan-1; Permian; Ordos Basin

1 Introduction

Global economic growth has recently intensified the demand for oil and gas resources [1,2,3,4]. The gradual exhaustion of conventional reserves and the difficulties in their exploration and development are no longer adequate to satisfy the pressing demands of countries worldwide.

Consequently, unconventional oil and gas resources, exemplified by tight sandstone reservoirs, have gained prominence due to their substantial exploration and development potential [2–4].

China’s Ordos Basin, a vast reservoir of hydrocarbons, is rich in oil and gas resources [4]. The Longdong area, located in the basin’s southwestern region, is a promising frontier for tight sandstone oil and gas exploration and development. This research primarily focuses on the sedimentary system, source rock, and parent rock characteristics, reservoir formation, and structural features of the Shan-1 reservoir [3,5–19]. However, a deeper understanding of the reservoir’s pore structure and its key influencing factors is essential. It is also crucial to identify the main factors affecting reservoir performance across different pore structures. As a crucial component of the Permian system, the pore structure of the Shan-1 reservoir significantly influences oil and gas storage, presenting a promising avenue for discoveries and advancements in the field.

At present, the analysis of pore structure is primarily based on mercury intrusion curves, with insufficient discussion on the main controlling factors for different pore structures. The analysis is mainly based on the mercury intrusion curve, lacking discussion on the main controlling factors for different pore structures. This study investigates the pore structure characteristics and controlling factors of tight reservoirs within the Shan-1 Member in the southwestern Ordos Basin. Utilizing techniques such as cast thin sections, scanning electron microscopy (SEM), high-pressure mercury intrusion (MICP), and analysis of reservoir physical properties, our research systematically explores pore types, the influence of rock components on pore structure, and diagenetic factors. A deeper understanding of micro-pore characteristics and their impact in tight sandstone reservoirs will enhance the efficiency of oil and gas resource development in this region, providing valuable theoretical insights and practical applications.

2 Geological setting

The study area, an enchanting landscape, lies to the southwest of the Ordos Basin (Figure 1(a)), covering an extensive 50,000 km2 [1]. To the west, the Tianhuan Depression, a prominent geological feature, abuts the western edge of the thrust belt. To the east, the southwestern part of the Shaanbei Slope, and to the south, the Weibei Uplift collectively shape the distinctive geological features of the region. The Shanxi Formation (P1s) is part of the Lower Permian strata, with its base in contact with the Taiyuan Formation (P1t) and its upper portion interfacing with the Middle Permian Shihezi Formation (P2h) [8].

Figure 1 The geographical location of the study area (a) and the lithological properties of the target layer (b).
Figure 1

The geographical location of the study area (a) and the lithological properties of the target layer (b).

The Shanxi Formation, a key component of this study, has a basal boundary marked by the “Beichagou Sandstone” and an upper boundary marked by the “Camel Neck Sandstone,” with an overall thickness of about 70 m. It represents a terrestrial lake delta sedimentary system, a significant geological feature. Based on sedimentary sequences and lithological associations, the study area is divided into the Shan-1 and Shan-2 Members (as depicted in Figure 1b) [8].

The Shan-1 Member primarily consists of sand and mudstone deposited by distributary channels. The sandstone in this region comprises medium- to fine-grained lithic quartz sandstone with an approximate thickness of 30 m.

The Shan-2 Member mainly comprises a series of delta coal-bearing strata, typically featuring 3–5 coal-forming periods. Rivers and delta sand bodies are interspersed within the coal-bearing strata, which is predominantly composed of gray, dark gray, or gray-brown fine-grained sandstone and siltstone interbedded with black mudstone. This section has a thickness of 30–40 m.

3 Methods

To achieve our research objectives, we utilized various experimental techniques, including SEM, thin section analysis, petrophysical property testing, and mercury injection capillary pressure (MICP). The thin section casting, SEM, MICP, and petrophysical property tests were carried out at the Shaanxi Key Laboratory of Oil and Gas, Xi’an Shiyou University.

The investigation of reservoir rocks and pores involved analyzing 34 thin sections to gain insights into mineral composition. Additionally, we examined four SEM images to study pore and mineral characteristics. A comprehensive petrophysical analysis utilized 620 rock property test results. To determine rock porosity and permeability, we employed the CMS-300 overburden porosity and permeability tester in conjunction with the HBYQ-VI reservoir-forming physical simulation platform, observing nitrogen flow. Finally, we explored the impact of micropore throat structure on tight sandstone permeability by integrating thin sections, SEM, and 34 MICP samples. These MICP experiments were conducted using an Aspe-730 from Coretest Systems Inc.

4 Results

4.1 Analysis of rock types

Analysis of rocks and minerals indicates that the clastic components of the Shan-1 reservoir are mainly composed of quartz, with the presence of lithic fragments. Feldspar particles are observed only occasionally and in small amounts, with an average content of less than 5%. The reservoir exhibits an average quartz content of 72.2%, with feldspar comprising 0.75%, lithic fragments accounting for 11.1%, and interstitial material constituting 13.2% (Figure 2).

Figure 2 Ternary diagram of sandstone composition of the Shan-1 reservoir.
Figure 2

Ternary diagram of sandstone composition of the Shan-1 reservoir.

4.2 Variations in filler types

The primary cement in the interstitial material of the Shan-1 reservoir is predominantly illite (2.57%), followed by average contents of siliceous minerals (2.14%), kaolinite (1.66%), siderite (1.39%), and chlorite (1.20%) (Figure 3). Chlorite is observed at the pore scale edges, exhibiting excellent anti-compaction properties that mitigate the impact of weak compaction on intergranular pores and necked throats. Illite content is notably high, accounting for over 20% of the total interstitial composition. Illite fills the pores in the form of silk strands, leading to pore shrinkage and blockage, which facilitates the formation of various complex pipe bundle throats [20].

