Modification of dolomitization on reservoir spaces in reef–shoal complex: A case study of Permian Changxing Formation, Sichuan Basin, SW China

Qiang Zhang; Lizhi Liu; Honglu Song; Chaosu Jin; Na Qiao; Lin Qi

doi:10.1515/geo-2022-0734

Article Open Access

Modification of dolomitization on reservoir spaces in reef–shoal complex: A case study of Permian Changxing Formation, Sichuan Basin, SW China

Qiang Zhang , Lizhi Liu , Honglu Song , Chaosu Jin , Na Qiao and Lin Qi

Published/Copyright: December 5, 2024

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From the journal Open Geosciences Volume 16 Issue 1

Abstract

The porosity of the reef–shoal complex increases with the increase of dolomite content, and it is worth investigating whether dolomitization leads to the formation of new pores or maintains the original porosity. The reef–shoal complex of the Permian Changxing Formation in the Sichuan Basin is selected to reveal the modification effect of dolomitization on the reef–shoal complex. As important reservoirs of hydrocarbons and carbon dioxide, reef–shoal complexes are composed of grainstone and framestone on both sides of the Kaijiang-Liangping Trough during the Late Permian. The difference in the development degree of sparry calcite cement determines whether the framestone on both sides of the Trough undergoes dolomitization or not. When the Mg²⁺ supply is insufficient, the dolomitization fluid prioritizes the replacement of the lime mud matrix. When the Mg²⁺ supply is sufficient, the reef-building organisms and bioclastic grains are completely replaced by dolomite. Statistical results show that when the dolomite content increases from 0 to 40%, there is no significant change in porosity. When dolomite content increases from 40 to 60%, the porosity gradually increases slightly. When dolomite content exceeds 60% and continues to increase, the porosity increases sharply. For reef–shoal complex reservoirs, dolomitization did not lead to the formation of new pores, but effectively preserved primary intergranular pores and freshwater leaching-induced pores. The porosity increases with the increase of dolomite content, partly due to pore preservation and partly due to the more effective anticompaction (and pressure solution) effect of dolomite compared to calcite, reflecting the overall effect of dolomite pore preservation. Although early freshwater leaching played a positive role in the formation of dolomite reservoirs, the preservation effect of dolomite pores contributes more to the current porosity and permeability of reservoirs, resulting in the highest quality reservoirs of reef–shoal complexes often developing at a certain distance below the exposed surface, and the best reservoir is dolomite in the upper part of the reef–shoal complexes, rather than dolomite interval located at the top.

Keywords: reef; shoal complex; dolomitization; diagenetic sequence; pore preservation; Changxing Formation; Sichuan Basin

1 Introduction

The dolomitization process is a metasomatism reaction in which dolomite minerals gradually replace calcite or aragonite minerals, manifested microscopically as a coupled process of dissolution of reactants such as calcite and aragonite, accompanied by precipitation of dolomite [1]. Dolomite reservoirs are widely distributed in major oil and gas basins around the world, generally with higher porosity than limestone, and are important oil and gas exploration targets in various basins [2,3,4,5,6,7]. Taking the Sichuan Basin as an example, the proven geological reserves of natural gas in dolomite formations are close to 2 × 10¹² m³ [8,9]. Therefore, research on the genesis of dolomite pores has attracted great attention, and how dolomitization affects the formation and evolution of carbonate reservoir pores is currently a controversial scientific issue [10,11,12,13]. In recent years, three mainstream views have been proposed. The first view is that the process of transforming calcite and aragonite into dolomite is a process of reducing the molar volume, which is conducive to generating new reservoir spaces [10,11]. The second viewpoint is that dolomitization only enhances the anti-compaction ability of reservoir spaces and cannot add new reservoir spaces [2,14,15]. The third viewpoint is that a strong dolomitization process can reduce reservoir spaces and cause pore cementation [16,17]. Therefore, most studies believe that different diagenetic environments, dolomitization patterns, degrees of dolomitization, and differences in the structure of limestone parent rocks may result in different alteration outcomes [7,18].

