Pore Fractal and Structure Analysis of Pore-Filling Chlorite in Continental Shales: A Case Study from the Qingshankou Formation in the Gulong Sag

Abstract

The successful exploration and development of shale oil in the clay-rich Gulong shale have sparked increased research into the influence of clay minerals on shale reservoirs. However, compared to chlorite in sandstones, limited studies have focused on the occurrence of chlorite in continental shales and its effects on shale reservoir properties. This study offers a comprehensive analysis of chlorite in Gulong shale samples from three wells at different diagenetic stages. Four primary chlorite occurrences are identified in the Gulong shale: Type I, which is chlorite filling dissolved pores in carbonate; Type II, which is isolated chlorite; Type III, which is chlorite filling organic matter; and Type IV, which is chlorite filling authigenic microquartz. Types I and III chlorites exhibit higher porosity, offering more storage space for shale reservoirs. Chlorites of Types I, III, and IV, filled with other substances, display higher fractal dimensions, indicating more complex pore structures. These complex pores are favorable for oil adsorption but hinder oil seepage. The processes of organic matter expulsion and dissolution, which intensify with increasing diagenesis, promote the development of Types I and III chlorites, thereby positively influencing the shale reservoir porosity of Gulong shale. This study underscores the influence of chlorite occurrences on shale reservoir properties, providing valuable insights for the future exploration and development of shale oil and gas.

1. Introduction

In recent years, the Cretaceous Qingshankou Formation of the Gulong Sag in the Songliao Basin, known as the Gulong shale, has made breakthroughs in the exploration and development of shale oil [1,2,3,4]. The Gulong shale has a high clay mineral content (>40%) and a high organic matter content (>2%) [2]. As major minerals, clay minerals play a key role in shale reservoirs and are important components of micro- and nanoscale pore networks [5,6]. Chen, et al. (2016) [7] noted that clay minerals contribute to a total porosity of up to 16% in shale reservoirs. Moreover, certain clay minerals, such as montmorillonite, have large surface areas and strong adsorption capacities, which are crucial for preserving organic matter during sedimentation and diagenesis [8,9,10,11].

As a key clay mineral, chlorite is significantly influenced by diagenetic processes, making its study crucial for understanding shale diagenesis [12,13]. Furthermore, the tendency of chlorite precipitation within pore spaces highlights its significant impact on porosity, thus becoming an important element in studying shale reservoir evolution [14,15]. In shales, chlorite also exists in diverse occurrences. He, et al. (2022) [16] and Sun, et al. (2024) [17] reported the formation of chlorite in the Gulong shale during early diagenesis, forming films along detrital grain surfaces in shales with high silt content. In the Nandan region, black shale chlorite appears as small particles (<1 μm) embedded within the matrix [18]. Different occurrences of chlorite may have varying effects on shale reservoir properties as it is in sandstone.

Currently, the influence of chlorite on shale pore spaces remains contentious. In clay minerals from the Longmaxi Formation marine shale, Li, et al. (2015) [19] observed a negative correlation between chlorite content and the specific surface area of micropores, suggesting a destructive effect of chlorite on shale micropores. Conversely, research by Chang, et al. (2022) [20] indicates a weak correlation between chlorite content and pore structure parameters or fractal dimensions. The effect of chlorite on shale reservoirs appears insignificant, possibly due to low chlorite content [20]. In transitional (marine–continental) shale, higher proportions of illite and chlorite in the Lower Yangtze Region’s Permian shale correspond to reduced contributions to the pore system by clay minerals [21]. In the Linxing area’s transitional shale, chlorite promotes the development of inorganic pores and complicates pore structure [22]. In continental shale, Hou, et al. (2022) [23] observed that clay minerals significantly contribute to pore development in the Gulong shale, with chlorite playing a particularly crucial role and exhibiting a positive correlation. Wang, et al. (2023) [2] discovered a nonlinear relationship between pore volume and chlorite content in Gulong shale. While certain forms of chlorite protect porosity, excessive growth can compromise pore integrity. Several scholars have proposed upper limits of chlorite content conducive to pore preservation, with suggested thresholds ranging from 5% to 10% [24,25,26]. Research on the Cretaceous Qingshankou Formation shale from the continental setting reveals that clay minerals reduce the pore fractal dimensions [27]. However, in the marine Bakken Formation shale, Longmaxi Formation shale, the Niutitang Formation shale, and the transitional Longtan Formation shale, the fractal dimension of the pores increases [20,28,29,30]. Chang, et al. (2022) [20] suggested that this difference may be related to the types of clay minerals present. For instance, fractal dimension exhibits a strong positive correlation with illite, while showing a negative correlation with kaolinite. These findings underscore the ongoing debate over clay’s influence on shale pore spaces across marine, transitional, and continental settings, necessitating further research. Therefore, direct pore and fractal analysis of individual clay minerals is of significant importance.

