The reconstruction of geological process of hydrocarbon accumulation is one of the bottleneck technical problems in petroleum exploration and evaluation, especially, the reconstruction of the geological process of hydrocarbon accumulation spanning tectonic phase of the ancient marine carbonate rocks in the lower structural layers of the superimposed basins in China. The determination of the time of hydrocarbon accumulation is the key for reconstructing the geological process of hydrocarbon accumulation. Previous researchers generally used comprehensive geological analysis, inclusion homogenization temperature, isotopic dating of diagenetic minerals and direct dating of hydrocarbon products (bitumen, crude oil and natural gas) to determine the time of hydrocarbon accumulation.

The comprehensive geological analysis^[1⇓⇓-4] mainly analyzes the hydrocarbon generation and expulsion time of source rocks and hydrocarbon accumulation periods based on the structure-burial history, geothermal history and trap formation time of a basin. The inclusion homogenization temperature method^{[5⇓⇓⇓-9]} mainly determines the temperature of hydrocarbon accumulation based on the capture temperature of hydrocarbon inclusions, and then analyzes the time and period of hydrocarbon accumulation in combination with the structural-burial history and geothermal history of a basin. Although it is a popular and effective hydrocarbon accumulation time analysis method, it still has limitations. Due to the instability of organic matter in hydrocarbon inclusions, the measured homogenization temperature of this kind of inclusion isn’t reliable. However, the gas-liquid two-phase brine inclusions have higher stability and the measured homogenization temperature of this kind of inclusion has high reliability. Therefore, the homogenization temperature of gas-liquid two-phase brine inclusions is usually used to replace the capture temperature of hydrocarbon inclusions. This requires that hydrocarbon inclusions and gas-liquid two-phase brine inclusions are symbiotic or formed during the same period. However, some diagenetic minerals of hydrocarbon reservoirs lack hydrocarbon inclusions, or it is difficult to find symbiotic hydrocarbon inclusions and gas-liquid two-phase brine inclusions in the same diagenetic mineral. In addition, it is necessary to establish reliable paleo-geothermal history and burial history curves of the target strata to use comprehensive geological analysis method and inclusion homogenization temperature method. However, there are many uncertainties due to different understandings of stratum denudation thickness and tectonic movement etc. Moreover, the same homogenization temperature may correspond to multiple hydrocarbon accumulation times on the paleo-geothermal history and burial history curves, which further increases the uncertainty^{[10⇓⇓-13]}. The reported isotopic dating methods of diagenetic minerals include illite K(Ar)-Ar and zircon U-Pb isotopic dating methods. But they are mainly suitable for clastic and igneous reservoirs, but not for carbonate reservoirs. Although the direct dating method of hydrocarbon products (bitumen and crude oil)^{[14⇓⇓⇓-18]} can be used for quantitative analysis of hydrocarbon generation and cracking time, due to the possibility of Re-Os isotopic system reset in the process of hydrocarbon migration and transformation, the geological meaning of Re-Os isotopic age still has divergence. Moreover, Re-Os dating has high requirements for samples, thus it has low success rate of test.

In recent years, the laser in-situ U-Pb isotope dating technology for carbonate minerals^[19-20] and clumped isotope (Δ₄₇) temperature measurement technology^[21-22] provided solutions for determining the hydrocarbon accumulation time of ancient marine carbonate rocks in the lower structural strata of superimposed basins in China. The core of this technology is to analyze hydrocarbon charging and crude oil cracking time, and the capture temperature and time of hydrocarbon inclusions comprehensively to reconstruct the geological process of hydrocarbon accumulation by establishing diagenetic sequences of bitumen and carbonate minerals and measuring U-Pb isotopic age and Δ₄₇ temperature of carbonate minerals containing hydrocarbon inclusions. This technology makes it possible to analyze the history of hydrocarbon accumulation in absolute age coordinate system, and solves the ambiguity problem of hydrocarbon accumulation time caused by the difference between paleo-geothermal history and burial history. Taking the reconstruction of the geological process of hydrocarbon accumulation of the Sinian Dengying Formation gas reservoir in the Central Sichuan paleo-uplift as an instance, this paper presents the application of laser in-situ U-Pb isotopic dating and clumped isotope (Δ₄₇) temperature measurement of carbonate minerals in the reconstruction of geological process of hydrocarbon accumulation spanning tectonic phases of ancient marine carbonate rocks in China.

