Dolomite genesis has been studied for more than 200 years. Grain dolomite, as one of the most important reservoirs of oil and gas, is a hot spot, focus and difficulty in dolomite genesis research field. Since 2010, large-scale doloarenite which can form high-quality reservoirs was discovered in the Ordovician Majiagou Formation of Ma₅⁵ submember, Ordos Basin^[1]. In addition, this kind of dolomite was also found in the Lower Triassic Maocaopu Formation in the northern Guizhou province of the Yangtze area, the Lower Triassic of Jialingjiang Formation in the southwestern Hubei province, the Middle-Upper Cambrian in the Upper Yangtze area, and the 4^th Member of Sinian Dengying Formation (Z₂dn₄) in Sichuan Basin^[2,3,4,5,6]. But different researchers have different views on the origin of the grain dolomite. Li Guorong and Chen Zhiyuan believed that it was the product of mixed water dolomi-tization (freshwater and seawater in early diagenetic stage)^[6,7]. Yang Hua and Huang Zhengliang considered that it’s probably the product of coupling seepage reflux dolomitization and burial dolomitization during the burial stage^[8,9,10]. Liu Deliang thought that this grain dolomite was directly related to diagenetic and buried dolomitization^[11]. Zhang Yongsheng, Wang Baobao, Suzhong Tang, and Fu Jinhua et al proposed that the origin of this grain dolomite was the result of buried dolomitization and local hydrothermal dolomitization^{[12,13,14,15]}. Petrash D A, Chen Yana et al suggested that early low temperature microbial action could also produce grain dolomite beach^[16,17].

Many researchers believe that the grain dolomite in M₅⁵ submember of Ordovician Majiagou Formation in northwestern Ordos Basin is the product of seepage reflux dolomitization together with burial dolomitization during the burial stage^{[7-11, 18]}. But is the grain dolomite from grain limestone dolomitization, or particle recrystallization of grain dolomite or microbial action? Although several industrial gas wells with a daily gas output of over 1 million cubic meters each have been found in O₂m₅⁵ of northwestern Ordos Basin, the origin of the grain dolomite remains to be answered. To solve this problem, besides conventional methods like microscopic observation for drilling core, cathodoluminescence, x-ray diffraction, and homogenization temperature of inclusions, the authors also conducted microsampling trace element analysis and laser microsampling stable isotope analysis to get accurate data to answer the origin of grain dolomite^[19].

Ordos Basin is located in the west of North China Craton^[20]. There are 6 tectonic units in this basin, Yimeng uplift, western Shanxi fold belt, Yishaan slope, Tianhuan depression, western margin thrust belt and Weibei uplift. The study area is located in Yishaan slope in the central part of the basin. The Majiagou Formation of middle Ordovician is divided into 6 members, O₂m₁, O₂m₂, O₂m₃, O₂m₄, O₂m₅, and O₂m₆ from bottom to top. O₂m₅ is mainly composed of dolomite, followed by limestone, mudstone and evaporite. O₂m₅ is further divided into 10 sub-members (from O₂m₅¹ to O₂m₅¹⁰) from top to bottom. More importantly, O₂m₅⁵ is the interest of this research. The basin was a semi-limited platform during the deposition of O₂m₅⁵, where micro-sedimentary facies of gypsum dolomitic flat (rich in gypsum concretion), limestone dolomitic flat (rich in lime dolomite) and limestone flat (rich in limestone) developed in turn from the northwest margin to the eastern part of the basin (Fig. 1). In O₂m₅⁵, grain dolomite is distributed like a ring on the plane in the supratidal-intertidal zone. There are 3 rock types, dolomicrite, limestone and grain dolomite on the section of O₂m₅⁵. Of them, dolomicrite is the surrounding rock of grain dolomite and its main mineral component is micrite dolomite. Limestone mainly occurs at the bottom of O₂m₅⁵ and is largely composed of calcite (Fig. 2). Grain dolomite is composed of spherical particles and interstitial material between spherical particles. The spherical particles are composed of micrite to silt dolomite and the interstitial material is composed of fine to medium dolomite (Fig. 3). For the sake of brevity, several key words which appear frequently in the following passage are explained as follows. Micrite dolomite refers to the mineral component of dolomicrite in surrounding rock, calcite is the mineral component of limestone, and micrite to silt dolomite is mineral component of the spherical particles in grain dolomite, and fine to medium dolomite is the mineral component of interstitial material between spherical particles in grain dolomite.

