A method for evaluating the composition and structure of paleolate lake communities

Abstract

The present invention provides a method for evaluating the composition and structure of ancient lake biomes, relating to the fields of paleoecology and oil and gas geology, and comprising the following steps: S1, obtaining two rock samples from lacustrine fine-grained sedimentary rocks; S2, performing kerogen separation and enrichment on one rock sample, and determining the major categories of ancient lake organisms under microscopic observation; S3, based on the major categories of organisms, performing gas chromatography, gas chromatography-mass spectrometry, and isotope mass spectrometry analysis on the other rock sample to identify the composition and content of biomarker compounds with biogenic significance and to identify the organism type; S4, based on the preservation efficiency of the identified biomarker compounds in the environment and the quantitative generation relationship between the biomarker compounds and their parent sources, restoring the original content of the compounds and determining the structure of the ancient lake biomes. The present invention has the following beneficial effects: accurate conclusions, high feasibility, and solves the problems of multi-source interpretation of the same type of biomarker compounds and restoration of the original content of the biomarkers.

Description

Technical Field

[0001] The present invention relates to the technical field of paleoecology and oil and gas geology, and in particular to a method for evaluating the composition and structure of ancient lake biological communities. Background Art

[0002] Fine-grained lacustrine sedimentary rocks form the material foundation of many major oil and gas fields both domestically and internationally. Their formation is controlled by paleolan productivity, preservation conditions, and sedimentation rates, with productivity and preservation conditions being the most important. Indicators of preservation conditions are currently under extensive research, with numerous indicators developed, including paleontological, mineral, and organic-inorganic geochemical indices. However, due to the complex composition of paleolan productivity, its quantitative assessment has been challenging. Modern lake productivity can be assessed by investigating the composition and structure of lake biomes. Ancient lakes also harbored biological communities, so theoretically, the composition and structure of paleolan biomes can quantitatively indicate paleoproductivity. However, this method presupposes an assessment of the composition and structure of these communities. Therefore, assessing the composition and structure of paleolan biomes is crucial for predicting the distribution of fine-grained lacustrine sedimentary rocks.

[0003] Currently, the main methods for assessing the composition and structure of ancient lake communities include micropaleontology and biogeochemistry. Micropaleontology can be affected by the absence or incomplete preservation of fossils in rocks, and it cannot identify prokaryotes such as bacteria and archaea that were present in ancient lakes. Biogeochemical results are often ambiguous because many biomarker compounds have multiple parental sources, meaning that multiple types of organisms can produce the precursors of the same biomarker compound. Summary of the Invention

[0004] In view of this, in order to solve the problems of incompleteness and inaccuracy in the current evaluation methods of the composition and structure of ancient lake biomes, an embodiment of the present invention provides a method for evaluating the composition and structure of ancient lake biomes.

[0005] An embodiment of the present invention provides a method for evaluating the composition and structure of ancient lake biomes, comprising the following steps:

[0006] S1. Obtain two rock samples from lacustrine fine-grained sedimentary rocks;

[0007] S2. After separating and enriching the organic matter in one of the rock samples, observe the sample under an optical microscope or a scanning electron microscope to determine the major types of paleo-lake organisms contained in the lacustrine fine-grained sedimentary rock;

[0008] S3. Based on the major biological categories contained in the lacustrine fine-grained sedimentary rocks, another rock sample is analyzed by gas chromatography, gas chromatography-mass spectrometry, and isotope mass spectrometry. The obtained gas chromatography, gas chromatography-mass spectrometry, and isotope results are used to identify the composition and content of biomarker compounds with biogenic significance, identify the types of organisms contained in each of the major biological categories of the paleolakes, and determine the composition of the paleolake biological community;

[0009] S4. Based on the preservation efficiency data of biomarker compounds in the environment and the quantitative generation relationship between biomarker compounds and their parent sources, the original content of characteristic biomarker compounds is restored to determine the community structure of ancient lakes.