The Sinian Dengying Formation is a very important exploration series in the Sichuan Basin. Nearly one trillion cubic meters of natural gas reserves have been found in the central part of the Sichuan Basin (Central Sichuan for short). The black mudstone of Lower Cambrian Qiongzhusi Formation is deemed the main hydrocarbon source rock^[23]. Reconstruction of geological process of hydrocarbon accumulation is the key to exploration field expansion and evaluation. The reservoirs of the Dengying Formation mainly occur in the second and fourth members of the Dengying Formation (Deng 2 Member and Deng 4 Member for short, respectively). The reservoirs are mainly microbial dolomite, which is locally recrystallized into power crystalline and fine crystalline dolomite. The reservoir space consists of mainly algal framework pores and dissolution vugs^[24]. There are multi-stage dolomite and bitumen developing in the dissolution vugs. The dolomite crystals contain liquid and gaseous hydrocarbon inclusions, which lays a foundation for determining the time and stage of gas accumulation in the Dengying Formation based on U-Pb isotopic dating and clumped isotopic (Δ₄₇) temperature measurement of dolomite minerals containing hydrocarbon inclusions.

The Caledonian paleo-uplift experienced five tectonic evolution stages since the deposition of the Dengying Formation^[25]: (1) Early Caledonian cycle: Two tectonic movements, Tongwan Episode I and Tongwan Episode II occurred, which resulted in the uplift and denudation of the Deng 2 Member and Deng 4 Member respectively, and the buried depth of the Dengying Formation bottom reached 1500 m. (2) Middle-Late Caledonian cycle: During Cambrian-Ordovician, three times of overlapping deposition and three times of uplift and denudation took place, which are Xingkai Movement, Yunan Movement and Duyun Movement respectively. The Guangxi Movement at the end of Silurian led to the overall uplift and denudation of Caledonian paleo-uplift in Central Sichuan, then the buried depth of the Dengying Formation bottom reached 3500 m. (3) Hercynian period: The Devonian-Carboniferous Systems were uplifted and denuded on the whole. At the end of Carboniferous, affected by Yunnan Movement, Central Sichuan suffered further denudation, and the buried depth of the Dengying Formation bottom returned to 2000 m. During Permian, the main part of Central Sichuan was in subsidence and sedimentation period, and the buried depth of the Dengying Formation bottom increased to 3500 m again. (4) Indosinian-Yanshanian period: The middle Indosinian Movement at the turn of Middle and Late Triassic completed the transformation of the Central Sichuan paleo-uplift from marine facies to continental facies, and the overlying Mesozoic continental deposits made the buried depth of the Dengying Formation bottom reach nearly 7000 m. (5) Himalayan period: The Gaoshiti- Longnüsi area in the eastern part of the paleo-uplift was relatively stable, where the buried depth of the Dengying Formation bottom was nearly 6000 m, while the Leshan-Ziyang area in the western part of the paleo-uplift was strongly folded and uplifted, where the buried depth of the Dengying Formation bottom was nearly 3000 m.

Previous researchers have reconstructed the geothermal history of the Central Sichuan uplift by using several paleo-temperature scale methods, and obtained the geothermal history of the Dengying Formation by combining the sedimentary burial history (Fig. 1)^[26⇓-28]. The geothermal history of the Dengying Formation shows that the temperature of the Dengying Formation decreased obviously during 300-400 Ma, which must be related to the tectonic uplift in Hercynian period; the temperature reached the highest at the end of Early Cretaceous, and then gradually decreased due to the uplift during late Yanshanian-Himalayan tectonic movements. Anyue gas field has always been in the area with the highest paleo-geotherm. The structural-burial history and paleo-geothermal history of the Sinian Dengying Formation in the Central Sichuan paleo-uplift controlled the accumulation history of the Dengying Formation gas reservoir^[29-30].

The samples with well-developed vugs filled by car-bonate diagenetic minerals, crude oil or bitumen were selected to ensure that the carbonate diagenetic minerals contain hydrocarbon inclusions, and diagenetic sequence between the carbonate diagenetic minerals and bitumen can be established easily. The samples were prepared according to the following steps.

(1) The sample was cut into a cylinder about 1.5-2.5 cm in diameter and about 0.8 cm thick, and then the sample was cut along the section into two parallel samples.

(2) One parallel sample was made into 100 μm thick slice A and 30 μm thick slice B; another parallel sample was made into a 100 μm thick slice C. The rest solid samples were left for powder samples.

(3) Through microscopic observation of slices A, B and C, the similarity of their petrological characteristics were confirmed to ensure that carbonate diagenetic minerals with the same occurrence and the same forming period can be easily found in these slices and the residual solid samples. Study on the characteristics of hydrocarbon inclusions in the slice A under microscope shows that the host mineral age represents the capture age of hydrocarbon inclusions. The diagenetic sequences of bitumen and carbonate diagenetic minerals were established by observing the slice B under microscope, and carbonate diagenetic minerals with the same occurrence as the slice B were found in slice C and used for laser in-situ U-Pb isotopic dating. By observing the slice C under microscope, the carbonate diagenetic minerals with the same occurrence as host minerals in slice A were found for laser U-Pb isotopic dating.

