Genesis of dolomite in the upper assemblage of the Ordovician Majiagou Formation in the southeastern Sulige gas field, Ordos Basin, China: Evidence from C, O, and Sr isotopes and major and trace elements

Typical dolomite reservoirs exist in the upper assemblage of the Ordovician Majiagou Formation in the southeastern Sulige gas field, Ordos Basin, however, the current understanding of dolomite genesis needs to be clarified. This study confirms the dolomitization characteristics, diagenetic environment, and genesis of dolomite through the use of core observations, thin section identification, and geochemical data (carbon and oxygen stable isotopes, strontium isotopes, and major and trace element contents). The results showed that dolomite samples from the study area includes micritic dolomite, powder crystal dolomite, and micrite to powder crystal dolomite, in which the main reservoir space consisted of intergranular pores, intercrystalline pores and various dissolved pores. The δ13C value of the dolomite samples ranged from −7.26‰ to1.28‰ with an average of −0.45‰, which is equivalent to that of seawater during the Ordovician. The δ18O value ranged from −9.94‰ to −6.32‰ with an average of −7.86‰, which is clearly more negative than that of the Ordovician seawater. The 87Sr/86Sr ratio varied from 0.70867 to 0.71033 with an average of 0.70946, which is significantly more positive than that of Ordovician seawater. The Mg/Ca ratio was lower than that of the seawater and close to 1:1. Overall, all of the samples were characterized by high Fe and Mn contents, while low Sr and Na contents. These results clearly indicate that the dolomitized fluid was closely related to seawater or a sea-source fluid. Furthermore, the restored paleosalinity, paleotemperature, and paleodepth also indicate that the dolomite was formed in a marine reducing environment. Combined with the petrological characteristics and geochemical indicators, two dolomitization models (mixed water dolomitization and burial dolomitization) were established for the upper assemblage of the Ordovician Majiagou Formation in southeastern Sulige gas field. We infer that mixed water dolomitization was dominant before the Carboniferous, whereas burial dolomitization was dominant after the Early Permian.


Introduction
Carbonate rocks (especially dolomite) are characterized by solubility and variability. Accordingly, the final hydrocarbon accumulation space essentially comprises a variety of secondary pores, which are mainly produced during diagenesis (James and Choquette, 1984;Roehl and Choquette, 1985;Ronchi et al., 2010;Yang et al., 2012;Wang et al., 2015). It is therefore particularly important to accurately understand the diagenetic environment. Carbon and oxygen isotopic analyses are common methods used to study the diagenetic environment and diagenesis of marine carbonate rocks. Many previous investigations have demonstrated that the stable isotope composition of carbonate rocks is closely related to the diagenetic environment, and can approximately reflect the stable isotopes composition of ancient seawater, as well as providing important information regarding paleosalinity, paleotemperature, paleoclimate, and sea level changes. Therefore, the composition of stable isotopes is of great significance to the diagenetic environment of carbonate rocks, and is also important for distinguishing the genesis of dolomite (Brand and Veizer, 1980;Derry et al., 1989Derry et al., , 1994Huang, 1997;James and Choquette, 1984;Keith and Weber, 1964;Morrow, 1982). The carbon isotope composition is mainly influenced by the organic carbon content and degasification, while the oxygen isotope composition is mainly influenced by meteoric water, temperature, and diagenesis (Lohmann, 1988;Madhavaraju et al., 2004). The migration and enrichment of elements in strata not only depends on the physical and chemical properties of the elements themselves, but are also greatly influenced by the extraneous conditions (e.g., paleoclimate and paleoenvironment); thus, the distribution of major and trace elements in strata reflects the changes of the paleoclimate and diagenetic environment to a certain extent (Jia and Liu, 2000).
As an important natural gas producing layer in the Ordos Basin, the genesis of dolomite in the Ordovician Majiagou Formation has always been a focus of geologists. Ordovician strata could be divided into three gas-bearing assemblages (upper, middle, and lower) during the long-term exploration practice. The upper assemblage of the Majiagou Formation consists of Ma5 1 to Ma5 4 , the middle assemblage consists of Ma5 4 to Ma5 10 , and the Ma4 member and those below it belong to the lower assemblage ( Figure 1; Yang and Bao, 2011). Due to the dynamics and continuity of diagenetic evolution, the genesis of dolomite is relatively complicated (Eric and Qing, 1992;Warren, 2000). The genetic model of dolomitization in the Majiagou Formation is still controversial due to the variation of the geological conditions between regions and the limited number of samples. It is generally believed that i) the northern part of Tianhuan sag was mainly affected by mixed water, burial, and hydrothermal dolomitization, ii) the southwestern margin of the basin was mainly affected by penecontemporaneous, seepage reflux, and burial dolomitization, iii) the eastern margin of the basin was mainly affected by penecontemporaneous, mixed water, burial, and hydrothermal dolomitization, and iv) the central Ordos Basin was mainly affected by penecontemporaneous and mixed water dolomitization (Bao et al., 2017;Chen et al., 1998;He et al., 2014;Li et al., 2005;Liu et al., 2017;Qin and Zeng, 1994;Zhao et al., 2005). In conclusion, the model of penecontemporaneous, seepage reflux and burial dolomitization is widely accepted for the entire Ordos Basin.
