Method for measuring oil content of oil-based mud polluted rock core (rock debris)

Abstract

The invention relates to the technical field of oil-gas exploration, in particular to a method for measuring the oil content of an oil-based mud polluted rock core (rock debris). The method comprises the following steps: oil-based mud fluorescence quantification, sample pretreatment, ultrasonic oscillation extraction, extraction liquid fluorescence analysis, wellhead oil solvent fluorescence quantification and oil content calculation. The method comprises the following steps: collecting oil-based mud and wellhead oil on an exploration site, treating the oil-based mud and the wellhead oil with pesticide residue-grade dichloromethane, and determining respective fluorescence fingerprint spectrograms and characteristic peaks through three-dimensional fluorescence detection; the method comprises the following steps: grinding a rock core (rock debris) sample to 60-80 meshes, carrying out ultrasonic extraction to obtain an extract, detecting a spectrogram, identifying and deducting an oil-based mud pollution peak, and calculating the oil content by combining the correlation between the fluorescence intensity and concentration of wellhead oil. The method can effectively distinguish the oil-based mud from the oil content of the sample, realizes accurate detection of the oil content of the polluted sample, makes up for insufficient core data, is suitable for evaluation of conventional, unconventional, deep and ultra-deep oil and gas reservoirs, and provides reliable support for oil and gas exploration and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration, and more particularly to a method for determining the oil content of cores (cuttings) contaminated by oil-based mud. Background Art

[0002] In the field of oil and gas exploration, accurately evaluating the oil content of rock samples is of great significance for oil and gas exploration, sweet spot evaluation, and resource calculation. With the continuous deepening of unconventional oil and gas resource exploration and the advancement of deep and ultra-deep oil and gas resource exploration, the importance of this evaluation has become increasingly prominent. Due to the difficulty of obtaining cores during drilling and the increase in coring costs with increasing depth, low coring rates, missing coring in key layers, and excessively high coring compositions often occur. In these cases, well logging is often relied upon to assess the oil content of the formation. However, this method lacks laboratory data calibration and is mainly based on empirical methods, which leads to high uncertainty. Oil content is a qualitative or semi-quantitative term used to describe whether a rock contains oil and its relative abundance.

[0003] Rock cuttings, as an important alternative to core samples, offer the advantages of easy acquisition and low cost. Oil content analysis can compensate for insufficient core data and improve the accuracy of formation oil content assessments. Furthermore, pressure-maintained coring is a primary method for accurately evaluating reservoir oil characteristics and preventing oil and gas loss. However, both rock cuttings and pressure-maintained coring often use oil-based mud (mostly diesel) to maintain formation pressure during drilling, which inevitably leads to mud contamination of the core samples.

[0004] At present, the main methods for evaluating oil content in rocks include Soxhlet extraction, dry distillation, nuclear magnetic resonance, pyrolysis, fluorescence, etc. Among them: The Soxhlet extraction method uses the principle of "like dissolves like" and uses polar organic solvents to extract core samples to obtain and quantify the oil content. However, since the components of oil-based mud are also mainly hydrocarbon compounds, this method cannot distinguish between the original oil in the sample and the oil-based mud.

[0005] Both the dry distillation method and the pyrolysis method quantitatively analyze the fluid contained in the sample by heating and evaporating it. The dry distillation method directly heats the sample at a high temperature of over 100°C for a long time, recovers the evaporated fluid components through a condensation device, and uses a calibration method to evaluate the fluid content; the pyrolysis method uses program temperature control, heating from 300°C to 600°C at a heating rate of 25°C / min, and uses infrared and hydrogen flame ionization detectors to quantitatively detect the volatile fluids at different temperature stages to determine the oil content. However, these two methods are also unable to separate and evaluate oil-based mud contaminants.

[0006] Nuclear magnetic resonance (NMR) is primarily based on the spin differentiation of hydrogen-containing fluids in a magnetic field, enabling quantitative characterization of fluid content, including kerogen, solid asphalt, oil, and water. However, existing NMR techniques are unable to quantitatively distinguish hydrocarbons in oil-based muds, whose NMR response characteristics are similar to those of oil signals in rock samples.