(4) Laser in-situ U-Pb isotopic dating technology was used to date the carbonate diagenetic minerals with the same occurrence and forming stage in the slice C with the host minerals in the slice A, and carbonate diagenetic minerals associated with bitumen in the slice B.

(5) By observing the residual solid samples under microscope, carbonate diagenetic minerals with the same occurrence and the same forming period with the host minerals in the slice A were found, and 10 mg powder sample was drilled, and the clumped isotope (Δ₄₇) thermometry technology was used to determine the Δ₄₇ temperature of the powder sample.

Although the hydrocarbon inclusions are small individually (2-5 μm), they are not directly used for test, but are used to find host minerals for dating. Therefore, it is very important to study abundance of hydrocarbon inclusions under microscope, while the size of inclusions has no substantial impact on the research.

In theory, the age of host minerals is the most reliable substitute for the capture time of hydrocarbon inclusions. Testing one age data needs about 30-50 laser spots on the host mineral. When the laser beam hits hydrocarbon inclusion, there will be data inflection point, so it is difficult to obtain reliable age data because data of the host mineral containing inclusion have too many inflection points. The solution of this study is to find dolomite minerals without hydrocarbon inclusions (mainly fine-medium crystalline dolomite, coarse crystalline dolomite and saddle-shaped dolomite) corresponding to the host minerals containing hydrocarbon inclusions on the basis of detailed mineralogical and petrographic observation of the host minerals containing hydrocarbon inclusions. The age of the dolomite mineral without inclusions is used to represent the age of the host mineral containing inclusions. This way, the accuracy of age data reliability can be greatly improved. For dolomite minerals with smaller particle size, several dolomite particles with the same forming period were selected for laser spotting. Therefore, examining and comparing the characteristics and distribution of hydrocarbon inclusions and petrographic correlation of host minerals under microscope is the key to obtain reliable age data.

The samples were firstly analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP- MS) for trace and rare earth elements mapping. In particular, ²³⁸U, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb contents help us judge whether the samples are contemporaneous and suitable for laser in-situ U-Pb isotopic dating. The working conditions of the laser system were the output energy of 3 J/cm², the diameter of the ablation beam spot of 100-300 μm depending on the size of the diagenetic fabric and the content of U (the beam spot diameter can be reduced if the content of U is high), and the ablation frequency of 10 Hz. After loading the sample target, the sample pool was continuously flushed with gas for about 2 h to remove the common Pb that may exist in the sample pool and gas path. The sampling method was single point ablation, and the single point analysis time was 3 min. Before the single point ablation, pre-ablation was done to the sample point for 2 s to eliminate surface Pb pollution. The ²³⁸U with higher signals was received; ²⁰⁷Pb, ²⁰⁶Pb, ²⁰⁸Pb, ²³²Th, ²⁰⁴Pb(+²⁰⁴Hg) and ²⁰²Hg with lower signals were received, and the signals of ²³⁸U with super-low content were received by IC5. Before connecting the laser, it is necessary to calibrate the mass, optimize the cup structure and lens parameters of mass spectrometer Nu Plasma II with the mixed test solution of Pb, Th and U, and optimize the cup structure and lens parameters. After connecting the laser, the instrument parameters were adjusted by line scanning of NIST612, so that the instrument reached the highest sensitivity.

The data processing can be done online or offline. Firstly, the original data were processed with Iolite 3.6 software of Paton et al.^[31] to obtain the corresponding isotope ratios. Later, the concordia plot and age calculation were completed by Isoplot 3.0 software of Ludwig et al.^[32].

Clumped isotope (Δ₄₇) is a new isotope geochemical index emerging in recent ten years, which is widely used in paleo-temperature reconstruction and diagenetic fluid tracing. Its testing method is more complex than the traditional stable isotopes, and is mainly divided into two parts: CO₂ purification and testing. The CO₂ purification was carried out on the pretreatment line of stainless steel purification, which mainly included the following steps: (1) 10 mg of powder sample was dissolved in 3.5 mL of super concentrated phosphoric acid (105%) for 30-40 min; the released CO₂ gas was condensed in a U-tube with liquid nitrogen (about -280 °C); liquid nitrogen was replaced by methanol (-90 °C) to separate the CO₂ from water in the U-tube. (2) the released CO₂ gas was placed in PorpakTM with methanol (-30 °C) to remove the impure organic gas. In this process, liquid nitrogen (about -280 °C) should be placed in the next U-tube to prevent the gas from back flowing that could lead to secondary pollution. The whole process took 30-40 min. The purified CO₂ gas was transferred to the gas sealing plug with liquid nitrogen (about -280 °C) for testing.

