A stress gradient analysis method based on the directional change of saline inclusions in FIP

Abstract

The present invention belongs to the research fields of geochemistry and structural geology, and discloses a stress gradient analysis method based on the directional change of brine inclusions in FIP. The present invention systematically collects and quantitatively analyzes the size, morphology and distribution characteristics of brine inclusions in FIP in different directions, constructs a stress gradient evolution model, and thus reveals the spatial variation of the local tectonic stress field. Compared with the traditional stress analysis method mainly based on macroscopic structures, the present invention focuses on the stress response behavior at the microscopic scale, combines various precise testing means such as microscopic observation, pressure measurement, and cathodoluminescence, and realizes the technical path of directly inverting the tectonic stress gradient from the microscopic structure. This method can be widely applied to geological research in tectonic environments such as fault zones and shear zones, provides a new technical means for crustal stress field analysis, and has important research value and broad application prospects.

Description

Technical Field

[0001] The present invention relates to the fields of geochemistry and structural geology, and particularly relates to a stress gradient analysis method based on the directional change of brine inclusions in FIP (Fluid Inclusion Plane). Background Art

[0002] Fluid inclusions are tiny fluids trapped by minerals during their formation, which can record the temperature, pressure and chemical composition of geological fluids and have important value in studying the evolution of crustal fluids, ore-forming processes and tectonic activities. The Fluid Inclusion Plane (FIP) is the fluid activity trace formed by the penetration and migration of fluids into deformation structures and microfractures during geological processes and then their healing and sealing, and is linearly arranged in the microfractures of mineral grains. FIP reflects the fluid penetration of geological bodies under different tectonic actions, and its occurrence, trace length, density and other parameters are often used to infer fault activities, tectonic evolution and regional stress states. However, current research mainly focuses on these parameter characteristics, and there is less research on the characteristics of inclusions inside FIP. Since stress changes will affect the opening and closing of fractures and the crystallization process of fluids in them, the size of inclusions in FIP may show a certain directional change law with the local stress gradient, thereby recording the change information of the stress field.

[0003] Traditional tectonic stress analysis methods mainly rely on the observation of macroscopic geological structures and rock mechanics experiments, such as fracture structure geometric analysis, paleostress inversion and numerical simulation, etc. However, these methods usually lack stress information at the microscale and are difficult to directly reveal the subtle stress change characteristics. In contrast, fluid inclusions can provide a more intuitive record of the stress environment. Especially for brine inclusions, their size, shape and spatial distribution are affected by tectonic stress and can be used to indicate stress changes. Although existing research involves techniques such as microscopic observation and temperature-pressure analysis of fluid inclusions, there is still a lack of systematic research on the directional change of the size of inclusions in FIP and its relationship with the stress gradient. There is a need for a stress gradient analysis method to make up for the deficiencies of existing stress research at the microscale and provide new data support for the study of fracture structure evolution, ore deposit genesis and regional stress field analysis. Summary of the Invention

[0004] The present invention aims to develop a stress gradient analysis method based on the directional change of brine inclusions in FIP to make up for the deficiencies of existing stress research at the microscale. To achieve the above object, the present invention adopts the following technical solutions:

[0005] S1. Selection of experimental samples

[0006] Select areas with obvious structural deformation and easy to conduct analysis of saline inclusions in FIP. Collect samples along directions perpendicular and parallel to the main structural line and mark the direction information. Subsequently, make oriented slides according to the structural direction to ensure that the directional characteristics of saline inclusions in FIP can be accurately identified in microscopic analysis;

[0007] S2. Measurement and statistics of saline inclusions in FIP

[0008] Through technical means such as microscopic observation, temperature measurement and cathodoluminescence, systematically analyze the size, shape and directional changes of saline inclusions in FIP, and identify their systematic differences in different directions;

[0009] S3. Classification and processing of data

[0010] Conduct directional classification and statistics on parameters such as volume, oblateness, density, temperature and pressure, and position of saline inclusions in FIP. Draw rose diagrams and calculate the mean value and standard deviation to provide data support for stress gradient analysis;

