Method for in-situ observation of light characteristic change of hydrocarbon source rock organic maceral in thermal evolution process

Abstract

The invention provides a method for in-situ observation of optical characteristic changes of organic macerals of a hydrocarbon source rock in a thermal evolution process, and the method analyzes the optical characteristic changes of the same organic macerals after thermal simulation by observing the characteristics of the organic macerals before and after thermal simulation of a hydrocarbon source rock sample. According to the method, the dynamic change of a sample in the thermal evolution process is obtained through a microscopic picture shot under a microscope, the optical property change of the hydrocarbon source rock organic macerals in the thermal evolution process is accurately obtained through an image analysis method, and the difference of thermal evolution behaviors of different organic macerals is determined. According to the method, the defects of a traditional thermal simulation experiment are overcome, and the optical characteristics of the organic maceral are intuitively obtained by adopting an image analysis method.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas geological exploration, and particularly to a method for in-situ observing the changes in the optical properties of organic macerals in source rocks during the thermal evolution process. Background Art

[0002] A type of rock rich in organic matter in sedimentary rocks and capable of forming oil and gas is called a source rock. Oil and gas are mainly formed by the transformation of some organic macerals in the source rock under suitable temperature and pressure conditions. Therefore, the composition and thermal evolution characteristics of the organic macerals in the source rock determine the generation potential and properties of oil and gas. The thermal evolution characteristics are the key parameters for understanding the degree of transformation of sedimentary organic matter into oil and gas. However, the thermal evolution of organic matter composed of different macerals varies greatly due to differences in chemical composition and structure. Generally, researchers can better understand the kerogen hydrocarbon generation process and hydrocarbon generation potential by analyzing the changes in physical and chemical properties during the thermal evolution of organic macerals. Traditional studies on the thermal evolution of organic matter mainly focus on the evaluation of organic matter type and maturity, while there are few in-situ observations and quantitative analysis studies on changes in microscopic structure, optical properties, etc. during the thermal evolution process.

[0003] At present, it is difficult to accurately study the change law of optical properties of the same organic maceral particle during the experiment, and the samples in traditional thermal simulation experiments are usually damaged, losing the information of in-situ organic matter evolution. The existing technology lacks a method for in-situ observing the thermal evolution of the same organic maceral, and cannot meet the requirements of real-time tracking of optical property changes, retaining the original structural information, and realizing the in-situ observation of the thermal evolution process of organic macerals in source rocks.

[0004] Therefore, in order to deeply reveal the thermal evolution mechanism of source rocks and evaluate the potential of oil and gas resources, it is crucial to develop a method for in-situ observing the changes in the optical properties of organic macerals in source rocks during the thermal evolution process. Summary of the Invention

[0005] To solve the deficiencies of the existing technology, the main object of the present invention is to provide a method for in-situ observing the changes in the optical properties of organic macerals in source rocks during the thermal evolution process. This method uses a microscope and thermal simulation as the main experimental means, takes the source rock maceral particles as the research object, accurately observes the changes in the optical properties of organic macerals during the thermal evolution process, clarifies the differences in the thermal evolution behaviors of different organic macerals, and avoids the drawback that traditional thermal simulation experiments cannot be used for in-situ observation because it is difficult to find the same maceral.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: A method for in-situ observing the changes in the optical properties of organic macerals in source rocks during the thermal evolution process, comprising the following steps: S1. Prepare the source rock sample into a polished thin section of rock block; S2. Observe the characteristics of organic macerals in the source rock through an optical microscope and measure the reflectivity; S3. Take out the source rock sample from the polished thin section of rock block; S4. Conduct artificial thermal simulation on the source rock sample; S5. Observe the changes in the optical properties of the same organic maceral particles after thermal simulation through an optical microscope.

[0007] Preferably, in step S1, preparing the source rock sample into a polished thin section of rock block includes: S11. Process the collected source rock sample into a block and select a sample with regular shape; S12. Consolidate the source rock sample with epoxy resin and grind and polish to obtain a polished thin section of rock block.