[0010] Furthermore, the step S2 specifically includes:

[0011] S201, crushing the rock sample;

[0012] S202, adding hydrochloric acid to the crushed rock sample to remove carbonate minerals in the rock sample;

[0013] S203, washing the rock sample until it is neutral, adding hydrofluoric acid, and then washing the solution until it is neutral;

[0014] S204. The washed rock sample is transferred to a beaker, concentrated hydrochloric acid is added, and the mixture is heated to boiling. After cooling, the sample is washed with water until neutralized, and impurities are filtered out. The sample is shaken in an ultrasonic cleaner, and the sample is transferred to a test tube and centrifuged for dehydration.

[0015] S205. Add glycerol to the test tube to prepare thin sections, observe under a microscope and a scanning electron microscope, and determine the major categories of ancient lake organisms contained in the lake-facies fine-grained sedimentary rock based on its morphology.

[0016] Furthermore, in step S201, the rock sample is crushed into powder with a particle size of 3 to 5 mm, and the powder with a particle size of less than 1 mm is repeatedly sieved.

[0017] Furthermore, the step S202 is specifically as follows: adding 10% hydrochloric acid to the crushed rock sample, stirring thoroughly, and letting it stand until no more bubbles are generated, thereby removing carbonate minerals in the rock sample.

[0018] Furthermore, the major biological categories include phytoplankton, benthic algae, higher plants and bacteria.

[0019] Furthermore, the step S3 specifically includes:

[0020] S301, drying and crushing the rock sample, heating the crushed rock sample, and extracting it with dichloromethane to obtain a concentrated extract;

[0021] S302, separating asphaltene from the concentrated extract, preparing a chromatography column, and eluting the chromatography column to separate saturated hydrocarbons;

[0022] S303, injecting saturated hydrocarbons into the gas chromatograph vaporization chamber and the gas chromatograph mass spectrometer vaporization chamber, starting a temperature rise program, obtaining analysis results, and extracting saturated hydrocarbons to perform carbon isotope online analysis using an isotope mass spectrometer;

[0023] S304. Analyze the obtained gas chromatography, chromatography-mass spectrometry and carbon isotope results to identify the composition and relative content of biomarker compounds with biological provenance indication, as well as their isotope ratios, and identify the types of organisms contained in each major biological category based on this.

[0024] Furthermore, the rock sample in step S301 is crushed into powder with a particle size of 0.18 mm.

[0025] Furthermore, the step 302 is specifically as follows: adding n-hexane to the concentrated extract to separate asphaltene, concentrating the concentrated extract after separating the asphaltene through a rotary evaporator, preparing a chromatography column composed of chromatographic silica gel and neutral alumina, eluting with n-hexane, and separating saturated hydrocarbons.

[0026] Furthermore, the quantitative production relationship between the biomarker compound and its parent source is determined by measuring the cellular production of the characteristic biomarker compound through a culture test, and the preservation efficiency of the biomarker compound in the environment is estimated by the degradation rate equation of organic compounds in seawater.

[0027] Furthermore, the maturity Ro of the lacustrine fine-grained sedimentary rock is less than 0.6%.

[0028] The beneficial effects brought about by the technical solution provided by the embodiments of the present invention are:

[0029] 1. The present invention provides a method for evaluating the composition and structure of paleolate lake communities. After separating and enriching kerogen from rock samples, the paleontological fossils in the microscopic rock samples are observed under optical and scanning electron microscopes. Biotypes are identified and counted based on differences in fossil morphology, thereby determining the major paleolate lake biological categories contained within the lacustrine fine-grained sedimentary rocks. Based on this, the rock samples are subjected to extraction, separation, gas chromatography, gas chromatography-chromatography, and isotope chromatography to identify the types of organisms within each major category of paleolate lake communities. Based on data on the preservation efficiency of biomarker compounds in the environment and the quantitative relationship between biomarker compounds and their parent sources, the original content of characteristic biomarker compounds is restored to determine the composition and structure of paleolate lake communities. This method provides accurate and highly feasible conclusions on the composition and structure of paleolate lake communities.

