Comprehensive evaluation method for influence of natural fractures on gas-bearing property of shale

Abstract

The invention discloses a comprehensive evaluation method for the influence of natural fractures on the gas-bearing property of shale, which comprises the following steps of: dividing shale fracture types by observing a rock core, counting development characteristics of different types of fractures, and establishing effective fracture development strength indexes of different types of fractures; dividing crack forming periods, judging inclusion components through laser Raman spectrum analysis and inclusion microscopic temperature measurement, recovering paleo-temperature characteristics during formation of cracks of different types and different periods, and determining a relative relation between the formation periods of the cracks of different types and the shale gas generation period by combining the stratum burial history and the thermal history; crack filling minerals and carbon, oxygen and strontium isotopes and rare earth elements of surrounding rocks on the two sides of the crack are tested, the material source of a crack filling vein body is judged, and the sealing performance of a shale system when the crack is formed is analyzed; and comprehensively judging the influence of different types and different periods of cracks on the gas-bearing property of shale. The shale gas reservoir capacity and migration capacity of different types of fractures can be reflected more accurately.

Description

Technical Field

[0001] The present invention relates to the technical field of geological exploration and development, and in particular to a comprehensive evaluation method for the influence of natural fractures on the gas-bearing property of shale. Background Art

[0002] Compared with conventional oil and gas reservoirs, marine shale gas reservoirs are characterized by low porosity and ultra-low permeability, making them challenging to develop. They require artificial volume fracturing to achieve industrial production capacity. Natural fractures serve as important reservoir spaces and seepage pathways for shale gas, significantly influencing its migration, enrichment, preservation, and effectiveness of fracturing. The marine shales of the Sichuan Basin have undergone multiple phases of tectonic movement, resulting in multi-phase and multi-type natural fracture development. These fractures, formed at different times, have different development characteristics, and scales, resulting in significant variations in their impact on shale gas production. This makes accurate assessment of their impact difficult. Currently, researchers primarily analyze the impact of natural fractures on shale gas content based on geostatistical methods, resulting in low accuracy. A scientific and effective method for accurately evaluating the impact of different fracture types and phases on shale gas production has yet to be established. Summary of the Invention

[0003] In view of this, the present invention provides a comprehensive evaluation method for the influence of natural fractures on the gas content of shale, which can accurately evaluate the influence of natural fractures of different types, different periods and different scales on the gas content of shale, and can provide a geological basis for the optimization of unconventional oil and gas sweet spots and reservoir fracturing transformation, which is of great significance for guiding the exploration and development of unconventional oil and gas.

[0004] The present invention discloses a comprehensive evaluation method for the influence of natural fractures on the gas-bearing property of shale, which comprises:

[0005] Step 1: Macroscopically, shale fracture types are classified by observing cores, the development characteristics of different fracture types are statistically analyzed, and effective fracture development intensity indices for different fracture types are established;

[0006] Step 2: Microscopically, use a microscope and cathodoluminescence to observe the microstructure and cutting relationship of the fracture-filling veins, classify the fracture formation stages, and determine the inclusion composition through laser Raman spectroscopy and inclusion microthermometry. Reconstruct the paleotemperature characteristics of different types and stages of fracture formation, and combine the burial history and thermal history of the formation to determine the relative relationship between the formation period of different types of fractures and the period of shale gas generation.

[0007] Step 3: Conduct carbon, oxygen, strontium isotope, and rare earth element testing on the fracture-filling minerals and the surrounding rocks on both sides of the fracture to determine the source of the fracture-filling veins and analyze the sealing capacity of the shale system when the fractures were formed;

[0008] Step 4: Comprehensively identify the impact of different types and stages of fractures on shale gas content.

[0009] Furthermore, the step 1 includes:

[0010] Macroscopically, fractures are divided into j types by observing the cores. The top depth, bottom depth, inclination, aperture, and filling degree of each fracture on the cores are statistically analyzed, and the effective fracture development intensity index of each fracture type is calculated using the following formula:

[0011]

[0012] Among them, EFII j is the effective crack development intensity index of the jth crack, n is the total number of cracks, H i2 is the top depth of the i-th crack, H i1 is the bottom depth of the i-th crack, θ i is the inclination angle of the i-th crack, D i is the opening of the i-th fracture, H3 is the core length, E i is the filling degree of the i-th crack, which can be divided into unfilled, partially filled, half filled and fully filled.

