2.1.1 Genetic types of dolomite

According to the genetic mechanism, dolomite can be divided into primary sedimentary dolomite, secondary metasomatic dolomite, and microbial-induced dolomite. Primary sedimentary dolomite requires special environmental conditions for its formation, such as closed, high salinity, strong evaporation, and alkaline lake environments, and is extremely rare in the long geological history (Liu, Xu, et al., 2019; Wanas & Sallam, 2016). Compared to primary sedimentary dolomite, secondary metasomatic dolomite in rock records is more widely developed. To explain the origin of secondary metasomatic dolomite, geologists have proposed various dolomitization models to explain the geological process and sedimentary environment of dolomite formation, including near-surface dolomitization (Warren, 2000), buried dolomitization (Jiang et al., 2016; Xiong et al., 2018), and hydrothermal dolomitization (Davies & Smith, 2006). Although dolomite cannot be successfully synthesized in the laboratory under near-surface low-temperature inorganic conditions (Rodriguez-Blanco et al., 2015), recent studies have shown that microorganisms play a certain role in inducing the formation of dolomite at low temperatures and have been effectively validated in experiments (Bontognali et al., 2012; Liu, Yu, et al., 2019; Petrash et al., 2017; Vasconcelos & McKenzie, 1997; Warthmann et al., 2000).

Dolomitization is a multistage comprehensive process involving different geological conditions and periods. In many cases, the formation of dolomite is influenced by factors such as fluid chemistry, thermodynamics, and kinetics (Peng et al., 2018). It is difficult to fully explain the genesis of dolostone using a single dolomitization model or research method. Therefore, it is necessary to analyze the trend of changes in rock geochemical characteristics over time and the changes in the diagenetic environment to provide a more comprehensive and reasonable explanation for the genesis of dolomite.

2.1.2 New research methods for the genesis of dolomite

In recent years, with the rapid development of nontraditional methods for studying the genesis of dolomite, such as Mg isotopes, clumped isotopes, dolomitic crystal structure, and laser in situ U-Pb dating, more useful information has been gained for the study of formation and evolution of dolomites. The analytical procedure for Mg isotopes consists of two primary stages: sample pretreatment and isotope determination. Sample pretreatment involves converting the sample into a suitable solution or solid for analysis by mass spectrometry, encompassing processes such as sample digestion, separation, purification, and drying. Isotope determination entails measuring the relative abundance of different Mg isotopes in the sample using a high-resolution multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) to calculate the isotopic composition of Mg. During the measurement process, appropriate standard materials are utilized for calibration and correction to mitigate instrument mass deviation and fractionation effects (Riechelmann et al., 2020). The laser in situ U-Pb isotope dating technique offers several advantages, including microarea in situ analysis, a straightforward sample preparation process, minimal sample consumption, low background interference, high spatial resolution, and rapid analysis speed, all of which contribute to its unique utility. The experimental setup comprises a laser ablation device and a high-resolution MC-ICP-MS. During the experimental analysis, the laser beam is employed to selectively remove the target component of the dolomite sample at a microscale. The material liberated by this process is then ionized in an argon plasma, and the resulting isotope ratio is measured using a mass spectrometer. Subsequently, the measured sample's isotope ratio and that of a standard sample are used to calculate the related element content and the isotope age of the measured sample (Elisha et al., 2021). The clumped isotope test method consists of two primary stages: CO2 purification and testing. The purification process involves the reaction of phosphoric acid with dolomite to generate CO2. The reaction temperature of phosphoric acid is maintained at 90°C, and the isotope value is subsequently determined based on the CO2 produced. As experimental conditions may vary across different laboratories, it is essential to standardize the experimental results to enable the conversion of the clumped isotope value (∆47) under diverse experimental conditions. Consequently, it is imperative to adjust the ∆47 values obtained at varying reaction temperatures (Murray et al., 2016).

Mg isotopes can effectively trace the source of Mg-rich fluids and reconstruct the evolution process of dolomitization (Huang et al., 2015; Ning et al., 2020; Peng et al., 2016). Clumped isotope temperature measurement technology has significant implications for analyzing the diagenetic temperature of dolomite, restoring oxygen isotope values (combined with oxygen isotope values) for the study of diagenetic environments and fluids (Shenton et al., 2015). The dolomitic crystal structure preserves unique evidence of the environment, crystallization, crystal growth, fluids, and other aspects during its formation process, which can be used as a means to study the formation environment and mechanism of dolomite (Yang, Zhu, et al., 2022). Laser in situ U-Pb dating of absolute age is of great value for understanding the diagenetic process and pore evolution of dolomite (Godeau et al., 2018).

