Paleotemperature measurement method combining isotopic dating and cluster isotopic thermometry
By combining isotope dating and cluster isotope thermometry, analyzing fluorescence background signals and fitting Raman peak shapes, the uncertainty in the calculation of paleotemperature and paleopressure of shale oil was solved, and higher precision salinity and pressure measurements were achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- DAQING OILFIELD CO LTD
- Filing Date
- 2025-12-26
- Publication Date
- 2026-07-21
AI Technical Summary
Existing technologies suffer from problems such as uncertainty in the division of fluid inclusion stages and fluorescence signals masking the true signals in the calculation of paleotemperature and paleopressure during the formation of shale oil, resulting in low calculation accuracy.
By combining isotope dating and cluster isotope thermometry, the Raman spectra of calcite vein shale samples were obtained, the fluorescence background signal was analyzed and subtracted, the Raman peak shape was fitted using the Voigt function, and the paleopressure of brine inclusions was calculated by combining the salinity calibration line.
This improved the accuracy of paleotemperature and paleopressure calculations during shale oil formation, reduced fluorescence interference, and ensured the accuracy of salinity calculations.
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Figure CN121409948B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of oil and gas extraction technology, specifically to a paleobaric measurement method that combines isotope dating and cluster isotope thermometry. Background Technology
[0002] Shale oil, as an important unconventional oil and gas resource, occupies an increasingly important position in the global energy structure. Its formation mechanism is complex, involving a series of closely coupled geological processes, including the generation, expulsion, sealing, and retention of shale oil in source rocks. Temperature and pressure are crucial information for revealing shale oil formation. Accurately reconstructing the paleotemperature and paleopressure at the time of shale oil formation is a core scientific issue for understanding its formation process and evaluating its resource potential.
[0003] Currently, fluid inclusion methods are commonly used to evaluate paleotemperature and paleopressure during shale oil formation. However, due to uncertainties in fluid inclusion phase classification, coexisting brine inclusions, the ambiguity of homogenization temperature, and the uncertainty in reconstructing burial and thermal evolution histories, as well as the fact that the fluorescence signal of organic matter in the dark, finely fragmented minerals of the shale matrix can mask the true signal, leading to distorted salinity calculations, these factors all affect the accuracy of subsequent paleopressure calculations. Summary of the Invention
[0004] To address the technical problems affecting the accuracy of subsequent paleopressure calculations, such as the uncertainty in fluid inclusion phase classification and the ability of fluorescence signals from organic matter in dark, finely fragmented shale matrix to mask the true signal, this invention aims to provide a paleobaric measurement method combining isotope dating and cluster isotope thermometry. The specific technical solution adopted is as follows:
[0005] Calcite vein shale samples were obtained, and Raman spectra of brine inclusions in the shale samples were obtained at different irradiation time points.
[0006] Based on the differences between Raman spectra at different times, the spectral signal to be measured is obtained; based on the similarity between the spectral signal to be measured and the Raman spectrum, the fluorescence background signal is determined, and the fluorescence background signal is subtracted from the Raman spectrum to obtain the fluorescence subtracted signal of brine inclusions in the shale sample;
[0007] In the fluorescence subtracted signal, the interval to be fitted in the fluorescence subtracted signal is determined based on the variation characteristics and difference characteristics of the intensity values under different wavenumbers; when fitting the interval to be fitted, the optimal fitting curve corresponding to each interval to be fitted is determined based on the deviation characteristics between the intensity values before and after fitting.
[0008] A salinity calibration line was obtained based on Raman spectral data of multiple sets of solutions with known salinity. The salinity of brine inclusions in shale samples was obtained based on the Raman shifts between the intensity values in the optimal fitting curve and the salinity calibration line, and the paleopressure of the brine inclusions was used to calculate the brine inclusions.
[0009] Furthermore, the method for acquiring the spectral signal to be measured includes:
[0010] Arrange the Raman spectra at all times in chronological order to obtain the spectral sequence;
[0011] In the spectral sequence, for each pair of adjacent Raman spectra, the difference between the previous Raman spectrum and the next Raman spectrum is calculated to obtain the difference spectrum;
[0012] All the difference spectra are arranged according to the time sequence of their corresponding Raman spectra to form a difference sequence;
[0013] The inverse difference method is used to calculate the difference sequence, and the curve obtained by integration is used as the spectral signal to be measured.
[0014] Furthermore, the method for obtaining the fluorescence background signal includes:
[0015] In the spectral signal to be measured, the signal within a preset wavenumber range is extracted and used as a fluorescence contrast signal;
[0016] Align the starting positions of the fluorescence contrast signal and the spectrum signal to be measured, and then extract a signal segment of the same length as the fluorescence contrast signal from the spectrum signal to be measured as the signal segment to be measured.
[0017] The Euclidean distance between the signal segment to be tested and the fluorescence contrast signal is calculated, negative correlation mapping is performed, and normalization is applied to obtain the similarity.
[0018] If the similarity is greater than a preset similarity threshold, the spectral signal to be tested is used as the fluorescence background signal corresponding to the shale sample; otherwise, the mean value of all intensities in the fluorescence contrast signal is obtained as the mean intensity, and the fluorescence contrast signal is smoothly fitted using a quadratic polynomial. The resulting fitted curve is used as the fluorescence background signal corresponding to the brine inclusions in the shale sample.
[0019] Furthermore, the method for obtaining the fluorescence subtraction signal includes:
[0020] In the Raman spectrum at the first moment and the fluorescence background signal, the difference between the intensity value in the Raman spectrum at each wavenumber and the intensity value in the fluorescence background signal is used to form the fluorescence subtraction signal of the brine inclusions in the shale sample.
