Dolomite reservoirs are important reservoirs for oil and gas. According to the statistics^[1], among 226 large and medium oil and gas fields of carbonate rock all over the world that contain 90 percent of the total hydrocarbon reserves of carbonate rock, 102 oil and gas fields and half of the reserves are distributed in dolomite reservoirs. In the Sichuan Basin, over 90 percent of natural gas reserves are enriched in the Sinian, Cambrian, Carboniferous, Permian and Triassic dolomite reservoirs. The major reservoir in the Jingbian gas field of Ordos Basin is dolomite reservoir of Ordovician Majiagou Formation. The Tarim Basin has rich dolomite reservoirs in the Cambrian and Middle-Lower Ordovician strata, where oil and gas reservoirs Yingmai-32 represented by Well Yingmai-32 and Shan-1 have been discovered. Because of the great importance of dolomite reservoir in oil and gas exploration, the origin of dolomite, precipitation of primary dolomite, and the formation of pores in dolomite have been hot topics in geological research^{[2,3,4,5,6,7,8,9]}.

(1) Origin of dolomite. Numerous dolomitization models were proposed previously, including eight mainstream dolomitization models, seepage-reflux dolomitization^[10], capillary concentration dolomitization^[11], sabkha dolomitization^[12], mixing-water dolomitization^[13], adjustment-compact dolomitization^{[3, 14]}, burial-compact dolomitization^[15], seawater thermal convection dolomitization^[16], and structure-controlled hydrothermal dolomitization^[17]. The dolomitization occurs in pene-contemporaneous or burial stages, no matter how many dolomitization models there are. The existing genetic models only explain the origin of one or some types of dolomite, however, due to the lack of continuity and systematicity, the genetic model-based classification of dolomite hasn’t been establish so far.

(2) Precipitation of primary dolomite during (pene-)contemporaneous stage. The results of lab experiment show that single inorganic precipitation reaction as long as 32 years is unable to generate dolomite under surface temperature and pressure (≤ 50 °C, pressure at depth of meters)^[18]. In Abu Dhabi Sabkha, Coorong Lagoons, Lagoa Vermelha and other modern salty coasts, spherical protodolomite can be precipitated through the action of sulfate reducing bacteria^{[19,20,21,22,23]}, halophilic bacteria^[24,25,26] and methane-producing bacteria^[26,27,28] at 35-40 °C. Warthmann et al. succeeded in gaining primary dolomite by precipitation in the laboratory^[21], found that the primary dolomite they cultured has similar spherical shape and low ordering degree to that produced in Lagoa Vermelha salty coast, and thus pointed out specific types of microbes (such as sulfate reducing bacteria, halophilic bacteria, methane-producing bacteria, etc.), certain salinity (35‰- 100‰) and alkalinity, certain temperature (30-45 °C), low concentration of sulfate radical ion, and high concentration of carbonate and magnesium ions are conditions for precipitation of primary dolomite. Extracellular polymers (EPS) generated from the microbial activity are one of the main contributors to dolomite precipitation^[23].

(3) Origin of pores in dolomite. The role of dolomitization in porosity construction and destruction has been a debatable issue for a long time^[29,30,31]. As the reservoir space is mainly developed in various types of dolomite, and even the reservoir space of reef shoal facies is mainly developed in dolomitized deposits. The mainstream view is that pores in the dolomite reservoir are the product of dolomitization. Based on principle of mass conservation, Weyl^[32] proposed that if dolomitization was a process of replacement between molecules, and source of CO₃^2- was very limited, when calcite was transformed to denser dolomite, the porosity could increase 13% theoretically. However, Lucia and Major^[33] pointed out that the theory was not applicable to formation mechanism of all pores in carbonate rocks, and porosity of dolomite was always equal to or less than that of its original rock, suggesting the characteristics of original rock might be a key factor for porosity variation in the process of dolomitization. Purser et al. held a middle view that although the characteristics of original rock (limestone) have an important effect on dolomite porosity, the source of CO₃^2- and restricted diagenetic environment are of equal importance, only in such diagenetic environment, can dolomitization lead to an increase of porosity^[34]. The porosity loss associated with dolomitization, especially precipitation of saddle dolomite blocking pores, is commonly seen in burial diagenetic environment^{[30, 35]}. In conclusion, no consensus on the contribution of dolomitization to formation of the porosity in dolomite has reached.