Samples were collected from the core of the Ma₅⁵ in 14 wells of the northwestern Ordos Basin (see Fig.1 for the sampling well No.). According to different depths, 5 limestone samples were taken from Well SD39-62C, 7 limestone samples from Well SD39-57 (Fig. 3a), and 3 dolomicrite samples from ell SD39-62C1 (Fig. 3c). The above samples were prepared by conventional methods. Grain dolomite samples were taken from different depths of the remaining 12 wells (Fig. 3e).

Micro sampling technology was applied to extract micrite to silt dolomite. Microscopic drilling tool was used for trace element analysis, while Laser microsampling was used for stable isotope of carbon and oxygen analysis. The micro sampling equipment can extract micrite to silt dolomite in spherical particles and fine to medium dolomite between different spherical particles precisely to ensure the purity of the samples and the accuracy and scientificalness of the test result. Micro-drill tool was operated by computer and electron microscope during trace element sample preparation. Micrite to silt dolomite and fine to medium dolomite were precisely extracted by the micro-drill tool (Fig. 3e-3f).

Core observation, ordinary thin section and cast thin section observation, X-ray diffraction, inclusion, carbon and oxygen isotope analysis were carried out. Scanning electron microscope analysis used Quantan 250 FEG SEM (with Oxford INCAx-max 20 EDS) manufactured by FEI Company of the US. The fluid inclusion analysis instrument was MDS 600 heating/cooling stage (with Carl-Zeiss Axio-plan 2 microscope) manufactured by the UK Linkam Company. Major and trace elements of Na, Sr, Ba and Mn were tested by inductively coupled plasma atomic emission spectrometry, and Fe was tested by dichromate titration.

Laser microsampling instrument was used for stable isotope of carbon and oxygen analysis. The core samples were made into thin sections of about 80 microns thick. The microsampling surface of the thin section was not polished, and the polished surface was bonded to glass with non-epoxy resin material to avoid releasing gas under vacuum condition, which would affect the accuracy of the analysis results of δ¹³C. Laser beams (wavelength 1046) was focused on the thin section in the vacuum sample box via a microscopic lens system. The sample decomposed to produce CO₂ gas when the temperature reached 1500 °C. Then the purified CO₂ was imported into isotope mass spectrometer (MAT 252) to test δ¹⁸O and δ¹³C. Laser microsampling technology for stable isotope analysis greatly improves the spatial resolution of samples, and makes the analysis accuracy of δ¹⁸O and δ¹³C higher.

Several groups of samples were analyzed by laser pyrolysis method and acid etching method, and the results show that δ¹⁸O from laser pyrolysis method has obvious distillation, while δ¹³C from laser pyrolysis method has little distillation, calcite and dolomite from laser pyrolysis method are 1.66‰ negative and 1.41‰ negative in δ¹⁸O than the corresponding data from acid etching method. Therefore, to make the test data from laser pyrolysis method comparable with the data from other methods, the δ¹³C and δ¹⁸O from laser pyrolysis method were corrected and are shown in Table 1. Micrite dolomite in dolomicrite (wall rock) and calcite in limestone were all rock samples. Micrite to silt dolomite in spherical particles of grain dolomite and fine to medium dolomite filled between spherical particles were micro-samples.

Grain dolomite usually has spherical particles or particle phantom structure, and the fillings between particles are mainly fine to medium dolomite. This light gray to gray color grain dolomite mainly occurs in the middle and upper part of Ma₅⁵ (Fig. 2 and Fig. 4a). The grain dolomite often remains the original spherical structure, and the spherical particles are mainly made up of micrite to silt dolomite (Figs. 3e and 4b), and gypsum pseudo crystals well preserved can be seen in the spherical particles (Figs. 3e-3f and Fig. 4f). Due to recrystallization, micrite to silt dolomite at the edge of the spherical particles are turned into fine to medium dolomite in subhedral crystals largely with rich intergranular pores (Figs. 3e-3f and Fig. 4b). Spherical particles are cut by cracks when tectonic activity is very strong, so the formation fluid can infiltrate into the spherical particles more easily, creating good conditions for recrystallization; and micrite to silt dolomite of the whole spherical particles would turn into fine-medium dolomite with abundant intergranular pores (Figs. 4c, 4d). Only the phantom of the spherical particle can be observed (Fig. 4c-4e). The grey to dark grey dolomicrite is the surrounding rock of grain dolomite and occurs in the whole Ma₅⁵ member (Fig. 2). Dolomicrite is made up of micrite dolomite (Figs. 3c-3d and 5), with some primary sedimentary structures such as bird-eye structure, salt pseudo-crystal and gypsum pseudo-crystal well preserved (Fig. 5a-5d). Light grey tight limestone mostly emerges at the bottom of Ma₅⁵, and is made up of crystal calcite (Fig. 3a-3d).