Figure 3 Histogram of interstitial material content in the Shan-1 reservoir of the Longdong area.
Figure 3

Histogram of interstitial material content in the Shan-1 reservoir of the Longdong area.

4.3 Variations in physical characteristics

In the Shan-1 reservoir, porosity predominantly exceeds 2.0% (Figure 4a), with an average value of 4.03%. Additionally, permeability consistently surpasses 0.01 mD, averaging 0.18 mD (Figure 4b).

Figure 4 Physical properties of Shan-1 reservoir in Longdong area, Ordos basin: (a) porosity distribution, (b) permeability distribution, and (c) cross-plot of porosity and permeability.
Figure 4

Physical properties of Shan-1 reservoir in Longdong area, Ordos basin: (a) porosity distribution, (b) permeability distribution, and (c) cross-plot of porosity and permeability.

The performance of the Shan-1 reservoir is significantly influenced by its physical properties, particularly porosity and permeability. Although these factors exhibit a general correlation, substantial variations in permeability can occur for the same porosity value. This underscores the importance of pore and throat characteristics within the sandstone matrix. Additionally, the presence of microcracks significantly enhances reservoir permeability (Figure 4c).

4.4 Variations in the pore architecture

4.4.1 Variations in the pore type

Based on the analysis of 34 thin sections, it is evident that the predominant lithology in the Shan-1 reservoir sand bodies consists of lithic sandstone and lithic quartz sandstone. The sandstone reservoir has experienced intense compaction and pressure solution during a prolonged diagenesis process, significantly affecting the reservoir’s porosity and permeability. Clastic particles, particularly quartz and lithic fragments, show linear and concave–convex contacts, often with varying degrees of quartz regrowth. This regrowth has led to the loss of many primary intergranular pores, a significant consequence of the diagenesis process.

The diagenetic examination of the Shan-1 reservoir has revealed a fascinating array of authigenic minerals – such as kaolinite, illite, calcite, and ferrocalcite – that have precipitated within the primary pores (Figure 3). These pores have subsequently experienced siliceous cement infilling, significantly reducing the original intergranular pore space in the sandstone. Observations from thin sections and SEM analysis of the Shan-1 reservoir have identified several distinct pore types, including intergranular pores, feldspar dissolution pores, lithic fragment dissolution pores, impurity dissolution pores, and intergranular micropores (Figure 5).

Figure 5 Distribution map illustrating the frequency of various pore types in the Shan-1 reservoir.
Figure 5

Distribution map illustrating the frequency of various pore types in the Shan-1 reservoir.

4.4.1.1 Intergranular pores

The dominant intergranular pores include quartz-enlarged intergranular pores and intergranular dissolution pores.

4.4.1.1.1 Intergranular pores after quartz overgrowth

The quartz grains develop overgrowths, but these overgrowths do not fill the intergranular pores; they only significantly reduce the original pore spaces (Figure 6a). This type of pore shape is regular, mostly triangular, quadrilateral, or polygonal, with straight edges. The pore size is moderate, ranging from 0.01 to 0.1 mm (Figure 6b). These pores are typically found in quartz sandstone and lithic quartz sandstone.

Figure 6 Microscopic characteristics of intergranular pores in the Shan-1 reservoir in the Longdong area. (a) Well Lian54 at 3949.8 m: coarse-grained quartz sandstone with well-developed intergranular pores. (b) Well Qt4 at 4326.2 m: altered tuffaceous, unevenly grained quartz sandstone showing intergranular pore development.
Figure 6

Microscopic characteristics of intergranular pores in the Shan-1 reservoir in the Longdong area. (a) Well Lian54 at 3949.8 m: coarse-grained quartz sandstone with well-developed intergranular pores. (b) Well Qt4 at 4326.2 m: altered tuffaceous, unevenly grained quartz sandstone showing intergranular pore development.

4.4.1.1.2 Intergranular dissolution pores

Partial dissolution of clastic grain edges occurs based on the original intergranular pores. The pore shapes are irregular, with jagged or bay-like edges, and the pores are relatively large, generally exceeding 0.05 mm (Figure 7a). These pores have a mixed origin, predominantly primary, as they result from slight dissolution enlarging the original intergranular pores (Figure 7b). The enlarged portion due to dissolution accounts for about 10% of the total pore space. When the intergranular pore edges lack chlorite rims or quartz overgrowths, the grain edges are more susceptible to dissolution, forming intergranular dissolution pores. Intergranular dissolution pores serve as a primary type of reservoir space, frequently coexisting with intragranular dissolution pores.

Figure 7 Microscopic characteristics of intergranular dissolution pores in the Shan-1 reservoir in Longdong area. (a) In well L14 at a depth of 3938.4 m, filamentous illite fills the spaces between debris particles and intergranular pores. Additionally, we observed intergranular microcracks and dissolution pores. (b) In well L19 at a depth of 3879.1 m, sheet-like kaolinite aggregates and filamentous illite occupy the intergranular pores, along with intergranular dissolution pores.
Figure 7

Microscopic characteristics of intergranular dissolution pores in the Shan-1 reservoir in Longdong area. (a) In well L14 at a depth of 3938.4 m, filamentous illite fills the spaces between debris particles and intergranular pores. Additionally, we observed intergranular microcracks and dissolution pores. (b) In well L19 at a depth of 3879.1 m, sheet-like kaolinite aggregates and filamentous illite occupy the intergranular pores, along with intergranular dissolution pores.