Based on reviewing previous research results, outcrop, core, thin section, and cathodoluminescence observations and other petrographic analysis were performed on samples taken from the key dolomite layers in the Sichuan, Tarim and Ordos Basins, and more than 600 geochemical tests including the ordering degree, carbon and oxygen isotopes composition, Sr isotopes ratio, trace elements, rare earth elements (REE), fluid inclusions, isotope geothermometry (D47), and isotopic dating were conducted. The genetic classification for dolomites based on forming environment variation and time sequence is proposed, petrographic and geochemical identification marks for different genetic types of dolomite are established, and the contribution of dolomitization to the formation of porosity is re-evaluated. These new understandings are of great theoretical significance, and realistic significance to the prediction of dolomite reservoir distribution.

Although there are many classifications for carbonate rocks^{[36, 37]}, they are predominately based on the rock texture, and a genetic classification widely agreed is absent. For example, the classification proposed by Folk is mainly based on the grain size, roundness, sorting, stacking pattern and grain components^[35]. The one proposed by Dunham^[37] mainly based on the occurrence of identifiable primary structural components. He named carbonate rocks with identifiable primary structural components the micrite, wackestone, packstone, grainstone and bondstone, and named those with unidentifiable ones crystalline limestone, and left the classification of dolomite uncovered. According to Shen Anjiang^[38], there are two main types of dolomite with identifiable and unidentifiable structural components of primary rock, respectively (Table 1). For dolomite with identifiable primary structural components, the classification of limestone is applicable by just changing the limestone in the limestone structure classification table into dolomite. Such dolomite is often considered as the product of pene-contemporaneous or contemporaneous precipitation or replacement. While the dolomite with unidentifiable primary structural components, such as fine, medium crystalline dolomites, are often considered as the product of secondary replacement or recrystallization, and named the very fine, fine, medium, coarse and macro crystalline dolomite, respectively, according to grain size, which correspond to the crystalline limestone classified by Dunham. The dolomite classification schemes described above are only descriptive ones based on the structure and grain size, not a genetic one. Warren summarized and discussed ten dolomitization models^[5], and analyzed the development mechanisms and background of different dolomitizations, but failed to establish a systematical and mature genetic classification.

Based on petrographic features, forming environment and time sequence, the authors classified dolomite into three types and six sub-types in this work (Table 2). This classification proposes that the type of dolomite is a function of time sequence and forming environment evolution. As time progresses (from early to late, from contemporaneous stage to burial stage) and diagenetic environment evolves (from wet to dry, from low temperature-pressure to high temperature-pressure, from fresh water, seawater to burial, hydrothermal diagenetic medium), different types of dolomite appear in sequence, showing clear boundaries in diagenetic and characteristic realms and clear evolving clues between different types of dolomite. This classification has more systematicity and better continuity. By contrast, the existing dolomitization models only explain the origin of one or some types of dolomite in the classification.

In terms of formation stage, dolomite is generally formed in (pene-)contemporaneous and burial stages.