Reef–shoal complex reservoirs have been proven in many cases to be important storage carriers for hydrocarbons and carbon dioxide worldwide [19,20,21,22]. The strong heterogeneity of reef reservoirs originates from the uncertainty of pore distribution, which directly affects hydrocarbon production or the effectiveness of carbon dioxide sequestration. The reported research results mostly use CT [23], mercury intrusion, and other methods to characterize heterogeneity [24,25] or use geophysical methods to characterize macroscopic distribution characteristics [26,27]. From the perspective of genesis, there is a lack of research on the heterogeneity of reef reservoirs. In the Eastern Sichuan Basin, statistical data show that after the dolomitization degree of the Changxing Formation reef–shoal complex reservoirs exceeds 50%, there is a similar positive correlation between porosity and dolomite content as mentioned above [28]. For dolomite reservoirs, the differences in the internal structure are reflected in the differences in porosity and permeability. Due to the continuous destruction process of precursor structure, it will inevitably be accompanied by changes in the internal structure. Analyzing the evolution process of the dolomite structure is beneficial for exploring the alteration effect of dolomite diagenesis on the reef–shoal complex. At the end of the Permian, platforms in the Eastern Sichuan Basin were separated by the Kaijiang–Liangping Trough. The reef–shoal complex reservoirs on the east and west sides of the Trough had different degrees of dolomitization and internal structures. The framestone on the west side of the Trough has basically preserved the original rock structure. This study compares the similarities and differences in dolomite types, cement types, and porosity characteristics of reef–shoal complex reservoirs on both sides of the Trough. Combining geochemical characteristics, it explores the evolution of the dolomite structure and pore structure and quantitatively estimates the contribution of dolomitization to reservoir pores.

2 Geological background

The study area is located in the Eastern Sichuan Basin (Figure 1a). The Sichuan Basin, located between Sichuan Province and Chongqing City, SW China (Figure 1b), is a nearly diamond-shaped intracratonic basin with pre-Sinian basement, covering an area of 230,000 km² with 12 km-thick sedimentary rocks (Figure 1c). The eastern region of the Sichuan Basin is a key area for natural gas exploration in China. The studied strata (Changxing Formation) date back to 254.1–251.9 Ma, which have been deposited in South China. In recent years, the Permian Changxing Formation reef–shoal complex gas reservoirs discovered are mainly distributed around the Kaijiang–Liangping Trough [29,30]. Controlled by the structural characteristics of the Trough, there are significant differences in the sedimentary features of the Changxing Formation on its east and west sides [31]. The east side of the Trough is a fault block boundary composed of horsts and trenches, while the platform margin is of the fault step slope type [32,33]. The platform on the east side is relatively high, and the sedimentary water bodies are shallower than those on the west side (Figure 2). The platform margin belongs to the ramp type, with a gradual transition from the interior of the platform [13]. The Changxing Formation is in conformity contact with the underlying Permian Longtan Formation and the overlying Triassic Feixianguan Formation (Figure 1c). The Permian–Triassic boundary lies between the Changxing and Feixianguan formations, which marks the largest mass extinction in world history [18].

Figure 1

(a) The spatial distribution map showing the study area and drilling wells in the Eastern Sichuan Basin. (b) Distribution map showing the sedimentary facies of Changxing Formation (modified from the study of Shen et al. [13]). (c) Generalized stratigraphy and tectonic history of Eastern Sichuan Basin (modified from the study of Hao et al. [35]). Sym. = Symbol.

Figure 2

Diagram indicating platform-margin types for the Kaijiang–Liangping Trough in the Eastern Sichuan Basin (modified from the study of Shen et al. [13]).

3 Samples and methods

At first, 268 samples from drilling cores of the Changxing reef–shoal complex were doubly polished, followed by blue epoxy impregnation and staining with Alizarin red for identification of calcite and dolomite. Then, petrographic relationships of calcite and dolomite were delineated using a CL8200MK5 cathodoluminescence (CL) microscope in the PetroChina Key Laboratory of Unconventional Oil and Gas Resources. The classification of Dunham (1962) was used for microscopic identification [34]. Sr isotope data were measured using a Finnigan Triton thermal ionization mass spectrometer (TIMS). The C–O stable isotope data were measured by a MAT253 isotope mass spectrometer in the PetroChina Key Laboratory of Unconventional Oil and Gas Resources. The δ¹⁸O and δ¹³C data are normalized to the Vienna Pee Dee Belemnite (V-PDB) standard and corrected by fractionation factors proposed by Fairchild and Spiro [36]. Homogenization temperatures (Th) of 32 samples were measured through a Linkam THMSG600 system (testing range: −196°C–600°C, accuracy: 0.2°C). A total of 352 drilling core samples were measured for their content of dolomite and calcite by the X-ray diffraction system made by the Netherlands Panalytical Company. Correspondingly, 352 porosity values and 183 permeability data were measured by high-pressure porosity tester and high-pressure permeability tester at the PetroChina Key Laboratory of Unconventional Oil and Gas Resources.