[0030] 2. The present invention provides a method for evaluating the composition and structure of ancient lake biological communities. Based on the determination of the major categories of ancient lake organisms through microscopic observation, rock samples are subjected to extraction, separation, gas chromatography, gas chromatography-chromatography, and isotope chromatography analysis. The original parent material sources of characteristic biomarker compounds are determined by combining gas chromatography, gas chromatography-chromatography, and isotope characteristics, thus solving the problems of multi-source interpretation of the same type of biomarker compounds and restoring the original content of biomarkers in ancient lakes. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a flow chart of a method for evaluating the composition and structure of ancient lake biological communities according to the present invention. DETAILED DESCRIPTION

[0032] To make the objectives, technical solutions, and advantages of the present invention more apparent, embodiments of the present invention will be further described below with reference to the accompanying drawings. The following describes a preferred embodiment of the present invention among multiple possible embodiments, which is intended to provide a basic understanding of the present invention but is not intended to identify the key or decisive elements of the present invention or to limit the scope of protection.

[0033] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not limiting. Therefore, other examples of the exemplary embodiments may have different values.

[0034] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered part of the specification.

[0035] It should be noted that similar reference numerals and letters represent similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further discussed in the subsequent drawings. At the same time, it should be understood that for ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual scale.

[0036] Please refer to Figure 1 , an embodiment of the present invention provides a method for evaluating the composition and structure of ancient lake biomes, comprising the following steps:

[0037] S1. Obtain two rock samples from lacustrine fine-grained sedimentary rocks;

[0038] When selecting lacustrine fine-grained sedimentary rocks, samples with lower maturity should be selected, generally with a maturity Ro < 0.6%, and no later modification by organic matter should be found. These samples meet the basic conditions for using the content of characteristic biomarker compounds to restore the size of biological populations.

[0039] S2. After separating and enriching the organic matter in one of the rock samples, the samples were observed under an optical microscope and a scanning electron microscope to determine the major categories of ancient lake organisms contained in the lake-facies fine-grained sedimentary rock.

[0040] The processing of the rock sample in step S2 may refer to the petroleum and natural gas industry standard SY / T5915-2018 "Analysis and Identification of Fossil Pollen". The details are as follows:

[0041] S201. Crush the rock sample: crush the rock sample into powder with a particle size of 3 to 5 mm, weigh 50 to 100 g, and repeatedly sieve to remove powder with a particle size of less than 1 mm.

[0042] S202. Add hydrochloric acid to the crushed rock sample to remove carbonate minerals in the rock sample.

[0043] Specifically: Add 10% hydrochloric acid to the crushed rock sample and stir it thoroughly with a glass rod. Add hydrochloric acid repeatedly according to the reaction situation and let it stand for 2 to 3 hours until there is no more bubbling. This will remove the carbonate minerals in the rock sample.

[0044] S203, washing the rock sample until it is neutral, adding hydrofluoric acid, and then washing the solution until it is neutral.

[0045] Specifically: Wash the rock sample treated with hydrochloric acid with clean water until it is neutral, then add hydrofluoric acid, stir thoroughly until there is no more foaming, let it stand for 12 to 24 hours, and then wash the solution until it is neutral.

[0046] S204. Transfer the washed rock sample into a beaker, add concentrated hydrochloric acid, heat to boiling, cool and wash with water until neutralized, filter out impurities, vibrate in an ultrasonic cleaner, transfer to a test tube and centrifuge for dehydration.

[0047] Specifically: transfer the washed sample into a beaker, add concentrated hydrochloric acid, heat to boiling, cool and wash with water to neutralize, sieve out impurities with a filter, vibrate in an ultrasonic cleaner for 30 minutes, transfer to a test tube and centrifuge for dehydration.

[0048] S205. Add glycerol to the test tube to prepare a thin section, observe the section under a microscope and a scanning electron microscope, and determine the major categories of paleo-lake organisms contained in the lacustrine fine-grained sedimentary rock based on its morphology. The major categories of organisms generally include phytoplankton, benthic algae, higher plants, and bacteria.

