A method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shales

Abstract

The present invention provides a method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale. The method of the present invention comprises the steps of: obtaining shale samples by coring under pressure; quickly crushing the shale samples to 20-60 meshes, placing them in TD-GC for component analysis, and obtaining the molecular composition of light components of free oil in connected pores; crushing the shale samples to 2.5-5 meshes, and extracting them with n-hexane and dichloromethane in turn; then crushing the residual shale samples to 60-80 meshes, and extracting them with dichloromethane; finally, crushing the residual shale samples to 200-250 meshes, and extracting them with a mixed solvent of dichloromethane and methanol; and performing molecular composition analysis on the obtained extracts. The method of the present invention takes into account the loss of light components, greatly improving the reliability and persuasiveness of the evaluation results; not only the molecular composition of shale oil in different occurrence states is clarified, but also the differences in the molecular composition of shale oil in different pore spaces are clarified, so as to realize the systematic evaluation of shale oil components.

Description

Technical Field

[0001] The invention relates to a method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale, and belongs to the technical field of oil and gas exploration. Background Art

[0002] The molecular composition of shale oil is not only the basic parameter for conducting research on shale oil water-rock interaction, shale oil microscopic occurrence state, fluid phase characteristics under nano-confinement effect, and seepage mechanism, but also an indispensable data for shale oil resource evaluation and development plan optimization. Due to the complex shale pore structure, the large number of nanopores, the strong interaction between fluid and pore wall (including minerals and organic matter), and the particularity of shale oil retention and enrichment process, the molecular composition of shale oil in different pore spaces and different occurrence states shows obvious differences.

[0003] The shale core is extracted with different particle sizes and different organic solvents (this technology is called graded extraction or step-by-step extraction), and the obtained extracts are subjected to organic geochemical analysis after a series of steps such as concentration and constant weight. This is a common method for obtaining shale oil components in different occurrence states (Qian et al., 2017; Pan et al., 2018). However, due to the presence of gaseous hydrocarbons (C 1 ~C 5 ) and light hydrocarbons (C 6 ~C 14 ) is lost in large quantities, resulting in this method obtaining mainly the heavy components (C 15+ ), gaseous hydrocarbons and light hydrocarbon components that are considered to have a high mobile ratio cannot be evaluated, and the evaluation results are very different from the in-situ conditions. The gaseous hydrocarbon content of the high-mature shale powder sample decreased by 91.02% after 2 minutes of storage, and nC 8Almost all of them are lost, the total hydrocarbon content is reduced by 76.92%, and the carbon number of residual hydrocarbons is greater than 9. Therefore, the loss of light components must be considered when evaluating the components of medium- and high-maturity shale oil (Li Ming et al., 2023). Although patent CN115856267A provides a method for analyzing the properties of shale oil in pores with different connectivity, it does not consider the loss of light components and only evaluates the residual oil with limited contribution to production capacity. The practical significance is insufficient, and this method is not suitable for medium- and high-maturity shale oil. In addition, existing research results have not linked the extracts to the pores where shale oil is retained. The differences in the pore spaces where different extracts are retained have not been clarified, and it is impossible to effectively guide the optimization of shale oil sweet spots and the formulation of development plans, nor can it support the revelation of the formation and enrichment process of shale oil. Due to the high degree of thermal evolution of medium- and high-maturity shales, organic matter is cracked to generate a large amount of gaseous hydrocarbons and light hydrocarbons. During conventional coring, the light components are rapidly lost due to changes in temperature and pressure. In addition, light components inevitably volatilize during the indoor placement, storage and pre-experimental pretreatment of the cores. In addition, the cores need to be flushed with heated organic solvents in the graded extraction experiment, which further aggravates the loss of light components. Therefore, the loss of light components must be considered when evaluating the components of medium- and high-maturity shale oil.

[0004] For the restoration of molecular composition of light components of shale oil, the current methods include e-index fitting method, produced oil comparison method and empirical formula method. Among them, the relationship between the molar amount and carbon number of the single e-index fitting often has a large deviation, and the use of double-stage or multi-stage fitting makes the prediction of model coefficients more complicated and not very operable; the premise of using the produced oil comparison method is to assume that the produced oil is consistent with the shale oil composition under the in-situ conditions, but in the process of shale oil flowing from the matrix through the artificial fracture network to the wellhead, the pressure continues to decrease, and a large amount of gaseous hydrocarbons are precipitated. In addition, the shale itself has a "molecular sieve" effect, and the component differentiation phenomenon is particularly obvious, resulting in a significant difference between the produced oil and the shale oil composition under the in-situ conditions, that is, the produced oil cannot truly reflect the component characteristics of shale oil under the in-situ formation conditions; the molecular composition of the light components of shale oil is closely related to the type of organic matter, the degree of thermal evolution and the history of hydrocarbon generation and expulsion. The shale oil composition in different basins and different layers is obviously different. The empirical formula method has shortcomings and limited applicability.

