Using Clumped Isotopes to Reconstruct the Maximum Burial Temperature: A Case Study in the Sichuan Basin

For strata that have experienced continual burial in the early stage and uplift in the late stage, the present-day temperature is lower than the maximum burial temperature (MBT), which is a key parameter for studying the hydrocarbon generation history of source rocks in petroliferous basins. In this paper, a new method for reconstructing the MBT is proposed based on the solid-state reordering model of carbonate clumped isotopes (Δ 47). The MBT reconstructed using the Δ 47 was compared with the MBT constrained using the traditional Easy%Ro model. The clumped isotope temperature (TΔ47) of the Permian micritic limestone from the Xibeixiang outcrop (about 62°C) is much higher than its initial formation temperature (20–25°C), suggesting that the limestone experienced partial solid-state reordering during the late burial process. The MBT of the calcite obtained from the solid-state reordering model is 139–147°C, which is quite similar to the MBT determined using the Easy%Ro model (139.5–147.5°C). TΔ47 of the Permian and Triassic limestone and calcite cements in the Puguang gas field are 150–180°C, while TΔ47 of the micritic dolostone is about 70°C, suggesting that the Δ47 of the limestone and calcite cements experienced complete solid-state reordering and the dolostone only experienced partial solid-state reordering. The MBT of the dolomite determined using the solid-state reordering model is 200–220°C, which is also similar to the MBT determined using the Easy%Ro model (202–227°C). Therefore, the case studies from the Sichuan Basin suggest that Δ47 can be used to reconstruct the MBT of ancient carbonate strata lacking vitrinite and detrital zircon data. However, different types of carbonate samples should be used to reconstruct the MBT for strata that have experienced different temperature histories. Micritic limestone and very finely crystallized dolostone can be used to reconstruct the MBT of strata that have experienced MBTs of <150–200°C and >200–250°C, respectively.


INTRODUCTION
The burial temperature (especially the maximum burial temperature, MBT) of certain strata in a particular sedimentary basin is closely related to the maturity of the organic matter and the hydrocarbon generation of the source rocks (Hunt, 1996;Helgeson et al., 2009;Qiu et al., 2012), and it can also affect the phases in oil and gas reservoirs (Barker, 1990;Hill et al., 2003;Zhu et al., 2018). As a result, the MBT is one of the key parameters for basin modeling and the evaluation of oil and gas resources (Kinley et al., 2008;Qiu et al., 2012;Wood, 2018). For sedimentary basins that have experienced continuous subsidence and burial, the burial depth of certain strata was the largest during the burial process, and its present-day burial temperature is the MBT, which can be determined using current heat flow or geothermal gradient models (Zuo et al., 2011). However, for sedimentary basins that subsided in the early stage and were uplifted and exhumed in the late stage, the present-day burial temperature of certain strata is usually lower than the MBT (Hao et al., 2008;English et al., 2016;Dou et al., 2021), which cannot be directly determined using current heat flow or geothermal gradient models. As a result, reconstructing the MBT of certain strata in a basin that experienced late uplift and exhumation is key to the evaluation of the oil and gas exploration potential, and it is also a research focus in basin modeling.
Common methods of MBT reconstruction include the maturity index of the organic matter (Héroux et al., 1979;Hackley et al., 2015;Luo et al., 2020b) and low temperature thermochronology (Carlson, 1990;Craddock and Houseknecht, 2016;Ehlers and Farley, 2003;Yamada et al., 2007). The maturity index of the organic matter is commonly characterized by the vitrinite reflectance (Mukhopadhyay, 1994;Katz and Lin, 2021), which is mainly controlled by the temperature (Hunt, 1996). Therefore, the current vitrinite reflectance (R o ), combined with an organic matter maturation model (such as the Easy%R o model; Sweeney and Burnham, 1990), can be used to determine the MBT. For strata without vitrinite data, the bitumen reflectance and graptolite reflectance can be used to calculate the equivalent R o (Jacob, 1989;Luo et al., 2020b), but different conversion methods exist, and they have different application scopes (Schmidt et al., 2019;Katz and Lin, 2021). Low temperature thermochronology mainly uses annealing kinetic models of fission tracks in minerals (such as apatite and zircon) and (U-Th)/He thermochronology to determine the temperature evolution (Carlson, 1990;Craddock and Houseknecht, 2016;Ehlers and Farley, 2003;Yamada et al., 2007). The annealing kinetic model of apatite fission tracks is only applicable at relatively low temperatures (below 125°C; Carlson, 1990). For strata that have experienced temperatures of >150°C, only the annealing of zircon fission tracks can be used (Yamada et al., 2007), but a unified annealing kinetic model of zircon has not yet been established. The (U-Th)/He thermochronology of zircon can be used to determine the corresponding MBT and time (Reiners, 2005), but for ancient marine strata, detrital zircons are often not easy to obtain, which limits the use of this method. Therefore, for ancient marine strata, both the maturity index of the organic matter and low temperature thermochronology have certain limitations in MBT reconstruction.