Spherical particles of grain dolomite and dolomicrite (surrounding rock) have the same mineral components, and primary sedimentary structures such as salt pseudo-crystal and gypsum pseudo crystal preserved (Fig. 3e-3f and Fig. 5b-5d). In contrast, there are few sedimentary structures in limestone. Therefore, it can be inferred that the matrix of spherical particles in grain dolomite and dolomicrite are the same, and come from micrite to silt dolomite formed during penecontemporaneous.

This section mainly elaborates geochemical characteristics, including x-ray diffraction, cathodoluminescence, major and trace elements and δ¹³C, δ¹⁸O isotopes of several main minerals, including micrite to silt dolomite in the spherical particle of grain dolomite, fine to medium dolomite filling between the spherical particles, micrite dolomite in micrite dolomite of surrounding rock and calcite in limestone.

3.2.1. X-ray diffraction and cathodoluminescence

X-ray diffraction analysis shows that micrite dolomite and micrite to silt dolomite have an order degree of 0.57-0.82, and 0.75 on average, and the average molar ratio of Ca of 49.2%. Both micrite dolomite and micrite to silt dolomite have a lower order degree and molar ratio (Ca) value than dolomite associated with non-evaporative environment (Table 1). Besides, Dolomite is dark brownish red color under cathodoluminescence (Fig. 5f). This kind of dolomite is considered to be the product of the quasi-syngenetic stage of the Sabha environment^[2]. The fine to medium dolomite has an order degree of 0.88-0.93, 0.90 on average, and an average molar ratio of Ca of near 50% (Table 1). The fine to medium dolomite has a better order degree and a standard Mg/Ca value (1:1). The crystals are polyhedral, dirty inside, and clean at edges, with few associated minerals. The dolomite has enough space to grow freely, and the MgCO₃ and CaCO₃ layers are in almost ideal arrangement. Under cathodoluminescence, it is dark orange red with a thin bright orange-yellow rim at the edge, which indicates recrystallization (Fig. 4f).

3.2.2. The major and trace elements

3.2.2.1. Mn, Fe and Sr

The sedimentary and diagenetic environment of dolomite can be reflected by trace elements. Dolomite formed in early oxidizing environment near surface has lower contents of Mn and Fe than dolomite formed in deep burial reducing environment^[20,21,22]. Dolomite continuously exchanges material with pore fluid in burial environment. According to the principle of chemical equilibrium, when the reaction pressure increases, the reversible reaction will proceed in the direction of volume reduction. In other words, as the formation pressure increases gradually during burial compaction, the cations of larger radius in dolomite lattice will be preferentially replaced by smaller ones. Therefore, the content of larger ion metal elements (Sr) will decrease and that of smaller elements (Fe, Mn) will increase in the process of burial diagenesis^[23].

The micrite dolomite and micrite to silt dolomite have lower contents of Mn and Fe than fine to medium dolomite (Fig. 6), which shows that micrite dolomite and micrite to silt dolomite experienced very weak burial diagenesis, while fine to medium dolomite experienced deep burial depth diagenesis. Micrite dolomite can approximately represent the quasi-syngenetic dolomite of sedimentary period, which is the precondition for comparative analysis of geochemical parameters of micrite dolomite and micrite to silt dolomite. Mn/Sr value less than 3 is also an important basis indicating no obvious burial diagenesis of dolomite. The larger the Mn/Sr value, the stronger the burial diagenesis the dolomite has experienced^[24,25,26]. The fine to medium dolomite has lower content of Sr, but higher content of Mn than calcite and micrite to silt dolomite in the study area (Fig. 7); consequently, the fine to medium dolomite has a Mn/Sr value of about 5-8, while calcite and micrite to silt dolomite have a Mn/Sr value of about 0-2 (Fig. 8), indicating that the fine to medium dolomite experienced strong burial diagenesis. Besides, the micrite dolomite and micrite to silt dolomite have similar contents of Mn and Fe and Mn/Sr value, indicating that they are from the same quasi-syngenetic dolomite. Although experiencing burial diagenesis, the micrite to silt dolomite with compact lithology is difficult for the diagenetic fluid to seep in without fracture cutting through, so it experienced weak alteration by diagenetic fluid. In places near cracks where the diagenetic fluid had good material exchange conditions, the dolomite comes in larger crystal, showing higher degree of recrystallization (Fig. 4g, 4h).