There are some limitations in previous studies, for example, previous studies regarding the mechanism of dolomitization in the Majiagou Formation mostly took all of the layers in the Ma5 member as a whole, or mainly focused on the middle assemblage, On the other hand, few studies have been conducted for the upper assemblage, where it is believed that there is little burial dolomite. However, studies have reported obvious characteristics of burial dolomitization in the upper assemblage of Ordovician Majiagou Formation in southeastern Sulige gas field. Based on the petrological and geochemical characteristics samples from this region, the aims of this study are to: (1) analyze the distribution and evolution characteristics of C, O, and Sr isotopes as well as major and trace elements; (2) discuss the geochemical characteristics and diagenetic environment of dolomite in the upper assemblage of the Ordovician Majiagou Formation; (3) develop a further understanding of the genesis of dolomitization. It is expected that our findings will provide a more powerful theoretical support for the study of the dolomite reservoir of the Ordovician Majiagou Formation in southeastern Sulige gas field.

Geological setting
The Sulige gas field is the largest gas field in China, with an estimated total natural gas reserve of 4.46 Â 10 12 m 3 (Yang et al., 2016). It has been found that there is a good exploration potential and development prospects in the lower Paleozoic carbonate strata, especially in the dolomite reservoirs of the upper assemblage of the Ordovician Majiagou Formation. The study area is located in the northern part of the Yishan Slope of the Ordos Basin (Figure 1), adjacent to the Jingbian gas field, where there is a transition zone of karst highland and karst slope with obvious paleokarst characteristics (Zhu et al., 2019).
The Ordos Basin was located in an epicontinental marine environment during the Ordovician, when there was a large amount of shallow carbonate sediments. The Majiagou Formation is dominated by carbonate rocks with a small amount of evaporite rocks, and has experienced three transgression periods and three regression periods (Xia et al., 1999). The Majiagou Formation can be divided into six major members from the bottom upwards. The Ma1, Ma3 and Ma5 members are mainly dolomite and gypsum rock, which were deposited in in conditions of restricted shallow water and dry climate. The Ma2, Ma4 and Ma6 members, were deposited in an open environment with relatively humid climate conditions, and mainly comprise limestone mixed with a small amount of dolomite. The Ma5 member can be divided into 10 sub-members (Ma5 1 -Ma5 10 ) from the top downwards, whereof the upper assemblage (Ma5 1 -Ma5 4 sub-members; Figure 1) is the main target of the paleokarst gas field (Yang and Bao, 2011). The Majiagou Formation gradually thinned out from the east to the west in the study area, and the Ma6 member had been eroded completely. The weathering crust paleokarst gas field is mainly developed in the upper assemblage of the Ordovician Majiagou Formation. A large number of dissolved pores, caves, and fractures formed by weathering denudation and eluviation are all favorable spaces for the accumulation of natural gas.
During the burial diagenesis process, the strata experienced compaction, pressure dissolution, filling, cementation, recrystallization, and metasomatism. The primary pores of dolomite are less preserved, and most of the pores are dissolution pores, caves and fractures formed in secondary process (Wei et al., 2015;Fu et al., 2019). Core observation and thin section identification in the present study revealed that dolomite samples from the upper assemblage of Majiagou Formation mainly included micritic dolomite, powder crystal dolomite, and micrite to powder crystal dolomite. The pores of the dolomite samples included fractures, dissolved pores, intergranular pores, and intragranular pores. The typical features are as follows. Fractures were either partially or totally filled with mud, gypsum, or calcite (Figure 2(c), (d), (f), (h)). Pinhole-like dissolved pores (Figure 2(a)) and reticular dissolved cracks (Figure 2(d)) were formed by bedding dissolution. Some lyotropic particles were completely dissolved, and then the intergranular dissolved pores (Figure 2(b)) and intragranular pores (Figure 2(g)) then formed. The intergranular pores were mainly distributed between dolomite crystals with a better automorphic degree. A small amount of pyrite could be seen in thin sections, and was mostly located at the edge of dissolved pores (Figure 2(b) and (c)). The cathodoluminescence of dolomite was generally dark red, with local bright red spots indicating the presence of Mn. Parts of the dissolved pores were observed to be filled by calcite and showed a bright orange cathodoluminescence.