In this work, the instrument for testing the purified CO₂ gas was Thermo MAT-253 mass spectrometer. Using dual sample inlet method, the standard gas and the CO₂ gas were alternately tested for 14 times at the ion beam intensity of 12 000 mV. PBL method was used to calibrate ion beam background of the mass spectrometer. The measured Δ₄₇ values were standardized according to the standardization method of Hunting et al.^[33]. The normalized initial value of Δ₄₇ was converted by using the carbon dioxide gas balance conversion scale between clumped isotope laboratories proposed by Dennis et al.^[34], to facilitate the data comparison between laboratories. It should be noted that the CDES of Stable Isotope Laboratory (SIL) of University of Miami in this clumped isotope test was based on the results of heating equilibrium gas at 1000 °C, 50 °C and 25 °C. The average calibration time was 20-30 d. Please refer to Murray et al.^[35] for detailed test and processing flow. Finally, the temperatures corresponding to the measured and processed Δ₄₇ values were calculated by the empirical formula proposed by Swart et al.^[36].

The samples used for dating and temperature measurement of host minerals containing hydrocarbon inclusions in this paper are from the Deng 2 Member in Well Gaoshi 6 (depth of 5363.04 m), the Deng 4 Member in Well Gaoshi 1 (depth of 4985.00 m), the Deng 2 Member in Well Moxi 22 (depth of 5418.70 m and 5416.90 m), the Deng 2 Member in Well Moxi 9 (depth of 5422.10 m), the Deng 2 Member on Ebian Xianfeng section, the Deng 2 Member on Nanjiang Yangba section, and the Deng 2 Member in Wangchang Gucheng section. We totally prepared 45 parallel slices from 15 samples.

The dissolution vugs in dolomite reservoirs of the Dengying Formation are generally filled with dolomite cements, bitumen, quartz and other hydrothermal minerals of different stages. Although the cementation sequence in single dissolution vugs is incomplete, a complete diagenetic sequence can be established based on the observation of a large number of rock slices and the contact relationship between cements of different stages. From the edge of a vug to the center, the diagenetic sequence is: radial dolomite cement→foliated dolomite cement→fine-medium crystalline dolomite cement→coarse crystalline dolomite cement→saddle-shaped dolomite cement→quartz (Fig. 2).

It is very difficult to find enough host minerals with hydrocarbon inclusions meeting the requirements of dating and temperature measurement. Moreover, it is hard to obtain reliable age data by direct dating of host minerals with enriched inclusions. A large number of slices must be prepared for microscopic observation to find minerals without inclusions corresponding to host minerals with inclusions for dating, which is also the reason for making parallel samples and cutting parallel slices. The microscopic observation of the characteristics and distribution of hydrocarbon inclusions was done in the CNPC Key Laboratory of Carbonate Reservoirs.

Except for the radial dolomite cement, the other four stages of dolomite cements and quartz are rich in inclusions. Except the foliated dolomite cement which only contains gas-liquid two-phase brine inclusions (Fig. 3a), the fine-medium crystalline dolomite cement, coarse crystalline dolomite cement, saddle-shaped dolomite cement and quartz contain both gas-liquid two-phase brine inclusions and hydrocarbon inclusions. The main component of hydrocarbon inclusions is methane by laser Raman spectroscopy testing. The fine-medium crystalline dolomite cement, coarse crystalline dolomite cement and saddle-shaped dolomite cement filling in fractures also contain abundant gas-liquid two-phase brine inclusions and hydrocarbon inclusions.

Microscopic observation reveals that there are two kinds of hydrocarbon inclusions (liquid and gaseous) in the dolomite cement. There are two types of liquid hydrocarbon inclusions: (1) hydrocarbon inclusions that are colorless-light yellow under plane polarized light, and yellow green fluorescence under ultraviolet light, they mostly occur in fine-medium crystalline dolomite cement (Fig. 3b-3d), representing middle and low maturity hydrocarbon inclusions; (2) Hydrocarbon inclusions that are light yellow under plane polarized light, and bright yellow green fluorescence under ultraviolet light, they mostly occur in coarse crystalline dolomite cement (Fig. 3e, 3f), representing middle and high maturity hydrocarbon inclusions. The gaseous hydrocarbon inclusions are black under plane polarized light, and have no fluorescence under ultraviolet light, mainly occurring in saddle-shaped dolomite cement (Fig. 3g, 3h) and quartz host minerals, representing high and over-mature maturity hydrocarbon inclusions. These hydrocarbon inclusions are smaller in size (2-5 μm) and are mainly sub-rounded and irregular in shape, with the characteristics of primary hydrocarbon inclusions. According to the result of laser Raman spectroscopy testing (Fig. 3g-3i), the composition of the gaseous hydrocarbon inclusions is CH₄. The U-Pb isotopic age and clumped isotopic (Δ₄₇) temperature of host minerals can represent the capture age and temperature of the hydrocarbon inclusions.