FE-SEM can provide high-resolution (up to 100 nm and higher) 2D images [31]. Two-dimensional pore and fractal analysis provides an intuitive way to directly observe the morphological features and complexity of chlorite porosity, which is of significant importance for understanding the impact of clay minerals on reservoir pore structure. By analyzing the fractal dimension of the pores, this method allows for a quantitative description of the pore morphology, size distribution, and complexity. This, in turn, provides a scientific basis for assessing the pore structural characteristics and permeability of the shale reservoir. Due to its high brightness and distinct boundary features under FE-SEM, chlorite can be effectively analyzed for its fractal dimension and pore structure using FE-SEM. Currently, the analysis of the pore structure of clay minerals in the Gulong shale mainly focuses on the overall rock samples, with limited studies on the specific pore structure of individual clay minerals. Given that chlorite is easily identifiable under electron microscopy and has the characteristic of filling pores, it is necessary to conduct a detailed pore structure analysis of chlorite.

To investigate the effect of chlorite on shale reservoirs, this study examined samples from different maturity stages of Gulong shale, utilizing scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and two-dimensional (2D) porosity and fractal dimension analysis. It summarizes the various occurrences of chlorite in Gulong shale and studies the pore structures and evolutionary characteristics associated with different occurrences of chlorite. Detailed analysis of chlorite occurrences in shale and their effect on pore structures offers valuable insights into the clay evolution mechanisms and reservoir effects of continental shale.

2. Geological Setting

The Songliao Basin, situated in northeast China, comprises six primary tectonic units (Figure 1a,b) [32]. Gulong Sag, located in the western part of the central depression (Figure 1b), represents a long-standing deep-water sag. The sedimentary sequence within Gulong Sag consists of rift, depression, and inversion structural layers from bottom to top (Figure 1b) [1]. However, the deepest Gulong Sag has experienced no significant uplift [1,2]. The Qingshankou Formation is stratified into three members: Qing-1 Member (K₂qn¹), Qing-2 Member (K₂qn²), and Qing-3 Member (K₂qn³) (Figure 1c). Among these, the K₂qn¹ serves as a primary source rock and shale oil reservoir in Gulong Sag, characterized by substantial thickness and high organic carbon content. It averages 70–80 m in thickness, with total organic carbon ranging from 1% to 5%, averaging 2.9%. Vitrinite reflectance (Ro) varies between 0.72% and 1.5%, averaging 0.91% [2]. Diagenesis processes predominantly include mechanical compaction, pressure dissolution, cementation, and dissolution, occurring primarily in the middle diagenesis phase A2-B [7].

The study primarily focuses on the K₂qn¹ and K₂qn² shales, renowned as the Gulong shale in China, which is a prominent area for terrestrial shale oil exploration [1,32]. The Gulong shale exhibits a broad thermal maturity spectrum ranging from immature to highly mature conditions (Figure 1b). Organic matter in the shale predominantly consists of lacustrine Type I kerogen, with organic macerals primarily composed of lamalginite and alginite-derived amorphous organic matter. Gulong shale predominantly comprises shale (>95% of total thickness), predominantly fine-grained (<0.0039 mm) clastic minerals such as quartz, clay minerals, feldspar, and minor calcite. Clay mineral content is generally high, while carbonate minerals are sparse and felsic minerals are prominent. Intermingled are thin layers of siltstone, dolomite, and shell limestone, typically 1–5 cm thick per layer. For instance, the Q1 oil layer measures 14.6 m in thickness, comprising 51 layers of gray-black shale (13.36 m), eight layers of siltstone (0.52 m), four layers of dolomite (0.49 m), and one layer of shell limestone (0.23 m). Organic carbon content in the Q1 layer ranges from 0.8% to 4.5%, averaging 2.6%. Mineral content analysis indicates that the Gulong shale primarily consists of felsic, clayey, and mixed shale (Figure 1c), with clay shale exhibiting comparatively higher total organic carbon (TOC) content.

