Determination method of diagenetic time under constraint of laser in-situ u-pb isotope dating

Abstract

This invention relates to the field of diagenetic mineral dating technology, specifically disclosing a method for determining diagenetic time under the constraint of laser in-situ U-Pb isotope dating. Using laser in-situ U-Pb isotope dating as its core, the method achieves diagenetic time determination and geological application in three steps. The first step involves sample preparation, target area location, and setting an analysis sequence including background sampling and sample ablation. Single-point or multi-point analysis of the target mineral is performed, and key signals are monitored to obtain reliable U-Pb isotope data. The second step, based on the obtained raw data, involves background subtraction, ratio calculation, standard sample correction, and apparent age calculation. Data harmony is evaluated using charts to determine the diagenetic age and its error. The third step integrates the multi-dimensional age data obtained in the previous step, combined with petrological and burial history data, to establish a diagenetic event sequence, pinpoint the timing of diagenetic processes, and clarify the key period of effective porosity development, providing time constraints for reservoir evaluation and hydrocarbon migration analysis.

Description

Technical Field

[0001] This invention relates to the field of diagenetic mineral dating in Earth sciences, specifically to a method for determining diagenetic time under the constraint of laser in-situ U-Pb isotope dating. Background Technology

[0002] Geochronology is one of the core fields of Earth science, and its goal is to accurately determine the absolute age of rocks, minerals, and their formation events (such as diagenesis, metamorphism, and mineralization). Isotope dating techniques, especially U-Pb isotope dating, have become one of the most accurate and reliable methods for determining the age of ancient geological events (especially the Precambrian) due to their long half-life and relatively closed system.

[0003] Diagenetic mineral dating is a core method for reconstructing diagenetic sequences and pore evolution history. In the field of carbonate rocks, existing dating techniques mainly include: U-Th solution method: suitable for very young carbonate minerals (such as cave sediments and corals), with an accuracy of 1-2 years. However, its applicability is extremely limited and cannot be applied to ancient carbonate rocks formed in geological history (such as the Paleozoic era).

[0004] U-Pb isotope dilution method: This method has been applied to the dating of Mesozoic and Cenozoic cavities and cave infills. It involves dissolving, chemically purifying, and isotopically diluting selected minerals or infills, followed by high-precision measurements using thermal ionization mass spectrometry or multi-collector inductively coupled plasma mass spectrometry.

[0005] For dating ancient marine carbonate rock diagenetic minerals (such as dolomite, calcite cement, and pore filling materials), the commonly used method includes the carbonate mineral U-Pb isotope dilution solution method. However, this method generally results in low U content (due to high sample quantity requirements) and low test success rate, which is time-consuming and labor-intensive. The uranium (U) content of carbonate minerals (especially dolomite) is usually much lower than that of minerals such as zircon, and the natural background lead content also constitutes interference.

[0006] To obtain sufficient uranium and lead signals and a reliable error range, multiple parallel samples must be analyzed. This results in a lengthy experimental procedure (sample preparation, dissolution, chemical separation, purification, and instrument testing), is time-consuming (weeks to months), and costly (reagents, instrument time, and manpower). The end result is a low overall success rate, a very limited range of applicable samples, difficulty in meeting the needs of large-scale, systematic studies, and an inability to be widely adopted.

[0007] Lack of suitable standards (cause) + sample heterogeneity (cause) → challenges to data reliability (effect): Solution methods obtain the average age of multiple selected mineral grains or a large area. If the target mineral itself exhibits micro-regional age differences (e.g., multi-stage cementation, recrystallization), or if the sample contains trace amounts of minerals from different stages, the direct consequence is that the obtained age value may be a mixed age, failing to reflect the true, specific timing of the diagenetic event. Furthermore, carbonate U-Pb dating lacks widely accepted standard materials that match the matrix of the mineral being tested, further complicating data quality control and accuracy assessment.

[0008] Unable to obtain micro-area / in-situ information → difficult to accurately constrain the timing of specific diagenetic events: the solution method destructively mixes all materials in the selected area.

[0009] The direct consequence is the complete loss of age distribution information within minerals (such as cementation zones) or small regions (such as single fracture filling or specific pore cementation). The ultimate result is the inability to accurately correlate age with specific, spatially identifiable diagenetic events (such as a particular cementation phase or post-dissolution filling), thus limiting its application in the study of fine diagenetic sequences and pore evolution history.