4 Results

4.1 Petrological characteristics

4.1.1 Framestone on the west side of the Trough

The framestones on the west side of the Trough are mainly distributed in the middle and upper parts of the reef–shoal complex and are mostly located in the upper part of the meter-level cycles that become shallower upwards (Figure 3). According to the observation results of cores and thin sections, the species diversity and abundance inside the framestone are very high, and the reef community is composed of reef-building organisms (including frame-building organisms and encrusting organisms) and reef-dwelling organisms. The frame-building organisms are composed of a large number of Sphinctozoa and Parauvanella (Figure 4a), among which Parauvanella has the highest relative abundance (Figure 4b and c). The encrusting organisms are composed of Archaeolithoporella wrapped around the frame-building organisms. Reef-dwelling organisms in Changxing Formation are composed of Tubiphytes and Stictoporellidae. The multi-stage sparry calcite cement (SCC) not only occupies the internal space of frame-building organisms (Figure 4d and e) but also occupies almost all the pore and vug spaces between the organisms (Figure 4f and g). The bioclastic grainstone at the top of the framestone has undergone strong dolomitization. The intensity of dolomitization gradually decreases until it disappears from the top to the middle of the reef–shoal complex, so the framestone has not undergone significant dolomitization and exhibits dull or non-luminescence under CL.

Figure 3

Macroscopic characteristics of the reef–shoal complex in Changxing Formation and thin-section photomicrographs on the west and east sides of the Kaijiang–Liangping Trough. (a) and (b) Well LH002-X2, 3890.82 m. Plane polarized light (PPL) and CL. (c) Well TS5, 3143.35 m PPL. (d) Well TS5, 3143.35 m PPL. (e) Well TS5, 3149.4 m PPL. (f) Well TS5, 3156.93 m PPL. (g) Well LG84, 4515.69 m PPL. (h) Well LG84, 4517 m PPL. (i) Well QLB 2, 5427.66 m PPL. (j) Well F003-X3, 4849.5 m PPL. (k) Well B001-1, 4109.37 m PPL. (l) Well B001-1, 4108.43 m PPL. (m) Well YA002-5, 4569.8 m PPL. (n) Well YA002-5, 4605.67 m PPL. (o) Well YA002-5, 4610.51 m PPL. (p) Well YA002-5, 4611.5 m PPL.

Figure 4

Thin-section photomicrographs of framestone from the Changxing Formation. (a) Sphinctozoa and Tubiphytes, Well TS5, 3097.31 m. (b) Vug between frame-building organisms is cemented by second-stage SCC, Well TS5, 3149 m. (c) Vugs in Parauvanella are cemented by second-stage SCC, Well TS5, 3149 m PPL. (d) and (e) Framestone exhibits non-luminescence under CL, Well TS5, 3097.31 m PPL and CL. (f) and (g) Framestone exhibits non-luminescence, Well TS5, 3149 m PPL and CL. (h) Framestone on east side of the Trough, Well QLB2, 5441.72 m. (i) Framestone composed of Solenopora, Well YA002-5, 4613.35–4613.44 m. (j) Framestone composed of Peronidella, Well YA002-5, 4611.66 m. (k) and (l) Framestones are partially dolomitized, Well YA002-5, 4613.44 m PPL and CL. (m) and (n) Framestone are partially dolomitized, Well YA002-5, 4611.66 m PPL and CL.

4.1.2 Framestones on the east side of the Trough

The framestones on the east side of the Trough are mainly distributed in the middle and lower parts of the reef–shoal complex and are mostly located in the upper part of the meter level cycle that becomes shallower upwards (Figure 3). According to the observation results of cores and thin sections, species diversity and abundance inside the framestone are relatively lower than on the west side. The frame-building organisms are composed of a large number of Parauvanella (Figure 4h), Solenopora (Figure 4i), and Peronidella (Figure 4j). Among the building organisms, Solenopora has the highest relative abundance (Figure 4j), indicating a relatively arid sedimentary environment. Archaeolithoporella, which wraps reef-building organisms, constitute encrusting organisms. Tubiphytes and Stictoporellidae constitute reef-dwelling organisms. The lime mud matrix only occupies the space between the frames. The bioclastic grainstone at the top of the framestone has undergone strong dolomitization. The intensity of dolomitization from the top to the middle of the reef–shoal complex has not weakened, so the framestone has been dolomitized, and the part that has been replaced by dolomite under CL exhibits bright red luminescence.