3. Samples and Methods

3.1. Samples

The core samples were obtained from the Qingshankou Formation shale of GY 8 wells with high maturity (Ro = 1.3–1.5%), ZY1 well with medium maturity (Ro = 0.9–1.0%), and C21 well with low maturity (Ro = 0.6–0.7%) [2]. Comprehensive whole-rock, clay analysis, and SEM-EDS analysis were carried out on all shale samples.

3.2. Methods

(1) 

X-ray diffraction (XRD)

XRD analysis of shale samples was conducted on both whole-rock (bulk) and clay fractions (<2 μm) using a TTR-type X-ray diffractometer manufactured by Rigaku (Japan). Prior to analysis, samples were pulverized to a 200-mesh powder. Whole-rock XRD patterns were acquired over a range of 5 to 65° with a step size of 0.02° and a scanning speed of 2°/min, utilizing Cu-Kα radiation (λ = 1.5418 Å). Experimental conditions included a 40 kV applied voltage and a 40 mA current, with a scattering slit of 1° and a receiving slit of 0.3 mm. Before analyzing clay minerals via XRD, carbonates and organic matter were removed using 0.3 M acetic acid and 10% hydrogen peroxide (H₂O₂), respectively. Clay fractions were isolated through suspension precipitation and subsequently transferred to silicon wafers to prepare oriented samples. The identification and quantification of clay minerals involved multiple treatments, including air drying (25 °C) (N sheet), ethylene glycol saturation (E sheet), and heating (550 °C) (T sheet). XRD data were collected over a 2.5–30° 2θ range. Mineral phases were identified and quantitatively assessed in weight percent (wt.%) using the Pearson VII fitting function on the Jade 6.0 software platform. Quantitative phase analysis was accomplished via Rietveld refinement, employing customized clay mineral structure models. The mineral compositions and relative mineral percentages of the shale samples were determined according to the Chinese oil and gas industry standard SY/T 5163-2018 [33].

(2) 

Field-Emission Scanning Electron Microscopy (FE-SEM)

All shale samples were cut into relatively flat 10 × 10 × 5 mm squares and manually polished with 800 mesh, 1000 mesh, and 2000 mesh sandpaper, respectively. A Leica argon ion polishing instrument was used to polish the circumferences of the samples to ensure smooth surfaces before carbon plating. After polishing, the sample surface was treated with carbon coating. The morphology, occurrence, dissolution, and pore characteristics of minerals and OM were imaged at a temperature and humidity of 22 °C and 40%, respectively. FE-SEM observation was performed using an FEI Helios Nano-Lab 650 (America) equipped with an energy-dispersive spectrometer (EDS). FE-SEM imaging was operated with a chamber pressure of 8–10 mbar, a working distance of 3–15 mm, and a voltage of 5–15 kV. EDS was operated with a working distance of 8–10 mm, a voltage of 10–20 kV, and a detection depth of 1–5 μm. EDS spectra were then used to determine mineral types.

(3) 

Pore structure Measurement and Fractal Analysis

The 2D pore structure and fractal dimension analysis were conducted using ImageJ 1.54d software (see Figure 2). The 2D porosity refers to the porosity of a chlorite aggregate, which consists of visible needle-like chlorite particles clustered together. First, we extracted the outline of the chlorite aggregate and then used the difference in grayscale values between pores and chlorite to extract the pores of the chlorite aggregate (Figure 2). The ratio of the pore area of the chlorite aggregate to the total area of the chlorite aggregate represents the chlorite porosity in this study (see formula (1)). This calculation provides a quantitative measure of the porosity specific to the chlorite aggregate under study.

$$Porosity={\displaystyle \frac{PoreArea}{ChloriteAssembleArea}}$$

(1)

In addition, the pore size and pore aspect ratio were statistically analyzed, with the aspect ratio defined as the ratio of the long axis to the short axis of the pores.

Fractal dimension analysis by Image J was implemented in the box-counting method, which involves several steps after binarizing the image (Figure 2). The basic principle of two-dimensional fractal dimension is based on the formula (2) [34]. With the Image J software, this is mainly realized through the following steps: First, the image is covered with boxes of various sizes (ε), and then each box is checked to count how many contain at least one foreground pixel (pixel value = 1). The number of such boxes (N(ε)) is calculated for each box size ε. Next, a double logarithmic graph of ε versus N(ε) is plotted. Finally, a linear regression analysis is performed on the log-log plot to determine the fractal dimension D, where the slope of the regression line (typically D) provides the fractal dimension.

$$D=\underset{\mathsf{\epsilon }\to \mathrm{\infty }}{\lim }{\displaystyle \frac{\ln N\mathsf{\epsilon }\left(F\right)}{-\ln \mathsf{\epsilon }}}$$

(2)

D is the fractal dimension of F; F is the object being measured; ε is the size of the box; and N is the number of similar elements calculated for each box size ε.