Carbonate minerals (such as calcite and dolomite) are commonly present in ancient marine strata. Carbonate clumped isotopes (Δ 47 ), developed in recent years, can be used to determine the formation temperature of carbonate minerals (T Δ47 ), which have been called clumped isotope thermometers (Ghosh et al., 2006;Eiler, 2007). Unlike traditional oxygen isotope (δ 18 O) thermometers, clumped isotopes can be used to directly determine the formation temperature of carbonate minerals using the abundance of 13 C-18 O in the carbonate minerals, without relying on the δ 18 O of the fluid from which the carbonate minerals grew (Eiler, 2007). However, T Δ47 determined from the clumped isotopes can differ from the original formation temperature due to solid-state reordering. If the carbonate minerals experienced high temperatures during late burial, the 13 C and 18 O underwent solid-state diffusion, resulting in changes in the abundance of 13 C-18 O in the minerals and T Δ47 of the minerals (Passey and Henkes, 2012;Stolper and Eiler, 2015). Kinetic models of the solid-state reordering of carbonate minerals have been developed in recent years (Passey and Henkes, 2012;Stolper and Eiler, 2015;Lloyd et al., 2018;Hemingway and Henkes, 2021). The carbonate minerals in ancient carbonate strata have usually experienced high temperatures and solid-state reordering of 13 C and 18 O. Therefore, the solid-state reordering of calcite and dolomite in ancient carbonate strata can be used to reconstruct the MBT according to the established solid-state reordering models for calcite and dolomite (Passey and Henkes, 2012;Stolper and Eiler, 2015;Lloyd et al., 2018). This provides a potential new method for reconstructing the MBT of ancient marine carbonate strata lacking vitrinite and detrital zircon data.
The purpose of this study was to reconstruct the MBT using the clumped isotopes of the Permian and Triassic limestone and dolostone samples from the Sichuan Basin and to compare the MBT determined using carbonate clumped isotopes and the Easy %R o model in order to verify the reliability of using the carbonate clumped isotope method to reconstruct the MBT and to provide a good case study for MBT reconstruction of ancient marine carbonate strata.

GEOLOGIC SETTING
The Sichuan Basin is a multi-cycle basin located in southwestern China (Figure 1), with the Ediacaran to Quaternary sediments. It experienced six main tectonic cycles: the Yangtze (before 630 Ma), Caledonian (630-320 Ma), Hercynian (320-252 Ma), Indosinian , , and Himalayan (65-0 Ma) movements (Zhai, 1989). Among them, the marine deposits were mainly developed from the Yangtze to the Early Indosinian, while the continental deposits were mainly developed from the Late Indosinian to the Himalayan.
The Sichuan Basin experienced early extension and late compression and uplift (Zhai, 1989). From the Yangtze to the Early Indosinian, two weak extension events resulted in the development of two intracratonic sags in the Sichuan Basin (the Early Cambrian Mianyang-Changning sag and the Late Permian Kaijiang-Liangping sag) (Liu et al., 2021). Strong compression and uplift have occurred since the Late Indosinian, and the Sichuan Basin has evolved into a foreland basin. In particular, the Longmanshan, located in the western boundary of the Sichuan Basin, uplifted gradually from the northeast to the southwest (Burchfiel et al., 1995). During the Yanshanian, many large northeast-southwest striking folds and faults were formed in the eastern Sichuan Basin due to regional northwest-southeast compression (Zhai, 1989). Then, rapid uplift and exhumation occurred during the Late Yanshanian and Himalayan (Deng et al., 2013). It is a typical basin that has experienced early subsidence and late uplift and exhumation in western China.
