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Article

Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China

by
Quanzhong Guan
1,*,
Dazhong Dong
2,
Bin Deng
1,
Cheng Chen
1,
Chongda Li
1,
Kun Jiao
1,
Yuehao Ye
1,
Haoran Liang
1 and
Huiwen Yue
1
1
College of Energy, Chengdu University of Technology, Chengdu 610059, China
2
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Fractal Fract. 2025, 9(7), 410; https://doi.org/10.3390/fractalfract9070410
Submission received: 19 May 2025 / Revised: 19 June 2025 / Accepted: 23 June 2025 / Published: 25 June 2025
(This article belongs to the Special Issue Pore Structure and Fractal Characteristics in Unconventional Oil and Gas Reservoirs)
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Abstract

Organic matter pores constitute a significant storage space in shale gas reservoirs, contributing to approximately 50% of the total porosity. This study employed a comprehensive approach, utilizing scanning electron microscopy, low-pressure N2 adsorption, thermal analysis, image statistics, and fractal theory, to quantitatively characterize the structure and complexity of organic matter pores in the Wufeng–lower Longmaxi Formations (WLLFs). The WLLFs exhibit a high organic matter content, averaging 3.20%. Organic matter pores are typically well-developed, predominantly observed within organic matter clusters, organic matter–clay mineral complexes, and the internal organic matter of pyrite framboid. The morphology of these pores is generally elliptical and spindle-shaped, with the primary pore diameter displaying a bimodal distribution at 10~40 nm and 100~160 nm, potentially influenced by the observational limit of scanning electron microscopy. Shales from greater burial depths within the same gas well contain more organic matter pores; however, the development of organic matter pores in deep gas wells is roughly equivalent to that in medium and shallow gas wells. Fractal dimension values can be utilized to characterize the complexity of organic matter pores, with organic matter macropores (D>50) being more complex than organic matter mesopores (D2–50), which in turn are more complex than organic matter micropores (D<2). The development of macropores and mesopores is a key factor in the heterogeneity of organic matter pores. The complexity of organic matter pores in the same well increases gradually with the burial depth of the shale, and the complexity of organic matter pores in deep gas wells is roughly equivalent to that in medium and shallow gas wells. The structure and fractal characteristics of organic matter pores in shale are primarily controlled by components, diagenesis, tectonism, etc. The lower Longmaxi shale exhibit a high biogenic quartz content and robust hydrocarbon generation from organic matter. This composition effectively shields organic matter pores from multi-directional extrusion, leading to the formation of macropores and mesopores without specific orientation. High-quality shale sections (one and two sublayers) have relatively high fractal dimension D2–50 and D>50 values of organic matter pores and gas content. Consequently, the quality parameters of shale and fractal dimension characteristics can be comprehensively evaluated to identify high-quality shale sections.

1. Introduction

The “North American Shale Revolution” and advancements in horizontal well and hydraulic fracturing technologies have brought the vast potential of shale oil and gas resources to the fore [1,2]. Given China’s significant reliance on foreign oil and gas, there are substantial security concerns. However, the robust development of shale gas can effectively mitigate issues related to resource scarcity, environmental degradation, and economic growth [3]. The basins in China contain three types of organic-rich shales: marine facies, marine–continental transition facies, and continental facies [4]. At present, the industrial production of marine facies shale gas has been realized, with a projected total output of 25 billion cubic meters in 2023. The primary producing strata are the black shales of the WLLFs in the southern region of the Sichuan Basin.
The primary distinction between shale and conventional reservoirs lies in the presence of micro–nano organic matter pores [5,6,7]. Since their discovery, organic matter pores have attracted significant attention. Research on shale organic matter pores predominantly centers on their origin, preservation, and characterization [8,9,10,11,12,13,14,15]. The formation of organic matter pores is influenced by the content, type, and thermal evolution level of the organic matter. Organic matter with a moderate content (total organic carbon (TOC) < 5.5%), favorable type (Types I and II1), and a higher degree of thermal evolution (Ro) typically exhibit well-developed pores [10,11,14,16,17,18]. The effective preservation of these pores post-formation is also vital, primarily governed by diagenetic processes and mineral composition [19,20,21]. Autogenic minerals establish rigid frameworks during diagenesis, which, coupled with overpressure formed by volume expansion after organic matter pyrolysis, prevent the collapse or even erasure of organic matter pores [22,23,24]. Organic matter pore characterization typically involves scanning electron microscopy, gas adsorption, and rock physics model analysis methods [25,26,27,28]. However, each testing method has its limitations, making it challenging to employ a unified parameter to characterize the complexity of the organic matter pore structure [8,26,29].
In recent years, the application of fractal theory on organic matter pores in shales has gradually emerged [30,31]. The fractal dimension, as an effective indicator describing the complexity and irregularity of pore structure, can reveal its intrinsic relationship with the storage and migration performance of shale gas. Research indicates that the fractal characteristics of pore surfaces are closely related to the adsorption capacity of shale gas, while the fractal structure of pore networks has a significant impact on the seepage process [19,24,26,32]. However, the strong heterogeneity of shale leads to significant differences in the pore structure of shale organic matter in different regions. In addition, the diversity of fractal calculation methods and the applicability of models under complex geological conditions still need further optimization.
Previous studies have shown that the organic matter pores in the “sweet spot” of the WLLFs account for about 50%, which are the main storage spaces for free gas and adsorbed gas, and have been used as one of the preferred indicators for target layers [18,33]. The complexity of these organic matter pores, however, remains to be quantitatively characterized, as well as the factors that control it. This study employs a comprehensive approach, utilizing scanning electron microscopy, low-pressure N2 adsorption, thermal analysis, image statistical processing, and fractal theory on the WLLFs. The objectives are to (1) digitally characterize the development characteristics of organic matter pores, such as pore size and direction; (2) quantitatively analyze the complexity of these pores; and (3) identify the controlling factors of the heterogeneity of organic matter pores and their geological significance.