4. Results

4.1. Mineralogy

As can be seen in Figure 3, the collected shale samples consist mainly of quartz, feldspar, and clay minerals, of which the clay minerals are mainly I/S mixed layer, illite, and chlorite. According to Hou, et al. (2022) [23] and Wang, et al. (2023) [2], clay minerals in the Gulong shale are dominated by illite (11–94%, 63% on average), followed by illite/smectite mixed layer (I/S) (3–68%, 22% on average), chlorite (1–59%, 12% on average) and kaolinite (1–26%, 7.4% on average). Notably, the content of chlorite increases with increasing depth, reflecting thermal maturity change [2,23]. As one of the major clay mineral types in the Gulong shale, coupled with its tendency to fill pore growths, it is important to study the effect of chlorite on shale reservoirs.

4.2. Chlorite Occurrence

Under SEM, four types of chlorite occurrences were identified (Figure 4): Type I is the chlorite filling the dissolved pore of carbonate, Type II is the isolated chlorite, Type III is the chlorite filling organic matter, and Type IV is the chlorite filling authigenic quartz. These four chlorite types exhibit distinct crystallographic states and pore characteristics. Type I chlorite shows coexistence with carbonate minerals, linked to carbonate dissolution pores (Figure 4a₁–a₃,f). This type of chlorite displays irregularly shaped pores, including dissolution pores, as well as encapsulated or triangular pores between chlorite crystals. Type II chlorite appears as needle-like or hair-like chlorite filling intergranular pores supported by particles such as feldspar and quartz (Figure 4b₁–b₃,e). The interior of these chlorite intergranular pores appears relatively clean, predominantly structured with triangular or slit-shaped pores where chlorite crystals interlock. Type III chlorite features acicular chlorite crystals filling intergranular pores enriched with flowing organic matter (Figure 4c₁–c₃). Type IV chlorite reveals microcrystalline quartz wrapped by chlorite, with triangular intergranular pores between the wrapped quartz and chlorite (Figure 4d₁–d₃,g). The wrapped microcrystalline quartz typically exhibits well-defined authigenic morphology, with grain sizes generally in the range of a few microns.

The distribution of intergranular pores in the four types of chlorites also varies. According to the International Union of Pure and Applied Chemistry (IUPAC), the classification of pore size is as follows: micropores (pore size < 2 nm), mesopores (pore size ranging from 2 to 50 nm), macropores (pore size > 50 nm) [35]. As shown in Figure 5, Type I chlorite predominantly consists of macropores, with 16.1% of mesopores developed, mainly distributed between 50 and 500 nm. Type II chlorite is also dominated by macropores, with 19.3% of mesopores developed, and the pore size mainly ranges from 100 to 500 nm. Type III chlorite is composed entirely of macropores, while Type IV chlorite predominantly features macropores and mesopores, with the main pore size ranging from 2 to 100 nm.

4.3. Distribution of Pore Structure and Fractal Dimension of Four Types of Chlorite

In total, we counted the occurrence characteristics of chlorite in 118 samples from three wells and analyzed the pore structure and fractal dimension of chlorite in four occurrences (Figure 6).

The pore structures of the four chlorite types exhibit distinct characteristics (Figure 6). Porosity analysis reveals that Type I and Type III chlorites possess higher porosity, with average values of 14.1% and 16.4%, respectively. Type II and Type IV chlorites have lower porosity, with average values of 6.51% and 6.64%, respectively. Type IV chlorite shows relatively higher porosity compared to Type II, possibly due to support from authigenic quartz.

Two-dimensional fractal analysis results indicate that Type II chlorite has the lowest fractal dimension, averaging 1.26 (Figure 6). Type I, Type II, and Type III chlorites exhibit relatively higher fractal dimensions, with average values of 1.38 each. Among these, Type I chlorite shows the highest overall fractal dimension, possibly reflecting the complexity of the dissolution processes involved.

