Stress gradient analysis method based on directional change of saline inclusion in FIP

Abstract

The invention belongs to the field of geochemistry and tectonic geology research, and discloses a stress gradient analysis method based on directional change of saline inclusion in FIP. According to the method, the size, shape and distribution characteristics of the saline inclusion in the FIP in different directions are acquired and quantitatively analyzed through the system, and a stress gradient evolution model is constructed, so that the spatial change of a local tectonic stress field is revealed. Compared with a traditional stress analysis method mainly based on a macrostructure, the stress analysis method focuses on stress response behaviors under a microscopic scale, combines various precise test means such as microscopic observation, pressure measurement and cathode luminescence, and realizes a technical path of directly inverting the tectonic stress gradient from the microstructure. The method can be widely applied to geological research of structural environments such as fault zones and shear zones, provides a new technical means for crust stress field analysis, and has important research value and wide 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 in 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 healed and sealed, showing a linear arrangement in the microfractures of mineral grains. FIP reflects the fluid penetration in 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 within FIP. Since stress changes can affect the opening and closing of fractures and the crystallization process of fluids therein, 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: S1. Selection of experimental samples 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, prepare 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; S2. Measurement and statistics of saline inclusions in FIP Through technical means such as microscopic observation, temperature measurement, and cathodoluminescence, systematically analyze the size, morphology, and directional changes of saline inclusions in FIP, and identify their systematic differences in different directions; S3. Classification and processing of data Conduct directional separation and statistics on parameters such as the 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; S4. Analysis of directional changes Quantitatively judge the change trends of parameters such as the 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. Joint analysis of multiple parameters helps to comprehensively determine the type of tectonic environment; S5. Stress gradient analysis Perform 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; S6. Result verification and application 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.

[0005] The present invention has the following beneficial effects: 1. By analyzing the directional changes in the size and morphology 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, the microscopic differences in the stress field in different tectonic units can be identified; 2. According to the quantitative study of the directional changes in the size and morphology of inclusions, reflecting the 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; 3. Analyses were carried out by combining various technical means such as microscopic observation, cathodoluminescence, and temperature measurement, ensuring the high precision and high reliability of the measurement results. By comprehensively using multiple techniques, the size, morphology, and spatial distribution characteristics of the inclusions were more comprehensively described, providing strong data support for inferring the local stress gradient. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 It is a flow chart for the overall study of a stress gradient analysis method based on the directional change of saline inclusions in FIP; Figure 2 It is a flow chart for the specific calculation of a stress gradient analysis method based on the directional change of saline inclusions in FIP; Figure 3 It is a schematic diagram for measuring the morphology of saline inclusions in FIP; Figure 4 It is a rose diagram of the long-axis distribution of saline inclusions in FIP; Figure 5 It is a schematic diagram of the stress gradient change. SPECIFIC EMBODIMENTS

[0007] 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.

[0008] In this embodiment, for the Archaean granite gneiss in the western slope area of the Bozhong Sag in the Bohai Bay Basin, well data with complete information were selected as the research object for detailed description. Microfractures and metamorphic tectonic planes are widely developed in this area. The granite 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.

[0009] The specific steps of the present invention are as follows: S1. Selection of experimental samples

[0010] Select areas with obvious tectonic deformation characteristics and easy to carry out inclusion analysis, sample in the directions perpendicular and parallel to the main tectonic line, mark directional marks on the samples to indicate the main tectonic direction, slice according to the direction information during sampling, check the directional distribution of inclusions under the microscope, and mark direction identifiers on each thin section.

[0011] S11. Selection of the study area The present invention selects typical geological structure areas with obvious stress field differences, such as fault zones, shear zones or ore-forming zones, 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, priority is given to areas with obvious tectonic deformation characteristics and easy to conduct inclusion analysis, such as tectonites or veins containing abundant quartz and calcite.