4.1.3 Bioclastic grainstone

Bioclastic grainstones are mainly distributed at the top and bottom of the reef–shoal complex. Bioclastic grainstone is a particle structure composed of shallow water benthic bioclastic grains or intact individuals, which are then filled with lime mud matrix between the grains.

4.1.4 Dolomite phase

4.1.4.1 D1 dolomite

Microscopically, the D1 dolomite consists of subhedral to anhedral dolomite crystals with a size range from 20 to 100 μm (average 70 μm) (Figure 4k). D1 dolomite characterized by planar-S texture selectively replaced lime mud between bioclasts and frame-building organisms (Figure 4l and m). D1 dolomites lack undulatory extinction and generally exhibit non-luminescence under a CL microscope (Figure 4n).

4.1.4.2 D2 dolomite

Microscopically, the D2 dolomite consists of euhedral to subhedral dolomite crystals with a size range from 150 to 250 μm (average: 200 μm) (Figure 5a and b). The D2 dolomite, characterized by planar-E texture, selectively partially replaced first-stage SCC and exhibits dull-red luminescence under a CL microscope (Figure 5c and d). Under conditions of low dolomite content, this fabric also replaced cement between bioclasts (Figure 5e–g). When the content of dolomite is high, the pore types and structures formed earlier have been fully preserved (Figure 5h). The precursor grains and frame-building textures are partially to completely obliterated (Figure 5i–l).

Figure 5

Thin-section photomicrographs of dolomite from the Changxing Formation. (a) D1 dolomite, first-stage SCC and second-stage SCC, Well TS5, 3104.72 m PPL. (b) D1 dolomite, first-stage SCC and D2 dolomite, Well TS5, 3117.3 m PPL. (c) and (d) D2 dolomite, Well LH002-X2, 3871 m PPL and CL. (e) Lime mud matrix in framestone dolomitized by D1 dolomite, Well LG84, 4516.05 m PPL. (f) Lime mud matrix in bioclastic grainstone is dolomitized by D2 dolomite, Well M4, 3468.24 m PPL. (g) Lime mud matrix in bioclastic grainstone is dolomitized by D2 dolomite, Well LG84, 4514.1 m PPL. (h) Bioclastic grainstone is completely dolomitized by D1 dolomite, Well LG84, 4507.77 m PPL. (i) Framestone is completely dolomitized by D1 dolomite, Well B001-1, 4105.41 m PPL. (j) Framestone is completely dolomitized by D1 dolomite, Well B001-1, 4105.41 m PPL. (k) Mold pores in D2 dolomite, Well C24, 2674.50 m. (l) Intergranular pores in D2 dolomite, Well B001-1, 4108.67 m PPL.

4.2 Comparison of diagenesis on both sides of the Trough

4.2.1 Diagenetic sequence

Based on previous reports [28,29,30,31,32], this study proposes that the reef–shoal complexes on both sides of the Kaijiang–Liangping Trough have undergone similar meteorological freshwater leaching after sedimentation; however, the scale and intensity of leaching on the east and west sides are different. The energy of seawater on the west side of the Trough is higher, and the first-stage of submarine cementation represented by SCC is stronger (Figure 5a and b). However, on the east side of the Trough, lime mud cementation is dominant, and SCC is rare (Figure 5i and j). The reef–shoal complexes on both sides of the trough have undergone similar hydrocarbon charging histories. However, based on the differences in rock types between the reef–shoal complexes on both sides of the Trough mentioned above, further diagenetic observations indicate different types of cement, indicating significantly different diagenetic histories.