[0005] Therefore, it is of great significance to develop a method to evaluate the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale. The in-situ conditions emphasized in the present invention are the physical and chemical states of shale naturally existing underground, including the real pressure, temperature, geostress and pore fluid characteristics of the formation, which directly affect the composition and occurrence state of shale oil. At present, the evaluation of the components of medium- and high-maturity shale oil is carried out under ground conditions. The temperature and pressure of the samples are greatly reduced compared with the formation conditions. If the influence of the loss of light components is not considered, the evaluation results are very different from the actual values, and it is impossible to truly guide the exploration and development of shale oil. The present invention combines pressure coring with graded extraction experiments for the first time to establish a method for evaluating shale oil components in different occurrence states under in-situ conditions of medium- and high-maturity shale. This method takes into account the loss of light components and directly uses pressure coring samples to obtain the molecular composition of light components in shale oil, rather than using traditional methods such as e-index fitting, comparison of produced oil components and empirical formulas, which greatly improves the reliability and persuasiveness of the evaluation results. It not only clarifies the molecular composition of shale oil in different occurrence states, but also clarifies the differences in the molecular composition of shale oil in different pore spaces, and realizes a systematic evaluation of shale oil components. By revealing the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale, including the molecular composition of free oil in connected pores, adsorbed oil in connected pores, adsorbed oil in confined pores, and adsorbed oil in closed pores, it can not only provide basic data for the study of oil-water-rock interaction, micro-nano pore fluid seepage laws, and shale oil development plans, but also help deepen the understanding of shale oil formation and enrichment process, and then reveal the enrichment mechanism of continental shale oil, serving the exploration and development of shale oil.

[0007] The technical solution of the present invention is as follows:

[0008] A method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale, comprising the steps of:

[0009] (1) Obtain shale samples by coring under pressure and store them in liquid nitrogen in a sealed container;

[0010] (2) The shale samples cored under pressure are quickly crushed to 20-60 meshes and placed in a TD-GC (thermal desorption-gas chromatography) instrument for component analysis to obtain the molecular composition of the light components of free oil in the connected pores;

[0011] (3) The shale sample was crushed to 2.5-5 mesh, and the shale sample was first extracted with n-hexane. The extract solution was allowed to stand at room temperature until the organic solvent was completely volatilized to obtain Extract A. Subsequently, the residual shale sample after extraction was extracted with dichloromethane. The extract solution was allowed to stand at room temperature until the organic solvent was completely volatilized to obtain Extract B. The residual shale sample after extraction was then crushed to 60-80 mesh, and extracted with dichloromethane again. The extract solution was allowed to stand at room temperature until the organic solvent was completely volatilized to obtain Extract C. Finally, the residual shale sample after extraction was crushed to 200-250 mesh, and extracted with a mixed solvent of dichloromethane and methanol. The obtained extract solution was allowed to stand at room temperature until the organic solvent was completely volatilized to obtain Extract D. The above-obtained extracts were subjected to molecular composition analysis, thereby obtaining the composition of shale oil molecules under different occurrence states of medium- and high-maturity shale in situ conditions.

[0012] According to the present invention, in step (1), coring under pressure is the best method to obtain in-situ fluid data of the formation.

[0013] Preferably, in step (2), the crushing process is controlled within 15 seconds, and liquid nitrogen is sprayed on the shale sample during the crushing process to maintain low temperature and prevent the loss of light components in the shale sample. The cutting, packaging, and removal, crushing, and weighing of the pressure-maintained coring shale sample should be carried out quickly and should be kept in an ultra-low temperature environment, for example, by continuously spraying liquid nitrogen to prevent the volatilization of the fluid in the shale sample.

