Carbonates play an important role in global oil and gas exploration, as nearly 50% of oil and gas resources are distributed in carbonates. Reservoir genesis and distribution prediction are among the key problems in carbonates hydrocarbon exploration. Previous researchers have done a lot of studies in this domain and gained many knowledge. As for the pore genesis of carbonates, Kerans et al.^[1] and Moore et al.^[2] proposed that early diagenesis and high chemical activity lead to the formation of secondary pores in carbonates, and they are mainly caused by burial dissolution; James and Choquette^[3], Lucia et al.^[4] suggested that carbonates are dominated by sedimentary primary pores. As for the contribution of dolomitization to pore formation, because the reservoir physical properties of dolomite are generally better than that of limestone and reservoirs mainly developed in dolomite, the idea that dolomitization is critical for pore formation is prevailing^[5]; Lucia et al.^[6] and Purser et al.^[7] believed that only in the diagenetic environment where the source of CO₃^2- is extremely limited, can dolomitization lead to the increase of pores. For the contribution of hydrothermal process to pore formation, Davis et al.^[8] considered that the hydrothermal activity controlled by structure movements results in the development of dolomite reservoirs. For the distribution of reservoirs in sequence framework, Moore^[2] points out that platform margin and evaporation platform are the most favorable facies for carbonate reservoirs.

Marine carbonates in China have the characteristics of old age, burying deeply and multi-stage diagenetic superimposition and transformation^[9], and have quite complicated process of reservoir formation and hydrocarbon accumulation. Exploration practices have proved that the intervals with well-developed high-quality reservoirs are not always oil and gas reservoirs, but also may be water or dry layers. Besides the lack of hydrocarbon source, this can also related to the mismatch between pore development time and hydrocarbon migration time. Therefore, it is necessary to study the diagenesis-porosity evolution of reservoirs, evaluate the effective pores before hydrocarbon migration. To determine the absolute ages of carbonate diagenetic minerals is the key to restore the diagenesis-porosity evolution history of reservoirs. U-Th ID method can be used to determine the absolute ages of carbonate minerals from 0 to 500000 years with the precision of 1-2 years^[10,11]. Prior studies also reported ages of vug cements and speleothems from Mesozoic and Cenozoic by U-Pb ID method^{[12,13,14,15,16]}. However, due to the low U content of ancient carbonates, limited reference material and difficulty in microsampling monogeneration cement (6-8 parallel samples, 200 mg per sample), dating ancient marine carbonates by ID method often faces problems such as long testing period, difficulty in properly sampling and low success rate, which limited the extensive use of carbonates U-Pb ID method.

In this study, by establishing laser MC-ICPMS in-situ U-Pb isotope dating technique for carbonate minerals, and the development and calibration of new laboratory reference material, the technical problems in sampling and testing in ID method have been solved, and a new isotope dating technique suitable for ancient marine carbonates has been established. This technique has been applied to the study of diagenesis-porosity evolution of dolomite in Sinian Dengying Formation of Sichuan Basin. The obtained understandings are consistent with the tectonic-burial history and thermal evolution history of the study area, proving the reliability of dating data and the validity of laser in-situ U-Pb isotope dating technique. This method provides technical support for the study of diagenesis-porosity evolution of ancient marine carbonate rock and porosity evaluation.

The Sinian Dengying Formation in the Sichuan Basin can be divided into Deng 1, Deng 2, Deng 3 and Deng 4 members from the bottom to the top ^[17], and is mainly platform facies deposit^[18,19]. The deposit of the Deng 1 Member is the product of transgression during the early Late Sinian. It is mainly composed of 300-450 m thick light-dark grey layered micrite-crystal powder dolomite, with interbedding dolarenite and algal dolomite. It is in conformable or disconformable contact with the Lower Sinian Doushantuo Formation. From early to late period, the deposit of the Deng 2 Member changes from algal lamina and algal-dolarenite dolomite (turning into powder to fine-grained dolomite after recrystallization) in shallow water platform facies to gypsum-dolomite and gypsum-salt rock. The increase of seawater salinity was conducive to the reproduction of microorganisms, as a result, grape lace texture and residual vugs developed in Deng 2 Member. Affected by the episode I of the Tongwan movement, the Deng 2 Member was uplifted and subjected to weathering and denudation, forming an erosion valley in near SN strike^[20]. Deng 2 Member, 400-800 m thick, is in disconformable contact with the overlying strata. Transgressive mudstone developed during early Deng 3 Member, which gradually thinned and disappeared in the SW direction, and the shallow-water platform micrite-crystal powder dolomite and granular shoal deposits developed during the late stage. The depositional period of the Deng 4 Member was the peak period of intra-platform rift development^[21], when microbial mound-shoal complexes on platform margin and in platform formed. The lithology is mainly algal lamina or algal stromatolith dolomite, with rich matrix pores and vugs. Affected by the episode II of the Tongwan movement, the Deng 4 Member, suffered different degrees of leaching and denudation, is in disconformable contact with the overlying formation, and is 30-400 m in residual thickness. Structural-lithofacies paleogeographic characteristics of the Dengying Formation play an important role in controlling reservoir formation.