In the reef–shoal complex on the west side of the Trough, the intergranular cement of the top bioclastic shoal is composed of a lime mud matrix. The cement between the frame-building organisms is a three-stage SCC. The cement in the reef–shoal complex on the east side of the Trough is composed of lime mud, and the SCC is rare.
The framestones on the east side of the Trough have clearly undergone strong dolomitization, while no large-scale dolomitization has been observed on the west side. Before the dolomitization process, the frame-building organisms and their cementing structures controlled the distribution of dolomite. The bioclastic shoal on the upper part of the framestones on both sides of the Trough has been completely altered by dolomitization. The development position and crystal characteristics of dolomite crystals are controlled by the lime mud matrix. The crystal size of dolomite is controlled by the strength of recrystallization.
A large number of mold pores and intergranular dissolution pores were observed, indicating that the dolomite reservoirs on the east side of the Trough has undergone strong fabric-selective dissolution. The pore morphology is closely related to the size of bioclasts. There was no occurrence of fabric-selective dissolution in the framestones on the west side of the Trough, but there were abundant fabric-selective dissolution phenomena in the top shoal.

4.2.2 Geochemistry and fluid-inclusion characteristics

The δ¹³C of D1 dolomite on the east side ranges from 1.39 to 3.80‰, with an average of 2.62‰ (n = 20). The average value of δ¹³C in D2 dolomite on the east side is 3.22‰ (n = 5) [32]. The δ¹³C of D1 dolomite on the west side ranges from 3.32 to 4.53‰, with an average of 3.80‰ (n = 11). The average value of δ¹³C in D2 dolomite on the west side is 3.36‰ (n = 4). The average δ¹³C values of different types of dolomites are basically the same, mainly ranging from 2.60‰ to 3.80‰ (Figure 6a). The range of carbon isotope values is basically consistent with that of coeval seawater sediments (Figure 6b). The δ¹⁸O of D1 dolomite on the east side ranges from −6.64 to 3.37‰, with an average of −4.32‰ (n = 20). The average value of δ¹⁸O in D2 dolomite on the east side is −5.30‰ (n = 5) [32]. The δ¹⁸O of D1 dolomite on the west side ranges from −5.02 to 3.44‰, with an average of −4.08‰ (n = 11). The average value of δ¹⁸O in D2 dolomite on the west side is −7.86‰ (n = 4). There are significant differences in the distribution range of oxygen isotopes among different types of dolomites (Figure 6a): it is significantly lower in D2 dolomite than in D1 dolomite. The average value of D2 on the west side is the lowest, with a decrease of 1.72‰ and 0.25‰, respectively, in the average values of framestone (−6.14‰, n = 8) and bioclastic grainstone (−7.61‰, n = 2). Considering the influence of oxygen isotope fractionation between dolomite/calcite and fluids [43,44], a difference of less than 2‰ in the isotopic mean values does not necessarily indicate different sources of fluids with δ¹⁸O values [45]. The difference in the isotopic mean values is attributed to the difference in diagenetic strength. The oxygen isotope composition shows a trend of increasing negative bias with increasing diagenetic strength. Thus, the dolomitization intensity of D2 dolomite on the west side is the strongest, while the diagenesis intensity of D1 dolomite on both sides of the Trough is similar. The conversion process from D1 to D2 has undergone different processes on different sides of the Trough.

Figure 6

(a) Cross-plot of δ¹³C and δ¹⁸O values for dolomite and limestone from the Changxing Formation. (b) Typical ranges of carbon isotope values from Changhsing carbonate sediments.

As shown in Figure 7a, the D1 dolomites on both sides of the Trough are close to well-preserved brachiopod fossils from the same period, but the average value is slightly lower than that of the fossils. Based on the Sr isotope curves of seawater during the Permian Changhsingian stage [46], the main range of ⁸⁷Sr/⁸⁶Sr values for calcite cement (0.7076–0.7080) is significantly higher than that of seawater during the Changhsingian stage (0.7070–0.7073) [46,47,48], which is consistent with the variation range of ⁸⁷Sr/⁸⁶Sr values for seawater or marine-derived fluids during the Early–Middle Permian and Triassic periods (Figure 7b).

Figure 7

(a) Cross-plot of δ¹⁸O vs ⁸⁷Sr/⁸⁶Sr for different fabrics of the Changxing Formation. (b) ⁸⁷Sr/⁸⁶Sr record of different geological ages, green curve with a light green 95% confidence envelope [50]. The values of well-preserved Changxingian brachiopods and bulk carbonates are from the study of Korte and Ullmann [50].