The pore size distribution shows that Type I and Type III chlorites have the largest average pore sizes, with Type I chlorite having an average pore size of 123 nm and Type III chlorite having an average pore size of 120 nm (Figure 6). Type II and Type IV chlorites have smaller pore sizes, with average pore sizes of 85 nm and 94.8 nm, respectively. In terms of the aspect ratio of the pores, Type I and Type III chlorites have smaller length-to-width ratios, averaging 2.11 and 2.4, respectively, indicating more regular pore shapes (Figure 6). In contrast, Type II and Type IV chlorites have larger length-to-width ratios, averaging 2.78 and 3.56, respectively, suggesting that their pores are more elongated.

5. Discussion

5.1. Genesis of Pore-Filling Chlorite

Type I chlorite exhibits associated occurrences within dissolution pores filled with carbonate minerals (Figure 4a₁–a₃). Wang, et al. (2023) [2] found that chlorite and carbonate show significant associations in the Gulong shale. During carbonate dissolution, Fe²⁺ and Mg²⁺ ions are released, facilitating the co-occurrence of chlorite and carbonate [36,37,38,39]. As dissolution intensifies, carbonate gradually dissolves completely, allowing chlorite to fill the pores progressively [2].

Type II chlorite grows between mineral grains such as feldspar and quartz (Figure 4b₁–b₃). Crystallization of chlorite on the surfaces of feldspar or quartz particles occurs when the ion concentration and activation energy conditions are met in the external fluid [12,15,39,40]. Pores within this chlorite type remain unfilled by other substances, possibly due to rapid fluid exchange rates in the larger intergranular pores [41,42,43].

Type III chlorite’s interstitial pores are filled with organic matter (Figure 4c₁–c₃). During thermal maturation, organic matter expelled during hydrocarbon generation is retained within pre-existing authigenic chlorite’s triangular or slit-like pores. According to He, et al. (2023) [44], hydrocarbon charging peaks in the Gulong Sag during moderate maturity stages, favoring the early-formed chlorite interstitial pores to be filled with organic matter. Alteration in the chemical composition of fluid after charging may halt further growth of this type of chlorite [45].

Type IV chlorite’s interstitial pores develop microcrystalline quartz (Figure 4d₁–d₃). During the transformation of minerals like smectite into chlorite, quartz precipitates (see formula (3)) [46]. When fluid transport of Si⁴⁺ is hindered, microcrystalline quartz often forms in association with authigenic crystals [46,47]. Limited by crystalline space and time, this quartz crystal typically exhibits small grain sizes but distinct hexagonal self-growth patterns [46,48].

2.4Ca_0.1Na_0.2Fe_1.1MgAlSi_3.6O₁₀(OH)₂ (Smectite) + 0.88H₂O + 1.44H⁺ ⇒ Fe_2.6Mg_2.2Al_2.4Si_2.8O₁₀(OH)₈ (Chlorite) + 0.24Ca²⁺ + 0.48Na⁺ + 0.04Fe²⁺ + 0.20Mg²⁺ + 5.84SiO₂ (Quartz)

The pore structures and characteristics of these four chlorite types are influenced by the different diagenetic processes they undergo. Thus, they may have distinct contributions and evolutionary characteristics to shale reservoirs [49,50].

5.2. Evolution of Pore Structure and Fractal Dimension for Pore-Filling Chlorite

The fractal dimension can characterize the complexity or irregularity of pore distribution [28]. A higher fractal dimension typically indicates a more intricate and irregular pore network, which may influence the shale’s ability to retain or transmit fluids—an essential factor in hydrocarbon reservoir studies [51,52,53]. In contrast, a lower fractal dimension suggests a simpler, smoother pore structure, often associated with a less interconnected pore network and potentially lower permeability [20].

In analyzing the porosity characteristics of four chlorite types, it is evident that Type I and Type III chlorites exhibit higher porosity (Figure 6). This implies that dissolution processes and the thermal evolution of organic matter may promote the formation of interstitial pores within chlorite crystals. The cementation between chlorite and quartz inhibits the development of interstitial pores. Compared to Type II chlorite, the other three types display larger fractal dimensions and more complex porosity (Figure 6). The filling of other substances leads to increased complexity in the internal pore structure of chlorite. Additionally, as shown in Figure 6, Type I and Type III chlorites exhibit more regular pore shapes, while Type II and Type IV chlorites have more elongated pores. The elongated pores in Type II and Type IV chlorites are less conducive to the percolation of oil and gas fluids.