2. Geologic Setting and Well Description

The Sichuan Basin, situated on the northwestern periphery of the Yangtze Plate and encompassed by several mountain ranges, is a substantial superimposed basin [2,34]. Since the Precambrian era, the Sichuan Basin has experienced numerous tectonic activities, resulting in approximately 10 km of strata deposition and the formation of several organic-rich shale sets, including the Doushantuo Formation (Ediacaran), Qiongzhusi Formation (Cambrian), Wufeng Formation–Longmaxi Formation (Ordovician to Silurian), Wujiaping Formation (Permian), and Ziliujing Formation (Jurassic) [35,36]. At present, the Wufeng Formation–Longmaxi gas shales in southern Sichuan are being produced on a large scale, leading to the formation of significant shale gas fields such as Changning-Weiyuan, Fuling, and Luzhou.
The Sichuan Basin is a secondary craton basin within the Yangtze Plate, which was located in the equatorial sea area on the northern edge of the Gondwana continent during the late Ordovician period. The convergent extrusion from the southeast Cathaysian plate caused uplift along the southern margin of the Yangtze land, and it connected with the ancient Kangdian land in the southwest to form the ancient Yunnan-Guizhou-Guangxi land; at the same time, the passive continental margin in the north subducted towards the North China plate, and the scope of the ancient uplift in central Sichuan on the western edge of the plate also continuously expanded [37]. This process has led to the formation of a semi-enclosed stagnant sea basin, sandwiched by ancient uplifts in central Sichuan and central Guizhou-Xuefeng, a phenomenon referred to as the “three uplifts sandwiched by one depression” [1,22]. From the Late Ordovician to the Early Silurian period, large-scale transgression deposited a series of organic-rich shale strata—the Wufeng Formation and Longmaxi Formation—on the deep-water shelf (Figure 1). These shales are consistently distributed throughout the Sichuan Basin, characterized by their substantial sedimentary thickness and high organic matter content, each with a sedimentary center located in the eastern and southern Sichuan Basin [2].
The Weiyuan shale gas field, situated in the southern region of the Sichuan Basin (Figure 1b), is primarily characterized by the Weiyuan anticline, where the top strata have undergone erosion [34,38]. Wells W202 and W205 are both located on the southeastern wing of this anticline. The depths of the organic-rich shale sections from the Wufeng Formation to the Longmaxi Formation in wells W202 and W205 range from 2560 m to 2580 m and 3687.5 m to 3710 m, respectively (Figure 1c). The bottom boundary of these formations is marked by the nodular limestone of the Baota Formation. During the Wufeng period, underwater uplifts resulted in the absence of the Wufeng black shale and Guanyinqiao layer in well W205. However, both wells have successfully developed their primary shale sections. The lithology of the Longmaxi Formation changes from bottom to top as carbonaceous shale, calcareous shale, and clay shale, which are laterally continuous in the southern Sichuan area.

3. Samples and Analytical Methods

3.1. Samples

In the Weiyuan shale gas field, 18 shale core samples were procured from the WLLFs in wells W202 (depth < 3500 m) and W205 (depth > 3500 m). Given the necessity for multiple supporting experiments on the samples, and considering their lithology, stratigraphic position, and sampling depth data, efforts were made to reduce the influence of heterogeneity within individual samples. Consequently, a subset of 7 samples was chosen for a comprehensive analysis (Figure 1c).

3.2. TOC Content and Mineralogy

All samples were subjected to crushing and processing in accordance with relevant industry standards prior to TOC content and mineral composition analyses. The TOC contents were ascertained using an LECO carbon–sulfur analyzer, boasting a precision of 1%. The mineralogical compositions were identified utilizing a Brucker D8 Advance X-ray diffractometer operating at 40 kV and 30 mA, employing Cu radiation. For comprehensive details on the experimental procedures, please refer to reference [39].