[0013] Furthermore, the step 2 includes:

[0014] The filling veins of different types of fractures and fractures of the same type but different periods on the core were collected and made into thin sections. The mineral composition of the veins was determined by microscopy and cathode luminescence. The cutting relationship and microstructural characteristics of the veins were observed to determine the order of fracture formation. The microstructural characteristics include crystal morphology, grain size, growth mode and mineral filling order. On this basis, the fluid inclusion composition, homogenization temperature, freezing point temperature and salinity in the fracture filling veins were tested by laser Raman spectroscopy and inclusion microthermometry. Combined with the burial history and thermal evolution history of the formation, the relative relationship between the formation period of different types and different periods of fractures and the period of shale gas generation was determined.

[0015] Furthermore, the step 3 includes:

[0016] Samples of fracture veins of different types and periods and the surrounding rocks on both sides of the veins were collected, ground into mineral powder, and tested for carbon, oxygen, strontium isotopes and rare earth elements. The differences in isotopes and rare earth elements between the veins and the surrounding rocks were compared to analyze the sources of vein-forming materials during the growth of fracture veins; the distribution patterns and characteristic parameters of rare earth elements between the veins and the surrounding rocks were compared.

[0017] Furthermore, the comparison of the distribution pattern and characteristic parameter differences of rare earth elements between the vein and the surrounding rock includes:

[0018] If the difference in isotopes and rare earth elements between the fracture vein body and the surrounding rock is small, it means that the fracture vein-forming material comes from the surrounding rock on both sides of the fracture, and the fracture vein-forming material has undergone short-distance migration. When the fracture is formed, the shale system is not in communication with the outside world, and the shale has good sealing properties. If the difference in isotopes and rare earth elements between the fracture vein body and the surrounding rock is large, it indicates that the fracture vein-forming material has undergone long-distance migration, the fluid inside the shale is in communication with the outside, and the shale has poor sealing properties.

[0019] Furthermore, rare earth elements include 15 lanthanide elements from La to Lu, and can be divided into light rare earth elements, medium rare earth elements and heavy rare earth elements according to atomic number from small to large; light rare earth elements include La, Ce, Pr and Nd, medium rare earth elements include Sm, Eu, Gd, Tb, Dy and Ho, and heavy rare earth elements include from Er to Lu.

[0020] Furthermore, the rare earth element distribution pattern can be expressed by the content of a single element. If Pr N / Tb N >1, indicating that light rare earth elements are enriched relative to medium rare earth elements, and vice versa, indicating that light rare earth elements are depleted relative to medium rare earth elements; if Pr N / Yb N >1, indicating that the medium rare earth elements are enriched relative to the heavy rare earth elements, and vice versa, indicating that the medium rare earth elements are depleted relative to the heavy rare earth elements; if Tb N / Yb N >1, indicating that the medium rare earth elements are enriched relative to the heavy rare earth elements, and vice versa, indicating that the medium rare earth elements are depleted relative to the heavy rare earth elements; N , Tb N , Yb N The values ​​of Pr, Tb, and Yb are normalized by the Australian Post-Archaean shales or North American shales.

[0021] Furthermore, the characteristic parameters include: total rare earth element ∑REE, Ce anomaly value, Eu anomaly value and Y anomaly value;

[0022] When Ce is abnormal, δCe=Pr N +(Pr N -Nd N ); When Eu is abnormal, δEu=Sm N ×(Sm N / Nd N ) 1 / 2 ; When Y is abnormal, δY=(0.5Dy N +0.5Ho N ), where δCe, δEu, and δY represent the Ce, Eu, and Y abnormal values, respectively. N 、Nd N 、Sm N 、DyN 、Ho N are the values ​​of Pr, Nd, Sm, Dy, and Ho normalized by the Australian Post-Archaean shale or North American shale.

