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Article

Heterogeneity and Cause Analysis of Organic Pore in Upper Permian Shale from Western Hubei, South China

by
Yang Liu
1,
Yuying Zhang
1,*,
Zhiliang He
2,
Shuangfang Lu
3,
Rui Yang
2 and
Yifei Li
1
1
School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China
2
School of Earth Resources, China University of Geosciences, Wuhan 430074, China
3
Sanya Offshore Oil & Gas Research Institute, Northeast Petroleum University, Sanya 572025, China
*
Author to whom correspondence should be addressed.
Fractal Fract. 2025, 9(11), 731; https://doi.org/10.3390/fractalfract9110731
Submission received: 26 September 2025 / Revised: 3 November 2025 / Accepted: 9 November 2025 / Published: 12 November 2025
(This article belongs to the Special Issue Fractal Analysis in Unconventional Reservoirs: Theory and Applications)
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Abstract

Organic pores serve as crucial storage spaces for shale gas, whose morphology and structure vary significantly among different types of organic matter, directly influencing the storage and seepage capacity of shale gas. The Upper Permian shale in the Western Hubei Trough formed in diverse sedimentary facies and has undergone multiple geological activities, resulting in strong heterogeneity of organic pores across different strata and regions. To figure out the heterogeneous characteristics of organic pores and the forming reason, the occurrence state of organic matter, pore morphology, and structural parameters (pore size, specific surface area, pore volume, and fractal dimension) of the Upper Permian shale in Western Hubei, have been discussed in detail, based on the data of field emission scanning electron microscopy and low-temperature nitrogen adsorption experiments conducted on the extracted organic matter. On this basis, fractal dimension theory was applied to discuss the heterogeneity of organic pores in different layers, and the reason for heterogeneity has been analyzed in detail. The results indicate that the occurrence mode of organic matter in different layers presents various characteristics: in the Gufeng Formation, the organic matters distribute primarily dispersed in flocculent state; at the bottom of Wujiaping Formation, they occur as isolated individuals, while the organic matters turn into discontinuous laminated distribution in the middle and upper Wujiaping Formation; in the Dalong Formation, the organic matters show continuous parallel banded distribution. Moreover, the morphology and structural parameters of organic pores exhibit obvious changes from the Gufeng Formation to the Dalong Formation: (a) the pore morphology shows the changed trend as extremely complex-simple-complex; (b) the specific surface area and pore volume follow the trend as large-small-large; (c) the pore size distribution displays in the pattern of bimodal-unimodal-bimodal; (d) the data of fractal dimension show the variation of high–low–high. Overall, the various sedimentary environments during the Upper Permian shale depositional period determined the differences in organic sources, which dominated the heterogeneity of organic pores in shale. These data clarify the development and variation characteristics of organic matter pores under different depositional environments, providing a theoretical basis for shale gas exploration and development during the transition from marine to marine–continental facies.

1. Introduction

The organic pores in shale are considered as important part of shale reservoirs, since they control the gas content, methane adsorption capacity and porosity of shale [1,2,3]. The content of adsorbed gas in shale can reach as high as 20% to 85%, being adsorbed in the pores and on the surfaces of organic matter and other minerals [4,5]. Previous studies have suggested that the development of organic pores could enhance the storage and adsorption capacity of shale, as well as provide effective migration pathways for the desorption and seepage of shale gas through interconnection with inorganic pores [6,7,8]. Therefore, the development characteristic of organic pores structure in shale is the key indicator for assessing the potential of shale gas, as it determines the quality of the shale reservoir. [9,10]. Organic pore structure can be represented by parameters such as the spatial shape, size/diameter, connectivity, and configuration. Moreover, organic pores are generally considered to form during the processes of hydrocarbon generation and expulsion from organic matter [11,12]. Different types of organic matter vary in their hydrocarbon generation potential, which leads to differences in the development of organic pore [13,14,15]. Therefore, in order to evaluate the adsorption capacity of shale, it is essential to analyze the development and structure of organic pores, as well as the relationship between heterogeneity and the sources of organic matter.
The Permian shale gas resources in the Western Hubei region show enormous potential, and significant progress has been achieved in shale gas exploration [16,17]. The shales in Gufeng Formation, Wujiaping Formation and Dalong Formation serve as the critical exploration strata and have become key research targets [18]. Previous studies indicate that the Upper Permian shale in this area deposited in multiple types of sedimentary facies with large depositional thickness, including shelf, coastal swamp, and carbonate platform [19,20]. The organic matters in Permian shales are predominantly Type I–II with significant differences in the sources of organic matter across different strata [21,22,23]. Therefore, it is inferred that the development characteristics of organic pores may also vary significantly among different layers in the Western Hubei region. However, current research on pores in Permian shale reservoirs mostly focuses on the developmental characteristics of individual formation, with limited comparative studies on the differences between different layers, thereby constraining the understanding of the reasons behind the heterogeneity in organic pore development.
To solve this issue, the morphology and other characteristics of organic pores in Upper Permian shales from the Western Hubei region (Tiewuji outcrop), have been investigated, based on observations under field emission scanning electron microscopy (FE-SEM). In addition, organic matter extracted from the shale samples was subjected to low-temperature nitrogen adsorption experiment to analyze the structural characteristics of organic pores, such as pore size distribution, pore volume, and specific surface area. On the basis of N2 adsorption data, the fractal dimensions of shale organic pores are calculated based on the FHH model to characterize the heterogeneity of organic pores in different layers. Finally, the heterogeneity of organic pores in Upper Permian rocks in the Western Hubei region and its causes have been analyzed by integrating all these data. This study could contribute to understanding the differences in organic pores and their controlling factors in Upper Permian shale reservoirs in the Western Hubei region. Furthermore, it will provide a theoretical basis for assessing the potential of shale gas resources in other regions or formations.