2.2.1 Microorganism–metazoan transformation mechanism

The global biogenic sedimentary record during the late Precambrian and Phanerozoic indicates that Earth's sedimentary and ecological systems have undergone at least five major microorganism–metazoan transitions from the Late Precambrian to the present (Chen et al., 2019). Each major transition period witnessed the transition process from a microbial-dominated sedimentary system to a metazoan-dominated sedimentary system. The first transformation occurred during the late Sinian, which recorded the birth, gradual development, and flourishing of relatively advanced and complex multicellular organisms in marine ecosystems dominated by microbial mats. The algal-mat ecosystem dominated by low-level bacteria and algae provided a suitable marine environment for the emergence and flourishing of metazoans during this period, and the participation of microorganisms also played an important role in establishing a metazoan-dominated ecosystem. Since the explosion of life during the Cambrian, the seafloor of the worldwide shallow waters has frequently changed from a microbial mat substrate to a substrate heavily disturbed by metazoans. The frequent transformation of these two types of marine substrates occurred throughout the evolutionary process of marine ecosystems during the early to middle Cambrian.

2.2.2 The mechanism of large-scale formation of ecosystems and microbial mats

The Precambrian ecosystem is characterized by the flourishing of microorganisms, especially photosynthetic cyanobacteria. As pioneer organisms, microorganisms first occupied ecosystems in harsh environments. Mat-growing microorganisms altered the physical and chemical properties of sedimentary substrates, occupying large carbonate platforms with microbial sedimentary structures in clastic rocks and microbialites in carbonate rocks (Jing et al., 2022; Yang, Chen, et al., 2022). During the Sinian, with the increase of oxygen in the ocean and the gradual improvement of pH, temperature, and other conditions, metazoans began to evolve and first appeared in clusters, leaving reliable solid fossil records in the strata. However, at this time, metazoans often moved on or below the microbial mat at a very shallow position, and their feeding strategy mainly manifested as foraging on the surface of the microbial mat or benthic herbivory at a very shallow position below the sediment (Chen et al., 2018). Overall, the Precambrian is characterized by microbial structures and the development and flourishing of microbial mats. Although metazoans appeared in large numbers during the Sinian, their disturbance and transformation of sediments and their role in the development and utilization of sedimentary substrates can be ignored (Buatois et al., 2016).

The Cambrian relic fossils record a completely different scene, revealing several complex relic communities and more extensive behavioral patterns. The Cambrian biological explosion event led to the differentiation of bilaterally symmetric animals and active transformation of sedimentary substrates by metazoans, continuous creation of new ecological spaces (Mángano & Buatois, 2014). Under the influence of biological perturbations, the sedimentary substrate changed from a microbial mat to a mixed substrate, resulting in a significant increase in oxygen and nutrient cycling between sediments and seawater. These changes acted as positive feedback, further increasing the number of microorganisms and enhancing the ability of benthic organisms to transform the sedimentary substrate. It provided the substrate for the Cambrian biological explosion, ultimately creating a very different ecological landscape from the Precambrian (Mángano & Buatois, 2017). Erwin et al. (2011) suggest that the middle–late Cambrian, following the Cambrian explosion, did not significantly increase the differentiation and diversity of solid fossils compared to the late stage of the early Cambrian. Fan et al. (2020) found an insignificant change in carbon isotope positive shift during the late Cambrian and a significant decrease in the number of solid fossil genera and species. Based on global trace fossil data, Mángano and Buatois (2014) suggest that although the diversity of the fourth-stage trace fossil increased significantly over millions of years during the Cambrian, there was no significant change in trace fossil differentiation and diversity compared to the third stage.

Based on the analysis of the above-mentioned solid and trace fossil data, the evolutionary radiation of benthic organisms stagnated during the Cambrian after the biological explosion. It resulted in various phyla of metazoans, although present in the Cambrian, having low differentiation and abundance and insufficient burrowing ability to occupy all ecological niches in benthic ecosystems (Chen et al., 2019). Microorganisms thrived in ecological spaces that metazoans could not occupy, resulting in various structures of microbial origin (microbialites or sedimentary structures of microbial origin). It led to the flourishing of both Cambrian microbial structures (microbial mat floor) and metazoan disturbance structures (mixed substrate). In addition, as the microorganisms grew, they secreted sticky EPS that bonded with other microorganisms and sediment grains to form microbial mats, and in the early stages, calcification also occurred, making the sedimentary substrate very tough. It is an insurmountable barrier for metazoans with weak burrowing abilities during the Cambrian. Therefore, no direct disturbance or destruction of microbial mats by metazoans has been found in the Cambrian strata.