3.2.2.2. Mg and Ca

Analysis results of major elements show that the calcite samples have MgO and CaO contents of 0.32-7.68 and 41.34- 59.51 respectively, the micrite dolomite samples have MgO and CaO contents of 17.21-19.22 and 36.21-38.14 respectively, and micrite to silt dolomite samples 16.28-26.37 and 27.61-37.72 respectively, and fine to medium dolomite samples 19.31-31.42 and 18.31-32.31 respectively (Table 1). As shown in Fig. 9, the calcite has low MgO content and high CaO content, which indicates no or weak dolomitization. The micrite dolomite and micrite to silt dolomite have similar contents of MgO and CaO, suggesting that spherical particles of grain dolomite and dolomicrite of surrounding rock have similar mineral components and are quasi-syngenetic dolomite. The fine to medium dolomite has high MgO content and low CaO content which are in linear negative correlation, indicating recrystallization^[18]. But the recrystallization degree of grain dolomite is related to the growth of crack. In the sites with fractures, the diagenetic fluid would flow freely and providing good conditions of material exchange, so the recrystallization degree is higher, the dolomite comes in large crystals, with intercrystalline pores in between (Fig. 4g, 4h). On the contrary, in places with no fractures, the dolomite appears in small crystals, reflecting lower recrystallization degree.

3.2.3. δ¹³C and δ¹⁸O

Test data of δ¹³C and δ¹⁸O shows that micrite to silt dolomite has δ¹³C about -2.0‰--0.9‰, -1.6‰ on average, and δ¹⁸O about 7.7‰--4.4‰, -6.3‰ on average; fine to medium dolomite has δ¹³C of about -2.8‰--1.6‰, -2.1‰ on average, and δ¹⁸O of about -11.7‰--7.9‰, -9.9‰ on average, and calcite has δ¹³C of about -3.2‰--2.3‰, -2.7‰ on average, and δ¹⁸O of about 10.1‰--7.2‰, -8.9‰ on average (Table 1, Fig. 10). Temperature and δ¹⁸O in formation liquid are important factors directly determining δ¹⁸O in dolomite^{[9, 27]}. when the fresh seawater is insufficient and the salinity of seawater increases, δ¹⁸O and δ¹³C in the sediment increase^[28]. Therefore, the micrite dolomite and micrite to silt dolomite, formed in strong evaporation and high salinity environment have higher δ¹⁸O and δ¹³C than calcite and fine to medium dolomite. In addition, the contents of ¹⁸O and ¹³C in minerals decrease with the change of mineral facies during diagenesis; on the contrary, ¹⁸O and ¹³C in pore water increase because ¹⁸O and ¹³C in minerals are taken away by pore water. This is why δ¹⁸O and δ¹³C in minerals are often negative after facies change^[29]. Micrite to silt dolomite in spherical particles of grain dolomite experienced recrystallization during burial diagenesis, turning into fine to medium dolomite with intergranular pores, with contents of ¹⁸O and ¹³C dropping and δ¹⁸O and δ¹³C in the minerals becoming negative. Obviously, micrite to silt dolomite turning into fine to medium dolomite is reasonable and consistent with the theory above. In contrast, compared with fine to medium dolomite, calcite has similar δ¹⁸O and positive δ¹³C, suggesting the possibility of calcite turning into fine to medium dolomite through burial diagenesis is low (Fig. 10).