Methods and results
The dolomite samples for the analyses were chosen from the Ma5 4 to Ma5 1 sub-members of 38 wells (the locations of the sampling wells are shown in Figure 1). Calcite veins were avoided during sampling to ensure the purity of dolomite samples. There are 43 samples for carbon and oxygen isotopes analysis, 31 samples for strontium isotope analysis, and 56 samples for major and trace elements analysis. The division of dolomite according to the classification standard of "Thin section examination of rock" (SY/T 5368-2000) (micrite < 0.03 mm, 0.03 mm<powder crystal < 0.1 mm). The carbon and oxygen isotopes analysis was performed at the stable isotope analytical laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences. The analytical instrument was a MAT253 gas isotope mass spectrometer (Thermo Scientific corporation, USA) at a mass number range of 1-150 Dalton. According to "Analysis method for carbon and oxygen isotope in organic matter and carbonate" (SY/T 5238-2008), the samples were dried after grinding, and then reacted with 100% phosphoric acid at 25 5 for 12 hours to produce CO 2 . The carbon and oxygen isotope compositions were then determined after drying and removal of impurities. During the analysis, one laboratory standard sample, IVA-CO-1 (d 13 C ¼ 2.21(V-PDB), d 18 O ¼ À1.9(V-PDB)) was tested every six samples to ensure the working status of the instrument. The analytical accuracy of carbon and oxygen was less than 0.06& and 0.15& respectively. The acid fractionation coefficient of dolomite was 1.01109 (25 5), and all the samples were normalized to the isotopic value of 25 5 with the standard value of NBS-19 marble (d 13 C NBS- , whereby the analysis error was within 0.2&. All the results (V-PDB) are shown in Table 1.
The strontium isotope analysis was completed at the analytical laboratory of the Beijing Research Institute of Uranium Geology. According to "Determinations for isotopes of lead, strontium and neodymium in rock samples" (GB/T 17672-1999), the samples were ground to powder, and then dissolved by hydrochloric acid before being separated and purified using an ion exchange column. The experiment was conducted at 20 0 and 45% humidity. The analytical instrument was PHOENIX-9444 with a test error of AE2r. The results are shown in Table 1.
The major and trace element analyses was conducted at the State Key Laboratory of Continental Dynamics, Northwest University. The contents of major elements were determined using the X-ray fluorescence glass melting method. The analytical instrument was RIX2100 X-ray fluorescence spectrometer (RIGAKU, Japan), with a determination ranging from 10 À6 to 100% and an accuracy better than 2%. The content of trace elements was determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500a, Agilent, USA), with a relative error less than 2%$5%. The results are shown in Table 2.

Evaluation of data validity
The validity of the research cognition depends on whether the samples retain the isotopic composition of the original sedimentary environment. According to previous studies, the Mn/Sr ratio, oxygen isotope composition, and correlation between the d 13 C and d 18 O were used to comprehensively determine the validity of the experimental results in this study.
Mn/Sr ratio. The loss of Na and Sr and the enrichment of Fe and Mn are affected by the meteoric water cycle during the later stage of carbonate sedimentation (Brand and Veizer, 1980;Huang, 1997), therefore, the Mn/Sr ratio of the samples can be used as an indicator to study the diagenesis and alteration. It is generally believed that when the Mn/Sr ratio is <10, the sample retains the original isotope characteristics, and the carbonate rocks do not suffer severe alteration, while when the Mn/Sr ratio is <2-3, it implies that the sample have inherited the isotopic composition of the original seawater (Derry et al., 1989(Derry et al., , 1994Huang et al., 2008;Kaufman et al., 1993;Yan et al., 2005). With the except of two samples (Sh181: Mn/Sr ¼ 11.12; T51: Mn/Sr ¼ 13.16), the Mn/Sr ratios of 56 dolomite samples were all <10, among which, 30 samples with the Mn/Sr ratios <3, thus indicating that the carbonate rocks have not undergone intense alteration.
Characteristics of oxygen isotope. The oxygen isotope in carbonate rocks is relatively sensitive and can be reduced by temperature, salinity, and water-rock reactions.   (Yan et al., 2005). The correlation between the d 13 C and d 18 O values of the samples was very poor (correlation coefficient of only 0.17; Figure 3); thus, it can be inferred that dolomite essentially inherited the carbon and oxygen isotopic characteristics of the original diagenetic environment.
In conclusion, the samples have basically retained the original diagenetic environment information, although they suffered weak alteration during later diagenesis; hence, the data can be completely used to analyze the original diagenetic environment and the genesis of dolomite.