3.3. Field Emission-Scanning Electron Microscopy (FE-SEM) for Organic Matter Pore Analysis

The organic porosity in shale oil/gas reservoirs predominantly exists at the micro–nano scale. To facilitate a more precise identification and observation of these pores, samples are initially subjected to physical and argon ion polishing treatments prior to analysis via a scanning electron microscope. It is important to note that all samples are deliberately not gold or carbon plated to avoid any potential interference with the accurate observation of organic porosity. For more information on the imaging process, please refer to reference [40].
To effectively extract information from organic matter pores in shale, JMicroVision 1.2.7 software is employed for processing. The relatively large grayscale values of both the organic matter and its pores enable a clear distinction from inorganic minerals and pores. Consequently, JMicroVision can accurately determine parameters such as the area, perimeter, length, width, equivalent circle diameter, azimuth angle, and the elongation of the organic matter pores (as shown in Figure 2).

3.4. Low-Pressure N2 Gas Adsorption for OM Extraction

The procedures for organic matter extraction and nitrogen adsorption experiments can be found in Guan et al., 2019 [10]. The BET-BJH model was employed to determine pore volumes, pore size distributions, and specific surface areas during low-pressure N2 adsorption.

3.5. Fractal Theory

Organic matter pores in shale, ranging from a few nanometers to several micrometers, exhibit complex and highly heterogeneous morphologies due to the combined effects of diagenesis and tectonics. These characteristics make them challenging to characterize using traditional Euclidean geometry. Consequently, fractal theory has been employed to analyze such intricate and heterogeneous objects, including organic matter pores [19,26]. Fractals capture the self-similarity of an object’s local parts to its overall structure, which can be quantitatively represented by the fractal dimension [41]. This dimension provides a measure of the complexity and self-similarity of fractal objects, offering insights into how they occupy space. Common methods for calculating the fractal dimension include the Frenkel–Halsey–Hill (FHH) model, the micropore fractal model, and image analysis.
In this research, the FHH model is employed to fractalize the low-pressure N2 adsorption curves of organic matter extraction, subsequently calculating the fractal dimension D. The mathematical formula for the FHH model is presented below:
ln(V/V0) = k + (D − 3)ln[ln(P0/P)]
where V is the volume of gas adsorption at equilibrium pressure P; V0 is the monolayer coverage volume; k is a constant; P0 is the saturation pressure; P is the equilibrium pressure; and D is the fractal dimension.

4. Results

4.1. TOC Content and Mineral Composition

In the study area, 52 core samples from five wells were analyzed for TOC content and XRD. However, due to the aforementioned reasons, only seven representative samples were selected for this study. The TOC content of the WLLFs in the Weiyuan Shale Gas Field varies between 1.05% and 8.10%, with an average of 3.20%. In both wells, it was observed that the TOC content increases progressively with depth, as presented in Table 1.
The WLLF components are predominantly quartz and clay minerals, with average concentrations of 23.29% and 38.0% respectively, accounting for more than 60% of all components. The content of quartz in shale exhibits a complementary relationship with that of clay minerals, particularly within the “sweet intervals”, where the quartz content exceeds 35% (Table 1). The remaining minerals include feldspar, calcite, dolomite, and pyrite. Some shale layers have a higher concentration of carbonate rock minerals, such as sample 2-14, potentially due to their location within a calcium-rich deep-water shelf sedimentary system.

4.2. FE-SEM Observation and Analyses

Using SEM to observe a large number of shale samples from the WLLFs in the study area, it was found that organic matter pores are generally well-developed (Figure 3). In the shale “sweet interval”, organic matter pores account for more than 50% of the total pores and are the main storage space for shale gas. The shapes of organic matter pores in the WLLFs vary, mainly being elliptical and spindle-shaped (Figure 3), with some showing directionality (Figure 3e), possibly due to tectonic compression. Under SEM observation, the occurrence of organic matter pores is diverse, mainly developed in independent organic matter clusters (Figure 3a,e,f), organic matter–clay mineral complexes (Figure 3d), and the internal organic matter of pyrite framboid (Figure 3b,c). The diameter of organic matter pores varies greatly, with the smallest reaching about 1nm, mainly affected by the resolution of the instrument [8,29]. The organic matter pores in shale have strong heterogeneity; even within the same organic matter, there are significant differences in the size, shape, and even organic matter pore development (Figure 3).
To mitigate the impact of heterogeneity, a large number of SEM observations were conducted on the organic matter within each sample to analyze the development characteristics of organic matter pores. Based on factors such as the degree and clarity of organic matter pore development in SEM images, 85 photographs were selected for analysis, with statistical results shown in Table 2. The area distribution of shale organic matter pores under the SEM microscope in the study area ranged from 49.38 nm2 to 45.50 µm2. The existence of ultra-large organic matter pores may be due to the fusion of a large number of “organic matter primary pores” during the hydrocarbon evolution process of organic matter (Figure 3a,d,f). The perimeter of organic matter pores ranged from 18.30 nm to 62,729.13 nm, with the complexity of pore morphology determining the size of the perimeter. The long and short axis sizes of organic matter pores were used to calculate the elongation rate of the pores, quantitatively characterizing the degree of change in pore morphology. Herein, the diameter of organic matter pores is represented by the equivalent circle diameter, distributed between 7.93 nm and 7608.62 nm, but mainly concentrated between 10~40 nm and 100~160 nm, as shown in Figure 4. Overall, various parameters of shale organic matter pores show a trend of gradually increasing with the increase in sample burial depth.