Marine carbonates and shales were deposited during the Permian and Early Triassic. From the bottom to top, the Permian can be divided into the Liangshan (P 1 l), Qixia (P 1 q), Maokou (P 1 m), Longtan (P 2 l)/Wujiaping (P 2 w), and Changxing (P 2 c)/Dalong (P 2 d) formations, and the Lower Triassic can be divided into the Feixianguan (T 1 f) and Jialingjiang (T 1 j) formations (Zhai, 1989). The P 2 l/P 2 w and P 2 d mainly consist of shales, which are the main source rocks of the P 2 c and T 1 f gas reservoirs (Li et al., 2005;Yu et al., 2021), while the P 2 c and T 1 f mainly consist of dolostone and limestone, which are the main reservoirs rocks of the main large gas fields (such as the Puguang, Yuanba, and Longgang gas fields) discovered in recent years (Ma et al., 2007;Du et al., 2010;Guo et al., 2018). The dolostone in the P 2 c and T 1 f was mainly developed on the platform margins of the Kaijiangliangping trough, and it was formed in a near surface to quite shallow burial environment. The dolomitization fluids were sourced from the T 1 f evaporated sea water (Jiang et al., 2014;Li et al., 2021).
All of the shale samples were analyzed for R o using conventional microphotometric methods (Stach et al., 1982) at the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing). The number of readings was generally >20 for the random reflectance, and the error of each sample was within 0.2%.
Thin sections of all of the limestone and dolostone samples were prepared for petrographic observations. Then, all of the limestone and dolostone samples and calcite cement samples were powdered to less than 0.15 mm (100 mesh) for δ 13 C, δ 18 O, and clumped isotope (Δ 47 ) analyses at the California Institute of Technology (Caltech). The detailed analytical procedures have been described in previously published papers (Ghosh et al., 2006;Huntington et al., 2009;Ryb and Eiler, 2018;Li et al., 2020). First, about 10 mg of carbonate sample was reacted with 103% phosphoric acid at 90°C, and the CO 2 produced was purified using a gas-chromatography column. Then, the 44-49 masses of the CO 2 were measured using a Thermo MAT253 isotope ratio mass spectrometer. Then, the δ 13 C, δ 18 O, and Δ 47 values were calculated using the methods described by Huntington et al. (2009). Finally, the Δ 47 values were converted into the absolute reference frame (Dennis et al., 2011), and the Frontiers in Earth Science | www.frontiersin.org September 2021 | Volume 9 | Article 759372 clumped isotopes temperature (T Δ47 ) of each sample was calculated using the temperature calibrations developed by Bonifacie et al. (2017). Most of the samples were measured two or three times. The standard deviation of the δ 13 C and δ 18 O measurement is <0.01‰, and that of the Δ 47 measurements is <0.02‰. The standard errors of the in-house standard (CIT Carrara and TV04) were <0.01‰. The raw data of d45-d79, D47 of samples, standards, equilibrated gases and heated gases were listed in the Supplementary Table 1.

Petrography
The crystal size of the carbonate minerals can be divided into micritic (<50 µm), very fine (50-100 µm), fine (100-250 µm), medium (250-500 µm), and coarse (>500 µm) (Bissell and Chilingar, 1967). All of the limestone samples collected from the XBX outcrop are bioclastic limestone, the cement between the bioclastics is micritic calcite with crystal sizes of <10 μm, and no obvious crystallization was observed in the bioclastics and calcite cement ( Figure 2A). The limestone samples collected from the Puguang gas field include bioclastic limestone ( Figure 2B) and micritic limestone ( Figure   There is a large difference in T Δ47 of the four dolostone samples from the Puguang gas field. T Δ47 of the two micritic dolostone samples are 71.7 ± 8.9°C (D90) and 72.5 ± 4.5°C (D83), that of the very fine crystallized dolostone is 88.5 ± 5.4°C, and that of the fine crystallized dolostone is 126.3 ± 7.6°C (D99).

