An Experimental Study of Dolostone Recrystallization Under Fluid Overpressure and Its Implication for Reservoir Property Improvement

Abstract

Recrystallization is an important diagenetic process that widely occurs in carbonate rocks as a result of increasing temperature and pressure during burial. Exploration has revealed that carbonate rocks affected by recrystallization vary in their reservoir properties. To investigate the favorable development conditions of recrystallized carbonate reservoirs, we selected the crystalline dolomite reservoir of the Majiagou Formation in the Ordos Basin as the object for experimental simulation and conducted a series of dolostone recrystallization simulation experiments under various fluid pressures at 120 °C with an axial load of 55 MPa. The fluid overpressure experiments showed extensive recrystallization growth of crystals with uniform grain size and a high degree of euhedral development, and they retained the optimal porosity and pore connectivity. However, the hydrostatic pressure group had a lower degree of recrystallization and inferior physical properties compared to the overpressure group. The results indicate that fluid overpressure is a key preservation condition for deep recrystallized carbonate reservoirs. Fluid overpressure effectively preserves the initial porosity and facilitates the euhedral growth of dolostone crystals, thereby enhancing pore connectivity. Furthermore, the Kozeny–Carman equation was employed to verify that recrystallization of dolostone under fluid overpressure significantly enhances reservoir permeability by adjusting the pore structure, specifically the pore tortuosity and the pore–throat ratio. Our study demonstrates that fluid overpressure is an important prerequisite for the development of deep recrystallized carbonate reservoirs. The search for carbonate formation units with overpressure conditions such as the presence of dense layer capping may be a new direction for deep carbonate oil and gas exploration.

1. Introduction

The formation and preservation mechanism of high-quality, large-scale, deep carbonate reservoirs has consistently been a focal point in oil and gas exploration [1,2,3]. Recrystallization, as a prevalent diagenesis during the burial of carbonate rocks, significantly influences the evolution of reservoir pore structures [4,5]. Recrystallization adjusts the size and arrangement of primary mineral grains and reshapes the pore spaces between mineral crystals, which is of significant importance for the development and modification of reservoirs [6,7]. The geochemical characteristics are also accompanied by the migration and enrichment of elements such as Ca and Mg and trace elements like Sr, Fe, and Mn [8,9,10]. Dolomites with well-developed intercrystalline pores formed by recrystallization, as well as granular limestones or dolomites that preserve intergranular pores, are extremely high-quality types of carbonate reservoir rocks [11,12]. It is evident that, starting from the reaction mechanisms of recrystallization, investigating the favorable factors that lead to the development of porosity is of great significance for the exploration and prediction of deep carbonate reservoirs.

Putnis (2002) described a type of fluid-mediated reaction that occurs at the molecular level and is widely found in various geological processes such as weathering, mineralization, and metasomatism [13]. This process involves the dissolution of the parent phase driven by fluid flow and the subsequent reprecipitation of the product phase, often leading to structural diversity in the products. This type of reaction where the dissolution process of the parent phase and the precipitation process of the product phase are coupled in space and time is referred as a coupled dissolution–reprecipitation reaction (CDR) [14,15]. Carbonate rock recrystallization is a type of coupled dissolution–reprecipitation reaction, characterized by the continuous exchange of ions such as Ca²⁺ and Mg²⁺ between minerals and fluids. Changes in physical and chemical conditions at the solid–liquid interface disrupt the original equilibrium system, driving ion exchange reactions along the chemical potential gradient from the solid–liquid contact to the mineral interior until a new equilibrium is reached [16]. The product phase of recrystallization exhibits higher compositional and structural stability than the reactant phase [17]. Taking dolostone as an example, as recrystallization proceeds, the lattice defects in the original dolostone crystals are replaced by the growth of regular layers, resulting in lower surface free energy and higher stability [18]. In terms of elemental composition, the stoichiometry of dolostone and the degree of cation ordering become closer to those of ideal dolostone with an increasing degree of recrystallization [19,20]. Factors influencing the coupling between the physical transport of fluids and solutes and changes in rock properties (such as porosity and permeability) have been a focus of research [17,21].