2. Geological Setting

The study area is located in Western Hubei Province, at the northern margin of the Yangtze Block, and belongs to the central–upper Yangtze region. Following the relatively stable cratonic evolution during the Caledonian period (Z–S), the area shifted into an extensional regime during the Hercynian period (D–T2) [24]. In the Permian, driven by the expansion of the Paleo-Tethys Ocean, the South Qinling Ocean basin underwent rifting, leading the northern margin of the Yangtze Block to evolve into a passive continental margin. Under this tectonic–sedimentary framework, a northward-opening carbonate ramp system developed in Western Hubei, within which the E’Xi Trough was formed (Figure 1) [24]. At the end of the Middle Permian, the central–upper Yangtze Block experienced large-scale regional uplift (the Dongwu Movement) and the emplacement of the Emeishan Large Igneous Province. These events induced intense tectono-sedimentary differentiation, resulting in the development of multiple phases of marine and transitional depositional systems within the E’Xi Trough [25]. During the deposition of the Gufeng Formation, the trough continued to subside and water depth gradually increased, developing basin facies. Later, the Dongwu Movement caused regional uplift of the Yangtze Block, resulting in erosion at the top of the Gufeng Formation and the formation of an unconformity. At the early stage of Wujiaping Formation deposition, activity of the Emeishan Large Igneous Province reached its peak, causing local uplift of the trough and a fall in sea level, producing a complex paleogeographic pattern: shelf facies developed in the northwestern part of Western Hubei, while tidal-flat facies developed in the eastern part [19,25,26]. During the late deposition of the Wujiaping Formation to the deposition of the Dalong Formation, the system evolved into ramp facies controlled by large-scale transgressive events in the South China region. The Dalong Formation is dominated by platform-basin facies and, in the late stage of deposition, developed outer ramp facies [19].
The Upper Permian strata of the E’Xi Trough, from oldest to youngest, consists of the Gufeng, Wujiaping, and Dalong Formations (Figure 1, right). The Gufeng Formation was mainly deposited in a deep-water basin facies environment, with lithologies dominated by siliceous rocks and siliceous shales, and a total thickness of approximately 23 m. At the transition between the Middle and Late Permian, the Dongwu Movement altered the paleogeographic framework of the Yangtze region, leading to partial erosion at the top of the Gufeng Formation 1924, with a ~1 m-thick Wangpo Shale developing between the Gufeng and Wujiaping Formations. The lower Wujiaping Formation is dominated by tidal-flat facies, with lithology primarily consisting of aluminous shale. The upper Wujiaping Formation is characterized by siliceous rocks and limestones, with some intervals interbedded with volcanic ash, and a total thickness of approximately 76.4 m. The Dalong Formation mainly comprises siliceous shales and marlstones, with a thickness of about 43.95 m.

3. Samples and Methods

Rock samples used in this study were collected from the Tiewuji section in Enshi Prefecture, Western Hubei. A total of 57 samples from different stratigraphic levels were analyzed for total organic carbon (TOC) content. To investigate the characteristics of organic matter pore structures, low-temperature nitrogen adsorption experiments were conducted on the organic matter extracted from eight selected samples representing different stratigraphic levels. The samples were processed and analyzed using the following procedures. First, the powdered shale samples were pretreated by soaking in distilled water for 2–4 h to allow clay minerals to swell, followed by removal of the supernatant. Then, the samples were sequentially treated with 6 mol/L HCl and 40% HF at 60–70 °C under stirring for 1–2 h to dissolve carbonates and silicate minerals. The acid treatment was repeated several times, with intermediate washing by 1 mol/L HCl and distilled water until the samples became weakly acidic. Subsequently, the residues were treated with 0.5 mol/L NaOH for 30 min to remove humic substances, followed by repeated washing with distilled water until neutral. For pyrite-bearing samples, a mixture of 6 mol/L HCl and zinc granules was added repeatedly until no H2S odor was detected. The cleaned residues were then subjected to a heavy-liquid flotation method using a ZnCl2 solution with a density of 2.0–2.1 g/cm3. The suspension was ultrasonically dispersed and centrifuged at 2000–3000 rpm for 20 min to separate kerogen from mineral matter. The upper kerogen fraction was collected, washed repeatedly with distilled water until no halide ions remained, and finally freeze-dried below 60 °C to obtain relatively pure kerogen for nitrogen adsorption experiments. Low-temperature nitrogen adsorption experiments were conducted on the extracted organic matter to acquire adsorption–desorption isotherms. Prior to the measurements, the samples were vacuum dried at 110 °C for 18 h to remove adsorbed water and volatile components from the surface, while ensuring that the organic matter pores remain stable without structural alteration. Nitrogen adsorption experiments were performed using an Autosorb IQ Station 1 instrument (Quantachrome Instruments, Boynton Beach, FL, USA). The N2 adsorption–desorption isotherms were analyzed using the Quantachrome Autosorb iQ analyzer and ASiQwin software (version 5.25). The specific surface area was calculated using the BET model. Pore size distributions were obtained using the non-local density functional theory (NLDFT) model with the kernel “N2 at 77 K on silica (cylindrical pore, adsorption branch model)” [28,29].
To investigate the morphological characteristics of organic matter pores in rock samples from different stratigraphic intervals, the rocks were sliced along the bedding planes, mechanically polished, and subsequently coated with a thin layer of gold to enhance conductivity. Scanning electron microscopy (SEM) observations were carried out at the East Engineering Research Center using a Thermo Scientific Apreo 2 field-emission scanning electron microscope (FE-SEM). High-resolution images were obtained at an accelerating voltage of 15 kV, with a maximum pixel resolution of 0.9 nm. A total of five representative samples from typical stratigraphic intervals were selected for observation.
To quantitatively analyze the structural complexity and heterogeneity of organic matter pores, this study introduces fractal theory. Fractal theory is a mathematical method for investigating the self-similarity of irregular objects and can be used to quantitatively assess the complexity, similarity, or heterogeneity of shale pore structures [30]. Based on nitrogen adsorption data, the FHH model was employed to calculate the fractal dimension, as proposed by Pfeifer et al. [31].
l n V = D n − 3 l n [ l n ( P 0 / P ) ] + C
In the equation, V represents the amount of nitrogen adsorbed at pressure P, Dₙ denotes the fractal dimension, P0 is the saturation vapor pressure, and C is a constant. According to the adsorption–desorption isotherms, the hysteresis loop disappears when the relative pressure is lower than P/P0 = 0.45, indicating that the adsorption behavior changes significantly across this point. Therefore, a relative pressure of P/P0 = 0.45 was selected as the boundary for calculating the fractal dimensions. When P/P0 < 0.45, nitrogen molecules are primarily controlled by van der Waals forces and undergo monolayer and multilayer adsorption; this region represents the surface fractal dimension (D1), which reflects the roughness and irregularity of the pore surface. When P/P0 > 0.45, capillary condensation becomes dominant, and the structural fractal dimension (D2) characterizes the complexity and heterogeneity of the internal pore structure [32]. The fractal dimensions in the low-pressure and high-pressure regions, denoted as D1 and D2, were calculated separately for each sample. D1 reflects the roughness of the pore surface, while D2 characterizes the complexity of the internal pore structure [33].
Statistical analyses and data visualization were performed using OriginPro 2022 (OriginLab Corporation, Northampton, MA, USA) and Python 3.10. The Python workflow utilized the pandas, numpy, matplotlib, and seaborn libraries for data processing and visualization. Final figure layouts were refined using CorelDRAW 2022 (Corel Corporation, Ottawa, ON, Canada).