The development of low-temperature dolomite during (pene-)contemporaneous stage is closely related to the evolution of paleoclimate and paleoenvironment. As the paleoclimate varies from wet to dry, with gradual increase of temperature, salinity and alkalinity, seawater (island) dolomite, microbial dolomite, evaporated (Sabkha or seepage reflux) dolomite develop in sequence, finally to layered gypsum - salt deposits. Associated gypsum-salt nodules or fillings are rarely seen in microbial dolomite, but common in evaporated dolomite. This is because the most suitable temperature, salinity, alkalinity for microbes living are 30-45 °C, 35‰-100‰, and over 8.5^[21], respectively, microbes are hard to survive when they get higher, and microbial dolomite is replaced by evaporated dolomite, and when salinity exceeds 350‰ and PH exceeds 10, evaporated dolomite is replaced by gypsum-salt layers. The seawater (island) dolomite in this work should be defined as the early low-temperature dolomite formed in wet climate. Since a large number of examples of this kind are from the island environment of modern oceans, the dolomite is named seawater (island) dolomite, but its formation is not limited to the island environment, and considered associated with the seawater dolomitization induced by geothermal convection or topography driven flows^[4,5]. The low-temperature dolomite has two kinds of origins, precipitation (protodolomite) and replacement. The microbial dolomite is the protodolomite originated from precipitation, while the seawater (island) dolomite and evaporated dolomite are generated from replacement, which are dominated by algal dolomite, micritic - very fine cystal dolomite and reef (mound) bank dolomite composed of micrites and very fine crystals, with primary texture remained. In a complete early low-temperature sequence, the seawater (island) dolomite, microbial dolomite, evaporated dolomite, gypsum-salt layers should appear orderly as climate varies from wet to dry. However, due to incomplete evolution of paleoclimate and paleoenvironment, the three types of dolomite and gypsum-salt layer don’t necessarily appear in succession in geologic records. The Leikoupo Formation in Sichuan Basin develop multiple cycles of microbial dolomite, evaporated dolomite and gypsum - salt layer from the bottom up, without early seawater (island) dolomite representing humid climate. While in the Dengying Formation in Sichuan Basin, only microbial dolomite occurs, with absence of seawater (island) dolomite representing humid climate and evaporated dolomite and gypsum-salt layers representing extreme drought climate. Multiple cycles composed of evaporated dolomite and gypsum-salt layers are found in the Majiagou Formation in Ordos Basin, with a small amount of microbial dolomite, reflecting intermittent desalination under extreme drought climate. While in the Rock Island of Xisha Islands, South China Sea, the seawater (island) dolomite only exists in the Mio-Pliocene strata, indicating the paleoclimate didn't reach drought stage (Fig. 1).

The burial dolomite with residual grain texture and crystal grain texture can be evolved from the early low-temperature dolomite or limestone by replacement, recrystallization and overgrowth, where residual grains are composed of crystalline dolomite. With the increase of burial dolomitization degree (increasing burial depth, rising temperature and prolonging time of dolomitization), the crystal size of dolomite increases gradually. When it is larger than the primary grain size, the primary grain texture is difficult to maintain, in contrast, when the crystal size is smaller than the primary grain, the primary grain texture is likely to be kept. This is the fundamental reason that some crystalline dolomites have residual primary grain texture, but some don't (fine, medium, coarse, macro dolomite). Obviously, the finer the primary grains, and the stronger the burial dolomitization the primary rock experiences, the more unlikely the primary grain texture would be kept. This also explains the reason why the residual grain texture is common in fine - medium crystalline dolomite.

Structurally controlled-hydrothermal dolomite is formed by tectonic and thermal fluid activities during burial stage. It could deviate from the normal burial evolution sequence in time and burial depth, and is related to tectonic activities. It often comes in two occurrences: the dolomite developing along the deep fluid pathway such as fracture systems and unconformities, in lenticular, porphyritic and fence distribution, which is dominated by medium-coarse crystalline dolomite originated from limestone or dolomite through replacement or recrystallization; coarse-macro crystalline dolomite distributed along fracture systems, unconformities, dissolved pores, dominated by precipitated saddle dolomite, with limestone or dolomite as host rock. It generally appears in association with hydrothermal minerals such as quartz, fluorite, sulfides (pyrite, galena, sphalerite).

In conclusion, the classification described above should represent a genetic classification, which reveals that as time goes on and diagenetic environment changes, different types of dolomite are formed in succession, making the evolving clues between different types of dolomite clear. All types of dolomite in nature can be found a corresponding position in the above-mentioned time sequence and forming environment, that is, they can be found in the classification in Table 2.

The classification of dolomite in Table 2 shows clear boundaries in the petrographic features, environments and time sequence between different types of dolomite, which laid a solid foundation for identification of dolomites of different origins based on petrological, geochemical features. This paper discussed petrographic and geochemistry identification markers of the three types and six subtypes of dolomite listed in Table 2, and analyze the petrological and geochemical differences of different genetic types of dolomite using many cases.