[0007] The existing fluorescence method for measuring the oil content of core samples uses the principle that hydrocarbon compounds fluoresce and the fluorescence intensity is positively correlated with the oil content. First, a non-fluorescent organic solvent is used to extract the crude oil in the rock sample through ultrasonic vibration to obtain an extract of a certain concentration. At the same time, wellhead oil is collected to prepare standard solutions of different oil concentrations. The oil content of the rock sample is quantified by determining the conversion formula between oil content and fluorescence intensity. Because the method of obtaining crude oil from the rock is similar to the extraction method, and oil-based mud also has fluorescence characteristics, it is also impossible to remove the influence of oil-based mud contaminants.

[0008] In summary, up to now, there has been a lack of effective methods and means for analyzing the oil content of core (cuttings) samples affected by oil-based mud. Existing technologies cannot overcome the influence of oil-based mud contamination to achieve accurate detection and analysis of the oil content of samples. Summary of the Invention

[0009] In view of this, the present invention provides a method for determining the oil content of cores (rock cuttings) contaminated with oil-based mud, aiming to solve the problem that existing rock oil content evaluation and testing methods cannot distinguish the original oil content in the core (rock cuttings) samples from the oil-based mud, and thus cannot accurately quantify the oil content characteristics of the samples contaminated by the oil-based mud, thereby improving the accuracy of oil content detection and analysis of core (rock cuttings) samples contaminated by oil-based mud, pointing out the direction for conventional, unconventional and deep and ultra-deep oil and gas exploration, providing data support for the correct evaluation of reservoir oil content, thereby making up for the shortcomings of core data and improving the reliability of formation oil content assessment.

[0010] In order to achieve the above object, the present invention adopts the following technical solutions: A method for determining the oil content of cores (cuttings) contaminated by oil-based mud, characterized by comprising the following steps: (1) Fluorescence quantification of oil-based mud: The oil-based mud at the exploration site is collected and diluted with an organic solvent. Its three-dimensional fluorescence fingerprint spectrum, maximum excitation wavelength, maximum emission wavelength and typical characteristic peaks are determined by three-dimensional fluorescence detection; (2) Sample pretreatment: Take core or cuttings samples and grind them to a certain particle size; (3) Ultrasonic extraction of samples with organic solvents: Take the pretreated sample, add organic solvent and perform ultrasonic extraction, and take the supernatant as the extraction liquid after standing; (4) Fluorescence quantitative analysis of sample extracts: taking the extracts, diluting them with an organic solvent, and obtaining their three-dimensional fluorescence spectra by three-dimensional fluorescence detection; (5) Fluorescence quantification of wellhead oil solvents at different concentrations: Take wellhead oil and prepare wellhead oil solvents at different concentrations with organic solvents. Determine the fluorescence intensity of wellhead oil solvents at different concentrations through three-dimensional fluorescence detection, and establish the correlation between fluorescence intensity and wellhead oil concentration. (6) Calculation of oil content: Based on the three-dimensional fluorescence characteristics of the oil-based mud determined in step (1) and the fluorescence characteristics of the wellhead oil determined in step (5), the characteristic peaks of oil-based mud contamination are identified and subtracted from the three-dimensional fluorescence spectrum of the sample extract obtained in step (4), and the oil content of the sample is calculated based on the correlation established in step (5).

[0011] In a specific embodiment, in step (1), the organic solvent is pesticide-residue grade dichloromethane.

[0012] In a specific embodiment, in step (2), the particle size of the sample after grinding is 60-80 mesh.

[0013] In a specific embodiment, in step (3), the specific operation of the ultrasonic vibration extraction includes: taking about 1g of the pretreated sample, adding 20ml of organic solvent, shaking 3 times with an interval of 20min, and then letting it stand for 1 hour.

[0014] In a specific embodiment, in step (3), during the ultrasonic vibration and standing process, the mixture is covered with tin foil and refrigerated to prevent the solvent from volatilizing.