Characteristics and geological significance of C, O, and Sr isotopes
Isotopic distribution law. The geological age of the upper assemblage of the Majiagou Formation is consistent with the international geological age of the Late Dapingian to Early Darriwilian of the middle Ordovician. There is an obvious global comparability of the Middle Darriwilian inorganic carbon excursion (MDICE). (Sial et al., 2013).   The d 13 C value of normal marine carbonate rocks ranges from À5& to 5& (Veizer and Hoefs, 1976), the d 13 C value of the samples varied between À7.26& and 1.28& with an average of À0.45&, which is consistent with the d 13 C value of Ordovician seawater (-2.0& to 0.5&, after Allan and Wiggins, 1993). The samples were concentrated near the Ordovician carbon isotope curve (Figure 4), thus revealing that dolomitic fluid should be related to the seawater in the same period.
The d 18 O value of samples ranged from À9.94& to À6.32& with an average of À7.86&, which are significantly negative than that of Ordovician seawater (-6.6& to 4.0&, after Allan and Wiggins, 1993). The d 18 O value of carbonates during the Early Darriwilian was between À8.00& and À5.40& with an average of À6.60& (Shields et al., 2003), while that of the dolomite samples from the Majiagou Formation ranged from À9.90& to À6.30& with an average of À8.00&; thus, the maximum, minimum and mean value were all significantly lower than those of the global oxygen isotope (Figure 4). There are many reasons for the negative d 18 O values. One is the leaching of meteoric water, whereby the mixing of meteoric water and seawater dolomitization can theoretical lead to negative d 18 O values. Alternatively, it may have been caused by the increased temperature during burial because the d 18 O value is sensitive to temperature change and decreased with increased temperature. Another possibility is that it might been caused by recrystallization in the burial conditions. The 87 Sr/ 86 Sr ratio of samples varied between 0.70867 and 0.71033 with an average of 0.70946, which is obviously higher than that of Ordovician seawater of 0.7087 (Burke et al., 1982;Shields et al., 2003). The increase of the 87 Sr/ 86 Sr ratio is directly related to 87 Sr-riched fluid, whereby the evolution of marine strontium isotopes reflects sea levels changes (Shields et al., 2003). Terrigenous-strontium fluid was dominant during the regressive period, which results in an increased 87 Sr/ 86 Sr ratio in seawater. In addition, the poor exchange of brine and seawater in a restricted environment could also have led to the increased 87 Sr/ 86 Sr ratio.
Geochemical characteristics and geological implications. The d 13 C and d 18 O value of samples depend on the d 13 C and d 18 O values of the diagenetic fluid, and are also influenced by the salinity and temperature of the diagenetic fluid. The carbon isotope composition is mainly affected by meteoric water and organic carbon, while the oxygen isotope composition is mainly influenced by meteoric water, the burial effect, and temperature. The strontium isotope composition is affected by the diagenetic fluid and by the fluctuation of the sea level. Therefore, the paleosalinity, paleotemperature and sea level changes can be quantified by the composition of C, O, and Sr isotopes, hence, the characteristics of the primitive diagenetic fluid can be qualitatively or semi-quantitatively, and then the diagenetic environment of dolomite will become clearer finally.
Paleosalinity. In general, the d 13 C and d 18 O values increase with the increasing salinity of the diagenetic fluid; therefore, these are widely used to reflect the changes of a sedimentary environment. Keith and Weber (1964) proposed an empirical formula (equation (1)) to calculate the isotope coefficient (Z) by using the d 13 C and d 18 O values, the Z value can used to distinguish the diagenetic environment of carbonate (freshwater facies or marine facies) in Jurassic and later strata. The Z value of the carbonate rocks that formed in a meteoric water diagenetic environment is <120, while it is >120 in seawater and a burial diagenetic environment. The empirical formula can also be used in the strata those are older than Jurassic according to the exploration experiences in the Tarim Basin, Qaidam Basin, and Ordos Basin in China (Wang, 2000). The Z coefficient is also a good indicator of paleosalinity, with a higher Z value corresponding to a higher paleosalinity.
The Z value of the 43 samples (Table 1) ranged from 107.62 to 125.95 with an average of 122.46, which indicates that dolomite in the upper assemblage of the Ordovician Majiagou Formation formed in a stable marine environment. The authors believe that all of the samples were marine facies, although 4 samples (Sh196, Sh225, T36-3, and To4; Table 1) with a Z value of <120 were outliers and may have been affected either by the meteoric water or isotopic fractionation under a high temperature as a regional hydrothermalism. The salinity increased due to the combined effect of the dry climate and restricted sea environment during the upper assemblage of the Majiagou Formation, whereby the carbon isotope curve shows an increasing trend that corresponds to the lowering of the global sea level.