4. Results

4.1. TOC Content and Variations

The TOC content shows considerable variability among the four formations (Figure 2). The Gufeng Formation displays the highest TOC values, ranging from 0.70% to 16.09%, with an average of 5.80%. The wide interquartile range (IQR ≈ 7.9%) indicates strong heterogeneity in TOC distribution within this formation. The Lower Wujiaping Formation shows TOC values between 0.07% and 5.12%, with an average of 0.96% and a relatively narrow IQR of 0.85%, suggesting a more uniform but generally low TOC level. The middle to upper Wujiaping Formation has TOC values ranging from 0.29% to 7.57%, with an average of 3.15% and an IQR of about 3.7%, indicating moderate variability. The Dalong Formation exhibits TOC values ranging from 0.17% to 6.40%, with an average of 1.93% and an IQR of approximately 1.6%. Overall, the Gufeng Formation shows the greatest variation in TOC, while the Lower Wujiaping Formation presents the lowest and most stable values among the studied intervals.

4.2. FE-SEM Images

To investigate the morphological characteristics of organic matter pores in shales from different stratigraphic intervals, five representative samples were selected from the Gufeng, lower and upper Wujiaping, and Dalong Formations for field emission scanning electron microscopy (FE-SEM) observations. In low-magnification mode, the occurrence state of organic matter was examined, whereas in high-magnification mode, the specific morphologies of organic matter pores were observed (Figure 3). The selected samples are as follows: Gufeng Formation (TWJ-001-g), lower Wujiaping Formation (TWJ-008-w), upper Wujiaping Formation (TWJ-014-w), and Dalong Formation (TWJ-019-d). These results will be further discussed in Section 5.1.1.

4.3. Low-Temperature Nitrogen Adsorption Experiments

Low-temperature nitrogen adsorption experiments can effectively characterize the structural features of organic matter pores, with key parameters including specific surface area (SSA), pore size, and pore volume. (Figure 4) shows the N2 adsorption–desorption isotherms of organic matter samples from different stratigraphic intervals. According to the BET and NLDFT model calculations (Table 1), the specific surface areas of the Gufeng Formation, upper Wujiaping Formation, and Dalong Formation samples range from 156.3 m2/g to 225.6 m2/g, which are much higher than those of the lower Wujiaping Formation (7.4 m2/g to 40.4 m2/g) Overall, the adsorption capacity of the lower Wujiaping Formation samples is significantly lower than that of the Gufeng, upper Wujiaping, and Dalong Formations. The higher adsorption capacities observed in these latter three formations indicate a much stronger development of organic pores compared with the lower Wujiaping Formation.
In terms of pore volume, the samples exhibit a trend consistent with that of specific surface area. The total pore volume of the Gufeng Formation, the upper Wujiaping Formation, and the Dalong Formation ranges from 0.28 to 0.50 cm3/g, whereas the lower Wujiaping Formation is significantly lower, only 0.01–0.15 cm3/g. Moreover, from a vertical perspective, the pore volume of the Gufeng Formation, the upper Wujiaping Formation, and the Dalong Formation shows a gradual increase from bottom to top.
In terms of average pore diameter, the Gufeng Formation, the upper Wujiaping Formation, and the Dalong Formation range between 6.50 and 15.83 nm, whereas the lower Wujiaping Formation is slightly higher, at 8.86–17.79 nm. Except for the lower Wujiaping Formation, the average pore diameter also exhibits an upward-increasing trend from bottom to top.

4.4. Fractal Dimensions of Organic Matter Pores

To further characterize the structural complexity of organic matter pores in shale, the Frenkel–Halsey–Hill (FHH) model (Equation (1)) was applied to fit the nitrogen adsorption isotherms in both the low-pressure region (P/P0 = 0.05–0.45) and the high-pressure region (P/P0 = 0.45–0.95). The corresponding fractal dimensions, D2 (low-pressure) and D1 (high-pressure), were obtained (Figure 5; Table 2) to reflect the roughness of the pore surface and the complexity of the internal pore structure, respectively. For organic matter pores in the Gufeng, Wujiaping, and Dalong Formations, the high-pressure fractal dimension Dn1 ranges from 2.525 to 2.786 with an average of 2.686, while the low-pressure fractal dimension Dn2 ranges from 2.454 to 2.675 with an average of 2.577.