Vitrinite Reflectance (R o )
The R o values of the Permian shales from the XBX outcrop and the Puguang gas field are presented in Table 2. The R o values of the two P 2 d shale samples from the XBX outcrop are 0.92 and 0.96%, while the R o values of the P 2 w shales from the Puguang gas field are 2.39-3.11% (average of 2.80%).

DISCUSSION EASY%R o Model and Maximum Burial Temperature Reconstruction for the Xibeixiang Outcrop and Puguang Gas Field
The maturity of organic matter, which is usually characterized by R o and/or equivalent R o (such as the solid bitumen reflectance or graptolite reflectance), is a commonly used method for paleotemperature reconstruction (Mukhopadhyay, 1994;Hackley et al., 2015;Luo et al., 2020b). The maturity evolution of organic matter is an irreversible process and can be characterized by a first-order chemical kinetics reaction. According to the Arrhenius equation, the maturity of organic matter has an exponential relationship with temperature and a linear relationship with time. As a result, temperature is the most important parameter affecting the maturity evolution of organic matter (Hunt, 1996). Theoretically, the MBT can be obtained based on a model of the evolution of the organic matter's maturity with temperature (such as the EASY%R o model; Sweeney and Burnham, 1990).  The EASY%R o model is a widely accepted model for the evolution of the maturity of organic matter, and the organic matter maturity (R o ) can be calculated for a given heating rate (Sweeney and Burnham, 1990). According to apatite fission track analysis (Deng et al., 2013), the Permian and Triassic strata in the XBX outcrop and the Puguang gas field reached their maximum burial depths at about 80 and 120 Ma, respectively (Figure 3). The area of the XBX outcrop was uplifted in the Late Indosinian (Burchfiel et al., 1995), and the R o of the Permian shale near the XBX outcrop in the NW Sichuan Basin is <1.3% (Fu et al., 2010), so the MBT of the Permian shale should be less than 160°C (Figure 4). A lot of solid bitumen with a relatively high maturity (R o >1.7%) is commonly present in the Permian and Triassic gas reservoir in the Puguang gas field, suggesting that large scale oilcracking occurred in this gas field, and the maximum burial temperature may have exceeded 200°C (Hao et al., 2008). In addition, the heat flow has decreased gradually since the Late Permian (Zhu et al., 2016). Thus, the burial temperature of the Permian and Triassic strata in the XBX outcrop and the Puguang gas field should have reached the maximum values when the burial depths reached the maximum values at about 80 and 120 Ma (Figure 3), respectively. As a result, the heating rates should be about 1.0°C/Ma (150°C/(260-80 Ma) 0.83°C/Ma) and 2.0°C/Ma (220°C/(250-120 Ma) 1.7°C/Ma) for the XBX outcrop and the Puguang gas field, respectively.
The evolution of R o with temperature based on the EASY%R o model for heating rates of 1.0 and 2.0°C/Ma is shown in Figure 4. According to this model, R o of the P 2 d shale in the XBX outcrop is 0.92-0.96%, so the MBT of the P 2 d should be 132-140°C, while R o of the P 2 w shale in the Puguang gas field is 2.39-3.11%, so the MBT of the P 2 w should be 207-232°C (Figure 4). The thicknesses of the intervals between the P 1 q limestone and the P 2 d shale in the XBX outcrop and between the P 2 c-T 1 f dolostone and the P 2 w shale in the Puguang gas field are about 300 and 200 m (Figure 3), respectively. In addition, the paleotemperature gradient in the Late Cretaceous was about 25°C/km (Zhu et al., 2016). As a result, the MBTs of the P 1 q limestone in the XBX outcrop and the P 2 c-T 1 f dolostone in the Puguang gas field should be 139.5-147.5 and 202-227°C, respectively. The MBT determined using the   (Sweeney and Burnham, 1990

Solid-State Reordering of Calcite and Maximum Burial Temperature of P 1 q in the Xibeixiang Outcrop
Carbonate clumped isotopes (Δ 47 ) can be used to directly determine the formation temperature or the equilibrium temperature of carbonate minerals, and they are independent of the δ 18 O of the fluid from which the carbonate minerals were deposited. This is called the clumped isotope thermometer (Ghosh et al., 2006;Eiler, 2007). The clumped isotope thermometer is mainly based on the exchange reactions of the 13 C and 18 O in carbonate minerals, and the abundance of 13 C-18 O in carbonate minerals is a function of temperature (Eiler, 2007). However, the clumped isotope temperature (T Δ47 ) may be altered by high temperatures and/or recrystallization during the late burial process and can be completely different from the original formation temperature (Passey and Henkes, 2012). Ancient carbonate minerals have usually experienced relatively high temperatures during late burial, which resulted in solid-state exchange (reordering) of the 13 C-18 O in the carbonate minerals. Consequently, the abundances of 13 C-18 O in carbonate minerals will differ from the original abundances when the carbonate minerals were formed, and the T Δ47 values should also be different from the original formation temperature of the carbonate minerals. In addition, recrystallization after mineral formation can also change the abundance of 13 C-18 O and T Δ47 of the carbonate minerals . Therefore, for ancient carbonate minerals, the present-day T Δ47 is often the result of the combined effect of recrystallization and solid-state reordering. The solid-state reordering of Δ 47 can be characterized using a kinetic model, and the MBT of the carbonate minerals can be reconstructed if the carbonate minerals experienced no or limited recrystallization.