The 87 Sr/ 86 Sr ratio can also be used to distinguish marine facies from non-marine facies because the ratio varies between sources and sedimentary environments. The evolving curve of the 87 Sr/ 86 Sr ratio during different geological period has been established by previous studies (Figure 4). There is an inverse correlation between the 87 Sr/ 86 Sr ratio and sea level, which is helpful to distinguish the diagenetic environment. The strontium isotope in seawater includes mantle-derived strontium and terrigenous strontium, with the 87 Sr/ 86 Sr ratio of the latter being relatively high (Li et al., 2000). The upper assemblage of the Majiagou Formation was deposited in a regressive period, when a considerable amount of terrigenous strontium flowed into the ocean as a result of weathering and denudation, thus increasing the 87 Sr/ 86 Sr ratio in seawater. Veizer and Demovic (1974) believed that the minimum 87 Sr/ 86 Sr ratio of a given set of samples could represent the 87 Sr/ 86 Sr ratio of seawater at that time. The minimum value of the 87 Sr/ 86 Sr ratio in the present study (0.7087) is consistent with the strontium isotope baseline (Figure 4), thereby indicating the genetic background of marine facies. The 87 Sr/ 86 Sr ratio of almost all of the samples were significantly higher than the baseline 87 Sr/ 86 Sr ratio, which might have been affected by fluid rich in high radioactivity 87 Sr during the diagenetic evolution process.
Paleotemperature and paleodepth. The oxygen isotopic composition of carbonate rocks is influenced by the equilibrium exchange reaction between fluid and rock as well as thermal isotopic fractionation. Heavy oxygen isotopes enter the diagenetic fluid, while light oxygen isotopes enter the dolomite lattices. Temperature is an important factor affecting the oxygen isotope composition of carbonate when the paleosalinity remains essentially constant. As an increased temperature leads to a decrease in the oxygen isotope value, oxygen isotopes can be used to restore the paleotemperature (Zhang, 1985). However, not all oxygen isotopes can be used to infer the paleotemperature, and a precondition is that the calcium carbonate content should be >25% (Wang, 2000), otherwise the paleotemperature will be extremely unreliable. The calcium carbonate content of 56 samples in the present study were all >25%; hence, these data could be used to calculate the paleotemperature.
The isotopic fractionation of oxygen between calcium carbonate and water depends on temperature, the temperature of the ancient ocean can be determined by the oxygen isotope composition, Craig (1965) established an empirical formula (equation (2)) to calculate the paleotemperature as follows:  Lea (2003) found that the oxygen isotopes compositions changed by 0.20& with each 1 a increment. The oxygen isotopes of carbonate rocks would have been lost due to the leaching of freshwater during the process of diagenesis alteration. Accordingly, the d 18 O value of samples may be 1&-2& lower than that of the original sedimentary environment, which would introduce a deviation of 5-10 0 to the paleotemperature. In view of this, a correction amount of 1-2& was first added to the oxygen isotope value of dolomite before the paleotemperature were calculated (Table 1, Figure 5(a)). The final paleotemperatures ranged from 34.85 to 55.37 7, with a main distribution range of 36-48 8, which are significantly higher that the usual formation temperature of dolomite (28-35 5). the calculated paleotemperatures satisfy the crystallization requirement of dolomite, thus indicating the high temperature genesis.
The surface temperature was 15 5 and the geothermal gradient was 4.02 0di0 À2 /m at that time according to Liu et al. (2006), and the calculated depth of dolomitization was 417.46-1065.85m (Table 1, Figure 5(b)). Huang (2010) believed that the depth of the transition zone between near-surface zone and deep burial zone was 600-1000 m, which can be considered as the middle burial environment. According to the thermal history and burial history of Sc1 (Figure 5(c)), there was a near-surface diagenetic environment during the Early Paleozoic and then became the deep burial environment after Permian. Both the calculated paleotemperature and the paleodepth have a good correspondence with the results of basin simulation (shadow zone in Figure 5(c)). Considering the burial depth of Ordovician Majiagou formation and the diagenetic sequences of Ma5 member, it is believed that the burial dolomitization of Ordovician Majiagou formation mainly occurred during the late diagenetic stage with a middle burial depth.