[0021] Furthermore, the method for obtaining the interval to be fitted includes:
[0022] In the fluorescence subtraction signal, all extreme points are obtained, and the fluorescence subtraction signal between each two adjacent minimum points is taken as a test interval;
[0023] Within each test interval, the absolute value of the difference between the intensity values between two minimum points is calculated as the intensity difference, and the absolute value of the difference between the wavenumbers between two minimum points is calculated as the wavenumber difference. The ratio of the intensity difference to the wavenumber difference corresponding to each test interval is normalized and used as the change intensity of each test interval.
[0024] The test interval where the intensity of change is greater than a preset change threshold is used as the fitting interval in the fluorescence subtraction signal.
[0025] Furthermore, the method for obtaining the optimal fitting curve includes:
[0026] Set the range of values for the mixing coefficients. Under each mixing coefficient, fit the data values in each interval to be fitted based on the Voigt function to obtain the fitted data. Calculate the residual between the fitted data and the unfitted data in each interval to be fitted.
[0027] The sum of squares of the residuals corresponding to all intervals to be fitted is taken as the objective function value under each mixing coefficient;
[0028] Set the maximum number of iterations and use the Particle Swarm Optimization (PSO) algorithm to find the optimal value of the objective function within the range of the mixing coefficients, thereby obtaining the optimal mixing coefficients;
[0029] Then, with the optimal mixing coefficient, the data values in each fitting interval are fitted based on the Voigt function to obtain the optimal fitting curve for each fitting interval.
[0030] Furthermore, the method for obtaining the salinity calibration line includes:
[0031] In the Raman spectral data of each known salinity, all peaks within a preset range are obtained as comparison peaks. The absolute value of the difference in wavenumber between each two adjacent comparison peaks is calculated as the shift value. The mean of the shift values corresponding to all comparison peaks is combined with the known salt concentration as a binary pair.
[0032] The least squares method is used to fit a straight line to all the pairs, and the resulting fitted line is used as the salinity calibration line.
[0033] Furthermore, the method for obtaining the salinity of the brine inclusions in the shale sample includes:
[0034] In each optimal fitting curve, all peak values are obtained, and the absolute value of the difference in wavenumber between each two adjacent peak values is calculated as the displacement to be measured. The mean of all displacements to be measured is used as the mean displacement corresponding to each optimal fitting curve.
[0035] The mean of the displacements of all the best-fit curves is substituted into the salinity calibration line to obtain the salinity of the brine inclusions in the shale sample.
[0036] Furthermore, the method for obtaining the paleopressure includes:
[0037] Obtain the homogenization temperature of brine inclusions in shale samples;
[0038] The capture pressure is calculated based on the Zhang-Frantz isochoric equation, the formula model of which includes: ; ; ; ; in, Indicates capture pressure; This indicates the capture temperature, which is 15°C higher than the homogenization temperature. The temperature represents the homogenization temperature; m represents the molar concentration of the salt; w represents the salinity. Represents the intercept coefficient of the isochoric form; The slope coefficient of the isochoric equation; It is a constant;
[0039] The calculated capture pressure was used as the paleopressure of the calcite vein shale sample.
[0040] Furthermore, the absolute temperature of the shale sample was determined by carbon-oxygen cluster isotope analysis as the paleotemperature of the calcite vein shale sample.
[0041] The present invention has the following beneficial effects:
[0042] First, Raman spectra of brine inclusions in calcite shale samples were obtained at multiple time points. Since the fluorescence signal of organic matter in shale often masks the true Raman characteristic peaks of brine inclusions, it can cause errors in salinity calculations. However, because the fluorescence intensity of organic matter decreases to some extent with prolonged irradiation time, and after several seconds or even hours of irradiation, the fluorescence may drop to extremely low levels, the fluorescence background signal was determined by analyzing the differences in Raman spectra at different time points and subtracted from the Raman signal to obtain the fluorescence-subtracted signal of brine inclusions in the shale sample. Given the large range of characteristic subbands and complex envelope peaks of brine inclusions, and considering that the peak intensity of the Raman spectrum can reflect the composition of brine inclusions, the intensity value variation and difference characteristics were analyzed in the fluorescence-subtracted signal to determine the fitting interval—that is, the interval with significant intensity changes that requires focused attention. Next, when performing curve fitting on the fitting interval, the deviation of the intensity values before and after fitting under different fitting conditions was analyzed to select the optimal fitting curve. Thus, the optimal fitting curve can more accurately characterize the characteristic peaks in the fitting interval. Finally, a salinity calibration line is constructed based on the Raman spectral data of known salinity to characterize the relationship between salinity and Raman shift. Then, the accurate salinity of brine inclusions in shale samples can be obtained based on the Raman shift between the intensity values of the optimal fitting curve and the aforementioned salinity calibration line, which can be used to calculate the paleopressure of the brine inclusions. Attached Figure Description
[0043] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or 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 only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0044] Figure 1 This is a flowchart illustrating a paleobaric measurement method combining isotope dating and cluster isotope thermometry, as provided in one embodiment of the present invention. Detailed Implementation
[0045] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation, structure, features, and effects of a paleothermobaric measurement method combining isotope dating and cluster isotope thermometry proposed according to the present invention. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.
[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.
[0047] The following description, in conjunction with the accompanying drawings, details a specific scheme for a paleobaric measurement method combining isotope dating and cluster isotope thermometry provided by this invention.