[0010] In summary, existing U-Pb dating techniques for carbonate rocks suffer from technical problems such as low success rate, inability to obtain in-situ age information of mineral interiors or products from different diagenetic stages, and difficulty in accurately characterizing diagenetic sequences. Summary of the Invention

[0011] The purpose of this invention is to provide a method for determining the diagenetic time under the constraint of laser in-situ U-Pb isotope dating, in order to solve the technical problems in the existing carbonate rock U-Pb dating technology, which has low test success rate, cannot obtain in-situ age information of minerals or products of different diagenetic stages, and is difficult to accurately characterize the diagenetic sequence.

[0012] To solve the above-mentioned technical problems, the present invention specifically provides the following technical solution: The method for determining diagenetic time based on laser in-situ U-Pb isotope dating includes the following steps: Step 100: Place the prepared sample thin section / light section into the laser ablation sample cell and locate the target analysis area under the microscope. The target analysis area is set as multiple target points on a single mineral grain or diagenetic minerals from different periods of the same sample thin section. Sample background acquisition and sample target analysis area ablation were carried out within a set target time period. During the ablation process, U and Pb isotope signal data were acquired using the MC-ICP-MS system to obtain the signal intensity, signal stability and isotope ratio of the target isotopes, forming a basic U-Pb isotope dataset. Step 200: Subtract the background signal from the acquired basic U-Pb isotope dataset and calculate the stable signal segment during the erosion process of the target analysis area. and The ratio results are corrected by mass discrimination correction factor and element fractionation correction factor determined by standard samples. Then, the apparent age is calculated by combining the decay constant of U and the data harmony is evaluated by Tera-Wasserburg diagram or Concordia diagram. The lower intersection age or harmony age is calculated to form a quantitative geological age dataset. Step 300: By combining the obtained quantitative geological age dataset with petrological observation results, establish a diagenetic event sequence, and based on coupled pore evolution analysis, identify the key periods for effective pore development; By comparing the reservoir cementation and sealing time with the peak hydrocarbon generation and expulsion period of source rocks in petrological observations using the time window analysis method, the possible time periods for hydrocarbon migration are determined. The effectiveness of the migration channel is verified by combining the fracture formation age. The hydrocarbon migration path is constrained according to the diagenetic sequence, providing key time constraints for reservoir evaluation.

[0013] As a preferred embodiment of the present invention, a diagenetic event sequence is established by combining the obtained quantitative geological age dataset with petrological observations, and the specific method based on coupled porosity evolution analysis includes: Step 301: By combining the obtained quantitative geological age dataset with petrological observation results, spatial matching is performed to establish the correspondence between the mineral formation sequence and the quantitative geological age dataset; Step 302: Construct a computational model to quantitatively correlate diagenesis with porosity; ; in, This is expressed as the current porosity; Expressed as volume of cementitious material; Expressed as the volume of pores created by dissolution; Furthermore, by constructing a time axis using a quantitative geological age dataset and combining it with thermal history simulation, the inflection point of porosity change was determined. Step 303: Use K-means clustering to screen for results that match the pore development period that overlaps with the oil and gas charging period, and identify the key period for effective pore development.

[0014] As a preferred embodiment of the present invention, it includes standard sample calibration: Based on the analysis of working standards for carbonate minerals developed in the laboratory, the instrument's mass discrimination correction factors for U and Pb isotopes, as well as elemental fractionation correction factors, were calculated and obtained. Specifically: ; in, This is represented as a quality discrimination correction factor; Represented as true Isotope ratio; Represented as measurement Isotope ratio; Represented as corresponding The ratio of the mass numbers of isotopes; ; in, Represented as the elemental fractionation correction factor; This indicates the corrected U and Pb isotope ratio. This represents the actual ratio of U to Pb isotopes.

[0015] As a preferred embodiment of the present invention, the laser parameters are adapted according to the mineral type as follows: For fine minerals such as zircon and monazite, the following parameters are used: Cluster spot, Energy density, Erosion frequency and single-point erosion mode; for coarse-grained carbonates or cements such as calcite cement, the following methods were adopted. Cluster spot, Energy density, 5-7Hz erosion frequency and Line scan mode with high scanning speed; for quartz secondary enlarged edges, use... Cluster spot, Energy density, Erosion frequency and single-point erosion mode.