Pore fluid pressure is a non-negligible factor in mineral–fluid reactions, and the mass transfer and volume changes that occur under varying stress conditions during fluid-driven reactions significantly influence the morphology and structure of the resulting reaction product phases [21,22]. Minerals are more prone to dissolution in high-stress areas and then precipitate at low-stress positions [23], which is associated with the increased solubility of minerals under high stress. In addition, deformation caused by high stress can induce “dynamic recrystallization” in dolostones, which is a mechanism driven by surface energy to accommodate strain, resulting in changes in grain size related to the level of differential stress [24,25]. Duan (2011) observed that under varying pore fluid pressures, calcite can produce two distinctly different product phases: pore-developed magnesite and dense, platy brucite [26]. This experiment corresponds with our research focus on the differentiated pore structure of carbonate rocks subsequent to recrystallization. Furthermore, carbonate reservoirs with potential for hydrocarbon exploration frequently display evidence of fluid overpressure. For instance, the Cambrian Dengying Formation gas reservoir in the Anyue gas field in Sichuan is a supergiant gas field formed under the seal of high-pressure mud shale [27]. The Feixianguan Formation, Changxing Formation, and the underlying Longtan Formation and Maokou Formation in the Yuezhai structure of Eastern Sichuan constitute an overpressure–strong overpressure system with a pressure coefficient greater than 1.5 [28]. And the intercrystalline pore-bearing microbial dolomite reservoirs in the western part of the Qaidam Basin also commonly exhibit overpressure characteristics [29]. It is evident that pore fluid pressure plays a crucial role in the evolution of pore structures subsequent to recrystallization in carbonate formations.

In the context of mineral–fluid reaction systems, laboratory-scale simulation experiments have emerged as a robust and well-established methodology [30,31,32,33]. To elucidate the influence of pore fluid pressure on the recrystallization of carbonate rocks, we designed a dual-pressure reaction vessel apparatus with independently controllable temperature, axial pressure, and fluid pressure to simulate the recrystallization process of carbonate minerals during burial. To ensure that the experimental conditions more closely approximate geological realities, we selected the sub-salt crystalline dolomite reservoir of the Majiagou Formation in the Ordos Basin as the subject of our simulation. The Majiagou Formation hosts a crystalline dolomite reservoir that exhibits favorable exploration potential, attributed to the presence of an overlying saline layer [34,35]. The presence of a tight cap rock also constitutes a foundation for the development of fluid overpressure conditions [36,37]. In this study, We conducted a series of experiments under different fluid pressure conditions with dolostone samples reacting with Mg-Ca system solutions. Scanning electron microscopy (SEM) and computed tomography (CT) scanning were utilized to qualitatively and quantitatively characterize the crystal morphology and pore structure of experimental products after the reactions. Additionally, we integrated modeling calculations to assess the impact of recrystallization on reservoir permeability, thereby clarifying the favorable fluid pressure conditions for the development of recrystallized carbonate reservoirs.

2. Methods

2.1. Experimental Setup

Our simulation experiments on carbonate rock recrystallization utilized a custom-designed dual-pressure reaction vessel apparatus (Figure 1), which is capable of exerting axial pressures up to 200 MPa, fluid pressures up to 40 MPa, and temperatures up to 200 °C. This parameter range enables the simulation of mineral–fluid rock reaction conditions for carbonate rocks at burial depths ranging from 0 to 5 km.

The dual-pressure reaction vessel apparatus consists of (1) a heating system, which is composed of a sample chamber and an external sleeve-type heating device. The sample chamber can accommodate powder and fluid samples, and the temperature control of the samples is achieved through the external sleeve heating device. It also consists of (2) an axial pressure loading system, which is achieved by a servo-controlled pump and a control circuit board. When axial pressure is applied, the movable grain samples within the chamber transmit the axial load to the surrounding areas in the form of force chains through compression. Under the interaction with the rigid inner walls, a confining pressure is generated that approximates the conditions of burial [38]. Next, the apparatus has (3) a fluid pressure controlling system, composed of a hand pump and a back pressure valve. Before the experiment, a pre-set fluid pressure was introduced into the back pressure valve using a hand pump. As the piston incrementally applies pressure to the sample within the chamber, once the fluid pressure exceeds the set value within the back pressure valve, the fluid will be released to depressurize, thereby stabilizing the fluid pressure within the chamber at the predetermined range. Additionally, a filter plate was installed within the chamber to prevent the expulsion of solid samples along with the fluid. Finally, the apparatus has (4) a measurement system, composed of an external computer and pressure–temperature sensors, which is capable of precisely controlling and recording the temperature, axial pressure, and fluid pressure within the sample chamber during the experimental process.