[0048] Please see Figure 1 The diagram illustrates a flowchart of a paleobaric measurement method combining isotope dating and cluster isotope thermometry, according to an embodiment of the present invention. The method includes the following steps:
[0049] Step S1: Obtain calcite vein shale samples and obtain Raman spectra of brine inclusions in the shale samples at different irradiation time points.
[0050] Shale samples with calcite veins were selected, and the macroscopic occurrence characteristics and microscopic mineral characteristics of the calcite veins were characterized in detail using core analysis, polarized light microscopy, and other methods. The focus was on whether the veins contained hydrocarbon residues and fluid inclusions, which are key genetic markers, to make a preliminary judgment on their relationship with oil and gas activity.
[0051] Based on macroscopic and microscopic feature identification, cathodoluminescence and other analytical methods were used to distinguish and classify calcite veins from different periods, clarifying their relative formation sequence. Polarizing and fluorescence microscopy were employed to locate and identify hydrocarbon inclusions associated with calcite. This is crucial direct evidence demonstrating the correlation between calcite veins and hydrocarbon injection events, ensuring that subsequent dating, thermometry, and pressure measurements focus on shale samples with calcite veins from the same period and of the same origin, all related to shale oil formation.
[0052] Fluid inclusion analysis is currently the most mainstream method for evaluating paleotemperature and paleopressure. Its principle is to study fluid inclusions trapped in mineral crystals during growth or healing. Therefore, thin sections prepared from shale samples are selected, and the petrographic characteristics of the inclusions are observed using a polarizing microscope. This includes the type, size, morphology, and distribution characteristics of the fluid inclusions (typically >5 μm), allowing suitable inclusions to be selected for testing.
[0053] Based on the aforementioned process, shale samples with calcite veins related to shale oil formation were selected. Pure calcite veins were extracted and prepared into thin sections for detailed microscopic observation. Micro-areas without cracks, without later dissolution or replacement, with low inclusion content and relatively high U content were selected as analytical targets to minimize the influence of common lead and ensure reliable radiogenic lead. Before analysis, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to perform trace and rare earth element laser surface scanning imaging analysis on the shale samples, especially the contents of 238U, 206Pb, 207Pb, and 208Pb, to help determine whether the shale samples were contemporaneous and suitable for in-situ laser U-Pb isotope dating. The laser system operated at an output energy of 3 J / cm³. 2 The diameter of the ablation beam spot is selected from 100 to 300 μm depending on the size of the structural components and the U content (the beam spot diameter can be reduced for high U content), and the ablation frequency is 10 Hz. After loading the sample target, the sample cell is continuously flushed with gas for about 2 hours to remove any common Pb that may be present in the sample cell and gas path. The sampling method is single-point ablation, and the single-point analysis time is 3 minutes. Before single-point ablation, the sample point is pre-ablated for 2 seconds to remove surface Pb contamination. The higher signal 238U is received using a Faraday cup, while the lower signals 206Pb, 207Pb, 208Pb, 232Th, 204Pb, and 202Hg are received using an ion counter. The signal of ultra-low 238U content is received using IC5. Before connecting the laser, the mass spectrometer NuPlasmaII must be calibrated and the cup structure and lens parameters optimized using a mixed test solution of Pb, Th, and U. After connecting the laser, the instrument parameters are adjusted by scanning the NIST612 line to achieve the highest instrument sensitivity. Data processing can be done online or offline. First, Iolite 3.6 software is used to process the raw data to obtain the corresponding isotope ratios. Then, Isoplot 3.0 software is used to draw concordance diagrams and calculate ages, ultimately determining the absolute age of shale oil formation. Following this, in-situ U-Pb isotope testing is performed on the selected contemporaneous calcite veins. Sample elemental isotope fractionation correction and ordinary lead correction are applied to the test data to eliminate testing errors and interference from primary lead. The absolute age of calcite vein crystallization is directly determined, thus identifying the time when the inclusions were captured, i.e., the "reservoir formation time."
[0054] Carbon-oxygen cluster isotope mass spectrometry analysis was performed on calcite samples from the same batch. Data accuracy was ensured through standard sample calibration, allowing direct calculation of the absolute temperature of calcite formation. This temperature is unaffected by fluid composition and requires no pressure correction, offering significantly higher accuracy and reliability than traditional methods. Therefore, carbon-oxygen cluster isotopes (ΔC0) were utilized. 47The absolute temperature of calcite veins was determined by temperature measurement. The carbon and oxygen cluster isotope analysis mainly consisted of two parts: CO2 purification and testing. CO2 purification was carried out on a stainless steel pretreatment line, mainly including the following steps: ① 10 mg of powder sample was dissolved in 3.5 mL of ultra-concentrated phosphoric acid (105%) for 30–40 min. The released CO2 gas was condensed in a U-shaped tube using liquid nitrogen (approximately -280°C). Then, -90°C methanol was used to replace the liquid nitrogen, separating the CO2 gas from the water in the U-shaped tube; ② The released CO2 gas was placed in a Porpak™ container of -30°C methanol to remove organic gaseous impurities. During this process, liquid nitrogen (approximately -280°C) was placed in the next U-shaped tube to prevent gas backflow and secondary contamination. The entire process took 30–40 min. The purified CO2 gas was transferred to a gas-sealed plug using liquid nitrogen (approximately -280°C) for testing. The purified CO2 gas was tested using a ThermoMAT-253 mass spectrometer, employing a dual-inlet method. The standard gas and the tested CO2 gas were alternately tested 14 times, with an ion beam intensity of 12000 mV. Background correction for the mass spectrometer ion beam was performed using the PBL method. The measured Δ... 47 The values were standardized using Hunting's standardization method. The standardized Δ... 47 Initial values were converted using the interlaboratory carbon dioxide gas balance conversion scale for cluster isotopes proposed by Dennis to facilitate data comparison between laboratories. Finally, the measured and processed Δ... 47 The temperature corresponding to the value is calculated using the empirical formula proposed by Swart, yielding Δ. 47,CDES The final value is absolute temperature, which can be used as paleotemperature.
