Calculation method for chlorite coating coverage rate

Abstract

A chlorite coating coverage rate calculation method relates to the technical field of geological exploration and comprises the following steps: collecting a typical rock core sample of a clastic rock stratum in a research area, observing sedimentary characteristics, judging sedimentary microfacies, testing physical properties of the rock core sample and chlorite content data, and preparing a sample slice for high-precision observation. Identifying and shooting the particles with chlorite coatings, calculating the coating coverage rates of the sample sheet particles by adopting a cross proportion method, a perimeter proportion method and an angle proportion method respectively, and then taking the mean value of the coating coverage rates of the three methods as the total coating coverage rate of the sample particles; carrying out linear coupling on the calculated total coating coverage rate of the sample particles and the chlorite content, and establishing a relationship between the chlorite coating coverage rate and the chlorite content; according to the method, under the condition that comprehensive actual geological conditions of the clastic rock reservoir containing the chlorite coating are fully considered, the content calculation method of the chlorite coating in the clastic rock reservoir is emphatically solved, and the coverage degree of the chlorite coating can be accurately judged.

Description

Technical Field

[0001] The invention relates to the technical field of geological exploration, and in particular to a method for calculating the coverage rate of chlorite coating. Background Art

[0002] As a common clay mineral, chlorite is widely distributed in reservoirs such as sedimentary rocks and volcanic rocks. It has important application value in the fields of oil exploration, underground reservoir research, and environmental geology. The presence of chlorite has a great influence on the physical properties of the reservoir, and has an important influence on the physical parameters such as the porosity, permeability, and pore structure of the reservoir. Its different existence modes have different effects on the physical properties of the reservoir. In clastic rocks, chlorite can exist in different forms, including chlorite shells, pore-filling chlorite, clastic components, minerals, and metasomatic products. Among them, chlorite shells, that is, chlorite exists on the surface of clastic particles in the form of inclusions. These chlorite inclusions form the outer envelope of the particles and cover the surface of the clastic particles. Encapsulated chlorite can affect the porosity, permeability, and physical stability of the particles. Therefore, studying and understanding the degree of development of chlorite envelopes is of great significance for the evaluation and development of clastic reservoirs.

[0003] Studies have shown that the presence of chlorite coatings will lead to a decrease in porosity, permeability and pore throat radius, as well as an increase in the unevenness of porosity distribution; but relatively speaking, the inhibitory effect of chlorite coatings on quartz cementation can also provide support for the pore wall to a certain extent, affecting the physical stability of the reservoir and achieving the effect of preserving pores. The effectiveness of this inhibitory effect depends more on the coverage of the chlorite coating, rather than just the chlorite content. In other words, the key to discussing the control effect of clay coatings in sandstone on diagenetic events and reservoir quality is how to accurately quantify and characterize the coverage of clay coatings. Therefore, knowing the coverage of chlorite coatings and coupling it with the physical properties of the reservoir, such as pores and permeability, can quickly and effectively obtain the correlation between reservoir physical properties and chlorite coating coverage, providing research materials for clarifying the influence of chlorite coatings on the densification process of sandstone, which has important scientific and application value.

[0004] At present, the main quantitative analysis method for rock mineral composition is X-ray diffraction analysis, but X-ray diffraction analysis can only test the total relative content of chlorite minerals, and cannot distinguish the content of chlorite in different forms, such as the relative content of chlorite coatings covering the surface of particles, and cannot provide a basis for judging the correlation between the coverage of chlorite coatings and reservoir quality. Therefore, a quick method for calculating the coverage of chlorite coatings is needed to explore the relationship between the coverage of coatings and the physical properties of sandstone reservoirs. Summary of the invention

[0005] In view of this, the present invention proposes a method for calculating the coverage of chlorite coating, which can directly observe, record and calculate the relative content of chlorite coating, and combine thin section observation, calculation results and X-ray diffraction analysis data to provide a data basis for interpreting its correlation with reservoir physical properties.