[0014] Preferably, in step (2), in the TD-GC test, the thermal desorption temperature is 280-320°C, and the heating time is 2-4 minutes. The thermally evaporated components during the heating process represent the light components of free oil in the interconnected pores of the shale. The TD-GC test can obtain the composition of the light components of free oil in the interconnected pores.

[0015] According to the preferred embodiment of the present invention, in step (3), the extraction method is Soxhlet extraction; the Soxhlet extraction method can be carried out according to the existing method, which is as follows: the shale sample is wrapped with a filter paper tube and placed in the sample chamber of the Soxhlet extractor, and an organic solvent and a copper sheet are added to the bottom of the bottle, and the amount of organic solvent added should be 1 / 2 to 2 / 3 of the volume capacity of the bottom bottle, and the organic solvent is heated until it boils, and then the extraction is started. The heating temperature should not be too high to prevent the organic solvent from excessively boiling and volatilizing.

[0016] Preferably according to the present invention, in step (3), during the process of obtaining extracts A, B and C, the extraction time is 6-10 hours; during the process of obtaining extract D, the extraction time is greater than or equal to 36 hours, preferably 36 to 45 hours.

[0017] Preferably according to the present invention, in step (3), in the mixed solvent of dichloromethane and methanol, the volume ratio of dichloromethane to methanol is 8:1 to 9.5:1, preferably 9:1.

[0018] Preferably, in step (3), extract A comes from the heavy components of free oil in the connected pores, extract B comes from the adsorbed oil in the connected pores, extract C comes from the adsorbed oil in the restricted pores, and extract D comes from the adsorbed oil in the closed pores. Shale nanopores are developed in large quantities and have poor pore connectivity. The pore walls have a strong adsorption effect on crude oil. According to the shale pore structure and shale oil occurrence characteristics, shale pores are divided into connected pores, restricted pores and closed pores. The crude oil retained in the connected pores can flow in the connected pores and has good mobility. It is an important component of the oil produced under the existing development process. Under the combined influence of hydrocarbon generation and diagenesis, some pore throats in shale become smaller, and the strong interaction between the pore walls and hydrocarbon fluids makes it difficult for the crude oil retained in the pores to flow effectively under in-situ conditions. This type of pore is called a restricted pore. A closed pore refers to a completely isolated pore, that is, the pore throat coordination number is 0, and the crude oil retained in it cannot flow. From connected pores to restricted pores and then to closed pores, the pore connectivity gradually deteriorates and the pore diameter generally tends to decrease gradually.

[0019] Preferably, according to the present invention, in step (3), the molecular composition analysis method is one or a combination of two or more of total hydrocarbon gas chromatography analysis, chromatography-mass spectrometry analysis, stable isotope analysis or Fourier transform ion cyclotron resonance mass spectrometry analysis.

[0020] The technical features and beneficial effects of the present invention are as follows:

[0021] 1. The present invention targets medium- and high-maturity shale oil layers, takes into account the loss of light components, combines pressure-maintained coring with graded extraction experiments for the first time, and establishes a method for evaluating the molecular composition of shale oil in different occurrence states and different pore spaces of medium- and high-maturity shale under in-situ conditions. The method clarifies the components of free oil in the connected pores of shale, adsorbed oil in the connected pores, adsorbed oil in the confined pores, and adsorbed oil in the closed pores under in-situ conditions, comprehensively and systematically reveals the molecular composition of shale oil in different occurrence states and different pore spaces, and innovatively solves the problem that the prior art cannot obtain light components in the connected pores of shale and cannot evaluate the shale oil components in different pore spaces.

[0022] 2. For medium-high maturity shale oil layers, the molecular composition of light components of free oil in the connected pores of medium-high maturity shale can be obtained by pressure-maintaining coring (C 1 ~C 15 ), rather than using traditional methods such as e-index fitting, oil production component comparison and empirical formula, this method is more accurate and greatly improves the reliability and persuasiveness of the evaluation results. The extracts obtained by graded extraction can successively obtain the molecular composition of the heavy components of free oil in the shale interconnected pores (C 15+), the composition of adsorbed oil molecules in connected pores, the composition of adsorbed oil molecules in confined pores and the composition of adsorbed oil molecules in closed pores, thereby obtaining the composition of shale oil molecules under different occurrence states of in-situ conditions of medium- and high-maturity shale.

