A combined analysis method for the organic matter type and the development characteristics of micropores in rocks

Abstract

The present invention relates to the technical field of energy exploration and analysis, and particularly to a method for jointly analyzing the organic matter type and micropore development characteristics in rocks, comprising the following steps: Step 1: After drying the massive or granular rock samples, cold embedding is carried out using epoxy resin mixed with conductive powder to make cylindrical samples; Step 2: After demolding the cylindrical samples, the first mechanical polishing is carried out; Step 3: When observing the samples under a microscope, an etching objective lens is used to mark the sample surface; Step 4: The samples obtained in Step 3 are subjected to the second mechanical polishing and argon ion polishing; Step 5: Rapid positioning observation is carried out using the scanning electron microscope navigation photos and the photos of the marked samples. The present invention applies the method of etching and marking to the marking of organic matter microscopic observation. After the secondary treatment process is completed, the samples can be subjected to various scanning electron microscope analyses, realizing the combined use of analysis techniques for multiple characteristics at multiple scales from the micron level to the nanometer level.

Description

Technical Field

[0001] The present invention relates to the technical field of energy exploration and analysis, and in particular to a method for jointly analyzing the type of organic matter and the development characteristics of micropores in rocks. Background Art

[0002] Unconventional oil and gas resources such as shale oil and shale gas are important alternative energy sources worldwide, and their huge resource potential may become the main force for oil production growth in the next 20 years.

[0003] Shale oil and shale gas have the characteristics of self-generation and self-storage. Compared with conventional oil and gas reservoirs, after shale oil and gas are generated from shale rich in organic matter, the pores in the shale become their main storage space. Research shows that the connectivity of organic matter pores is better than that of intra-mineral grain pores, and the three-dimensional effective pore network formed by the interconnection of organic matter particles will have a significant impact on the oil and gas storage capacity and permeability of thermally mature shale. Therefore, the fine characterization of organic matter pores is the key to evaluating the oil and gas storage capacity of shale. At the same time, the type of organic matter is one of the important factors affecting the development of organic matter pores. The microscopic component composition of organic matter and the type of bitumen will also have a significant impact on organic matter pores. For example, due to its good oil and gas generation ability, the development potential of organic matter pores of type I kerogen is much higher than that of type III kerogen. The sapropel group with strong hydrocarbon generation ability develops rich organic pores, while the pores in the vitrinite group are less developed. The pore development of bitumen at different evolution stages is different. Therefore, while analyzing the development characteristics of organic matter pores, it is very important to clarify the type of organic matter particles, and then distinguish the original sedimentary organic pores and the pyrolysis bitumen organic pores after migration, because the final organic pore network and connection path in mudstone mainly depend on the proportion of different states of organic matter distribution.

[0004] At present, the characterization methods of pore types in organic-rich mudstones can be mainly divided into four categories: imaging method, fluid invasion method, adsorption method, and scattering method. Among them, the imaging method is the only one that can directly observe the development characteristics such as the size, morphology, and distribution of micropores in organic matter in rocks. The existing imaging method analysis techniques mainly include field emission scanning electron microscopy (FE-SEM), focused ion beam scanning electron microscopy (FIB-SEM), etc., which can observe the pore size, morphology, and distribution characteristics in shale samples at the nanoscale, and are one of the effective means for finely characterizing the pore characteristics of organic matter. However, there are great deficiencies in the discrimination of organic matter types by the scanning electron microscopy observation method. This is mainly because the classification of organic matter types under the scanning electron microscope can only be based on their morphological characteristics, and it is very difficult to distinguish between amorphous sapropel group, exinite, and exudatinite with similar morphology, and it is also impossible to distinguish organic matter with different degrees of evolution. Only under the optical microscope can the oil immersion reflected light optical properties and fluorescence characteristics be used for discrimination. The existing scanning electron microscopy micropore observation method cannot realize the micropore observation of different types of microscopic components and bitumens, which seriously affects the research on the generation and enrichment laws of oil and gas in shales and the evaluation of reservoir capacity. Therefore, there is an urgent need to develop a combined observation and analysis method for the optical properties and scanning electron microscopy characteristics of specific observation targets in rock samples. Summary of the Invention

[0005] The purpose of the present invention is to provide a combined analysis method for the types of organic matter and the development characteristics of micropores in rocks, which can solve the above technical problems.