[0006] In order to solve at least one of the above technical problems, the technical solution provided by the present invention is a method for calculating the coverage rate of chlorite coating, comprising the following steps: Step S1: Collect typical core samples of clastic rock layers in the study area, observe sedimentary characteristics, and determine sedimentary microfacies; Step S2: testing the core samples to obtain physical properties and chlorite content data, and preparing sample slices; Step S3: Observe the sample slices with high precision, identify and photograph the particles with chlorite coating; Step S4: using the cross ratio method, the perimeter ratio method and the angle ratio method to calculate the coating coverage of the sample thin slice particles respectively, and then taking the average of the coating coverage of the particles by the three methods as the total coating coverage of the sample particles; Step S5: Linearly couple the calculated total coating coverage of the sample particles with the chlorite content to establish a relationship between the chlorite coating coverage and the chlorite content.

[0007] The technical effects of the present invention are: (1) The present invention fully considers the comprehensive actual geological conditions of clastic reservoirs containing chlorite coatings, focuses on solving the calculation method of the content of chlorite coatings in clastic reservoirs, and fits it with the XRD test results to obtain a function model of the special existence form of chlorite coatings, which can accurately determine the coverage degree of chlorite coatings.

[0008] (2) The present invention statistically analyzes the chlorite coating content and establishes the law of reservoir property changes under different chlorite coating contents, thereby providing a quantitative data basis for judging reservoir properties under this geological background and providing research materials for studying the influence of chlorite coating on the sandstone densification process. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0010] Figure 1 It is the overall flow chart of the present invention; Figure 2This is a porosity and permeability cross-plot of clastic reservoirs in a certain area of ​​the Sichuan Basin in the present invention; Figure 3 This is a sample analysis diagram of the chlorite shell development characteristics in a certain area of ​​the Sichuan Basin in the present invention; Figure 4 Figure a is a schematic diagram of calculation of the cross ratio method in the present invention; Figure 4 b is a schematic diagram of calculation of the perimeter ratio method in the present invention; Figure 4 Figure c is a schematic diagram of calculation of the angle ratio method in the present invention; Figure 5 It is a graph showing the intersection of chlorite content and chlorite coating data in a certain area of ​​Sichuan Basin in an embodiment of the present invention; Figure 6 a is a graph showing the relationship between the chlorite coating coverage and the reservoir porosity in an embodiment of the present invention; Figure 6 b is a graph showing the relationship between the chlorite coating coverage and the reservoir permeability in an embodiment of the present invention; Figure 6 c is a graph showing the relationship between chlorite content and reservoir porosity in an embodiment of the present invention; Figure 6 Figure d is a graph showing the relationship between chlorite content and reservoir permeability in an embodiment of the present invention. DETAILED DESCRIPTION

[0011] The present invention will be further described in detail below in conjunction with the embodiments and drawings.

[0012] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are 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 creative work are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention.

[0013] A method for calculating the coverage rate of chlorite coating comprises the following steps: Step S1: Collect typical core samples of clastic rock layers in the study area, observe sedimentary characteristics such as grain size, color, and structural structure, and determine sedimentary microfacies.

[0014] Step S2: Conduct tests on core samples to obtain physical property and chlorite content data, and prepare sample slices, including physical property tests consisting of porosity tests and permeability tests and XRD tests.

[0015] Step S3: Use an optical microscope or a scanning electron microscope to observe the sample slice with high precision, identify and photograph the particles with chlorite coating.

[0016] Under a polarizing microscope, chlorite forms a thin film perpendicular to the particle surface and covers the outer edge of the particle; under a scanning electron microscope, leaf-shaped chlorite is generally seen attached to the particle surface. The field of view should be based on the principle of being able to clearly see the particle morphology. Using a Zeiss Axioscope A1 APOL. digital transflective polarizing microscope, a typical particle can be selected with a 10x or 20x lens and placed in the center of the field of view to take a single particle image.

[0017] Step S4: Calculate the coating coverage of the particles of the sample sheet using the cross ratio method, perimeter ratio method and angle ratio method respectively, and then take the average of the particle coating coverage of the three methods as the total particle coating coverage of the sample.