[0011] S4. Analysis of directional changes

[0012] Quantitatively judge the change trends of parameters such as volume, oblateness, density, temperature and pressure of saline inclusions in FIP in different directions, identify the principal stress direction and stress characteristics, and thus provide a basic basis for stress gradient analysis. The multi-parameter joint analysis helps to comprehensively determine the type of tectonic environment;

[0013] S5. Stress gradient analysis

[0014] Conduct normalized calculations on the pressure, oblateness and volume of saline inclusions in FIP, construct a stress gradient calculation model, and use the data statistically analyzed in S3 to calculate and draw a schematic diagram of stress gradient changes to quantify the spatial distribution characteristics of the local stress field and assist in identifying the tectonic deformation mechanism;

[0015] S6. Result verification and application

[0016] On the basis of considering the influence of measurement errors and data deviations, quantify the influence of volume, oblateness and pressure on the stress gradient by the normalized weight method, and conduct result verification in combination with the geological background and measurement errors.

[0017] The present invention has the following beneficial effects:

[0018] 1. By analyzing the directional changes in the size and shape of saline inclusions in FIP, it is possible to quantitatively study the spatial distribution of the stress field at the microscopic scale, providing an important supplement for macroscopic tectonic stress analysis. By analyzing the characteristics of saline inclusions in FIP, it is possible to identify the microscopic differences in the stress field in different tectonic units;

[0019] 2. By quantitatively studying the directional changes in the size and morphology of inclusions to reflect different directions and intensities of stress, the present invention can accurately reveal the gradient and spatial differences of the local stress field. Inclusions of different morphologies can reveal different states of the stress they are subjected to, thus enriching the dimensions of stress field research, especially in tectonic environments such as fault zones and shear zones;

[0020] 3. By combining various technical means such as microscopic observation, cathodoluminescence, and temperature measurement for analysis, the high precision and high reliability of the measurement results are ensured. By comprehensively using multiple technologies, the size, morphology, and spatial distribution characteristics of inclusions are more comprehensively described, providing strong data support for inferring local stress gradients. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flowchart of the overall research of a stress gradient analysis method based on the directional changes of saline inclusions in FIP;

[0022] Figure 2 It is a flowchart of the specific calculation of a stress gradient analysis method based on the directional changes of saline inclusions in FIP;

[0023] Figure 3 It is a schematic diagram of the measurement of the morphology of saline inclusions in FIP;

[0024] Figure 4 It is a rose diagram of the long-axis distribution of saline inclusions in FIP;

[0025] Figure 5 It is a schematic diagram of the stress gradient change. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] In this embodiment, for the Archaean granitic gneiss in the western slope area of the Bozhong Sag in the Bohai Bay Basin, well data with complete information are taken as the research object for detailed description. Microfractures and metamorphic structural planes are widely developed in this area. The granitic gneiss has a dense structure, is rich in quartz and feldspar components, and has good mechanical response characteristics, providing a good data basis for the present invention.

[0028] The specific steps of the present invention are as follows:

[0029] S1. Selection of experimental samples

[0030] Select areas with obvious structural deformation characteristics and easy to conduct inclusion analysis. Sampling is carried out in the directions perpendicular and parallel to the main structural lines. Directional markers are marked on the samples to indicate the main structural direction. Slicing is performed according to the direction information during sampling. The directional distribution of inclusions is examined under a microscope, and direction identifiers are marked on each thin section.

[0031] S11. Selection of the study area

[0032] The present invention selects typical geological structural areas with obvious stress field differences, such as fault zones, shear zones or ore-forming belts, etc., to ensure that the directional changes in the size and morphology of brine inclusions in the FIP can be observed. Due to the action of forces in different directions in these tectonic environments, the distribution, size and morphological characteristics of inclusions change in different regions. Especially in areas with significant stress gradients such as both sides of faults, the core and wings of folds, the development characteristics of inclusions may be more representative. Therefore, when selecting samples, areas with obvious structural deformation characteristics and easy to conduct inclusion analysis are preferentially selected, such as tectonites or veins containing abundant quartz and calcite.