[0023] Furthermore, the step 4 includes:

[0024] Comprehensively analyze the intensity of fracture development, the stages of fracture formation and evolution, and the history of shale hydrocarbon generation and expulsion to clarify the relative time relationship between the formation period of different types of fractures and the period of shale gas generation; combine the source of fluids when fracture-filling minerals are formed to judge the closure of the shale system, and comprehensively judge the impact of fractures of different types and stages on the gas content of shale.

[0025] Furthermore, the basis for comprehensive judgment is as follows: ① If the fractures are formed before the shale gas generation stage and the filling degree is high, the fractures have almost no effect on the shale gas production capacity; ② If the fractures are formed during the shale gas generation stage or the gas reservoir adjustment stage, and the source of the vein-forming fluid is exogenous fluid, the fractures are connected to the outside world, and the shale gas migrates a long distance along the fractures, serving as shale gas loss channels. The development of fractures will destroy the sealing of the shale gas reservoir. The larger the effective fracture development index, the stronger the destructive effect on the gas reservoir, the lower the shale gas content, and the lower the production capacity; ③ If the fractures are formed during the shale gas generation stage or the gas reservoir adjustment stage, and the source of the vein-forming fluid is endogenous fluid, such fractures are not connected to the outside world at the time of formation. The development of fractures can provide a large amount of storage space, which is conducive to the enrichment of shale gas. The larger the effective fracture development intensity index, the stronger the destructive effect on the gas reservoir, the higher the shale gas content, and the greater the production capacity.

[0026] Due to the adoption of the above technical solution, the present invention has the following advantages:

[0027] 1. The effective fracture development intensity index developed in this paper simultaneously considers the impact of natural fracture type and fracture filling degree on shale gas. This allows for quantitative evaluation of the development intensity of different fracture types while also assessing changes in fracture effectiveness. Therefore, it more accurately reflects the storage and migration capacity of different fracture types for shale gas. Whether natural fractures affect shale gas after formation depends on the spatial and temporal alignment between the fracture formation period and the shale gas generation period, while the extent of this impact is determined by the development intensity of the effective fractures and the containment of the shale gas system at the time of fracture formation.

[0028] 2. This comprehensive evaluation method, developed by comprehensively considering the spatiotemporal relationship between fracture formation and shale gas generation, the closure of the shale system at the time of fracture formation, and the intensity of effective fracture development, can more accurately assess the impact of different fracture types and stages on shale gas content. Compared with existing technologies, this method offers diverse technical approaches, strong scientific and systematic research, and reliable research results, making it highly transferable. The research results can provide guidance for shale gas exploration and development in different regions, strata, and types, and provide technical support and geological basis for the exploration and development of other unconventional oil and gas resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments described in the embodiments of the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0030] Figure 1 Flowchart for comprehensive evaluation of the impact of natural fractures on shale gas content;

[0031] Figure 2 Development characteristics of different types of fractures in the Wufeng Formation-Longmaxi Formation;

[0032] Figure 3 A diagram to judge the sequence of fracture formation based on the cutting relationship of natural fracture veins;

[0033] Figure 4 Image of liquid oil inclusions developed in dolomite-filled network fractures;

[0034] Figure 5 Pure gas-phase methane inclusions developed in quartz-filled bedding-parallel fractures and the laser Raman spectrum of methane inclusions;

[0035] Figure 6 Burial history of the Wufeng-Longmaxi shale in a block of the Sichuan Basin and the formation stages of different types of fractures. DETAILED DESCRIPTION

[0036] The present invention will be further described with reference to the accompanying drawings and embodiments. The embodiments described are only a part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by those skilled in the art should fall within the scope of protection of the embodiments of the present invention.

[0037] See also Figure 1 The present invention provides an embodiment of a comprehensive evaluation method for the influence of natural fractures on the gas-bearing property of shale, which comprises:

[0038] Step 1: Classify and count the development characteristics of natural fractures and establish the shale effective fracture development intensity index;

[0039] Macroscopically, fractures are classified into j types through core observation. The top depth, bottom depth, inclination, aperture, and filling degree of each fracture on the core are statistically analyzed, and the effective fracture intensity index (EFII) of each fracture type is calculated:

[0040]

[0041] n is the total number of cracks; H i2 is the top depth of the i-th crack; H i1 is the bottom depth of the i-th crack; θ i is the inclination angle of the i-th crack; D i is the opening of the i-th fracture; H3 is the core length; E i is the filling degree of the i-th crack, which is divided into unfilled (1), partially filled (0.8), half filled (0.5) and fully filled (0); EFII j Represents the effective crack development intensity index of the j-th crack.