4.4.1.1.3 Moldic pores

Moldic pores, a significant feature in our geological context, are formed by the complete dissolution of detrital particles, leaving only the external shape or minimal remnants of the particles [21]. In some cases, only the green clay coating of the particles remains. These pores, which are thoroughly dissolved from feldspar, rock fragments, and other components, are well-developed in our study area (Figure 8). They have a tabular appearance due to feldspar and are generally larger than 0.1 mm. However, their abundance is relatively low, and their pore volume rarely exceeds 2%, making them secondary reservoir spaces.

Figure 8 Microscopic characteristics of intragranular dissolution pores in the Shan-1 reservoir in Longdong area. (a) Well Lian1, 3469.8 m, rock debris dissolution pore; (b) Well Lian1, 3470.3 m, feldspar dissolution pore, and rock debris dissolution pore; (c) Well L19, 3879.1 m, potassium feldspar particles are dissolved along the cleavage, and sheet-like kaolinite aggregates and filamentous illite are filled in the intergranular pores; (d) Well L7, 4156.35 m, feldspar particles have been dissolved, broken, and mixed with filamentous illite.
Figure 8

Microscopic characteristics of intragranular dissolution pores in the Shan-1 reservoir in Longdong area. (a) Well Lian1, 3469.8 m, rock debris dissolution pore; (b) Well Lian1, 3470.3 m, feldspar dissolution pore, and rock debris dissolution pore; (c) Well L19, 3879.1 m, potassium feldspar particles are dissolved along the cleavage, and sheet-like kaolinite aggregates and filamentous illite are filled in the intergranular pores; (d) Well L7, 4156.35 m, feldspar particles have been dissolved, broken, and mixed with filamentous illite.

4.4.1.2 Intercrystalline micropores

Intercrystalline micropores primarily occur within fine clay minerals and are commonly developed in interlayer pores of kaolinite [22] (Figure 9), intergranular illite inclusions, and mudstone matrix. Illite intercrystalline micropores are visible in this area due to feldspar alteration, rock fragment dissolution, and hydration processes. Additionally, there are micropores within the mudstone matrix and mudstone fragments. These micropores have diameters smaller than 0.01 mm, representing a common pore type in this region.

Figure 9 Microscopic characteristics of intergranular pores of kaolinite in the Shan 1 reservoir in the Longdong area. (a) Well L19, 3879.1 m, intergranular pores in kaolinite; (b) Well Qt4, 4374.8 m, intergranular pores of kaolinite.
Figure 9

Microscopic characteristics of intergranular pores of kaolinite in the Shan 1 reservoir in the Longdong area. (a) Well L19, 3879.1 m, intergranular pores in kaolinite; (b) Well Qt4, 4374.8 m, intergranular pores of kaolinite.

Therefore, in the Shan-1 reservoir interval of the Longdong area, intragranular dissolution pores resulting from diagenetic alteration are the dominant pore type. The development of intergranular pores and intercrystalline micropores further enhances the reservoir properties.

4.4.2 Characteristics of pore structures

The pore structure, a multifaceted concept encompassing the size, shape, connectivity, arrangement, and evolutionary characteristics of pores and their connecting throats, is fundamental to our understanding [23,24]. Pores, serving as fluid storage spaces, and throats, which significantly impact rock permeability, are its core elements. The sizes and shapes of these pores and throats are intricately linked to particle size, shape, contact type, and cementation. Crucially, the size and connectivity of throats directly determine rock permeability, a key parameter in reservoir engineering. Hence, the extent of pore and throat development, along with their interplay, significantly influences the distribution of oil within reservoirs.

Various conventional methods for studying rock pore structures, including MICP, thin section, SEM, and image analysis, each offer distinct advantages. However, the mercury intrusion capillary pressure method stands out as the most commonly used technique. Its long-standing presence in reservoir research has now made it a classic approach, underscoring its significance and relevance in our study.

In applying our classification and evaluation criteria for the reservoir pore structures, specifically Shan-1, four distinct pore structures were identified, as presented in Table 1 and Figure 10.

Table 1

Characteristics of pore throats in the Shan-1 reservoir of the Longdong area

Summary data Reservoir properties Pore throat size Pore throat connectivity characteristics Pore throat distribution features
Porosity (%) Permeability (mD) Threshold pressure (MPa) Median pressure (MPa) Median radius (μm) Maximum mercury saturation (%) Sorting coefficient Skewness coefficient Mean coefficient
Minimum value 0.700 0.016 0.200 1.654 0.007 25.050 1.029 −4.206 0.070
Maximum value 7.900 15.430 6.568 109.553 0.444 98.944 5.800 2.030 13.833
Average 4.005 0.538 1.877 24.458 0.160 61.828 2.290 −0.071 8.100
Figure 10 Characteristics of MICP in the Shan-1 reservoir in the Longdong area. (a) Type I pore structure; (b) Type II pore structure; (c) Type III pore structure; and (d) Type IV pore structure.
Figure 10

Characteristics of MICP in the Shan-1 reservoir in the Longdong area. (a) Type I pore structure; (b) Type II pore structure; (c) Type III pore structure; and (d) Type IV pore structure.

The mercury intrusion curve for Type I pore structure, a vital tool for understanding pore structures, is platform-like, indicating excellent pore throat connectivity, low skewness, and low displacement pressure, typically less than 1.0 MPa. Within the reservoir, the median pore radius exceeds 0.1 µm, while mercury-withdrawal efficiency consistently surpasses 35%. Porosity generally exceeds 8%, and permeability typically surpasses 0.4 mD (Figure 10a).

For Type II pore structure, the mercury intrusion curve exhibits a platform-like shape, albeit with a noticeable slope. This suggests relatively limited pore throat connectivity and moderate skewness. Characterized by a displacement pressure range of 1–2 MPa, a median pore radius exceeding 0.05 µm, mercury-withdrawal efficiency exceeding 30%, and typical porosity falling between 6 and 8%, this type exhibits permeability ranging from 0.2 to 0.4 mD (Figure 10b).