[0033] S12. Directional sampling and indoor marking

[0034] To ensure obtaining inclusion samples with directional change characteristics, the present invention adopts a systematic directional sampling method. In the selected area with obvious tectonic activities, sampling is carried out in the directions perpendicular and parallel to the main structural lines (such as fault zones, shear zones), and a geological compass is used to measure information such as its strike, dip, and structural plane orientation to analyze the development law of inclusions in different directions. At least 5 - 10 typical samples are selected in each direction. Directional markers are marked on the samples to indicate the main structural direction, and detailed sampling information is recorded simultaneously to ensure the representativeness and statistical reliability of the data. Subsequently, during the indoor processing stage, all samples are systematically numbered, and the direction information, tectonic background, and number information of the sampling points are entered into the sample management system to ensure that the directional characteristics of the samples can be accurately traced during subsequent analysis.

[0035] (3)Orientation of specimen preparation

[0036] During specimen preparation, to maintain the direction consistency of fluid inclusion analysis, slicing is strictly carried out according to the direction information during sampling. According to the structural characteristics of the rock, the specimen preparation direction is determined, such as slicing along the direction of the maximum principal stress or perpendicular to the strike of the fault zone, so as to accurately analyze the size, morphology and distribution characteristics of inclusions in different directions. Subsequently, the directional distribution of inclusions is examined under a microscope, and direction identifiers are marked on each thin section to ensure that the directional changes of inclusions can be accurately identified during subsequent analysis and are consistent with the original structural direction. Through reasonable specimen preparation techniques for orientation, the comparability of data can be improved, ensuring the accuracy of subsequent experimental analysis;

[0037] S2. Measurement and Statistics of Fluid Inclusions

[0038] After the preparation of oriented thin sections, the analysis methods for fluid inclusions mainly include microscopic observation, temperature measurement, and cathodoluminescence. By studying the size, shape, and directional changes of brine inclusions, the stress gradient can be inferred. In this study, different technical means are combined to comprehensively analyze the spatial distribution, physical and chemical properties, and morphological characteristics of inclusions, so as to improve the accuracy and reliability of the research.

[0039] (1) Microscopic Measurement

[0040] Through polarized light and fluorescence microscopic observations, the morphology, size, and distribution characteristics of brine inclusions in thin sections are measured. During the measurement process, data need to be collected in multiple directions to analyze the systematic changes in parameters such as the size and morphology of brine inclusions in the FIP in different directions, so as to reveal the characteristics of the stress gradient. For the samples after the preparation of oriented thin sections, FIPs are systematically measured in different directions ( Figure 3 ), and the relevant data of the inclusion size (volume), morphology (flattening ratio), and distribution density are recorded, and the change trend along a specific direction is observed and recorded to judge whether there is an influence of the stress gradient. If the size, morphology, or density of the inclusions shows a gradual change trend along a specific direction, such as gradually increasing or decreasing, it may reflect the characteristics of the stress gradient in that direction. Combining the distribution of the FIP trace and the tectonic background, the direction and variation law of the local stress field are further inferred. At the same time, the stress characteristics of the inclusions can also be analyzed through their morphological characteristics. For example, elliptical inclusions may reflect the action of shear stress, while larger inclusions may form in low-stress areas.

[0041] (2) Microscopic Temperature Measurement Analysis

[0042] The homogenization temperature (Th) and freezing point temperature (Tm) of inclusions are measured using a heating and cooling stage to obtain information on the temperature, salinity, and pressure of the ore-forming fluid recorded by the inclusions. In the samples of oriented thin sections, representative FIPs and the brine inclusions therein are selected, and inclusions in different directions are preferably measured. Through the changes in the homogenization temperature of inclusions in different directions, the influence of the local stress field on the fluid temperature distribution can be inferred, and stress gradient analysis is carried out in combination with the directional changes in the inclusion size. For example, in the high-stress direction, the inclusions may record a higher homogenization temperature, indicating that this direction may have experienced higher pressure-temperature conditions. Combining experimental calculations of the sealing pressure and using the isotherm method or the PVT calculation model, the thermal-fluid evolution characteristics of the local stress field are inferred.