[0055] When measuring paleopressure, the metastable temperature range of salinity inclusions is very narrow, and bubbles appear extremely rapidly during cooling, making phase transition points difficult to detect visually, thus hindering the accurate acquisition of key change point data. Furthermore, existing techniques utilize an empirical conversion table between freezing point temperature and salinity proposed by Bodnar to derive the salinity of the liquid phase within the inclusion for calculating the salinity of the fluid within the inclusion. This method introduces calculation errors in actual experiments, and the presence of fluorescent interference from hydrocarbon organic matter in brine inclusions associated with hydrocarbon inclusions in calcite veins further complicates the process. Therefore, in this invention, Raman spectra of brine inclusions in shale samples at multiple time points are obtained. Analysis of the Raman spectra eliminates the influence of fluorescence background interference. Simultaneously, segmented fitting of the brine inclusion spectral signals is performed to obtain Raman peak information in key regions, and this Raman peak information is used to correct the inclusion salinity. Compared to salinity obtained through traditional empirical conversion tables, this method offers higher accuracy.
[0056] Specifically, after obtaining brine inclusions in the shale sample, a single-crystal silicon wafer (520.7 cm²) was used.-1 Using an absolute Raman wavelength standard, the Raman spectrometer is calibrated to eliminate mechanical drift caused by environmental temperature changes and ensure Raman shift accuracy. The brine inclusions are placed in a hot-cold stage, and a laser with a wavelength of 785 nm is used to acquire the Raman spectrum of the fluid inclusions. Simultaneously, due to environmental interference during signal acquisition, noise is present in the original signal. Therefore, the original brine inclusion signal is used as input to a second-order Savitzky-Golay smoothing algorithm with a window length of 1 / 10 of the entire signal length and a time interval of 1 second. Several images (10 images in this embodiment) are continuously extracted, and the processed results are recorded as the Raman spectrum of the brine inclusions in the shale sample. The horizontal axis of the Raman spectrum represents the Raman shift, with wavenumber as the unit, and the vertical axis represents the intensity.
[0057] Step S2: Based on the differences between Raman spectra at different times, the spectral signal to be measured is obtained; based on the similarity between the spectral signal to be measured and the Raman spectrum, the fluorescence background signal is determined, and the fluorescence background signal is subtracted from the Raman spectrum to obtain the fluorescence subtracted signal of brine inclusions in the shale sample.
[0058] During Raman spectroscopy analysis of brine inclusions, the presence of aromatic compounds, unsaturated hydrocarbons, and other organic substances within these inclusions leads to intense fluorescence under visible laser excitation. The fluorescence background intensity is several orders of magnitude higher than the Raman scattering signal, causing the Raman characteristic peaks to be submerged and difficult to identify. Therefore, it is necessary to analyze and calculate the Raman spectral data of brine inclusions to minimize or even eliminate the interference of the high-intensity fluorescence background on the Raman spectra.
[0059] During the spectral analysis of brine inclusion samples, a baseline drift occurs initially during irradiation. This is because the fluorescence intensity of organic matter decreases with prolonged irradiation. After several seconds or even hours of irradiation, the fluorescence may drop to extremely low levels. Therefore, by utilizing this characteristic, we can first analyze the differences between Raman spectra at different times to obtain the target spectral signal. The target spectral signal can characterize the intensity changes of fluorescent substances over a continuous period of time. By analyzing the similarity between the Raman spectrum and the target spectral signal, we can more accurately determine the true fluorescence background signal. This helps to subtract the fluorescence background signal from the Raman spectrum, resulting in a fluorescence subtracted signal that truly reflects the spectral characteristics of brine inclusions in shale samples.
[0060] Preferably, in one embodiment of the present invention, the method for obtaining the fluorescence subtraction signal includes the following steps:
[0061] Step S201: Based on the differences between Raman spectra at different times, perform inverse difference calculations to obtain the spectral signal to be measured.
[0062] Since the fluorescence intensity decreases to some extent over time, while the Raman peak remains basically unchanged, the Raman spectra at all times are arranged in chronological order to obtain a spectral sequence.
[0063] Then, in the spectral sequence, for each two adjacent Raman spectra, the difference between the previous Raman spectrum and the next Raman spectrum is calculated (the intensity at the same wavenumber is subtracted) to obtain the difference spectrum. Since the Raman peak remains basically unchanged, the fluorescence intensity changes with time. Therefore, the difference spectrum at this time mainly reflects the instantaneous change of fluorescence, that is, the fluorescence attenuation value in the Raman spectral signal. All the difference spectra are arranged according to the time sequence of the corresponding Raman spectra to form a difference sequence. The inverse difference method is used to calculate the fluorescence interference spectrum of organic matter in the original signal of brine inclusions in the shale sample. Therefore, the curve obtained by the final integration is taken as the spectral signal to be measured, reflecting the fluorescence interference intensity in the Raman spectrum.
[0064] Step S202: Determine the fluorescence background signal based on the similarity between the spectral signal to be measured and the Raman spectrum.
