The cores of the well Xike 1 was observed and the dolomite samples were collected at different depths. More than 200 fresh carbonate samples, including dolomite samples and a small amount of limestone samples, were collected by drilling cores. All samples are immediately stored in plastic bags to reduce pollution and oxidation. At the institute of sedimentary geology, Chengdu university of technology, 160 carbonate samples were observed by a polarizing microscope. Fifty-one dolomite samples of different layers were selected to analyze the contents of carbon, oxygen, hydrogen isotopes and REE.

An Leica polarizing microscope (Leica-Dmrx) was used for optical microscopic identifications, and the thin sections of dolomite cores were observed by transmission light. In addition, this paper also selected some samples for cathodoluminescence, which was completed on the basis of identification and statistics of stained cast thin sections. The instrument used was a CL8200MK5 cathodoluminescence instrument (equipped with Leica polarizing microscope) of the exploration and Development Research Institute of Sinopec Southwest Oil and Gas Company. Considering the comparability of samples, all samples adopted the same test conditions, i.e., beam voltage of 15.8 kv and beam current of 304 μA. The exposure duration is 1.8 s.

Carbon and oxygen isotope analysis is mainly completed by a MAT-253 isotope mass spectrometer of the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences. Kiel IV Carbonate Device sample preparation system is used. Reference standard: GBW-04405. It is based on DZ/T0184.17-1997 “Phosphoric acid method for determination of carbon and oxygen isotopes in carbonate minerals or rocks.” Analysis accuracy: The standard deviations of the measured values of δ¹³C (PDB) and δ¹⁸O (PDB) are less than 0.040‰ and 0.080‰.

Hydrogen isotopic analyses were conducted on the fluid inclusions hosted in the mineral particles (dolomite). Samples were first degassed of labile volatiles by heating under vacuum to 150°C for 3 h. Water was released by heating the samples to approximately 500°Cby means of an induction furnace. Water was converted to hydrogen by passage over heated zinc powder at 400°C (Friedman, 1953), and the hydrogen was analyzed with a MAT-252 mass spectrometer. The analyses of standard water samples suggest a precision for δD of ±3‰ (1σ). The hydrogen isotopic analyses were performed at the Analytical Laboratory of Beijing Research Institute of Uranium Geology (ALBRIUG).

REE concentrations were determined at the Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences. About 40 mg of each sample was first digested with 0.6 ml HNO₃ and 2 ml HF in a Teflon beaker. After standing with seal-capping for 2 h, the beaker was placed on a hotplate at about 150°C for 24 h. A 0.25 ml HClO₄ was added to the residue, and it again was heated to near dryness in an open-top beaker on a hotplate at about 150°C. The residue was redissolved with 1 ml HNO₃+1 ml H₂O and sealed again, and it remained on a hotplate at about 120°C for an additional 12 h. The residue was diluted by 1% HNO₃ solution to produce a volume of 40 g, and it was subsequently analyzed by ICP-MS to determine the REEs in the sample solution.

From the core observations, the crystal diameter of dolomite increases with the increase of depth, and the proportion of crystal dolomite in the rock increases with the increase of depth, in the form of yellowish brown sugar.

The geochemical characteristics of dolomites are an important information carrier for the genesis and formation mechanism of dolomites. The enrichment and loss, migration and preservation of elements are not only the records of sedimentary environments, but also an important calibration of geological processes (Hong H, 2004; Ni S et al., 2009; Hu Zuowei, 2010). For carbon, oxygen and hydrogen isotopes, carbon isotopes, they are mainly used to study paleotemperature, salinity and fluid properties, and hydrogen isotopes are mainly used to trace fluid properties in combination with the covariance law of oxygen isotopes (Craig, 1961; Holser, 1979; Richard, 2013).

The carbon and oxygen isotopes of 51 carbonate rock samples were analyzed in Well Xike 1. Kaufman et al. found that in order to ensure the effectiveness of data, the value of δ¹⁸O cannot be less than −10‰, and Mn/Sr <10 can be used as a basis to determine whether the carbon isotope maintains the original composition (Kaufman A J et al., 1995).