[0043] (3) Cathodoluminescence Analysis

[0044] Using a cathodoluminescence microscope (CL), analyze the microstructural characteristics inside minerals and inclusions, and identify the fracture propagation, grain boundary migration, and microfracture filling caused by stress. For example, if inclusions in a certain direction are concentrated at microfractures or mineral grain boundaries, it indicates that this direction has experienced strong stress and may represent the direction of tectonic activity. Combining cathodoluminescence with the size, shape distribution characteristics of inclusions can further analyze the influence of stress gradient on rock deformation and fluid activity. In addition, some minerals (such as calcite, quartz) may show different cathodoluminescence colors or intensity changes under different stress environments, and this information can be used to infer the evolution process of the stress field;

[0045] S3. Classification and Processing of Data

[0046] After completing the measurement of the above fluid inclusions, it is necessary to classify and process the inclusion data in different directions for subsequent applications in analyzing directional changes and stress gradients. First, record parameters such as the size (volume), shape (flatness ratio), density, temperature, pressure, and position of the inclusions separately, and classify them according to the direction. The directionality of the inclusions can be plotted as a rose diagram based on the distribution of their long-axis directions to determine whether there is a preferred orientation.

[0047] Volume formula of ellipsoid: , where a, b, and c are the long axis, short axis, and empirically inferred height of the inclusion respectively (since the data obtained by microscopic observation is planar data, so ).

[0048] Flatness Ratio (FR) formula of inclusion: , where a and b are the long axis and short axis of the inclusion respectively.

[0049] Density formula of inclusion: , Density of inclusions in the i-th direction, Number of inclusions counted in this direction, Statistical area in this direction.

[0050] Secondly, calculate statistical indicators such as the mean and standard deviation of the data in each direction to quantify their directional differences.

[0051] Among them, the mean formula: , is the mean, N is the total number of data, is the i-th observed value.

[0052] Standard deviation formula: , is the standard deviation, is the i-th observed value, is the mean, N is the total number of data.

[0053] In addition, to ensure the representativeness of the data, outliers should be removed, and the reliability of the data should be tested by fitting the normal distribution using statistical methods. Finally, the data in different directions are organized into a table or database format for subsequent visual analysis and stress gradient speculation, ensuring the systematicness and comparability of the data and providing a solid foundation for subsequent research;

[0054] S4. Analysis of directional changes

[0055] The analysis of directional changes is the core step of this method, aiming to analyze the changing trends of data such as the size, shape, density, temperature, and pressure of brine inclusions in the FIP in different directions, and to speculate on the characteristics of the local stress field and stress gradient.

[0056] (1) Analysis of the directional change trend of a single parameter

[0057] We analyze the directional change trends according to four parameters: size, shape, density, and temperature / pressure respectively.

[0058] First, for the analysis of the directional change trend of size. The change in the size of the inclusion mainly refers to the change in volume, and usually the change in volume is estimated based on the changes in the long and short axes. The change in the size of the inclusion in different directions can reveal the influence of the local stress field. The directional judgment of the inclusion size can be used to draw a rose diagram according to the characteristics of the long axis, such as Figure 4, the figure shows that the long axis of the inclusion body is mainly distributed between 60° and 120°, especially concentrated at 90°. The distribution of the surface long axis has obvious directionality, which may be closely related to the direction of tectonic stress. For a specific direction, if there is a significant change in the inclusion volume, it indicates that the stress action in this direction is stronger. For example, in the tensile zone, the inclusion body will increase in volume in the tensile direction, while in the compression zone, the volume may decrease. Since the changes in both the long and short axes will affect the volume change, we need to consider the change characteristics of the long and short axes. Here are five possible cases: ① Both the long and short axes increase, and the volume increases. It may be caused by uniform tensile stress or increased fluid pressure, commonly found in tensile fractures, tectonic tensile zones, or areas with strong fluid activity; ② The long axis increases, the short axis changes little, and the volume change is not obvious. It may be caused by unidirectional tensile deformation and shear stress, typical of ductile shear zones or stress tectonic environments with obvious directionality; ③ The short axis increases, the long axis changes little, and the volume may increase. It may be caused by lateral extrusion resulting in local expansion, which may reflect a local high-pressure environment, such as thrust faults or tectonic compression zones; ④ The long axis changes significantly, the short axis remains almost unchanged, and the volume change is small. It may be caused by enhanced shear deformation resulting in an elongated shape, commonly found in shear zones in the direction of tectonic tension or ductile rheological deformation environments; ⑤ Both the long and short axes change, but the volume hardly changes. It may be caused by non-uniform deformation, affected by shear or dissolution recrystallization, distributed in shear zones, complex stress fields, or locally deformed areas.