[0008] Preferably, in step S2, the steps of observing the characteristics of organic macerals in the source rock and measuring the reflectivity include: S21. Turn on the power supply and the instrument switch in sequence, adjust the voltage and current, and preheat to make the instrument reach a stable working state; S22. Select the magnification of the objective lens and the eyepiece; S23. Select a known reflectivity standard sample for reflectivity calibration under oil immersion conditions; S24. Drop immersion oil on the polished surface of the thin section of rock block and place the thin section of rock block on the microscope stage; S25. Scan each organic maceral in the thin section of rock block under reflected light and fluorescence conditions to obtain reflected light and fluorescence spectral data; S26. Measure the random reflectivity of each maceral in the thin section of rock block under reflected light conditions.

[0009] Preferably, in step S3, taking out the source rock sample from the polished thin section of rock block includes: S31. Cut off the solidified epoxy resin around the sample; S32. Soak and demold the residual epoxy resin wrapping the sample with an organic solvent to facilitate the complete extraction of the source rock sample.

[0010] Preferably, the device for conducting artificial thermal simulation on the source rock sample in step S4 includes: a vacuum pumping device, a high-temperature reaction kettle, and a heating device. The left side of the high-temperature reaction kettle is connected to an inert gas delivery pipeline through a first pipeline, and the right side is connected to the vacuum pumping device through a second pipeline. Valves are provided on both the first pipeline and the second pipeline. A digital display pressure gauge is connected to the top of the high-temperature reaction kettle. A sample chamber for placing the source rock sample is arranged inside the high-temperature reaction kettle, and the bottom of the high-temperature reaction kettle is connected to the heating device.

[0011] Preferably, the artificial thermal simulation of the hydrocarbon source rock sample includes the following steps: S41. Place the demolded hydrocarbon source rock sample into the sample chamber of the high-temperature reaction kettle; S42. Evacuate and fill the high-temperature reaction kettle with inert gas through a vacuum pumping device and an inert gas delivery pipeline. After the high-temperature reaction kettle is in a vacuum state, set the heating program, start heating, and conduct the thermal simulation experiment.

[0012] Preferably, before the artificial thermal simulation, the airtightness of the high-temperature reaction kettle is tested.

[0013] Preferably, the airtightness test of the high-temperature reaction kettle includes: First, evacuate the high-temperature reaction kettle using a vacuum pumping device; Second, inject inert gas into the high-temperature reaction kettle through the inert gas delivery pipeline. If the internal pressure of the high-temperature reaction kettle remains stable, the airtightness of the device is good.

[0014] In the present invention, before the thermal simulation starts, the high-temperature reaction kettle is evacuated and filled with inert gas to further exhaust the air in the reaction kettle, eliminating the influence of air on the thermal simulation of the hydrocarbon source rock, and making the entire thermal simulation process carried out under sealed conditions, which is convenient for accurately observing the changes in the optical properties of organic macerals during the thermal evolution process.

[0015] Preferably, in the method of the present invention, the equipment for in-situ observing the changes in the optical properties of organic macerals in the hydrocarbon source rock includes: an optical microscope and a thermal simulation device. The thermal simulation device includes a high-temperature reaction kettle, a vacuum pumping device, and a heating device; The optical microscope is used to observe the optical properties of organic macerals in the hydrocarbon source rock sample before and after thermal simulation; The high-temperature reaction kettle is used to provide a high-temperature and high-pressure environment for the hydrocarbon source rock sample to promote the pyrolysis of organic matter; The vacuum pumping device is used to evacuate the high-temperature reaction kettle to create an oxygen-free thermal evolution environment; The heating device is used to heat the hydrocarbon source rock sample in the high-temperature reaction kettle to cause the hydrocarbon source rock sample to undergo pyrolysis.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention retains the evolution trajectory of the same particle through the epoxy resin consolidation and positioning shooting technology, adopts the vacuum pumping-inert gas replacement process to ensure an oxygen-free thermal simulation environment, and truly restores the underground thermal evolution environment; the analysis of optical properties and the determination of reflectivity data improve the accuracy of maturity evaluation, providing an efficient and reliable technical means for the evaluation of unconventional oil and gas resources.