[0016] As a preferred embodiment of the present invention, the MC-ICP-MS system has low... Isotope signal enhancement, including: against Channel settings Amplifier for gain calibration; The channel integration time is extended to 100-200ms, while the integration time for conventional isotope channels is set to 50ms; mass discrimination is enabled for filtering. Interference signal.

[0017] In a preferred embodiment of the present invention, the isotope ratio is calculated using a weighted average method, with the weight being the reciprocal of the signal strength; the error assessment for age calculation includes an evaluation based on signal statistical error. The standard deviation and external error based on the repeatability error of the standard sample were used to calculate the uncertainty of the final age through analysis of covariance and Monte Carlo simulation.

[0018] As a preferred embodiment of the present invention, in the mineral co-occurrence sequence determination, if the age of early quartz cement is 120±3 Ma and the age of late calcite cement is 85±2 Ma, the quartz cement can be inferred to have formed during the rapid burial period, and the calcite cement is related to the uplift event, based on the burial history curve. In the determination of the key period of effective porosity, if the hydrocarbon generation window corresponding to the homogenization temperature of hydrocarbon inclusions is 90-60 Ma, and matches the dissolution and porosification period of 85-70 Ma, then this stage is determined to be the effective porosity formation period.

[0019] As a preferred embodiment of the present invention, in the evaluation of the matching of oil and gas migration time, if the reservoir cementation and sealing time is 70 Ma and the simulated peak period of hydrocarbon generation and expulsion in the source rock is 65 Ma, then it is determined that the oil and gas migration occurred between 70 and 65 Ma; if the early quartz cementation forms a tight zone and the late fracture filling calcite age is 65 Ma, then it is determined that the oil and gas migrated through the fracture system after 65 Ma.

[0020] Compared with the prior art, the present invention has the following advantages: This invention achieves multi-dimensional technical effects through precise U-Pb isotope data acquisition, reliable age calculation, and in-depth geological applications. First, by utilizing in-situ laser analysis and real-time signal monitoring, high-fidelity U-Pb isotope data of target minerals is efficiently acquired, avoiding sample contamination and signal interference, thus laying a solid data foundation for age calibration. Second, through background subtraction, standard sample correction, and graphical data verification, the absolute age of diagenesis is accurately calculated, solving the problem of insufficient accuracy in traditional dating methods and achieving quantitative constraints on diagenetic time. Finally, by integrating multi-dimensional age data and coupling it with petrological, burial history, and other data, a clear sequence of diagenetic events is constructed, accurately calibrating the time nodes of each diagenetic process and identifying the key period of effective porosity development. This provides core temporal evidence for reservoir evaluation and hydrocarbon migration matching analysis, significantly improving the scientific rigor and accuracy of geological interpretation and providing reliable technical support for oil and gas resource exploration and development. Attached Figure Description

[0021] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the overall process of an embodiment of the present invention. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] like Figure 1 As shown, this invention provides a method for determining diagenetic time under the constraint of laser in-situ U-Pb isotope dating, comprising the following steps: Step 100: Place the prepared sample thin section / light section into the laser ablation sample cell and locate the target analysis area under the microscope. The target analysis area is set as multiple target points on a single mineral grain or diagenetic minerals from different periods of the same sample thin section. Sample background acquisition and sample target analysis area ablation were carried out within a set target time period. During the ablation process, U and Pb isotope signal data were acquired using the MC-ICP-MS system to obtain the signal intensity, signal stability and isotope ratio of the target isotopes, forming a basic U-Pb isotope dataset. Step 200: Subtract the background signal from the acquired basic U-Pb isotope dataset and calculate the stable signal segment during the erosion process of the target analysis area. and The ratio results are corrected by mass discrimination correction factor and element fractionation correction factor determined by standard samples. Then, the apparent age is calculated by combining the decay constant of U and the data harmony is evaluated by Tera-Wasserburg diagram or Concordia diagram. The lower intersection age or harmony age is calculated to form a quantitative geological age dataset. Step 300: By combining the obtained quantitative geological age dataset with petrological observation results, establish a diagenetic event sequence, and based on coupled pore evolution analysis, identify the key periods for effective pore development; By comparing the reservoir cementation and sealing time with the peak hydrocarbon generation and expulsion period of source rocks in petrological observations using the time window analysis method, the possible time periods for hydrocarbon migration are determined. The effectiveness of the migration channel is verified by combining the fracture formation age. The hydrocarbon migration path is constrained according to the diagenetic sequence, providing key time constraints for reservoir evaluation.