The Caledonian paleo-uplift in the Sichuan Basin has undergone five stages of tectonic evolution since the Dengying Formation deposited ^[22]. (1) Early tectonic evolution stage of Caledonian Cycle: two stages of tectonic movements, episode I and episode II of the Tongwan movement, resulted in the uplift and denudation of Deng 2 and Deng 4 members, respectively. (2) Middle-late tectonic evolution stage of Caledonian Cycle: Three times of overlapping deposition and three times of uplift and denudation occurred during the Cambrian-Ordovician (Xingkai movement, Yunan movement and Duyun movement, respectively). The Guangxi movement during the Late Silurian resulted in the uplift and denudation of the Caledonian paleo-uplift in central Sichuan Basin, which was in parallel unconformity with the Permian system. (3) Hercynian tectonic evolution stage: Devonian-Carboniferous systems were uplifted and denuded in the Upper Yangtze region. Affected by Yunnan movement at the end of Carboniferous, the central Sichuan Basin was further denuded. During Permian, the main part of the Sichuan Basin was in the period of subsidence and sedimentation, and the Dongwu movement led to the denudation of the Permian Maokou Formation. (4) Indosinian-Yanshanian tectonic evolution stage: The Indosinian movement at the turn of the Middle and Late Triassic made the Sichuan Basin transform from marine to continental deposits, and the Middle-Lower Triassic system was denuded to different degrees. (5) Himalayan tectonic evolution stage: Gaoshiti-Longnüsi region in the eastern part of the paleo-uplift was relatively stable and buried deep, while Leshan-Ziyang region in the western part of the paleo-uplift strongly folded with small burial depth. Tectonic evolution plays an important role in controlling dolomite reservoir transformation, hydrocarbon accumulation, and evolution of the Dengying Formation in the Sichuan Basin^{[19, 23]}.

The dolomite in the Dengying Formation is an important oil and gas reservoir in the Sichuan Basin. It is mainly developed in the Deng 2 and Deng 4 members, with a cumulative thickness of 20-100 m. The stratum thickness in platform margin is much larger than that inside platform. The main reservoir space are pores (algal lattice pore, intergranular pore), vugs and fractures which are filled by asphalt^[19]. We selected grape lace dolomite, algae lamina lattice dolomite and algal-dolarenite dolomite with multiple stages of cements in the above reservoir spaces as testing samples to carry out dating. It can provide important information for the establishment of diagenesis-porosity evolution history and the determination of effective pore before hydrocarbon migration. The information on the source, horizon, occurrence and purpose of the dating samples are shown in Table 1 and Fig. 1.

If the traditional isotope dilution method is used to date carbonate minerals, the samples to be measured should have high enough U and Pb contents, and a sufficient number of small samples have to be obtained from a hand specimen; moreover, the U/Pb ratio of this set of small samples has to have a variation scope big enough to fit an isochrone curve between ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb. The age at the lower intersection point of the isochrone curve and Tera-Wassenburg concordia line fitted by the data is calculated to represent the crystallization age of the carbonate mineral. However, carbonate minerals usually are complex in origin, formed in multiple stages, and susceptible to later reformation. Therefore, it is very difficult to find ideal dating samples that are from the same origin at the same period in a closed system and have a variation scope of U/Pb ratio big enough to fit the isochrone curve. Moreover, the traditional isotope dilution method is not suitable for low-uranium carbonate minerals: because (1) the samples to be tested must be large in quantity, but the U/Pb ratios after dissolution of the samples are normalized, while U/Pb ratios of various small samples are small in variation scope, making it difficult to construct an ideal isochrone curve on Tera-Wassenburg concordia diagram to obtain the precise age of the lower intersection point. (2) Contamination is highly likely during sampling and chemical separation process, and the requirements for ultra-clean laboratory background are very high. Therefore, although this method was used in a few examples successfully in a few research fields over the past 20 years^[13,15], it is difficult to be applied to ancient marine carbonate rock dating due to the limitations in U content, sample quantity, U/Pb ratio and laboratory background etc. So far, little report has been found in this field.