[0023] 3. The method of the present invention can comprehensively and effectively evaluate the composition of shale oil molecules in different occurrence states under in-situ conditions of medium- and high-maturity shale. The evaluation results are accurate and can accurately analyze the composition of shale oil molecules under in-situ conditions of medium- and high-maturity shale. The shale oil components obtained by the method of the present invention can not only provide basic data for the study of oil-water-rock interaction, the flow law of shale micro- and nano-pore fluids, and shale oil development plans, but also deepen the understanding of shale oil retention and enrichment processes, and further reveal the enrichment mechanism of continental shale oil, serving the exploration and development of shale oil.

[0024] 4. According to the characteristics of shale oil components in different occurrence states and the interaction between shale oil and pore walls, the present invention selects n-hexane, dichloromethane, dichloromethane and dichloromethane + methanol mixed solvents in turn, and ensures that the polarity of the organic solvents used gradually increases in the experiment. If dichloromethane with a stronger polarity is selected first, all the heavy components of free oil and adsorbed oil in the connected pores will be extracted, and shale oil in different occurrence states cannot be distinguished. In addition, the particle size of shale crushing has an important influence on the experimental results. The larger the mesh size of the shale sample, the smaller the radius of the pores opened. When the shale sample is crushed to 2.5-5 mesh, organic solvents can enter most of the connected pores, and the diameter of the connected pores is greater than 500nm (mostly greater than 1μm); when the shale sample is crushed to 60-80 mesh, the restricted pores are opened, and the restricted pore diameters are mainly distributed in 100-500nm, and the restricted pores are mainly adsorbed oil; when the shale sample is crushed to 200-250 mesh, the closed pores in the shale are opened, and the closed pore diameters are generally less than 100nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a flow chart of the experimental evaluation of shale oil components in different occurrence states under in-situ conditions of high-maturity shale in the embodiment;

[0026] Figure 2 The thermal desorption-gas chromatogram (a) and the corresponding normal alkane content graph (b) of the pressure-maintained cored shale sample in Example 1;

[0027] Figure 3 The gas chromatogram of the total hydrocarbons of the extract A in Example 1 (a), the gas chromatogram of the total hydrocarbons of the extract B (b), the gas chromatogram of the total hydrocarbons of the extract C (c), and the gas chromatogram of the total hydrocarbons of the extract D (d).

[0028] Figure 4It is the proportion of the extractable components in the test example, including 2.5-5 mesh powder sample n-hexane extraction product (a), 2.5-5 mesh powder sample dichloromethane extraction product (b), 60-80 mesh powder sample dichloromethane extraction product (c). DETAILED DESCRIPTION

[0029] In order to better understand the present invention, it is further described below in conjunction with specific embodiments.

[0030] The experimental methods used in the examples are conventional methods unless otherwise specified. The materials, reagents, etc. used in the examples are all commercially available unless otherwise specified.

[0031] Example 1

[0032] A method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale, such as Figure 1 As shown, the steps include:

[0033] (1) The pressure-maintained coring shale samples obtained from the well site (the medium-to-high-maturity shale oil layer of the first member of the Qingshankou Formation in the Gulong Sag of the Songliao Basin, with a vitrinite reflectance of R o The value is about 1.55%) and is quickly placed in a liquid nitrogen barrel and sealed for storage;

[0034] (2) The shale sample stored in the liquid nitrogen barrel was quickly taken out, and the block core was quickly hit with a geological hammer to split it into two. A small piece of the sample was selected for the following operation, and the other piece was quickly placed in the liquid nitrogen barrel. The small piece of sample was quickly crushed to 20-60 mesh, and the crushing time was controlled within 15 seconds. During the crushing process, liquid nitrogen was sprayed into the shale sample to maintain a low temperature to prevent the light components in the shale sample from being lost. Then, 30-60 mg of powder sample was immediately weighed and placed in the crucible of the thermal desorption-gas chromatography to start the experiment. In the thermal desorption-gas chromatography experiment, the sample heating temperature was 300°C and the heating time was 3 minutes. The chromatograph model was Shimadzu GC-14B, and the chromatographic column model was HP-5petroleum50m×0.20mm×0.5μm. The above steps of taking out the core stored in the liquid nitrogen barrel, quickly hitting it with a geological hammer, crushing it, weighing it and placing it in the crucible of the thermal desorption-gas chromatograph should all be carried out quickly, with the total time controlled within 20 seconds, and it needs to be kept in an ultra-low temperature environment, for example, by continuously spraying liquid nitrogen to prevent the volatilization of the fluid in the shale sample.