[0018] Among them, the specific steps of the cross ratio method are: A certain number of particles within the field of view are selected, and rectangular coordinate axes are set at their center points respectively. The extension lines of the rectangular coordinate system 0°, 90°, 180°, and 270° are made to intersect with the particle boundaries. The development of chlorite coating, pores, cementation and filling materials, and particle contact at the intersections are observed respectively. The points with chlorite coating are scored as 1 point, and the remaining points are scored as 0 points. The percentage of the average score is taken as the single-particle coating coverage rate, and the mean particle coating coverage rate within the field of view is calculated. It is worth noting that in order to avoid the statistical weight of the uncovered part being too high and increasing the error, the uncovered part is defined as an invalid segment, and particles with invalid segments exceeding 2 / 3 will not be included in the statistical range.

[0019] The specific steps of the perimeter ratio method are: Select the same particles as those in the cross ratio method, observe the clay coating coverage at the edge of the particles, define the part with chlorite coating as the coverage section, and calculate the single particle coating coverage rate and the particle coating coverage rate within the field of view, as shown in formulas (1) and (2): Single particle coating coverage = 100% (1) Particle coating coverage within the field of view = 100% (2) In formula (2), i is the data of a single particle.

[0020] The specific steps of the angle ratio method are: Select the same particles as in the circumference ratio method, observe the clay coating coverage at the edge of the particles, define the part with chlorite coating as the coverage segment, count the angles between the starting point to the end point of all coverage segments in the particles and the center point of the particles, calculate the proportion of the sum of the angles between the starting point to the end point of all coverage segments and the center point of the particles in the circumference, and get the single particle coating coverage rate. On this basis, calculate the particle coating coverage rate within the field of view.

[0021] To ensure the consistency and reliability of statistical data, the number of particles selected within the field of view should be no less than 20.

[0022] Step S5: Linearly couple the calculated total coating coverage of the sample particles with the chlorite content to establish a relationship between the chlorite coating coverage and the chlorite content.

[0023] The model specifically uses the chlorite content of the research object in the study area as the horizontal coordinate and the capsule coverage rate as the vertical coordinate. Data processing software is used to make data intersection diagrams, establish mathematical models, and determine relevant relationships.

[0024] On this basis, data processing software was used to draw correlation diagrams between the chlorite coating coverage and reservoir porosity, and between the chlorite coating coverage and reservoir permeability, to determine their correlations and clarify the relationship between the chlorite coating coverage and reservoir physical property changes in the study area, providing research materials for studying the influence of chlorite coating on the sandstone densification process.

[0025] Example 1: The specific technical solution of the present invention is illustrated by taking the clastic rock reservoir in a certain area of ​​the Sichuan Basin as an example.

[0026] Step 1: First, select typical drilling cores of clastic reservoirs in the study area, observe their sedimentary characteristics such as grain size, color, and structural structure, and determine their sedimentary microfacies: The study area has gray fine sandstone intercalated with black carbonaceous layers, gray-black and black mudstone, parallel bedding, trough cross-bedding, and sand-pattern cross-bedding; positive grain sequence is dominant, which is a multi-stage river channel superposition, and occasionally reverse grain sequence is seen, which is a river mouth bar, far sand bar, sheet sand, etc. Based on the laws of sedimentology, a preliminary judgment is made on its physical property changes.

[0027] Step 2: Drill core plug samples (2.5 cm in diameter, 5 cm in length), grind thin slices, and perform physical property tests and XRD tests: The sample preparation and testing process includes: slicing the corresponding samples of the clastic reservoir in the study area (25mm in diameter, 2~3.5mm in thickness); using a casting instrument to vacuum and pressurize the rock samples; using a grinder to coarsely grind, medium grind, fine grind, and fine grind the sample surface; using solid fir glue to glue the central part of the slide with one side ground into a rough surface to the finely ground rock sample, and gently squeeze the slide with tweezers until the glue layer is thin and uniform without bubbles; coarsely grind the glued rock sample on the grinder to 0.28~0.40mm, finely grind to 0.12~0.18mm, and finely grind to 0.04~0.05mm; drip an appropriate amount of fir glue on the casting slice, heat it slightly, put the cover slip on it, and gently squeeze it with tweezers to remove bubbles.