[0049] S3. Based on the major biological categories contained in the lacustrine fine-grained sedimentary rocks, another rock sample is subjected to gas chromatography-mass spectrometry and chromatography-mass spectrometry / mass spectrometry analysis. The obtained gas chromatography, chromatography-mass spectrometry and carbon isotope results are used to identify the composition and relative content of biomarker compounds with provenance-indicating significance, and to recognize the types of organisms contained in each of the major biological categories of the ancient lakes.

[0050] S301, drying the rock sample and then crushing it, heating the crushed rock sample and extracting it with dichloromethane to obtain a concentrated extract.

[0051] The concentrated extract is obtained in accordance with the petroleum and natural gas industry standard SY / T5118-2021 "Determination of Extract Content in Rocks", specifically:

[0052] The rock sample is dried in an oven at 40-45° C. for more than 4 hours, the dried rock sample is crushed with a sample crusher at a temperature not exceeding 50° C., and the sample powder is sieved with a standard sieve with an aperture of 0.18 mm; an appropriate amount of the crushed sample is weighed, wrapped with filter paper or a filter paper tube, and placed in the sample chamber of the extractor, and dichloromethane and a copper sheet are added to the bottom bottle; a constant temperature water bath is turned on for heating, the heating temperature is below 60° C., and the extraction is completed when the fluorescence of the extract dripping from the sample chamber is weakened to below level 3 under fluorescent light; the extract solution is concentrated to 5-10 ml, and the heating temperature of the dichloromethane extract during the concentration process is not higher than 50° C.

[0053] S302. Separate asphaltene from the concentrated extract, prepare a chromatography column, and elute the chromatography column to separate saturated hydrocarbons: add n-hexane to the concentrated extract to separate asphaltene, concentrate the concentrated extract after separating the asphaltene through a rotary evaporator, prepare a chromatography column composed of chromatography silica gel and neutral alumina, and elute with n-hexane to separate saturated hydrocarbons.

[0054] The separation of saturated hydrocarbons can refer to the petroleum and natural gas industry standard "Analysis of Soluble Organic Matter in Rocks and Crude Oil Group Components", as follows:

[0055] Weigh 15-50 mg of the concentrated extract into a stoppered weighing flask, add about 30 ml of n-hexane, and dissolve it by ultrasonication. After the sample and n-hexane are completely mixed, let it stand for more than 12 hours to fully precipitate the asphaltenes in the sample; filter the asphaltenes, collect the filtrate in a stoppered conical flask, and wash the stoppered weighing flask and absorbent cotton with n-hexane until the filtrate is colorless; concentrate the filtrate washed with n-hexane to 2-3 ml using a rotary evaporator, and use it for column chromatography separation; fill the bottom of the chromatography column with a small amount of absorbent cotton, add 3-4 g of chromatography silica gel and 2-3 g of neutral alumina, and add an appropriate amount of n-hexane to wet the stationary phase in the chromatography column; elute with 3-5 ml of n-hexane each time, for a total of 30 ml to separate the saturated hydrocarbons.

[0056] S303. Inject saturated hydrocarbons into the gas chromatograph vaporization chamber and the gas chromatograph mass spectrometer vaporization chamber, start the temperature rising program, obtain the analysis results, and extract saturated hydrocarbons to perform carbon isotope online analysis using an isotope mass spectrometer.

[0057] The saturated hydrocarbon sample was drawn up with a microsyringe and injected into the vaporization chamber of the gas chromatography-mass spectrometer. The temperature ramp was initiated, and data acquisition and processing were performed on a computer workstation. The method followed the national standard GB / T 18340.5-2010, "Organic geochemical analysis of geological samples - Part 5: Analysis of saturated hydrocarbons in rock extracts and crude oil - Gas chromatography."

[0058] Saturated hydrocarbon samples were collected and analyzed online for elemental carbon isotope composition using an isotope mass spectrometer. Reference was made to the national standard GB / T 18340.2-2010, "Methods for organic geochemical analysis of geological samples - Part 2: Determination of stable carbon isotopes in organic matter - Isotope mass spectrometry."