[0012] S12, Directional Sampling and Indoor Marking 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, samples are taken in directions perpendicular and parallel to the main tectonic lines (such as fault zones, shear zones), and information such as their strike, dip, and tectonic plane orientation is measured using a geological compass to analyze the development law of inclusions in different directions. At least 5 - 10 typical samples are selected in each direction, and directional marks are marked on the samples to indicate the main tectonic direction. At the same time, detailed sampling information is recorded to ensure the representativeness and statistical reliability of the data. Subsequently, in 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.

[0013] (3) Oriented Section Preparation During the section preparation process, to maintain the direction consistency of fluid inclusion analysis, sections are cut strictly according to the direction information during sampling. According to the tectonic characteristics of the rock, the section preparation direction is determined, such as cutting along the maximum principal stress direction 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 checked under the 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 tectonic direction. Through reasonable oriented section preparation technology, the comparability of data can be improved, ensuring the accuracy of subsequent experimental analysis; S2, Measurement and Statistics of Fluid Inclusions

[0014] After the oriented section preparation is completed, the analysis methods for fluid inclusions mainly include microscopic observation, temperature measurement, and cathodoluminescence, to study the size, morphology, and directional changes of brine inclusions, and then infer the stress gradient. This study combines different technical means to comprehensively analyze the spatial distribution, physical and chemical properties, and morphological characteristics of inclusions to improve the accuracy and reliability of the research.

[0015] (1)Microscopic measurement Through polarized light and fluorescence microscopic observations, the morphology, size, and distribution characteristics of saline 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 saline inclusions in the FIP in different directions, in order to reveal the stress gradient characteristics. For the samples after directional section preparation, systematically measure the FIP in different directions ( Figure 3 ), record the relevant data of the inclusion size (volume), morphology (flattening ratio), and distribution density, and observe and record their changing trends along specific directions to determine whether there is an influence of stress gradient. If the size, morphology, or density of the inclusions show a gradual change trend along a specific direction, such as gradually increasing or decreasing, it may reflect the stress gradient characteristics in that direction. Combining the distribution of the FIP trace and the tectonic background, further speculate on the direction and variation law of the local stress field. At the same time, the stress characteristics of the inclusions can also be analyzed through their morphological characteristics. For example, elliptical inclusions may reflect shear stress, while larger inclusions may form in low-stress areas.

[0016] (2)Microscopic thermometry analysis Use a heating and cooling stage to measure the homogenization temperature (Th) and freezing point temperature (Tm) of the inclusions to obtain information on the temperature, salinity, and pressure of the ore-forming fluid recorded by the inclusions. In the samples with directional section preparation, select representative FIP and the saline inclusions therein, and try to measure the inclusions in different directions as much as possible. Through the changes in the homogenization temperature of the inclusions in different directions, the influence of the local stress field on the fluid temperature distribution can be speculated, and stress gradient analysis can be 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. Combine experimental calculations to seal the pressure, and use the isotherm method or the PVT calculation model to speculate on the thermal-fluid evolution characteristics of the local stress field.

[0017] (3)Cathodoluminescence analysis Use a cathodoluminescence microscope (CL) to analyze the microstructural characteristics inside the minerals and inclusions, and identify the crack propagation, grain boundary migration, and microcrack filling caused by stress. For example, if the inclusions in a certain direction are concentrated in microcracks or grain boundaries of minerals, it indicates that this direction has experienced strong stress, which may represent the direction of tectonic activity. Combining cathodoluminescence with the size, morphology, and distribution characteristics of inclusions, the influence of stress gradient on rock deformation and fluid activity can be further analyzed. 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 speculate on the evolution process of the stress field; S3. Classification and processing of data

[0018] 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 of directional changes and stress gradient analysis. First, record parameters such as the size (volume), shape (flatness ratio), density, temperature and pressure, and position of the inclusions respectively, and classify them according to the direction. The directionality of the inclusions can be judged by drawing a rose diagram based on the distribution of the long axis direction to determine whether there is a preferred orientation.

[0019] Ellipsoid volume formula: , 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 ).

[0020] Flatness Ratio (FR) formula of the inclusion: , where a and b are the long axis and short axis of the inclusion respectively.