[0035] Thermal release hydrocarbon chromatogram Figure 2 (a), the carbon number distribution in the chromatogram is C 1 ~C 23 , the corresponding n-alkane content is shown in Figure 2 (b) The gaseous hydrocarbon content is high. The hydrocarbon components of the thermal release of the pressure-maintained coring reflect the molecular composition of the light components of the free oil in the interconnected pores of the shale.

[0036] (3) The remaining shale sample was first crushed to 2.5-5 mesh, 100 g of the sample was weighed and placed in a Soxhlet extractor and extracted with n-hexane for 8 hours, and the filtrate and shale sample residue were obtained by filtration. The filtrate was allowed to stand at room temperature until the organic solvent evaporated, and the obtained soluble organic matter was recorded as extract A. Then the solvent was changed, and dichloromethane was used to continue to extract the above residual sample for 8 hours, and the filtrate and shale sample residue were obtained by filtration. The filtrate was allowed to stand at room temperature until the organic solvent evaporated, and the obtained soluble organic matter was recorded as extract B. The above residual sample was crushed to 60-80 mesh, and dichloromethane was used to extract for 8 hours, and the filtrate and shale sample residue were obtained by filtration. The filtrate was allowed to stand at room temperature until the organic solvent evaporated, and the obtained soluble organic matter was recorded as extract C. Finally, the residual sample was crushed to 200-250 mesh, and a more polar mixed solvent of dichloromethane and methanol (v:v=9:1) was selected for extraction for 36 hours, and the filtrate and the residual shale sample were filtered. The filtrate was allowed to stand at room temperature until the organic solvent evaporated, and the obtained soluble organic matter was recorded as extract D. The above extraction methods are all Soxhlet extraction; the Soxhlet extraction method is as follows: the shale sample is wrapped with a filter paper tube and placed in the sample chamber of the Soxhlet extractor, and the organic solvent and copper sheet are added to the bottom of the bottle. The amount of organic solvent added should be 1 / 2-2 / 3 of the volume of the bottom bottle, and the organic solvent is heated to boiling to start extraction. Subsequently, the above extracts were subjected to total hydrocarbon gas chromatography experiments to obtain the extract components. The chromatograph model used was Agilent 7890A gas chromatograph, the carrier gas was 99.999% nitrogen, the chromatographic column was HP-1 elastic quartz capillary column (60m×0.25mm×0.25m), and the carrier gas flow rate was constant 1ml / min.

[0037] The chromatogram of extract A obtained in the fractional extraction is attached. Figure 3 (a) represents the molecular composition of the heavy components of free oil in the interconnected pores of shale. It can be clearly seen from the chromatogram that n-alkanes with a carbon number less than 15 are lost to varying degrees, and n-alkanes with a carbon number less than 10 are almost all volatilized; the chromatogram of extract B is attached. Figure 3 (b) represents the composition of adsorbed oil molecules in the interconnected pores; the chromatogram of extract C is attached. Figure 3 (c), represents the molecular composition of the adsorbed oil in the restricted pores, and extract D represents the molecular composition of the adsorbed oil in the closed pores.

[0038] Test example

[0039] To further illustrate the importance of sample crushing particle size and organic solvent selection in the present invention, a medium-high maturity shale sample from the Qing section of Gulong Depression was selected and crushed to 2.5-5 mesh (2 portions) and 60-80 mesh (1 portion) for standby use. Two portions of 2.5-5 mesh powder samples were extracted with n-hexane and dichloromethane solvents, respectively, and one portion of 60-80 mesh powder sample was extracted with dichloromethane solvent. The extraction time was 8 hours. The extraction method was as described in Example 1, and the three extracts were separated by group components to obtain the contents of saturated hydrocarbons, aromatic hydrocarbons, colloids and asphaltene.