4.3. Pore Size Distribution, Pore Volume, and Special Surface Area (SSA)

Generally speaking, low-pressure N2 adsorption experiments are relatively difficult for characterizing pores smaller than 2 nm. However, in conjunction with the SEM observations used, due to the inherent resolution of SEM and the identification level of JMicroVision software [28], this study adopts low-pressure N2 adsorption experiments to analyze organic matter pores. The summary curves of low-pressure N2 adsorption experiments on shale organic matter extracts are shown in Figure 5. The adsorption curve of each sample has three stages: a rapid rise under P/P0 < 0.1, a slow rise from 0.1 to 0.9, and a sharp rise above 0.9. According to the IUPAC classification, all samples’ adsorption curves exhibit distinct type II and type IV characteristics, indicating that the structure of the extracted organic matter is primarily composed of micropores and mesopores, with a small amount of microfractures or macropores [42]. Moreover, due to capillary condensation, the N2 adsorption–desorption curves of each sample form a “hysteresis loop”. These “hysteresis loops” are essentially close to types H3 and H4 in IUPAC classification, suggesting that the main pore types in the organic matter extracts are plate-slit pores, ink bottle pores, and tubular pores, with the main pore diameters being in the micropore–mesopore (type H4) or macropore (type H3) range.
The experimental results show that the SSA and pore volume of shale organic matter extracts are 60.1268~163.0092 m2/g and 0.2151~0.3972 mL/g, respectively, with an average of 101.28 m2/g and 0.2986 mL/g, all of which are significantly higher than the overall SSA and pore volume of the shale (Table 3). The average diameter distribution of organic matter pores is between 10.64 and 22.18 nm, which is consistent with the main organic matter pore distribution observed under the SEM microscope (Figure 4). However, the experimental analysis shows that the development of organic matter pores mainly concentrates on 2~4 nm, indicating that a certain amount of macropores develop in shale organic matter pores, and also reveals that both SEM and JMicroVision recognition and analysis of organic matter pores are constrained by their own conditions [8,29].

4.4. Fractal Analysis

The organic matter pores, at the micro–nano level, can be quantitatively characterized through various analytical tests. However, these tests often struggle to fully highlight the complexity and heterogeneity of organic matter pores. A more precise method for characterizing this complexity is fractal dimension analysis. Figure 6 illustrates the key process of calculating the fractal dimension using the FHH model, based on low-pressure N2 adsorption curves. To ensure consistency with the pore size distribution of the low-pressure N2 adsorption curve, we investigate the characteristics and significance of the fractal dimension under varying pore size distribution conditions. This study employs the Kelvin equation to calculate the radius of pores under relative pressure, thereby obtaining the fractal dimension characteristics of <2 nm, 2~50 nm, and >50 nm.
Table 4 reveals the pore structure characteristics of organic matter extracts from all samples. There is a significant difference in the fractal dimensions of different organic matter pores. The fractal dimensions of macropores, mesopores, and micropores are distributed at 2.51~2.76, 2.52~2.68, and 2.01~2.03, respectively, with corresponding average values of 2.61, 2.60, and 2.02. Overall, within the same shale sample, the fractal dimension of organic matter pores is D>50 > D2–50 > D<2, possibly due to the more complex structural features of macropores in organic matter pores, followed by mesopores, while micropores are closer to elliptical. Longitudinally, well W202 also shows an increasing trend in fractal dimensions with depth, possibly due to factors such as TOC content and tectonic activities. However, the change in the fractal dimensions of deep shale in well W205 is not significant, possibly due to sampling distribution issues. Comparing the two wells indicates that the complexity of the pore structure of deep shale organic matter pores is roughly equivalent to that of mid-shallow shale pores.