The characteristics and distribution of bitumen and dolomite cements in slice B were examined under microscope to sort out the diagenetic sequence of dolomite cements and bitumen. The bitumen has two occurrences: (1) free film in pores (Fig. 2g, 2h), which is mainly related to the oxidation of crude oil^[37]; (2) Retained between the two stages of dolomite in patch (Fig. 2d, 2e, 2i), which is mainly related to the cracking of crude oil^[38]. Although there are several stages of dolomite cements filling the vugs, the dolomite cements associated with the bitumen are mainly fine-medium crystalline dolomite, coarse crystalline dolomite and saddle-shaped dolomite.

The film bitumen free in pores (Fig. 2g②, 2h②) is largely associated with fine-medium crystalline dolomite and coarse crystalline dolomite. According to the occurrence and mutual cutting relationship of the bitumen and dolomite, the time of hydrocarbon charging and crude oil oxidation is later than the filling time of fine-medium crystalline dolomite (Fig. 2g①). Bitumen film can be seen around crystals of fine crystalline dolomite and in the intercrystalline pores between fine crystalline dolomites. In addition, the time of the hydrocarbon filling and crude oil oxidation should be earlier than filling time of the coarse crystalline dolomite (Fig. 2h③).

The patchy bitumen has two types of occurrences: (1) retained between two stages of coarse crystalline dolomite (Fig. 2i②), indicating the time of hydrocarbon charging and crude oil cracking is between the first stage of coarse crystalline dolomite cement (Fig. 2i①) and the second stage of coarse crystalline dolomite cement (Fig. 2i③); (2) associated with coarse crystalline dolomite, saddle-shaped dolomite and quartz (Fig. 2d, 2e), indicating that the time of crude oil cracking is later than the filling time of the saddle-shaped dolomite. Fig. 2d-2e reveals that the quartz was subjected to dissolution, forming harbor-like edge, and bitumen filled in the harbor-like dissolution pores, and the time of crude oil cracking was later than the formation of quartz. Moreover, according to the contact relationship between the quartz and saddle-shaped dolomite, the saddle-shaped dolomite was formed earlier than quartz. Therefore, it is made sure that the time of crude oil cracking was later than the filling time of saddle-shaped dolomite.

The establishment of diagenetic sequence of the bitumen and dolomite cements lays a foundation for the determination of hydrocarbon charging and crude oil oxidation or cracking time based on dating of dolomite minerals in the Dengying Formation gas reservoir of Central Sichuan paleo-uplift.

Thin slice observation under microscope has revealed that the diagenetic sequence of the vug fillings is radial dolomite cement→foliated dolomite cement→fine-medium crystalline dolomite cement→coarse crystalline dolomite cement→saddle-shaped dolomite cement→quartz and other hydrothermal minerals; and the hydrocarbon inclusions are mainly distributed in fine-medium crystalline dolomite, coarse crystalline dolomite, saddle-shaped dolomite and quartz. Meanwhile, using thin slices A and C, the mineralogy and petrography of host minerals were examined and compared under microscope to search for fine-medium crystalline dolomite, coarse crystalline dolomite and saddle-shaped dolomite suitable for laser in-situ U-Pb isotopic dating. The laser in-situ U-Pb isotopic dating of dolomite minerals was carried out in LA-ICPMS at CNPC Key Laboratory of Carbonate Reservoirs. The samples with ultra-low U content were tested by LA-MC-ICPMS at the University of Queensland, Australia (Fig. 4).

Four measured age data were measured for fine-medium crystalline dolomite: (482±14) Ma, (472±21) Ma, (468±12) Ma and (416±23) Ma (Fig. 4a-4d). Based on the petrographic analysis of the host minerals of hydrocarbon inclusions, (416±23) Ma represents the age of the host minerals rich in hydrocarbon inclusions, and also the age of hydrocarbon inclusion captured during the main hydrocarbon accumulation period. But the initial time of hydrocarbon generation and expulsion of Qiongzhusi Formation source rock can be dated back further to (482±14) Ma (Fig. 5).

Three age data were measured for coarse crystalline dolomite, namely (248±27) Ma, (246.3±1.5) Ma and (216.4±7.7) Ma (Fig. 4e-4g). According to the mineralogical and petrographic analysis of the host minerals, (248±27) Ma and (246.3±1.5) Ma represent the age of host minerals rich in liquid hydrocarbon inclusions, and also represent the age of hydrocarbon inclusions captured during the main hydrocarbon accumulation period, but the hydrocarbon generation and expulsion time could last up to (216.4±7.7) Ma (Fig. 5).