5. Discussion

5.1. Morphology of Organic Pores

5.1.1. Organic Pore Morphology Under FE-SEM

Gufeng Formation: (Figure 3a) The organic matter is widely dispersed, mainly occurring as flocculent or fine-grained detrital particles, distributed among inorganic minerals such as quartz. (Figure 3b–d) The organic pores exhibit a complex structure, typically showing several micrometer-scale macropores, within which numerous nanoscale (<10 nm) pores develop in a bottle-shaped configuration, indicating that the organic pores in this horizon are well developed and structurally complex.
Lower Wujiaping Formation: (Figure 3e,f) The organic matter mainly occurs as isolated particles, often in contact with inorganic minerals forming shrinkage cracks. (Figure 3g,h) The organic pores in this sample are few, mostly spherical and isolated, with smooth surfaces and simple structures.
Upper Wujiaping Formation: (Figure 3i) The organic matter displays a discontinuous banded or sub-laminated distribution, intermediate between fully dispersed and bedding-enriched types. (Figure 3j–l) The organic pores are mainly fissure-like and channel-shaped, with rough pore walls, but overall pore development is less than that of the Gufeng Formation. Pyrite and inorganic dissolution pores are observed in some mineral phases.
Dalong Formation: (Figure 3m) The organic matter mainly occurs as continuous bands parallel or sub-parallel to bedding, associated with pyrite and well-developed inorganic pores. (Figure 3n–p) Solid bitumen fills interbedded fractures or dissolution pores, and occasionally organic pores up to several micrometers in diameter are observed, with rough pore walls, although overall organic pore development is not prominent.

5.1.2. Organic Pore Morphology Reflected by N2 Adsorption–Desorption

The morphology of nitrogen adsorption isotherms can be used to characterize the pore structures of complex porous media and has already been widely applied in the pore characterization of shale gas reservoirs [34,35,36]. The adsorption isotherms of the Gufeng Formation, the upper Wujiaping Formation, and the Dalong Formation generally resemble the Type IV isotherm in the IUPAC classification [36], exhibiting a typical “S-shaped” pattern (Figure 6). The explanation of the Type IV isotherm is as follows: at relative pressures below 0.05, the adsorption curve rises very steeply. According to the micropore filling theory, the potential energies of gas–solid interactions from adjacent walls within micropores overlap, significantly enhancing the adsorption capacity of micropores. As a result, adsorption increases rapidly at low pressures, and the adsorption mechanism differs from that on open surfaces. This indicates the presence of a large number of molecular-scale micropores in the organic matter. At relative pressures of 0.05–0.8, the isotherm rises gradually, corresponding to monolayer and multilayer adsorption, suggesting the development of mesopores within the organic matter. At relative pressures greater than 0.8, the amount of adsorption increases sharply, and the isotherm exhibits an inflection point. Even near the saturation vapor pressure, adsorption continues to rise significantly. This stage corresponds to capillary condensation occurring within macropores of the organic matter, indicating the presence of macropores. This indicates that micropores (<2 nm), mesopores (2–50 nm), and some macropores (>50 nm) are simultaneously developed in the organic matter of the Gufeng, upper Wujiaping, and Dalong Formations. Such isotherm characteristics suggest that the pore system possesses a certain degree of multiscale structure and storage potential. In contrast, the isotherm shape of the lower Wujiaping Formation samples more closely resembles Type III, with a lower slope in the low-pressure region and a generally smaller adsorption capacity, indicating a significantly lower abundance of micropores.
In nitrogen adsorption–desorption isotherms, the commonly observed hysteresis loops mainly result from differences in the gas–liquid interface during adsorption and desorption, which lead to distinct diffusion pathways of nitrogen molecules [37,38]. In typical ink-bottle-shaped pores, nitrogen molecules gradually undergo capillary condensation inside the pore during adsorption. During desorption, however, due to the geometric restriction of the narrow neck, nitrogen molecules must first evaporate through the neck, thereby delaying the release of the gas. According to capillary condensation theory, the curvature radius of the gas–liquid interface within the pores can be expressed by the Kelvin equation:
γ k = − 2 σ 1 V m l R T b l n p ∕ p 0
σ 1 : surface tension of the liquid condensate; V m l : molar volume of the liquid condensate; R : ideal gas constant; T b : analysis temperature; γ k : curvature radius of the adsorbed gas condensed in the pore; p 0 : saturation vapor pressure of nitrogen at liquid nitrogen temperature; p : equilibrium adsorption pressure of nitrogen.
The curvature radius γ k exhibits a monotonic increase with relative pressure p ∕ p 0 . When the curvature radius of the interface within the pore is small, nitrogen molecules can only evaporate at lower relative pressures. Therefore, at the same relative pressure, the gas content during the desorption process is typically higher than that during the adsorption process, ultimately resulting in an adsorption–desorption hysteresis loop. Similarly, for cylindrical pores open at both ends, the gas–liquid interface during adsorption is a continuously varying cylindrical surface, whereas during desorption it becomes spherical. Therefore, analyzing the area and shape of hysteresis loops can reflect pore types. However, such hysteresis loops may be influenced by the superposition of multiple pore morphologies and thus can only indicate the dominant pore type. Accordingly, SEM images are needed to accurately determine pore types. According to the IUPAC classification of hysteresis loops [36], samples from the Gufeng Formation generally exhibit H2-type hysteresis loops, characterized by a steep drop of the desorption branch at high relative pressures and large loop areas (Figure 6). This indicates that their pore structures contain numerous “ink-bottle” pores, i.e., complex pores with narrow necks and wide bodies. Such structures cause pronounced hysteresis during desorption due to changes in the gas–liquid interface, where gas molecules have difficulty escaping through the narrow pore throats, resulting in “desorption delay.” This feature is consistent with the organic pore morphology observed in the SEM images of Gufeng Formation sample TWJ-001-g (Figure 3b–d and Figure 6d), where micron-scale macropores contain abundant “pores within pores” of the ink-bottle type (diameter < 10 nm), with nanoscale organic pores well developed, densely clustered, and interconnected.
The lower Wujiaping Formation samples exhibit inconspicuous hysteresis loops, with two samples showing almost no hysteresis (Figure 6c). This indicates that their pore structures are mainly dominated by cylindrical-like pore with two open ends or capillary-like isolated pores, characterized by poor connectivity and limited storage capacity, consistent with the isolated cylindrical pores observed in the SEM images of the lower Wujiaping Formation sample TWJ-008-w (Figure 3e–h and Figure 6c).
In contrast, the upper Wujiaping Formation and Dalong Formation samples display H3-type hysteresis loops, with relatively smooth desorption branches and loops that are small in area and gently shaped, suggesting pore structures dominated by slit-shaped pores, parallelplate-like pores, and other open pores (Figure 6a,b). This feature agrees well with the slit-shaped and open-type pores shown in (Figure 3k,l and Figure 6a,b). Such pores generally exhibit good channel connectivity, make a significant contribution to adsorption capacity, but show relatively weak hysteresis. Moreover, as seen in (Figure 3g,h), the organic pores are relatively few in number, typically occurring as isolated spherical pores with simple internal structures.
In summary, the combined analysis of low-temperature nitrogen adsorption isotherms and SEM observations provides a more comprehensive understanding of the systematic differences in pore types, structural complexity, and connectivity of shale organic matter formed under different depositional settings. The deep-marine environments (Gufeng Formation, upper Wujiaping Formation, and Dalong Formation) are more favorable for the development of complex pore systems, characterized by the coexistence of ink-bottle and slit-shaped pores and pronounced hysteresis loops. In contrast, the lower Wujiaping Formation deposited in a coastal swamp environment exhibits simpler pore structures with weak hysteresis loops. These findings are consistent with previous studies [39]. The hysteresis loop analysis and SEM observations are mutually corroborative, providing a reliable basis for the refined characterization of organic matter pores in different reservoirs.