2.2.3 Microorganisms induce large-scale dolomite formation

Environmental conditions played an important role in controlling their occurrence from the development and evolution of microbial structures and metazoan disturbance structures during the Cambrian (Zhang, Algeo et al., 2022). When the environment is conducive to microbial mats or microbial membranes (such as hard substrate and stable hydrodynamic conditions), especially after the deposition of stormy calcirudite, microbialites or microbial structures can develop in large quantities. When the environment is not suitable for the development of microbial mats or membranes (such as soft substrate and sustained high-energy conditions) due to the lack of protection from tough microbial mats in the sedimentary substrate, metazoans begin to burrow in the sediments, leaving behind various types of biological disturbance structures. Due to frequent sea-level fluctuations and hydrodynamic conditions during the Cambrian, neither side of the ecosystem could fully occupy the entire ecological space between microorganisms and metazoans. Their interaction led to the alternating development of microbial and metazoan perturbation structures in the Cambrian. The abundant fossils of cyanobacteria that developed in the Cambrian dolomite indicate that Cambrian microorganisms contributed to the formation of the dolomite.

2.3.1 Temperature

Under low-temperature conditions, there is a barrier to dolomitization where the Mg2+ ion is solvated to form Mg2+-H2O, making it difficult to collide effectively with calcite. The conditions for dolomitization are insufficient, which explains why seawater supersaturates dolomite for a long time without precipitation (Lippmann, 1982; Yang, Yu, et al., 2022). As the temperature increases, on the one hand, it can accelerate the rate of molecular motion, leading to an increase in the effective collision between Mg2+ and calcite within a certain period. On the other hand, the tightness of the bond between Mg2+ and water decreases, making Mg2+ more easily desolvated. Free Mg2+ enters the calcite lattice and replaces Ca2+ in the lattice, promoting dolomite formation (Gaines, 1980; Mei, 2012; Zhang et al., 2009). Previous experiments have shown that the maximum dissolution rate of aqueous CO2 solution on rocks is between 60 and 90°C (Fan et al., 2007; Jiang et al., 2008; Yang et al., 2014; Zhang et al., 2008). Yang, Yu et al. (2022) conducted experiments on dolomitization under different temperature conditions and obtained the dissolution law of calcite in CO2 aqueous solution in the temperature range of 60–95°C. The dissolution rate of calcite is negatively correlated with temperature, with the highest at 60°C and the lowest at 95°C. This conclusion is in line with actual geological phenomena. In fact, the higher the temperature of carbonate layers, the lower the amount of mineral dissolution (Shou et al., 2016). Therefore, the occurrence time of dolomitization is speculated to be during the sedimentary burial period when the formation temperature is below 60°C.

2.3.2 Fluid properties

The MMg/MCa ratio in the fluid is one of the decisive factors for the formation rate of dolomite, which has an important impact on dolomite formation. The higher the MMg/MCa ratio in the solution, the shorter the induction period and the faster the dolomitization rate (Etschmann et al., 2014; Kaczmarek & Sibley, 2011). Within the range of 0.15 < MMg/MCa < 9.09, the higher the MMg/MCa ratio in the solution, the more likely it is to undergo dolomitization. However, if the MMg/MCa ratio is too high (greater than 9.09), magnesite may be formed (Fan et al., 2012).

Dolomitization occurs under sufficient Mg2+ supply conditions. An increase in Mg2+ concentration can reduce the thermodynamic stability of calcite, increase its dissolution rate, and reduce its effective supersaturation (Davis et al., 2000). Usdowski (1994) demonstrated through experiments that dolomite can be generated at temperatures above 60°C. However, in Yang, Yu et al.'s (2022) experiment, dolomite was not generated at temperatures of 80 and 60°C. The analysis found that the experimental solution of Yang, Yu et al. (2022) had the molar concentration C(Ca2+) + C(Mg2+) of approximately 0.057 mol/L, while the experimental solution of Usdowski (1982) had C(Ca2+) + C(Mg2+) of approximately 7–9 mol/L, which is about 150 times higher than the former. It is the fundamental reason for the differences in results. Increasing the concentration of Mg2+ in the solution can increase the effective collision between Mg2+ and calcite, thereby promoting the dolomitization process. The evaporation-pumping dolomitization can be analyzed using the theory of increasing Mg2+ concentration. When the basin is limited, seawater evaporates, salinity increases, and Mg2+ concentration increases. After Ca2+ precipitates with CO32− and SO42−, MMg/MCa in the solution increases, generating dolomite (Yang et al., 2023; Zhang et al., 2007).