2.1.1. Seawater (island) dolomite

According to the elucidation above, the seawater (island) dolomite should be defined as the early low-temperature dolomite formed under humid climate. Since a large number of cases are from the island environment of modern ocean, the dolomite is called seawater (island) dolomite, but it is not formed in island environment only. The origin of such dolomite was explained as the mixed water dolomitization^[13] and seawater thermal convection dolomitization^[16] previously.

Such dolomite is young, Neogene to Modern in age, and the dolomitization occurred in penecontemporaneous stage and in a relatively simple diagenetic environment surrounded by seawater (isolated platform). The dolomite or its associated rocks are new, and have not experienced late burial diagenetic transformation, with the dolomitization temperature close to the surface temperature. With an example of the Miocene- Pliocene strata on Shidao Island of Xisha Islands, South China Sea, the petrographic and geochemical features of such dolomite are illustrated^[39]. The dolomites with similar features and origins are also found in the Miocene-Pliocene strata in Bahamas Bank^[16], Pacific Enewetak Atoll^[40] and Caribbean Cayman Islands^[41].

On Shidao Island, in Well Xike 1, there are sedimentary hiatus surfaces (exposure surfaces) and associated bioclastic dolomite and algal dolomite (Fig. 2a, 2b), which are 150 m and 10 m thick respectively, at the top of the Miocene Xuande Formation and Pliocene Yongle Formation. The dolomite has well preserved primary texture, and primary sedimentary pores and early supergene enlarged pores largely. X-ray diffraction analysis shows this kind of dolomite is pure and low in order degree (around 0.4). It has positive δ¹³C and δ¹⁸O of (1‰-3‰ PDB) which are not correlated, this is completely different from mixed water dolomite. The variation trend of its δ¹³C is the same as that of the global water at the same stage. Its Sr isotope ratio is in the range of and has the same variation trend with that of the same stage seawater. It has low contents of Mn (less than 40×10^-6) and Fe (less than 200× 10^-6), and higher Sr content of ((150-250)×10^-6). There is no correlation between its Sr content and Fe content, and between Sr content and Mn content. Its rare earth element distribution pattern is similar to that of modern seawater. The geothermometer of oxygen isotope (D47) reveals the dolomite is formed between 20-35 °C. The isotopic absolute age of the dolomite cement is 5±0.2 Ma. All these prove that the dolomitization happened soon after the deposition, and in the diagenetic environment not completely away from seawater.

2.1.2. Microbial dolomite

As mentioned above, the microbial dolomite refers to the primary dolomite formed by precipitation of low-temperature dolomite related to microbial activities. The specific types of microbes (sulfate-reducing bacteria, methane-producing archaea, cyanobacteria, etc.), certain salinity (35‰-100‰) and alkalinity (pH value more than 8.5), certain temperature range (30-45 °C), low concentration of sulfate ion, high concentrations of carbonate ion and magnesium ion are necessary conditions for development of low-temperature dolomite in nature. The extracellular polymers (EPS) produced by microbial activities is one of the main contributors to precipitation of dolomite^[23], which is facilitated by carboxyl functional groups on the cell surface of the halophilic archaea in high salinity environment^[42]. Naturally precipitated dolomite and that produced by lab culture experiments have similar sphere, ellipsoid and dumbbell shapes, which are the important identification marks of microbial dolomite^[23]. The modern microbial dolomite is widely distributed in Sabkha high salinity areas such as Abu Dhabi, Coron Lagoon and Lagoa Vermelha, etc. Microbial dolomites spread extensively in the Sinian Dengying Formation and Triassic Leikoupo Formation in Sichuan Basin, the Sinian Qiaganbulake Formation and Cambrian Xiaoerbulake Formation in Tarim Basin, and the Jixianian Wumishan Formation in North China as important oil and gas reservoirs^[43]. With an example of the primary dolomite precipitated in microbial mats on the Sabkha coast and microbial dolomite in the Fourth Member of Dengying Formation in the Sichuan Basin, petrological and geochemical features of such dolomite are elaborated.