[0017] 2. The present invention realizes the direct observation and quantitative analysis of the thermal evolution process of organic macerals in source rocks through non-destructive sample treatment, high-precision experimental control, simulation of real geological conditions, and in-situ dynamic tracking, etc. The present invention takes the microscope and thermal simulation as the main experimental means, takes the maceral particles of source rocks as the research object, obtains the dynamic changes of the sample during the thermal evolution process through the microscopic pictures taken under the microscope, accurately obtains the changes in the optical properties of the organic macerals in source rocks during the thermal evolution process by image analysis methods, and clarifies the differences in the thermal evolution behaviors of different organic macerals. This method is simple to operate and low in cost, providing a theoretical basis for the exploration and development of oil and gas resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further described below with reference to the drawings: Figure 1 is a schematic flow chart of the present invention; Figure 2 is a comparison of reflected light (a, c, e) and fluorescence (b, d, f) photographs of the Upper Devonian Ohio Shale in the same field of view before and after different thermal simulation temperatures in Example 2; Figure 3 is a comparison of reflected light (a, c, e) and fluorescence (b, d, f) photographs of the Upper Triassic Yanchang Formation Shale in the same field of view before and after different thermal simulation temperatures in Example 3. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] The present invention realizes the direct observation and quantitative analysis of the thermal evolution process of organic macerals in source rocks by truly restoring the environment under geological conditions and adopting in-situ continuous observation of the reflectivity, morphological structure, and fluorescence characteristics of organic macerals in source rocks before and after thermal simulation by a microscope. This method is simple to operate and low in cost, provides key in-situ thermal evolution information for source rock samples, helps to more accurately predict the distribution and state of oil and gas, and provides a theoretical basis for the exploration and development of oil and gas resources.

[0020] The present invention will be further described in detail below with reference to the drawings and specific embodiments, but the protection scope of the present invention is not limited thereby.

[0021] Example 1: This example provides a method for in-situ observing the changes in the optical properties of organic macerals in source rocks during the thermal evolution process, as Figure 1 shown, the method includes the following steps: S1. Prepare the source rock sample into a polished rock slice; S2. Observe the characteristics of the organic macerals in the source rock through an optical microscope and measure the reflectivity; S3. Take out the source rock sample from the polished rock slice; S4. Perform artificial thermal simulation on the source rock sample; S5. Observe the changes in the optical properties of the same organic maceral particles after thermal simulation through an optical microscope.

[0022] In a preferred embodiment, step S1 includes: S11. Process the collected hydrocarbon source rock samples into blocks along the vertical / parallel bedding direction and select samples with regular shapes. S12. Consolidate the hydrocarbon source rock samples with epoxy resin and polish them using a Buehler automatic polishing machine to obtain polished rock blocks.

[0023] In a preferred embodiment, for step S2, the reflectance measurement is completed on an optical microscope equipped with a Hilgers Technisches Büro Fossil system, including: S21. Turn on the power and the switches of various electrical components related to the instrument in sequence, adjust the voltage and current to the specified values, and preheat for 0.5 h to 1.0 h to make the instrument reach a stable working state. S22. Select the magnification of the objective lens and the eyepiece. S23. Select a known reflectance standard sample and perform reflectance calibration under oil immersion conditions. S24. Drop immersion oil on the polished surface of the polished rock block and place the polished rock block on the microscope stage. S25. Scan each organic maceral in the polished rock block under reflected light and fluorescence conditions to obtain reflected light and fluorescence spectral data. S26. Measure the random reflectance of each maceral in the polished rock block under reflected light conditions.

[0024] In a preferred embodiment, step S3 includes: Cut off the cured epoxy resin around the sample, soak and demold the remaining epoxy resin wrapping the sample with an organic solvent to completely remove the hydrocarbon source rock sample.