The mercury intrusion curve for the Type III pore structure is platform-like with a distinct slope, reflecting poor pore throat connectivity and fine skewness. The relatively high displacement pressure, generally over 2 MPa, presents potential challenges. The median radius is less than 0.05 µm, mercury-withdrawal efficiency is above 30%, porosity generally ranges from 4 to 6%, and permeability exceeds 0.1 mD (Figure 10c).

For Type IV pore structure, the mercury intrusion curve exhibits an absence of a platform, indicating very unfavorable physical properties and high displacement pressure, typically exceeding 2 MPa. These characteristics, coupled with a porosity usually less than 4% and permeability below 0.1 mD (Figure 10d), present significant challenges but also unique opportunities for further research and understanding.

5 Discussion

5.1 Influence of rock composition on the pore structure

The analysis of mercury pressure data in the Shan-1 reservoir has underscored the significant impact of rock composition on the pore structure. Notably, a negative correlation between the quartz content and displacement pressure has been observed (Figure 11a). When the quartz content is less than 65%, the predominant pore structure types are III and IV. In the 65–80% range, the predominant types are I and II. Conversely, rock debris content shows a positive correlation with displacement pressure (Figure 11b). When the rock debris content is below 10%, the predominant pore structures are Types I and II. However, when the rock debris content exceeds 10%, the central pore structures shift to Types III and IV. These findings can be directly applied in the field to enhance our understanding of reservoir properties and optimize extraction processes.

Figure 11 The influence of quartz and rock debris content on reservoir structural parameters in the Shan-1 reservoir in the Longdong area. (a) the quartz content and displacement pressure; (b) the rock debris and displacement pressure; (c) the average pore throat and quartz content; and (d) the average pore throat and rock debris content.
Figure 11

The influence of quartz and rock debris content on reservoir structural parameters in the Shan-1 reservoir in the Longdong area. (a) the quartz content and displacement pressure; (b) the rock debris and displacement pressure; (c) the average pore throat and quartz content; and (d) the average pore throat and rock debris content.

Moreover, the data reveal a distinct correlation between the average pore throat radius (APTR) and quartz content, as illustrated in Figure 11c. As quartz content increases, the APTR consistently increases. For quartz content below 65%, pore structures predominantly exhibit Types III and IV, whereas within the 65–80% range, they shift to Types I and II. In contrast, the APTR exhibits a clear negative correlation with rock debris content (Figure 11d). When rock debris content is below 10%, pore structures are dominated by Types I and II; however, when it exceeds 10%, Types III and IV become prevalent. These findings, supported by meticulous data collection and analysis, provide a robust foundation for further research and practical applications in the fields of geology and petroleum engineering.

5.2 Impact of diagenesis on the pore characteristics

5.2.1 Impact of compaction on the pore structure

While the feldspar content in the Shan-1 reservoir is relatively low, the proportion of components such as quartz and rock debris ranges from 70 to 90%. The apparent compaction rate in reservoirs reflects the speed at which rocks compact during diagenesis, leading to porosity reduction in sandstone reservoirs [25], and is calculated using the following formula:

(1) Apparent compaction rate = Primary porosity-intergranular volume Primary porosity × 100 .

In equation (1), the intergranular volume is calculated as the sum of the total volume of existing pores and the cement content minus the volume of dissolution pores. The primary porosity can be expressed as 20.19 + 22.90/S0, where S0 represents the Trask sorting coefficient.

The Shan-1 reservoir in the Longdong area exhibits an apparent compaction rate ranging from 39.7 to 77.6%, with an average of 58.1%. These data suggest significant compaction, as depicted in Figure 12. The compaction rate varies among reservoirs with different pore structures, with reservoirs characterized as having Type III (moderate compaction) and IV (severe compaction) pore structures exhibiting higher apparent compaction rates than those with Type I (no compaction) and II (mild compaction) pore structures (Figure 13).

Figure 12 Distribution of apparent compaction, cementation, and dissolution rates in the Shan-1 reservoir.
Figure 12

Distribution of apparent compaction, cementation, and dissolution rates in the Shan-1 reservoir.

Figure 13 Histogram of average apparent compaction, cementation, and dissolution rates across different pore structure types in the Shan-1 reservoir.
Figure 13

Histogram of average apparent compaction, cementation, and dissolution rates across different pore structure types in the Shan-1 reservoir.

The APTR, a measure of the size of the pathways through which fluids move in the reservoir, shows a significant decreasing trend with increasing apparent compaction rate, particularly within the 40–70% range, demonstrating a strong negative correlation (Figure 14a). Figure 14b illustrates a positive correlation between the apparent compaction rate and displacement pressure, which measures the pressure needed to displace fluids from the reservoir. For Type IV pore structures, the displacement pressure increases sharply with higher apparent compaction rates, especially in the 80–100% range, where the change is most drastic. Owing to solid compaction, significant differences in pore structures exist in the Shan-1 reservoir, with Type III and IV pore structures exhibiting more intense compaction and higher displacement pressures compared to Types I and II.

Figure 14 Correlation between pore structure parameters and apparent compaction rate of the Shan-1 reservoir in the Longdong area. (a) the apparent compaction rate and displacement pressure of different pore structures; (b) the apparent compaction rate and average pore radius of different pore structures.
Figure 14

Correlation between pore structure parameters and apparent compaction rate of the Shan-1 reservoir in the Longdong area. (a) the apparent compaction rate and displacement pressure of different pore structures; (b) the apparent compaction rate and average pore radius of different pore structures.

5.2.2 Impact of cementation on the pore structure

Cementation is a crucial diagenetic process in which sediments are transformed into sedimentary rocks. During diagenesis, pores and throats are significantly reduced or even eliminated, which detrimentally affects the pore structure of reservoir rocks.