[0025] By combining the obtained quantitative geological age dataset with petrological observations, a sequence of diagenetic events is established, and specific methods based on coupled porosity evolution analysis include: Step 301: By combining the obtained quantitative geological age dataset with petrological observation results, spatial matching is performed to establish the correspondence between the mineral formation sequence and the quantitative geological age dataset; Step 302: Construct a computational model to quantitatively correlate diagenesis with porosity; ; in, This is expressed as the current porosity; Expressed as volume of cementitious material; Expressed as the volume of pores created by dissolution; Furthermore, by constructing a time axis using a quantitative geological age dataset and combining it with thermal history simulation, the inflection point of porosity change was determined. Step 303: Use K-means clustering to screen for results that match the pore development period that overlaps with the oil and gas charging period, and identify the key period for effective pore development.

[0026] Including standard calibration: Based on the analysis of working standards for carbonate minerals developed in the laboratory, the instrument's mass discrimination correction factors for U and Pb isotopes, as well as elemental fractionation correction factors, were calculated and obtained. Specifically: ; in, This is represented as a quality discrimination correction factor; Represented as true Isotope ratio; Represented as measurement Isotope ratio; Represented as corresponding The ratio of the mass numbers of isotopes; ; in, Represented as the elemental fractionation correction factor; This indicates the corrected U and Pb isotope ratio. This represents the actual ratio of U to Pb isotopes.

[0027] Laser parameters are adapted according to mineral type as follows: For fine minerals such as zircon and monazite, the following parameters are used: Cluster spot, Energy density, Erosion frequency and single-point erosion mode; for coarse-grained carbonates or cements such as calcite cement, the following methods were adopted. Cluster spot, Energy density, 5-7Hz ablation frequency and Line scan mode with high scanning speed; for quartz secondary enlarged edges, use... Cluster spot, Energy density, Erosion frequency and single-point erosion mode.

[0028] The low-power MC-ICP-MS system Isotope signal enhancement, including: against Channel settings Amplifier for gain calibration; The channel integration time is extended to 100-200ms, while the integration time for conventional isotope channels is set to 50ms; mass discrimination is enabled for filtering. Interference signal.

[0029] The isotope ratios are calculated using a weighted average method, with the weights being the reciprocal of the signal strength; the error assessment for age calculation includes factors based on signal statistical errors. The uncertainty of the final age was calculated using standard deviation and external error based on standard repeatability error, through analysis of covariance and Monte Carlo simulation.

[0030] In mineral co-occurrence sequence determination, if the early quartz cement age is 120±3 Ma and the late calcite cement age is 85±2 Ma, the quartz cement can be inferred from the burial history curve to form during the rapid burial period, and the calcite cement is related to the uplift event. In determining the key period of effective porosity, if the hydrocarbon generation window corresponding to the homogenization temperature of hydrocarbon inclusions is 90-60 Ma, and matches the dissolution and porosification period of 85-70 Ma, then this stage is determined to be the effective porosity formation period.

[0031] In the evaluation of the time matching of oil and gas migration, if the reservoir cementation and sealing time is 70 Ma and the simulated peak period of hydrocarbon generation and expulsion in the source rock is 65 Ma, then the oil and gas migration is determined to have occurred between 70 and 65 Ma. If the early quartz cementation forms a tight zone and the late fracture filling calcite age is 65 Ma, then the oil and gas are determined to have migrated through the fracture system after 65 Ma.

[0032] Laser parameter optimization: such as specific combinations of laser beam size (30-80μm), energy density, ablation frequency, and ablation mode (single point vs. line scan) to maximize ablation efficiency and signal strength.

[0033] Mass spectrometry parameter optimization: such as plasma power, gas flow rate, lens parameters, and multi-receiver configuration (especially detector selection and gain settings for low Pb signal strength).

[0034] Data processing algorithms: background subtraction and isotope ratio calculation for low signal intensity backgrounds (e.g.) 207 Pb / 235 U, 206 Pb / 238 U), and a specific process for error assessment (including fractionation correction introduced by laser ablation).