[0042] Step 2: Divide the fracture stages and clarify the relative relationship between the formation period of different types of fractures and the period of shale gas generation;

[0043] Thin sections were prepared from different types of fractures and veins filling the same type of fractures from different periods. The vein mineral composition (such as calcite, quartz, pyrite, and organic matter) was determined using microscopy and cathodoluminescence. The vein cutting relationships and microstructural characteristics (crystal morphology, grain size, growth pattern, and mineral filling sequence) were observed to determine the order of fracture formation. Laser Raman spectroscopy and inclusion microthermometry were then used to measure the composition, homogenization temperature, freezing point, and salinity of the fluid inclusions within the fracture-filling veins. Furthermore, combined with the burial and thermal evolution history of the formation, the relative relationship between the formation period of different types and periods of fractures and the period of shale gas generation was determined.

[0044] Step 3: Determine the sealing properties of the shale system when different types and stages of fractures are formed;

[0045] Samples of fracture veins of different types and periods and the surrounding rocks on both sides of the veins were collected and ground into mineral powder. Carbon, oxygen, strontium isotope and rare earth element tests were carried out. The differences in isotopes and rare earth elements between the veins and the surrounding rocks were compared to analyze the source of the vein-forming materials during the growth of the fracture veins.

[0046] Rare earth elements (REEs) include the 15 lanthanide elements from La to Lu. Based on atomic number, REEs can be divided into light rare earth elements (LREEs, La, Ce, Pr, and Nd), medium rare earth elements (MREEs, Sm, Eu, Gd, Tb, Dy, and Ho), and heavy rare earth elements (HREEs, Er to Lu). The analytical results were standardized using the North American Shale Values ​​(NASC) (Haskin et al., 1968) to further compare the REE distribution patterns and characteristic parameters between the veins and the surrounding rocks.

[0047] The rare earth element distribution pattern can be expressed by the content of a single element. If Pr N / Tb N >1, indicating that LREE is enriched relative to MREE, and vice versa, indicating that LREE is depleted relative to MREE; if Pr N / Yb N >1, indicating that MREE is enriched relative to HREE, and vice versa, indicating that MREE is depleted relative to HREE. N / Yb N >1, indicating that MREE is enriched relative to HREE, and vice versa, indicating that MREE is depleted relative to HREE. N , Tb N , Yb N are the values ​​of Pr, Tb, and Yb normalized by the Post-Archaean Australian Shale (PAAS) or the North American Shale (NASC), respectively.

[0048] Among them, the characteristic parameters include: total rare earth element ∑REE, Ce anomaly value, Eu anomaly value and Y anomaly value;

[0049] When Ce is abnormal, δCe=Pr N +(Pr N -NdN); when Eu is abnormal, δEu=Sm N ×(Sm N / Nd N ) 1 / 2 ; When Y is abnormal, δY=(0.5Dy N +0.5Ho N ), where δCe, δEu, and δY represent the Ce, Eu, and Y abnormal values, respectively. N 、Nd N 、Sm N 、Dy N 、Ho N are the values ​​of Pr, Nd, Sm, Dy, and Ho normalized by the Post-Archaean Australian Shale (PAAS) or the North American Shale (NASC), respectively.

[0050] If the difference in isotopes and rare earth elements between the fracture vein body and the surrounding rock is small, it indicates that the fracture vein-forming material originated from the surrounding rock on both sides of the fracture, and the fracture vein-forming material migrated over a short distance. When the fracture was formed, the shale system was not connected to the outside world, and the shale has good sealing properties. If the difference in isotopes and rare earth elements between the fracture vein body and the surrounding rock is large, it indicates that the fracture vein-forming material migrated over a long distance, the fluid inside the shale communicated with the outside world, and the shale has poor sealing properties.