The microscopic temperature measurement results of fluid inclusions of different minerals are shown in Figure 8. The number of fluid inclusions in the first and second stages of SCC is rare, and two-phase inclusions suitable for temperature measurements have not been observed, suggesting relatively low temperatures of the corresponding diagenetic fluids. Microscopic observations indicate that the presence of calcite in these two stages hindered the further occurrence of dolomitization. The homogenization temperature of the third-stage SCC is mainly distributed between 100 and 120°C [49], indicating that the crystallization process of calcite occurred during the burial diagenesis stage. The fourth-stage SCC exhibits multiple homogenization temperature ranges, namely 120–140°C, 120–140°C, 140–160°C, and 160–180°C, suggesting multiple episodes of saturated calcium carbonate fluid (Figure 8).

Figure 8

Homogenization temperature (Th) distribution of fluid inclusions for different SCC in the Changxing Formation of Eastern Sichuan Basin (the homogenization temperature is from the study of Pan et al. [49] and this study).

5 Discussion

5.1 Factors controlling dolomite texture

5.1.1 Sedimentary characteristics

During the depositional period, the reef–shoal complexes on the east and west sides of the Trough exhibit similar sedimentary cycle characteristics, and the vertical stacking relationship of sediments is roughly the same [32]. Framestones are developed in relatively high-energy areas, while relatively low-energy bioclastic grainstones are developed in the lower part of the reef–shoal complex [27,28,29,30]. The dolomite that preserves the original structure of the frame-building organisms on the east side may have corresponded to the framestone as its precursor. These corresponding rocks often have similar grain sizes, shapes, structures, and pore types, providing key evidence for the above inference. The preservation of the original structure in dolomite may be related to the small particle size and numerous nucleation sites within the precursor limestone. However, sedimentary structures cannot explain the distribution of residual structures and the destruction of original structures.

5.1.2 Meteoric fresh water leaching

The degree of atmospheric freshwater leaching has an impact on the mineral composition of the cement between bioclastic grains and may have a significant influence on the development of the dolomite structure. Under the influence of freshwater, the submarine cement undergoes an alteration from aragonite to low magnesium calcite, and a relatively stable freshwater calcite cement is inevitably formed on its outer rim, resulting in differences in the mineral composition and stability inside and outside these bioclastic grains. The differences in the mineral composition have a strong impact on the subsequent diagenetic pathways, fluid evolution, and pore development processes. In the process of dolomitization, the cement of bioclastic grains is unstable micritic aragonite crystals, which have a much larger specific surface area than bioclastic grains. All these conditions contribute to the rapid selective dolomitization of micritic aragonite cement. The specific surface area of bioclastic grains or frame-building organisms is relatively small, which is not conducive to the continuous occurrence of dolomitization but is beneficial for the preservation of the original structure.

In sharp contrast, in the grainstone interval far away from the exposed surface, intergranular pores continue to retain the original seawater. The neomorphism of aragonite grains and intergranular aragonite submarine cement occurs simultaneously, with the same mineral composition inside and outside the bioclastic grains. Under these conditions, there are no selective dolomitization characteristics. The original internal structure has not been preserved.

The grain shoals and reefs near the exposed surface are subjected to strong freshwater selectively dissolution, forming a large number of intergranular pores or mold pores. These pores are well inherited in D1 and D2 dolomites, including similar developmental positions, sizes, and morphological characteristics. The presence of these pores provides a channel for the supply of magnesium-rich fluids, and D1 dolomite is more prone to recrystallization to form D2. For reef–shoal complexes far away from the exposed surface, the complex that did not undergo leaching was first replaced by D1 dolomite. Subsequent dolomitization fluids cannot enter D1 for recrystallization but instead choose a larger specific surface area lime mud matrix for dolomitization. The structure of dolomite depends on the mineral size and shape characteristics of the limestone precursor. Whether the reef–shoal complex is subjected to freshwater leaching will play a key role in the structural evolution of dolomite.

5.1.3 Recrystallization and late dissolution

The structural differences in the precursor limestone result in a diversity of initial dolomite crystal sizes and structures. This diversity may have a significant impact on the subsequent degree of recrystallization of dolomite. Previous studies have reported a close relationship between the microstructure of different dolomite crystals and recrystallization, confirming that the dolomite structure and fluid geochemistry control the degree of recrystallization of dolomite [51,52]. For the initially formed D1 dolomite, the small and dense crystals determine the relatively insufficient recrystallization fluid and weak water–rock interaction. The changes in the original crystal and structure during subsequent diagenesis are weak. There are also corresponding reports in previous studies on dolomite in other cases. For D1 dolomite with mold pores and intergranular pores from the beginning, abundant pore water will greatly increase the water–rock ratio [2]. The strong recrystallization process formed a more distinct dolomite crystal structure.