5. Discussion

5.1. Effect of Shale Compositions on Organic Matter Pore Structure

The organic matter and mineral compositions of shale significantly influence the pore structure characteristics of organic matter pores [10,22,43,44]. These pores, which primarily form during the kerogen generation and hydrocarbon expulsion process, as well as the secondary cracking of crude oil, constitute the main storage space in marine shale reservoirs [13,18]. They also include a small quantity of primary biological pores [45]. Organic matter is the material basis for the formation of organic matter pores. This study reveals that TOC content has a certain positive correlation with D2–50 and D>50, and a negative correlation with D<2, indicating that the organic matter pores are mainly organic mesopores and macropores in the WLLFs, which is consistent with observations under SEM (Figure 3 and Figure 4), as shown in Figure 7. This may be due to the fusion of generated organic matter micropores into mesopores or macropores. However, there is a good positive correlation between quartz and D<2, which should be related to biogenic quartz at the bottom of the Longmaxi Formation. Previous studies have revealed that the biogenic quartz content in the organic-rich shale of the Longmaxi Formation exceeds 50%, with small particle size, and particles filled with organic matter [22,43]. These types of organic matter clumps are relatively small, with limited internal development space, mainly honeycomb-like organic matter pores. The clay mineral content is roughly equivalent to that of quartz, but they are complementary. The figure shows that clay minerals have basically no correlation with D>50, D2–50, and D<2, possibly because the organic matter–clay mineral complex promotes the development of organic matter pores but is difficult to preserve [19,46].
The shale in the study area contains relatively few other minerals, but all of them, except for feldspar, have varying degrees of impact on the organic matter pore structure (Figure 7). Organic acids emitted during the hydrocarbon generation and expulsion can dissolve carbonate minerals, resulting in relatively large dissolution pores. In the later stage, some of these pores are filled by migrating oil/bitumen, leading to organic matter pore development with secondary cracking [43,44,47]. Therefore, calcite and dolomite show a negative correlation with D<2, and even dolomite shows a positive correlation with D>50. Most of the pyrite framboid is filled by organic matter, and the inter-crystalline pores of pyrite framboid are generally larger. The internal organic matter pores have a high degree of development and large diameters, showing a positive correlation with both D>50 and D2–50 (Figure 3 and Figure 7).

5.2. Effect of Diagenesis on Organic Matter Pore Structure

The diagenesis of shale is a highly intricate process, involving physical, chemical, and biological interactions [17,22,43]. The most notable of these are compaction, dissolution, and hydrocarbon generation and explosion from organic matter, with the dissolution having been previously detailed in 5.1. This section aims to investigate the influence of compaction and hydrocarbon generation and explosion on the pore structure of the organic matter itself.
Research suggests that organic matter inherently possesses low hardness and high plasticity, facilitating its flattening during the compaction process [23,48]. This results in the formation of elongated pores within the organic matter. However, observations from wells W202 and W205 show that the elongation rates of organic matter pores in both shallow and deep shale layers are concentrated between 0.4 and 0.7, unaffected by depth (Figure 8). On the contrary, the development of organic matter pores is better within some deeply buried shales (Figure 3 and Figure 4). This may be due to a large amount of rigid minerals forming an effective framework that inhibits compaction [10,22,43]; on the other hand, during the hydrocarbon generation and expulsion process within the shale’s organic matter, there is a sharp decrease in mass (Figure 9), but a rapid increase in volume (oil/gas density < water density < kerogen density), causing general overpressure within the pores, effectively counteracting some or even all of the compaction effects [23,49].
The hydrocarbon generation from organic matter is the primary process of pore formation in organic matter, except for some internal pores within primitive organisms [45]. Organic matter is a high molecular weight polymer, and changes occur within its molecules during the hydrocarbon generation process, resulting in changes to its own elastic modulus. Through the application of TG-DSC analysis, it was determined that phase transitions could occur during the heating reaction process of organic matter. These transitions could potentially lead to glass transitions, thereby enhancing resilience performance. Coupled with the previously mentioned overpressure, this comprehensive function can effectively protect the pores in organic matter.

5.3. Effect of Tectonism on Organic Matter Pore Structure

The Sichuan Basin has experienced numerous tectonic events, with the WLLFs having been subjected to late Caledonian, Hercynian, Indosinian, and Yanshan-Himalayan orogenic movements [36,50]. These have had a certain impact on organic matter pore structure. Core observation of the Wufeng Formation–Longmaxi Formation shale revealed evidence of multiple tectonic activities in both the Wufeng Formation and the bottom of the Longmaxi Formation (Figure 10). The Wufeng Formation shale core exhibited high-angle fractures, some of which were intersected by another set of fractures, all filled with multi-stage vein bodies, suggesting multi-stage tectonic-fluid action. Low-angle and high-angle fractures were both present at the bottom of the Longmaxi Formation shale, likely formed due to slippage at the shale’s base, and step phenomena were observed on the core cross-section (Figure 10a,b). However, the upper part of the Longmaxi Formation was less affected by tectonic activity.
Tectonic compression can cause changes in the morphology of organic matter pores, with a certain directionality in their long axes. Through the analysis of SEM images, it is found that tectonic activity has a weak effect on the organic matter pores (Figure 10c). The organic matter pore size in the lower Longmaxi Formation does not have obvious directionality, and it is larger than that in the upper part (Figure 10, Table 3). However, all organic matter pores have a certain elongation rate, indicating that they are indeed affected by tectonism (Figure 8, Table 2). On the other hand, some pressure is offset by the shale itself (such as pore pressure increasing by the hydrocarbon generation of organic matter, autogenic mineral-forming rigid frameworks, etc.), allowing for an effective preservation of organic matter pores (Figure 3).