[0021] Inclusion density formula: , Density of inclusions in the i-th direction, Number of inclusions counted in this direction, Statistical area in this direction.

[0022] Secondly, calculate statistical indicators such as the mean and standard deviation of the data in each direction to quantify their directional differences.

[0023] Among them, the mean formula: , is the mean, N is the total number of data, is the i-th observed value.

[0024] Standard deviation formula: , is the standard deviation, is the i-th observed value, is the mean, and N is the total number of data.

[0025] In addition, to ensure the representativeness of the data, outliers should be excluded, and the reliability of the data should be tested by normal distribution fitting using statistical methods. Finally, organize the data in different directions into a table or database format for subsequent visualization analysis and stress gradient speculation, ensuring the systematicness and comparability of the data, and providing a solid foundation for subsequent research; S4. Analysis of directional changes 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 speculate on the characteristics of the local stress field and stress gradient.

[0026] (1)Analysis of the directional change trend of a single parameter We analyze the directional change trend according to four parameters: size, shape, density, and temperature and pressure respectively.

[0027] First, 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 changes in the size of the inclusion in different directions can reveal the influence of the local stress field. The directional judgment of the size of the inclusion can be used to draw a rose diagram according to the characteristics of the long axis. For example Figure 4 , this figure shows that the long axes of the inclusions are mainly distributed between 60° and 120°, and are particularly concentrated at 90°. The surface long axis distribution has obvious directionality, which may be closely related to the direction of tectonic stress. In a specific direction, if there is a significant change in the volume of the inclusion, it indicates that the stress in that direction is strong. For example, in the tensile zone, the inclusion 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 change in volume, we need to consider the change characteristics of the long and short axes. Here, there are 5 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, and is 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 unidirectional tensile deformation caused by 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 shear deformation resulting in enhanced slender morphology, commonly found in shear zones or ductile rheological deformation environments in the tectonic tensile direction; 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, and is distributed in shear zones, complex stress fields, or locally deformed areas.

[0028] 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 presents an elongated or flattened shape, indicating that it has experienced a strong heterogeneous 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, density change, and temperature and pressure parameters of the inclusion 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 the inclusion and its spatial distribution, it is divided into the following five types to assist in judging the geological environment in which it forms and deforms: 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 tensile or shear deformation and is common in ductile shear zones or tectonic tension zones; ② The inclusion is flattened and the oblateness is significantly reduced. The inclusion presents a flattened or sheet-like shape and is 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 subjected to uniform deformation, 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 occurs, mainly occurring in areas that have not been transformed by tectonic stress or have not experienced strong stress action.

[0029] Then, the directional change trend of the density of the number of inclusions is analyzed. The density of the number of inclusions can reveal the degree of local crack development, and then reflect the spatial change trend of the stress field. Different change patterns can determine the direction of crack development, the main control direction of stress and the difference in local stress field. This part can be divided into 4 parts for discussion: ① The high-density and low-density areas are obvious and distributed in a directional manner. The cracks in a specific direction are more developed and the density is higher. There are fewer cracks perpendicular to this direction and the density is lower. It may be formed in a tectonic tension zone, shear zone or regional tectonic stress field; ② Uniform distribution, no obvious directional difference. The stress is relatively uniform, or the inclusion has undergone fluid recrystallization after formation, resulting in the weakening of the original distribution, and it is formed in a tectonic stable area or an area with weak fluid activity; ③ The high-density area extends in a certain direction, and the density change is distributed in a gradient. Due to the action of stress gradient, the density changes, mainly in the transition zone from the inside to the edge of the shear zone, and the fault zone with strong tectonic activity; ④ Local high-density areas appear, and the overall density is low. It may be caused by local stress concentration or local dissolution and recrystallization, but the overall tectonic stress is weak. It is commonly seen in fault intersection areas, brittle-ductile transition zones, or local tectonic fracture zones with strong fluid activity.