5.2. Size Distribution of Organic Matter Pores

To further elucidate the structural characteristics of organic matter pores in shale, the pore size distribution of the samples was calculated from low-temperature nitrogen adsorption data using the NLDFT (non-local density functional theory) model, and the relationship curves between dv/dD and pore size were plotted (Figure 7). The results indicate that the pore size of organic matter varies significantly among different stratigraphic intervals.
As shown in (Figure 7), the pore size distribution curves of the Gufeng, upper Wujiaping, and Dalong samples exhibit a distinct bimodal pattern. The primary peak is mainly distributed within the 1–2 nm range, while the secondary peak occurs between 3–8 nm, indicating the coexistence of micropores and mesopores in these samples, which is favorable for gas molecule adsorption and storage. Such bimodal structures are commonly associated with a higher abundance of planktonic algae [39]. The bimodal pore-size distribution observed in the Gufeng, upper Wujiaping, and Dalong Formations can be attributed to the thermal evolution of algal-derived kerogen. In the early stages of maturation, the conversion of kerogen to bitumen and the expulsion of liquid hydrocarbons generate abundant nanoscale micropores within the organic matrix. With increasing maturity, secondary cracking of retained bitumen produces gaseous hydrocarbons and pyrobitumen, and the associated structural shrinkage and devolatilization induce mesopores. The coexistence of these two stages of pore formation results in a hierarchical organic pore system characterized by both micropores and mesopores, forming a distinct bimodal distribution pattern [40]. In contrast, the lower Wujiaping samples display a unimodal distribution, predominantly concentrated in the range of 7–50 nm, corresponding to mesopores to macropores, with a clear deficiency of micropores. This observation is consistent with their low adsorption capacity on isotherms and the relatively smooth, isolated occurrence of organic matter observed in SEM images (Figure 3h). The organic matter of the lower Wujiaping Formation is mainly derived from higher plants, which tend to generate and expel small hydrocarbon molecules during hydrocarbon generation, leaving behind aromatic frameworks and thus resulting in a narrower pore size range. In terms of distribution width, samples from deep-marine environments (Gufeng, upper Wujiaping, and Dalong Formations) exhibit broader pore size distributions with wider ranges, reflecting a more complex pore system and stronger structural heterogeneity, whereas the paralic swamp deposits of the lower Wujiaping Formation show a highly concentrated pore size distribution, indicating a simpler pore system with a single pore-size structure.

5.3. Specific Surface Area and Pore Volume of Organic Matter Pores

5.3.1. Specific Surface Area and Pore Volume

The specific surface area of the samples from the Gufeng Formation, the upper Wujiaping Formation, and the Dalong Formation is generally high (Table 2, Figure 8), ranging from 156.3 to 225.6 m2/g, indicating that organic matter pores in these intervals are well developed. In contrast, the specific surface area of the lower Wujiaping Formation samples is significantly lower, only 7.4–40.4 m2/g. For all samples, micropores and mesopores contribute more than 90% of the specific surface area (Figure 9), but the contribution ratios differ among Formations: Gufeng Formation is dominated by micropore contributions, whereas the Wujiaping and Dalong Formations are characterized by a larger contribution from mesopores. This difference may be related to the higher abundance of smaller pores in the Gufeng Formation (Figure 7), as smaller pore sizes can provide larger specific surface areas. SEM images further confirm this, showing that the organic matter pores in the Gufeng Formation have rougher surfaces, where numerous smaller pores are developed (Figure 3d).
The distribution trend of total pore volume is generally consistent with that of specific surface area (Figure 8). The pore volume of the Gufeng Formation, the upper Wujiaping Formation, and the Dalong Formation is relatively large, ranging from 0.283 to 0.500 cm3/g, while that of the lower Wujiaping Formation is only 0.01–0.15cm3/g, showing a significant difference (Table 1). Notably, in the Gufeng, upper Wujiaping, and Dalong Formations, pore volume gradually increases upward with stratigraphic depth (Figure 8). In terms of pore volume distribution, except for the Gufeng Formation, the pore volume of organic matter in other intervals is mainly contributed by mesopores (~70%) and macropores (~20%). In contrast, the Gufeng Formation shows a distinct feature, with micropores contributing about 10%, which is related to its complex surface morphology (Figure 9).
The average pore size shows a general increasing trend from the lower Gufeng Formation to the upper Dalong Formation, with a sudden increase in the lower Wujiaping sample TWJ-009-g, which may be attributed to rapid and complex changes in depositional environments that led to differences in organic matter sources (Figure 8). The relatively smaller average pore size of the Gufeng Formation may be due to the large number of micropores, which reduces the overall average pore size (Figure 8).