Characteristics and geological significance of major and trace elements
MgO and CaO contents and Mg/Ca ratio. The change of the main elements in dolomite is usually expressed by the CaO and MgO contents. The CaO:MgO mole ratio of ideal dolomite is 1:1, and the mass fractions are 30.4% and 21.9%, respectively. The mass fraction of CaO in all of the samples was between 19.93% and 50.85% with an average of 31.25%, which is close to CaO content of the ideal dolomite. The mass fraction of MgO ranged from 3.52% to 21.72% with an average of 19.23%, which is slightly less than the MgO content of the ideal dolomite. The variation of the CaO and MgO contents in dolomite can reflect the extent of metasomatic dolomitization. A positive correlation between CaO and MgO infers that dolomite is of a sedimentary origin, while a negative correlation indicates that dolomite was formed by metasomatism or recrystallization (Hu et al., 2010). There is an obviously negative linear correlation between the MgO and CaO (Figure 6(a)). Most of the samples plotted close to the metasomatic line and only a few samples plotted close to the sedimentary line, thus indicating that dolomite in the study area is primarily of a metasomatic origin. The area was a restricted sea when the strata of the upper assemblage of Ordovician Majiagou Formation was deposited in the Ordos Basin, and a large amount of Mg 2þ was retained in the formation water. Metasomatic dolomitization occurred when the formation water mixed with meteoric water.
The Mg/Ca ratio is also an important parameter reflecting the diagenetic environment of dolomite, and the Mg/Ca ratio of dolomite associated with evaporite minerals is relatively high. Generally, the Mg/Ca ratio in a high salinity environment is >5 (or even 10), while normal seawater has a Mg/Ca ratio of $3-4. Underground fluid with a proportion of sea water of 5%-30% is already oversaturated with respect to Mg 2þ , whereas it is unsaturated with respect to Ca 2þ ; hence, metasomatic dolomitization can occur (Badiozamani, 1973). The Mg/Ca ratio of the samples varied between 0.1 and 1.04 with an average of 0.88 ( Figure 6(b)); thus, these were not associated with near-surface dolomitization because this would correspond to a higher Mg/Ca ratio. These low Mg/Ca ratios indicate that dolomite formed in a shallow burial mixed water environment.
Characteristics of Fe, Mn, Sr, and Na. The Fe, Mn, Sr, and Na contents can be used to distinguish the diagenetic environment and fluid properties of carbonate rocks. Fe and Mn are sensitive elements indicating the redox environment, and the Fe and Mn contents of dolomite that formed in an oxidizing environment are lower than those formed in a reducing environment. At the same time, the Fe and Mn contents can also reflect the diagenetic intensity and burial depth; the deeper the burial depth is, the higher the diagenetic intensity. Sr is the most important trace element in seawater and seawater-derived fluids. Na can directly reflect the salinity of diagenetic fluids. Generally, the Fe and Mn contents are much higher than the Sr and Na contents in carbonate minerals that reformed during the late diagenesis. The Sr isotopic content of carbonate minerals in the early diagenesis is basically the same as that of the original seawater. Some meteoric water mixed into diagenetic fluid due to uplift and denudation, thus resulting in a significant increase in the Fe and Mn contents, a significant decrease in the Sr and Na contents, and an increased 87 Sr/ 86 Sr ratio. High Fe and Mn contents are effective indicators for identifying burial dolomite, and are usually more effective than the Sr and Na contents. Although the Fe and Mn contents in seawater are generally quite low, they are relatively enriched in underground diagenetic fluid, which can effectively distinguish the type of dolomitization environment (Veizer, 1983). The Fe/Ca ratio and Mn/Ca ratio in diagenetic fluid are 1000 times higher than those in seawater (Allan and Wiggins, 1993), which means that Fe and Mn are more enriched in burial dolomite than in primary dolomite. This is because only reductive Fe 2þ can be replaced with Ca 2þ and/or Mg 2þ to be stably preserved in carbonate minerals. The Fe and Mn contents of most primary dolomite is <50 Â 10 À6 , whereas the Fe content in the samples is (1400-35900)Â10 À6 with an average of 7930 Â 10 À6 and the Mn content is (0-1300)Â10 À6 with an average of 275 Â 10 À6 . These high Fe and Mn contents indicates a burial environment for dolomite.
Low Sr and Na contents are also an obvious feature of burial dolomite, and indicate mixed water dolomitization. This is because the Sr and Na contents are very high in seawater and underground diagenetic fluids, while they are quite low in meteoric water. The loss of Sr and Na in early dolomite is usually caused by leaching of meteoric water. Further, Sr 2þ is a product of high salinity fluids that are usually enriched in a high salinity environment, and mainly replaces Ca 2þ in lattices, however, the Sr content would be reduce in a meteoric water environment as a result of recrystallization (Brand and Veizer, 1980). Therefore, the change of Sr content can well reflect the variation of salinity in sedimentary and diagenetic environment. A study found that the average Sr content is 23.25 Â 10 À6 in the mixed water dolomite and 175 Â 10 À6 in the burial dolomite (Qin and Zeng, 1994). The Sr content of the samples was (47.30-140)Â10 À6 with an average of 80.80 Â 10 À6 , thus suggesting that a large amount of Sr was lost due to the leaching of meteoric water during burial dolomitization. Early dolomitized marine carbonates are rich in Sr, while late dolomitized carbonates are poor in Sr (Tucker and Wright, 1990). Warren (2000) reported that the Na content of ancient dolomite was (900-1000)Â10 À6 according to Warren's research (2000), in contrast, the content of Na in the samples is only (100-1000)Â10 À6 with an average of 389 Â 10 À6 , it was obviously reduced due to the influence of meteoric water.