In microbial mats on the modern coast of Abu Dhabi (99.60% microbes are Desulfovibrio brasiliensis), the precipitated primary dolomite oxygen isotope composition value is positive to low negative (-3‰-7‰ PDB), while the precipitated primary carbon isotope value is low positive (0-2‰ PDB)^[22]. This type of dolomite often features laminar algal, stromatolite algal, framework algal, algal arenite, clot algae dolomites (Fig. 2c). It has sphere shape and extracellular polymer (EPS), and well-preserved primary texture, and its porosity is mainly composed of sedimentary primary pores and dissolution enlarged pores due to exposure, showing features of early stage low temperature precipitation dolomite related to evaporated environment and microbial activities^[23]. The microbial dolomite in the Fourth Member of Dengying Formation in Sichuan Basin has a degree of order of 0.4-0.5, negative oxygen isotope composition value (-8‰ - 4‰ PDB), low positive carbon isotope ratio (0-2‰ PDB), Sr isotope ratio value equivalent with the reference value of seawater in the same period^[44], abundance of Fe, low content of Sr and Mn, dark orange light of cathodoluminescence, forming temperature of 30-35 °C revealed by oxygen isotope geothermometer (D47), absolute age from isotopic dating of 542-555 Ma, which is consistent with the corresponding formation age. These features are similar to features of modern microbial dolomite on Abu Dhabi Coast (Fig. 2d, 2e), but with more negative oxygen isotopic composition value, which is related to the burial diagenetic alternation.

2.1.3. Evaporated dolomite

Early low-temperature dolomite includes primary dolomite associated with microbial activities, and evaporated dolomite derived from diagenetic replacement. Previously, researchers explained the origin of this kind of dolomite with seepage reflux dolomitization^[10], capillary enrichment dolomitization^[11] and evaporating pump dolomitization^[12]. The evaporated dolomite, which has nothing to do with microbial activities (high salinity leads to death of microbes), is formed in a higher salinity (100‰-350‰) and alkalinity (pH value more than 9) environment than the microbial dolomite under arid climate. Thus, it is often associated with gypsum nodules or precipitation of cements, and distributed between microbial dolomite and gypsum-salt rock vertically. The dolomite related to evaporative environment is developed extensively in various stages (Table 4). With an example of the Lower Cambrian dolomite in the Tarim Basin (Fig. 2f, 2g), its petrographic and geochemical features are elaborated. This kind of dolomite also exists in the Majiagou Formation of Ordos Basin (Fig. 2h) and the Leikoupo Formation of Sichuan Basin (Fig. 2i), and acts as important oil and gas reservoir.

The evaporated dolomite in the Lower Cambrian strata, Tarim Basin is dominated by gypsum nodule-bearing micritic dolomite and grain dolomite. Covered by gypsum-salt layers, it has well-preserved primary texture, and its porosity is mainly contributed by gypsum-moldic pores, grain moldic pores, and residual intergranular pores (Fig. 2f, 2g). The dolomite has an order degree of 0.6-0.7, low positive - low negative carbon isotopic value (-2‰-2‰ PDB) and low negative oxygen isotopic value (-8‰--4‰ PDB). It has higher Sr isotope value and similar REE distribution pattern with those of seawater in the same period, generally low content of Fe and Mn, nonluminescence or dark orange cathode luminescence, forming temperature of 35-60 °C revealed by oxygen isotope geothermometer (D47), and forming time equivalent with or slightly later than the corresponding formation.

Geochemical features and change trends of the three types of low-temperature dolomite during (pene-)contemporaneous stage are shown in Fig. 3.

Dolomitization during burial stage generates two types of dolomite, crystalline texture dolomite and dolomite with primary texture. The dolomite with crystalline texture is dominated by fine and medium crystalline dolomites, and a small amount of coarse crystalline dolomite. Coarse and marco crystalline dolomites are primarily observed in the structurally controlled - hydrothermal dolomite, with intercrystalline pores and intercrystalline dissolved pores developed. The dolomite with primary texture is dominated by grain dolomite, in which grains consist of fine, medium crystalline dolomites.