2.3.3 Crystal nucleus promotes dolomitization

The presence of dolomite crystal nuclei can promote the dissolution of calcite, and Mg2+ in dolomite diffuses into the calcite lattice, leading to a decrease in the order of calcite. The particles within an ordered crystal structure are regularly distributed and relatively stable, requiring high energy to break the lattice. In a disordered crystal, the distribution of particles lacks regularity, displaying varying energy levels that render it unstable and susceptible to disruption (Gong & Huang, 1984; Zhu et al., 2015). As a result, more Mg ions enter the calcite lattice, promoting dolomitization (Hong et al., 2016; Yang et al., 2020; Yang, Zhu et al., 2022; Zhang et al., 2009). When the Ca2+ concentration in the solution is not saturated, Mg2+ ions in the dolomite diffuse into the calcite lattice, accelerating calcite dissolution. When the Ca2+ concentration in the solution reaches saturation, Mg2+ ions in the dolomite diffuse into the calcite lattice, accelerating dolomitization, shortening the dolomitization time, and promoting dolomite growth. Zhu et al. (2015) conducted dissolution experiments on carbonate rocks in acetic acid and hydrochloric acid under normal temperature and pressure conditions, and the results showed that adding a small amount of dolomite to limestone can increase the solubility of calcite. Yang, Yu et al. (2022) conducted dolomitization experiments with different initial mineral compositions and found that pure calcite took 9–12 days to dolomitize, while rocks with 5% dolostone took 6–9 days to dolomitize (Figure 1). The addition of dolostone nuclei shortened the dolomitization time and promoted the growth of dolostone. Therefore, small amounts of dolostone in limestone are an essential driving force for its rapid dolomitization.

2.3.4 Lattice defects

The minerals' lattice defects can reflect the crystal formation environment and the growth process (Zhang, Glasser, et al., 2014). The dolomitization process does not occur independently but is usually accompanied by calcite dissolution, which is a calcium precipitation reaction. Dissolution occurs first at lattice defects with high surface free energy. The grain boundaries of calcite are broken, Ca2+ ions are expelled, and Mg2+ ions are transferred from the outer layer of calcite to the inner layer (Sorai, 2021). When Mg2+ ions enter the interior of the calcite crystal and combine with the crystal lattice, they further destroy the crystal structure of calcite, causing many dislocations and defects. After the calcite dissolves to equilibrium, more Mg2+ ions enter the calcite crystal to form dolostone (Pinto et al., 2017).

In the process of geological evolution, the boundaries of crystals are often incomplete, with lattice defects that are more susceptible to dissolution, due to insufficient material sources or the space limitations of crystallization (Zhang et al., 2018). It is also an important reason why the boundaries and corners of mineral crystals are often the preferred sites for dissolution, which can result in rounded boundaries and corners. The reason why dolomitization mostly develops within the platforms or ancient karst unconformities is because the surface free energy of calcite at the sequence interface or unconformity surface is high, and there are a large number of lattice defects, which are conducive to the occurrence of dolomitization. It reveals the important reasons for the high degree of dolomitization at the sequence interface and the good properties of dolomite reservoirs.

The close contact between the lattice of two minerals leads to the diffusion of Mg2+ and Ca2+ ions in the lattice. Mg2+ ions in dolomite diffuse through dolomite/calcite into the calcite lattice, replacing Ca2+ ions in the lattice, and the calcite lattice transforms into a dolomite lattice. The surface of dolomite, devoid of Mg2+, has an adsorption effect on Mg2+ in solution. Compared with hydrated Mg2+-H2O, the active Mg2+ in the solution is more likely to adsorb on the surface of dolomite, enter the dolomite lattice, supply the missing Mg2+, and release Ca2+. During this process, the original dolomite crystal boundary in the rock sample acts as a fluid channel, and more Mg2+ diffuses through calcite/dolomite into the calcite lattice. After the transformation of calcite into dolomite, the porosity increases, and fluids enter the calcite/dolomite through the pores, increasing the contact area between calcite and fluids and promoting dolomitization. Yang, Yu et al. (2022) used multiphase flow reaction solute transport simulation technology to simulate dolomitization under heterogeneous conditions. The numerical simulation results showed that dolomitization first occurred at the boundary of calcite and dolostone (referred to as calcite|dolostone), and as the reaction progressed, the boundary effect promoted dolomitization, which was particularly significant for the changes in calcite, verifying the Mg2+ diffusion hypothesis of calcite|dolostone (Figure 2). The experimental results of Yang, Zhu et al. (2022) indicate that dolostone can be generated along the dissolution zone of calcite, and there is no dolostone in the flat area, nor is there any obvious dissolution feature. It also confirms that Mg2+ can dissolve through calcite into the calcite lattice, producing metasomatic dolostone. The formation of dolomite driven by microorganisms during the Precambrian–Cambrian is due precisely to the compaction process after sedimentation, which tightly binds calcite and dolostone produced by microorganisms, thus promoting dolomitization.