5.4. Relationship Between Fractal Characteristics of Organic Matter Pores and Shale Gas Occurrence

Natural gas primarily exists in shale reservoirs in two forms: free or adsorbed. Parameters of pore structure such as the specific surface area and pore volume can effectively reflect the storage capacity of shale reservoirs. Fractal dimension is an important indicator characterizing the non-homogeneity of porous media surfaces and pore structures. A higher fractal dimension indicates a more complex pore structure, which is often not conducive to gas permeation and diffusion but is favorable for gas storage. The larger the fractal dimension value, the rougher the surface of the shale pores, increasing adsorption sites and hindering gas diffusion. As the fractal dimension value increases, the complexity of the internal pore space in shale increases, the connectivity between pores becomes more tortuous, and the permeation capability weakens. Therefore, without other factors, shale gas reservoirs with high fractal dimension values have relatively weak self-permeation capabilities, enhancing the enrichment effect of shale gas.
Compared with the shallow shale reservoirs in the Wufeng Formation–Longmaxi Formation of the study area, the deep shale reservoirs have roughly equivalent parameters, and the distribution range of fractal dimension values of organic matter pores is almost the same. Combining gas content data, it is found that the high-quality shale sections (one and two sublayers) in the Weiyuan area have relatively high fractal dimension D2–50 and D>50 values of organic matter pores and gas content, indicating that shale with high silica content has a complex three-dimensional “pore-crack” connectivity system (Figure 1 and Figure 11). This is related to the fact that the marine shale in the study area is rich in biogenic silica and has its own overpressure. The high TOC content facilitates the formation of a rich network of organic matter pores, while the overpressure effectively preserves the development of internal pores. In addition, simulation experiments also show that the 3~4 nm pores in the deep shale reservoirs are mainly free gas; thus, a relatively large pore volume contains more free gas, which is beneficial for large-scale mining in the later stage. Moreover, siliceous shale is mainly composed of brittle minerals, which can easily form a complex pore-crack system through fracturing modification in the later stage.

6. Conclusions

Based on the above comprehensive research, the following conclusions are drawn:
(1) The organic matter pores of the WLLFs in the Weiyuan Shale Gas Field exhibit strong heterogeneity. Organic matter pores are generally found in organic matter clusters, organic matter–clay composites, and the internal organic matter of pyrite framboid. Morphologically, they are mainly elliptical and spindle-shaped, with significant differences in pore arrangement direction, and a small amount show orientation. The structure of organic matter pores in shale at different depths varies greatly.
(2) The organic matter pore structures in the WLLFs are relatively complex, with fractal dimensions D>50 > D2–50 > D<2, mainly influenced by mesopores and macropores. The organic matter pores are primarily mesopores and macropores, with the main pore diameter distribution in the range of 10~40 nm and 100~160 nm, unaffected by the depth of shale burial. However, the fractal dimension values of organic matter pores show an increasing trend with burial depth, and the macropore diameter of organic matter pores at the bottom of the Longmaxi Formation shale relatively increases.
(3) The complexity of organic matter pore structure is influenced by various factors such as the composition of authigenic minerals, the physicochemical properties of organic matter, diagenesis, and tectonism. TOC contents, quartz, and clay minerals have a significant impact on the structure of organic matter pores. Compaction and tectonic activities can destroy organic matter pores, mainly reflected in the directional characteristics of organic matter pore development. However, the hydrocarbon generation and explosion of organic matter and its own phase transition, as well as the framework of rigid minerals, play a good protective role in preserving organic matter pores.
(4) The organic-rich siliceous shale deposited during the 1st and 2nd sublayers has a high TOC content, large fractal dimension values of organic matter pores, and complex organic matter pore structures, forming a three-dimensional connected pore system. Therefore, it has both numerous gas adsorption sites to store adsorbed gas and a large pore space to store free gas, resulting in high gas content. This is the “sweet spot” for shale gas enrichment. Therefore, the quality parameters of shale and fractal dimension characteristics can be comprehensively evaluated to identify high-quality shale sections.

Author Contributions

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

Funding

This research was funded by the National Natural Science Foundation of China, grant number 42202192, and supported by the Sichuan Science and Technology Program, grant number 2024NSFSC0811.