[0065] Given that saltwater inclusions are located at 1800-2800 cm -1 Within the wavenumber range, most brine inclusions exhibit almost no characteristic peaks in their Raman spectra, resulting in extremely weak signals. This wavenumber range represents the Raman spectral quiescent region for brine inclusions; that is, the spectral response in this quiescent region is entirely contributed by fluorescence. Furthermore, under normal circumstances, the spectral intensity change in the quiescent region should correspond to the fluorescence intensity change. Therefore, in the spectral signal to be measured, a preset wavenumber range (1800-2800 cm⁻¹) should be included. -1 The signal within the range is extracted and used as a fluorescence contrast signal.
[0066] Since the lengths of the fluorescence contrast signal and the target spectral signal are inconsistent, the starting positions of the fluorescence contrast signal and the target spectral signal are aligned. Then, a segment of the same length as the fluorescence contrast signal is extracted from the target spectral signal, starting from the starting position, as the target signal segment. The Euclidean distance between the target signal segment and the fluorescence contrast signal is calculated. The smaller the Euclidean distance, the higher the similarity between the target spectral signal (reflecting the fluorescence interference intensity in the Raman spectrum, calculated by the inverse difference method) and the fluorescence contrast signal (characterizing the fluorescence intensity in the silent region). Therefore, the Euclidean distance is negatively correlated and normalized to correct the logical relationship and obtain the similarity score. The higher the similarity score, the more accurately the target spectral signal reflects the fluorescence characteristics. The negative correlation mapping and normalization can be performed using the formula... ,in, Let x represent an exponential function with the natural constant e as the base, and let x represent the independent variable.
[0067] Therefore, if the similarity is greater than the preset similarity threshold, the spectral signal to be tested can be directly used as the fluorescence background signal corresponding to the shale sample; if the similarity is less than or equal to the preset similarity threshold, it indicates that the spectral signal to be tested contains more effective Raman spectral information. In this case, the mean value of all intensities in the fluorescence contrast signal is obtained as the mean intensity, and the fluorescence contrast signal is smoothly fitted using a quadratic polynomial. Finally, the obtained fitted curve can be used as the fluorescence background signal corresponding to the brine inclusions in the shale sample.
[0068] It should be noted that the preset similarity threshold in this embodiment of the present invention is 0.85. The specific value can be adjusted according to the implementation scenario and is not limited here. However, in order to ensure accuracy, it should not be lower than 0.7.
[0069] Step S203: Subtract the fluorescence background signal from the Raman spectrum to obtain the fluorescence subtracted signal of the brine inclusions in the shale sample.
[0070] Based on the aforementioned processes of finding differences and inverse difference, the fluorescence background signal in the Raman spectrum can be obtained. However, the fluorescence background signal can mask or distort the true Raman characteristic peak information of the brine inclusions. Therefore, in this sub-step, the fluorescence background signal can be subtracted from the Raman spectrum to obtain a fluorescence subtracted signal that fully characterizes the Raman features.
[0071] In the Raman spectrum and fluorescence background signal at the first time point, the difference between the intensity value in the Raman spectrum at each wavenumber and the intensity value in the fluorescence background signal is used to form the fluorescence subtraction signal of brine inclusions in the shale sample. At this time, the fluorescence subtraction signal effectively eliminates the interference of fluorescence background and can more accurately reflect the composition, structure and other characteristics of brine inclusions, providing more reliable data support for subsequent analysis.
[0072] Step S3: In the fluorescence subtracted signal, based on the variation and difference characteristics of the intensity values at different wavenumbers, determine the interval to be fitted in the fluorescence subtracted signal; when fitting the interval to be fitted, based on the deviation characteristics between the intensity values before and after fitting, determine the optimal fitting curve corresponding to each interval to be fitted.
[0073] The characteristic subbands of brine inclusions span a wide range and have complex envelope peaks. The peak intensities of the Raman spectra can reflect the composition of the brine inclusions, such as the Raman band of the stretching vibration of OH in water (3000-3700 cm⁻¹). -1 However, since peak overlap is unavoidable in Raman spectroscopy, affecting peak characteristic parameters, further calculation and analysis of the Raman spectral signal of salt water inclusions are necessary. Therefore, the variation and difference characteristics of intensity values in the fluorescence subtraction signal were analyzed first to determine the fitting interval, that is, the interval with more obvious intensity changes that needs more focus.
[0074] Preferably, in one embodiment of the present invention, the method for obtaining the interval to be fitted includes:
[0075] Extreme points in fluorescence subtraction signals often represent points of significant intensity change, which may correspond to characteristic peaks or important turning points in the spectrum. These positions usually contain key information about shale samples. Therefore, in fluorescence subtraction signals, all extreme points are obtained, and the fluorescence subtraction signal between each two adjacent minimum points is taken as a test interval.
[0076] Then, within each test interval, the absolute value of the difference between the intensity values between two minimum points is calculated as the intensity difference. The intensity difference reflects the amplitude of the signal intensity change within the test interval; the larger the value, the more significant the change. The absolute value of the difference in wavenumber between two minimum points is calculated as the wavenumber difference, reflecting the span of the test interval on the wavenumber axis. The ratio of the intensity difference to the wavenumber difference is calculated, and the normalized value of the ratio is used as the change intensity of each test interval. The change intensity is a rough estimate of the rate of change of intensity within the test interval. The larger the change intensity, the more drastic the signal change within the test interval, and the higher the complexity may be. Therefore, it should be given more attention for subsequent analysis. Thus, the test intervals with change intensity greater than a preset change threshold are designated as the fitting intervals in the fluorescence subtraction signal. Normalization is a technique well-known to those skilled in the art. The choice of normalization function can be linear normalization or standard normalization, etc., and the specific normalization method is not limited here.