Our findings from meticulous thin-section identification experiments reveal that the clay minerals in the Shan-1 reservoir are primarily kaolinite, illite, and chlorite. The kaolinite content varies from 0 to 6%, with an average of 1.8%, while the illite content ranges from 0.5 to 11%, with an average of 4.4%. These values are particularly significant. The content of carbonate cement, varying from 0 to 9% with an average of 2.2%, and siliceous cement, ranging from 0.5 to 10% with an average of 3.5%, also provide valuable insights (Figure 3).

The apparent cementation rate within reservoirs quantifies the strength of cementation during diagenesis [25]. This parameter offers essential insights into the cementation process, enhancing our understanding of reservoir properties and their hydrocarbon extraction potential. The calculation formula is as follows:

(2) Apparent cementation rate = Cementation volume Original intergranular volume × 100 .

The apparent cementation rate of the Shan-1 reservoir exhibits considerable variation, spanning from 13.5 to 51.6%, with an average value of 36.2%. These data suggest a relatively uniform level of cementation strength, as depicted in Figure 15. Type III and IV reservoirs exhibit lower apparent cementation rates, while Class I and II reservoirs exhibit higher rates. The primary reason is that Type III and IV reservoirs undergo strong compaction, which lowers their cementation rates (Figure 15). Cementation nearly eliminates macropores, mesopores, and some throats, leaving only a few delicate pores, micropores, and throats, dramatically reducing the available pore space and severely damaging the connectivity between pores and throats.

Figure 15 Correlation between pore structure parameters and effective cementation rate in the Shan-1 reservoir of the Longdong area. (a) the apparent cementation rate and displacement pressure of different pore structures; (b) the apparent cementation rate and average pore radius of different pore structures.
Figure 15

Correlation between pore structure parameters and effective cementation rate in the Shan-1 reservoir of the Longdong area. (a) the apparent cementation rate and displacement pressure of different pore structures; (b) the apparent cementation rate and average pore radius of different pore structures.

A sample with a Type I pore structure has a relatively high apparent cementation rate and a large APTR (sample QT14263.9 m), mainly due to high quartz content, low compaction rate, and high dissolution rate. Other Type I pore structures show a less distinct negative correlation with apparent cementation rate. The median radius of Type II–IV pore structures decreases with increasing apparent cementation rate, especially in the 20–50% range (Figure 15a). The displacement pressure of Type I and II pore structures does not change significantly with increasing apparent cementation rate, displaying a straight curve. However, the displacement pressure of Type III and IV pore structures decreases with increasing apparent cementation rate, showing a strong negative correlation (Figure 15b). In summary, cementation significantly influences the variations in pore structures within the Shan-1 reservoir.

5.2.3 Impact of dissolution on the pore structure

The dissolution of soluble components, including volcanic rock fragments and small amounts of feldspar, within the sandstone clastic particles, matrix, cement, and replacement minerals has created secondary porosity. This process, along with the transformation of kaolinite into intercrystalline pores, has significantly enhanced the rock’s storage space. The porosity formed under different diagenetic mechanisms has also expanded the intergranular pores. The dissolution process has favorably modified the pore structure, a finding that has direct implications for understanding and managing reservoir permeability.

The apparent dissolution rate within reservoirs describes the speed at which pure substances dissolve under constant surface area conditions. To quantitatively characterize reservoir dissolution intensity, we calculate the apparent dissolution rate using the following formula:

(3) Apparent dissolution rate = Dissolution porosity Total porosity × 100 .

In equation (3), the dissolution porosity is the percentage of the total pore area that is composed of dissolution pores.

The dissolution alteration of the Shan-1 reservoir is significant, with an apparent dissolution rate ranging from 0 to 10.8%, averaging 2.14%, indicating a relatively high dissolution intensity (Figure 14). The dissolution rate in Type I and II pore structures surpasses that in Types III and IV (as demonstrated in Figure 13). The APTR of the pore structures tends to increase with an increasing apparent dissolution rate. In contrast, the APTR of Type III–IV pore structures shows no significant change with increasing apparent dissolution rate, as indicated by the low slope of the curve (Figure 16a). The displacement pressure of Type I and II pore structures generally decreases with an increasing apparent dissolution rate, which has significant implications for reservoir management. In contrast, the relationship between displacement pressure and apparent dissolution rate in Type III–IV pore structures is insignificant (Figure 16b).

Figure 16 The relationship between apparent dissolution rate and pore structure parameters of the Shan-1 reservoir in the Longdong area. (a) the apparent dissolution rate and displacement pressure of different pore structures; (b) the apparent dissolution rate and average pore radius of different pore structures.
Figure 16

The relationship between apparent dissolution rate and pore structure parameters of the Shan-1 reservoir in the Longdong area. (a) the apparent dissolution rate and displacement pressure of different pore structures; (b) the apparent dissolution rate and average pore radius of different pore structures.

6 Conclusions

  1. The evolution of pore structures in the Shan-1 reservoir is significantly influenced by the rock composition and diagenetic processes. Higher quartz content helps preserve pore throats, with a positive correlation between quartz levels and the APTR. Specifically, when quartz content exceeds 70%, Type I and II pore structures, characterized by larger pore throats, are predominant. Conversely, rock fragment content shows a negative correlation with the pore throat size. When rock fragment content is below 10%, Type I and II structures with larger median pore-throat radii are prevalent. In contrast, Type III and IV pore structures are typically associated with higher rock fragment content and smaller pore throats.

  2. Impact of diagenetic processes: Compaction is identified as the primary diagenetic process modifying pore structures within the Shan-1 reservoir. Additionally, cementation and dissolution also play substantial roles in defining the pore characteristics. The effects of compaction and cementation vary among different pore structures, most significantly affecting Type IV, followed by Types III, II, and I. On the other hand, dissolution processes enhance the pore structures, with the greatest improvements observed in Type I, followed by Types II, III, and IV.