[0040] The experimental results show that for two 2.5-5 mesh powder sample extracts, the saturated hydrocarbon content of the product obtained by extraction with n-hexane solvent is 89.71%, and the colloid content is 3.05% ( Figure 4 a), while the proportion of saturated hydrocarbons in the product obtained by dichloromethane extraction dropped to 82.67%, and the gum content increased to 11.71% ( Figure 4 b), indicating that dichloromethane can extract the colloid components distributed in the interconnected pores. A large number of studies have shown that colloids mainly include sulfur-containing, oxygen-containing and nitrogen-containing compounds. Since the above compounds have a large molecular weight and contain a large number of polar functional groups, they are easily adsorbed on the surface of the pore wall and are the main components of the adsorbed oil. In other words, dichloromethane can extract a large amount of adsorbed oil. When obtaining free oil, a solvent with weak polarity must be used to avoid large amounts of adsorbed oil being extracted, otherwise the method will fail. Therefore, the present invention uses n-hexane solvent to extract the powder sample to obtain the heavy components of the free oil in the interconnected pores, and uses dichloromethane solvent to extract the powder sample to obtain the adsorbed oil components in the interconnected pores. Comparing the product components obtained by dichloromethane extraction of 2.5-5 mesh and 60-80 mesh powder samples, the proportion of saturated hydrocarbons in the extract of the 60-80 mesh powder sample decreased, and the proportion of aromatic hydrocarbons increased significantly ( Figure 4 c), indicating that when the sample is crushed to 60-80 mesh, "new" pores are opened, and the fluids in such pores are contacted with organic solvents and then extracted. The pores opened in this process are restricted pores. If the sample is crushed to too small a mesh, the restricted pores cannot be effectively opened; if the crushed particle size is too large, some closed pores may be released; if the mesh sizes of the two crushings are close, it is impossible to distinguish between different pore fluids. Therefore, the crushing mesh sizes of the present invention are 2.5-5 mesh, 60-80 mesh, and 200-250 mesh, respectively.

Claims

1. A method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale, comprising the steps of: (1) Obtain shale samples by coring under pressure and store them in liquid nitrogen in a sealed container; (2) The shale samples were quickly crushed to 20-60 mesh and placed in a TD-GC (thermal desorption-gas chromatography) instrument for component analysis to obtain the molecular composition of the light components of free oil in the interconnected pores; (3) The shale sample was crushed to 2.5-5 mesh, and firstly, the shale sample was extracted with n-hexane, and the extract solution was allowed to stand at room temperature until the organic solvent was completely volatilized to obtain Extract A; then, the residual shale sample after extraction was extracted with dichloromethane, and the extract solution was allowed to stand at room temperature until the organic solvent was completely volatilized to obtain Extract B; then, the residual shale sample after extraction was crushed to 60-80 mesh, and extracted with dichloromethane again, and the extract solution was allowed to stand at room temperature until the organic solvent was completely volatilized to obtain Extract C; finally, the residual shale sample after extraction was crushed to 200-250 mesh, and extracted with a mixed solvent of dichloromethane and methanol, and the obtained extract solution was allowed to stand at room temperature until the organic solvent was completely volatilized to obtain Extract D; the molecular composition of the above-obtained extracts was analyzed respectively, so as to obtain the composition of shale oil molecules under different occurrence states of medium- and high-maturity shale in situ conditions.

2. The method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale according to claim 1, characterized in that: In step (2), the crushing process is controlled within 15 seconds. During the crushing process, liquid nitrogen is sprayed on the shale sample to maintain a low temperature and prevent the light components in the shale sample from being lost.

3. The method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale according to claim 1, characterized in that: In step (2), in the TD-GC test, the thermal desorption temperature is 280~320°C and the heating time is 2~4 minutes.

4. The method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale according to claim 1, characterized in that: In step (3), the extraction method is Soxhlet extraction.

5. The method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale according to claim 1, characterized in that: In step (3), in the process of obtaining extracts A, B and C, the extraction time is 6 to 10 hours; in the process of obtaining extract D, the extraction time is greater than or equal to 36 hours.

6. The method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale according to claim 1, characterized in that: In step (3), in the mixed solvent of dichloromethane and methanol, the volume ratio of dichloromethane to methanol is 8:1 to 9.5:

1.

7. The method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale according to claim 1, characterized in that: In step (3), extract A comes from the heavy components of free oil in the interconnected pores, extract B comes from the adsorbed oil in the interconnected pores, extract C comes from the adsorbed oil in the restricted pores, and extract D comes from the adsorbed oil in the closed pores.

8. The method for evaluating the molecular composition of shale oil in different occurrence states under in-situ conditions of medium- and high-maturity shale according to claim 1, characterized in that: In step (3), the molecular composition analysis method is one or a combination of two or more of total hydrocarbon gas chromatography analysis, chromatography-mass spectrometry analysis, stable isotope analysis or Fourier transform ion cyclotron resonance mass spectrometry analysis.

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