[0025] In a preferred embodiment, step S4 includes: S41. Place the demolded hydrocarbon source rock sample into the sample chamber of the high-temperature reactor. S42. Evacuate and fill the high-temperature reactor with inert gas through a vacuum pump and an inert gas delivery pipeline. After the high-temperature reactor is in a vacuum state, set the heating program, start heating, and conduct a thermal simulation experiment.

[0026] In a preferred embodiment, in step S4, the artificial thermal simulation experiment of the hydrocarbon source rock sample includes the following operations: S41. Place the demolded hydrocarbon source rock sample into the sample chamber of the high-temperature reactor. S42. Evacuate the high-temperature reactor using a vacuum pump. After completion, close the valve between the vacuum pump and the high-temperature reactor. S43. Fill the high-temperature reactor with inert gas through an inert gas supply pipeline. After completion, close the valve between the inert gas supply pipeline and the high-temperature reactor. S44. Evacuate the high-temperature reactor again. After completion, close the valve between the vacuum pump and the high-temperature reactor. S45. Repeat steps S43 - S44 twice. S46. After the high-temperature reactor is in a vacuum state, set the heating program and start heating.

[0027] The thermal simulation environment of the source rock sample is completed through S41 - S46, truly restoring the underground thermal evolution environment of the source rock sample.

[0028] In a preferred embodiment, in step S5, microcomponent photos of the same position of the source rock sample before and after thermal simulation are taken through an optical microscope, and the optical property changes of the organic microcomponents of the source rock during the thermal evolution process are visually compared.

[0029] Example 2: This example is further described in combination with Example 1. In this example, a Devonian Ohio shale sample is selected for illustration.

[0030] A method for in-situ observing the optical property changes of organic microcomponents in a source rock during the thermal evolution process includes the following steps: S1. Prepare a rock block polished section from the source rock sample: Process the collected source rock sample into blocks along the vertical / parallel bedding direction, and select block samples with regular shapes. Prepare a mixture of epoxy resin and hardener in a ratio of 3:1, and homogenize it through ultrasonic oscillation. Then bond it to the surface of the above-mentioned block sample and let it stand for curing. Rigorously perform rough grinding, fine grinding, and polishing on the cured sample to make a rock block polished section.

[0031] S2. Observe the characteristics of the organic microcomponents in the source rock through an optical microscope and measure the reflectivity. Complete the reflectivity measurement on an optical microscope equipped with a Hilgers Technisches Büro Fossil system. S3. Take out the source rock sample from the rock block polished section. Cut off the cured epoxy resin around the rock block polished section, soak the remaining epoxy resin wrapping the sample with dichloromethane for demolding, and completely take out the source rock sample. S4. Place the source rock sample in a high-temperature reactor for artificial thermal simulation. Vacuum the high-temperature reactor and fill it with inert gas. Set the temperatures to 300 °C and 310 °C respectively. S5. After the thermal simulation experiment is completed, take micro-component photos of the same position of the source rock sample before and after thermal simulation through an optical microscope, and observe the changes in the optical properties of the same organic micro-component after thermal simulation. The experimental results are as Figure 2 shown.

[0032] As Figure 2 can be seen, in the microscopic component observation of the original sample, Tasmanites in the Upper Devonian Ohio Shale shows a complete cystic structure, solid bitumen appears grayish white, and the reflectivity is 0.55% ( Figure 2 a). The characteristic yellow fluorescence of Tasmanites indicates that the sample has not experienced significant thermal alteration ( Figure 2 b). After thermal simulation at 300 °C ( Figure 2 c), Tasmanites shrinks, and a substance similar to solid bitumen forms on the surface, showing grayish black, and the fluorescence intensity significantly decays to brown ( Figure 2 d). The reflectivity of the original solid bitumen increases to 1.02%; under the condition of 310 °C ( Figure 2 e), Tasmanites has basically been completely transformed into solid bitumen, forming a network structure intertwined with the shale mineral matrix, and the fluorescence response completely disappears ( Figure 2 f). The reflectivity of the original solid bitumen rises to 1.39%. The experiment shows that hydrogen-rich components (such as Tasmanites) undergo in-situ transformation to form solid bitumen during the thermal evolution process. In addition, the regular change of the reflectivity of solid bitumen in the sample with the increase of temperature indicates that the reflectivity evolution can effectively characterize the thermal maturity process of the source rock.