[0015] In a specific embodiment, in step (4), the specific operation of the dilution includes: taking a certain volume of the extract and placing it in a cuvette, adding 2 ml of organic solvent to dilute it; if the fluorescence intensity exceeds the detection limit of the instrument, continue to dilute it with organic solvent and record the dilution multiple.

[0016] In a specific embodiment, in step (5), the organic solvent is pesticide-residue grade dichloromethane.

[0017] In a specific embodiment, in step (5), establishing the correlation between fluorescence intensity and wellhead oil concentration includes: determining the fluorescence intensity of wellhead oil solvents with different concentrations through three-dimensional fluorescence detection to obtain a linear relationship between fluorescence intensity and wellhead oil concentration.

[0018] In a specific possible implementation scheme, in step (6), the identifying and subtracting characteristic peaks of oil-based mud pollution include: comparing the maximum excitation light wavelength, maximum emission light wavelength and typical characteristic peaks of the oil-based mud and the wellhead oil, determining the position of the characteristic peak of oil-based mud pollution in the three-dimensional fluorescence spectrum of the sample extract, and reading the maximum fluorescence intensity of the oil peak outside the characteristic peak.

[0019] In a specific embodiment, in step (6), the calculation of the oil content of the sample includes: combining the dilution factor in step (4) and the correlation established in step (5), and converting the read maximum fluorescence intensity of the oil peak into the oil content of the sample.

[0020] Compared with the prior art, the method for determining the oil content of cores (cuttings) contaminated by oil-based mud disclosed in the present invention is used for accurately detecting and analyzing the oil content of core (cuttings) samples contaminated by oil-based mud. The method constructs a technical process based on three-dimensional fluorescence quantitative analysis, including the steps of fluorescence quantification of oil-based mud, sample pretreatment, ultrasonic oscillation extraction of the sample with an organic solvent, fluorescence quantitative analysis of the sample extract, fluorescence quantification of solvents with different wellhead oil concentrations, and oil content calculation. Taking into account the characteristic that oil-based mud and wellhead oil have different three-dimensional fluorescence fingerprint spectra and characteristic peaks, the method first determines the three-dimensional fluorescence fingerprint spectra and typical characteristic peaks of the oil-based mud and wellhead oil, then identifies and subtracts the characteristic peaks of oil-based mud contamination from the three-dimensional fluorescence spectrum of the sample extract, and calculates the oil content of the sample with reference to the linear relationship between wellhead oil concentration and fluorescence intensity. This allows for accurate evaluation of the oil content of samples contaminated by oil-based mud, effectively improving the accuracy of oil content detection in core (cuttings) samples contaminated by oil-based mud, and has the following beneficial effects: Compared with traditional oil content evaluation methods, this method takes into account the fluorescence characteristics of mud contamination solvents and crude oil, and can accurately determine the oil content of mud contaminated samples, providing new methods and new ideas for determining formation oil content evaluation parameters; by accurately evaluating the oil content of widely existing rock cuttings samples, it can realize predictive analysis of oil content in the absence of conventional core samples, thereby providing data support for conventional, unconventional and deep and ultra-deep oil and gas exploration.

[0021] Compared with traditional methods such as Soxhlet extraction, dry distillation, pyrolysis and existing fluorescence methods that cannot distinguish the oil content of oil-based mud and samples, this method achieves the subtraction of contamination effects by identifying the different fluorescence characteristics of the two, providing an effective means for oil content analysis of mud-contaminated samples and opening up a new path for the determination of formation oil content evaluation parameters; at the same time, with the help of accurate analysis of easily obtained rock cuttings samples, it can make up for the shortcomings of core data and achieve reliable prediction of oil content in the absence of conventional core samples, providing accurate data support for reservoir oil content evaluation in conventional, unconventional and deep and ultra-deep oil and gas exploration, and helping to improve the accuracy of formation oil content assessment. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0023] Figure 1 The figure is an overall flow chart of a method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to the present invention.

[0024] Figure 2 The three-dimensional fluorescence spectrum of oil-based mud. The three-dimensional fluorescence fingerprint of oil-based mud in the example is used to judge and identify the oil-based mud signal in the three-dimensional fluorescence spectrum of the contaminated sample based on its characteristic peak distribution position and fluorescence characteristic parameters.