[0077] It should be noted that the method for obtaining the preset change threshold here is as follows: under the same experimental conditions, for a known single-peak spectrum (such as the ice peak at 3098 cm⁻¹), -1 The calculation is performed to obtain the intensity of the change in the known single-peak spectral signal, and this intensity is used as the change threshold.
[0078] At this point, all the regions to be fitted in the fluorescence subtraction signal can be screened out.
[0079] Typically, the true shape of Raman peaks within the fitting range of brine inclusions is Lorentz-shaped. However, due to factors such as instrument broadening and sample inhomogeneity, the observed peak shape is closer to Gaussian. The Voigt function, a convolution of a Gaussian function and a Lorentz function, can more accurately describe the Raman peak shape. The Voigt function controls the proportion of the Lorentz component through a mixing coefficient, thereby achieving the fitting of Raman peak shapes at different peak positions. In actual calculations, the calculation of the Lorentz half-width is complex and difficult to calculate accurately; therefore, the mixing coefficient... Since precise evaluation is not possible, in this embodiment of the invention, when fitting the interval to be fitted, the optimal fitting curve corresponding to each interval to be fitted can be determined based on the deviation characteristics between the intensity values before and after fitting.
[0080] Preferably, in one embodiment of the present invention, the method for obtaining the optimal fitting curve includes:
[0081] The formula model for the Voigt function includes: ; in, This represents the Raman peak shape fitting function. or Represents the mixing coefficient. For Lorentz function, A Gaussian function centered at the origin. This is the set of parameters for the region to be fitted by the brine inclusions, specifically wavenumber and light intensity.
[0082] By mixing coefficient or By controlling the proportion of the Lorentz component, the Raman peak shape at different peak positions can be fitted. The specific mathematical expressions of the Lorentz function and the Gaussian function are known functions and will not be elaborated here.
[0083] The range of the mixing coefficient is set to [0,1]. Under each mixing coefficient, the data values in each interval to be fitted are fitted based on the Voigt function to obtain the fitted data. The residual between the fitted data and the unfitted data in each interval to be fitted is calculated. Then, the sum of the squares of the residuals corresponding to all intervals to be fitted is used as the objective function value under each mixing coefficient.
[0084] The maximum number of iterations is set to 20, and the Particle Swarm Optimization (PSO) algorithm is used to find the optimal value of the objective function within the range of the mixing coefficients, thereby obtaining the mixing coefficients. or The optimal value is determined so that the fitted shape remains consistent with the experimental observation data. In other words, the Particle Swarm Optimization (PSO) algorithm is considered to have converged when it reaches its maximum number of iterations, and the converged mixing coefficients are considered the optimal mixing coefficients.
[0085] Finally, with the optimal mixing coefficient, the data values in each fitting interval are fitted based on the Voigt function to obtain the optimal fitting curve for each fitting interval.
[0086] It should be noted that the Voigt function and the Particle Swarm Optimization (PSO) algorithm are well-known techniques, and the specific process will not be elaborated here.
[0087] Step S4: Obtain the salinity calibration line based on the Raman spectral data of multiple sets of solutions with known salinity; based on the Raman shifts between the intensity values in the optimal fitting curve and the salinity calibration line, obtain the salinity of the brine inclusions in the shale sample for calculating the paleopressure of the brine inclusions.
[0088] During microthermometry calculations, rapid cooling can lead to metastable phenomena in brine inclusions. In this case, bubbles appear extremely quickly during cooling, making phase transition points difficult to detect visually. This results in inaccurate freezing point information from microthermometry, leading to errors in the calculated capture pressure. Raman spectroscopy, a spectroscopic analysis technique based on molecular vibration and rotation, exhibits different characteristic peak positions, intensities, and widths in Raman spectra due to differences in molecular composition and structure. Therefore, this invention utilizes Raman spectral data from multiple sets of solutions with known salinity to obtain a salinity calibration line. Based on the Raman shifts between intensity values in the optimal fitting curve and the salinity calibration line, the salinity of brine inclusions in shale samples is calculated, thus aiding in the calculation of paleopressure.
[0089] Preferably, in one embodiment of the present invention, the method for obtaining the salinity calibration line includes:
[0090] Solutions with varying salinities were prepared, and their Raman spectra were acquired. Then, for each known salinity, all peak values within a predetermined range were selected as comparative peaks. This is because the salinity of the brine inclusions affects the OH stretching vibrations of water molecules, and the OH stretching vibration band is located at approximately 3000-3700 cm⁻¹. -1 The broad peaks within this range, with their complex peak shapes, are the result of the superposition of water molecule vibrational states under different hydrogen bonding environments. Therefore, the preset range here is chosen to be 3000-3700 cm⁻¹. -1 Calculate the absolute value of the wavenumber difference between any two adjacent contrast peaks as the displacement value, and then combine the mean of the displacement values corresponding to all contrast peaks with the known salt concentration as a binary pair. This will yield a series of values in the form of (…). The tuple of (w), where, The value represents the mean shift value corresponding to the peak, and w represents the salinity.
[0091] Finally, the least squares method is used to fit a straight line to all the pairs, and the resulting fitted line is used as the salinity calibration line.
[0092] It should be noted that in this embodiment of the present invention, the salinity solution can be set to 5%, 10%, 15%, 20% and so on; the least squares method is a well-known technique, and the specific process will not be described in detail here.
[0093] Because of the specific physicochemical relationship between salinity and pressure, accurate salinity data is fundamental to establishing a correct pressure calculation model. Therefore, the accuracy of salinity in brine inclusions in shale samples directly affects the accuracy of subsequent paleopressure calculations. Here, by utilizing the Raman shift corresponding to the intensity value in the optimal fitting curve, combined with the aforementioned salinity calibration line, the salinity of brine inclusions in shale samples can be accurately determined.