The genesis of dolomite in the Ma5 member of Ordovician Majiagou Formation in eastern Sulige gas field was analyzed by Li et al. (2016). The authors considered that there are two types of dolomite in that area, namely penecontemporaneous dolomite and burial dolomite, respectively, and the natural gas reservoirs mainly exists in the burial dolomite. Their research results can be used as a reference (Table 3), because there is some overlap with our study area, and the paleokarst background is also similar. Accordingly, we found that the dolomite in the study area has typical characteristics of the burial dolomite. Fu et al. (2019) summarized the diagenesis and diagenetic evolution of the Ma5 member in the east-central Ordos Basin. The authors believed that the compaction during late diagenesis period after the Carboniferous led to the release of Mg 2þ rich fluid, which resulted in a strong burial dolomitization.

Discussions
Diagenetic environment and dolomitization model Allan and Wiggins (1993) (Figure 7(a)) and high temperature dolomite (Figure 7(b)). These findings suggest that the dolomitization of dolomite in the upper assemblage of Ordovician Majiagou Formation occurred in a burial environment with a high temperature.
The C and O isotopes characteristics of fluids vary between different diagenetic environments. Nelson and Smith (1996) established a d 13 C-d 18 O diagram that can be used to distinguish the original diagenetic environment of carbonate rocks (Figure 7(c)). As shown in Figure 7(c), >90% of the samples fell in the area of marine limestone. We note that this is Figure 7. Comprehensive discrimination of the carbonate diagenetic environment (a,b) after Allan and Wiggins (1993); (c) after Nelson and Smith (1996). not contradictory to the fact that the samples are dolomite, because the carbon and oxygen isotope values of marine limestone remain basically unchanged after dolomitization. A few samples fell in the overlap area of marine limestones and meteoric cements, which indicate that they may have experienced the leaching of meteoric water.
Based on the comprehensive analysis of the influence of salinity, temperature, and sea level changes on the C, O, and Sr isotopic composition and major and trace element characteristics of carbonate rocks in the study area, it is concluded that the although dolomite in the upper assemblage of the Majiagou Formation underwent a weak alteration during the late diagenetic process, it still inherited information of original diagenetic environment. The evidence is as follows. The d 13 C value was found to be relatively stable and maintains the characteristics of the Ordovician seawater, thus indicating that that the dolomitization occurred before the oil-generating window and the d 13 C value was unaffected by organic carbon. With the increase of burial depth, the increase of temperature and the leaching of meteoric water, the value of d 18 O is obviously lower than that of the Ordovician seawater. The Z value of samples ranged from 107.62 to 129.92 and was partly influenced by meteoric water, thus indicating a marine facies background. The paleotemperature ranged from 29.79 9 to 55.37 7, which reveals that the high burial temperature was beneficial to the occurrence of dolomitization. The burial depth was between 367.85 m and 1004.28 m, thereby suggesting that the dolomitization was not completely separated from the influence of the surface water. The 87 Sr/ 86 Sr value varied from 0.7087 to 0.7103 with an average of 0.7094, which is greater than the isotopic background value of 0.7087 in geological history, and was influenced by terrigenous strontium and the burial effect. The relatively low Mg/Ca ratios of samples reflect the burial characteristics, and the scatter plot of CaO and MgO reveals metasomatism. The high Fe and Mn content indicate a reducing and burial environment, while low Sr and Na contents reflect a low salinity. The Mn/Sr ratio of samples was >3, thus indicating a weak diagenetic alteration, which may have been influenced either by dolomitization during the burial period or by meteoric water during the epigenetic period. A considerable amount of authigenic pyrite in the thin sections not only evidences a high Fe content, but also reflects the strong reducing environment of the burial conditions. The dolomite was observed to be dark red under the cathode luminescence, which demonstrates that Fe 2þ and Mn 2þ had replaced Ca 2þ and Mg 2þ , which also reveals a reducing environment. In summary, the dolomite in the upper assemblage of Ordovician Majiagou Formation in the study area formed in a high temperature reduction environment with a middle burial depth. Dolomite mainly experienced mixed water dolomitization before the Carboniferous (Figure 8(a)) and burial dolomitization after the Early Permian (Figure 8(b)).