[0030] Finally, the directional change trend of temperature and pressure is analyzed. This patent mainly analyzes stress gradients, so the focus of the analysis is on the directional change of pressure. The directional change of temperature will affect the rheological characteristics of rocks and fluid activities, which needs to be discussed separately, and the coupling relationship between the two will indicate the characteristic tectonic environment, which also needs to be considered. The changes in pressure in different directions are mainly divided into three categories: ① Pressure increase. It may be affected by compression and is mainly formed in thrust faults, thrust structures or tectonic compression zones; ② Pressure decrease. It may be affected by the tension environment and is mainly common in normal faults, tension cracks or pull-apart basins; ③ The pressure changes are large. It may indicate complex tectonic movements, which occur in strike-slip faults, shear zones or ductile-brittle transition zones. The directional change of temperature is also considered from three aspects: ① Temperature increase. It indicates that it is affected by hydrothermal fluids and may occur in deep shear zones, high-temperature hydrothermal systems or deep tectonic zones; ② Temperature decrease. It may be affected by cooling and mainly occurs in areas such as the upper plate of the fault or the hydrothermal cooling edge zone; ③ The temperature change is not obvious. It shows that there is no obvious thermal anomaly in the temperature field, and it mainly exists in a stable magma-tectonic environment. Considering temperature and pressure at the same time, there are mainly three situations: ① High temperature and high pressure. It may exist in deep shear zones or high-temperature metamorphic zones; 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 structure, and the rock is uplifted but has not undergone high-temperature metamorphism. Low temperature and low pressure have little effect in stress gradient analysis, and the directional information is weak, so it will not be discussed here.

[0031] (2) Analysis of directional change trends of multiple parameters In stress analysis, since the four parameters of size, shape, density, and temperature and pressure often interact with each other, comprehensively considering their effects is more helpful for 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 and 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 common 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 the tectonic compression zone; ③ Shear-dominated environment. The formed inclusions are elongated along the shear direction, the shape is irregular, the volume changes little, the density may increase, and the temperature and pressure are locally unevenly distributed. It is common in deformation environments such as shear zones or rheological zones.

[0032] In the process of multi-parameter comprehensive analysis, considering the subsequent calculation amount and the correlation of variables, here we choose 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; S5. Analysis of stress gradient

[0033] The calculation of stress gradient is of great significance in the study of geology and tectonic evolution. As microscopic recorders 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.

[0034] 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: ; 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 The formula for pressure change is: ; where is the average pressure of multiple inclusions in a certain direction, is the pressure of the i-th inclusion.

[0035] The formula for shape (flattening ratio) change is: ; Among them, is the average flattening ratio of multiple inclusions in a certain direction, is the flattening ratio of the i-th inclusion.

[0036] The volume change of the inclusion is usually caused by the pressure change. The volume change formula is: ; Among them, is the average volume of multiple inclusions in a certain direction, is the volume of the i-th inclusion.

[0037] This patent uses the normalized weight method for calculation. Combining the calculation data and formulas in the above directional change analysis: ; Among them, 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.

[0038] Finally, the above formulas are integrated as: ; Among them, is the change in the relative position of the inclusion in the measurement direction (μm 3 ), and N is the number of inclusions.

[0039] 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 Figure 5 shown. The abscissa of this figure represents the measurement direction of 0° - 180°, and the ordinate represents the pressure gradient. The main peak is around 90°. At this 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 the tectonic deformation mechanism. At the same time, it is necessary to make a comprehensive judgment in combination with the geological background. However, in the actual application process, it is necessary to pay attention to the influence of measurement errors on the calculation results, such as microscopic measurement accuracy, inclusion temperature and pressure measurement, etc.; S6, Result verification and application

[0040] The logic of this method is based on the geomechanics theory, and the influences of volume, morphology, and pressure on the stress gradient are reasonably quantified by the normalized weight method. The rationality of the results can be verified by comparing the calculated results with existing tectonic stress analysis methods, such as paleostress restoration and rock mechanics experiment data. At the same time, the result verification should also be combined with the geological background analysis to evaluate whether the calculated 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.

[0041] 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.