2.2. Experimental Condition

The starting material was natural dolostone, which had not been calcined or acid-etched, with a purity of up to 99.5%. The dolostone was crushed and ground in an agate mortar and then sieved to ensure the grain size was ground to 200 mesh (≤75 μm). The starting fluid was a solution containing 0.08 M MgCl₂, 0.06 M CaCl₂·2H₂O, and 0.28 M NaCl [20,39]. This particular solution composition was preferred as (1) it has an ionic strength similar to that of sea-water and (2) Kaczmarek and Sibley (2011) showed that dolomite recrystallization occurs more rapidly with such solution compositions than for solutions containing a lower Mg:Ca ratio [30].

During the experiment, 133.7 g dolostone powder was placed into the sample chamber, followed by the addition of 150 mL of the prepared solution to thoroughly mix with the powder. After filling, a high-temperature ring pressure sleeve paired with a heat-resistant rubber ring served as the sealing component, preventing leakage at the interface between the pressure sleeve and the fluid composition and ensuring the integrity of the seal within the entire chamber. All experiments were allowed to react continuously for 6 days.

The settings for temperature and pressure were designed to simulate the geological conditions at a burial depth of 2000 m in the Majiagou Formation of the Ordos Basin. The experimental temperature was established at 120 °C, referencing the thermal evolution history of the Mahuangou Formation [40]. The axial pressure was calculated based on the actual burial depth, rock frame density, and other parameters, using the formula ρ₀ = 10⁻³H[(1 − ϕ)ρ_g + ρ_f] g (MPa) [41], where ρ_g is the skeletal density of the overlying rock, g/cm³; ρ_f is the interlayer pore fluid density, g/cm³; H is the burial depth, m; and ϕ is the porosity. In this experiment, an approximate axial pressure value of 55 MPa was determined. The hydrostatic pressure at a certain burial depth could also be derived from the formula P_f = 10⁻³Hgρ_f (MPa). The hydrostatic pressure is about 20 MPa at a burial depth of 2000 m. When the pore fluid pressure exceeds the hydrostatic pressure at that depth during the burial process, it is considered to be in a state of fluid overpressure [42]. The pressure coefficients (pore pressure/hydrostatic pressure) of carbonate reservoirs with fluid overpressure in realistic exploration are generally above 1.3 [28,43]. Therefore, we set the pressure coefficient above 1.3 in the experimental groups of fluid overpressure.

In order to compare the experimental results of dolostone recrystallization under hydrostatic pressure and fluid overpressure, we conducted three types of experiments. Experiment 1 served as a blank control group, where only axial pressure and temperature were applied to the dolostone powder without the addition of solution. Hence, the dolostone in this group did not undergo recrystallization. Experiment 2 was the hydrostatic pressure group with the solution added, and thus the dolostone underwent recrystallization under normal fluid pressure. Experiments 3 and 4 were the fluid overpressure groups, where all conditions were set the same as the hydrostatic pressure group except for the pore pressure. The overpressure coefficients for the two experiments were designated as 1.4 and 1.8, respectively.

2.3. Characterization of Reaction Products

After each experiment, some of the experiment products were selected for SEM observation. The fragments were washed with distilled water and dried at room temperature for 24 h. The small block samples were embedded into an epoxy resin block and polished using diamond suspension sizes ranging from 9 to 1 µm. The microstructures and compositions of the reacted fragments were analyzed by JCM-7000 NeoScope (JEOL Limited, Tokyo, Japan) at PKU with a beam current set at a voltage of 15 kV and a working distance of ca. 12.5 mm with a high-vacuum mode.

For the experiment samples used in the CT analysis, untreated products with a certain degree of consolidation after reaction were used. The CT characterization was performed using the VtomeX, manufactured in Germany, which is distinguished by the following features: ① dual X-ray sources (180 kV nanofocus X-ray source and 240 kV microfocus X-ray source), enabling the examination of samples of varying sizes; and ② a high-precision flat-panel detector with a spatial resolution capable of reaching 0.27 μm. The samples from Experiments 2, 3, and 4 were prepared into cylindrical specimens measuring 25 mm × 40 mm. Therefore, a scanning resolution of 20 μm was selected to directly scan the cylindrical specimens in order to acquire higher-resolution micro-porous and throat structural parameters.