2.4.1 Dolomitization processes during the quasi-contemporary period

The development of in situ laser U-Pb isotope dating technology for carbonate minerals not only solves the problem of the absolute age of carbonate minerals but also reconstructs the diagenesis-pore evolution history of reservoirs through the absolute age and content of carbonate cement. The effective porosity and reservoir formation effectiveness before oil and gas migration can be evaluated by combining the structural–burial history and hydrocarbon generation history. Taking the Sinian dolomite in the Sichuan Basin as an example, in situ laser U-Pb isotope dating of surrounding rocks and different stages of dolomite cement was performed to determine the dolomitization time.

Many dating studies have been carried out on the late Sinian Dengying Formation. In the most recent study, Huang et al. (2020) reported a new secondary ion mass spectrometry U-Pb age of (543.4 ± 3.5) Ma (with a mean square of weighted deviates of 1.2) for the tuff layer in the upper Dengying Formation (i.e., Baimatuo Member) of the Zhoujiaao profile in the Three Gorges. Zhou et al. (2018) conducted a chronological study using two types of k-bentonite from the Fanglong profile in South China. The U-Pb age of the k-bentonite in the lower Dengying Formation was (557 ± 3) Ma. These data are consistent with the sedimentation time limit of 560–541 Ma determined from the Shangmiaohe Biota and Sinian Biota production beds (Zhao et al., 2022). The age data of the four surrounding rocks measured in this research are (543 ± 24) (548 ± 12) (521 ± 15) and (530 ± 13) Ma, which are equivalent to the age of the Dengying Formation and can represent the stratigraphic age, proving the reliability of U-Pb dating data of carbonate rocks (Figures 3 and 4).

The ages of four fine-grained dolostones measured in this study are (514 ± 19) (514.8 ± 4.6) (500 ± 16) and (503.11 ± 11) Ma, respectively, which differ from the age of the Dengying Formation by about 30 Ma. It indicates that this age series may reflect the early genesis of dolomitization in the Dengying 2 Member (Figures 3 and 4). Therefore, the dolomitization occurred in the postsedimentary quasi-contemporaneous period with a burial depth of less than 500 m. The dolomitization time was very fast, less than 30 Ma. Hu et al. (2020) found that the age data of algal laminae or algal stromatolite dolostone are 488–491 Ma by dating the surrounding dolostones of the lower Cambrian Shoerbluk Formation in the Tarim Basin, reflecting that the dolomitization occurred in the early stage. The algal information in the sedimentary period is still preserved in the dolomite, further proving that the dolomitization occurred in the early stage. The occurrence time is close to the age of the layer.

2.4.2 Rapid dolomitization during the quasi-contemporaneous period

A corresponding change in the degree of cation order inevitably accompanies the transition of dolostone from a disordered to an ordered state. Therefore, with the order degree parameter measured during the artificial synthesis of dolomite under laboratory conditions, it is possible to conduct a good study of the quasi-contemporaneous dolomitization period. There have been successful cases of experiments simulating the formation process of secondary metasomatic dolomite. Kell-Duivestein et al. (2019) reacted calcite or aragonite in an excess solution of MgCl2 and NaHCO3 for 360 days, observed the degree of ordering under different temperature conditions, and established a linear relationship between temperature and dolomitization time. With this relationship, it was inferred that the time required for complete dolomitization of aragonite at 25 and 50°C was 6.8 and 1.4 Ma, respectively. The higher the temperatures, the faster the dolomitization process. It is estimated that a significant amount of low-order dolomite (with a crystal order degree [COD] of 0.3) can be formed within a few hundred to several thousand years at both 50 and 25°C. This conclusion is in the same order of magnitude as the prediction of Kaczmarek and Thornton (2017) and is consistent with the argument of Usdowski (1989, 1994) that dolostone can be formed within 7 years under laboratory conditions at 60°C (Figure 5).