In recent years, a genetic types of grain dolomite reported mostly included dolomitization of grain limestone and microbial action^[30]. The formations of grain dolomite formed by grain limestone dolomitization are mainly as follows: the Lower Triassic Jialingjiang Formation in Enshi, southwestern Hubei province ^[2], the Lower Triassic Maocaopu Formation in the northern Guizhou province of the Yangtze area^[3], the Middle-Upper Cambrian in the Upper Yangtze area^[4], the Middle-Upper Cambrian in Shimen, Hunan^[5], Middle-Lower Ordovician in Tazhong Area, Tarim Basin^[31], Cambrian-Ordovician in the north-central part of North China Platform, the Lower Cambrian Longwangmiao Formation in eastern Sichuan basin^[32,33], the Permian-Triassic Khuff Formation in central Saudi Arabia and the Upper Jurassic Arab Formation in southern United Arab Emirates^[34,35]. The grain limestone, which turns into grain dolomite through dolomitization, usually deposits in open subtidal high-energy zone. Microbial dolomite is grain dolomite which is related to microbial action. This kind of dolomite is widely distributed in the world. For example, it has been found in Triassic Leikoupo Formation and Sinian Dengying Formation in Sichuan basin, Cambrian Sholbrack Formation in Tarim Basin^[36], modern sediments in Coorong Area, Southern Australia and the Triassic in Zsámbék Basin, north-central Hungary^[37,38]. Zhao Wenzhi et al proposed that microbial dolomite was formed in shallow sea (lake) tidal flat environment with specific temperature (30-45 C), salinity (3.5%- 10%) and alkalinity (pH value larger than 8.5); the 3 indicators too high or too low weren’t conducive to the formation of microbial dolomite, and the 3 indicators too high were favorable for the formation of evaporite^[30].

Firstly, gypsum pseudo-crystal and salt pseudo-crystal were observed in spherical grains of grain dolomite in Ma₅⁵, which are indicators of the exposure environment of the supratidal zone (Fig. 3f and Fig. 4c). Obviously, it is not a favorable sedimentary environment for grain limestone. What’s more, grain dolomite is mainly distributed in dolomitic flat, not in limestone flat that is favorable to grain limestone deposition (Fig. 1). Therefore, the original composition of spherical grains of grain dolomite is micrite to silt dolomite precipitating in dolomitic flat in quasi-syngenetic period, rather than calcite precipitating in limestone flat. Grain dolomite in Ma₅⁵ is not the product of grain limestone dolomitization. Besides, gypsum pseudo-crystals can be observed in grain dolomite of Ma₅⁵, while this kind of gypsum-salt associates is rarely seen in microbial dolomite, so microbial origin of grain dolomite doesn’t hold water.

In this study, the temperature and salinity of sedimentary period and geotemperature and salinity of formation fluid in diagenetic period were restored from fluid inclusions. Fluid inclusions preserve the original metallogenic fluids, record the forming conditions and history of minerals, and reflect the nature of metallogenic fluids. There are many fluid inclusions in the calcite of limestone, quartz of micrite to silt dolomite and fine to medium dolomite of spherical grains. Fluid inclusions in calcite of limestone, quartz of micrite to silt dolomite, fine to medium dolomite of grain dolomite have an average homogenization temperature of 60.4 °C, 117.4 °C and 146.3 °C, and average salinity of 9.75%, 24.73% and 24.39% (Fig. 11). The formation temperature in Ordos Basin increases with the increase of burial depth, showing a stable linear relationship. Nowadays, the geothermal gradient is about 2.4-3.1 °C/100 m, higher in the east and lower in the west, and 2.5 °C/100 m in the northwest^[39]. Based on the buried depth of 3 100 m, the temperatures when the calcite in limestone, quartz in silt-micritic dolomite and fine to medium dolomite between spherical grains formed were 14.1, 24.9 and 70.7 C respectively. Therefore, it can be inferred that calcite in limestone was formed in normal seawater with low salinity, micrite to silt dolomite in spherical grains of grain dolomite was formed in supratidal zone, high temperature and evaporation environment, fine to medium dolomite filling between spherical particles was formed in burial environment with much higher temperature than ground conducive to recrystallization. Besides, the salinity of fine to medium dolomite is similar to micrite to silt dolomite, indicating the nature of diagenetic liquid in burial period was similar to that of the seawater in quasi-contemporaneous period, and the Mg²⁺ was from reflux seepage of seawater^[15].

The micrite to silt dolomite in spherical grains deposited under temperature and salinity similar with micrite dolomite, but quite different from calcite in limestone, which proves that micrite to silt dolomite in spherical grains is homologous with micrite dolomite in surrounding rock. They are both from micrite dolomite formed in quasi-syngenetic period, but from source different with the calcite, indicating that fine to medium dolomite is the product of recrystallization of silt-micritic dolomite. Besides, although grain dolomite can be formed by microbial action in the limited sea (lake) of arid tidal flat, the temperature and salinity in the sedimentary period of the grain dolomite were 24.9 °C and 24.39% respectively, this lower temperature and higher salinity environment are not suitable for microbial action, so the microbial origin of the grain dolomite is ruled out.