[0006] The present invention provides a combined analysis method for the types of organic matter and the development characteristics of micropores in rocks, including the following steps:

[0007] Step 1: After drying the massive or granular rock sample, cold embedding is carried out using epoxy resin mixed with conductive powder to make a cylindrical sample;

[0008] Step 2: After demolding the cylindrical sample, perform the first mechanical polishing;

[0009] Step 3: When observing the sample under the microscope, mark the sample surface using an etching objective lens;

[0010] Step 4: Perform the second mechanical polishing and argon ion polishing on the sample obtained in Step 3;

[0011] Step 5: Use the scanning electron microscopy navigation photo and the marked sample photo for rapid positioning observation.

[0012] Preferably, the specific steps of step 1 are as follows: Dry the prepared rock blocks and debris for later use. Prepare a cold mounting mold, apply a release agent to the inner surface of the mold, and wait for the release agent to dry. Then, place the rock blocks or debris into the cold mounting mold respectively. Add a curing agent to the epoxy resin and mix evenly. Add conductive powder to the epoxy resin and stir well. Pour the mixed conductive epoxy resin into the cold mounting mold, let it stand until cured, and then demold to obtain cylindrical samples for later use. The mixing ratio of the conductive powder to the epoxy resin and the mixing ratio of the epoxy resin to the curing agent are determined according to the requirements of different product types.

[0013] Preferably, the specific steps of step 2 are as follows: In the first step, use 400 or 600 - mesh sandpaper to grind the rock surface on an automatic polishing machine. If it is a granular sample, it is advisable to make the edges of the sample outcrops closely connected. In the second step, use 1200 - mesh sandpaper for fine surface grinding to reduce the scratch depth. In the third step, polish the sample surface with alumina polishing liquid and a matching polishing cloth. In the fourth step, polish the sample surface with alumina polishing liquid and a matching polishing cloth with a smaller particle size than that in the third step.

[0014] In the third and fourth steps of step 2, polish with 0.3μm and 0.05μm alumina polishing liquid respectively.

[0015] Preferably, the standard of the sample obtained in step 2 is that the surface is flat under a microscope, and it is advisable that no scratches can be seen under a magnification of 500 times.

[0016] Preferably, the specific steps of step 3 are as follows: Under oil - immersion reflected light, conduct an observation and analysis of the organic matter type according to the standard method to clarify the main organic matter type characteristics in the sample. Select typical organic matter particles and use an etching objective lens to draw a circle mark on the observation target.

[0017] Preferably, the lower part of the etching objective lens is a large cylindrical structure, and a movable small steel cylindrical structure (diameter 2.5mm) is nested in the large cylindrical structure. The small cylindrical structure is located on one side of the center point of the large cylindrical structure, and the steel vertex sharpened at the edge 0.25mm from the small cylindrical structure. When the large cylindrical structure is rotated, the small cylindrical structure will rotate together with the large cylindrical structure driven by the large cylindrical structure, and the steel vertex will form a circular mark on the sample with the center point of the large cylindrical structure as the center. When only the small cylinder is rotated, the distance between the steel vertex and the center point of the large cylinder gradually increases to 2.25mm. Therefore, when the large cylinder is rotated, the radius of the etched circle is 0.25mm - 2.25mm, and the diameter of the formed circular mark is 0.5mm - 4.5mm.

[0018] After completing the microscopic observation of organic matter particles, clarifying the type of organic matter and its optical properties, and etching and marking the particles of interest, it is necessary to remove the surface immersion oil and etching debris, and meet the flatness requirements for scanning electron microscopy observation. The surface of the sample needs to be polished mechanically for the second time and polished by argon ion.

[0019] Preferably, the method of the second mechanical polishing in step 4 is as follows: the rotation speed of the chassis of the automatic polishing machine is 160 rpm, the rotation speed of the mechanical head is 70 rpm, the single-point pressure is 10 N. Under the condition of 0.05 μm alumina polishing solution and supporting polishing cloth, polish for 30 - 50 seconds, then place the surface of the sample downward into the cutting fixture for bottom cutting to make the top and bottom surfaces of the sample parallel and the height < 1 cm.

[0020] Preferably, to ensure the observation effect of the scanning electron microscope and remove the mechanical scratches on the surface of the organic matter, argon ion polishing needs to be carried out. In the experiment, it is necessary to adjust the ion beam voltage and current and set the polishing program to avoid the damage of the high temperature during the polishing process to the organic matter on the surface of the sample while achieving a satisfactory polishing effect. The appropriate voltage, polishing time and polishing program can be adjusted according to the hardness and thermal evolution degree of the sample. The argon ion polishing program in step 4 is as follows: a. 4.5 kV - 6.5 kV, 30 min - 120 min; b. 2.5 kV - 3 kV, 10 min - 15 min.