In recent 20 years, with the development of laser ablation technique, laser in-situ U-Pb isotope dating technique has been widely used to test the high-precision age of high-U minerals (such as zircon, monazite, castelnaudite, titanolite, rutile, apatite and garnet), and has become the most commonly used dating method in the field of geochronology. In recent years, laser in-situ U-Pb dating of some low-U minerals has also attracted more and more attention, especially carbonate minerals^{[24,25,26,27,28,29,30]}. Compared with U-Pb dating by isotope dilution solution method, LA-(MC)-ICP-MS carbonate mineral micro-area U-Pb dating technique has the advantages of in situ and simple sample preparation process, low sample consumption, low background, high spatial resolution and fast analysis speed (only 3-5 minutes for single point analysis). Most carbonate minerals have obvious low U content (usually 2-4 orders of magnitude lower than zircon, that is, the measured signal is only 1/10 000-1/100 of zircon), so it is very difficult to detect. However, the uranium content of carbonate rock itself varies in several orders of magnitude, ranging from (1-2)×10^-6 mg/g to (1-10)×10^-3 mg/g, generally in the range of (0.05-0.50)×10^-3 mg/g.

Researchers used sector-field single-receiver ICP-MS (such as Elements 2, Elements XR, and Attom), or multiple-receiver MC-ICPMS (such as Nu Plasma or Neptune) to conduct U-Pb isotope dating for carbonate minerals previously, and achieved initial success^{[24,25,26,27,28,29,30]}. But the sensitivity of the sector-field single-receiver ICP-MS is not as high as that of MC-ICPMS, and the efficiency of peak-hopping method is low. Though most MC-ICPM can realize efficient static measurement of ²³⁸U-²⁰⁸Pb-²⁰⁷Pb-²⁰⁶Pb, only ²⁰⁸Pb-²⁰⁷Pb-²⁰⁶Pb are measured in ion counter, ²³⁸U is often measured in 10¹¹ Ω universal Faraday cup. Generally, when U content is less than 0.2×10^-3 mg/g, it is very difficult to accurately measure ²³⁸U. In order to solve this problem, we installed a high sensitivity preamplifier (10 times higher sensitivity) and a discrete dynode multiplier for static measurement of ²³⁸U isotope in the highest quality end H10 Faraday cup of Nu Plasma II MC-ICPMS in the University of Queensland. The former makes the sensitivity of testing ²³⁸U ion current 10 times higher than that of the ordinary Faraday cup, and the latter 100 times higher. When U content is high enough (e.g. greater than 0.1×10^-3 mg/g), the high sensitivity Faraday cup was used to measure; when U content is low (e.g. less than 0.1×10^-3 mg/g), the multiplier was used to measure. As the background of the multiplier is very low, it can accurately measure the U content when the U content is higher than 1×10^-6 mg/g. In addition, our NU Plasma II MC-ICPMS is equipped with five multipliers for static measurement of ²⁰⁸Pb, ²⁰⁷Pb, ²⁰⁶Pb, ²⁰⁴Pb (and ²⁰⁴Hg interference peak) and ²⁰²Hg in the low mass range, respectively. At present, if the U/Pb ratio of a sample to be tested is high enough, even if its U content is as low as 1×10^-6 mg/g, we can date it effectively.

The laser ablation system ASI RESOlution SE we used is made by Australian Scientific Instruments (ASI), and composed of 193 nm ArF excimer laser and Laurin Technic double-chamber sample room. Firstly, we used the RESOlution SE laser and Thermo iCap-RQ quadrupole ICPMS to pre-scan the contents of trace elements in the sample target, which can quickly and clearly identify the variation ranges of U, Pb and U/Pb ratio in the sample at the micro-scale. Secondly, we selected the samples with high U/Pb ratio and low common Pb content, then selected the laser spot diameter according to the contents of U and Pb to conduct dating on Nu Plasma II MC-ICPMS.