  3. Optimal conditions for high-quality pore structures: A combination of moderate compaction and cementation, along with higher dissolution rates, contributes to the formation of high-quality pore structures. When the apparent compaction rate is below 55%, favorable structures, mainly Types I and II, are predominant. There is an inverse relationship between the apparent cementation rate and the APTR. Optimal pore structures occur within a cementation range of 30–50%, where Type I and II structures prevail. However, if the apparent cementation rate exceeds 50%, intense cementation can severely damage pore structures, leading to a dominance of Type III and IV structures. Conversely, when cementation rates fall below 30%, the adverse effects of strong compaction become pronounced. Additionally, the apparent dissolution rate inversely correlates with displacement pressure, and when this rate exceeds 2%, the reservoir is characterized by favorable pore structures with a larger APTR, primarily corresponding to Type I and II structures.

Acknowledgments

This research was supported by the National Science and Technology Major Project of China (2016ZX05556). The authors would like to thank the staff of all laboratories that cooperated in performing the tests and analyses. We are also grateful to the anonymous reviewers whose comments improved the quality of this manuscript.

  1. Author contributions: Guangjun Xu: conceptualization, formal analysis, investigation, and writing – original draft. Hao Li: writing-review. Lei Tang: editing. Xiaoke Gong: drawing. Yuxin Xie, Danni Zhao, and Jianmeng Chen: resources. Qinlian Wei: conceptualization and writing – review and editing.

  2. Conflict of interest: The authors declare that there is no conflict of interest.

  3. Data availability statement: Data sharing is not applicable to this article, mainly due to commercial restrictions.

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Received: 2024-08-12
Revised: 2024-10-05
Accepted: 2024-11-03
Published Online: 2024-12-27

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/geo-2022-0746
Keywords for this article
tight reservoir; pore structure; Shan-1; Permian; Ordos Basin
Creative Commons
BY 4.0