[0035] Before analyzing the samples, a series of laboratory-developed working standards of carbonate minerals (such as “Carbonate_Std_A”, “Carbonate_Std_B”, etc.) were analyzed. These standards have been precisely labeled with their U-Pb ages and isotopic compositions using solution methods (ID-TIMS or solution MC-ICP-MS). By analyzing standard samples, we established the instrument's mass discrimination correction factor for U and Pb isotopes, as well as the elemental fractionation correction factor (Pb / U ratio).

[0036] Frequency of standard analysis: In the sample analysis sequence, 1-2 standard points are usually analyzed after every 5-10 unknown sample points to monitor and correct instrument drift.

[0037] Common lead correction: Correction is necessary because ancient carbonate rocks may contain nadir lead (non-radiogenic Pb). Commonly used 204 Pb is used to estimate the composition of the initial lead (e.g., using Pb). 208 Pb / 206 Pb- 207 Pb / 206 Pb or 207 Pb / 206 Pb- 204 Pb / 206 The initial lead ratio can be estimated using the Pb isochron method or by using known crustal lead evolution models.

[0038] This implementation method creatively proposes and establishes a technical system for "laser ablation multi-receiver inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) in-situ U-Pb isotope dating", and develops laboratory working standards suitable for U-Pb dating of ancient carbonate rocks.

[0039] The core of this implementation method lies in: In-situ micro-area analysis: Using high spatial resolution laser beams (such as 193nm ArF excimer lasers or femtosecond lasers) to directly erode target diagenetic mineral regions on thin rock sections, light sections, or mineral grains, achieving in-situ sampling and element / isotope analysis at the micrometer scale (typical erosion pit diameter: 30-80μm).

[0040] High-sensitivity, high-precision mass spectrometry detection: The aerosol generated by laser ablation is directly introduced into an inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with a multi-collector. MC-ICP-MS can simultaneously and accurately determine uranium (…). 238 U, 235 U) and lead (204 Pb, 206 Pb, 207 Pb, 208 The signal intensity ratio of Pb isotopes was used to overcome the problem of weak signals in samples with low U content.

[0041] Optimize erosion and analysis parameters: In view of the characteristics of low U content and potentially significant matrix effects in ancient carbonate rocks (especially dolomite), the system optimizes the combination of key parameters such as laser energy, erosion frequency, beam spot size, carrier gas flow rate, and ICP power to achieve the best signal strength, stability, and isotope ratio accuracy.

[0042] The desired effect or objective: 1) Significantly reduced sample requirements: Single-point analysis requires only a small amount of sample (nanogram level), and can perform non-destructive or minimal-destructive analysis directly on small diagenetic mineral areas (such as cement edges and pore fillings) on rock thin sections.

[0043] 2) Significantly improves analytical efficiency and success rate: Eliminates cumbersome sample dissolution and chemical separation steps, and has a short single-point analysis time (~1-2 minutes), which can quickly obtain a large number of data points, especially suitable for samples with low U content.

[0044] 3) Achieve high spatial resolution: High-density point analysis or line scanning (such as along growth zones) can be performed inside minerals to reveal the mineral growth history or age information of different diagenetic stages, and to finely characterize the diagenetic sequence.

[0045] 4) Provides reliable age data: Through matching standard samples and optimized parameters, high-precision and accurate U-Pb isotope age data are obtained (typical accuracy: 1-3%). This provides key time points for establishing the diagenesis-pore evolution history.

[0046] 5) Solving the problem of dating ancient carbonate rocks: Successfully applied to ancient marine carbonate rocks (such as the dolomite of the Dengying Formation of the Sinian System) that are deeply buried and have undergone multiple phases of alteration in areas such as the Sichuan Basin, obtaining diagenetic ages that are consistent with the geological background (tectonic-burial history, thermal history).

[0047] The following are the specific implementation process and key parameters of this invention: Prepare samples suitable for laser in-situ analysis. Select core or outcrop samples containing target diagenetic minerals (such as dolomite cement, calcite filling pores). Prepare standard rock thin sections (30 μm thick) or polished thick sections. Requirements: smooth surface, no scratches, clean and uncontaminated. Observe in detail under a microscope (reflected light, cathodoluminescence CL, fluorescence microscope, etc.), clearly identify the target mineral areas requiring dating (such as cement from different generations, pore edges, specific diagenetic mineral grains), and mark their locations (photographs can be taken for recording).