[0028] To determine the porosity, a plunger core porosimeter was used to test the porosity of the selected core samples at room temperature and pressure. The test process includes: measuring the length and diameter of the prepared core sample with a vernier caliper to obtain the total volume of the rock; adjusting the pressure regulator to read the pore volume under the corresponding gas input pressure, and comparing the standard curve to obtain the solid phase volume of the rock sample. The rock porosity is quantitatively calculated according to Boyle's law as shown in formula (3), where the difference between the total rock volume and the particle volume is the porosity volume.

[0029] (3) In formula (3): V b is the total volume of rock; V g is the particle volume; V p is the porosity volume; φ is the porosity.

[0030] In order to determine the permeability, the selected core samples were measured using the STY-Ⅲ gas permeability meter. The test process includes: measuring the length and diameter of the prepared core sample with a vernier caliper and calculating the cross-sectional area; allowing a gas of known viscosity (such as helium) to flow through a rock sample of known size to measure the pressure difference and flow rate; the inlet and outlet pressures and outlet pressures are measured using a manometer, and the gas flow rate is measured by a calibrated exhaust hole, thereby calculating the gas permeability of the rock sample, as shown in formula (4). Through experiments, the values ​​of reservoir porosity and permeability at normal temperature and pressure are obtained, and the Origin drawing software is used to draw a porosity and permeability intersection comparison chart of the clastic reservoir in a certain area of ​​the Sichuan Basin with porosity as the horizontal coordinate and permeability as the vertical coordinate, as shown in the figure below. Figure 2 shown.

[0031] (4) In formula (4), K is the gas permeability of the rock sample, μm 2 ; A is the cross-sectional area of ​​the rock sample, cm 2; L is the length of the rock sample, cm; μ is the gas viscosity, mPa·S; Q is the gas flow rate at absolute atmospheric pressure, cm 3 / s; P0 is atmospheric pressure, MPa; P1 is the inlet pressure of the rock sample, MPa; P2 is the outlet pressure of the rock sample, MPa.

[0032] Since the crystals of each mineral have a specific X-ray diffraction spectrum, the intensity of the characteristic peak in the spectrum is positively correlated with the content of the mineral in the sample. Therefore, the positive correlation between the content of a certain mineral and the intensity of its characteristic diffraction peak - K value can be determined by experimental methods, and then the content of the mineral can be calculated by measuring the intensity of the characteristic peak of the mineral in the unknown sample. This is the "K value method" in X-ray diffraction quantitative analysis. The content of common non-clay minerals and the total amount of clay minerals in sedimentary rocks can be determined by the K value method. The relative content of each clay mineral type in clay minerals is determined by the diffraction peak area difference subtraction method of natural slices, ethylene glycol saturated slices and high-temperature slices. Under the conditions of 40kV in-tube voltage and 150mA in-tube current, X'Pert PRO MPD X-ray diffractometer was used for XRD clay quantitative analysis. The detection standard was SY / T 5163-2010. X-ray clay analysis data of sandstone reservoir minerals in a certain area of ​​Sichuan Basin were obtained, and 5 groups of samples with better chlorite development were selected to prepare thin sections for subsequent testing.

[0033] Step 3: Use tools such as optical microscopy or scanning electron microscopy (SEM) to observe the sample with high precision and identify and photograph the particles with chlorite coating.

[0034] According to the identification guide for clastic rock thin sections, under a polarizing microscope, chlorite can be seen to form a thin film covering the outer edge of the particles in a manner perpendicular to the surface of the particles. Figure 3 As shown ( Figure 3 (a)-(c) in the figure), and the blade-like chlorite attached to the particle surface can be clearly seen under the scanning electron microscope ( Figure 3 A Zeiss AxioscopeA1 APOL. digital polarized light microscope was used to select typical particles at a 10x or 20x magnification and place them in the center of the field of view to take single particle images. Figure 5 shown.

[0035] Step 4: Process the observed images obtained and calculate the chlorite coating content of the particles using statistical methods; The image processing was performed using the "cross ratio method", "perimeter ratio method" and "angle ratio method", and the average chlorite coating coverage obtained by the three methods was calculated. In the statistical process, all particles with chlorite coatings on the edges can be used as statistical objects, regardless of the size of the coating, while particles without coatings are not included in the statistical range.