[0059] S304. Analyze the obtained gas chromatography, chromatography-mass spectrometry, and isotope results to identify the composition and relative content of biomarker compounds with biogenic significance, as well as their isotope ratios. Based on these, the organisms within each major biological category are identified. Here, saturated hydrocarbon gas chromatography, gas chromatography-mass spectrometry, and individual hydrocarbon carbon isotopes are combined to determine the original parent material source of the characteristic biomarker compounds. This determines the organismal type of each characteristic biomarker compound, resolving the issue of multiple sources for the same biomarker compound.

[0060] S4. Based on data on the preservation efficiency of biomarker compounds in the environment and the quantitative relationship between biomarker compounds and their parent sources, the original concentrations of characteristic biomarker compounds were restored to determine the composition and structure of the ancient lake community. This approach combines the preservation efficiency of biomarkers in the environment and the quantitative relationship between biomarker compounds and their parent sources to restore the original concentrations of biomarker compounds, thus solving the problem of restoring the original concentrations of biomarkers in ancient lakes.

[0061] The quantitative production relationship between the biomarker compound and its parent source is determined by measuring the cellular production of the characteristic biomarker compound through culture experiments. The preservation efficiency of the biomarker compound in the environment is estimated using the degradation rate equation for organic compounds in seawater. The degradation rate equation for organic compounds in seawater can be referenced in the academic paper "Degradation and Patterns of Marine Sedimentary Organic Matter" (Jiang Yuxuan et al., Marine Geology and Quaternary Geology, 2014, Vol. 34, No. 4, pp. 173-179).

[0062] The above method for evaluating the composition and structure of paleolan communities was used to evaluate the composition and structure of paleolan communities in the third member of the Hetaoyuan Formation in the Paleogene of the Biyang Sag. The application example is as follows:

[0063] Two rock samples were prepared from organic-rich shale samples (TOC > 2.0%) from the He-3 Member of the Biyang Sag, a lacustrine fine-grained sedimentary rock. Because the lacustrine shale samples from the He-3 Member of the Biyang Sag selected for this study have low maturity (Ro < 0.6%) and no evidence of later organic matter alteration, they meet the basic conditions for restoring biological population sizes using characteristic biomarker compound content.

[0064] One rock sample was examined using optical microscopy and scanning electron microscopy to identify the types of organic matter it contained. The sample was first enriched with kerogen and then sliced. Observation under transmitted light and fluorescence revealed two main types of organic matter: the first, amorphous matter, which can be further divided into sapropelic amorphous matter and humic amorphous matter based on morphological characteristics and origin. Sapropelic amorphous matter is primarily formed by the sapropelization of the remains of lower aquatic organisms, such as algae, under oxygen-deficient conditions, mediated by microorganisms. Humic amorphous matter is primarily formed by the complete microbial degradation of epidermal, vascular, or basal tissues (which may also contain small amounts of lower organisms) of higher plants (terrestrial or aquatic). The second largest category is particulate organic matter. Algae identified in the lacustrine shales of the Biyang Depression include various subtypes, such as coccoliths, benthic red algae, Chlorella, and Volvox. The habitats of these different algae species often vary widely. For example, the vast majority of coccoliths inhabit seawater of normal salinity and are primarily found in modern open-sea and pelagic environments. Benthic red algae and brown algae are mostly marine, found near tropical and subtropical coasts, with a smaller number being freshwater species, mostly found in rapids, waterfalls, and mountain waters with cold air circulation. Both Volvox and Chlorella belong to the Chlorophyta and primarily inhabit freshwater, but have also been found in mixed freshwater and saltwater waters. In addition to algae, pollen and plant debris from higher plants, as well as ostracods, which can inhabit freshwater, brackish water, and normal seawater, were also identified in the samples. The above analysis of paleontological findings in the rock samples indicates that during the development of the lacustrine shale in the He-3 Member of the Biyang Sag, the paleo-lake community was primarily composed of phytoplankton, benthic algae, higher plants, and zooplankton. Furthermore, microfossils suspected to be bacteria and archaea were discovered under a scanning electron microscope, indicating that microorganisms such as bacteria and archaea likely existed in the saline lake basin at the time.