A problem in the dating of U-Pb isotopic age of carbonate minerals by LA-MC-ICP-MS method is to find suitable natural mineral samples. In addition, in order to avoid matrix effect between different minerals, the used reference material should be as close as possible in composition and texture to the samples to be measured. At present, the two reference material ASH15E (stalagmites from Negev Desert, Israel)^[28,31-32] and WC-1 (calcite vein from Walnut Canyon, 0.5 km west of Whites City, New Mexico City, USA) ^{[24-25,27,30]} are used for carbonate mineral dating. But there are some limitations in using them to correct ancient marine carbonate samples. The calibrated age of the ASH15E reference material by isotope dilution solution method is only 3.001 Ma, which is much younger than that of ancient marine carbonate rock samples and is not an ideal reference material for ancient marine carbonate rock samples. The recommended age of the WC-1 is 254.4 Ma. Although it is similar to the age of the ancient marine carbonate rock, the heterogeneity of WC-1 reference material may lead to uncertainty of 3%-5%^[26,28]. In addition, these two international reference material are extremely limited, thus they can’t be used as the reference material of our laboratory, as the consumed quantity is much larger.

However, we found an AHX-1 reference material more suitable for dating ancient marine carbonate rock from the lower Cambrian Xiaoerblak Formation in Aksu area, Tarim Basin. After calibration, it can be used as an internal laboratory standard. The AHX-1 is pure calcite crystal filling vug along fracture. The sample is pure, homogeneous and widely distributed, and is a diagenetic product of the same period and same origin. In the past year, we have repeatedly detected the contents of trace elements and U-Pb isotope ratio in this sample by LA-(MC)ICP-MS, and obtained thousands of sets of data (Table 2). We found that this sample is very suitable for laser in-situ U-Pb isotope dating. Its content of ²⁰⁸Pb in most parts is almost zero, which indicates that the sample is free of the effect of common Pb, and ²⁰⁶Pb and ²⁰⁷Pb are both products of U decay. Its average content of ²³⁸U is about 0.15×10^-3 mg/g. Although the U content of zircon reference material 91500 is more than 70 times higher, this content of ²³⁸U meets the detection requirement completely, as the U content detection limit of our MC-ICPMS instrument is down to 1×10^-6 mg/g only.

In Nu Plasma II, we repeatedly measured ASH15E and AHX-1 for more than 20 times by laser method at different time intervals. We applied the recommended age 3.001 Ma of ASH15E^[24] to calibrate the age of AHX-1. The weighted average age of more than 20 groups we obtained is 209.8±1.3 Ma. Fig. 2a shows the ASH15E reference material. One of its tested age of AHX-1 is 209.1±1.2 Ma. At the same time, we presumed that AHX-1 was a reference material, then we measured the ages of ASH15E and WC-1 to be 2.960±0.025 Ma (Fig. 2b) and 259.0±4.1 Ma (Fig. 2c). The results of our several measurements show that AHX-1 with a low level of common Pb is a more ideal reference material for carbonate minerals. By using it as reference material, each measurement has high stability, most of the data points are located near the lower intersection of the concordia line, and a small number of data points are distributed on the isochrone curve, resulting in smaller age error and upper intersection point error (the upper intersection point represents the common Pb composition of the sample). However, this reference material is only used as laboratory work reference material at present, and the calibration between different dating methods (solution method and laser method) is under way. Taking AHX-1 as a hypothetical sample, we measured the calcite cement from the Devonian Guanwushan Formation in the northern Wuhuadong section, Sichuan Basin. Its average U content is 0.01×10^-3 mg/g, and the measured age is 244.3±2.1 Ma (Fig. 2d), similar to the measured age with ASH15E and WC-1 as reference material (246.1±2.3 Ma and 245.4±1.7 Ma, respectively), which further verifies the validity of the AHX-1 reference material.

The samples suitable for laser in-situ U-Pb isotope dating should be homologous calcite or dolomite crystals of the same source and same period. They can be surrounding rock or fillings in pores, vugs or fractures. We first selected the samples meeting the dating requirements, then cut and cleaned them before target preparation. The preparation of laser target in this study was completed in the Key Laboratory of Carbonate Reservoir, PetroChina Hangzhou Research Institute of Geology. The preparation method of laser ablation (LA) carbonate target is similar to that of SHRIMP zircon target^[33]. In order to eliminate Pb contamination during sample preparation, it is necessary to do super-clean treatment to the target in the super-clean chamber before sample testing.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to analyze in-situ trace and rare earth elements before testing, especially the contents of ²³⁸U, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb, to help us judge whether the sample was suitable for laser in-situ U-Pb isotope dating. The working conditions of the laser system were: the output energy of 3 J/cm²; the diameter of the laser spot depending on the component size of the texture and the content of U, usually 100 µm beam spot; and the ablation frequency of 10 Hz. After loading the sample target, the sample pool was flushed by helium gas continuously for about 2 hours to remove the common Pb that may exist in the sample pool and the gas path. Sampling method was single point ablation. The time of single point analysis was generally 3 minutes. Before single point ablation, the sample point was pre-ablated for 2 seconds to eliminate surface Pb contamination. The higher signal of ²³⁸U was received by Faraday cup, lower signals of ²⁰⁷Pb, ²⁰⁶Pb, ²⁰⁸Pb, ²³²Th, ²⁰⁴Pb (+²⁰⁴Hg) and ²⁰²Hg were received by discrete grabber multiplier, and ultra-low signal of ²³⁸U was received by IC5. Mass calibration and optimization of cup structure and lens parameters of the mass spectrometer Nu plasma II must be carried out by using a mixing testing liquid of Pb, Th and U before connecting the laser. After connecting the laser, the instrument parameters were adjusted by line scanning to NIST612, to make the Th/U ratio close to 1, the UO/U ratio less than 0.3%, and the sensitivity of the instrument was optimized.