[0059] Secondly, analyze the directional change trend of the morphology. To quantify the directional change of the morphology, analyze according to the characteristics of the oblateness mentioned above. When the FR value is close to 1, the inclusion approaches a circular shape, indicating that it is less stressed during formation or later deformation; when the FR value is significantly less than 1, the inclusion shows an elongated or flattened shape, indicating that it has experienced a strong inhomogeneous deformation process. The value of FR itself can only reflect the degree of deviation of the geometric shape and cannot independently identify the specific deformation mechanism. Therefore, it is necessary to comprehensively judge by combining information such as the directional distribution of inclusions, density changes, and temperature and pressure parameters to improve the recognition accuracy of the stress environment. In the directional analysis of the morphology, according to the combined relationship between the morphological characteristics of inclusions and their spatial distribution, they are divided into the following five types to assist in judging the geological environment in which they are formed and deformed: ① The inclusion is elongated and the oblateness is significantly reduced. The inclusion shows an obvious elongated shape in a specific direction, and the FR value is significantly less than 1. This feature usually indicates that the inclusion is subjected to unidirectional tension or shear deformation, commonly found in ductile shear zones or tectonic extension areas; ② The inclusion is flattened and the oblateness is significantly reduced. The inclusion shows a flattened or sheet-like shape, mainly compressed in one direction in a high-strain environment, mainly occurring in tectonic compression zones or ductile rheological environments; ③ The inclusion tends to be irregular in shape, but the change in oblateness is not obvious. It may be affected by multi-directional stress or dissolution and recrystallization, mainly occurring in tectonic composite zones and brittle-ductile transition zones where the stress action is relatively complex; ④ The shape changes significantly, but the overall shape is oval and the change in oblateness is not obvious. The inclusion is uniformly deformed, with a relatively high fluid pressure or affected by temperature, mainly occurring in weakly deformed areas and stable environments with a relatively high fluid pressure; ⑤ The change in the morphological direction is not obvious and the oblateness is nearly 1. Affected by a uniform stress field, no obvious stress action has occurred, mainly occurring in areas that have not been transformed by tectonic stress or have not experienced strong stress actions.

[0060] Then, the directional change trend analysis of the density of the inclusion number is carried out. The density of the inclusion number can reveal the development degree of local fractures, and then reflect the spatial change trend of the stress field. Different change patterns can be used to judge the fracture development direction, the main stress control direction and the local stress field difference. This part can be discussed in four parts: ① The high-density and low-density areas are obvious and distributed directionally. Fractures are more developed in a specific direction with a higher density, and fewer fractures are perpendicular to this direction with a lower density. It may be formed under the action of a tectonic tensile zone, a shear zone or a regional tectonic stress field; ② Uniform distribution, without obvious directional difference. The stress is relatively uniform, or the original distribution is weakened due to fluid recrystallization after the formation of the inclusions, and it is formed in a tectonically stable area or a region with weak fluid activity; ③ The high-density area extends along a certain direction, and the density change shows a gradient distribution. Due to the stress gradient, the density changes, mainly occurring in the transition zone from the inside to the edge of the shear zone and the fault zone with strong tectonic activity; ④ High-density areas appear locally, and the overall density is low. It may be caused by local stress concentration or local dissolution recrystallization, but the overall tectonic stress is weak, and it is common in fracture intersection areas, brittle-ductile transition zones or locally tectonically fractured zones with strong fluid activity.