There are two main kinetic models for characterizing the solid-state reordering of carbonate minerals: the transient defect/equilibrium defect model (Henkes et al., 2014) and the exchange-diffusion model (Stolper and Eiler, 2015). The best agreement between model predictions and natural dolomite marbles was found when using the exchange-diffusion model (Lloyd et al., 2018). Therefore, the exchange-diffusion model was used to in this study. According to the exchange-diffusion model, the variation of T Δ47 was shown in Figure 5, if the Permian-Triassic limestone was formed at about 30°C and reached a maximum temperature of about 220°C at 120 Ma. When the burial temperature was <100°C, no solid-state reordering occurred and T Δ47 of the calcite would be stable. When the burial temperature was between 100 and 200°C, partial solid-state reordering occurred and T Δ47 of the calcite increased as the burial temperature increased. When the burial temperature was >200°C, full solid-state reordering occurred and T Δ47 of the calcite reached equilibrium with the burial temperature. When the burial temperature was <200°C during the late uplift and exhumation, partial solid-state reordering occurred again and T Δ47 of the calcite decreased as the burial temperature decreased. If the burial temperature was decreased from above 200°C to below 150°C during the uplift, T Δ47 of the calcite would have decreased to a blocking temperature (final apparent temperature) of about 175-200°C (Henkes et al., 2014), and it did not decrease as the burial temperature decreased ( Figure 5). The solid-state reordering of dolomite is similar to that of calcite, but T Δ47 of dolomite is stable when the burial temperature is <150°C, and it does not reach equilibrium with the burial temperature when the burial temperature is <250-300°C (Lloyd et al., 2018).
T Δ47 of the limestone from the XBX outcrop is 62°C on average, which is significantly higher than the seawater temperature (20-30°C) during the deposition of P 1 q (Henkes et al., 2018), but is far lower than the blocking temperature after reaching the equilibrium temperature. According to the kinetic model of the solid-state reordering of calcite, it can be inferred that the limestone only underwent partial solid-state reordering, and it did not reach equilibrium. The petrological characteristics of the three bioclastic limestone samples (D1-D3) from the XBX outcrop show that the bioclastics were cemented with micritic calcite, and no obvious recrystallization was observed. Therefore, the T Δ47 values of the present-day limestone samples were mainly affected by the solid-state reordering process. As a result, the solid-state reordering model can be used to reconstruct the MBT of these limestone samples. Based on the solid-state reordering model for the calcite in the XBX outcrop, the relationship between T Δ47 and MBT is shown in Figure 6. T Δ47 of the present-day limestone is 62°C, and the corresponding MBT of the XBX limestone is about 139-147°C (Figure 6), which is basically consistent with the MBT constrained by the EASY% R o model (139.5-147.5°C).
FIGURE 5 | The general clumped isotope temperature (T Δ47 ) evolution of the P 2 c-T 1 f dolomite and calcite from the Puguang gas field in the Sichuan Basin, assuming that the maximum burial temperature reached 220°C and then decreased to 110°C during the late uplift event. The solid-state reordering models for calcite and dolomite are from Stolper and Eiler (2015) and Lloyd et al. (2018), respectively. FAT Final apparent temperature.