Data processing can be done online or offline. First, the original data was processed with Iolite 3.6 software^[34] to obtain the corresponding isotope ratios, and then the concordia diagram drawing and age calculation were completed by Isoplot 3.0 software^[35].

Taking the Dengying Formation in the Sichuan Basin as an example, we carried out laser in-situ U-Pb isotope dating analysis of surrounding rocks and dolomite cements formed in different periods, and binary isotope (D47), stable carbon and oxygen isotopes composition, trace and rare earth element, strontium isotope composition, cathodoluminescence and inclusion homogenization temperature analysis of parallel samples. The results of dating analysis are shown in Table 3, Fig. 3 and Fig. 4. The results of D47 or inclusion homogenization temperature, stable carbon and oxygen isotopes composition, cathodoluminescence analysis are shown in Table 3. The results of trace and rare earth elements, strontium isotope composition analysis are shown in Table 4. Trace and rare earth strontium isotope composition data were tested in the Radioisotope Laboratory of the College of Geosciences, University of Queensland. Binary isotope (D47) data was tested in the Isotope Laboratory of the Department of Earth and Space Sciences, University of California, Los Angeles. The tests of cathodoluminescence, stable carbon and oxygen isotopes, and homogenization temperature of inclusions were done in the Key Laboratory of Carbonate Reservoir of CNPC. Laser in-situ U-Pb isotope dating test was done in the Key Laboratory of Carbonate Reservoir of CNPC, and the Radioisotope Laboratory of College of Geosciences, University of Queensland. The two labs jointly developed in-situ U-Pb isotope dating technique. Dating of the reference material was completed by Australian MC-ICP-MS (Nu Plasma II). Dating of the Dengying Formation sample was completed on LA-ICP-MS (Element XR) of CNPC.

Table 4 shows the geochemical characteristics of trace and rare earth elements and strontium isotopes of only 6 dating samples, but they basically represent the geochemical characteristics of 20 samples for dating in this study. Geochemical characteristics in Table 4 show that the samples have fairly high ²³⁸U contents, with the average ²³⁸U contents of six types of texture components reaching 3.50×10^-3, 3.67×10^-3, 1.97×10^-3, 1.57×10^-3, 5.88×10^-3, 2.69×10^-3 mg/g, respectively, much higher than the detection limit; the contents of ²⁰⁶Pb and ²⁰⁷Pb from ²³⁸U decay are also high, indicating the samples are ideal dating samples. Only the samples have higher common Pb content, not as ideal as the AHX-1 reference material, so the interference of ordinary Pb should be considered when calculating age.

Dolomite reservoir space of the Dengying Formation in the Sichuan Basin is mainly developed in algal lamina, algal stromatolith and algal-dolarenite, being controlled by deposit facies. The major types include vug (2-100 mm), pore (0.01- 2.00 mm) and fracture (Fig. 3). There are three origins (primary, superficial dissolution and burial-hydrothermal dissolution) for both vug and pore^[19,36-37]. There is even less understanding on the period of fractures and the influence of fracture on the development and filling of vugs and pores. Based on laser in-situ U-Pb isotopic dating data of diagenetic products and geochemical characteristics of binary isotopes, strontium isotopes, trace elements and rare earth elements, combined with tectonic-burial history, thermal history of the basin and hydrocarbon generation history of source rocks, we analyzed the pore genesis and diagenesis-porosity evolution history of dolomite reservoirs in the Dengying Formation of the Sichuan Basin, which can provide evidence for effective pore evaluation before hydrocarbon migration.