[0033] Example 3: Further illustrate in combination with Example 1. In this example, the Upper Triassic Yanchang Formation shale sample in the Ordos Basin is selected for illustration.

[0034] A method for in-situ observing the changes in the optical properties of organic micro-components of source rocks during the thermal evolution process, including the following steps: S1. Prepare a rock block polished section from the source rock sample: Process the collected source rock sample into blocks along the vertical / parallel bedding direction, and select regular-shaped block samples; Prepare a mixture of epoxy resin and hardener in a ratio of 3:1, and homogenize it through ultrasonic oscillation. Then bond it to the surface of the above-mentioned block sample and let it stand for curing; Rigorously perform rough grinding, fine grinding and polishing on the cured sample to make a rock block polished section.

[0035] S2. Observe the organic maceral characteristics of the source rock through an optical microscope and measure the reflectivity of the samples; Measure the reflectivity of the samples on an optical microscope equipped with a Hilgers Technisches Büro Fossil system; S3. Take out the source rock samples from the polished rock slices; Cut off the solidified epoxy resin around the polished rock slices, soak the remaining epoxy resin wrapping the samples with dichloromethane for demolding, and completely take out the source rock samples; S4. Put the source rock samples into a high-temperature autoclave for artificial thermal simulation, evacuate the high-temperature autoclave and fill it with inert gas, and set the temperatures to 310 °C and 320 °C respectively; After the thermal simulation experiment is completed, take micrographs of the same position of the source rock samples before and after thermal simulation through an optical microscope, observe the changes in the optical properties of the same organic maceral after thermal simulation, and the experimental results are as Figure 3 shown.

[0036] Figure 3 clearly reveals the in-situ transformation behavior of bituminite in the shale of the Yanchang Formation in the Upper Triassic: under reflected light of the original sample, the bituminite appears grayish-black, and the reflectivity is 0.28% ( Figure 3 a), and the fluorescence image shows that it exhibits brown fluorescence ( Figure 3 b), indicating low maturity characteristics; after thermal simulation treatment at 310 °C, in the same field of view, the bituminite undergoes in-situ aggregation and structural reorganization, and the reflectivity jumps to 0.72% ( Figure 3 c), and the synchronous fluorescence response weakens and regional quenching occurs ( Figure 3 d); when the temperature rises to 320 °C, the bituminite is transformed into clearly defined solid bitumen masses, and the reflectivity is 1.04% ( Figure 3 e), and the fluorescence completely disappears ( Figure 3 f), and the newly formed solid bitumen strictly inherits the spatial distribution of the original organic matter. The regular change of its reflectivity with temperature can effectively characterize the thermal maturity process of the source rock, provide key in-situ thermal evolution information for predicting the occurrence phases of shale oil and gas. This in-situ observation excludes the interference of mineral migration, realizes the continuous phase change of bituminite in the Yanchang Formation shale, and the reflectivity jump and fluorescence weakening process provide key in-situ thermal evolution information for predicting the occurrence phases of shale oil and gas.

[0037] According to the above embodiments, this method can intuitively reveal the in-situ transformation process of hydrogen-rich components such as Tasmanites into solid bitumen, as well as the aggregation and reorganization of bituminite and the fluorescence quenching phenomenon. This method realizes the dynamic monitoring of the phase change behavior of organic macerals through image analysis, and has the advantages of simple operation, controllable experimental conditions, and strong data continuity.