Test data of δ¹³C and δ¹⁸O shows that δ¹³C and δ¹⁸O of micrite to silt dolomite in spherical grains of grain dolomite are almost the same with those of micrite dolomite in micrite dolomite (Fig. 10). This results show that the spherical grain and micrite to silt dolomite are homologous, and both originated from micrite dolomite depositing in the quasi-syngenetic period. They are both from micrite dolomite in quasi syngenetic period. δ¹³C and δ¹⁸O of fine to medium dolomite show negative offset. Only when micrite to silt dolomite turned into fine to medium dolomite, can negative offset of δ¹³C and δ¹⁸O occur (Fig. 10). Besides, the geochemical characteristics of major and trace elements, X-ray diffraction and so on also confirm that the micrite to silt dolomite in spherical grains is homologous with micrite dolomite, originating from micrite dolomite in quasi syngenetic period. But fine to medium dolomite is from different material source with calcite. These facts prove that the grain dolomite is the product of recrystallization of fine to medium dolomite under burial conditions.

In summary, the "prototype" of grain dolomite is composed of spherical particles of micrite to silt dolomite formed at the uplift location of sedimentary landform in evaporite tidal flat of restricted sea. Later, under burial conditions, the spherical particles experienced recrystallization in quasi-syngenetic period partially or totally, turning into fine to medium dolomite, and forming dolomite reservoir with intergranular pores. This process is named "dolomitic recrystallization" in this study. Based on study of grain dolomite in Ma₅⁵ of the northwest of Ordos Basin, we concluded that "dolomitic recrystallization" is the origin of grain dolomite. This conclusion is of great significance for predicting the distribution of grain dolomite reservoirs in this area. Primary micrite to silt dolomite grains mainly developed in dolomitic flat (Fig. 12), which, therefore, should be considered as exploration target area. Besides, the grain dolomite layers in Ma₅⁷⁺⁹, Ma₂ and Ma₄ of the Ordovician, Majiagou Formation in the central and eastern basin are all of this origin^[40,41,42].

Micrite to silt dolomite of Ma₅⁵, Ordovician, in the northwest of Ordos basin is the basis of grain dolomite. After deposition in quasi syngenetic period, micrite to silt dolomite exposed intermittently, forming dry debris (Fig. 13a-Ⅰ, 13a-Ⅱ). With the rise of sea level, the debris formed by dry cracking accumulated in disorder under the transport of seawater (Fig. 13a-Ⅲ). The debris particles collided with each other and turned gradually round, forming primary grain dolomite with the high frequency rise and fall of sea level (Fig. 13a-Ⅳ, 13a-Ⅴ). After the deposition of the M₅⁵ silt-micrite dolomite, dolomitic flat dolomite of Ma_1-4 deposited and capillary concentration and reflux osmosis of brine with high Mg²⁺ happened simultaneously (Fig. 13b-Ⅰ). Dolomite filling in intergranular pores between grains of dolomite experienced "dolomitic recrystallization" under reflux seepage and shallow burial, consequently, the dolomite grains grew in size, and in places with fractures easy for formation water to access, the recrystallization is stronger (Fig. 13b-Ⅱ and 13b-Ⅲ). In deep burial stage, formation temperature further rose (Fig. 13c-Ⅰ), and compaction released water seeped toward the dolomite area, causing recrystallization of dolomite and further increase of the crystals, thus the fine to medium dolomite reservoir with intergranular pores came up (Fig. 13b-Ⅱand 13b-Ⅲ).

The "prototype" spherical particles in grain dolomite were formed in the arid evaporation environment of restricted sea during the quasi-syngenetic period. Gypsum pseudo-crystals can be seen clearly in remained spherical particles. The micrite to silt dolomite making up the spherical particles has similar composition, trace elements and δ¹⁸O and δ¹³C with micrite dolomite in surrounding rock. Therefore, it can be inferred that micrite dolomite and micrite to silt dolomite were formed in the same sedimentary environment. Under burial conditions, micrite to silt dolomite in the grain dolomite recrystallize, forming grain dolomite reservoir with intergranular pores. Micrite to silt dolomite turning into fine to medium dolomite is reasonable, as proved by negative δ¹⁸O and δ¹³C in the fine to medium dolomite, not calcite. Spherical particles in grain dolomite were formed in a low temperature and high salinity environment which is impossible for microbial action, so the microbial origin of the particle is ruled out. Fine to medium dolomite is the product of "dolomitic recrystallization" of micrite to silt dolomite in spherical particles, therefore, the dolomitic flat should be taken as the key target area for the exploration of grain dolomite reservoir.