[0048] The CL microscope is particularly effective for identifying U-bearing minerals such as zircon and monazite, as well as calcite / dolomite cement sequences.

[0049] Instrument system preparation and standard calibration are performed to ensure instrument stability, and mass discrimination and fractionation corrections are conducted, specifically including: Turn on the laser ablation system (LA) and MC-ICP-MS, preheat and stabilize.

[0050] Choose a suitable combination of laser parameters: Laser wavelength: 193nm ArF excimer laser or femtosecond laser (e.g., 266nm, 213nm). Recommendation: 193nm ArF excimer laser (relatively smaller matrix effect).

[0051] Laser energy density: Adjust the laser energy to achieve an energy density of 2-8 J / cm². 2 (The specific range depends on the sample hardness and required ablation efficiency). Typical range: 3-6 J / cm 2 (Regarding dolomite and calcite).

[0052] Beam spot diameter: Selected based on the size of the target area and the required analysis resolution. Typical range: 30μm-80μm. For fine structures (such as thin cemented edges), 10μm-25μm can be selected (requiring higher energy density or femtosecond laser).

[0053] Etching frequency: 5-20Hz. Typical value: 10Hz.

[0054] Carrier gas: Helium (He) is usually used as the main carrier gas, mixed with a small amount of argon (Ar) or hydrogen (H2) to improve transmission efficiency and signal stability. He flow rate: ~0.4-0.8 L / min; Ar / H2 flow rate: ~0.05-0.1 L / min.

[0055] Optimize MC-ICP-MS parameters: RF power: 1200-1500W (to ensure complete ionization of the sample).

[0056] Sampling cone / Truncation cone: Select the appropriate cone type (e.g., Ni cone).

[0057] Gas flow rate (cooling gas, auxiliary gas): Set and optimize according to the instrument's recommended values.

[0058] Detector configuration: Configure appropriate receiver positions on the MC for simultaneous reception. 204 Pb, 206 Pb, 207 Pb, 208 Pb, 235 U,238 U's signal. 204 Pb (used for general lead correction) is typically corrected using an ion counter (CDD) or a Faraday cup (requiring high sensitivity).

[0059] The laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) in-situ U-Pb dating technology system used in this embodiment, combined with independently developed laboratory working standards suitable for ancient carbonate rocks, offers advantages over traditional U-Pb solution methods (isotope dilution methods): The sample requirement and loss are significantly reduced (micro-destructive / near-non-destructive analysis), enabling direct dating analysis of trace amounts and micro-areas of diagenetic minerals.

[0060] Traditional solution method: usually requires drilling 200 mg of pure mineral powder (about 6-8 parallel samples), which consumes a large amount of sample.

[0061] This implementation method achieves single-point analysis by creating only a micrometer-sized (typically 30-80 μm in diameter and 10-50 μm in depth) erosion pit on the mineral surface. This is based on the mineral density (e.g., dolomite ~2.8 g / cm³). 3 It is estimated that the sample consumption at a single point is in the range of micrograms (μg) to nanograms (ng) (e.g., a pit with a diameter of 50 μm and a depth of 20 μm consumes about 0.11 μg of sample).

[0062] This makes it possible to directly and non-destructively (compared to overall destructive sampling) date trace amounts, micro-areas, and precious diagenetic minerals, such as early cementitious materials and pore edge fillings, greatly expanding the range of samples that can be dated. This implementation method significantly shortens the sample pretreatment and analysis cycle and improves the test success rate.

[0063] The traditional solution method for sample pretreatment (drilling, dissolution, chemical separation and purification) is complex and lengthy, usually taking several days to several weeks. It is also prone to introducing contamination or causing loss of target elements (especially low-content Pb) during the process, resulting in a high test failure rate (especially in samples with U content <1ppm).

[0064] The sample pretreatment in this embodiment is simplified (only light or thin sections need to be prepared), eliminating the need for chemical separation. In-situ analysis speed is significantly improved, with single-point analysis time controllable within 1-3 minutes. Combined with an automated platform, dozens to hundreds of points can be analyzed per day. This significantly shortens the time to obtain valid age data, improving work efficiency and test success rate (especially in ancient carbonate rock samples with low U content).