[0038] The above embodiments are only the preferred technical solutions of the present invention and should not be regarded as limitations on the present invention. The protection scope of the present invention should be the technical solutions recorded in the claims, including equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for in-situ observing the changes in the optical properties of organic macerals in source rocks during the thermal evolution process, characterized in that It includes the following steps: S1. Prepare the source rock sample into a polished rock slice; S2. Observe the characteristics of organic macerals in the polished slice through an optical microscope and measure the reflectance; S3. Take out the source rock sample from the polished rock slice; S4. Conduct artificial thermal simulation on the source rock sample; S5. Observe the changes in the optical properties of the same organic maceral particles after thermal simulation through an optical microscope.

2. The method according to claim 1, characterized in that, In step S1, preparing the source rock sample into a polished rock slice includes: S11. Process the collected source rock sample into a block; S12. Consolidate the source rock sample with epoxy resin and polish it with a Buehler automatic polishing machine to obtain a polished rock slice.

3. The method according to claim 2, characterized in that, The above step S2 includes: S21. Turn on the power supply and the instrument switch in sequence, adjust the voltage and current, and preheat to make the instrument reach a stable working state; S22. Select the magnification of the objective lens and the eyepiece; S23. Select a known reflectance standard sample to calibrate the reflectance under oil immersion conditions; S24. Drop immersion oil on the polished surface of the polished slice and place the polished slice on the microscope stage; S25. Scan each organic maceral in the polished slice under reflected light and fluorescence conditions to obtain reflected light and fluorescence spectral data; S26. Measure the random reflectance of the macerals in the polished slice under reflected light conditions.

4. The method according to claim 3, characterized in that, In step S3, taking out the source rock sample from the polished rock slice includes: S31. Cut off the solidified epoxy resin around the sample; S32. Soak and demold the residual epoxy resin wrapping the sample with an organic solvent to facilitate the complete removal of the source rock sample.

5. The method according to claim 4, characterized in that, For step S4, the device for conducting artificial thermal simulation on the source rock sample includes: a vacuum pumping device, a high-temperature reaction kettle, and a heating device. The left side of the high-temperature reaction kettle is connected to an inert gas pipeline through a first pipeline, and the right side is connected to the vacuum pumping device through a second pipeline. Valves are provided on both the first pipeline and the second pipeline. A digital display pressure gauge is connected to the top of the high-temperature reaction kettle. A sample chamber for placing the source rock sample is arranged inside the high-temperature reaction kettle, and the bottom of the high-temperature reaction kettle is connected to the heating device.

6. The method according to claim 5, characterized in that, The artificial thermal simulation of the source rock sample includes the following steps: S41. Put the demolded source rock sample into the sample chamber of the high-temperature reaction kettle; S42. Vacuumize and fill the high-temperature reaction kettle with inert gas through the vacuum pumping device and the inert gas pipeline. After the high-temperature reaction kettle is in a vacuum state, set the heating program, start heating, and conduct a thermal simulation experiment.

7. The method according to claim 5, characterized in that, Before conducting the artificial thermal simulation, conduct an airtightness test on the high-temperature reaction kettle.

8. The method according to claim 7, characterized in that, The airtightness test includes: First, use the vacuum pumping device to vacuumize the high-temperature reaction kettle; Second, inject inert gas into the high-temperature reaction kettle through the inert gas pipeline. If the internal pressure of the high-temperature reaction kettle remains stable, the airtightness of the device is good.

9. The method according to any one of claims 1-8, characterized in that, The equipment used in the method includes: an optical microscope and a thermal simulation device. The thermal simulation device includes a high-temperature reaction kettle, a vacuum pumping device, and a heating device; The optical microscope is used to observe the optical properties of organic macerals in the source rock sample before and after thermal simulation; The high-temperature reaction kettle is used to provide a high-temperature and high-pressure environment for the source rock sample to promote the pyrolysis of organic matter; A vacuum pumping device is used to pump the vacuum of a high-temperature reaction kettle to create an oxygen-free thermal evolution environment. A heating device is used to heat the source rock sample in the high-temperature reaction kettle to cause pyrolysis of the source rock sample.

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