3. Results

Each set of experiments was reacted for 6 days according to the set conditions (Table 1), and the temperature and pressure values at any moment were recorded by the computer (Figure 2). The experiment products after reaction were divided into two parts for SEM observation and CT analysis, respectively.

3.1. SEM Features

The analysis of the SEM images from different experimental results reveals that the dolostone crystals in the blank group are fragmented and incomplete, and after mechanical compaction, they form a dense stack of lamellar aggregates (Figure 3b–d), with almost no pore space present. When fluid is involved, the hydrostatic pressure group and overpressure group both show different degrees of recrystallization. Specifically, the hydrostatic pressure group shows crystals with incomplete crystalline profiles (Figure 3f,g), with grain sizes up to about 100 μm, and signs of dissolution are visible at the crystal steps (Figure 3h). The overpressure groups exhibit a higher degree of recrystallization overall, with larger crystals and more complete crystal forms (Figure 3j,l). Experiment 3 shows traces of crystal growth along the lattice defects (Figure 3k), with a maximum grain size up to about 200 μm. Crystal growth at the microscopic level can be explained by the uneven distribution of matter within the crystal lattice, which generates stress within the lattice. When this stress exceeds a certain threshold, the lattice undergoes shear displacement along a particular plane, leading to the formation of a spiral dislocation. The crystal elements then grow outward along the concave angles located at the dislocations [17]. The Experiment 4 crystals generally exhibit a high degree of euhedral development (Figure 3n–p). Comparing the overpressure group with the hydrostatic pressure group, the crystals appeared to be different in grain size, euhedral development, and distribution arrangement, reflecting a trend that higher fluid pressure leads to a higher degree of recrystallization.

3.2. CT Analysis Results

The blank group only underwent mechanical compaction, resulting in extremely poor porosity; hence, only the three experiment groups with fluid involvement were subjected to CT scanning (Figure 4a–c) to obtain their pore structure characteristics (Table 2). Experiment 2 under the hydrostatic pressure condition has the highest number of pores, with an average pore radius of 64.25 μm and a porosity of 18.93%. Experiment 3 under the overpressure condition has fewer pores than Experiment 2, with an average pore radius of 86.53 μm and a slightly increased porosity of 21.98%. The results of Experiment 4 were similar to those of Experiment 3, with pore structure parameters significantly better than those of Experiment 2, having an average pore radius of 88.43 μm and a porosity of 23.09%.

Although Experiment 2 has a significantly higher number of pores compared to Experiments 3 and 4, the porosity in Experiments 3 and 4 is greater owing to the larger average pore radii. This could be explained by the fact that the crystal dissolution–reprecipitation process alters the distribution pattern of crystals and pores. As a result, the originally isolated small pores become reconnected, forming larger pores, which leads to an increase in porosity with a simultaneous reduction in the total number of pores. Pore connectivity is a crucial parameter for evaluating reservoir quality and significantly constrains the reservoir permeability [44]. The interconnection of previously isolated small pores directly enhances pore connectivity, which also accounts for the observed significantly higher pore connectivity in Experiments 3 and 4 compared to Experiment 2 (Figure 4e).

4. Discussion

4.1. Effect of Fluid Pressure on Product Structure

In our experiments, aside from the solution composition (which provides the essential Ca²⁺ and Mg²⁺ for the recrystallization of dolostone) [45], temperature and pressure almost entirely control the entire dissolution–reprecipitation reaction process. Temperature is the primary factor in controlling the rate of recrystallization. The Arrhenius equation intuitively reflects the exponential relationship between the reaction rate and temperature, and there is experimental evidence that increasing the temperature appropriately can greatly accelerate the recrystallization process of dolomite [46,47]. In our case, in Experiments 2, 3, and 4, the reaction temperatures were all set to 120 °C. After the same reaction duration, the dolostone crystals should theoretically exhibit the same degree of recrystallization. However, it can be observed that dolostone under fluid overpressure shows a superior recrystallization extent (Figure 3i,n) compared to the hydrostatic pressure group (Figure 3f,g). This indicates that in our experiments, the temperature setting merely served to reach the threshold for recrystallization to occur. Excessively high temperatures could lead to rapid crystal growth, conversely resulting in a disordered filling state. The true determinant of the product pore structure is the pore fluid pressure.