Hydrogen isotope test is carried out for dolomites in different layers of well Xike 1. The analysis results are shown in Table 3:

It can be seen from the analysis results that among the three samples in the shallow dolomites (579.26 m shallow), The maximum value of δD is −46.7‰ and the minimum value is −50.3‰; in the middle and deep dolomite interval (770.05–1257.4 m well section), the maximum value of δD is −48.1‰ and the minimum value is −57.0‰.

The detailed REE element parameters of shallow, middle and deep dolomite samples from the Well Xike 1 are shown in Table 4. The REE distribution model of seawater standardized by PAAS is mainly characterized by negative Ce anomaly and relative enrichment of heavy rare earths (Zhang J et al., 1996). After PAAS standardization, the REE test results of shallow, middle and deep dolomite samples in Well Xike 1 (a total of 20 samples) show that the samples of dolomites in different layers have good similarities with the PAAS distribution curve of modern seawater, which are highlighted by LREE loss, negative Ce anomaly and positive Y anomaly (Figures 7, 8).

(1) The hydrogen and oxygen isotopes of the present seawater are relatively stable. The fluctuation range of the δD value is 4‰, and that of the oxygen isotope value is 0.3‰ (Tian et al., 2016). The hydrogen isotopic composition of deep seawater are shown in Table 6.

The δD and δ¹⁸O values of fluids from each source are (Kyser et al., 1984; Zhang et al., 1993; Wang et al., 2011):

Magmatic water: δD −50‰∼−80‰; δ¹⁸O+6‰∼+8‰;

Present seawater: δD 0‰;

Atmospheric water (rainwater): δD −350‰∼+50‰; δ¹⁸O −44‰∼+10‰;

Among the three samples in the shallow buried dolomites (below 579.26 m), The maximum value of δD is −46.7‰, the minimum value is −50.3‰, and the average value is −48.3‰; while in the middle and deep buried dolomite interval (770.05–1257.4 m well section), the maximum value of δD is −48.1‰, the minimum value is −57.0‰, and the average value is 53.1‰. It can be seen that no matter whether it is shallow, middle or deep dolomites, the δD value is obviously less than the hydrogen isotope value of seawater, and is similar to that of limestone. In Well Xike 1, the average hydrogen isotope value of limestone is −55.67‰ (Yin He, 2020). Richard (2013) believes that the surface fluid δD value is −40‰ ∼ −70‰, it reflects that the diagenetic environment of dolomites is mainly near surface environment. While the hydrogen isotope composition in atmospheric precipitation varies greatly, the δD value ranges from −350‰∼ + 50‰. It is speculated that the dolomite may have been affected by the mixing of atmospheric precipitation and concentrated seawater during the quasi syngenetic period. However, Bi et al. (2018) thinks that the small hydrogen isotope value of the Well Xike 1 may also be related to the decomposition of organic matter.

The statistical results of REE parameters of the shallow, middle and deep dolomites are shown in Table 7. Compared with the continental crust (Taylor et al., 1985) and carbonate rocks in eastern China (Yan M et al., 1997), the dolomites in layer 7 of Well Xike 1 shows a typical low total rare earth (∑REE) -with an average value of 3.928 μg/g, the ΣREE values of most samples range from 1.74 μg/g to 5.47 μg/g, only a few samples showed high values, but none of them exceeded 100 μg/g. The distribution range of ΣREE indicates marine carbonate rocks (Qing et al., 1994). It can be seen from this feature that no other provenance is mixed into dolomites. In terms of light rare earth content, the average is 3.123 μg/g, and the ∑LREE value ranges from 1.746 μg/g to 7.190 μg/g. In terms of heavy rare earth content, the average value of light rare earth content is 0.805 μg/g, the ∑HREE value ranges from 0.380 μg/g to 1.829 μg/g. On the whole, the REE characteristics of the 7-layer dolomites are basically similar, indicating that there is a relatively similar environment in the diagenetic process.