Articles in the same Issue

  1. Regular Articles
  2. Theoretical magnetotelluric response of stratiform earth consisting of alternative homogeneous and transitional layers
  3. The research of common drought indexes for the application to the drought monitoring in the region of Jin Sha river
  4. Evolutionary game analysis of government, businesses, and consumers in high-standard farmland low-carbon construction
  5. On the use of low-frequency passive seismic as a direct hydrocarbon indicator: A case study at Banyubang oil field, Indonesia
  6. Water transportation planning in connection with extreme weather conditions; case study – Port of Novi Sad, Serbia
  7. Zircon U–Pb ages of the Paleozoic volcaniclastic strata in the Junggar Basin, NW China
  8. Monitoring of mangrove forests vegetation based on optical versus microwave data: A case study western coast of Saudi Arabia
  9. Microfacies analysis of marine shale: A case study of the shales of the Wufeng–Longmaxi formation in the western Chongqing, Sichuan Basin, China
  10. Multisource remote sensing image fusion processing in plateau seismic region feature information extraction and application analysis – An example of the Menyuan Ms6.9 earthquake on January 8, 2022
  11. Identification of magnetic mineralogy and paleo-flow direction of the Miocene-quaternary volcanic products in the north of Lake Van, Eastern Turkey
  12. Impact of fully rotating steel casing bored pile on adjacent tunnels
  13. Adolescents’ consumption intentions toward leisure tourism in high-risk leisure environments in riverine areas
  14. Petrogenesis of Jurassic granitic rocks in South China Block: Implications for events related to subduction of Paleo-Pacific plate
  15. Differences in urban daytime and night block vitality based on mobile phone signaling data: A case study of Kunming’s urban district
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  18. Spatial and temporal changes in ecosystem services value and analysis of driving factors in the Yangtze River Delta Region
  19. Deep fault sliding rates for Ka-Ping block of Xinjiang based on repeating earthquakes
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  23. Flood risk assessment, a case study in an arid environment of Southeast Morocco
  24. Lower limits of physical properties and classification evaluation criteria of the tight reservoir in the Ahe Formation in the Dibei Area of the Kuqa depression
  25. Evaluation of Viaducts’ contribution to road network accessibility in the Yunnan–Guizhou area based on the node deletion method
  26. Permian tectonic switch of the southern Central Asian Orogenic Belt: Constraints from magmatism in the southern Alxa region, NW China
  27. Element geochemical differences in lower Cambrian black shales with hydrothermal sedimentation in the Yangtze block, South China
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  29. Obliquity-paced summer monsoon from the Shilou red clay section on the eastern Chinese Loess Plateau
  30. Classification and logging identification of reservoir space near the upper Ordovician pinch-out line in Tahe Oilfield
  31. Ultra-deep channel sand body target recognition method based on improved deep learning under UAV cluster
  32. New formula to determine flyrock distance on sedimentary rocks with low strength
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  35. Detecting heterogeneity of spatial accessibility to sports facilities for adolescents at fine scale: A case study in Changsha, China
  36. Effects of freeze–thaw cycles on soil nutrients by soft rock and sand remodeling
  37. Vibration prediction with a method based on the absorption property of blast-induced seismic waves: A case study
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  41. Phase change of the Ordovician hydrocarbon in the Tarim Basin: A case study from the Halahatang–Shunbei area
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  50. Measuring urban growth dynamics: A study in Hue city, Vietnam
  51. Comparative models of support-vector machine, multilayer perceptron, and decision tree ‎predication approaches for landslide ‎susceptibility analysis
  52. Experimental study on the influence of clay content on the shear strength of silty soil and mechanism analysis
  53. Geosite assessment as a contribution to the sustainable development of Babušnica, Serbia
  54. Using fuzzy analytical hierarchy process for road transportation services management based on remote sensing and GIS technology
  55. Accumulation mechanism of multi-type unconventional oil and gas reservoirs in Northern China: Taking Hari Sag of the Yin’e Basin as an example
  56. TOC prediction of source rocks based on the convolutional neural network and logging curves – A case study of Pinghu Formation in Xihu Sag
  57. A method for fast detection of wind farms from remote sensing images using deep learning and geospatial analysis
  58. Spatial distribution and driving factors of karst rocky desertification in Southwest China based on GIS and geodetector
  59. Physicochemical and mineralogical composition studies of clays from Share and Tshonga areas, Northern Bida Basin, Nigeria: Implications for Geophagia
  60. Geochemical sedimentary records of eutrophication and environmental change in Chaohu Lake, East China
  61. Research progress of freeze–thaw rock using bibliometric analysis
  62. Mixed irrigation affects the composition and diversity of the soil bacterial community
  63. Examining the swelling potential of cohesive soils with high plasticity according to their index properties using GIS
  64. Geological genesis and identification of high-porosity and low-permeability sandstones in the Cretaceous Bashkirchik Formation, northern Tarim Basin
  65. Usability of PPGIS tools exemplified by geodiscussion – a tool for public participation in shaping public space
  66. Efficient development technology of Upper Paleozoic Lower Shihezi tight sandstone gas reservoir in northeastern Ordos Basin
  67. Assessment of soil resources of agricultural landscapes in Turkestan region of the Republic of Kazakhstan based on agrochemical indexes
  68. Evaluating the impact of DEM interpolation algorithms on relief index for soil resource management
  69. Petrogenetic relationship between plutonic and subvolcanic rocks in the Jurassic Shuikoushan complex, South China
  70. A novel workflow for shale lithology identification – A case study in the Gulong Depression, Songliao Basin, China
  71. Characteristics and main controlling factors of dolomite reservoirs in Fei-3 Member of Feixianguan Formation of Lower Triassic, Puguang area
  72. Impact of high-speed railway network on county-level accessibility and economic linkage in Jiangxi Province, China: A spatio-temporal data analysis
  73. Estimation model of wild fractional vegetation cover based on RGB vegetation index and its application
  74. Lithofacies, petrography, and geochemistry of the Lamphun oceanic plate stratigraphy: As a record of the subduction history of Paleo-Tethys in Chiang Mai-Chiang Rai Suture Zone of Thailand
  75. Structural features and tectonic activity of the Weihe Fault, central China
  76. Application of the wavelet transform and Hilbert–Huang transform in stratigraphic sequence division of Jurassic Shaximiao Formation in Southwest Sichuan Basin
  77. Structural detachment influences the shale gas preservation in the Wufeng-Longmaxi Formation, Northern Guizhou Province
  78. Distribution law of Chang 7 Member tight oil in the western Ordos Basin based on geological, logging and numerical simulation techniques
  79. Evaluation of alteration in the geothermal province west of Cappadocia, Türkiye: Mineralogical, petrographical, geochemical, and remote sensing data
  80. Numerical modeling of site response at large strains with simplified nonlinear models: Application to Lotung seismic array
  81. Quantitative characterization of granite failure intensity under dynamic disturbance from energy standpoint
  82. Characteristics of debris flow dynamics and prediction of the hazardous area in Bangou Village, Yanqing District, Beijing, China
  83. Rockfall mapping and susceptibility evaluation based on UAV high-resolution imagery and support vector machine method
  84. Statistical comparison analysis of different real-time kinematic methods for the development of photogrammetric products: CORS-RTK, CORS-RTK + PPK, RTK-DRTK2, and RTK + DRTK2 + GCP
  85. Hydrogeological mapping of fracture networks using earth observation data to improve rainfall–runoff modeling in arid mountains, Saudi Arabia