In the field of rock mechanics, the concept of effective pressure (Pe) is commonly used to quantify the degree of compaction, Pe = σ_n − aP_f, where σ_n is the overlying formation pressure (Pa), a is Biot’s coefficient (ranging from 0 to 1), related to the bulk modulus of the sample with and without empty pores [42,48], and P_f is the pore fluid pressure. In our case, it is appropriate to assume that Biot’s coefficient a is close to 1 [48]. In all experiments, the axial load could be approximated to the overlying formation pressure σ_n, which is uniformly 55 MPa. In Experiment 1, without fluid pressure protection, the effective pressure is 55 MPa, leading to intense compaction and severe pore destruction (Figure 3b). In Experiments 2, 3, and 4, the effective pressures are 35 MPa, 27 MPa, and 19 MPa, respectively. Theoretically, the fluid overpressure groups, with high pore fluid pressure protection, would weaken the compaction effect, thereby preserving more pore space (Figure 5), and the higher the fluid pressure, the greater the preservation of porosity. The CT scanning data of the experiment results (Table 2) precisely confirm this conclusion, with Experiment 4 having the highest porosity and Experiment 2 the worst.

High fluid pressure can resist compaction, thereby preserving better pore space. On one hand, the process of recrystallization requires continuous mass transport, and these pores provide unobstructed fluid migration pathways for ion exchange, ensuring the continuous progress of recrystallization. On the other hand, the euhedral growth of dolostone crystals also requires certain spatial constraints [49], and these pores provide ample space for the regular and orderly growth of dolostone crystals. The dissolution of small dolostone crystals and the formation of larger crystals with more uniform grain sizes and orderly distribution also optimizes pore connectivity to a certain extent as a consequence (Figure 5). It is apparent that in our experiments, fluid overpressure affects the mass transport channels and the space for crystal growth and arrangement, resulting in a better pore structure after recrystallization.

However, in natural geological settings, when the intensity of recrystallization reaches a certain level, the growing dolomite crystals can occlude pore space, which is also referred to as “over dolomitization” [50,51]. As the degree of recrystallization increases with progressive burial, planar-e dolomite with well-developed intercrystalline pores evolves into nonplanar dolomite with pore occlusion [52,53]. This could be attributed to the high ionic strength of basin brines and the episodic influx of external fluids, which have led to the excessive recrystallization of dolomite and even cementation. In contrast, our experiments simulate a closed burial system caused by the sealing effect of evaporite layers. The influx of external fluids is restricted. Meanwhile, the relatively high thermal conductivity of the evaporite layer can effectively reduce the diagenetic temperature of the underlying strata, thereby mitigating the intensity of diagenesis [54]. This ultimately restricts the degree of dolomite recrystallization and preserves a better pore structure.

4.2. Implication to Reservoir Property Improvement

Pore tortuosity (the ratio of the average effective flow path length to the straight-line length in the direction of flow) and the pore–throat ratio (the ratio of the radius of the pore to that of the throat) are important parameters characterizing the heterogeneity of the pore structure and significantly constrain the permeability of the stratum [55,56,57]. Recrystallization, by adjusting the morphology and arrangement of mineral crystals, significantly alters the pore tortuosity and pore–throat ratio, thereby affecting the permeability of the stratum.

To quantify the relationship between the changes in pore structure induced by recrystallization and permeability, we developed a theoretical calculation model. The Kozeny–Carman equation, as the fundamental basis for many seepage calculation models, has been widely applied in various fields such as subsurface seepage, oil and gas field development, chemical engineering, and electrochemistry [58,59]. Following Li’s (2007) modeling approach, we introduced pore tortuosity and the pore–throat ratio to establish the Kozeny–Carman (K-C) equation applicable to our study [60]. In a uniform capillary medium model (Figure 6), the K-C equation can be expressed as K = ${\displaystyle \frac{\varphi r^2}{8{\tau }^2}}$_, where φ is the porosity, r is the pore radius, and τ is the pore tortuosity [55]. This approach considers the pores as equal-diameter tubes. However, in actual reservoirs, pores are interspersed with varying sizes, where the larger compartments are classified as pores and the smaller ones as throats (Figure 6). To enhance the fidelity of our model to geological conditions, we introduced the pore radius r_p and the throat radius r_t to refine the equation. In the idealized model, the pores and throats are of equal length, and the pore–throat ratio is expressed as Ra = ${\displaystyle \frac{r_p}{r_t}}$. After adjusting the relative parameters such as the flow rate and porosity, the refined equation is K = ${\displaystyle \frac{\phi {R_a}^2 {r_p}^2}{2\left(1+{R_a}^4\right)\left(1+{R_a}^2\right){\tau }^2}}$ [60].