Frontiers in Earth Science | www.frontiersin.org September 2021 | Volume 9 | Article 759372 Solid-State Reordering of Dolomite and the Maximum Burial Temperature of the P 2 c-T 1 f in the Puguang Gas Field The MBT determined using the EASY%R o model is >200°C (202-227°C; Figure 4), which supports the fact that widespread oil cracking occurred in the Puguang gas field (Hao et al., 2008), while the present-day burial temperature of the P 2 c-T 1 f is 100-135°C. According to the solid-state reordering model of calcite ( Figure 5) and the temperature history of the Puguang gas field, the limestone or early-formed calcite cements should have experienced complete solid-state reordering, and the final apparent temperature of the limestone and calcite cement should be similar to the blocking temperature (150-160°C) because the present-day burial temperature is 100-135°C due to the late uplift and exhumation. T Δ47 of the micritic limestone and bioclastic limestone is 150-170°C, which is consistent with the blocking temperature predicted by the solid-state reordering model for calcite. The coarse calcite cements, associated with the solid bitumen and having more negative δ 13 C values (−1.108‰ to −0.227‰) than that of its bulk rock (dolostone), are considered to be the product of oil cracking and thermochemical sulfate reduction (TSR) processes (Hao et al., 2008), and they were deposited during the early burial process not during the late uplift and exhumation. T Δ47 of the calcite cements in the Puguang gas field are 145-156.5°C (average of 151.4°C), which is also very close to the blocking temperature of calcite predicted by the model. The small differences in the final apparent temperatures of the limestone and the coarse calcite cements in the Puguang gas field according to the uniform solid-state reordering model may be due to the different kinetic parameters used in the solid-state reordering models of the coarse calcite and micritic limestone, especially during the cooling process. However, no published paper has discussed this problem, and it is not discussed in detail in this paper. However, it is certain that the relatively high temperature (>200°C) in the Puguang gas field resulted in the complete solid-state reordering of the micritic limestone and the coarse calcite cements. Therefore, the micritic limestone and coarse calcite cements cannot be used to reconstruct the MBT in the Puguang gas field. However, the temperature of the onset of the partial reordering of dolomite is much higher than that of calcite, and the complete solid-state reordering temperature of dolomite can be as high as 250-300°C (Lloyd et al., 2018). As a result, the solidstate reordering of the dolostone samples can be used to reconstruct the MBT in the Puguang gas field. The T Δ47 values of the dolostone with different dolomite crystal sizes from the Puguang gas field are significantly different (Table 1). T Δ47 of the micritic dolostone (71.7-72.5°C) is the lowest, followed by T Δ47 of the very finely crystallized dolostone (88.5 ± 5.4°C), but T Δ47 of the two types of dolostone are very close to each other. T Δ47 of the finely crystallized dolostone (126.3 ± 7.6°C) is significantly higher than that of the micritic dolostone and the very finely crystallized dolostone, suggesting that T Δ47 of the finely crystallized dolostone was also affected by relatively strong recrystallization, while the very finely crystallized dolostone only experienced very limited recrystallization.
It is widely accepted that the micritic and very finely crystallized dolostones were caused by penecontemporaneous dolomitization, which occurred under near surface conditions or quite shallow burial conditions. As a result, the initial formation temperatures of the micritic and very finely crystallized dolostones should be similar to the temperature of the coeval seawater. The temperature (30-40°C) in Southern China was lethally hot during the Late Permian-Early Triassic (Sun et al., 2012), so the initial formation temperature of the micritic and very finely crystallized dolostones can be assumed to be 30-40°C. Assuming that the MBT reached 180°C, 200°C, 220°C, and 240°C, the corresponding T Δ47 evolution of the dolostone can be predicted using the solid-state reordering model for dolomite (Figure 7). If the MBT reached 180°C, only T Δ47 of the micritic dolostone (D83) and the very finely crystallized dolostone (D89) are within the predicted range (1.0 σ error included; Figure 7A), while T Δ47 of the finely crystallized dolostone (D99) is significantly higher than the predicted range. If the MBT reached 200°C or 220°C, only T Δ47 of the very finely crystallized dolostone (D89) is within the predicted range, and T Δ47 of the micritic dolostone and the finely crystallized dolostone (D99) are slightly lower and significantly higher than the predicted range, respectively ( Figures 7B,C). If the MBT reached 240°C, only T Δ47 of the finely crystallized dolostone (D99) is within the predicted range, and T Δ47 of the micritic dolostone and the very finely crystallized dolostone are lower than the predicted range ( Figure 7D). As was discussed above, T Δ47 of the limestone and calcite cements were similar to the blocking temperature during the late cooling process, suggesting that the calcite experienced complete solid-state reordering, and the MBT should be >200°C. Combined with the T Δ47 evolution shown in Figure 6, the MBT of the P 2 c-T 1 f in the PG gas field should be 200-220°C, which is similar to the MBT determined using the EASY%R o model (202-227°C).