2.2.1. Crystalline dolomite with primary grain texture

Such dolomite could be originated from either grain limestone or evaporated dolomite, and mainly appears in coarse grain limestone or evaporated dolomite, with grains of fine, medium crystalline dolomites, which are resulted from replacement, recrystallization and secondary overgrowth similarly with the dolomite with crystal grain texture. The preservation of primary grain texture is due to the large primary grain than dolomite grain. Depending on the preservation degree of primary grain texture, a series of transitional types are classified between the dolomite with crystal grain texture and crystalline dolomite with primary grain texture, thus, they are often associated or mixed with each other. With an example of the Feixianguan Formation oolitic dolomite, petrographic and geochemical features of such dolomite are illustrated.

The ooids constituting the oolitic dolomite are composed of fine and medium euhedral and semi-euhedral crystalline dolomites, with intergranular pores and oomold pores, and rare calcite and dolomite cements (Fig. 4a, 4b, 4c). The dolomite has a degree of order from 0.5 to 0.8, which increases with crystal size and euhedral degree; low positive carbon isotopic value (0.5‰ - 3‰ PDB), low negative-high negative oxygen isotopic value (-8‰ --4‰ PDB), which tends to be high negative with the increase of crystal size and euhedral degree. Its Sr isotope ratio is close to that of seawater during the same period. Its REE pattern with higher content of light elements than heavy elements reflects its burial origin^[45]. It has high contents of Fe and Mn, ranging from 629.2×10^-6 to 1 194.0×10^-6 and from 58.7×10^-6 to 83.8×10^-6, respectively, strong orange cathodoluminescence. The inclusion homogenization temperature (80-150 °C) highly consistent with that revealed by oxygen isotope geothermometer (D47) indicates the dolomite is formed in deep burial environment, and is considered as a result of multi-stage replacement or recrystallization. The isotopic dating reveals the absolute ages of dolomites formed in different stages are younger than the corresponding formation ages.

2.2.2. Dolomite with crystalline texture

As mentioned above, the dolomite with crystal grain texture is dominated by fine and medium crystalline dolomites. Its primary rock is grain limestone^[38]. When the crystal grain is smaller, the primary grain texture can be remained, but when the crystal grain is larger than the primary grain size, the primary grain texture is hard to be kept. As the buried depth increases and dolomitization goes on, the grain size of dolomite crystal increases gradually. Previously, researchers explained the origin of this type of dolomite with burial-compaction dolomitization^[15] and adjustment-compaction dolomitization^{[3, 14]}. This kind of dolomite is distributed widely in the Upper Cambrian, Ordovician Penglaiba Formation, the lower member of Yingshan Formation in the Tarim Basin, the middle Fourth and Fifth Members of Majiagou Formation in the Ordos Basin, and Longwangmiao Formation, Qixia Formation, Changxing Formation and Feixianguan Formation in the Sichuan Basin, and often associates with dolomite with primary grain texture.

With an example of the Qixia Formation in the Sichuan Basin, petrographic and geochemical features of such dolomite are described. Compared with the dolomite with primary grain texture, it has rare primary grain texture, and is dominated by larger xenomorphic-subhedral dolomites (Fig. 4d, 4e, 4f) in mosaic-like contact. It mainly contains intercrystalline pores and intercrystalline dissolved pores, and dolomite cements commonly. It has similar geochemical characteristics and change trend with dolomite with primary grain texture, which is characterized by a wide data change range, indicating it is the product of multiple-stage replacement and recrystallization.

In the Qixia Formation and Maokou Formation, Sichuan Basin, with the action of replacement and deposition, structurally controlled - hydrothermal dolomite appears in two kinds of occurrence. One is the quasi-layered, lenticular or patchy dolomite distributed along the fracture system, unconformity surface and other deep fluid channels, which experienced destructive effect and constructive effect to porosity, contains pores largely inherited from primary pores, and is composed of mainly medium coarse to coarse grain saddle dolomite (Fig. 4g). Deep-source magnesium-rich hydrothermal fluids with temperature 5 °C higher than the surrounding environment migrate upward through the unconformity surface and fracture system, leading to formation of dolomite through dolomitization by replacing the adjacent limestone. Previously, the origin of this dolomite was explained by hydrothermal dolomitization^[17].