Dolomite surrounding rocks: The measured ages of the two surrounding rocks (algal lamina or algal stromatolith dolomites) are 584±32 Ma and 592±24 Ma respectively, which are similar to ages of the Ediacaran system (542-635 Ma). They represent stratigraphic ages and may also reflect the age of early dolomitization. Dolomitization is considered to be related to the climatic background of evaporation during the same sedimentary period, preserving the protolithic structure of algal lamina and algal stromatolith^[38]. No luminescence, low positive value of carbon isotope (1‰-3‰), low negative value of oxygen isotope (-4‰ to -1‰) and mean value of strontium isotope of 0.708 718 (equivalent to seawater in the same period) under cathodoluminescence are typical geochemical characteristics of Sabha background of early drought and oxidation^[2].

Vug fillings: There are five stages of dolomite cements in vugs, which constitute grape lace structure and represent the complete cemented filling sequence. The cemented filling sequence of large vugs is complete, and there are even residual vugs. The cemented filling sequence of small vugs is incomplete, without residual vugs. Most scholars believe that grape lace structure is sedimentary origin, and the vugs are sedimentary primary pores, superficial dissolution enlargement pores or buried dissolution pores^[39,40,41].

The ages of 3 concentric ring-edge dolomite cement samples were measured at 546±7.6 Ma, 545±6 Ma and 545±12 Ma, respectively. The age of two radial dolomite cement samples were measured at 516±10 Ma and 514±14 Ma. The ages of two layered light-dark grey dolomite cement samples were measured at 482±14 Ma and 487±21 Ma. These three stages of cements were formed during the early Caledonian period, presumably related to the Xingkai movement, Yunan movement and Duyun movement in the study area. The medium-coarse crystalline dolomite cement was dated at the age of 248±27 Ma, indicating it may be related to hydrothermal activity caused by the Dongwu movement during the late Hercynian period. The age of two samples of the latest coarse crystalline dolomite cement were dated at 20±130 Ma and 115±69 Ma. Although the errors are very large (due to low U content but very high common Pb content), they both represent very young cements, which may be related to hydrothermal activity and filling caused by the strong fold of Leshan-Ziyang during Himalayan period. The late Caledonian to early Hercynian diagenetic products related to the Guangxi movement have not yet been found in the study area. These understandings indicate that the formation time of the vugs was earlier than the age of the concentric ring-edge dolomite cement, and should be before burial. Therefore, the vugs are the result of the stratum exposure and atmospheric freshwater leaching caused by the multi-stage Tongwan movement, rather than the product of burial or hydrothermal dissolution. The five stages of dolomite cements from early to late period get stronger in cathodoluminescence intensity, rise in D47 temperature or inclusion homogenization temperature, and become negative in oxygen isotope gradually (Table 3). These trends confirm the gradual increase of burial depth and rise of temperature during the filling process of vugs^[2], and reformation by hydrothermal activities during late Hercynian and Himalayan periods.

It should be pointed out that the strontium isotope changes of these five stages of cements reflect the hydrothermal activity traces of medium-coarse crystalline and coarse crystalline dolomite cements. The average strontium isotope values of the concentric ring-edge dolomite cement, radial dolomite cement and layered light grey-dark dolomite cement are 0.708 732, 0.709 351 and 0.709 186, respectively, which basically reflect the strontium isotope characteristics of contemporaneous seawater. But the average strontium isotope values of the medium-coarse crystalline dolomite cement and coarse crystalline dolomite cement are 0.711 473 and 0.712 029, respectively, which are obviously higher than that of contemporaneous seawater and is thus influenced by deep hydrothermal fluid. The deep hydrothermal fluid would be affected by the radiogenic Sr of the surrounding rock in the channel during its ascending process and be higher in Sr isotope value.

The absolute age data of 5-stage fillings in vugs provide evidence for a new explanation of the genesis of grape lacy dolomite and vugs. The grape lace structure is obviously not sedimentary origin, but the product of multi-stage cementation after burial. The vugs were formed in sedimentary period (primary pore) or underwent early superficial atmospheric freshwater dissolution (dissolution enlargement).

Pore fillings: there are three stages of dolomite cements in pores, partially or completely filling the pores. The measured age of the equiaxed dolomite cement is 545.7±8.5 Ma, which is equivalent to that of the concentric ring-edge dolomite cement. The measured age of the leaf-like dolomite cement is 499±25 Ma, equivalent to that of the radial dolomite cement filling vugs. The medium-coarse crystalline dolomite cement samples were measured at two ages, 457±17 Ma and 468±12 Ma, which are slightly earlier than the layered light grey-dark dolomite cement filling vugs. These three stages of cements were all formed during the early Caledonian stage, but reflect the continuous filling process. No pore fillings of late Caledonian, Hercynian and Himalayan hydrothermal minerals (saddle dolomite, sphalerite, etc.) have been observed. They have similar cathodoluminescence, D47 temperature or inclusion homogenization temperature, oxygen isotope characteristics with the vug fillings of the same period. The absence of cemented fillings of late Caledonian, Hercynian and Himalayan periods is because the pores are not as large as the vugs; the three stages of cements during early Caledonian period are sufficient to fill up the pores, and there was burial-hydrothermal activity in the same period in fact.