[0025] Figure 3 3D fluorescence spectrum of mud-contaminated sample. 3D fluorescence fingerprint of mud-contaminated sample in this example, including oil-based mud signal and oil signal.

[0026] Figure 4 The conversion relationship between wellhead oil concentration and fluorescence intensity is shown in Figure 2. The positive correlation formula between the wellhead oil concentration and fluorescence intensity of the shale sample in the example is used as the calculation formula for fluorescence intensity and oil content, and as the calculation formula for oil content of the shale sample.

[0027] Figure 5 is the three-dimensional fluorescence spectrum of the wellhead oil sample (concentration 1.1335 g / ml). is the three-dimensional fluorescence fingerprint of the shale sample corresponding to the wellhead oil sample in the example, and Figure 2 The most contrasting, used to distinguish Figure 3 Oil signal and mud signal of the sample. DETAILED DESCRIPTION

[0028] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0029] The present invention provides a method for determining the oil content of cores (cuttings) contaminated by oil-based mud. The method is used to accurately detect and analyze the oil content of core (cuttings) samples contaminated by oil-based mud. The core technical principle is based on the fact that oil-based mud and crude oil (represented by wellhead oil) have different three-dimensional fluorescence fingerprint spectra and characteristic peaks. The three-dimensional fluorescence quantitative analysis technology is used to distinguish between the two, and then the influence of oil-based mud contamination is eliminated to accurately quantify the oil content of the sample.

[0030] like Figure 1 As shown, the present invention provides a method for determining the oil content of a core (cuttings) contaminated by oil-based mud, wherein the core (cuttings) is a rock sample. The method is used to analyze the oil content of the rock sample and includes the following steps: Fluorescence quantification of oil-based mud: First, the oil-based mud collected at the exploration site is subjected to fluorescence quantitative analysis. After dilution with pesticide-grade dichloromethane, three-dimensional fluorescence detection is performed to determine its three-dimensional fluorescence fingerprint spectrum, maximum fluorescence wavelength, maximum emission wavelength, and typical characteristic peaks, providing a basis for subsequent identification of contamination peaks. Sample pretreatment and ultrasonic extraction of samples with organic solvents: Secondly, core (cuttings) samples were pretreated and ground to 60-80 mesh particles. Quantitative samples were extracted with pesticide-residue-grade dichloromethane by ultrasonic vibration. After standing, the supernatant was obtained as the extract. During the shaking and standing process, the samples were covered with tin foil and refrigerated to prevent solvent volatilization. Fluorescence quantitative analysis of sample extract: Afterwards, the sample extract was subjected to fluorescence quantitative analysis. Quantitative extracts were taken by pipetting, diluted with pesticide-grade dichloromethane until the fluorescence intensity was within the instrument detection limit, and its three-dimensional fluorescence spectrum was obtained using a fluorescence spectrophotometer. For specific test methods, see Liu Keyu, Lu Xuesong, Gui Lili, et al. Reservoir Quantitative Fluorescence Technology and Its Application in Oil and Gas Accumulation Research [J]. Earth Science, 2016, 41(03): 373-384. This document records the setting of test parameters and the parameter analysis method, which will not be described in detail in this invention. Fluorescence quantification of wellhead oil solvents at different concentrations: At the same time, fluorescence quantitative analysis of wellhead oil solvents at different concentrations was performed. Wellhead oil was quantitatively sampled and prepared with pesticide-grade dichloromethane to prepare solutions of different concentrations. The linear correlation between fluorescence intensity and concentration, as well as the three-dimensional fluorescence fingerprint spectrum and characteristic peak parameters were determined through three-dimensional fluorescence analysis to establish a conversion relationship between oil content and fluorescence intensity. Oil content calculation: Finally, in the oil content calculation step, the three-dimensional fluorescence fingerprint spectra and characteristic peak parameters of the oil-based mud and wellhead oil are compared to determine the position of the characteristic peak of oil-based mud contamination on the three-dimensional fluorescence spectrum of the sample extract. The maximum fluorescence intensity of the oil peak outside the characteristic peak is read. Combined with the dilution factor and the conversion relationship between the fluorescence intensity and concentration of the wellhead oil, the oil content of the sample after removing the oil-based mud contamination is calculated.