5.3.2. Correlation Among Surface Area, Pore Volume, and Average Pore Size

Overall, the samples from the Gufeng Formation, the upper Wujiaping Formation, the Dalong Formation, and the lower Wujiaping Formation all exhibit a clear positive correlation (Figure 10a), indicating that as pore number or connectivity increases, not only does pore volume increase, but the corresponding specific surface area also rises. Among them, the deep-water shelf samples (Gufeng, upper Wujiaping, and Dalong Formations) are characterized by higher specific surface areas and pore volumes, whereas the lower Wujiaping Formation samples deviate from this trend, with both specific surface area and pore volume being significantly lower, reflecting the poor development of organic pores in this interval.
In the Gufeng, upper Wujiaping, and Dalong Formations, a moderate negative correlation between specific area and average pore size is observed (Figure 10b): the smaller the average pore size, the larger the specific surface area. This pattern is consistent with the pore-size distribution curves, which show that these intervals are dominated by micropores with peak values concentrated in the 1–2 nm range, thereby providing larger specific surface areas. By contrast, the lower Wujiaping Formation does not follow this trend. Its average pore sizes fall mainly within the 7–50 nm range, dominated by mesopores and macropores, yet its specific surface area is markedly low. This suggests that micropores are scarce in this interval; even though larger pores are present, they contribute little to surface area, leading to a relatively simple pore system with weaker storage capacity.
In general, samples from all four intervals (including the lower Wujiaping Formation) display a positive correlation between average pore size and pore volume (Figure 10c), indicating that larger average pore sizes correspond to higher pore volumes, which agrees with the physical meaning of pore volume. The regression slopes are similar across intervals, showing a high degree of consistency. However, it is noteworthy that the pore volumes of the lower Wujiaping Formation samples remain consistently low; despite having larger average pore sizes, they fail to form sufficient pore capacity, reflecting sparse and scattered pores as well as poorly developed organic pore systems in this unit.

5.4. Heterogeneity of Organic Pores

The Gufeng Formation exhibits relatively high fractal dimensions Dn1 and Dn2, indicating strong heterogeneity of organic pores, with complex internal structures and rough pore surfaces (Figure 11a,b). This observation is consistent with the SEM images of sample TWJ-001-g (Figure 3a–d), which reveal rough pore surfaces and intricate “pore-in-pore” structures. In the lower Wujiaping Formation, the fractal dimension Dn1 in the high-pressure region differs between the two samples, with one showing a relatively higher value and the other a lower value,, which may be related to the rapidly changing types of organic matter leading to unstable pore structures. Meanwhile, the fractal dimension Dn2 in the low-pressure region is significantly smaller than that of other stratigraphic units, suggesting smoother pore surfaces. This feature is also reflected in the SEM images of sample TWJ-008-d (Figure 3e–h), where organic matter occurs in isolated distributions with few and regularly shaped surface pores. The upper Wujiaping and Dalong Formations show intermediate values of fractal dimensions Dn1 and Dn2, suggesting that their pore internal structures and surface roughness are between those of the Gufeng Formation and the lower Wujiaping Formation.
Further statistical analysis of the Dn1 + Dn2 values (Figure 11c) reveals a clear correspondence between fractal dimensions and depositional environments. Specifically, the Gufeng and Dalong Formations, representing typical deep-water shelf deposits, exhibit the highest total fractal dimensions; the lower Wujiaping Formation, deposited in a coastal marsh setting, shows the lowest values; whereas the upper Wujiaping Formation, formed in a transitional zone between marine and terrestrial environments, displays intermediate levels. These results indicate that depositional environment exerts significant control on the heterogeneity of organic matter pores: the deeper the depositional water, the more complex the organic pores, and the stronger their heterogeneity.
Fractal dimension analysis reveals the differences in pore surface roughness and internal structural complexity among different stratigraphic intervals and highlights their close relationship with the depositional environment. However, fractal dimension alone is insufficient to comprehensively reflect the interaction mechanisms within the pore system; therefore, it is necessary to further investigate the statistical relationships between fractal dimensions and pore structural parameters (Figure 12). Dn1, Dn2, and their sum (Dn1 + Dn2) show a very strong positive correlation, indicating that pore surface roughness and internal structural complexity are highly consistent, jointly controlling the heterogeneity of the pore system. Specific surface area (SSA) exhibits a moderate negative correlation with average pore diameter (Figure 10b), but a strong positive correlation with micropore volume and micropore SSA, suggesting that the abundance of micropores and complex surfaces are the key controlling factors for SSA. In contrast, pore volume is more closely related to mesopore and macropore volumes, indicating that the total volume is mainly contributed by medium and large pores. The TOC content shows a strong positive correlation with micropore volume and micropore specific surface area, but no obvious correlation with total pore volume or total SSA. This suggests that higher organic matter abundance mainly promotes the development of micropores within the organic matrix, whereas it has little influence on the formation of larger pores or the overall pore structure complexity. In summary, micropores dominate SSA, mesopores and macropores determine pore volume, while fractal dimensions comprehensively reflect the complexity of the pore system. These findings further demonstrate that depositional environment and organic matter type, by influencing pore size distribution and structural complexity, ultimately determine the heterogeneity and storage potential of shale reservoirs.