Two age data were measured for the saddle-shaped dolomite: (115±69) Ma and (41±10) Ma (Fig. 4h, 4i). The petrographic analysis of the host minerals shows that they represent the age of host minerals rich in gaseous hydrocarbon inclusions. Although the ages are larger in span and error, they represent the very young age of Yanshanian-Himalayan period and the formation time of the gas reservoir.

The areas corresponding to the host minerals with hydrocarbon inclusions in slice A were found in the rest core samples, and 10 mg of powdered samples of fine-medium crystalline dolomite, coarse crystalline dolomite and saddle-shaped dolomite were extracted with bur drill or micro drill to measure the temperature of clumped isotopes. The drilling of the powdered samples was completed in CNPC Key Laboratory of Carbonate Reservoirs. The temperature measurement of the clumped isotopes (Δ₄₇) was conducted on the Thermo MAT-253 mass spectrometer at the Stable Isotope Laboratory (SIL) of University of Miami. The test data are listed in Table 1.

Generally, the temperature of the clumped isotopes (Δ₄₇) in fine-medium crystalline dolomite is lower than that in coarse crystalline dolomite, and that in coarse crystalline dolomite is lower than that in saddle-shaped dolomite, which is highly consistent with the phase and maturity of hydrocarbon inclusions in dolomite host minerals.

Based on the ages and clumped isotope (Δ₄₇) temperatures of host minerals containing hydrocarbon inclusions, and the ages and established diagenetic sequence of dolomite minerals associated with the bitumen, the geological process of hydrocarbon accumulation in the Dengying Formation of Central Sichuan paleo-uplift has been comprehensively analyzed in this work. Firstly, based on the understanding of the regional geological background, tectonic movement and geothermal flow characteristics of the paleo-uplift in Central Sichuan, the normalized curves of paleo-geothermal history and burial history under the absolute age coordinate system were worked out with the constraints of measured U-Pb isotopic ages and clumped isotopic (Δ₄₇) temperatures of dolomite cements on burial depth, which provide key graphs for the reconstruction of the forming history of the Dengying Formation gas reservoir in Central Sichuan paleo-uplift. Through this reconstruction, it is concluded that the Dengying Formation gas reservoir in Central Sichuan paleo-uplift experienced three hydrocarbon accumulation stages: oil accumulation at the end of Silurian, oil accumulation from the end of Permian to Early Triassic, and gas accumulation in Yanshanian-Himalayan period.

During the oil accumulation at the end of Silurian represented by age of (416±23) Ma and clumped isotope (Δ₄₇) temperature of 84-120 °C, the buried depth of the Dengying Formation bottom reached 3500 m (Fig. 5). After subtracting the thickness of the Dengying Formation (nearly 1000 m), the Cambrian Qiongzhusi Formation was at the buried depth of 2500 m and the formation temperature of 70-100 °C at that time, just at the peak period of hydrocarbon generation. The difference in burial depth of nearly 1000 m between the Dengying Formation and the Qiongzhusi Formation is the main reason that the Δ₄₇temperature of host mineral (84-120 °C) in the Dengyign Formation is generally higher than that of the source rock (70-100 °C) in the Qiongzhusi Formation during the peak period of hydrocarbon generation. Based on the analysis of the age of fine-medium crystalline dolomite in the same forming period with the host minerals containing hydrocarbon inclusions, the initial hydrocarbon generation time of the source rock of the Qiongzhusi Formation can be dated further back to at least (482±14) Ma, at that time, the Qiongzhusi Formation was at the burial depth of about 1100 m and formation temperature of 40-50 °C. Similarly, for host minerals with Δ₄₇ temperatures of less than 100 °C, the capture temperature of hydrocarbon inclusions does not represent the temperature at the hydrocarbon generation peak, and the initial hydrocarbon generation time can be advanced to Ordovician. The massive uplift and denudation of the Devonian-Carboniferous made the buried depth of the Qiongzhusi Formation source rock return to 1000 m, and at the formation temperature of 30-40 °C, the Qiongzhusi Formation hydrocarbon generation of the source rock stopped.

The established diagenetic sequence and measured ages of bitumen and associated carbonate minerals further reveal that the first oil charging and oxidation in Central Sichuan paleo-uplift occurred between (468±12) Ma and (416±23) Ma. The precipitation of fine crystalline dolomite, represented by (468±12) Ma (Middle Ordovician) marks that the source rock of the Qiongzhusi Formation entering the period of hydrocarbon generation, expulsion and oil charging. The precipitation of coarse crystalline dolomite represented by (416±23) Ma (end of Late Silurian) marks the end of the peak period of hydrocarbon generation, expulsion and accumulation. The oxidation of crude oil not only formed thin-film bitumen free in the dissolution vugs, but also provided space for the precipitation of coarse crystalline dolomite. This stage of oil charging was obviously related to the continuous burial of the source rock of the Qiongzhusi Formation during the Cambrian-Silurian period. The stagnation of hydrocarbon generation and crude oil oxidation were related to the overall uplift and denudation of Caledonian paleo-uplift in Central Sichuan caused by Guangxi Movement at the end of Silurian.