[0031] Through the above technical means, the present invention can effectively distinguish between the original oil content and oil-based mud contamination in the sample, and realize accurate detection and analysis of the oil content of core (cuttings) samples contaminated by oil-based mud.

[0032] Exemplarily, the method of the present invention comprises the following steps: Fluorescence quantification of oil-based mud: collect oil-based mud from the exploration site, dilute it with pesticide-grade dichloromethane, and use three-dimensional fluorescence detection to determine the three-dimensional fluorescence fingerprint of the oil-based mud, determine the maximum fluorescence wavelength and the maximum luminescence wavelength, and determine its typical characteristic peaks; see Figure 2 As shown, its maximum fluorescence intensity peak has a maximum excitation wavelength of 234 nm and a maximum emission wavelength of 344 nm; Sample pretreatment: take about 10g of core sample, grind it into 60-80 mesh particles, and sample 5 times; Ultrasonic extraction and fluorescence quantification of the sample with organic solvent: Place about 1 g of pretreated 60-80 mesh sample particles in a beaker and record the mass m1. Ultrasonic extraction is performed using 20 ml of pesticide-grade dichloromethane, with three shakes at 20-minute intervals. Finally, after standing for 1 hour, the supernatant is taken as the extraction solution. During the shaking and standing process, the beaker is covered with tin foil and refrigerated to prevent solvent evaporation. Fluorescence quantification of sample extract: pipette a volume V1 of sample extract into a cuvette, add 2ml of pesticide-grade dichloromethane for dilution, and perform three-dimensional fluorescence analysis using a fluorescence spectrophotometer. If the fluorescence intensity exceeds the instrument detection limit, continue diluting with dichloromethane, record the dilution factor (w), and then perform fluorescence analysis to obtain a three-dimensional fluorescence spectrum of the extract to obtain the fluorescence intensity; see Figure 3 As shown, its maximum fluorescence intensity peak has a maximum excitation wavelength of 2258 nm and a maximum emission wavelength of 375 nm; Fluorescence quantification of wellhead oil solvents with different concentrations: quantitatively take wellhead oil of different masses, use 20ml of pesticide-grade dichloromethane to prepare wellhead oil solvents with concentrations of W1, W2, W3 and W4, and use fluorescence spectrophotometer to perform three-dimensional fluorescence analysis of wellhead oil solvents with different concentrations. First, compare the wellhead oil ( Figure 4 ) and oil-based mud ( Figure 2 ) 3D fluorescence test results, determine the maximum fluorescence wavelength and the maximum luminescence wavelength, and determine their typical characteristic peak distribution (Table 1); Table 1 Comparison of characteristic peak parameters of oil-based mud and wellhead oil

[0033] Secondly, the linear conversion relationship and calculation formula between fluorescence intensity Q1 and wellhead oil concentration were determined (Q1=A×W, Q1 is fluorescence intensity, unit pc, A is coefficient, W is wellhead oil solvent, unit g / ml; the conversion relationship between oil content and fluorescence intensity in this area is y=91178×W; see Figure 5 ) (see Table 2), and at the same time, the oil content is calculated. After obtaining the three-dimensional fluorescence spectrum of the mud pollution extractant, the three-dimensional fluorescence fingerprint spectra and the maximum fluorescence wavelength and the maximum luminescence wavelength of the oil-based mud and wellhead oil are compared (see Table 1). The difference between the mud characteristic peak position and the oil peak position on the three-dimensional fluorescence spectrum of the contaminated sample is determined, and the maximum fluorescence intensity (Q2) of the oil peak other than the oil-based mud pollution characteristic peak (wellhead oil characteristic peak position, excitation light wavelength 234nm, wavelength 375nm) is read. With reference to the dilution multiple, the sample fluorescence intensity Q is calculated (the fluorescence intensity of each gram of sample extracted with 20ml of pesticide-grade dichloromethane). Combined with the relationship between the wellhead oil solvent and the fluorescence intensity, the oil content W1 per gram of sample is calculated.