To visually delineate the impact of pore tortuosity and the pore–throat ratio on permeability, we took the Majiagou Formation in the Ordos Basin as a reference object and calculated a series of permeability values by considering pore tortuosity and the pore–throat ratio as single variables, respectively. By analyzing the physical data of the crystalline dolomite reservoirs of the Majiagou Formation, we adopted an average porosity of 2.5% and an average pore radius of 50 μm. When the pore tortuosity is considered as the sole variable, its value typically ranges from 1 to 5 [61]. Given an average pore–throat ratio of 20, we employed the Kozeny–Carman equation to determine the permeability across various degrees of tortuosity (Table 3). Similarly, when considering the pore–throat ratio as the single variable, we took an average tortuosity of 2.5 and calculated the corresponding permeability (Table 4).

Pore tortuosity is inversely proportional to permeability (Figure 7). When the pore paths are relatively straight (τ < 2), the permeability is significantly affected by tortuosity, and small changes in pore tortuosity can cause noticeable changes in permeability. However, when the pore paths are already quite tortuous (tortuosity > 2), although an increase in tortuosity still decreases permeability, the sensitivity of the response between the two gradually decreases. The pore–throat ratio has an almost destructive effect on permeability (Figure 7), with an increase in the pore–throat ratio leading to an exponential decrease in permeability. Corresponding to the actual geological situation, the larger the pore space, the higher the permeability should be, but if the throat is small, the large pore space cannot produce high formation permeability, which is the “bottleneck” effect in the formation rock [62].

Our experiments show that dolostone recrystallization under fluid overpressure leads to more regular crystal morphology and a homogeneous grain size. The straight crystal shapes and orderly arrangement result in more direct pore channels, reducing pore tortuosity (Figure 8). Similarly, the uniformity of crystal grain sizes reduces the pore–throat ratio (Figure 8). The optimization of these two pore structure parameters reduces the reservoir inhomogeneity and consequently largely improves the seepage capacity of the reservoir. This is well corroborated with the pore connectivity measured by our experimental products (Figure 4e). In summary, the recrystallization of carbonate rocks under fluid overpressure has a role in adjusting and improving pore structure, with a key focus on enhancing the reservoir permeability.

5. Conclusions

Recrystallization, as a fluid-mediated dissolution–reprecipitation reaction, is governed by ultra-local conditions [32,64]. Pore fluid pressure, a pivotal factor influencing mineral–fluid reaction interfaces, significantly impacts the morphological and structural evolution of the product phases. Furthermore, in the realm of hydrocarbon exploration, numerous crystalline dolomite reservoirs have been proven to have overpressure systems, indicating that pore fluid pressure is crucial in the recrystallization of carbonate rocks. Our experiments confirm that under fluid overpressure, compaction is mitigated, allowing for the better preservation of pore space. These pores act as conduits for mass transport during the reaction and ensure that minerals have adequate space for euhedral growth. Consequently, dolostones recrystallized under fluid overpressure demonstrate enhanced euhedral development and a uniform grain size distribution. Macroscopically, this results in a decrease in reservoir heterogeneity and an optimization of structural parameters, including pore tortuosity and the pore–throat ratio, thereby significantly enhancing reservoir permeability.

Our experiments provide an interpretation for the widespread development of intercrystalline dolomite reservoirs in the Majiagou Formation beneath the salt layers in the Ordos Basin. The sealing effect of the evaporite layer is a key factor in the development of crystalline dolomite reservoirs. On the one hand, the overpressure caused by the sealing of the evaporite layer is an important prerequisite for the development of intercrystalline pores after recrystallization, which has been confirmed by our experiments. Meanwhile, the lack of external fluid supply in a closed system prevents the occurrence of over recrystallization. On the other hand, the relatively high thermal conductivity of the evaporite layer can reduce the diagenetic temperature of the underlying strata, thereby slowing down the diagenetic intensity and, to some extent, avoiding pore occlusion caused by intense recrystallization or cementation. Evidently, this interpretation is not only applicable to the Majiagou Formation in the Ordos Basin but may also be relevant to other carbonate formations with evaporite seals, which could potentially serve as prospective reservoirs. This also offers an alternative approach for our carbonate hydrocarbon exploration, namely the search for stratigraphic units with the potential for overpressure, such as those with evaporite seals or shale layers.