The porosity of Type I chlorite initially increases and then gradually decreases with maturity (Figure 7), which may be due to the early thermal evolution of organic matter. The substantial organic acid release and subsequent dissolution of carbonate minerals increases the porosity. As organic matter matures, organic acid release diminishes, fluid pH shifts from acidic to mildly alkaline, and residual ions crystallize rapidly within pores [2,16,54], leading to chlorite filling and gradual porosity reduction. Liu, et al. (2019) [55] reported the existence of a change in fluid pH from acidic to moderately alkaline in the Gulong shale. Fractal characteristics of Type I chlorite show minimal variation with maturity (Figure 7), indicating dissolution processes have negligible impact on its pore structure.

Porosity and fractal characteristics of Type II chlorite show minimal change with thermal evolution (Figure 7 and Figure 8). A P value greater than 0.05 suggests no significant difference between the three box charts (Figure 7 and Figure 8). This may result from the gradual occupation of mineral surfaces available for chlorite crystallization. Once chlorite fills available pore space, subsequent crystallization sites become scarce, resulting in minimal changes in porosity and pore structure during thermal evolution [15].

As can be seen in Figure 7 and Figure 8, the porosity and fractal dimension of Type III chlorite increase gradually with maturity. During organic matter thermal evolution, kerogen begins hydrocarbon expulsion, progressively filling interstitial chlorite pores and expanding organic matter-occupied spaces, thereby enlarging chlorite pores continuously [56,57]. Organic matter occupies pore spaces, shielding interstitial chlorite pores from compaction and cementation effects of other minerals. Increased organic matter injection leads to more complex pore shapes, making Type III chlorite pores more intricate as maturity progresses (Figure 8).

Type IV chlorite exhibits an initial increase followed by a decrease in porosity (Figure 7). This may result from the authigenic microcrystalline quartz providing a framework support for chlorite [58]. Initially, chlorite forms numerous interstitial pores with authigenic quartz particles, facilitating early pore space development [58]. As chlorite continues to grow, these pore spaces gradually fill and are further compacted, resulting in a later-stage reduction in Type IV chlorite porosity. The fractal dimension of Type IV chlorite initially decreases and then increases, indicating a transition from simple to complex pore structures (Figure 8). The rigid support of regular, self-generated quartz contributes to simplified chlorite pores initially, but as chlorite grows and compaction takes effect, these pores become increasingly complex at higher maturity stages.

5.3. Pore Structure Versus Fractal Dimension of Pore-Filling Chlorite

As shown in Figure 9, the relationship between porosity and fractal dimension varies across different types of chlorites, each exhibiting distinct characteristics in their fitting curves. Type I chlorite shows a phenomenon where the fractal dimension first increases with porosity and then decreases after reaching a certain point (Figure 9a). This is due to the gradual dissolution of carbonate minerals: as porosity increases, the pore complexity decreases, and at high porosity, the carbonate is typically not replaced by chlorite, leading to a lower fractal dimension. Thus, high porosity in Type I is mainly attributed to dissolution processes. Type II chlorite, in contrast, demonstrates a positive correlation between porosity and fractal dimension, with both increasing together (Figure 9b). This behavior is primarily driven by the intercrystalline pores of chlorite, where higher porosity indicates a more complex pore structure, resulting in a higher fractal dimension. Type III chlorite exhibits no significant correlation between porosity and fractal dimension, showing an irregular curve that reflects a more complex and heterogeneous pore structure (Figure 9c). Finally, Type IV chlorite also shows a positive correlation between porosity and fractal dimension, similar to Type II (Figure 9d). The increase in porosity leads to more complex intercrystalline pores, which in turn increases the fractal dimension, indicating a more intricate pore structure.

In summary, the four types of chlorites differ in the relation between porosity and fractal dimension. Type I shows a decreasing fractal dimension at high porosity due to carbonate dissolution. Type II and Type IV display a positive correlation indicating more complex pore structures, while Type III shows a complex, irregular relationship without a clear trend.

Currently, studies based on BET average pore size obtained through nitrogen adsorption reveal a negative correlation between pore size and fractal dimension, with a general understanding that smaller pores lead to more complex pore structures (Figure 10b) [28,59,60]. However, in the chlorite samples from the study area, the fractal dimension and pore size show a positive correlation (Figure 10a). This suggests that larger chlorite pores possess higher pore complexity. The injection of organic matter and the dissolution of carbonate minerals lead to the expansion of intergranular pores. At the same time, this results in a more dispersed pore size distribution, making the pore structure more complex. As seen in Figure 6, chlorite with no fillings generally corresponds to lower fractal dimensions and smaller average pore sizes, whereas chlorite with fillings tends to have higher fractal dimensions and more dispersed pore size distributions. Therefore, for chlorite particles, an increase in the average pore size can lead to a more complex pore structure, primarily due to the larger average pore size indicating higher pore dispersion.