[0051] Step 4: Comprehensively identify the impact of different types and stages of fractures on shale gas content.

[0052] A comprehensive analysis of fracture development intensity, fracture formation and evolution stages, and shale hydrocarbon generation and expulsion history clarified the relative temporal relationship between the formation period of different fracture types and the period of shale gas generation. Furthermore, the source of fluids during fracture filling mineral formation was used to determine the sealing capacity of the shale system, comprehensively elucidating the impact of different fracture types and stages on shale gas content. The criteria for this distinction are as follows: ① If fractures form before the shale gas generation stage and are highly filled, they have little impact on shale gas production capacity. ② If fractures form during the shale gas generation stage or reservoir adjustment stage, and the source of the veining fluid is exogenous, the fractures communicate with the outside world, shale gas migrates long distances along the fractures, and they serve as dissipation pathways for shale gas. The development of fractures can compromise the sealing capacity of shale gas reservoirs. A higher effective fracture development index indicates a greater destructive effect on the gas reservoir, lower shale gas content, and lower production capacity. ③ If fractures are formed during the shale gas generation stage or the gas reservoir adjustment stage, and the source of the vein-forming fluid is endogenous fluid, such fractures are not connected with the outside world when they are formed. The development of fractures can provide a large amount of storage space, which is conducive to the enrichment of shale gas. The greater the effective fracture development intensity index, the stronger the destructive effect on the gas reservoir, and the higher the shale gas content, the greater the production capacity.

[0053] For ease of understanding, the present invention provides a more specific embodiment:

[0054] Taking the marine shale of the Longmaxi Formation in a block of the Sichuan Basin as an example, the shale fractures are divided into four types through core observation: network fractures, bedding fractures, stratum-controlled fractures, and translaminar fractures. Among them, the network fractures are irregular in shape, have no organization system, and have no direction ( Figure 2 A in the stratum); bedding fractures refer to fractures that are nearly parallel to the stratum or intersect at a small angle ( Figure 2 B in the stratum); Intralayer cracks refer to cracks that develop inside the stratum and end at both ends of the cracks at the layer level. Smaller cracks ( Figure 2 C in the stratum); translaminar fractures refer to fractures that cut through the stratum vertically and are larger in scale ( Figure 2 The top depth, bottom depth, dip angle, aperture, and filling degree of each fracture on the core are statistically analyzed, and the effective fracture intensity index (EFII) of four types of fractures is calculated respectively:

[0055]

[0056] n is the total number of cracks; H i2 is the top depth of the i-th crack; H i1 is the bottom depth of the i-th crack; θ i is the inclination angle of the i-th crack; D i is the opening of the i-th fracture; H3 is the core length; E i is the filling degree index of the i-th crack, which is divided into unfilled (1), partially filled (0.8), half filled (0.5), and fully filled (0). j = 1, 2, 3, 4. FII1 represents the intensity of cross-layer crack development; FII2 represents the intensity of intra-layer crack development; FII3 represents the intensity of parallel-bedding crack development; and FII4 represents the intensity of network crack development.

[0057] Secondly, the relationship between the fracture filling vein cutting was studied by microscope and cathode luminescence (see Appendix Figure 3 ) and growth patterns, and the formation stages of the cracks were divided. Through inclusion microthermometry and laser Raman spectroscopy analysis, the paleotemperature characteristics of the formation of different types and stages of cracks were restored, and the formation time of different types and stages of cracks was further determined by combining the burial history and thermal history of the formation. The homogenization temperature of the network crack inclusions in the study area is between 140℃ and 160℃, and liquid oil inclusions (attached) are developed in the veins. Figure 4 ), the cracks were formed during the oil generation stage; the homogenization temperature of the bedding fracture inclusions was between 190℃ and 210℃, and pure gas-phase methane inclusions were developed inside the vein body (attached Figure 5 ), indicating that the fractures were formed at the stage of hydrocarbon cracking and dry gas generation near the maximum burial depth; the homogenization temperature of the inclusions of intra-layer fractures and trans-layer fractures is between 120℃~140℃ and 160℃~180℃, and they were formed during the stage of tectonic compression and uplift (Appendix Figure 5 , attached Figure 6 ).