From this, it can be seen that the metasomatic dolomite can be formed rapidly in the quasi-contemporary period under shallow burial conditions. Taking the Sichuan and Tarim craton basins of China as an example, the geothermal gradient of the Sinian–Cambrian is (2.4–3.5)°C/100 m, and the sedimentary fluid of these strata was formed under the evaporation of seawater. With the continuous seawater concentration, a large amount of Ca2+ in the solution entered the solid phase, which promoted an increased MMg/MCa in the solution. At this time, the fluid state is rich in Mg2+, similar to that of Kell-Duivestein et al. (2019). In the simulation experiment, a sufficient Mg2+ solution condition was used with a deposition temperature of about 35°C, and the aragonite had sufficient Mg2+ supply after deposition to promote its dolomitization. It suggests that aragonite precipitated during the Sinian–Cambrian can be dolomized after deposition to form low-order dolostone, which can be further transformed into fully ordered dolomite within 6.8 Ma. Based on the burial history, the burial depth of these strata is within 500 m at this time. Considering the heating effect of deep burial and the surface temperature at this time, it is possible that a relatively high degree of dolomitization could have occurred at a depth of less than 400 m. This understanding again proves that quasi-contemporaneous dolomitization was completed during the shallow burial period. Therefore, the time required for dolomitization may be very short compared to geologic time scales.

2.4.3 Temperature of penecontemporaneous dolomitization

The clumped isotope temperature measurement technology has important significance in analyzing the diagenetic temperature of dolomite, restoring the oxygen isotope values (combined with oxygen isotope values) of dolomitized fluids, and analyzing diagenetic environments and fluids (Gasparrini et al., 2023). Ferry et al. (2011) calculated the formation temperature of dolomite in the Latemar carbonate assemblage of the Dolomites in Italy, from binary isotopes of carbonate rocks ranging from 40 to 80°C. The fluid's estimated temperature and oxygen isotope values are similar to those of diffusive fluids in modern mid-oceanic ridges. Millán et al. (2016) applied binary isotopes to study the formation temperature and dolomitization fluid composition of the Nisku Formation dolomite in the upper Devonian of the Alberta Basin, Canada. They clarified that the Nisku Formation underwent three stages of dolomitization and believed that the dolomitization stage was consistent with the diagenetic stage of the stratigraphic burial history, effectively proving that clumped isotope thermometers can reconstruct complex dolomitic diagenesis history.

Previously, we conducted clump isotope tests on dolomites from the Precambrian to the lower Paleozoic in China, and Zheng et al. (2017) conducted binary isotope thermometry studies on the middle-lower Cambrian dolomites in the Tarim Basin. The research shows that the granular dolomite was formed in a low-temperature, quasi-contemporaneous, and shallow burial environment, and the diagenetic fluid is seawater. The fine-grained dolomite may be formed in a deeply buried diagenetic environment where the original rock is transformed by high-temperature recrystallization, and the diagenetic fluid is hot underground brine. The dolostone cement in the pores and fractures is a product of the precipitation of magnesium-rich hot brine in deep diagenetic environments. Liu et al. (2020) argued that Δ47 has unique temperature-indicating properties and is not affected by the chemical and isotopic composition of the fluid during carbonate precipitation. The clumped isotope test and analysis were carried out for the lower Ordovician Yingshan dolomite in the central Tarim Basin. The dolomite within the Yingshan Formation underwent three distinct stages of diagenetic fluid evolution. Initially, the diagenetic fluid was characterized as shallow to medium-buried Ordovician reformed seawater, with a Δ47 distribution range of 69–94°C. Subsequently, the diagenetic fluid transitioned to late deep-buried brine, with a Δ47 range of 111–113°C. Finally, the third stage involved thermal fluid, with a Δ47 range of 130–147°C. It is suggested that the development of the Yingshan Formation dolomite reservoir is closely related to burial dissolution and thermal fluid activities and may also be superimposed by atmospheric water karstification. Chang et al. (2020) conducted detailed clumped isotope and petrological studies on the dolomite of the Sinian Doushantuo Formation in South China and found that the dolomite of the Doushantuo Formation was formed in a low-temperature environment, and the early low-temperature disordered dolostones were gradually transformed into ordered dolomite by later dissolution and recrystallization. Overall, the dolomite matrix mainly occurs in the low-temperature stage, and early rapid dolomitization is beneficial for preserving primary pores.