  86. Petrography and geochemistry of pegmatite and leucogranite of Ntega-Marangara area, Burundi, in relation to rare metal mineralisation
  87. Prediction of formation fracture pressure based on reinforcement learning and XGBoost
  88. Hazard zonation for potential earthquake-induced landslide in the eastern East Kunlun fault zone
  89. Monitoring water infiltration in multiple layers of sandstone coal mining model with cracks using ERT
  90. Study of the patterns of ice lake variation and the factors influencing these changes in the western Nyingchi area
  91. Productive conservation at the landslide prone area under the threat of rapid land cover changes
  92. Sedimentary processes and patterns in deposits corresponding to freshwater lake-facies of hyperpycnal flow – An experimental study based on flume depositional simulations
  93. Study on time-dependent injectability evaluation of mudstone considering the self-healing effect
  94. Detection of objects with diverse geometric shapes in GPR images using deep-learning methods
  95. Behavior of trace metals in sedimentary cores from marine and lacustrine environments in Algeria
  96. Spatiotemporal variation pattern and spatial coupling relationship between NDVI and LST in Mu Us Sandy Land
  97. Formation mechanism and oil-bearing properties of gravity flow sand body of Chang 63 sub-member of Yanchang Formation in Huaqing area, Ordos Basin
  98. Diagenesis of marine-continental transitional shale from the Upper Permian Longtan Formation in southern Sichuan Basin, China
  99. Vertical high-velocity structures and seismic activity in western Shandong Rise, China: Case study inspired by double-difference seismic tomography
  100. Spatial coupling relationship between metamorphic core complex and gold deposits: Constraints from geophysical electromagnetics
  101. Disparities in the geospatial allocation of public facilities from the perspective of living circles
  102. Research on spatial correlation structure of war heritage based on field theory. A case study of Jinzhai County, China
  103. Formation mechanisms of Qiaoba-Zhongdu Danxia landforms in southwestern Sichuan Province, China
  104. Magnetic data interpretation: Implication for structure and hydrocarbon potentiality at Delta Wadi Diit, Southeastern Egypt
  105. Deeply buried clastic rock diagenesis evolution mechanism of Dongdaohaizi sag in the center of Junggar fault basin, Northwest China
  106. Application of LS-RAPID to simulate the motion of two contrasting landslides triggered by earthquakes
  107. The new insight of tectonic setting in Sunda–Banda transition zone using tomography seismic. Case study: 7.1 M deep earthquake 29 August 2023
  108. The critical role of c and φ in ensuring stability: A study on rockfill dams
  109. Evidence of late quaternary activity of the Weining-Shuicheng Fault in Guizhou, China
  110. Extreme hydroclimatic events and response of vegetation in the eastern QTP since 10 ka
  111. Spatial–temporal effect of sea–land gradient on landscape pattern and ecological risk in the coastal zone: A case study of Dalian City
  112. Study on the influence mechanism of land use on carbon storage under multiple scenarios: A case study of Wenzhou
  113. A new method for identifying reservoir fluid properties based on well logging data: A case study from PL block of Bohai Bay Basin, North China
  114. Comparison between thermal models across the Middle Magdalena Valley, Eastern Cordillera, and Eastern Llanos basins in Colombia
  115. Mineralogical and elemental analysis of Kazakh coals from three mines: Preliminary insights from mode of occurrence to environmental impacts
  116. Chlorite-induced porosity evolution in multi-source tight sandstone reservoirs: A case study of the Shaximiao Formation in western Sichuan Basin
  117. Predicting stability factors for rotational failures in earth slopes and embankments using artificial intelligence techniques
  118. Origin of Late Cretaceous A-type granitoids in South China: Response to the rollback and retreat of the Paleo-Pacific plate
  119. Modification of dolomitization on reservoir spaces in reef–shoal complex: A case study of Permian Changxing Formation, Sichuan Basin, SW China
  120. Geological characteristics of the Daduhe gold belt, western Sichuan, China: Implications for exploration
  121. Rock physics model for deep coal-bed methane reservoir based on equivalent medium theory: A case study of Carboniferous-Permian in Eastern Ordos Basin
  122. Enhancing the total-field magnetic anomaly using the normalized source strength
  123. Shear wave velocity profiling of Riyadh City, Saudi Arabia, utilizing the multi-channel analysis of surface waves method
  124. Effect of coal facies on pore structure heterogeneity of coal measures: Quantitative characterization and comparative study
  125. Inversion method of organic matter content of different types of soils in black soil area based on hyperspectral indices
  126. Detection of seepage zones in artificial levees: A case study at the Körös River, Hungary
  127. Tight sandstone fluid detection technology based on multi-wave seismic data
  128. Characteristics and control techniques of soft rock tunnel lining cracks in high geo-stress environments: Case study of Wushaoling tunnel group
  129. Influence of pore structure characteristics on the Permian Shan-1 reservoir in Longdong, Southwest Ordos Basin, China
  130. Study on sedimentary model of Shanxi Formation – Lower Shihezi Formation in Da 17 well area of Daniudi gas field, Ordos Basin
  131. Multi-scenario territorial spatial simulation and dynamic changes: A case study of Jilin Province in China from 1985 to 2030
  132. Review Articles
  133. Major ascidian species with negative impacts on bivalve aquaculture: Current knowledge and future research aims
  134. Prediction and assessment of meteorological drought in southwest China using long short-term memory model
  135. Communication
  136. Essential questions in earth and geosciences according to large language models
  137. Erratum
  138. Erratum to “Random forest and artificial neural network-based tsunami forests classification using data fusion of Sentinel-2 and Airbus Vision-1 satellites: A case study of Garhi Chandan, Pakistan”
  139. Special Issue: Natural Resources and Environmental Risks: Towards a Sustainable Future - Part I
  140. Spatial-temporal and trend analysis of traffic accidents in AP Vojvodina (North Serbia)
  141. Exploring environmental awareness, knowledge, and safety: A comparative study among students in Montenegro and North Macedonia
  142. Determinants influencing tourists’ willingness to visit Türkiye – Impact of earthquake hazards on Serbian visitors’ preferences
  143. Application of remote sensing in monitoring land degradation: A case study of Stanari municipality (Bosnia and Herzegovina)
  144. Optimizing agricultural land use: A GIS-based assessment of suitability in the Sana River Basin, Bosnia and Herzegovina
  145. Assessing risk-prone areas in the Kratovska Reka catchment (North Macedonia) by integrating advanced geospatial analytics and flash flood potential index
  146. Analysis of the intensity of erosive processes and state of vegetation cover in the zone of influence of the Kolubara Mining Basin
  147. GIS-based spatial modeling of landslide susceptibility using BWM-LSI: A case study – city of Smederevo (Serbia)
  148. Geospatial modeling of wildfire susceptibility on a national scale in Montenegro: A comparative evaluation of F-AHP and FR methodologies
  149. Geosite assessment as the first step for the development of canyoning activities in North Montenegro
  150. Urban geoheritage and degradation risk assessment of the Sokograd fortress (Sokobanja, Eastern Serbia)
  151. Multi-hazard modeling of erosion and landslide susceptibility at the national scale in the example of North Macedonia
  152. Understanding seismic hazard resilience in Montenegro: A qualitative analysis of community preparedness and response capabilities
  153. Forest soil CO2 emission in Quercus robur level II monitoring site
  154. Characterization of glomalin proteins in soil: A potential indicator of erosion intensity
  155. Power of Terroir: Case study of Grašac at the Fruška Gora wine region (North Serbia)
  156. Special Issue: Geospatial and Environmental Dynamics - Part I
  157. Qualitative insights into cultural heritage protection in Serbia: Addressing legal and institutional gaps for disaster risk resilience
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