[0036] The specific operation of the cross ratio method is as follows: select the center point of the particle, place the rectangular coordinate axis, and intersect the particle boundary at 0°, 90°, 180°, and 270°. Record the development of clay coating, pores, cementing materials and filling materials, and particle contact at the intersection, and record the clay coating point as 1 point and other points as 0 points. Figure 4 In a, the percentage of the average score is the single particle coating coverage (0-100%), and the average of all particle coating coverages in the field of view is the average coating coverage of each sample.

[0037] Perimeter ratio method: The clay coating coverage of the particle edge is divided into covered section, uncovered section (particles are in contact with pores or siliceous cementation is developed at the particle edge), and compacted section (particles are in contact with particles or other filling materials), and marked with yellow, red, and blue dotted lines respectively, see Figure 4 Then, by measuring the length of each line segment, the average coating coverage of the single particle and the sample is calculated using formula (1) and formula (2): Single particle coating coverage = 100% (1) Particle coating coverage within the field of view = 100% (2) In formula (2), i is the data of a single particle.

[0038] (3) Angle ratio method. Similar to the former, the difference is that the angle is used to calculate the ratio of the clay coating covered section to the uncovered section ( Figure 4 c) Select the center point of the particle and use the 360° scale of the rock microscope circular rotating stage to count the covered section, uncovered section (particles are in contact with pores or siliceous cementation develops at the edges of particles), and compacted section (particles are in contact with particles or other filling materials) of the particles, and mark them with yellow, red, and blue dotted lines, respectively. The angle of the yellow section and the ratio of 360° are the coating coverage of a single particle.

[0039] The above method requires selecting at least 20 particles in the field of view under the microscope to ensure the consistency and reliability of the statistical data.

[0040] The rock sample at a depth of 5187.93m from Well 2 in a certain area of ​​the Sichuan Basin was taken as sample group 1, and 20 typical particles were selected for cross ratio method processing, and were processed by perimeter ratio method and angle ratio method respectively. After image processing and statistical analysis, the statistical results of the coating coverage of the three methods were obtained as shown in Table 1-3. In order to ensure the reliability of the data, the average value of the coverage obtained by the three methods was taken to obtain the chlorite coating coverage of Well 2-5187.93m, and then the same method was used to process the remaining four selected samples.

[0041] Table 1 Statistics of chlorite coating coverage of Well 2 (5187.93 m) in a certain area of ​​Sichuan Basin (cross ratio method)

[0042] Table 2 Statistics of chlorite coating coverage of Well 2 (5187.93 m) in a certain area of ​​Sichuan Basin (perimeter ratio method)

[0043] Table 3 Statistics of chlorite coating coverage of Well 2 (5187.93 m) in a certain area of ​​Sichuan Basin (angle ratio method, unit: °)

[0044] Step 5: Linearly couple the calculated chlorite coating content with the chlorite content tested by XRD, fit the correlation function between the two, and establish a mathematical model; Taking the chlorite content of the five groups of research objects in the study area as the horizontal axis and the coating coverage rate as the vertical axis, we can get the following: Figure 5 The data cross-plot shown is logarithmically related. R 2 =0.3379. It can be seen that in this depth section of the study area, the increase in chlorite content and the chlorite coating coverage have a certain increasing trend.

[0045] On this basis, the chlorite coating coverage data of a study area in the Sichuan Basin, together with the porosity, permeability and XRD clay analysis data, are summarized into Table 4, as shown below, to provide a data basis.

[0046] Table 4 Summary of chlorite coating coverage, physical properties, and XRD clay analysis data of samples from a certain area in the Sichuan Basin

[0047] Then, the scatter plots of the capsule coverage and reservoir porosity and permeability, and the chlorite content and reservoir porosity and permeability are plotted respectively, as shown in the following figure. Figure 6 As shown by Figure 6 A and Figure 6 As can be seen in Figure c, in the well section (depth) containing chlorite coating in the study area, the coverage and content of chlorite coating are logarithmically correlated with the porosity. R 1 2 =0.5644, R 2 2 =0.5536, and by Figure 6 As can be seen in Figure b, the chlorite coating coverage is logarithmically correlated with the permeability, and the correlation is relatively low relative to the porosity. R2 =0.4539, from Figure 6 From the middle d, it can be concluded that there is no obvious correlation between chlorite content and permeability.