[0021] If it is impossible to add conductive powder in the cold embedding step under the experimental conditions, a coating step can be added after the argon ion polishing step is completed. Coating is to meet the conductivity requirements of the surface of the sample for electron microscopy observations such as field emission scanning electron microscopy and focused ion beam scanning electron microscopy. The coating can choose carbon coating or platinum coating. The thickness of carbon coating is 3 nm - 5 nm, and the thickness of platinum coating is 2 nm - 3 nm. When it is necessary to carry out analyses with higher requirements for conductivity such as energy spectrum, the thickness of carbon coating is preferably 8 nm - 10 nm, and the thickness of platinum coating is 3 nm - 5 nm.

[0022] Preferably, the specific steps of step 5 are as follows: after completing the above sample observation and pretreatment steps, fix the sample on the sample stage according to the operation process of the field emission scanning electron microscope, focused ion beam scanning electron microscope or helium ion scanning electron microscope, put it into the sample chamber, rotate the sample to the same angle as when observing under the microscope, and take a navigation photo. When observing with the electron microscope, quickly locate to the position of the etching mark or the microscope photo by double-clicking the mouse, and carry out positioning analysis.

[0023] Beneficial effects:

[0024] The method of etching marks in the present invention is applied to the marking of organic matter microscopic observation. After the secondary treatment process is completed, the sample can be subjected to various scanning electron microscope analyses, and the marked objects can be located and observed for various characteristics, realizing the combined use of analysis techniques for multi-scale and multiple characteristics from the micron scale to the nanometer scale. It solves the problem that the parent material type of organic matter with pore development cannot be determined by conventional scanning electron microscope micropore observation. Through the technical process in the present invention, based on the analysis of the organic matter type, the micropore development characteristics of each type of organic matter can be located and observed, solving the problem that the organic matter type cannot be determined in electron microscope observation.

[0025] The combined method of the present invention can realize the observation and analysis of polished surfaces of blocky, granular and powdery samples by microscope-scanning electron microscope. In addition to carrying out the location analysis of organic matter type and micropore characteristics, this combined method can also be applied to other location analyses where it is not suitable to make marks with a pen. The processed sample is not only suitable for scanning electron microscope observation, but also can carry out other rock polished section analysis and testing items, such as laser Raman spectroscopy analysis, etc., and can also carry out analysis and testing of various accessories at the scanning electron microscope end, such as energy spectrum at the scanning electron microscope end, electron probe, cathode luminescence identification, EBSD analysis, secondary ion mass spectrometry, etc. At the same time, the marked sample can still be repeatedly subjected to microscope observation and analysis and can be permanently preserved. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0027] Figure 1 It is a schematic structural diagram of the bottom of the etching objective lens of the present invention;

[0028] Figure 2 It is a schematic structural diagram of the sample cutting fixture of the present invention;

[0029] Figure 3 It is a positioning photo of the present invention after marking under the microscope and during scanning electron microscope observation;

[0030] Figure 4 It is a comparison photo of the same marked area under the reflected light of the microscope and the scanning electron microscope of the present invention;

[0031] Figure 5 It is a comparison photo of the same area under the fluorescence of the microscope and the scanning electron microscope of the present invention;

[0032] Figure 6 For the present invention Figure 5 An enlarged schematic diagram of the left figure;

[0033] Figure 7 For the present invention Figure 6 The backscattered electron image of the scanning electron microscope corresponding to region 1 in the present invention;

[0034] Figure 8 For the present invention Figure 6 The backscattered electron image of the scanning electron microscope corresponding to region 2 in the present invention;

[0035] Figure 9 For the present invention Figure 6 The backscattered electron image of the scanning electron microscope corresponding to region 3 in the present invention;

[0036] Figure 10 For the present invention Figure 6 The backscattered electron image of the scanning electron microscope corresponding to region 4 in the present invention;

[0037] Figure 11 The structural schematic diagram of the etching objective lens used in the present invention.

[0038] Explanation of reference numerals: 1 - large cylindrical structure, 2 - small cylindrical structure, 3 - vertex, 4 - movable screw, 5 - threaded port, 6 - dial wheel. Detailed implementation manners

[0039] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the scope of protection of the present invention.