[0042]

[0043] 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 salt water inclusions in FIP, characterized in that: The following steps are involved: S1. Experimental sample selection Select an area with obvious structural deformation and easy to analyze the brine inclusions in the fluid inclusion trace surface FIP, collect samples in the direction perpendicular and parallel to the main structural line and mark the direction information, and then make directional sections according to the structural direction to ensure that the directional characteristics of the brine inclusions in the FIP are accurately identified in the microscopic analysis; S2. Measurement and statistics of saline inclusions in FIP Through microscopic observation, temperature measurement and cathodoluminescence technology, the size, morphology and directional changes of brine inclusions in the fluid inclusion trace surface FIP are analyzed to identify their systematic differences in different directions; S3. Data classification and processing The volume, flattening, density, temperature, pressure and position parameters of the brine inclusions in the fluid inclusion trace surface FIP are directionally separated and statistically analyzed, and rose diagrams are drawn and the mean and standard deviation are calculated to provide data support for stress gradient analysis. S4. Directional change analysis Quantitatively judge the changing trends of the volume, flattening, density, temperature and pressure parameters of the brine inclusions in the fluid inclusion trace surface FIP in different directions, identify the principal stress direction and stress characteristics, and provide a basic basis for stress gradient analysis; S5. Stress gradient analysis The pressure, flattening and volume of the brine inclusions in the fluid inclusion trace surface FIP were normalized and calculated, and a stress gradient calculation model was constructed. The stress gradient change diagram was calculated and drawn using the statistical data of 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 error and data bias, the normalized weight method is used to quantify the effects of volume, flattening and pressure on stress gradient, and the results are verified by combining the geological background and measurement error.

2. A stress gradient analysis method based on directional changes of salt water inclusions in FIP according to claim 1, characterized in that: When collecting samples in the direction perpendicular and parallel to the main structural line in step S1, 5 to 10 typical samples are selected in each direction, directional marks are marked on the samples to indicate the main structural direction, and detailed sampling information is recorded.

3. A stress gradient analysis method based on directional changes of salt water inclusions in FIP according to claim 1, characterized in that: In step S5, the pressure, flattening and volume of the brine inclusion in the fluid inclusion trace surface FIP are normalized and calculated. The pressure change of the brine inclusion in the fluid inclusion trace surface FIP is Δ P i The formula is: ； in, P avg is the average pressure of multiple inclusions in a certain direction, P i is the pressure of the ith inclusion; The flattening change of salt water inclusions in the fluid inclusion trace surface FIP△ FR i The formula is: ； in, FR avg is the average flattening of the salt water inclusions in multiple fluid inclusion trace surfaces FIP in a certain direction, FR i is the flattening of the brine inclusion in the i-th fluid inclusion trace surface FIP; Volume change of brine inclusions in the fluid inclusion trace surface FIP△ V i Volume change of the brine inclusion in the fluid inclusion trace surface FIP due to pressure change△ V i The formula is: ； in, V avg is the average volume of brine inclusions in multiple fluid inclusion trace surfaces FIP in a certain direction, V i is the volume of the brine inclusion in the i-th fluid inclusion trace surface FIP; The normalized weight method is used for calculation, combined with the calculation data and formula in the above directional change analysis: ； in, w 1 、w 2 、w 3 are the weights of volume, flattening and pressure in calculating stress gradient, σ V , σ FR , σ p are the standard deviations of volume, oblateness and pressure, respectively; Integrate the formula into a stress gradient calculation model: ； in, △x i is the change in the relative position of the brine inclusions in the fluid inclusion trace surface FIP in the measurement direction (μm 3 ); N is the number of salt water inclusions in the fluid inclusion trace surface FIP; w 1 、w 2 、w 3 are the weights of volume, flattening and pressure in calculating stress gradient; △v i is the volume change of the brine inclusion in the fluid inclusion trace surface FIP; △FR i is the flattening change of the salt water inclusion in the fluid inclusion trace surface FIP; △P i It is the pressure change of the brine inclusion in the fluid inclusion trace surface FIP.

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