[0048] Combining Table 4 and Figure 6 The results show that within the well section (depth) containing chlorite coatings in the study area, the increase in chlorite content has a certain positive effect on reservoir porosity, but has little contribution to permeability; the higher the coverage of the chlorite coating, the more obvious the increase in porosity and permeability, that is, within the study range, the existence of chlorite coatings has a good constructive effect on reservoir physical properties. Therefore, for the research objects in this study area, when observing thin sections, a higher coverage of chlorite coatings can indicate that its physical properties are relatively high and the reservoir quality is relatively good, which is an auxiliary means to quickly judge the quality of physical properties.

[0049] In summary, the present invention provides a fast and effective method for calculating the coverage of chlorite coating. In the process of reservoir evaluation, it is a more accurate, comprehensive and effective means to explore the destructive or protective effect of chlorite coating on reservoir quality. It provides research materials for clarifying the influence of chlorite coating on the densification process of sandstone, and has important scientific research and application value.

[0050] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the embodiments of the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for calculating the coverage of chlorite coating, characterized in that: The following steps are involved: Step S1: Collect typical core samples of clastic rock layers in the study area, observe sedimentary characteristics, and determine sedimentary microfacies; Step S2: testing the core samples to obtain physical properties and chlorite content data, and preparing sample slices; Step S3: Observe the sample slices with high precision, identify and photograph the particles with chlorite coating; Step S4: using the cross ratio method, perimeter ratio method and angle ratio method to calculate the coating coverage of the sample thin slice particles respectively, and then taking the average of the coating coverage of the particles by the three methods as the total coating coverage of the sample particles; Step S5: Linearly couple the calculated total coating coverage of the sample particles with the chlorite content to establish a relationship between the chlorite coating coverage and the chlorite content.

2. The method for calculating the coverage of chlorite coating according to claim 1, characterized in that: The deposition characteristics described in step S1 include particle size, color, and structural configuration.

3. The method for calculating the coverage of chlorite coating according to claim 1, characterized in that: The tests performed on the core samples in step S2 include porosity test, permeability test and XRD test.

4. The method for calculating the coverage of chlorite coating according to claim 1, characterized in that: The instrument for high-precision observation of the sample slice in step S3 is an optical microscope or a scanning electron microscope.

5. The method for calculating the coverage of chlorite coating according to claim 1, characterized in that: The specific steps of the cross ratio method described in step S4 are: A certain number of particles within the field of view were selected, and rectangular coordinate axes were set at their center points. The extended lines of the rectangular coordinate system 0°, 90°, 180°, and 270° were made to intersect with the particle boundaries. The development of chlorite coating, pores, cementation and filling materials, and particle contact at the intersections were observed. The points with chlorite coating were scored as 1 point, and the remaining points were scored as 0 points. The percentage of the average score was taken as the single-particle coating coverage rate. After excluding particles with invalid sections exceeding 2 / 3, the mean particle coating coverage rate within the field of view was calculated.

6. The method for calculating the coverage of chlorite coating according to claim 5, characterized in that: The specific steps of the perimeter ratio method described in step S4 are: Select the same particles as those in the cross ratio method, observe the clay coating coverage at the edge of the particles, define the part with chlorite coating as the coverage section, and calculate the single particle coating coverage rate and the particle coating coverage rate within the field of view, as shown in formulas (1) and (2): Single particle coating coverage = 100% (1) Particle coating coverage within the field of view = 100% (2) In formula (2), i is the data of a single particle.

7. A method for calculating the coverage of chlorite coating according to claim 6, characterized in that: The specific steps of the angle ratio method described in step S4 are: Select the same particles as those in the circumference ratio method, observe the clay coating coverage at the edge of the particles, define the part with chlorite coating as the coverage segment, count the angles between the starting point and the end point of all coverage segments in the particles and the center point of the particles, calculate the proportion of the sum of the angles between the starting point and the end point of all coverage segments and the center point of the particles in the circumference, and get the single particle coating coverage rate. On this basis, calculate the particle coating coverage rate within the field of view.

8. A method for calculating the coverage of chlorite coating according to any one of claim 5, characterized in that: The number of particles selected within the field of view is no less than 20.

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