The other is the saddle dolomite developing along the fracture system, unconformity surface, dissolved vugs and pores by precipitation^{[17, 46]}. It is composed of coarse to macro crystalline dolomite, and the host rock is limestone or dolomite (Fig. 4h, 4i). It generally comes in the form of cement filling vugs and pores, which mainly destroy porosity, forming residual pores, vugs and fractures. It could appear alone or in association with hydrothermal minerals such as quartz, fluorite, sulfides (pyrite, galena, sphalerite).

The structurally controlled - hydrothermal dolomite is characterized by large crystal size, curved crystal face, sweep extinction, and relatively low degree of order (0.6-0.8). The fluid inclusion homogenization temperature and isotope (D47) temperature (190-220 °C) are apparently higher than the formation temperature when the dolomite was formed. The fluid is abundant in volatile gas and ¹⁸O, high in salinity (12%-25%), and severely depleted in oxygen stable isotopes (-25‰ - -10‰ PDB). The dolomite has strong cathodoluminescences or alternation with nonluminescence, which is related to the change in Fe and Mn contents. Its Sr isotope ratio (⁸⁷Sr/⁸⁶Sr) is much higher than that of host formation of saddle dolomite, and the isotopic dating reveals its absolute age is younger than the formation.

Geochemical features and change trends of the three types of high-temperature crystalline dolomites in burial stage are shown in Fig. 5.

It should be pointed out that some geochemical indexes mentioned above may have deviation since the dolomite would experience various intensity of late digenetic transformation after forming, therefore, the analysis of geochemical data based on specific diagenetic evolution is essential in practical application. Regardless of this, the change trend of geochemical indicators between dolomites formed in time sequence can be preserved.

As mentioned above, the origin of pores in dolomite reservoir has been a hot topic in geological research for a long time. According to the relationship between development degree and types of pores and dolomitization in a large number of cases, it is concluded that the direct contribution of dolomitization to porosity has been exaggerated. The pores in large-scale dolomite reservoirs are mainly the inheritance of primary pores and resulted from supergene dissolution, partly derived from burial dissolution. The early dolomitization is conducive to preservation of pores, but doesn’t generate pores directly.

Based on the theoretical calculation results, dolomitization in the closed environment would lead to volume reduction. Most researchers considered the porosity in dolomite reservoir the product of dolomitization^{[32, 47-49]}. Huang Sijing et al. conducted geochemical analysis of crystalline dolomite reservoir in the Feixianguan Formation, Northeast Sichuan, and proposed that burial dolomitization in the closed system played an important role in reservoirs forming^[50]. Meanwhile, research shows that dolomitization also may destroy porosity^{[29, 33, 51-52]}. Therefore, the contribution of dolomitization to reservoir porosity is uncertain in general. The point generally accepted is that dolomitization can destroy, maintain or enhance the porosity development^[5] according to the difference in geological conditions and chemical kinetics reaction.

In this work, based on the case studies of dolomite reservoirs in three large basins, the diverse effects of dolomitization on porosity alteration have been reveals in mainly three aspects.

(1) Dolomitization can generate some pores. In partially dolomitized bioclastic limestone of the Changxin Formation in Longgang area, the sponge spicule and tissue have experienced different degrees of dolomitization (Fig. 6a, 6b) are mainly composed of high calcium dolomite, and distributed along the organism structure. Micro fissures and pores increased remarkably after the sponge tissue experienced dolomitization (Fig. 6c). It should be pointed out that, the contribution of dolomitization to porosity construction is very limited although proved by this case. The porosity generated by dolomitization may only provide seepage channels for late diagenetic fluid transformation, and can’t directly give rise to large-scale high-quality reservoirs.

(2) Early dolomitization is favorable for pore preservation. With strong resistance to compaction, dolomite can preserve the porosity formed early. For example, in supergene environment, the dissolving capacity of dolomite is far weaker than easily soluble minerals such as aragonite, high-magnesian calcite and gypsum, thus, the preservation of moldic pores formed by dissolution of the easily soluble minerals benefited from the framework made of the insoluble dolomite (Fig. 2f). In addition, due to the strong resistance to compaction of dolomite, pressolution products are hardly generated (sutures are mainly developed in limestone, rarely in dolomite). The lack of source for burial cementation is one of the important factors contributing to preservation of preexisting pores in the dolomite related to evaporation environment. The brittleness of dolomite is favorable for fracture development, which is also one of reasons why dolomite reservoirs generally have better quality than limestone reservoirs.