5.5. Causes of Heterogeneity in Organic Pores

During the depositional stage of Gufeng Formation, the study area experienced a relatively high sea level, developing siliceous deep shelf. During this period, organic matter was derived from plankton algae with minimal terrigenous input, and the organic matter type was primarily Type I [16]. At the bottom of Wujiaping Formation, the depositional environment shifted to coastal swamp with the decreasing of sea level. The content of plankton algae decreased, while higher plant debris increased significantly, resulting in organic matter being mainly Type II. In the middle and upper part of Wujiaping Formation, sea level rose again, restoring the deep-water shelf environment. The content of plankton algae rebounded, while contributions from higher plants and green algae diminished. Until the deposition of Dalong Formation, organic matter remained dominated by plankton algae, with Type I being the predominant organic matter type [22]. Previous studies have indicated that differences in depositional environments significantly influence the abundance and type of organic matter, thereby controlling the development characteristics of organic pores [1,41,42,43,44].
The analysis results in this study have revealed that the fractal dimensions in the Upper Permian shale show obvious correlation of coordinated variation with the evolution of depositional environment. In the Gufeng Formation and Dalong Formation (deep shelf), the organic pores exhibit high fractal dimensions. On the contrary, the bottom of Wujiaping Formation (coastal swamp) shows low fractal dimension. The fractal dimensions in middle and upper members of Wujiaping Formation lie in medium. This result demonstrates that the depositional environment exerts significant control on the heterogeneity of organic pore structure: as water depth increases and the proportion of algae-derived organic matter rises, the organic pore structure becomes more complex, and heterogeneity in organic pore correspondingly enhances.
According to existing regional stratigraphic correlations and lithofacies–paleogeographic analyses, the Tiewuji section is situated in the central part of the Western Hubei Depression. The Upper Permian Wujiaping and Dalong formations in this area display broadly continuous and similar lithofacies and depositional facies across the region. Surrounding areas, including Enshi, Badong, and Jianshi, exhibit comparable deep shelf to slope sedimentary characteristics, indicating that the Tiewuji section is regionally representative of the Upper Permian shale succession in Western Hubei. However, toward the basin margins, the depositional facies gradually shift into shallower environments, which may result in certain lateral variations in organic matter abundance and pore structure [17,19].

6. Conclusions

In the Upper Permian organic-rich shales of Western Hubei, the types and structures of organic pores have been strongly influenced by the depositional environment. In the deep shelf (Gufeng Formation, upper Wujiaping Formation, and Dalong Formation), organic pores are well developed, with an average specific surface area (SSA) of ~196 m2/g and an average pore volume of ~0.4 cm3/g. In contrast, the shale in tidal-flat (lower Wujiaping Formation) exhibit significantly lower organic pore development, with an average SSA of ~25 m2/g and an average pore volume of ~0.08 cm3/g. The organic pore sizes in the Gufeng Formation, upper Wujiaping Formation, and Dalong Formation display bimodal distribution, indicating the coexistence of multiscale pore systems, whereas samples from the lower Wujiaping Formation show poor pore development with unimodal distribution. The pore morphologies revealed by FE-SEM imaging are highly consistent with those inferred from low-temperature N2 adsorption experiments, mutually confirming the observations. The organic pores in the Gufeng Formation, upper Wujiaping Formation, and Dalong Formation exhibit complex structures, commonly developing multilevel pore types such as ink-bottle, parallel-plate, and slit-shaped pores. In contrast, the organic pores in the lower Wujiaping Formation are characterized by relatively simple pore structures, dominated by isolated spherical pores with poor connectivity.
Fractal dimensions D1 and D2 show obvious positive correlation, suggesting a strong coupling between pore surface roughness and internal structural complexity. The fractal dimension results also show a distinct depositional response. In the Gufeng Formation and Dalong Formation, which represent typical deep shelf deposits, both D1 and D2 values of organic pores are high—with the average value of D1 being 2.7124 and that of D2 being 2.6205—reflecting structurally complex pores with rough surfaces. In contrast, organic pores in the lower Wujiaping Formation, deposited in transitional marine–terrestrial environment, exhibits lower fractal dimensions, indicating smooth surfaces and simpl pore structures. Overall, the shale deposited in deep shelf has been favorable for the organic pores with rough surface and complex structure.

Author Contributions

Conceptualization, Y.L. (Yang Liu) and Y.Z.; methodology, Y.Z.; software, Y.L. (Yang Liu) and Y.L. (Yifei Li); validation, R.Y.; formal analysis, Y.Z.; investigation, Y.L. (Yang Liu); re-sources, Z.H.; data curation, Y.Z.; writing—original draft preparation, Y.L. (Yang Liu); writing—review and editing, S.L.; visualization, Z.H.; supervision, S.L.; project administration, Y.Z.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [Initiation and Cultivation Project of Northeastern University], “Evolution Process of Hydrocarbon Generation and Expulsion from Organic Matter in Paleogene Shale in Fushun Basin and Occurrence Characteristics of Shale Oil”, grand number [N25LPY035], and [Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology], titled “Characteristics of Organic Matter Sources in Permian Marine Shale in the Northern Sichuan—Western Hubei Region and Their Impact on Reservoir Development”, grand number [33550000-24-ZC0613-0045].