[0040] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0041] In addition, the terms "first" and "second" are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of the said features. In the description of the present invention, the meaning of "a plurality" is two or more, unless otherwise specifically defined. In addition, the terms "mounted", "connected", and "coupled" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0042] In view of the technical defect that it is difficult to determine the type of organic matter of the observation object in the existing analysis methods of organic matter micropore imaging, on the basis of in-depth research on the processes of two analysis methods and the sample pretreatment methods, the present invention proposes a combined analysis method in which the same sample is subjected to different pretreatment methods to simultaneously carry out microscopic observation under oil immersion conditions and scanning electron microscope observation under high vacuum conditions. The method has a simple process and reliable results, and is a non-destructive (without changing the sample characteristics and can be repeatedly analyzed and permanently preserved) combined analysis method for positioning the type of organic matter and the development characteristics of micropores.

[0043] Example 1

[0044] To highlight the applicability of the method to the sample, samples of two lithologies and properties, namely siliceous mudstone, carbonaceous mudstone, clastic particles and massive, were selected for the description of specific examples.

[0045] The prepared rock blocks (with one side planed) and clastic particles were dried and reserved. Prepare a cold embedding mold, apply a release agent on the inner surface of the mold, and wait for the release agent to dry. Place the rock block (plane down) and about 3 g of clastic particles into the mold respectively.

[0046] According to the product instructions, after uniformly mixing the curing agent and epoxy resin at a volume ratio of 1:2, add the conductive powder and epoxy resin at a weight ratio of 1:1 to the epoxy resin, stir well, pour the mixed epoxy resin into the cold embedding mold, let it stand for 9 hours, and then demold and reserve.

[0047] The demolded sample was polished according to the following method, and the sample was ultrasonically cleaned for 1 minute after completing the second and fourth steps. Place the polished sample in a desiccator for standby.

[0048]

[0049] Under a polarized fluorescence microscope, using a 50x oil immersion objective lens, the type characteristics of organic matter are determined by comparing the oil immersion reflected light and fluorescence optical properties of the organic matter in the sample. Select typical organic matter particles or regions of interest and mark them using an etching objective lens. Select a marking range with a diameter of 2 mm - 3 mm and etch 25 circles. After completing the marking, wipe off the immersion oil, take a photo of the marked sample, and record the marking order, such as Figure 3 shown. In addition, the uppermost end of the etching objective lens is a threaded port 5, which has the same threaded port specifications as that of a common objective lens. When in use, the etching objective lens and the common objective lens are installed on the objective lens turret together. The structural schematic diagram of the etching objective lens is Figure 11 shown. When in use, according to the hardness of the sample, the rotary dial 6 can be rotated 15 to 50 circles to leave a mark with an appropriate depth. Specifically, at 0.25 mm from the edge of the small cylindrical structure 2 with a diameter of 2.5 mm is a sharpened steel vertex 3. When the large cylindrical structure 1 is rotated, the small cylindrical structure 2 will rotate along with the large cylindrical structure 1 under the drive of the large cylindrical structure 1, and the steel vertex 3 will form a circular scratch on the sample with the center point of the large cylindrical structure 1 as the center. The etching objective lens actually used in the operation of this embodiment is Leica Object marker M 25, product number: 11505059.

[0050] Perform secondary treatment on the sample, and the specific process is as follows: a. Secondary polishing: Set the chassis rotation speed of the automatic polishing machine to 160 rpm, the fixture rotation speed to 70 rpm, and the single-point pressure to 10 N. Under the conditions of 0.05 μm alumina polishing solution and a supporting polishing cloth, polish for 30 - 50 seconds. b. Bottom cutting: Place the sample face down in a special cutting fixture and perform bottom cutting to make the sample meet the requirements of parallel top and bottom surfaces and a height < 1 cm. Argon ion polishing requires the bottom and top surfaces of the sample to be completely parallel and the thickness < 1 cm. Therefore, the sample needs to be cut twice. Use the Figure 2 sample cutting fixture in the appendix to cut the bottom surface of the sample. Specifically, use the Figure 2 sample cutting fixture in the appendix for secondary cutting. The inner diameter of the fixture is 26 mm - 28 mm, the side wall and bottom are 2 mm thick, the total height is 6 mm - 8 mm, there are three movable screws 4 on the side to fix the sample, and the bottom screw hole is used to fix the fixture. c. Argon ion polishing: Set the polishing conditions to 5.5 kV for 40 min; 3 kV for 15 min.