In the redox environment, the multivalent state characteristics of Ce and Eu will show sensitive changes. Calcium and magnesium ions (Ca²⁺ and Mg²⁺) in the lattice structures of dolomites can be replaced by rare earth elements. In the oxidation environment, Ce and Eu in dolomites are relatively less. This is because Ce and Eu ions can be trivalent in the oxidation environment, so they are difficult to replace divalent calcium and magnesium ions in the lattice of dolomites, which are easier to be extracted and exist in the fluids. Under low temperature and alkaline environment, Eu changes from trivalent state to divalent state, which is easier to dissolve and thus is migrated and depleted. According to the analysis of the measured sample data of dolomites in well Xike 1, the dolomite δCe negative anomaly is the lowest, and the δEu values of some dolomites have different negative anomaly characteristics from carbonate rocks in the eastern China, indicating that marine derived fluids are involved in the whole diagenetic process (including dolomitization) or that the diagenesis of dolomites is mainly affected by marine derived fluids. In dolomites the negative anomaly of δCe value reflects its diagenesis in the semi open environment of partial oxidation, while the diagenetic environment corresponding to the negative δEu value anomaly is low temperature and alkaline conditions.

The degree of Eu negative anomaly in shallow dolomite is slightly higher than that in other dolomites, indicating that it may have a lower ambient temperature during its formation-the oxygen isotope characteristics of foraminifera observed in ODP184 voyage sampling core also reflect the seawater cooling event in the South China Sea in the late Pliocene, which may be the reason for the lower diagenetic ambient temperature of shallow dolomite (Li Jianru et al., 2004).

In the South China Sea, in the early Miocene of about 5 Ma, there was obvious magmatic activities in the residual expansion center of the southwest basin (Meng L et al., 2014). From the late Miocene, the Dongsha Movement appeared in the northern South China Sea, and its activity stopped at the end of late Miocene/early morning Miocene (5.5 Ma) (Lin et al., 2009; Zhao et al., 2012). The large-scale dolomites in Wells Xike 1 and Xichen 1 is formed below the Miocene/Pliocene boundary.

The global paleoclimate background of dolomite deposits in Well Xike-1 during the Late Miocene-Pliocene is characterized by gradual cooling. In the middle and late Miocene, the global climate became significantly cooler, marine productivity increased sharply, and organic carbon buried rapidly, which further led to the decrease of bottom water temperature, and eventually led to the expansion and permanent formation of the Antarctic ice sheet at the end of the Miocene (Zachos et al., 2001; Lu and Chen, 2006).

From the end of the Miocene to the early Pliocene, the global climate fluctuated dramatically, experiencing several cooling (about 6.15–5.8 Ma in the Late Miocene), the latest in the Miocene to the Early Pliocene (about 5.7–5.46 Ma), warming (about 5.8–5.7 Ma in the Late Miocene), 5.46–4.9 Ma in the Early Pliocene (Hodell et al., 1986), and 4.6 Ma in the Early Pliocene. Therefore, from the late Miocene to the early Pliocene, the global climate is not always cold, but during the warming period, the ice sheet melts, the sea level rises, and the hydrothermal activity of the seabed is more active than that of the cooling period.

Due to the tectonic movement around 5.5 Ma, the thermal convection generated by deep thermal fluids migration is likely to affect the deposited Huangliu formation sediments. The mineral structures of dolomites in Well Xike 1 generally shows that the mineral structures of dolomites is relatively better in the first member of the Meishan Formation (576.5–758.4 m) with relatively low degree of dolomitization. The reason may be that the temperature of the diagenetic fluids is relatively high (the homogenization temperature of 82 dolomite fluid inclusions below 1064 m well depth tested by Shi et al. (2016) is 129.5°C–268.5°C, with an average of 184.6°C). When the buried depth increases gradually, the proportion of over dolomitized fog-bright edge dolomite and cement dolomite increases significantly.