[0065] This implementation significantly improves spatial resolution (micro-area in-situ analysis capability): it enables high-precision in-situ dating of diagenetic products from different regions or stages within a mineral. The laser ablation beam diameter can be adjusted to 8-80 μm (or even smaller, depending on the laser type and optical system). It allows for in-situ, high-spatial-resolution U-Pb dating of different micro-areas within a mineral, including growth zoning, cementation sequences, dissolution rims, and fracture fillings. It can finely characterize the time series within the same mineral or between adjacent diagenetic minerals, providing crucial data for accurately reconstructing the timeframe of complex diagenetic events (such as multi-stage cementation, dissolution, and dolomitization).

[0066] Application effects in diagenesis-pore evolution research: Successfully applied to practical geological problems, it provides a reliable time constraint for reconstructing pore evolution history.

[0067] This invention, by establishing a laser-guided in-situ U-Pb dating system and supporting standards, significantly reduces sample requirements (μg / ng vs. mg level), greatly improves analytical efficiency (minutes / points vs. days / weeks) and success rate (especially for low-U samples), achieves high spatial resolution (micrometer level) in-situ analysis, and ensures accurate and comparable data. These improvements in quantitative indicators make precise and efficient in-situ U-Pb dating of trace, micro-area, and multi-stage diagenetic minerals in ancient marine carbonate rocks a reality. This provides a reliable time-constrained means for finely reconstructing diagenetic sequences and pore evolution history, solving the core technical challenges faced by traditional solution methods in application.

[0068] In this embodiment: In laser in-situ U-Pb isotope dating, apparent age, intersection age, and harmonic age are specifically explained as follows: 1. Apparent Age This refers to direct processing without isotope fractionation correction. 206 Pb / 238 U or 207 Pb / 235 The preliminary age result is obtained from the U-ratio calculation. This value may contain systematic errors such as instrument quality bias and elemental fractionation effects, and needs to be calibrated with standard samples to obtain an accurate age.

[0069] 2. Intersection Age Specifically referring to the Tera-Wasserburg map ( 207 Pb / 235 Uvs. 206 Pb / 238In the equation (U), the age of the intersection point of the linear regression line and the concordia curve is used. This age reflects the time of the most recent thermal event experienced by the mineral.

[0070] 3. Concordant Age The age at which data points fall directly on the concordia curve (within the error range) in the Concordia plot indicates that the sample has not experienced subsequent geological disturbances.

[0071] In this embodiment, absolute time is defined as: a quantitative geological age in "millions of years (Ma)" directly calculated using the isotope decay law.

[0072] In this implementation: only the intersection age or harmony age can be called the absolute time (because it represents the true age of geological events). The apparent age is essentially uncorrected intermediate data and needs to be corrected before it can be used for absolute age calculation.

[0073] The above embodiments are merely exemplary embodiments of this application and are not intended to limit this application. The scope of protection of this application is defined by the claims. Those skilled in the art can make various modifications or equivalent substitutions to this application within its substance and scope of protection, and such modifications or equivalent substitutions should also be considered to fall within the scope of protection of this application.

Claims

1. A method for determining diagenetic time under the constraint of laser in-situ U-Pb isotope dating, characterized in that, Includes the following steps: Step 100: Place the prepared sample thin section / light section into the laser ablation sample cell and locate the target analysis area under the microscope. The target analysis area is set as multiple target points on a single mineral grain or diagenetic minerals from different periods of the same sample thin section. Sample background acquisition and sample target analysis area ablation were carried out within a set target time period. During the ablation process, U and Pb isotope signal data were acquired using the MC-ICP-MS system to obtain the signal intensity, signal stability and isotope ratio of the target isotopes, forming a basic U-Pb isotope dataset. Step 200: Subtract the background signal from the acquired basic U-Pb isotope dataset and calculate the stable signal segment during the erosion process of the target analysis area. and The ratio results are corrected by mass discrimination correction factor and element fractionation correction factor determined by standard samples. Then, the apparent age is calculated by combining the decay constant of U and the data harmony is evaluated by Tera-Wasserburg diagram or Concordia diagram. The lower intersection age or harmony age is calculated to form a quantitative geological age dataset. Step 300: By combining the obtained quantitative geological age dataset with petrological observation results, establish a diagenetic event sequence, and based on coupled pore evolution analysis, identify the key periods for effective pore development; By comparing the reservoir cementation and sealing time with the peak hydrocarbon generation and expulsion period of source rocks in petrological observations using the time window analysis method, the possible time periods for hydrocarbon migration are determined. The effectiveness of the migration channel is verified by combining the fracture formation age. The hydrocarbon migration path is constrained according to the diagenetic sequence, providing key time constraints for reservoir evaluation.