[0094] Preferably, in one embodiment of the present invention, the method for obtaining the salinity of brine inclusions in shale samples includes:
[0095] In each optimally fitted curve, all peak values are obtained, and the absolute value of the difference in wavenumber between any two adjacent peak values is calculated as the displacement to be measured. The mean of all displacements to be measured is taken as the mean displacement corresponding to each optimally fitted curve.
[0096] Finally, by substituting the mean of the displacements of all the best-fit curves into the salinity calibration line obtained above, the salinity of the brine inclusions in the shale sample can be obtained.
[0097] After obtaining the salinity of the brine inclusions, the paleopressure corresponding to the shale sample can be calculated.
[0098] Preferably, in one embodiment of the present invention, the method for obtaining ancient pressure includes:
[0099] First, the homogenization temperature of brine inclusions in shale samples was obtained: the homogenization temperature of brine inclusions was measured using a microscopic hot and cold stage. The heating rate was 15°C / min when the temperature was less than 80°C, 10°C / min when the temperature was greater than 80°C, and 5°C / min when the homogenization temperature was greater than 120°C. The homogenization temperature of brine inclusions associated with hydrocarbon inclusions in calcite veins was measured.
[0100] Existing techniques utilize microthermometry to analyze synthetic fluid inclusions, proposing that in chemical systems with different solutes, the homogenization temperature is related to the inclusion formation temperature, pressure, and salinity of the trapping fluid, thus summarizing relevant empirical formulas. In this embodiment of the invention, the homogenization temperature and salinity data of brine inclusions in shale samples are combined to obtain the brine inclusion trapping pressure. The brine system in the study area is a NaCl-H₂O brine system. The Zhang-Frantz isochoric equation calculation method is a well-known technique in the field, and the specific mathematical expression is as follows: ; ; ; ; in, This indicates the capture pressure, expressed in MPa. This indicates the capture temperature, in °C, which is 15 °C higher than the homogenization temperature. The homogenization temperature is expressed in °C; m represents the molality of the salt, expressed in mol / kg; and w represents the salinity (%). Represents the intercept coefficient of the isochoric form; The slope coefficient of the isochoric equation; It is a constant;
[0101] Finally, the calculated capture pressure was used as the paleopressure of the calcite vein shale sample.
[0102] It should be noted that the above formulas are all well-known technologies, and the specific methods for obtaining them will not be elaborated here.
[0103] In summary, Raman spectra of brine inclusions in calcite shale samples were first obtained at multiple time points. Since the fluorescence signal of organic matter in shale often masks the true Raman characteristic peaks of brine inclusions, it can cause errors in salinity calculations. However, because the fluorescence intensity of organic matter decreases to some extent with prolonged irradiation time, and after several seconds or even hours of irradiation, the fluorescence may drop to extremely low levels, the fluorescence background signal was determined by analyzing the differences in Raman spectra at different time points and subtracted from the Raman signal to obtain the fluorescence-subtracted signal of brine inclusions in the shale sample. Given the large range of characteristic subbands and complex envelope peaks of brine inclusions, and considering that the peak intensity of the Raman spectrum can reflect the composition of brine inclusions, the intensity value variation and difference characteristics were analyzed in the fluorescence-subtracted signal to determine the fitting interval, i.e., the interval with significant intensity changes that requires focused attention. Then, when performing curve fitting on the fitting interval, the deviation of the intensity values before and after fitting under different fitting conditions was analyzed to select the optimal fitting curve. Thus, the optimal fitting curve can more accurately characterize the characteristic peaks in the fitting interval. Finally, a salinity calibration line is constructed based on the Raman spectral data of known salinity to characterize the relationship between salinity and Raman shift. Then, the accurate salinity of brine inclusions in shale samples can be obtained based on the Raman shift between the intensity values of the optimal fitting curve and the aforementioned salinity calibration line, which can be used to calculate the paleopressure of the brine inclusions.
[0104] It should be noted that the order of the above embodiments of the present invention is merely for descriptive purposes and does not represent the superiority or inferiority of the embodiments. The processes depicted in the accompanying drawings do not necessarily require a specific or sequential order to achieve the desired result. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.
[0105] The various embodiments in this specification are described in a progressive manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.