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Depositional facies during Wujiaping Stage and location of study area. (b) Lithostratigraphic column of Tiewuji outcrop (Modified after [27]).
Figure 1. (a) Depositional facies during Wujiaping Stage and location of study area. (b) Lithostratigraphic column of Tiewuji outcrop (Modified after [27]).
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Figure 2. TOC content and variations in Upper Permian organic-rich shales in Tiewuji outcrop.
Figure 2. TOC content and variations in Upper Permian organic-rich shales in Tiewuji outcrop.
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Figure 3. FE-SEM images of organic matter and pore morphologies in different layers of Tiewuji Outcrop. (a–d) Gufeng Formation, (e–h) lower Wujiaping Formation, (i–l) upper Wujiaping Formation, and (m–p) Dalong Formation.
Figure 3. FE-SEM images of organic matter and pore morphologies in different layers of Tiewuji Outcrop. (a–d) Gufeng Formation, (e–h) lower Wujiaping Formation, (i–l) upper Wujiaping Formation, and (m–p) Dalong Formation.
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Figure 4. Nitrogen adsorption–desorption isotherms of organic matter in Tiewuji outcrop.
Figure 4. Nitrogen adsorption–desorption isotherms of organic matter in Tiewuji outcrop.
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Figure 5. Fractal characteristics of organic matter pores in different layers of Tiewuji outcrop.
Figure 5. Fractal characteristics of organic matter pores in different layers of Tiewuji outcrop.
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Figure 6. (a–d) Pore morphologies of organic matter in different layers.
Figure 6. (a–d) Pore morphologies of organic matter in different layers.
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Figure 7. Pore size distribution of organic matter in Tiewuji outcrop.
Figure 7. Pore size distribution of organic matter in Tiewuji outcrop.
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Figure 8. Structural parameters of organic pores in different layers of Tiewuji outcrop.
Figure 8. Structural parameters of organic pores in different layers of Tiewuji outcrop.
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Figure 9. (a) proportions of specific surface in different layers of Tiewuji outcrop; (b) proportions of pore volume in different layers of Tiewuji outcrop.
Figure 9. (a) proportions of specific surface in different layers of Tiewuji outcrop; (b) proportions of pore volume in different layers of Tiewuji outcrop.
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Figure 10. (a) Correlation between pore volume and specific surface area; (b) correlation between average pore size and specific surface area; (c) correlation between average pore size and pore volume.
Figure 10. (a) Correlation between pore volume and specific surface area; (b) correlation between average pore size and specific surface area; (c) correlation between average pore size and pore volume.
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Figure 11. Fractal dimensions of organic matter pores in different layers of Tiewuji outcrop. (a) Fractal dimension Dn1 in the high-pressure region; (b) Fractal dimension Dn2 in the low-pressure region; (c) Sum of fractal dimensions Dn1 + Dn2.
Figure 11. Fractal dimensions of organic matter pores in different layers of Tiewuji outcrop. (a) Fractal dimension Dn1 in the high-pressure region; (b) Fractal dimension Dn2 in the low-pressure region; (c) Sum of fractal dimensions Dn1 + Dn2.
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Figure 12. Correlation heatmap among different parameters.
Figure 12. Correlation heatmap among different parameters.
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Table 1. Pore structure parameters of organic matter in Tiewuji section.
Table 1. Pore structure parameters of organic matter in Tiewuji section.
SamplesDepth (m)Average Pore Diameter (nm)BET Specific Surface Area (m2/g)DFT Pore Volume (cm3/g)Formation
TWJ-019-d48.359.04225.60.45Dalong
TWJ-006-d50.5515.83156.30.50
TWJ-018-w54.6510.83194.10.45Upper Wujiapin
TWJ-014-w80.058.92194.00.38
TWJ-009-w128.6517.7940.40.15Lower Wujiapin
TWJ-008-w130.258.867.40.01
TWJ-003-g139.856.95199.90.30Gufeng
TWJ-001-g153.656.50205.10.28
Table 2. Fractal dimensions of organic matter pores in Tiewuji outcrop.
Table 2. Fractal dimensions of organic matter pores in Tiewuji outcrop.
Samples Relative Pressure (P/P0) = 0.45–0.95Relative Pressure (P/P0) = 0.05–0.45Stratigraphic
Fitting EquationR2High Relative Pressure D1Fitting EquationR2Low Relative Pressure D2
TWJ-019-dy = −0.301x + 4.3200.998252.6980y = −0.385x + 4.3230.997642.6148Dalong
Formation
TWJ-006-dy = −0.392x + 3.9550.994652.6077y = −0.471x + 3.9830.997492.5291
TWJ-018-wy = −0.342x + 4.2100.999632.6585y = −0.455x + 4.1990.997882.5453Upper Wujiaping Formation
TWJ-014-wy = −0.309x + 4.2130.998172.6910y = −0.410x + 4.1870.998262.5904
TWJ-009-wy = −0.475x + 2.6130.999062.5247y = −0.546x + 2.6450.998182.4536Lower Wujiaping Formation
TWJ-008-wy = −0.236x + 0.9830.990242.7642y = −0.453x + 0.9350.998622.5474
TWJ-003-gy = −0.242x + 4.2010.998982.7579y = −0.337x + 4.1810.997042.6634Gufeng
Formation
TWJ-001-gy = −0.214x + 4.2360.998252.7858y = −0.325x + 4.2030.998082.6747
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Liu, Y.; Zhang, Y.; He, Z.; Lu, S.; Yang, R.; Li, Y. Heterogeneity and Cause Analysis of Organic Pore in Upper Permian Shale from Western Hubei, South China. Fractal Fract. 2025, 9, 731. https://doi.org/10.3390/fractalfract9110731

AMA Style

Liu Y, Zhang Y, He Z, Lu S, Yang R, Li Y. Heterogeneity and Cause Analysis of Organic Pore in Upper Permian Shale from Western Hubei, South China. Fractal and Fractional. 2025; 9(11):731. https://doi.org/10.3390/fractalfract9110731

Chicago/Turabian Style

Liu, Yang, Yuying Zhang, Zhiliang He, Shuangfang Lu, Rui Yang, and Yifei Li. 2025. "Heterogeneity and Cause Analysis of Organic Pore in Upper Permian Shale from Western Hubei, South China" Fractal and Fractional 9, no. 11: 731. https://doi.org/10.3390/fractalfract9110731

APA Style

Liu, Y., Zhang, Y., He, Z., Lu, S., Yang, R., & Li, Y. (2025). Heterogeneity and Cause Analysis of Organic Pore in Upper Permian Shale from Western Hubei, South China. Fractal and Fractional, 9(11), 731. https://doi.org/10.3390/fractalfract9110731

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