[0065] Based on the microscopic identification results, another rock sample was analyzed by gas chromatography-mass spectrometry (GC-MS) and chromatography-mass spectrometry / mass spectrometry (GC-MS / MS). The results showed that C 15 -C 35 Normal alkanes, isoprenoids (mainly pristane and phytane), C 21-22 and C 27-29 Regular sterane, C 19 -C 30 Tricyclic terpanes, C 27 -C 35 Hopane, and β-carotene, C 24 Tetracyclic terpanes and gammaceranes were also identified. In addition, 24-n-propylcholestane was identified in GC-MS / MS from 414 to 217.

[0066] Based on the preservation efficiency data of biomarker compounds in the environment and the quantitative production relationship between biomarker compounds and their parent sources, the original content of characteristic biomarker compounds is restored to determine the composition and structure of the ancient lake community.

[0067] There are two key links: one is the cellular production of characteristic biomarker compounds, and the other is the preservation efficiency of biomarker compounds in the environment.

[0068] The production of characteristic biomarker compounds by cells was determined through experimental culture. In this example, several typical saline lake basin microalgae / bacteria were selected, primarily including Porphyridium (red algae), Emilianian (coccolithophores), Volvox (green algae), Microcystis (cyanobacteria), and Chlorobacter (green sulfur bacteria). The culture and measurement steps are briefly described as follows: ① Selection of biological sources: algae and bacteria are primarily isolated from lake water and factory wastewater (for Chlorobacter); ② Cultivation under appropriate temperature, CO2 concentration, and nutrient concentration conditions; ③ Measurement of cell growth; ④ Collection of cells, extraction with organic solvents, and measurement of specific biomarker content. The resulting data are then used to calculate the production of characteristic biomarker compounds by cells.

[0069] The preservation efficiency of biomarker compounds in the environment was estimated using the degradation rate equations of different types of organic compounds (such as steroids, terpenes, etc.) in seawater established by previous researchers.

[0070] Finally, the above-mentioned method for evaluating the composition and structure of ancient lake communities was used to reconstruct the ancient biological community structure during the development period of multiple pieces of lacustrine shales in the third member of the core of the Biyang Depression. It was determined that the content of phytoplankton (mainly coccolithophores and green algae) during the development period of the lacustrine shale in the third member of the core of the Biyang Depression was between 42% and 71% (an average of 63%), the content of bacteria and archaea (mainly green sulfur bacteria, purple sulfur bacteria, and halophilic archaea) was between 9% and 44% (an average of 21%), the content of benthic algae (mainly red algae and brown algae) was between 2% and 21% (an average of 11%), and the content of aquatic plants (represented by spores and plant debris) was less than 6%.

[0071] In this document, directional terms such as front, back, top, and bottom are defined based on the positions of components in the accompanying drawings and relative to each other, and are intended for clarity and convenience in describing the technical solution. It should be understood that these terms are relative and may vary depending on usage and placement. The use of these directional terms should not limit the scope of protection claimed in this application.

[0072] The above embodiments and features of the embodiments may be combined with each other unless they conflict. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for evaluating the composition and structure of ancient lake communities, characterized in that: The following steps are involved: S1. Obtain two rock samples from lacustrine fine-grained sedimentary rocks; S2. After separating and enriching the kerogen from one of the rock samples, the kerogen was observed under an optical microscope and a scanning electron microscope to determine the major types of paleo-lake organisms contained in the lacustrine fine-grained sedimentary rock; S3. Based on the major biological categories contained in the lacustrine fine-grained sedimentary rocks, another rock sample is analyzed by gas chromatography, gas chromatography-mass spectrometry, and isotope mass spectrometry. The obtained gas chromatography, gas chromatography-mass spectrometry, and isotope results are used to identify the composition and content of biomarker compounds with biogenic significance, identify the types of organisms contained in each of the major biological categories of the paleolakes, and determine the composition of the paleolake biological community; S4. Based on the preservation efficiency of biomarker compounds in the environment and the quantitative generation relationship between biomarker compounds and their parent sources, the original content of characteristic biomarker compounds is restored to determine the structure of ancient lake community.