2. The method for determining diagenetic time based on laser in-situ U-Pb isotope dating as described in claim 1, characterized in that, By combining the obtained quantitative geological age dataset with petrological observations, a sequence of diagenetic events is established, and specific methods based on coupled porosity evolution analysis include: Step 301: By combining the obtained quantitative geological age dataset with petrological observation results, spatial matching is performed to establish the correspondence between the mineral formation sequence and the quantitative geological age dataset; Step 302: Construct a computational model to quantitatively correlate diagenesis with porosity; ； in, This is expressed as the current porosity; Expressed as volume of cementitious material; Expressed as the volume of pores created by dissolution; Furthermore, by constructing a time axis using a quantitative geological age dataset and combining it with thermal history simulation, the inflection point of porosity change was determined. Step 303: Use K-means clustering to screen for results that match the pore development period that overlaps with the oil and gas charging period, and identify the key period for effective pore development.

3. The method for determining diagenetic time based on laser in-situ U-Pb isotope dating as described in claim 1, characterized in that, Including standard calibration: Based on the analysis of working standards for carbonate minerals developed in the laboratory, the instrument's mass discrimination correction factors for U and Pb isotopes, as well as elemental fractionation correction factors, were calculated and obtained. Specifically: ； in, This is represented as a quality discrimination correction factor; Represented as true Isotope ratio; Represented as measurement Isotope ratio; Represented as corresponding The ratio of the mass numbers of isotopes; ； in, Represented as the elemental fractionation correction factor; This indicates the corrected U and Pb isotope ratio. This represents the actual ratio of U to Pb isotopes.

4. The method for determining diagenetic time based on laser in-situ U-Pb isotope dating as described in claim 1, characterized in that, Laser parameters are adapted according to mineral type as follows: For fine minerals such as zircon and monazite, the following parameters are used: Cluster spot, Energy density, Erosion frequency and single-point erosion mode; for coarse-grained carbonates or cements such as calcite cement, the following methods were adopted. Cluster spot, Energy density, 5-7Hz erosion frequency and Line scan mode with high scanning speed; for quartz secondary enlarged edges, use... Cluster spot, Energy density, Erosion frequency and single-point erosion mode.

5. The method for determining diagenetic time based on laser in-situ U-Pb isotope dating as described in claim 1, characterized in that, The low-power MC-ICP-MS system Isotope signal enhancement, including: against Channel settings Amplifier for gain calibration; The channel integration time is extended to 100-200ms, while the integration time for conventional isotope channels is set to 50ms; mass discrimination is enabled for filtering. Interference signal.

6. The method for determining diagenetic time based on laser in-situ U-Pb isotope dating as described in claim 1, characterized in that, The isotope ratios are calculated using a weighted average method, with the weights being the reciprocal of the signal strength; the error assessment for age calculation includes factors based on signal statistical errors. The standard deviation and external error based on the repeatability error of the standard sample were used to calculate the uncertainty of the final age through analysis of covariance and Monte Carlo simulation.

7. The method for determining diagenetic time based on laser in-situ U-Pb isotope dating as described in claim 1, characterized in that, In mineral co-occurrence sequence determination, if the early quartz cement age is 120±3 Ma and the late calcite cement age is 85±2 Ma, combined with the burial history curve, it can be inferred that the quartz cement formed during the rapid burial period, and the calcite cement is related to the uplift event. In determining the key period of effective porosity, if the hydrocarbon generation window corresponding to the homogenization temperature of hydrocarbon inclusions is 90-60 Ma, and matches the dissolution and porosification period of 85-70 Ma, then this stage is determined to be the effective porosity formation period.

8. The method for determining diagenetic time based on laser in-situ U-Pb isotope dating as described in claim 1, characterized in that, In the evaluation of the time matching of oil and gas migration, if the reservoir cementation and sealing time is 70 Ma and the simulated peak period of hydrocarbon generation and expulsion in the source rock is 65 Ma, then the oil and gas migration is determined to have occurred between 70 and 65 Ma. If the early quartz cementation forms a tight zone and the late fracture filling calcite age is 65 Ma, then the oil and gas are determined to have migrated through the fracture system after 65 Ma.

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