[0058] Further carbon, oxygen, strontium isotope, and rare earth element testing of fracture-filling minerals and surrounding rocks on either side of the fractures was conducted to determine the source of the material filling the fracture veins and analyze the sealing capacity of the shale system at the time of fracture formation. The results show that translaminar fractures primarily form during the tectonic compression and uplift phase and after the period of significant shale gas generation. Furthermore, the vein-forming material primarily originates from external fluids. After fracture formation, they communicate with the external environment, leading to shale gas loss. A higher effective fracture development intensity index indicates a greater destructive effect on shale gas preservation conditions. Bedding-parallel and intralaminar fractures primarily form during the period of significant shale gas generation and tectonic compression and uplift. The vein-forming material originates from the surrounding rocks on either side, and fracture formation does not compromise shale gas preservation conditions. A higher effective fracture development intensity index indicates a higher shale gas enrichment. Network fractures form during the oil generation phase and are filled with minerals. Fracture effectiveness is poor, and the source of the vein-forming fluid is primarily endogenous, resulting in a low gas-bearing capacity in the shale.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A comprehensive evaluation method for the influence of natural fractures on shale gas content, characterized in that: include: Step 1: Macroscopically, shale fracture types are classified by observing cores, the development characteristics of different fracture types are statistically analyzed, and effective fracture development intensity indices for different fracture types are established; Step 2: Microscopically, use a microscope and cathodoluminescence to observe the microstructure and cutting relationship of the fracture-filling veins, classify the fracture formation stages, and determine the inclusion composition through laser Raman spectroscopy and inclusion microthermometry. Reconstruct the paleotemperature characteristics of different types and stages of fracture formation, and combine the burial history and thermal history of the formation to determine the relative relationship between the formation period of different types of fractures and the period of shale gas generation. Step 3: Conduct carbon, oxygen, strontium isotope, and rare earth element testing on the fracture-filling minerals and the surrounding rocks on both sides of the fracture to determine the source of the fracture-filling veins and analyze the sealing capacity of the shale system when the fractures were formed; Step 4: Comprehensively identify the impact of different types and stages of fractures on shale gas content.

2. The method according to claim 1, characterized in that The step 1 comprises: Macroscopically, fractures are divided into j types by observing the cores. The top depth, bottom depth, inclination, aperture, and filling degree of each fracture on the cores are statistically analyzed, and the effective fracture development intensity index of each fracture type is calculated using the following formula: Among them, EFII j is the effective crack development intensity index of the jth crack, n is the total number of cracks, H i2 is the top depth of the i-th crack, H i1 is the bottom depth of the i-th crack, θ i is the inclination angle of the i-th crack, D i is the opening of the i-th fracture, H3 is the core length, E i is the filling degree of the i-th crack, which can be divided into unfilled, partially filled, half filled and fully filled.

3. The method according to claim 1, characterized in that The step 2 includes: The filling veins of different types of fractures and fractures of the same type but different periods on the core were collected and made into thin sections. The mineral composition of the veins was determined by microscopy and cathode luminescence. The cutting relationship and microstructural characteristics of the veins were observed to determine the order of fracture formation. The microstructural characteristics include crystal morphology, grain size, growth mode and mineral filling order. On this basis, the fluid inclusion composition, homogenization temperature, freezing point temperature and salinity in the fracture filling veins were tested by laser Raman spectroscopy and inclusion microthermometry. Combined with the burial history and thermal evolution history of the formation, the relative relationship between the formation period of different types and different periods of fractures and the period of shale gas generation was determined.

4. The method according to claim 1, wherein The step 3 includes: Samples of fracture veins of different types and periods and the surrounding rocks on both sides of the veins were collected, ground into mineral powder, and tested for carbon, oxygen, strontium isotopes and rare earth elements. The differences in isotopes and rare earth elements between the veins and the surrounding rocks were compared to analyze the sources of vein-forming materials during the growth of fracture veins; the distribution patterns and characteristic parameters of rare earth elements between the veins and the surrounding rocks were compared.