Based on the model of the relationship between the final apparent temperature (present-day T Δ47 ) and the MBT (Figure 8), the MBTs determined using the present-day T Δ47 of the micritic dolostone (D83 and D90), the very finely crystallized dolostone (D89), and the finely FIGURE 6 | Relationship between the final apparent temperature (FAT) and the maximum burial temperature of the limestone from the Xibeixiang (XBX) outcrop, assuming that the calcite crystalized at 20 and 25°C at 270 Ma using Stolper and Eiler (2015) solid-state reordering model for calcite.
Frontiers in Earth Science | www.frontiersin.org September 2021 | Volume 9 | Article 759372 8 crystallized dolostone (D99) are 162-174, 171-220, and 245-252°C, respectively. Only the MBT determined using the finely crystallized dolostone falls within the temperature range determined using the EASY%R o model. The MBT determined using the micritic dolostone is slightly lower than the temperature range determined using the EASY%R o model. This may be due to the kinetic parameter of the solid-state reordering model for dolomite was constructed based on dolomite with some recrystallization. While the MBT determined using the finely crystallized dolostone is significantly higher than the temperature range determined using the EASY%R o model, which is most likely due to the occurrence of significant recrystallization. However, the time and temperature at which the recrystallization reached final equilibrium is unknown. Consequently, the present-day T Δ47 of the finely crystallized dolostone cannot be used to determine the MBT. Perhaps U-Pb dating of carbonate can be used to accurately determine the final equilibrium age of the dolostone (Pan et al., 2020), and combined with the general burial and thermal gradient information, it can be used to reconstruct the MBT based on the solid-state reordering model, which is a valuable research direction for future studies.

CONCLUSION
The maximum burial temperature (MBT) of the XBX outcrop and the Puguang gas field was reconstructed using the solid-state FIGURE 7 | The general clumped isotope temperature (T Δ47 ) evolution of the dolostone from the Puguang gas field, assuming that the dolomite crystalized at 30 and 40°C at 250 Ma, and the maximum burial temperature reached 180, 200, 220, or 240°C and then decreased to 110°C during the late uplift event. The solid-state reordering model for dolomite is from Lloyd et al. (2018). Note: the D83, D89, and D99 represent dolostone samples, the black horizontal lines and columns in the right side represent the T Δ47 and error bar of the samples D83, D89, and D99. Frontiers in Earth Science | www.frontiersin.org September 2021 | Volume 9 | Article 759372 9 reordering models for calcite and dolomite. The MBTs of the P 1 q limestone in the XBX outcrop and the P 2 c-T 1 f dolostone in the Puguang gas field determined using the solid-state reordering models for calcite and dolomite are 139-147 and 200-220°C, respectively, which are similar to the ranges determined using the EASY%R o model (139.5-147.5 and 202-227°C, respectively). Based on this study in the Sichuan Basin, the solid-state reordering models for calcite and dolomite provide a new potential method for reconstructing the MBT of ancient carbonate strata with MBTs of <150-200 and 250-300°C, respectively. In particular, the MBT determined using the solid-state reordering model for dolostone with limited crystallization is acceptable, while that of dolostone with obvious crystallization is significantly higher than the actual MBT. In addition, the solid-state reordering kinetic model for micritic dolostone also needs to be further optimized in future research.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding author.