Data Availability Statement

All data can be found in the paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Fractalfract 09 00410 g001
Figure 1. Location of the southern Sichuan research area (b) in the Sichuan Basin (a) and study wells stratum histogram (c) (modified after Ref. [34]). Samples marked with blue arrows and red arrows are from the W202 well and W205 well, respectively.
Figure 1. Location of the southern Sichuan research area (b) in the Sichuan Basin (a) and study wells stratum histogram (c) (modified after Ref. [34]). Samples marked with blue arrows and red arrows are from the W202 well and W205 well, respectively.
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Figure 2. Definitions and measurement methods of parameters.
Figure 2. Definitions and measurement methods of parameters.
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Figure 3. HIM-SEM images of organic matter pores. (a) Organic matter pores with different pore sizes. Large organic matter pores have formed by the fusion of small ones. The large organic matter pores are complex, containing many small organic matter pores that are interconnected, 2-11. (b) Organic matter present in the intercrystallite pore of pyrite framboid, with well-developed small organic matter pores, 2-5. (c) Organic matter distributed in contiguous patches. Internal pores have developed but with strong heterogeneity. Organic matter pores in the upper left corner are larger, 2-13. (d) Organic matter–clay composite distributed in flakes. Ultra-large organic matter pore development, 5-5. (e) Individual organic matter aggregates, with intricately developed pores with directionality compressed by rigid minerals, 2-14. (f) Single organic matter clusters, with highly developed pores, ellipsoidal and non-directional, 5-8.
Figure 3. HIM-SEM images of organic matter pores. (a) Organic matter pores with different pore sizes. Large organic matter pores have formed by the fusion of small ones. The large organic matter pores are complex, containing many small organic matter pores that are interconnected, 2-11. (b) Organic matter present in the intercrystallite pore of pyrite framboid, with well-developed small organic matter pores, 2-5. (c) Organic matter distributed in contiguous patches. Internal pores have developed but with strong heterogeneity. Organic matter pores in the upper left corner are larger, 2-13. (d) Organic matter–clay composite distributed in flakes. Ultra-large organic matter pore development, 5-5. (e) Individual organic matter aggregates, with intricately developed pores with directionality compressed by rigid minerals, 2-14. (f) Single organic matter clusters, with highly developed pores, ellipsoidal and non-directional, 5-8.
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Figure 4. Organic matter pore size distribution from different layers and wells in the study area.
Figure 4. Organic matter pore size distribution from different layers and wells in the study area.
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Figure 5. Low pressure N2 adsorption–desorption isotherms collected for shale organic matter extracts in two wells.
Figure 5. Low pressure N2 adsorption–desorption isotherms collected for shale organic matter extracts in two wells.
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Figure 6. Plots of ln(V/V0) vs. ln(ln(P0/P)) from low-pressure N2 adsorption isotherms using the FHH model.
Figure 6. Plots of ln(V/V0) vs. ln(ln(P0/P)) from low-pressure N2 adsorption isotherms using the FHH model.
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Figure 7. Correlation diagram between shale components and fractal dimension values of different organic matter pores.
Figure 7. Correlation diagram between shale components and fractal dimension values of different organic matter pores.
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Figure 8. Organic matter pore elongation value distribution from different layers and wells in the study area.
Figure 8. Organic matter pore elongation value distribution from different layers and wells in the study area.
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Figure 9. TG and DSC curves for sample 2-5 under air atmosphere. TG—blue solid line and DSC—red solid line.
Figure 9. TG and DSC curves for sample 2-5 under air atmosphere. TG—blue solid line and DSC—red solid line.
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Figure 10. Representative core photographs and rose diagrams. (a) High-angle fractures, filled by minerals, from the lower Longmaxi Formation, 2-5. (b) Low-angle translational fractures, steps, and mirror-smooth features, with obvious scratches, 2-13. (c) Rose diagrams of organic matter pores in length directions; the long axis of organic matter pores is distributed in all directions, but there is a slight dominant direction.