During the oil accumulation from the end of Permian to Early Triassic represented by (248±27) Ma and (246.3±1.5) Ma, and clumped isotope (Δ₄₇) temperatures of 105-180 °C, the buried depth of the Dengying Formation bottom of 3500 m, the source rock of the Qiongzhusi Formation entered the peak of hydrocarbon generation again, and the hydrocarbon generation and expulsion could last until (216.4±7.7) Ma (between Middle and Late Triassic); the Dengying Formation bottom could reach the maximum buried depth of 4500 m and the formation temperature of 160 °C; the source rock of the Qiongzhusi Formation reached the maximum buried depth of 3500 m and the formation temperature of 125 °C. The Indosinian Movement at the turn between the Middle and Late Triassic made the Central Sichuan paleo-uplift transform from marine facies to continental facies completely, at the same time, the source rock of the Qiongzhusi Formation shifted from oil generation to gas generation.

The diagenetic sequence and measured ages of bitumen and associated carbonate minerals further reveal that the second oil charging and cracking in Central Sichuan paleo-uplift took place between (246.3±1.5) Ma and (216.4± 7.7) Ma, forming the patchy bitumen in dissolution vugs. The interval from the first oxidation time of crude oil at (416±23) Ma to the second charging time of crude oil at (246.3±1.5) Ma was obviously related to the overall uplift and denudation of the area during Devonian-Carboniferous period. The rapid burial at the end of Carboniferous made the source rock of the Qiongzhusi Formation enter the hydrocarbon generation, expulsion and charging stage again. The precipitation of coarse crystalline dolomite represented by (246.3±1.5) Ma (Early Triassic) marks that the source rock in the Qiongzhusi Formation entered the period of hydrocarbon generation, expulsion and oil charging once again. The precipitation of coarse crystalline dolomite represented by (216.4±7.7) Ma (Late Triassic) marks the end of peak period of hydrocarbon generation, expulsion and accumulation. The cracking of crude oil not only formed patchy bitumen filling vugs, but also provided space for the precipitation of coarse crystalline dolomite. The Indosinian Movement at the turn of Middle and Late Triassic resulted in the transformation of the paleo-uplift from marine facies to continental facies, and the overlying Mesozoic thick continental deposits made the buried depth of the Dengying Formation reach nearly 7000 m, which is the main reason for the cracking of the oil accumulated in the second stage.

During the gas accumulation in Yanshanian-Himalayan period represented by age of (115±69) Ma and (41±10) Ma and clumped isotope (Δ₄₇) temperature of 200-220 °C, the buried depth of the Dengying Formation bottom reached 4500 m. Because of the rapid deposition of the overlying continental Middle Cenozoic strata, the Dengying Formation bottom quickly reached the buried depth of 7000 m and formation temperature of 160-200 °C; the source rock of the Qiongzhusi Formation also reached the maximum buried depth of 6000 m and the formation temperature of 170 °C. Therefore, the natural gas came mainly from the cracking of crude oil accumulated during the Late Permian-Early Triassic, secondarily from the direct gas generation of the source rock in the Qiongzhusi Formation at high-over mature stage. Quartz is the filling material during the last stage in the dissolution vugs. Although no age data of the quartz available, it must be formed later than the last stage of coarse crystalline saddle-shaped dolomite. In the quartz, the inclusions have homogenization temperature of 200-220 °C and contain abundant methane gas. This means that gas supply was a continuous process from the Late Triassic, and the major hydrocarbon accumulation took place during Yanshanian period, but could last to Himalayan period. In addition, Himalayan Movement led to adjustment of the pre-existing gas reservoirs and formation of secondary gas reservoirs.

In thin slices, association of bitumen, coarse crystalline dolomite, saddle-shaped dolomite and quartz could be found. It can be sure that the cracking time of crude oil was later than the filling time of the saddle-shaped dolomite. Two age data were measured for the saddle-shaped dolomite: (115±69) Ma and (41±10) Ma. Though large in span, they represent the age of Yanshanian-Himalayan period. Except Weiyuan-Ziyang area where the structure formed during Himalayan period made the Sinian buried depth at about 3000 m, the overall buried depth was at 6000-7000 m and in the stage of gas generation from kerogen in high-over mature source rock and gas generation from crude oil cracking, so gas charging dominated in this area during the Yanshanian-Himalayan period. The existence of this stage bitumen indicates that the cracking of crude oil could last until Himalayan period. The Δ₄₇ temperatures of coarse crystalline dolomite and saddle-shaped dolomite are generally 20 °C higher than the formation temperatures of the same period, which is related to hydrothermal fluid.