5.4. Evolution Model of Pore-Filling Chlorite

Based on the findings of these studies, the four chlorite types undergo distinct evolutions during diagenesis (Figure 1). Enhanced dissolution of carbonate promotes chlorite growth. Research by Liu, et al. (2019) [55] indicates that fluids in the Gulong Sag transitioned from early acidic to later mildly alkaline conditions, conducive to chlorite growth. Additionally, early dissolution released Fe²⁺ and Mg²⁺ that facilitated rapid chlorite growth in later stages, gradually occupying the crystal outlines of carbonate (Figure 11). Type II chlorite, lacking sufficient nucleation sites, shows minimal changes during diagenesis (Figure 11). Conversely, Type III chlorite, influenced by increasing organic matter expulsion, fills more interstitial spaces, resulting in stagnant growth, increased porosity, and complexification of interstitial pores (Figure 11). Type IV chlorite evolves through diagenesis with concurrent growth of authigenic quartz, leading to accelerated growth of chlorite in later stages and a rapid reduction in porosity at high maturity levels (Figure 11). Compression by authigenic quartz complicates the interstitial pore structure of chlorite at advanced maturity stages.

Researchers have found that fractal dimension is related to organic carbon content and microporous development [28,51,62,63]. Fractal dimension reflects the development degree of shale micropores. The more developed the micro-pore, the smaller the average pore size, the larger the specific surface area, and the larger the fractal dimension [62]. Yang, et al. (2014) [62] reported that the higher the organic carbon content, the larger the fractal dimension. The fractal dimension has different effects on the storage and transportation of hydrocarbons. The larger the fractal dimension, the pore structure tends to be more complex, which is favorable to the adsorption and storage of hydrocarbons, but not conducive to fluid seepage [28,62]. The Gulong shale is extensively developed with a significant amount of chlorite, which typically occurs in a pore-filling occurrence (Figure 2 and Figure 4). Type I and Type III chlorite have higher porosity, and these two types of yielding chlorite have higher contribution to the pore space. In addition, the presence of carbonate, cementation of authigenic quartz, and injection of organic matter can lead to chlorite pore complexity (Figure 6), which may adsorb hydrocarbons into the pore space and thus be detrimental to hydrocarbon seepage [47,64].

5.5. Chlorite in Gulong Shale Reservoir Characteristics

In the study of marine, continental, and transitional shale reservoirs, it has been found that the relationship between shale pore fractal dimensions and clay minerals is complex. In the study by Wang, et al. (2023) [2], it is observed that chlorite in Gulong shale exhibits a transition from initially negatively affecting porosity to later exerting a positive influence when its content exceeds 5%. This shift is likely associated with changes in chlorite occurrence. With increasing diagenesis, enhanced dissolution and hydrocarbon generation processes lead to an increasing presence of Type I and Type III chlorites due to carbonate dissolution and organic matter maturation, thereby preserving intercrystalline pores in chlorite. This phenomenon is supported by findings from Hou, et al. (2022) [23] regarding a positive correlation between chlorite and organic matter in Gulong shale and from Kang, et al. (2024) [4] regarding a negative correlation between chlorite and carbonate. Consequently, the transition in chlorite pore effects in Gulong shale may be attributed to an initial dominance of Type II and Type IV chlorites, followed by a gradual shift toward Type I and Type III chlorites, resulting in an initially negative and subsequently positive correlation with porosity. Furthermore, Type I and Type III chlorites exhibit higher 2D fractal dimensions. The increasing presence of these chlorites complicates shale pore structures, potentially hindering gas permeation while enhancing gas adsorption in Gulong shale reservoirs.

In summary, shales at or after the peak of hydrocarbon production may develop more Type I and Type III chlorite, and chlorite may contribute more positively to shale reservoirs at this stage. Therefore, during shale oil and gas exploration and development in shale reservoirs where a large amount of chlorite is developed, chlorite occurrence can be an important factor for consideration. The practical implications of this approach for hydrocarbon exploration and reservoir evaluation are significant. Understanding the role of chlorite, especially Type I and Type III, in shale reservoirs can guide more accurate reservoir assessments. Chlorite’s impact on pore structure, along with fractal analysis, helps identify areas with higher storage capacity and better pore connectivity, which directly influence fluid flow and hydrocarbon extraction efficiency.