Author Contributions

Conceptualization, writing—original, investigation, visualization, S.C.; writing—review and editing, project administration, B.L.; funding acquisition, resources, X.L., L.W. and H.B.; software, formal analysis, R.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

All data generated and analyzed during this study are included in this published article.

Acknowledgments

We are grateful to the reviewers for their helpful comments that improved the manuscript.

Conflicts of Interest

Authors Xinshe Liu, Liubin Wei and Hongping Bao are employed by the company PetroChina. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Schematic diagram of dual-pressure reaction vessel apparatus.

Figure 2. Temperature and pressure recordings during the experiment: (a) is blank group Experiment 1; (b) is hydrostatic pressure Experiment 2; (c) is overpressure Experiment 3; (d) is overpressure Experiment 4.

Figure 3. SEM images of experimental results: (a) blank group Experiment 1 product; (b–d) images of blank group, tightly stacked; (e) hydrostatic pressure experiment product; (f,g) low degree of crystal growth in the hydrostatic pressure; (h) dissolution at the crystal steps; (i) overpressure group Experiment 3 product; (j–l) euhedral crystals in Experiment 3; (k) crystal growth outward along the concave angles; (m) overpressure group Experiment 4 product; (n–p) crystals with significant recrystallization in Experiment 4; (p) locally enlarged image of n.

Figure 4. CT scanning results indicate the improvement of the pore structure: (a) CT imaging of Experiment 2; (b) CT imaging of Experiment 3; (c) CT imaging of Experiment 4, green represents the pore space; (d) comparison of connectivity porosity in three sets of experiments; (e) comparison of pore connectivity in three sets of experiments.

Figure 5. Formation of better pore structure facilitated by high fluid pressure.

Figure 6. Modeling of porous media. Left shows the uniform hair bundle tube model. Right shows pores and throats in the actual pore structure.

Figure 7. Relationship between pore tortuosity, pore–throat ratio, and permeability.

Figure 8. Improvement in pore tortuosity and the pore–throat ratio by recrystallization. The position of the dashed line shows a magnified view of the pore structure (modified from Chen et al., 2024) [63].

Table 1

Summary of dolostone recrystallization experiments and relative conditions.

	Blank Group	Hydrostatic Group	Overpressure Group
No.	No. 1	No. 2	No. 3	No. 4
Solid	Dolostone Particle	Dolostone Particle	Dolostone Particle	Dolostone Particle
Solid weight (g)	133.7	133.7	133.7	133.7
Initial liquid volume (mL)	/	150	150	150
Volume of liquid discharged (mL)	/	112	96	88
Volume of remaining liquid (mL)	/	38	54	62
T (°C)	120	120	120	120
P axial (MPa)	55	55	55	55
P hydrostatic	/	20	20	20
P pore	/	20	28	36
Pressure coefficient	/	1	1.4	1.8
Time (days)	6	6	6	6

Table 2

Pore structure parameters based on CT scanning.

Content			Hydrostatic PressureNo. 2	OverpressureNo. 3	OverpressureNo. 4
Sample	Resolution/μm		20	20	20
	Connected pores/%		17.87	21.70	22.94
	Porosity%		18.93	21.98	23.09
	Pore connectivity/%		94.41	98.73	99.35
Pore	Quantity		90,778	77,472	78,678
	Radius/μm	Max	382.31	402.38	413.44
		Min	24.79	24.82	25.33
		Average	64.25	86.53	88.43

Table 3

Calculated permeability for different degrees of pore tortuosity.

τ	1	1.5	2	2.5	3	3.5	4
K (mD)	0.195	0.087	0.049	0.031	0.022	0.016	0.012

Table 4

Calculated permeability for different degrees of pore–throat ratios.

Ra	1	2	5	10	50	100	200
K (mD)	1250	235	7.68	0.495	8 × 10−4	5 × 10−5	3 × 10−6

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