5. The method according to claim 4, characterized in that The comparison of the distribution pattern and characteristic parameter differences of rare earth elements between the vein and the surrounding rock includes: If the difference in isotopes and rare earth elements between the fracture vein body and the surrounding rock is small, it means that the fracture vein-forming material comes from the surrounding rock on both sides of the fracture, and the fracture vein-forming material has undergone short-distance migration. When the fracture is formed, the shale system is not in communication with the outside world, and the shale has good sealing properties. If the difference in isotopes and rare earth elements between the fracture vein body and the surrounding rock is large, it indicates that the fracture vein-forming material has undergone long-distance migration, the fluid inside the shale is in communication with the outside, and the shale has poor sealing properties.

6. The method according to claim 4 or 5, characterized in that Rare earth elements include 15 lanthanide elements from La to Lu. According to their atomic numbers, rare earth elements can be divided into light rare earth elements, medium rare earth elements and heavy rare earth elements. Light rare earth elements include La, Ce, Pr and Nd, medium rare earth elements include Sm, Eu, Gd, Tb, Dy and Ho, and heavy rare earth elements include Er to Lu.

7. The method according to claim 6, characterized in that The rare earth element distribution pattern can be expressed by the content of a single element. If Pr N / Tb N >1, indicating that light rare earth elements are enriched relative to medium rare earth elements, and vice versa, indicating that light rare earth elements are depleted relative to medium rare earth elements; if Pr N / Yb N >1, indicating that the medium rare earth elements are enriched relative to the heavy rare earth elements, and vice versa, indicating that the medium rare earth elements are depleted relative to the heavy rare earth elements; if Tb N / Yb N >1, indicating that the medium rare earth elements are enriched relative to the heavy rare earth elements, and vice versa, indicating that the medium rare earth elements are depleted relative to the heavy rare earth elements; N , Tb N , Yb N The values ​​of Pr, Tb, and Yb are normalized by the Australian Post-Archaean shales or North American shales.

8. The method according to claim 7, characterized in that Characteristic parameters include: total rare earth element ∑REE, Ce anomaly value, Eu anomaly value and Y anomaly value; When Ce is abnormal, δCe=Pr N +(Pr N -Nd N ); When Eu is abnormal, δEu=Sm N ×(Sm N / Nd N ) 1 / 2 ; When Y is abnormal, δY=(0.5Dy N +0.5Ho N ), where δCe, δEu, and δY represent the Ce, Eu, and Y abnormal values, respectively. N 、Nd N 、Sm N 、Dy N 、Ho N are the values ​​of Pr, Nd, Sm, Dy, and Ho normalized by the Australian Post-Archaean shale or North American shale.

9. The method according to claim 1, characterized in that The step 4 comprises: Comprehensively analyze the intensity of fracture development, the stages of fracture formation and evolution, and the history of shale hydrocarbon generation and expulsion to clarify the relative time relationship between the formation period of different types of fractures and the period of shale gas generation; combine the source of fluids when fracture-filling minerals are formed to judge the closure of the shale system, and comprehensively judge the impact of fractures of different types and stages on the gas content of shale.

10. The method according to claim 9, characterized in that The basis for comprehensive judgment is as follows: ① If fractures are formed before the shale gas generation stage and are highly filled, then the fractures have almost no impact on shale gas production capacity; ② If fractures are formed during the shale gas generation stage or the gas reservoir adjustment stage, and the source of the vein-forming fluid is exogenous fluid, the fractures are connected to the outside world, shale gas migrates long distances along the fractures, and they serve as dissipation channels for shale gas. The development of fractures will destroy the sealing of the shale gas reservoir. The larger the effective fracture development index, the stronger the destructive effect on the gas reservoir, the lower the shale gas content, and the lower the production capacity; ③ If fractures are formed during the shale gas generation stage or the gas reservoir adjustment stage, and the source of the vein-forming fluid is endogenous fluid, such fractures are not connected to the outside world at the time of formation. The development of fractures can provide a large amount of storage space, which is conducive to the enrichment of shale gas. The larger the effective fracture development intensity index, the stronger the destructive effect on the gas reservoir, the higher the shale gas content, and the greater the production capacity.

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