Porosity and permeability determine whether diagenetic fluids can flow in porous media and make sufficient contact with dolomite points. At the end of the earliest period of dolomitization, the differences in porosity and permeability of dolomite will trigger varying degrees of recrystallization [53]. At present, it is not possible to restore the initial porosity–permeability of the dolomite in the study area before recrystallization, but the differences in porosity–permeability between different types of dolomite will provide some clues. Framestone and grainstone dolomitized by D2 dolomite are characterized by the highest porosity–permeability value (Figure 9). The dolomite crystal is the coarsest and the original structure is completely destroyed, reflecting the high-intensity recrystallization effect.

Figure 9

Porosity and permeability for dolomite and limestone from the Changxing Formation.

As a transitional product, the physical properties and structural characteristics of grainstone dolomitized by D2 dolomite exhibit a moderate degree of recrystallization. Micrite limestone dolomitized by D1 has a relatively high porosity (Figure 9); however, the small crystals and well-preserved original structure indicate weak subsequent recrystallization. Moreover, the low permeability may affect the flow of the recrystallization fluids. Obviously, there is a good correspondence between the porosity–permeability characteristics of different types of dolomites before recrystallization and the current porosity–permeability characteristics. During the recrystallization process, the pre-existing primary and secondary pores provide space for the migration of dolomitization fluids. These dolomites continue to grow and point toward the center of the pores. As the degree of recrystallization increases, the dolomite crystals continue to grow, and the sedimentary structure gradually disappears. Similar structural characteristics and evolutionary processes of dolomite can also be observed in other cases with fine to medium crystalline dolomite reported by previous studies [54,55,56].

During the deep burial stage, accompanied by oil charging and subsequent oil cracking, the dolomite reservoirs are dissolved by acidic fluids mainly composed of organic acids. This process inhibits the precipitation of carbonate minerals in the reservoir spaces. The H₂S content in the natural gas of the Changxing Formation ranges from 1.80 to 6.32% [57], with an average value exceeding the maximum value of biogenic H₂S or the kerogen sulfo-compound cracking H₂S (3%) [58,59,60]. Thin sections and CL analysis show that the D2 dolomite crystals in the Changxing Formation have a cloudy surface and irregular shape, which is significantly different from the euhedral dolomite formed by dissolution and re-precipitation. Meanwhile, the rim of the D2 dolomite is mostly wrapped with solid bitumen, indicating that the space required for its growth does not come from the dissolution of dolomite itself during deep burial and dissolution but from the preservation of early pores.

5.2 Influence of dolomitization on reservoir space evolution

In terms of the dolomitization model in the reef–shoal complex, previous studies proposed several models, including the eodiagenetic model [29], shallow burial model [61], and reflux–seepage model [62]. However, studies on the effect of dolomitization on precursors are rare. To evaluate the effects of dolomitization on reservoir pores, this study selected core intervals from Well LH002-X2 and Well YA002-5 on the west and east sides of the Trough for comparative analysis of porosity and mineral content (Figure 10). The results indicate that the highest porosity value of the reef–shoal complex on the west side is developed on the top of the shallower upward grain shoal cycle, which means that early atmospheric freshwater has a significant impact on the alteration of grainstone. This effect is theoretically also reflected in the grain shoal of the east side, and the pore morphology (porosity and distribution characteristics) should have similar features. However, the vertical variation of porosity shows that the peak porosity of dolomite is usually not developed at the top of the reef–shoal complex, but at a certain distance from the top. The best type of reservoir rocks is the grainstone in the upper part of the grain shoal that has been replaced by D2 dolomite, rather than the grainstone at the top of the grain shoal that has been replaced by D1 dolomite.

Figure 10

Vertical sections of the dolomite content and porosity from typical wells around the Kaijiang–Liangping Trough from the Changxing Formation.