[0061] Finally, the directional change trend analysis of temperature and pressure is carried out. This patent mainly focuses on the analysis of the stress gradient, so the key analysis is on the directional change of pressure. The directional change of temperature will affect the rock rheological characteristics and fluid activity, which needs to be discussed separately, and the coupling relationship between the two will indicate a specific tectonic environment, which also needs to be considered. The changes of pressure in different directions are mainly divided into three categories: ① Pressure increases. It may be affected by compressive forces and is mainly formed in thrust faults, nappe structures or tectonic compression zones; ② Pressure decreases. It may be affected by a tensile environment and is mainly common in normal faults, tensile fractures or pull-apart basins; ③ The pressure changes greatly. It may indicate complex tectonic movements and occurs in strike-slip faults, shear zones or ductile-brittle transition zones and other areas. The directional change of temperature is mainly considered from three aspects: ① Temperature rises. It indicates being affected by hydrothermal fluids and may occur in deep shear zones, high-temperature hydrothermal systems or deep tectonic zones; ② Temperature drops. It may be affected by cooling and mainly occurs in the hanging wall of fractures or the hydrothermal cooling fringe zone and other areas; ③ The temperature change is not obvious. It indicates that there is no obvious thermal anomaly in the temperature field and mainly exists in a stable magma-tectonic environment. Considering the temperature and pressure simultaneously, there are mainly three situations: ① High temperature and high pressure. It may exist in deep shear zones or high-temperature metamorphic areas; ② High temperature and low pressure. It may be caused by hydrothermal or magmatic activities; ③ Low temperature and high pressure. It may reflect the situation of thrust structures, where the rock is uplifted but has not experienced high-temperature metamorphism. Low temperature and low pressure have little effect on the stress gradient analysis and the directional information is weak, so it will not be discussed here.

[0062] (2)Multi-parameter directional change trend analysis

[0063] In stress analysis, since the four parameters of size, shape, density, and temperature / pressure often interact with each other, comprehensively considering their effects is more conducive to the subsequent stress gradient analysis. We can first determine the formation environment of the study area based on the results of the directional change trend analysis of the four parameters of size, shape, density, and temperature / pressure judged above. Here, three main environments are mainly discussed: ① Tension-dominated environment. In this case, the inclusions show an obvious tensile shape, the shape tends to be elongated, the flattening ratio decreases, the volume may increase, the density decreases, the temperature increases, and the pressure decreases. It is commonly found in tectonic tension zones or fracture expansion zones; ② Compression-dominated environment. At this time, the inclusions may tend to be flat or irregular, the flattening ratio increases, the volume decreases, the density increases, the temperature decreases, and the pressure increases. The typical environment is the thrust fault zone or tectonic compression zone; ③ Shear-dominated environment. The formed inclusions are elongated along the shear direction, with an irregular shape, little change in volume, the density may increase, and the temperature and pressure show local non-uniform distribution. It is commonly found in deformation environments such as shear zones or rheological zones.

[0064] In the process of multi-parameter comprehensive analysis, considering the subsequent calculation amount and the correlation of variables, here we select size-shape-pressure as the main variables, because they directly reflect the direction and intensity of stress action, while density and temperature can be used as auxiliary analysis parameters;

[0065] S5. Analysis of stress gradient

[0066] The calculation of stress gradient is of great significance in the research of geology and tectonic evolution. As the microscopic recorder formed during the deformation of geological bodies, the characteristics of parameters such as the size, shape, temperature, and pressure of inclusions can reflect the changes in the local stress field. Therefore, quantitatively calculating the stress gradient can not only reveal the spatial distribution characteristics of the stress field but also reflect the tectonic deformation mechanism.