Researchers reconstructed the accumulation history of the Dengying Formation gas reservoir in Central Sichuan by comprehensive geological analysis, homogenization temperature of inclusions and formation time of paleo-uplift and trap methods before. Though they have obtained a lot of understandings, there are still larger divergences. Luo et al.^[39]and Yang et al.^[40] suggested that the accumulation history of Dengying Formation gas reservoir could be divided into four stages: the first oil generation stage from Ordovician to the end of Silurian, the second oil generation stage from Permian to Middle Triassic, the cracking stage of paleo-oil reservoir from Late Triassic, and the gas generation peak stage from Late Jurassic to Cretaceous. Liu et al.^[41]considered that the Dengying Formation oil reservoir was formed during the end of Silurian and Early-Middle Permian, was in the peak of secondary oil generation during the Late Triassic, and the paleo-gas reservoir was formed during the Yanshanian period. Wang et al.^[42] concluded that the Dengying Formation oil reservoir was formed during the Permian to the end of Triassic, and the cracking of paleo-oil reservoir and natural gas charging occurred during the Yanshanian-Himalayan period. Wang et al.^[43] thought that the Dengying Formation oil reservoir was formed during Permian-Middle Triassic, but the main hydrocarbon accumulation period was Middle-Late Triassic, and the gas reservoir was formed during Late Triassic-Cretaceous. Sun et al.^[44-45] believed that the paleo-oil reservoir was formed during Triassic-Early Jurassic, after Jurassic the crude oil cracked into natural gas. For the above views, the hydrocarbon accumulation stages are mainly inferred from geological knowledge. There are different opinion on the stages (one stage and two stages) and time of oil accumulation. They all think that there is only one gas accumulation stage, but they have different opinions on the time of gas accumulation. The differences in the time and stage of hydrocarbon accumulation are caused by different geological understandings.

Although our geologic understanding on the formation process of Dengying Formation gas reservoir in the Central Sichuan paleo-uplift is different from those of other researchers, it is based on the dating and temperature data of carbonate diagenetic minerals, and the analysis of the capture time and capture temperature of hydrocarbon inclusions, the time of hydrocarbon charging and crude oil cracking, thus the reconstructed hydrocarbon accumulation history of the Dengying Formation gas reservoir in the Central Sichuan paleo-uplift are mutually confirmed. The comprehensive analysis of tectonic burial history, geothermal history and hydrocarbon generation history of source rocks in the study area also strongly supports this viewpoint. This shows the reliability and applicability of this dating and temperature determination technology in the reconstruction of the geological process of hydrocarbon accumulation in ancient marine carbonate rocks.

Four key points determine the effect of the laser in-situ U-Pb isotope dating and clumped isotope (Δ₄₇) temperature measurement technology to reconstruct the geological process of hydrocarbon accumulation: (1) study on the characteristics and distribution of hydrocarbon inclusions under microscope, including the identification of liquid and gaseous primary hydrocarbon inclusions, the abundance and distribution pattern of hydrocarbon inclusions; (2) study on facies of host minerals containing hydrocarbon inclusions, to find the carbonate minerals with no or little hydrocarbon inclusions formed in the same period with the host minerals for dating and temperature measurement, which is also the reason why we need to make parallel samples and cut parallel slices;(3) establishment of diagenetic sequence of bitumen and carbonate diagenetic minerals under microscope; (4) establishment of normalized curves of paleo-geothermal history and burial history of the target layer based on the burial depth constrained by the U-Pb isotope age and clumped isotope (Δ₄₇) temperature.

By using the laser in-situ U-Pb isotope dating and clumped isotope (Δ₄₇) temperature measurement technology, and based on the the ages of bitumen and associated carbonate minerals and the establishment of diagenetic sequence, the time and stages of hydrocarbon accumulation in the Dengying Formation gas reservoir were reconstructed in absolute age coordinate system, eliminating the ambiguity in the reconstruction, and making the reconstruction of hydrocarbon accumulation geological process more precise. This technology provides a new means for the study of hydrocarbon accumulation history. Hydrocarbon accumulation in the Dengying Formation in Central Sichuan paleo-uplift experienced three stages. The first oil accumulation peak was at the end of Silurian. But the hydrocarbon supply from source rocks was a continuous process, so the hydrocarbon supply could be advanced to the Early-Middle Ordovician at most, which may be the main reason why many researchers thought this stage of hydrocarbon accumulation happened during Ordovician-Silurian. The second oil accumulation peak was from the end of Permian to Early Triassic, and the time of continuous hydrocarbon supply could be postponed to the turn of Middle and Late Triassic. The third stage gas accumulation peak was in Yanshanian period, but it could last to Himalayan period.