[0051] Fix the sample on the sample stage with conductive glue and place it in the sample chamber of the scanning electron microscope. Adjust the sample orientation and take a navigation photo. Observe the organic matter particles and regions of interest one by one from low magnification to high magnification under the scanning electron microscope. The observation effect is as Figure 4 - 10 shown, which reflects the oil immersion reflected light and fluorescence optical properties of sporinite and asphaltene and their micropore development characteristics.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for jointly analyzing the type of organic matter and micropore development characteristics in rocks, characterized in that: The following steps are involved: Step 1: After the block or granular rock sample is dried, it is cold-mounted using epoxy resin mixed with conductive powder to make a cylindrical sample; Step 2: After demoulding the cylindrical sample, perform the first mechanical polishing; the specific steps of step 2 are as follows: the first step is to grind the rock surface on an automatic grinding and polishing machine using 400 or 600 mesh sandpaper. If it is a granular sample, it is better to make the exposed edges of the sample closely connected; the second step is to use 1200 mesh sandpaper to finely grind the surface to reduce the depth of the scratch; the third step is to polish the sample surface using aluminum oxide polishing liquid and matching polishing cloth; the fourth step is to polish the sample surface using aluminum oxide polishing liquid with a smaller particle size than that in the third step and matching polishing cloth; Step 3: When observing the sample under a microscope, use an etching objective lens to mark the sample surface; the specific steps of step 3 are as follows: under oil-immersion reflected light, carry out observation and analysis of organic matter types according to standard methods, clarify the characteristics of the main organic matter types in the sample, select typical organic matter particles, and circle the observation targets using an etching objective lens; Step 4: The sample obtained in step 3 is subjected to a second mechanical polishing and argon ion polishing; the method for the second mechanical polishing in step 4 is as follows: the chassis speed of the automatic polishing machine is 160 rpm, the mechanical head speed is 70 rpm, the single point pressure is 10 N, and the polishing liquid and the matching polishing cloth are used for polishing for 30-50 seconds, and then the sample surface is placed in a cutting fixture with the surface facing downward, and the bottom surface is cut so that the top and bottom surfaces of the sample are parallel and the height is less than 1 cm; Step 5: Use the SEM navigation photos and the marked sample photos for quick positioning and observation.

2. The combined analysis method of organic matter type and micropore development characteristics in rock according to claim 1, characterized in that: The specific steps of step 1 are as follows: drying the prepared rock blocks and debris for use, preparing a cold mounting mold, applying a release agent on the inner surface of the mold, and after the release agent is dried, placing the rock blocks or debris into the cold mounting mold respectively; adding a curing agent to the epoxy resin and mixing evenly, adding conductive powder to the epoxy resin, stirring well, pouring the mixed conductive epoxy resin into the cold mounting mold, standing until solidified, and then demolding to obtain a cylindrical sample for use.

3. The combined analysis method of organic matter type and micropore development characteristics in rock according to claim 1, characterized in that: The third and fourth steps in step 2 are polished using 0.3 μm and 0.05 μm aluminum oxide polishing liquids respectively.

4. The combined analysis method of organic matter type and micropore development characteristics in rock according to claim 3, characterized in that: The standard of the sample obtained in step 2 is that the surface is flat under a microscope, preferably with no scratches visible at 500 times magnification.

5. The method for jointly analyzing the type of organic matter and micropore development characteristics in rocks according to claim 1, characterized in that: The lower part of the etching objective lens is a large cylindrical structure, in which a movable small steel cylindrical structure is embedded. The small cylindrical structure is located on one side of the center point of the large cylindrical structure, and the edge of the small cylindrical structure is a sharpened steel vertex at 0.25 mm.

6. The method for jointly analyzing the type of organic matter and micropore development characteristics in rocks according to claim 1, characterized in that: The argon ion polishing procedure in step 4 is: a. 4.5 kV-6.5 kV, 30 min-120 min; b. 2.5 kV-3 kV, 10 min-15 min.

7. The combined analysis method of organic matter type and micropore development characteristics in rock according to claim 1, characterized in that: The specific steps of step 5 are as follows: after completing the above-mentioned sample observation and pretreatment steps, fix the sample on the sample stage according to the operating procedures of the field emission scanning electron microscope, focused ion beam scanning electron microscope or helium ion scanning electron microscope, place it in the sample chamber, rotate the sample to the same angle as when observing under the microscope, and take a navigation photo. When observing under the electron microscope, double-click the mouse to quickly locate the location of the microscope photo and carry out positioning analysis.

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