[0067] In the calculation of stress gradient, we not only consider the pressure gradient but also comprehensively consider the shape and volume changes of inclusions to more comprehensively depict the spatial distribution of the stress field. Assuming that these elements can be represented by functions, then: ;

[0068] where P is the pressure of the inclusion (MPa), FR is the flattening ratio of the inclusion, and V is the volume of the inclusion (μm 3 ). To quantify the stress gradient, we define the normalized relative variables of each parameter

[0069] The formula for pressure change is: ;

[0070] where is the average pressure of multiple inclusions in a certain direction, and is the pressure of the i-th inclusion.

[0071] The formula for the change in shape (flattening ratio) is: ;

[0072] where, is the average flattening ratio of multiple inclusions in a certain direction, is the flattening ratio of the i-th inclusion.

[0073] The volume change of inclusions is usually caused by pressure change. The formula for volume change is: ;

[0074] where, is the average volume of multiple inclusions in a certain direction, is the volume of the i-th inclusion.

[0075] This patent uses the normalized weight method for calculation. Combining the calculation data and formulas in the above directional change analysis: ;

[0076] where, are the weights of volume, flattening ratio, and pressure respectively when calculating the stress gradient. are the standard deviations of volume, flattening ratio, and pressure respectively.

[0077] Finally, the above formulas are integrated as: ;

[0078] where, is the change in the relative position of the inclusion in the measurement direction (μm 3 ), and N is the number of inclusions.

[0079] We calculate based on the above statistical data of the volume, flattening ratio, pressure, and position of the inclusions, substitute the calculated data into the final stress gradient calculation formula, and can draw a schematic diagram of the stress gradient change, as shown in Figure 5 . The abscissa of this figure represents the measurement direction from 0° to 180°, and the ordinate represents the pressure gradient. The main peak is around 90°, at which time the pressure gradient is the largest, indicating that the stress change amount in this direction is the most intense, and it may be the main stress concentration area, which is consistent with the concentration direction of the rose diagram. There are underestimations near 50° and 135°, indicating that the stress diffuses or releases in this direction. The schematic diagram of the stress gradient change is in the shape of a "mountain", indicating that this area has obvious directional stress distribution characteristics. This method is applicable to different geological environments and deformation conditions, can effectively reflect the spatial distribution characteristics of the local stress field, and provides a quantitative basis for the study of tectonic deformation mechanisms. At the same time, it is necessary to make a comprehensive judgment in combination with the geological background. However, in the actual application process, attention needs to be paid to the influence of measurement errors on the calculation results, such as microscopic measurement accuracy, inclusion temperature and pressure measurement, etc.;

[0080] S6. Result Verification and Application

[0081] The logic of this method is based on the theory of geomechanics, and the influences of volume, morphology, and pressure on the stress gradient are reasonably quantified by the normalized weight method. The verification of the results can be carried out by comparing the calculation results with existing tectonic stress analysis methods to verify their rationality, such as data from paleostress restoration and rock mechanics experiments. At the same time, the result verification should also be combined with the analysis of the geological background to evaluate whether the calculation results conform to the tectonic deformation characteristics. Moreover, the influences of measurement errors and data deviations should be taken into account to improve the applicability and stability of this method.

[0082] This method can be applied to fields such as tectonic stress field analysis and research on deformation mechanisms. By quantitatively calculating the local stress gradient, the stress distribution characteristics under different tectonic environments can be revealed, assisting in judging the activity of faults, the deformation characteristics of shear zones, and the regional tectonic evolution process.