[0034] Table 2 Test results of different wellhead oil solution concentrations and fluorescence intensities

[0035] The calculation formula for the fluorescence intensity Q of the sample solution is: Q=Q2×w / M1 The calculation formula for the oil content of the sample after removing mud pollution is: W1=Q / A, see Table 3 for the results.

[0036] Table 3 Calculation results of oil content correction for oil-based mud contaminated samples

[0037] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same and similar parts between the various embodiments can be referred to each other. The above description of the disclosed embodiments enables professionals and technicians in this field to implement or use the present invention. Various modifications to these embodiments will be apparent to professionals and technicians in this field, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for determining the oil content of cores (cuttings) contaminated by oil-based mud, characterized in that: The following steps are involved: (1) Fluorescence quantification of oil-based mud: The oil-based mud at the exploration site is collected and diluted with an organic solvent. Its three-dimensional fluorescence fingerprint spectrum, maximum excitation wavelength, maximum emission wavelength and typical characteristic peaks are determined by three-dimensional fluorescence detection; (2) Sample pretreatment: Take core or cuttings samples and grind them to a certain particle size; (3) Ultrasonic extraction of samples with organic solvents: Take the pretreated sample, add organic solvent and perform ultrasonic extraction, and take the supernatant as the extraction liquid after standing; (4) Fluorescence quantitative analysis of sample extracts: taking the extracts, diluting them with an organic solvent, and obtaining their three-dimensional fluorescence spectra by three-dimensional fluorescence detection; (5) Fluorescence quantification of wellhead oil solvents at different concentrations: Take wellhead oil and prepare wellhead oil solvents at different concentrations with organic solvents. Determine the fluorescence intensity of wellhead oil solvents at different concentrations through three-dimensional fluorescence detection, and establish the correlation between fluorescence intensity and wellhead oil concentration. (6) Calculation of oil content: Based on the three-dimensional fluorescence characteristics of the oil-based mud determined in step (1) and the fluorescence characteristics of the wellhead oil determined in step (5), the characteristic peaks of oil-based mud contamination are identified and subtracted from the three-dimensional fluorescence spectrum of the sample extract obtained in step (4), and the oil content of the sample is calculated based on the correlation established in step (5).

2. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (1), the organic solvent is pesticide-residue grade dichloromethane.

3. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (2), the particle size of the sample after grinding is 60-80 mesh.

4. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (3), the specific operation of the ultrasonic vibration extraction includes: taking about 1g of the pretreated sample, adding 20ml of organic solvent, shaking 3 times at intervals of 20min, and then letting it stand for 1 hour.

5. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (3), during the ultrasonic vibration and standing process, the solution is covered with tin foil and refrigerated to prevent the solvent from volatilizing.

6. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (4), the specific operation of the dilution includes: taking a certain volume of the extract and placing it in a cuvette, adding 2 ml of organic solvent to dilute it; if the fluorescence intensity exceeds the detection limit of the instrument, continue to dilute it with organic solvent and record the dilution multiple.

7. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (5), the organic solvent is pesticide-residue grade dichloromethane.

8. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (5), establishing the correlation between fluorescence intensity and wellhead oil concentration includes: determining the fluorescence intensity of wellhead oil solvents with different concentrations through three-dimensional fluorescence detection, and obtaining a linear relationship between fluorescence intensity and wellhead oil concentration.

9. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (6), the identification and subtraction of the characteristic peak of oil-based mud pollution include: comparing the maximum excitation light wavelength, maximum emission light wavelength and typical characteristic peak of the oil-based mud and the wellhead oil, determining the position of the characteristic peak of oil-based mud pollution in the three-dimensional fluorescence spectrum of the sample extract, and reading the maximum fluorescence intensity of the oil peak outside the characteristic peak.

10. The method for determining the oil content of cores (cuttings) contaminated by oil-based mud according to claim 1, characterized in that: In step (6), the calculation of the oil content of the sample includes: combining the dilution factor in step (4) and the correlation established in step (5), and converting the read maximum fluorescence intensity of the oil peak into the oil content of the sample.

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