For the framestone interval on the west side, most of the interframe space is cemented by the first to second-stage SCC generated during the shallow submarine syngenesis stage (Figure 11). This pore destruction effect is further reflected by the fact that the current porosity of the framestone is less than 1.0% and has not been extensively replaced by dolomite (Figure 12a). However, similar calcite cement is usually not developed in the framestone interval on the east side, and a large amount of reservoir space is preserved (Figure 11). Some studies proposed that the absence of calcite cement in the dolomite interval may reflect that dolomite has stronger resistance to compaction and dissolution compared to limestone [2,13], which inhibits dissolution and leads to the production of calcite. Therefore, early calcite cement inhibited the occurrence of dolomitization, while early dolomitization inhibited the cementation of later calcite.

Figure 11

Diagenetic pathway of dolomitization in Changxing Formation, Eastern Sichuan Basin.

Figure 12

(a) Cross-plot of dolomite content vs porosity in the Changxing Formation. (b) Cross-plot of dolomite content vs permeability in the Changxing Formation.

The reservoir porosity in the study area increases with the dolomitization degree, mainly due to the preservation of corresponding primary intergranular pores and the inhibition of calcite cement caused by the pressure-solution effect. The statistical results show that when the dolomite content increases from 0 to 40%, there is no significant change in porosity. When the dolomite content increases from 40 to 60%, the porosity gradually increases slightly. When the dolomite content exceeds 60% and continues to increase, the porosity increases sharply (Figure 12a). It is worth noting that the above conclusion may not apply to changes in permeability. Except for a few samples with micro-fractures causing abnormally high permeability values, the permeability of the vast majority of samples changes dramatically when the dolomite content is between 20 and 50%, and there is a significant positive correlation. When the dolomite content exceeds 50% and continues to increase, there is no significant change in permeability, and its value stabilizes at about 4 mD. However, when the dolomite content exceeds 70%, the permeability shows a decreasing trend, which is attributed to the cementation of throats caused by over-dolomitization (Figure 12b).

6 Conclusions

For reef–shoal complexes, the dolomite with varying degrees of preservation of its original structure is controlled by the degree of recrystallization and deep burial dissolution. The sedimentary cycle essentially established an early frame for the distribution of dolomite with different structural types in reef-shoal complexes. The effects of early atmospheric water leaching on the structure, mineral composition, and stability of the reef–shoal complexes are important reasons for the structural differentiation of dolomite. Recrystallization and deep burial dissolution have intensified the destruction of the original structure.
The difference in the development degree of SCC determines whether the framestone on both sides of the Trough undergoes dolomitization or not. When the supply of Mg²⁺ is insufficient, the dolomitization fluid prioritizes the replacement of the lime mud matrix. When the Mg²⁺ supply is sufficient, the reef-building organisms and bioclastic grains are completely replaced by dolomite.
The synergistic growth relationship between dolomite content and porosity does not indicate that dolomitization has added new pores. For the reef–shoal complex reservoirs, dolomitization did not lead to the formation of new pores but effectively preserved primary intergranular pores and freshwater leaching-induced pores. The porosity increases with the increase of dolomite content, partly due to pore preservation and partly due to the more effective anti-compaction (and pressure solution) effect of dolomite compared to calcite, reflecting the overall effect of dolomite pore preservation.
Although early freshwater leaching played a positive role in the formation of dolomite reservoirs, the preservation effect of dolomite pores contributes more to the current porosity and permeability of reservoirs, resulting in the highest quality reservoirs of reef–shoal complexes often developing at a certain distance below the exposed surface, and the best reservoir is dolomite in the upper part of the reef–shoal complexes, rather than dolomite interval located at the top.

Acknowledgments

The authors are grateful to the reviewers whose comments and suggestions greatly improved the manuscript.

Funding information: This work was supported by the National Natural Science Foundation of China (Grant No. 41972165) and the National Natural Science Foundation of China (Grant No. 42202166).
Author contributions: Conceptualization: Q. Z. and L. L.; methodology: Q. Z. and H. S.; investigation: Q. Z, C. J., and N. Q.; data curation: N. Q. and L. Q.; writing – original draft preparation: Q. Z. and L. Q.; writing – review and editing: L. L. and N. Q.; and formal analysis: H. S., C. J., and L. Q.
Conflict of interest: Authors state no conflict of interest.

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Received: 2024-07-30

Revised: 2024-11-01

Accepted: 2024-11-01

Published Online: 2024-12-05

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

Articles in the same Issue

https://doi.org/10.1515/geo-2022-0734

Keywords for this article

reef; shoal complex; dolomitization; diagenetic sequence; pore preservation; Changxing Formation; Sichuan Basin

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