[0083]

[0084] Those of ordinary skill in the art should understand that the discussions of the above embodiments are only exemplary and are not intended to imply that the scope of the present invention is limited to these examples; under the concept of the present invention, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above, which are not provided in detail for the sake of brevity. Any omission, modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A stress gradient analysis method based on the directional change of saline inclusions in FIP, characterized in that It includes the following steps: S1. Selection of experimental samples Select a region with obvious structural deformation and easy to analyze the saline inclusions in the fluid inclusion trace plane FIP. Collect samples along the directions perpendicular and parallel to the main structural line and mark the direction information. Subsequently, prepare oriented specimens according to the structural direction to ensure accurate identification of the directional characteristics of the saline inclusions in the FIP during microscopic analysis; S2. Measurement and statistics of saline inclusions in the FIP Through microscopic observation, temperature measurement and cathodoluminescence techniques, analyze the size, shape and directional changes of the saline inclusions in the fluid inclusion trace plane FIP, and identify their systematic differences in different directions; S3. Classification and processing of data Conduct directional separation and statistics on the volume, oblateness, density, temperature and pressure, and position parameters of the saline inclusions in the fluid inclusion trace plane FIP, draw rose diagrams and calculate the mean value and standard deviation to provide data support for stress gradient analysis; S4. Analysis of directional changes Quantitatively judge the change trends of the volume, oblateness, density, temperature and pressure parameters of the saline inclusions in the fluid inclusion trace plane FIP in different directions, identify the principal stress directions and stress characteristics, and provide a basic basis for stress gradient analysis; S5. Stress gradient analysis Normalize the pressure, oblateness and volume of the saline inclusions in the fluid inclusion trace plane FIP, construct a stress gradient calculation model, calculate and draw a schematic diagram of stress gradient changes using the data statistically analyzed in S3 to quantify the spatial distribution characteristics of the local stress field and assist in identifying the structural deformation mechanism; S6. Result verification and application Considering the influence of measurement errors and data deviations, quantify the influence of volume, oblateness and pressure on the stress gradient by the normalized weight method, and verify the results in combination with the geological background and measurement errors.

2. The stress gradient analysis method based on the directional change of brine inclusions in FIP according to claim 1, characterized in that When collecting samples along the directions perpendicular and parallel to the main structural line in step S1, select 5 - 10 typical samples in each direction, mark the directional marks on the samples to indicate the main structural direction, and record the detailed sampling information at the same time.

3. A stress gradient analysis method based on the directional change of brine inclusions in FIP according to claim 1, characterized in that In step S5, normalization calculations are performed on the pressure, flattening, and volume of the saline inclusions within the fluid inclusion trace plane FIP. The pressure change Δ P i of the saline inclusions within the fluid inclusion trace plane FIP is given by the formula: ; Among them, P avg is the average pressure of multiple inclusions in a certain direction, P i is the pressure of the i-th inclusion; Change △ in the oblateness of brine inclusions within the fluid inclusion trace plane FIP FR i is given by the formula: ; Wherein, FR avg is the average oblateness of the saline inclusions within the multiple fluid inclusion trace planes FIP in a certain direction, FR i is the oblateness of the saline inclusion within the i-th fluid inclusion trace plane FIP; Volume change △ of brine inclusions within the fluid inclusion trace plane FIP V i Volume change △ of brine inclusions within the fluid inclusion trace plane FIP caused by pressure change V i The formula is: ; Wherein, V avg is the average volume of the saline inclusions within multiple fluid inclusion trace planes FIP in a certain direction, V i is the volume of the saline inclusion within the i-th fluid inclusion trace plane FIP; Use the normalized weight method for calculation, combining the calculation data and formulas in the above analysis of directional changes: ； Among them, w 1 、w 2 、w 3 are the weights of volume, oblateness, and pressure in calculating the stress gradient, σ V 、σ FR 、σ p are the standard deviations of volume, oblateness, and pressure respectively; Integrate the formulas into a stress gradient calculation model: ； wherein, △x i is the change in the relative position of the saline inclusions in the fluid inclusion trace plane FIP in the measurement direction, μm 3 ; N is the number of saline inclusions in the fluid inclusion trace plane FIP; w 1 、w 2 、w 3 are the weights of volume, flatness and pressure respectively when calculating the stress gradient; △v i is the volume change of the saline inclusions in the fluid inclusion trace plane FIP; △FR i is the flatness change of the saline inclusions in the fluid inclusion trace plane FIP; △P i is the pressure change of the saline inclusions in the fluid inclusion trace plane FIP.

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