Figure 10. Representative core photographs and rose diagrams. (a) High-angle fractures, filled by minerals, from the lower Longmaxi Formation, 2-5. (b) Low-angle translational fractures, steps, and mirror-smooth features, with obvious scratches, 2-13. (c) Rose diagrams of organic matter pores in length directions; the long axis of organic matter pores is distributed in all directions, but there is a slight dominant direction.
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Figure 11. The relationship between the fractal dimension of shale organic matter pores and gas content. 2-14 is special, from the non-producing interval.
Figure 11. The relationship between the fractal dimension of shale organic matter pores and gas content. 2-14 is special, from the non-producing interval.
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Table 1. Total organic carbon contents and mineral compositions of the shale samples from Wufeng and Longmaxi Formations in the two wells.
Table 1. Total organic carbon contents and mineral compositions of the shale samples from Wufeng and Longmaxi Formations in the two wells.
Sample No.TOC
%		Clay
%		Quartz
%		Feldspar
%		Calcite
%		Dolomite
%		Pyrite
%		Anhydrite
%		Siderite
%
2-52.54222192817120
2-112.481468673200
2-136.063441837223
2-142.29725235211000
5-11.0531391752231
5-53.0028399137220
5-82.8427339216220
Table 2. Quantitative organic matter pore parameters from FE-SEM images for shale samples of two wells.
Table 2. Quantitative organic matter pore parameters from FE-SEM images for shale samples of two wells.
Sample No.PoresArea/nm2Perimeter/nmLength/nmWidth/nmEquivalent Circular Diameter/nmElongation
2-51798277.00–153,739.61 (2580.92)33.62–5595.64 (184.25)5.26–1185.76 (65.74)4.08–488.47 (35.16)14.56–560.52 (45.01)0.09–1.00 (0.59)
2-111709650.36–2,087,905.77 (33,272.35)66.43–14,565.63 (744.00)8.06–3393.23 (253.60)4.49–1695.35 (132.44)28.78–1630.46 (167.10)0.05–1.00 (0.58)
2-131042226.76–238,962.07 (5347.66)39.22–4761.47 (242.49)7.41–1143.86 (86.03)4.76–467.86 (46.90)16.99–551.59 (59.66)0.12–1.00 (0.57)
2-142478910.67–45,500,000.00 (228,316.75)245.88–62,729.13 (1083.17)29.85–30,418.80 (449.83)26.93–1599.36 (159.12)106.51–7608.62 (239.24)0.02–1.00 (0.53)
5-1436142.40–32,566.75 (1886.11)31.43–2724.89 (215.87)10.67–634.58 (63.18)4.98–256.18 (34.26)13.47–203.63 (38.71)0.20–1.00 (0.59)
5-540549.38–27,577.32 (1376.73)18.30–2520.00 (174.39)6.67–397.78 (52.41)3.65–237.62 (29.48)7.93–187.83 (33.19)0.20–0.96 (0.63)
5-81307326.53–18,971.43 (1518.06)47.07–1386.75 (169.13)5.71–424.35 (57.52)5.71–165.77 (32.37)20.39–155.42 (39.99)0.16–1.00 (0.59)
Table 3. Quantitative pore parameters of the shale samples in two wells from low-pressure N2 gas adsorption analyses.
Table 3. Quantitative pore parameters of the shale samples in two wells from low-pressure N2 gas adsorption analyses.
Sample No.SSA (m2/g)Pore Volume (mL/g)Mean Radius (nm)Mode Radius (nm)
2-596.48380.2954489.142.89
2-1165.7830.25777510.242.97
2-13163.00920.2828155.322.88
2-14121.41830.2726466.723.12
5-160.12680.2151918.552.9
5-596.7340.39720311.092.95
5-8105.40490.3689979.822.85
Table 4. Fractal dimensions obtained from the FHH model of Wufeng–Longmaxi shales from two wells.
Table 4. Fractal dimensions obtained from the FHH model of Wufeng–Longmaxi shales from two wells.
Sample No.Slope>50R2>50D>50Slope2–50R22–50D2–50Slope<2R2<2D<2
2-5−0.390.9912.61−0.380.9962.52−0.990.9492.01
2-11−0.450.9932.55−0.440.9962.56−0.970.9432.03
2-13−0.240.9622.76−0.320.9922.68−0.980.9612.02
2-14−0.270.9892.73−0.350.9992.65−0.980.9462.02
5-1−0.370.9732.63−0.420.9992.58−0.970.9482.03
5-5−0.530.9922.47−0.410.9962.59−0.990.9362.01
5-8−0.490.9882.51−0.370.9982.63−0.990.9482.01
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Guan, Q.; Dong, D.; Deng, B.; Chen, C.; Li, C.; Jiao, K.; Ye, Y.; Liang, H.; Yue, H. Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China. Fractal Fract. 2025, 9, 410. https://doi.org/10.3390/fractalfract9070410

AMA Style

Guan Q, Dong D, Deng B, Chen C, Li C, Jiao K, Ye Y, Liang H, Yue H. Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China. Fractal and Fractional. 2025; 9(7):410. https://doi.org/10.3390/fractalfract9070410

Chicago/Turabian Style

Guan, Quanzhong, Dazhong Dong, Bin Deng, Cheng Chen, Chongda Li, Kun Jiao, Yuehao Ye, Haoran Liang, and Huiwen Yue. 2025. "Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China" Fractal and Fractional 9, no. 7: 410. https://doi.org/10.3390/fractalfract9070410

APA Style

Guan, Q., Dong, D., Deng, B., Chen, C., Li, C., Jiao, K., Ye, Y., Liang, H., & Yue, H. (2025). Structure and Fractal Characteristics of Organic Matter Pores in Wufeng–Lower Longmaxi Formations in Southern Sichuan Basin, China. Fractal and Fractional, 9(7), 410. https://doi.org/10.3390/fractalfract9070410

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