Wells Xichen 1 and Xike 1 have similar dolomite mineral types, including bright crystal cement, various mud crystal fossils, mud crystal debris and mud crystal particles (Wei et al., 2008a; Wei et al., 2008b; Wei et al., 2008c). Dolomite accounts for the majority of dolomites in Well Xichen 1, with a volume fraction of more than 91.1%, up to 100%. From these data, it can be seen that there are relatively loose carbonate rocks in the dolomitization process, which can be fully contacted with water and rock, and can last for a long time to dolomitize.

Among the three sections of dolomites in Well Xichen 1, both the δ¹³C and δ¹⁸O values are positive numbers. Among them, the average value of δ¹³C is 2.24‰, and the average value of δ¹⁸O is 2.56‰ (Wei et al., 2008a; Wei et al., 2008b; Wei et al., 2008c), and Wei et al. (2006a, 2006b) think that there is no obvious rainwater or underground fresh water leaching environment during the formation and evolution of dolomites in Well Xichen 1. However, if the dolomites in Well Xichen 1 undergoes alteration of lithogenic and hydrothermal fluids (Wei et al., 2006ab; Wei et al., 2007) during the evolution of metagenetic rocks, the carbon and oxygen isotopes (especially oxygen isotopes) of rocks may change, and the existing test values can not fully represent the characteristics of the original sedimentary environment. Wei et al. (2008a), Wei et al. (2008b), and Wei et al. (2008c) measured the fluid inclusions of authigenic bright dolomite minerals in rock pores or fractures. The results show that the fluid homogenization temperature during inclusion capture is 102.5–296°C, with an average of 156.3°C. The salinity ranged from 0.55 to 6.25%, with an average of 3.75%. The salinity slightly higher than that of the modern seawater reflects the fluid characteristics of dolomitization of brackish water or of medium salinity. The temperature measurement results of the inclusions are similar to the test results of Wells Sike 1 by Shi et al. (2016), which reflects that the similar diagenetic environments.

From the regional tectonic background, during the tectonic movement in the Xisha area in the late Miocene, the thermal convection caused by the upwelling of thermal fluids may have an impact on the deposited sediments of the Huangliu Formation, and the impact on the carbonate rocks of the Sanya Formation is more significant and extensive. High temperature is a necessary thermodynamic condition for the formation of dolomites (Machel, 2004; Huang, 2010). Therefore, the thermal fluids in this period undoubtedly promoted the occurrence of percolation and dolomitization. According to the research on dolomite structure, mineral composition, isotopic characteristics, inclusion type and characteristics of Well Xichen 1, it is shown that the hot water fluids played a role in the evolution process of carbonate rocks in the Xisha Islands. Under magmatism, the hydrothermal fluids can get enough heat, while the formation water and hydrothermal fluids have similar compositions, and dolomites are transformed by diagenetic fluids and hydrothermal fluids of imestone reservoirs.

Other scholars also systematically analyzed the δ¹³C and δ¹⁸O values of Well Xichen 1, and discussed the diagenetic model of dolomites of well Xichen 1 (Xisha Islands):

Previous studies suggested that that the δ18O value of dolomitse in Well xichen-1 is 2‰–5‰ higher than that of non-dolomitic rocks, and the δ¹³C value is 3‰ higher than that of the non-dolomitic rocks. It is believed that this feature of enrichment of oxygen carbon heavy isotopes is a strong evidence of the increase of seawater salinity beyond the normal seawater. Some studies believe that the values of δ¹⁸O of dolomite in Well Xichen 1 are normal, which cannot explain that the diagenetic medium has salinity anomaly higher than sea water, but the δ¹³C value of the Xisha dolomites is too high and the δ¹³C value of the symbiotic calcites is too light. The dolomite in Well Xichen can be explained by the mechanism of mixed water dolomitization (He and Zhang, 1990). Combined with the test results of inclusions, some studies thought that dolomites in Well xichen-1 show the characteristics of high temperature and salinity, and it is likely to be affected by hydrothermal activities when it is formed (Zhao, 2010).