6. Limitations and Future Research

This paper conducts a comparative analysis of the two-dimensional porosity and pore fractality of chlorite in shale samples with varying occurrences, aiming to deepen our understanding of the contribution of chlorite to shale reservoirs. By focusing on a large number of samples, this study provides valuable insights into the relationship between chlorite’s microstructure and its role in shaping the overall pore characteristics of shale. However, the image analysis method utilized in this study is limited by inherent issues related to image scale. Specifically, it is challenging to obtain large-scale images that can adequately represent the entire shale sample. As a result, the impact of factors such as depth on the pore characteristics of chlorite remains unresolved in the current analysis. To address this limitation, future work should focus on conducting large-scale analyses that incorporate a more comprehensive view of chlorite’s pore characteristics throughout the shale sample. In recent years, numerous scholars have delved into the scale problem associated with shale image analysis. Techniques such as box-counting and Z-contrast criterion have been proposed to facilitate the study of representative elementary areas in shale [65,66,67]. These approaches help overcome the challenge of capturing the full diversity of pore structures within the rock. Moreover, the 2D fractal employed in this study provides a clear and intuitive representation of the planar morphology of the pores. However, this approach is inherently limited in that it does not offer a complete understanding of the 3D pore structure, which is crucial for fully characterizing shale reservoir.

In light of these limitations, FIB/SEM and X-ray CT scanning emerge as a promising technique to address the issue of observing the pore structure in three dimensions [68]. FIB/SEM can produce such 3D images at high resolution (1 nm) [69], but it is labor-intensive, costly, and suffers from reconstruction uncertainty [70]. Three-dimensional imaging would allow for a more accurate representation of the complex spatial arrangement of pores and better capture the heterogeneous nature of the pore network. While X-ray CT scanning holds great potential, challenges remain in terms of mineral identification, resolution, and scale determination [68,71]. Therefore, further development of these methods, particularly in the context of accurately identifying minerals and establishing reliable scaling protocols, is essential to make three-dimensional pore analysis a viable and effective tool for future research in shale petrophysics [66].

By overcoming these technical limitations and incorporating three-dimensional analysis into the study of chlorite and shale porosity, we can gain a more comprehensive understanding of the factors influencing pore structure and ultimately improve our ability to model and predict the behavior of shale reservoirs.

7. Conclusions

This article analyzes different occurrences of chlorite in Gulong shale and their evolution using XRD, SEM, 2D porosity, and fractal analysis. It also investigates the impact of chlorite on shale reservoirs. The main conclusions drawn are as follows:

(1) Through SEM analysis, four types of chlorite occurrences were identified in Gulong shale: Type I fills dissolved carbonate pores, Type II exists without filling, Type III fills organic matter, and Type IV fills authigenic micro quartz.

(2) Porosity analysis reveals that Type I and Type III chlorites exhibit higher porosity, with average values of 14.1% and 16.4%, respectively. In contrast, Type II and Type IV chlorites show lower porosity, averaging 6.51% and 6.64%, respectively.

(3) Results from two-dimensional fractal analysis indicate that Type II chlorite has the lowest fractal dimension, averaging 1.26. Type I, Type II, and Type III chlorites exhibit relatively higher fractal dimensions for pore structures, each averaging 1.38. The presence of other substances contributes to pore complexity.

(4) As thermal maturation progresses, Type I and Type II chlorites show little change, while Type III chlorites gradually increase in porosity due to hydrocarbon expulsion from organic matter, leading to enhanced pore fractality and complexity. Type IV chlorites initially decrease in porosity, followed by an increase, with pore fractality increasing initially and then decreasing. During diagenetic evolution, quartz and chlorite continue to grow, and fluid pH shifts from acidic to neutral or alkaline, promoting rapid growth of late-stage chlorites. Consequently, the porosity of this chlorite type decreases rapidly in high maturity stages. Additionally, compression from authigenic quartz results in increased complexity of intercrystalline chlorite pores at high maturity stages.

(5) The transition in pore effects of chlorites in Gulong shale may be attributed to an initial dominance of Type II and Type IV chlorites, gradually shifting toward the predominance of Type I and Type III chlorites. This shift leads to chlorites initially showing a negative correlation with porosity and later transitioning to a positive correlation.