4.3.1 Dolomitization mechanism

The exploration and development of hydrocarbon is moving toward the deeper burial condition where calcite experiences more significant dolomitization (Debure et al., 2021). To investigate the influence of the dolomitization process on the storage of oil and gas resources, Zhu, Wei, Li, et al. (2023) for the first time, applied density functional theory (DFT) calculation to investigate the influence of orderly replacement of Ca atoms by Mg atoms with 10%, 20%, 30%, 40%, and 50% of the lattice volume, lattice stress, and the thickness of Mg monolayer (Figure 14). As the volume of calcite decreased by nearly 6.3 Å3 for each Ca atom replaced by Mg atom due to the smaller ionic radius of Mg than Ca (0.86 vs. 1.14 Å) (Cai et al., 2021), the volume of dolomitized calcite decreased with increased Mg proportion.

In addition, due to the unfavorable thermodynamic dolomitization process (Elstnerová et al., 2010), the formation of dolomite required stricter conditions such as high temperature and concentration, saturation, or salinity (Morse et al., 2007; Zhang, Nahi, et al., 2022). However, only 45% of Ca atoms can be replaced by Mg atoms even under very high pressure and temperature conditions due to the lattice stress (Hong et al., 2016; Zhang et al., 2010). Figure 14d showed that the lattice stress of Ca1.06Mg0.94(CO3)2 was nearly 68 times higher than that of Ca1.97Mg0.03(CO3)2. In addition, two different rules were found for the decreased thickness of Mg monolayer due to increased lattice stress, that is, rapid decrease when PMg ≤ 30% while slight decrease when 30% < PMg ≤ 50%, respectively.

As reported previously, the calcite volume decreased at increasing percentage of Mg replacement, especially leading to a constant decrease in interplanar distance of dolomite d104 value (Zhang et al., 2010). In addition, the value of d104 and PMg was negatively correlated, for example, d104 = 2.886 Å for PMg = 50% and d104 = 2.901 Å for PMg = 45% (Ivanishin & Nasr-El-Din, 2021) and a much higher d104 value of 3.019 Å for PMg = 5.25% (Zheng et al., 2021). According to Fick's diffusion law, higher mobility point defects due to the replacement of Ca atoms by Mg atoms exhibited a higher solubility (Stephenson et al., 2008). Furthermore, intergranular holes were formed during the dolomitization process due to the decreased volume and increased spherical local compressive stress (Hong et al., 2016).

Thus, based on the DFT calculations, Zhu, Wei, Li et al. (2023) revealed the dolomitization process of calcite from an atomic level, in terms of both thermodynamics and kinetics. Specifically, during this process, the dolomite with a lower PMg expands the crystal lattice and reduces the stability, thereby increasing the dissolution rate and giving a dissolution rate in an order of dolomite PMg50% < PMg40% < PMg30% < PMg20% < PMg10% < calcite.

4.3.2 Dissolution mechanisms

Zhu, Wei, Wu et al. (2023) reported that the dissolution rates of Ca2+ and Mg2+ in acetic acid solution were higher than those in carbonic acid solution, and the dissolution rates of both calcite and dolomite were increased with rising temperature in acetic acid solution (Figure 15a). In contrast, the dissolution rates of calcite were gradually decreased in the carbonic acid solution (Figure 15b). In addition, dolomite exhibited the highest dissolution rate at 373.15 K in carbonic acid, while the lowest being observed at 423.15 K (Figure 15b). It should be noted that Ca dissolved faster than Mg in both acetic and carbonic acid due to the higher chemical reactivity and faster diffusion rate for Ca atoms.

Although previous studies investigated the dissolution of calcite and dolomite (Lttge et al., 2003; Lund et al., 1973; Ruiz-Agudo et al., 2010; Taylor et al., 2004), that is, Perez-Fernandez et al. (2017) reported that the Ca dissolution rate decreased at increasing temperature, Pokrovsky et al. (2009) reported a similar experimental phenomenon due to the retrograde solubility of carbonates in temperature; the influence of surface reactivity and related dissolution kinetics for typical planes and boundaries were not fully understood. To reveal the underlying mechanisms, Zhu, Wei, Wu et al. (2023) investigated the surface energy of calcite and dolomite typical planes, giving an order of (110) > (116) > (101) > (113) > (018) > (104) for both calcite and dolomite, suggesting that the (104) plane was the most stable plane while (110) plane was the most active plane (Gao et al., 2017; Lan et al., 2017; Lardge et al., 2009; Tang et al., 2019). Thus, the (110) plane exhibited a higher dissolution rate while the (104) plane was normally presented in nature (Fenter & Sturchio, 2012; Heberling et al., 2013; Stipp & Hochella, 1991). In addition, the diffusion coefficients of the calcite (104)/(110) boundary and (110) plane were nearly 3.80 and 2.58 times than those of the (104) plane. Similar values of 3.76 and 2.55 times were found for dolomite. Thus, the boundary area was preferentially dissolved (Schusteritsch et al., 2021; Wu, Zhang, et al., 2022), leading to heterogeneous dissolution pores on the surface of carbonate reservoirs (Bouissonnié et al., 2018; Luttge et al., 2013).