Fracture fillings: The dolomite cements filling fractures were measured at four ages: 472±21 Ma, 20±130 Ma, 41±10 Ma and 41±35 Ma. The first age data represents the age of early Caledonian faulting activity and dolomite cements. The medium-coarse crystalline dolomite cements filling pores and the layered light grey-dark dolomite cement filling vugs may be related to this period of faulting activity. Although the latter three ages are quite different, they all reflect the ages of very young cements, and represent the ages of Himalayan faulting activity and dolomite cement. The latest coarse crystalline dolomite cements filling vugs may be related to this period of faulting activity. Although the age of dolomite cement filling fractures equivalent to the Late Hercynian age has not been measured, it does not mean that there is no late Hercynian faulting activity in the study area. It may be because corresponding samples have not been collected. The medium-coarse crystalline dolomite cement filling vugs should be related to the faulting activity in this period. The orange-yellow medium luminescence under cathodoluminescence, high D47 temperature and high negative value of oxygen isotope all confirm that the hydrothermal solution of dolomite cement filling fractures is controlled by faulting activity.

The occurrence and characteristics of three stages of dolomite cements during early Caledonian in vugs are quite different. Similarly, the occurrence and characteristics of dolomite cements filling fractures are quite different from those filling vugs, thus conventional techniques are difficult to correlate their genesis. Laser in-situ U-Pb isotope dating for carbonate minerals provides a technical means for comparative study of genesis and stages of cements with different occurrences and characteristics.

There is no absolute age of late Caledonian-early Hercynian dolomite cements in pores, vugs and fractures, which may be related to the overall uplift and denudation of the Silurian-Carboniferous Guangxi movement and Yunnan movement. It is characterized by parallel unconformity contact, and the tectonic compression is not strong. There is also no absolute age of Indosinian-Yanshanian dolomite cements in pores, vugs and fractures, which may be because that the Indosinian movement at the turn of the Middle and Late Triassic completed the transformation from marine to continental deposits in the Sichuan Basin, then entered a continuous and stable subsidence, and was filled by continental deposits. The structure was generally stable and the tectonic compression was not strong. Late Hercynian was the development period of the Sichuan Basin and even global volcanic rocks, and Himalayan was the structural adjustment period of the Sichuan Basin, with strong tectonic activity. The emergence of dolomite cements was inevitable.

The obtained absolute ages of dolomite cements provide new evidence for the genesis of dolomite reservoirs in the Dengying Formation of the Sichuan Basin, and provide a means for evaluating the diagenesis-porosity evolution history and effective pores of reservoirs. The storage space in the Dengying Formation dolomite reservoir was formed in the pre-burial sedimentary and supergenetic environment, including both sedimentary primary pores (algae lattice pores and intergranular pores) and dissolution enlarged vugs. The age of the first-stage dolomite cement filling the vugs is equal to the age of the stratum, sufficiently demonstrating that these vugs are not the product of burial dissolution. The burial process of the Dengying Formation dolomite reservoir was actually the gradual filling process of the pores and vugs. According to the absolute ages of dolomite cements, vug filling processes occurred in three stages: early Caledonian, late Hercynian and Himalayan; and pore filling processes mainly happened in early Caledonian. Fractures, as migration channels of diagenetic media, provided material sources for cements filling vugs and pores. Residual vugs, pores and fractures not filled with cements constitute the main reservoir space.

Referring to the initial porosity value of Clyde H Moore^[2] and the accumulation of residual pores and cement distribution area under microscope, the initial porosity is taken as 30%, the increased porosity by superficial dissolution is 10%, and the total porosity before burial is 40%. The average porosity was decreased from 40% to 15% by three stages of dolomite cements during early Caledonian. The average porosity was decreased from 15% to 10% by the dolomite cements during late Hercynian, which was remained till the end of the Yanshan movement. The average porosity was decreased from 10% to 8% by the dolomite cements during Himalayan. Fractures do not contribute much to porosity, mainly as burial-hydrothermal channels, and are mostly filled. Based on this, the diagenesis-porosity evolution history of the Dengying Formation dolomite reservoir is established (Fig. 5). Combining the tectonic-burial history of the Dengying Formation^[42], thermal history of the basin^[43], and hydrocarbon generation history of the Cambrian Qiongzhusi Formation source rock^[44], we can evaluate the migration time of oil and gas, pre-migration pores and reservoir formation stages.