(3) Associated dolomite cement and precipitation of saddle dolomite destroy porosity. After calcite is replaced by dolomite, over dolomitization or dolomite cementation would happen if there is a continuous fluids supply. For example, the dolomite cement precipitated in intergranular pores (Fig. 2b, 2i), and the dolomite precipitated by burial dolomitization and hydrothermal dolomitization when reforming parent rocks or surrounding rocks, destruct dissolved vugs and pores by filling.

Although the porosity of dolomite reservoir is not primarily resulted from dolomitization, dolomite is able to form high quality reservoir. Pores in dolomite reservoirs mainly come from inheritance of primary pores, secondarily from enlarged pores by dissolution of meteoric fresh water in the early supergene environment and dissolved vugs and pores formed by burial - thermal dissolution in the burial environment.

Dolomites with complete and part of primary grain texture have a large number of intergranular pores, framework pores (Fig. 2d, 2g, 2i), organism pores, moldic pores (Figs. 4c and 6d), and enlarged pores (further dissolution of primary pores) remained. The intergranular pores, framework pores, organism pores are primary pores formed in deposition, and the moldic pores and enlarged pores are related to dissolution of meteoric fresh water in early supergene stage. In burial environment, the burial-hydrothermal dissolution can form non-fabric selective pores (Fig. 6e, 6f), which are an important supplement to dolomite reservoir space. Pores of the three origins constitute the main dolomite reservoir space^[53].

The classification of dolomite based on petrography, environment and sequences has been proposed. Development of the penecontemporaneous low-temperature dolomite greatly depends on the paleoclimate and paleoenvironment. As the paleoclimate varied from wet to dry, when the temperature, salinity and alkalinity increased, seawater (island) dolomite, microbial dolomite, evaporated dolomite developed in sequence, finally to gypsum salt rock. Penecontemporaneous low-temperature dolomite mainly includes (gypsum) micritic dolomite and reef (mound) beach facies dolomite generated by precipitation (protodolomite) and replacement respectively, with the original texture well preserved. Three types of dolomite in burial stage are mainly the crystalline dolomite with original texture hardly preserved, and even if part grain texture is remained, the grains are composed of crystalline dolomite. Moreover, with increasing burial depth, diagenetic temperature, dolomitization time and varying diagenetic fluids, the dolomite grains turn coarser and more euhedral.

There are clear boundaries in geochemical characteristics and evolving clues between the six different types of dolomite, however, a lot of dolomite isn’t of single genesis, for example, the early low-temperature dolomite could receive transformation by burial dolomitization additionally, and the dolomite with crystal grain texture formed during burial stage could receive transformation by structural-hydrothermal dolomitization, which greatly increase the geochemical complexity of dolomite. It is necessary to conduct analysis on orderly geochemical and petrological variation corresponding to time and diagenetic environment to help make a more comprehensive and reasonable genetic explanation. Further studies on factors leading to geochemistry difference between different genetic types of dolomite are needed.

Since dolomite reservoirs account for a high proportion in hydrocarbon reservoirs, and have better reservoir properties to limestone, dolomitization is considered as an important contributor to porosity construction. This study shows dolomitization can obviously increase the micro- fissures but has little contribution to reservoir space, the micro-fissures in the dolomite can provide pathways for diagenetic fluids, laying a solid foundation for the supergene dissolution, large-scale dolomitization during burial stage and development of burial dissolved pores. The pores in dolomite are mainly inherited from the primary porosity, and some are from supergene dissolution and burial dissolution. Over-dolomitization would destroy porosity by precipitation of dolomite cement and saddle dolomite. Early dolomitization is conducive to the preservation of primary pores, by forming pore framework with high resistance to compaction, reducing source of cements, and generating brittle fractures.