Hydrothermal dolomite is related to tectonics and mostly exists near fracture systems such as faults, it is characterized by saddle dolomite and is often associated with pyrite and calcite (Davies and Smith, 2006). The Ma5 member dolomite of the Jingbian gas field has obvious hydrothermal dolomite characteristics (Yao et al., 2009). The possibility of hydrothermal dolomite in the study area cannot be ruled out because the southeastern part of the Sulige gas field is located to the north of the Jingbian gas field, and both gas fields are located in the Carboniferous to Permian basement fault zone. The calcite vein and pyrite in the pores and cracks of the thin sections indicate that there should be local hydrothermal action. The d 13 C andd 18 O values of the calcite vein in the same sample were both more negative than those of the matrix dolomite, which suggests that the formation temperature of calcite was higher than that of the dolomite. However, as the calcite vein formed later than dolomite and the formation time is unknown, it is impossible to determine whether the high temperature was caused by hydrothermal fluid or an increased burial temperature. Therefore, the existence of hydrothermal dolomitization and the extent of its influence require further studied. (a) During the shallow burial stage before the Carboniferous, calcite was replaced by dolomite, and dolomitization was dominated by mixed water dolomitization; The carboniferous sedimentary period was relatively short (about 19.1Ma), and the sedimentary thickness was comparatively small (maximum of 45m), as the shallow burial stage continued to the Ordovician. (b) After the Early Permian, the increased burial depth meant that compaction released water rich in Mg 2þ an extensively promoted burial dolomitization; (c) There were four main diagenetic stages, and dolomitization was dominated by mixed water dolomitization and burial dolomitization.

Petroleum geological significance
The dolomite of the Majiagou Formation in the study area mainly underwent four main diagenetic stages: the penecontemporaneous stage, shallow burial stage, epidiagenetic stage, and middle burial stage (Figure 8(c)), with obvious differences in the pore evolution of each stage. Diagenesis during the penecontemporaneous stage (O 1 þ 2 ) was dominated by filling and cementation, whereas, penecontemporaneous dolomitization was limited, and a small amount of interlayer dissolution made no obvious contribution to pore formation. The overlying sediments gradually thickened during the shallow burial stage (O 2 þ 3 ), and compaction resulted in a sharp decrease in porosity. This stage was relatively short and buried dolomitization was relatively less, and the early secondary intergranular pores were mainly formed by mixed water dolomitization. During the epidiagenetic stage (O 3 -C 1 ), the strata were uplifted and denuded, with a 130 Ma depositional hiatus. Under the action of meteoric water, a large number of dissolution pores, holes, and cracks were formed, which provided an effective space for the accumulation of natural gas. The middle burial stage (C 2 -P 3 ) was a critical period for the development of reservoir pores. There was a sufficient reaction between compaction-released acidic water and magnesium-rich brine and rocks, which, when coupled with burial dolomitization, led to the formation of more dissolution pores and intercrystalline pores. Hydrocarbons were generated and expulsed in huge quantities from the Late Jurassic to the Early Cretaceous (J 3 -K 1 ), after which, natural gas migrated and accumulated in dolomite reservoirs. Under the influence of the tectonic inverse of the Ordos Basin, a west-dipping monocline was formed at the end of the Early Cretaceous, which led to the readjustment of the existing gas reservoirs and formed the current distribution pattern (Figure 9). The variation in the physical properties of the reservoir resulted in significant differences between the gas reservoirs. The layers with well-developed dissolution holes and good physical properties in the upper Ordovician strata (especially in the Ma5 1 and Ma5 4 layers) are the favorable reservoirs for the accumulation of natural gas. At present, a number of high-yielded gas-bearing strata have been found in the upper assemblage of the Ordovician Majiagou Formation.

Conclusions
The diagenetic environment of the upper assemblage carbonate of Ordovician Majiagou Formation was effectively identified using carbon and oxygen stable isotopes, strontium isotope, and major and trace elements. The data revealed that the dolomite of the upper assemblage of the Majiagou Formation was formed in a reducing marine environment, and mainly includes micritic dolomite, powder crystal dolomite, and micrite to powder crystal dolomite. Two main models of dolomitization were determined for the study area: mixed water dolomitization and burial dolomitization. In the shallow burial stage before the Carboniferous, mixed water dolomitization was dominated by metasomatism and occurred in the initial marine limestone under the interaction of seawater and meteoric water. Then, during the middle burial stage after the Early Permian, burial dolomitization was widely developed along with an increased burial depth and temperature.