The hydrocarbons in the Dengying Formation gas reservoir in Gaoshiti-Moxi structure in the Sichuan Basin is considered to come from Cambrian Qiongzhusi Formation^[44]. With the continuous burial during the Caledonian period, the source rock of the Qiongzhusi Formation began to generate hydrocarbons at the end of Silurian, and primary migration and reservoir formation took place. At this time, the effective porosity could reach 15%, mainly residual vugs. At the end of Silurian, with the tectonic uplift from Devonian to Carboniferous, the crude oil cracked into gas, and escaped or accumulated to form gas reservoir, and the residual membrane asphalt was mainly distributed in the vugs. At this time, the porosity of the reservoir could reach 12%-15%. With the continuous burial at the end of the Carboniferous, the source rock of the Qiongzhusi Formation entered the peak of hydrocarbon generation, and the oil and gas generated accumulated during the late Hercynian-Indosinian (Late Permian-Triassic) period, which is the main period of reservoir formation. At this time, the porosity could reach 12%. With the continuous increase of burial depth, crude oil cracked, forming patchy asphalt filling vugs, and the cracking gas from crude oil escaped or accumulated to form Yanshanian gas reservoirs. Anyue gas field belongs to such gas reservoir finalized during Yanshanian. At this time, the porosity of the reservoir could reach 8%-10%. The Himalayan period was the adjustment period of gas reservoirs. The Yanshanian gas reservoirs adjusted due to Himalayan tectonic movement. Some of the gas was adjusted to accumulate in Himalayan structural traps. Weiyuan gas field belongs to such gas reservoir finalized during Himalayan period. At this time, the porosity of the reservoir could reach 8%.

The development of laser in-situ U-Pb isotope dating technique for carbonate minerals not only solves the problem of absolute age of carbonate minerals, but also can help restore the diagenesis-porosity evolution history of reservoirs on the basis of the absolute ages and contents of carbonate cements. Moreover, the effective porosity and reservoir formation effectiveness before hydrocarbon migration can be evaluated by combining the tectonic-burial history and hydrocarbon generation history. Some predecessors^[45,46] determined the reservoir-forming periods mainly by homogeneous temperature of oil and gas inclusions. The evaluation of effective porosity and reservoir-forming effectiveness before hydrocarbon migration opened up a more effective way to determine the reservoir-forming periods.

Through the development of AHX-1, a laboratory working reference material with a calibration age of 209.8 Ma, and the application of laser ablation sampling system and multi-receiver inductively coupled plasma mass spectrometer, the problems of dating ancient marine carbonate reference material, sampling and dating samples with ultra-low U and Pb contents (the limit value is greater than 1×10^-6 mg/g), difficult to do by solution method, have been solved. Laser in-situ U-Pb isotope dating technique for ancient marine carbonates has been established.

Based on the dating results of different stages of dolomite cements filling vugs, pores and fractures, it is found that the burial diagenesis of dolomite reservoir in the Dengying Formation are mainly the gradual filling of primary pores (algal lattice pore and intergranular pore) and supergenetic dissolution pores. The filling process of the vugs took place in three stages (early Caledonian, late Hercynian and Himalayan periods). The filling process of the pores mainly happened during the early Caledonian period. There is a high coincidence between the ages of three-stage dolomite cements filling in fractures and those of three-stage dolomite cements filling vugs and pores, then the genetic relationship of dolomite cements filling vugs, pores and fractures at various stages is clarified. The residual vugs, pores and fractures not filled with cements constitute main reservoir space. Based on this, the diagenesis-porosity evolution history of the Dengying Formation dolomite reservoir in central Sichuan has been established.

The understandings of diagenesis-porosity evolution history of dolomite reservoir of the Dengying Formation in the Sichuan Basin are highly consistent with the tectonic-burial history of the area and the thermal history of the basin. This confirmed the reliability of dating data and the validity of laser in-situ U-Pb isotope dating technique. Combining with the hydrocarbon generation history of Cambrian Qiongzhusi Formation source rocks, it provides a new method for determining the formation time of cements in ancient marine carbonate reservoirs, studying diagenesis-porosity evolution and evaluating effective pore before hydrocarbon migration.