In addition, the normalized diffusion coefficients for calcite were 1.54 times greater than that of dolomite in the general boundary model, while 1.62 times in the boundary-rich model (Figure 15c,d), suggesting that calcite was preferentially dissolved under deeply buried acid conditions (Hong et al., 2016; Offeddu et al., 2014). Furthermore, the diffusion coefficient was positively correlated with the surface area; thus, the diffusion coefficient of the (104)/(110) boundary in boundary-rich was higher than that in the general boundary for both calcite and dolomite, that is, 2.05 and 1.53 times for calcite and dolomite, respectively (Figure 15e,f). In this case, the boundary-rich area (e.g., closely metasomatized or high dislocation density carbonate reservoirs) was preferentially dissolved to form deeply dissolved pores (Koeshidayatullah et al., 2020; Urosevic et al., 2012), which was beneficial to the recovery of oil and gas resource.

By simulating the dissolution of typical crystal planes of calcite and dolomite in different solution systems and calculating molecular, the essence of heterogeneous dissolution and pore formation on typical crystal planes of calcite and dolomite was revealed. This research unveiled the mechanism behind the dolomitization of calcite, elucidated the characteristics of calcite and dolomite (104)/(110) grain boundaries, and clarified their dissolution process in carbonate solutions. It also sheds light on the factors constraining the dolomitization process and provides insights into the preservation mechanisms governing deeply buried dolostone reservoirs. The principle that deep dolostone reservoirs are superior to limestone reservoirs has been demonstrated.

5.3.1 The 10 000-m deep exploration potential in Tarim Basin

In the Sinian–Cambrian dolomite reservoirs of the Tarim Basin, several sets of reservoir–cap combinations are developed. Among them, the dolomite deposited on the semirestricted platform in the upper Sinian is transformed into good reservoir spaces by karstification, and the dolomite can be clamped by the Sinian and lower Cambrian source rocks, respectively, forming the best reservoir conditions. The effective reservoir area of the Qigebulak dolomite is 35 000 km2 with huge potential.

There are two main types of reservoir–cap combinations in the Cambrian. One is the reservoir–cap combination, where the lower Cambrian Xiaoerbrak granular beach is the reservoir, and the middle Cambrian gypsum salt layer is the cap. The Xiaoerbrak oolitic dolomite and sandy dolomite are distributed along the periphery of the South Tarim Ancient Land, with an area of 45 000 km2. Another set of reservoir–cap combinations comprises middle and upper Cambrian platform margin reef beach as the reservoir and upper platform slope mudstone as the cap, with an area of 40 000 km2. The development of high-quality source rocks in the lower Cambrian Yuertus Formation, together with these two reservoir sets, forms a lower generation and upper reservoir model with favorable reservoir conditions for oil and gas migration and accumulation. It is currently the best choice for a breakthrough at the 10 000 m level.

5.3.2 Deep exploration potential at depths of 10 000 m in Sichuan Basin

The Sichuan Basin has discovered two trillion cubic meters of gas in place in the Sinian and Cambrian, Anyue, and Penglai, showing enormous exploration potential for marine carbonate rocks. In the 10 000-m-deep strata of the Sichuan Basin, four types of beach bodies, namely platform margin beach, reef beach, high-energy beach, and margin or inter-salt beach, are developed, with three types of traps: lithologic, fault-controlled lithologic, and structural-lithologic traps (Huang et al., 2023). The Dengying Formation has developed two exploration fields: platform margin microbial (mound) beach and intraplatform microbial (mound) beach. High-quality reservoirs are widely distributed, and the Cambrian Qiongzhusi, Maidiping, and Doushantuo source rocks constitute high-quality hydrocarbon supply conditions with potential for large-scale oil and accumulation. The lower Cambrian Canglangpu source rocks, Longwangmiao, and Qiongzhusi formations form an accumulation combination of lower generation and upper storage, and natural gas accumulation is closely related to the distribution of high-energy granular beaches. Based on the 3D accumulation of the Sinian–lower Paleozoic around the center of hydrocarbon generation, with source–reservoir configuration and comprehensive analysis of various accumulation control conditions, the Sinian Dengying Formation and the lower Cambrian Canglangpu–Longwangmiao Formation are selected for exploration at a depth of 10 000 m, with a cumulative area of over 50 000 km2 and a huge exploration potential.