2. A method for evaluating the composition and structure of ancient lake communities according to claim 1, characterized in that: The step S2 specifically includes: S201, crushing the rock sample; S202, adding hydrochloric acid to the crushed rock sample to remove carbonate minerals in the rock sample; S203, washing the rock sample until it is neutral, adding hydrofluoric acid, and then washing the solution until it is neutral; S204. The washed rock sample is transferred to a beaker, concentrated hydrochloric acid is added, and the mixture is heated to boiling. After cooling, the sample is washed with water until neutralized, and impurities are filtered out. The sample is shaken in an ultrasonic cleaner, and the sample is transferred to a test tube and centrifuged for dehydration. S205. Add glycerol to the test tube to prepare thin sections, observe under a microscope and a scanning electron microscope, and determine the major categories of ancient lake organisms contained in the lake-facies fine-grained sedimentary rock based on its morphology.

3. The method for evaluating the composition and structure of ancient lake communities according to claim 2, wherein: In step S201, the rock sample is crushed into powder with a particle size of 3 to 5 mm, and the powder with a particle size of less than 1 mm is repeatedly sieved to remove the powder.

4. A method for evaluating the composition and structure of ancient lake communities according to claim 2, characterized in that: The step S202 specifically includes: adding 10% hydrochloric acid to the crushed rock sample, stirring thoroughly, and allowing to stand until no more bubbles are generated, thereby removing carbonate minerals from the rock sample.

5. The method for evaluating the composition and structure of ancient lake communities according to claim 1, wherein: The major biological categories include phytoplankton, benthic algae, higher plants and bacteria.

6. The method for evaluating the composition and structure of ancient lake communities according to claim 1, wherein: The step S3 specifically includes: S301, drying and crushing the rock sample, heating the crushed rock sample, and extracting it with dichloromethane to obtain a concentrated extract; S302, separating asphaltene from the concentrated extract, preparing a chromatography column, and eluting the chromatography column to separate saturated hydrocarbons; S303, injecting saturated hydrocarbons into the gas chromatograph vaporization chamber and the gas chromatograph mass spectrometer vaporization chamber, starting a temperature rise program, obtaining analysis results, and extracting saturated hydrocarbons to perform carbon isotope online analysis using an isotope mass spectrometer; S304. Analyze the obtained gas chromatography, chromatography-mass spectrometry and carbon isotope results to identify the composition and relative content of biomarker compounds with biological provenance indication, as well as their isotope ratios, and identify the types of organisms contained in each major biological category based on this.

7. A method for evaluating the composition and structure of ancient lake communities according to claim 6, characterized in that: In step S301, the rock sample is crushed into powder with a particle size of 0.18 mm.

8. The method for evaluating the composition and structure of ancient lake communities according to claim 6, wherein: The step 302 specifically includes: adding n-hexane to the concentrated extract to separate asphaltene, concentrating the concentrated extract after separating the asphaltene through a rotary evaporator, preparing a chromatography column composed of chromatographic silica gel and neutral alumina, eluting with n-hexane, and separating saturated hydrocarbons.

9. The method for evaluating the composition and structure of ancient lake communities according to claim 1, wherein: The quantitative production relationship between the biomarker compound and its parent source is determined by measuring the cell production amount of the characteristic biomarker compound through a culture test, and the preservation efficiency of the biomarker compound in the environment is estimated by the degradation rate equation of organic compounds in seawater.

10. The method for evaluating the composition and structure of ancient lake communities according to claim 1, wherein: The maturity Ro of the lacustrine fine-grained sedimentary rock is less than 0.6%.

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