Claims
1. A paleobaric measurement method combining isotope dating and cluster isotope thermometry, characterized in that the method... include: Calcite vein shale samples were obtained, and Raman spectra of brine inclusions in the shale samples were obtained at different irradiation time points. Based on the differences in Raman spectra at different times, the spectral signal to be measured is obtained; from the spectral signal to be measured, the signal within the preset wavenumber range is extracted as the fluorescence contrast signal; Align the starting positions of the fluorescence contrast signal and the spectrum signal to be measured, and then extract a signal segment of the same length as the fluorescence contrast signal from the spectrum signal to be measured as the signal segment to be measured. The Euclidean distance between the signal segment to be tested and the fluorescence contrast signal is calculated, negative correlation mapping is performed, and normalization is applied to obtain the similarity. If the similarity is greater than the preset similarity threshold, the spectral signal to be tested will be used as the fluorescence background signal corresponding to the shale sample. Otherwise, the mean value of all intensities in the fluorescence contrast signal is obtained as the mean intensity, and the fluorescence contrast signal is smoothly fitted using a quadratic polynomial. The resulting fitted curve is used as the fluorescence background signal corresponding to the brine inclusions in the shale sample. The fluorescence background signal was subtracted from the Raman spectrum to obtain the fluorescence subtracted signal of brine inclusions in the shale sample; In fluorescence subtraction signals, the fitting interval is determined based on the variation and difference characteristics of intensity values at different wavenumbers. When fitting the interval to be fitted, the optimal fitting curve corresponding to each interval is determined based on the deviation characteristics between the intensity values before and after fitting. Salinity calibration lines were obtained from Raman spectral data of multiple known salinity solutions. Based on the Raman shifts between intensity values in the optimal fitting curve and the salinity calibration lines, the salinity of brine inclusions in shale samples was obtained to calculate the paleopressure of the brine inclusions. The method for acquiring the spectral signal to be measured includes: arranging the Raman spectra at all times in time sequence to obtain a spectral sequence; in the spectral sequence, for each two adjacent Raman spectra, calculating the difference between the previous Raman spectrum and the next Raman spectrum to obtain the difference features; arranging all the difference features in time sequence according to the corresponding Raman spectra to obtain a difference sequence; calculating the difference sequence using the inverse difference method, and using the curve obtained by integration as the spectral signal to be measured. The method for obtaining the fitting interval includes: acquiring all extreme points in the fluorescence subtraction signal, and taking the fluorescence subtraction signal between each two adjacent minimum points as a testing interval; within each testing interval, calculating the absolute value of the difference between the intensity values between two minimum points as the intensity difference, calculating the absolute value of the difference between the wavenumbers between two minimum points as the wavenumber difference, and normalizing the ratio of the intensity difference to the wavenumber difference corresponding to each testing interval as the change intensity of each testing interval; and taking the testing interval with a change intensity greater than a preset change threshold as the fitting interval in the fluorescence subtraction signal. The method for obtaining the optimal fitting curve includes: setting the range of values for the mixing coefficients; fitting the data values within each fitting interval based on the Voigt function under each mixing coefficient to obtain the fitted data; calculating the residual between the fitted data and the unfitted data within each fitting interval; using the sum of squares of the residuals corresponding to all fitting intervals as the objective function value under each mixing coefficient; setting the maximum number of iterations; and using the particle swarm optimization (PSO) algorithm to find the optimal value of the objective function within the range of values for the mixing coefficients to obtain the optimal mixing coefficients; and fitting the data values within each fitting interval based on the Voigt function under the optimal mixing coefficients to obtain the optimal fitting curve corresponding to each fitting interval. The method for obtaining the salinity calibration line includes: in the Raman spectral data of each known salinity, all peaks within a preset range are obtained as comparison peaks; the absolute value of the difference in wavenumber between each two adjacent comparison peaks is calculated as the shift value; the mean of the shift values corresponding to all comparison peaks is combined with the known salt concentration as a binary pair; the least squares method is used to perform linear fitting on all binary pairs, and the obtained fitted line is used as the salinity calibration line. The method for obtaining the salinity of brine inclusions in shale samples includes: in each optimal fitting curve, obtaining all peak values, calculating the absolute value of the difference in wavenumber between each two adjacent peak values as the displacement to be measured, and taking the mean of all displacements to be measured as the average displacement value corresponding to each optimal fitting curve; substituting the mean of the average displacement values of all optimal fitting curves into the salinity calibration line to obtain the salinity of brine inclusions in shale samples. Methods for obtaining ancient pressure include: Obtain the homogenization temperature of brine inclusions in shale samples; The capture pressure is calculated based on the Zhang-Frantz isochoric equation. The formula model of the Zhang-Frantz isochoric equation includes: in, Indicates capture pressure; This indicates the capture temperature, which is 15°C higher than the homogenization temperature. The temperature represents the homogenization temperature; m represents the molar concentration of the salt; w represents the salinity. Represents the intercept coefficient of the isochoric form; The slope coefficient of the isochoric equation; It is a constant; The calculated capture pressure was used as the paleopressure of the calcite vein shale sample. The preset wavenumber range is 1800-2800; The preset similarity threshold is 0.85; A calcite vein shale sample was obtained, and the Raman spectra of brine inclusions in the shale sample were acquired at different irradiation time points. The process included: after obtaining the brine inclusions in the shale sample, a single-crystal silicon wafer was used as the absolute Raman wavelength standard to calibrate the Raman spectrometer; the brine inclusions were placed in a hot-cold stage, and a laser with a wavelength of 785 nm was selected to acquire the Raman spectrum of the fluid in the inclusions; simultaneously, the original signal of the brine inclusions was used as the input of a second-order Savitzky-Golay smoothing algorithm, with a window length of 1 / 10 of the entire signal length and a time interval of 1 second, and 10 processed results were continuously extracted and recorded as the Raman spectra of the brine inclusions in the shale sample. The method for obtaining the preset change threshold is as follows: under the same experimental conditions, the known single-peak spectrum is calculated to obtain the change intensity of the known single-peak spectrum signal, and this intensity is used as the change threshold.
2. The paleothermobaric measurement method combining isotope dating and cluster isotope thermometry according to claim 1, characterized in that, Methods for obtaining fluorescence subtraction signals include: In the Raman spectrum and fluorescence background signal at the first time point, the difference between the intensity value in the Raman spectrum at each wavenumber and the intensity value in the fluorescence background signal is used to form the fluorescence subtraction signal of the brine inclusions in the shale sample.
3. The paleobaric measurement method combining isotope dating and cluster isotope thermometry according to claim 1, characterized in that, The absolute temperature of shale samples was determined by carbon-oxygen cluster isotope analysis and used as the paleotemperature of calcite vein shale samples.