Method for representing microstructure of maceral in shale through combination of light microscope and electron microscope

Abstract

The invention provides a method for characterizing a microstructure of a maceral in shale by combining a light microscope and an electron microscope, which comprises the following steps: preparing a shale sample to be observed to obtain a sample with a required specification; abrasive paper of different specifications is used for polishing, and a sample with the thickness being 3-5 mm is obtained; performing argon ion polishing on a to-be-observed surface of the treated sample; straight lines which are perpendicular to each other and spaced by 1 mm in the transverse direction and the longitudinal direction are engraved on the polished to-be-observed face through a laser engraving machine, and the straight lines are numbered respectively; carrying out optical microscope observation on the processed sample, identifying the interested macerals, photographing, and recording the number of the laser engraving straight line closest to the macerals; and carrying out scanning electron microscope observation on the sample, repositioning the interested macerals according to the recorded numbers, and carrying out microstructure observation and photographing on the interested macerals. According to the method, the microstructure of the maceral in the shale can be rapidly, economically and effectively represented.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration, and particularly to a method for jointly using an optical microscope and an electron microscope to characterize the microscopic structure of macerals in shale. Background Art

[0002] The macerals in shale refer to organic matters that can be identified under an optical microscope. According to different biological sources and subsequent diagenetic evolution processes, the macerals can be divided into sapropel group, exinite group, vitrinite group, inertinite group, animal organic debris group, secondary organic group, etc. Under an optical microscope, different macerals can be identified according to the color, convexity and morphological characteristics of organic matters under reflected light, transmitted light and fluorescence. With the commercial development of shale gas, it is found that the nano-scale organic pores in macerals are important spaces for shale gas storage. Usually, a scanning electron microscope is used to observe the nano-pores in shale macerals, and it is found that there are great differences in the pore development degrees in different organic matters, which are mainly caused by the differences in the physical and chemical structures of different macerals. Therefore, in the evaluation of shale organic pores, it is very necessary to evaluate according to the maceral classification. However, different macerals cannot be effectively identified under a scanning electron microscope, and nano-scale organic pores cannot be observed under an optical microscope. Under the above background, jointly using an optical microscope and a scanning electron microscope becomes a necessary means to characterize the nano-pores of macerals in shale.

[0003] The joint use of an optical microscope and a scanning electron microscope involves repositioning the macerals identified by the optical microscope under the scanning electron microscope. The conventional method is to compare and reposition the target macerals under the scanning electron microscope according to the maceral photos taken by the optical microscope. A large number of practical operations show that this method is time-consuming, laborious, has high experimental costs, and has poor repositioning effects. According to the inventor's previous research experience, only about 10% of the macerals observed by the optical microscope can be repositioned under the scanning electron microscope. Another method is to make marks around the identified macerals by means of a marking objective lens configured on the optical microscope, and then locate the macerals according to the marks under the scanning electron microscope. Although this method greatly improves the success rate of maceral positioning compared with the first method, when it is necessary to observe multiple macerals in the same sample, the marks of each component are very similar, which will increase the difficulty of repositioning.

[0004] To solve this problem, a variety of commercial devices combining optical microscopes and scanning electron microscopes have emerged. Their core technology is to construct a common coordinate system and software recognition system for optical microscopes and scanning electron microscopes. Microconstituents are identified under the optical microscope, photographed, and their coordinates are recorded. Then, the sample is transferred to the scanning electron microscope, and by inputting the coordinates, the target microconstituent can be repositioned. This technology has well solved the problem of difficult repositioning in the combination of optical microscopes and scanning electron microscopes. However, the prices of these instruments are relatively high, and this positioning system can only be installed in the optical microscopes and scanning electron microscopes produced by this enterprise. Therefore, due to the high cost, it is difficult to promote and apply. Summary of the Invention

[0005] In view of the above analysis, the object of the present invention is to provide an efficient and economical method for characterizing the microstructure of microconstituents in shale by combining an optical microscope and a scanning electron microscope, which is used for evaluating the development degree of organic pores in shale.

[0006] The object of the present invention is mainly achieved through the following technical solutions:

[0007] A method for characterizing the microstructure of microconstituents in shale by combining a light microscope and an electron microscope, comprising:

[0008] Step S1: Take a shale sample to be observed. The specifications of the sample are determined by the observer according to the research purpose and in combination with the requirements of instruments such as ion sputtering instruments, optical microscopes, and scanning electron microscopes.

[0009] Step S2: Grind the upper and lower surfaces of the sample to be observed with 300 - 600 mesh sandpaper to a thickness of 3 - 5 mm. The surface to be observed needs to be polished with 2000 mesh, 3000 mesh, and 5000 mesh sandpaper for 3 minutes respectively.

[0010] Step S3: Ion-polish the surface to be observed with an ion sputtering instrument.

[0011] Step S4: Use a laser engraving machine to engrave straight lines with a transverse and longitudinal interval of 1 mm that are perpendicular to each other on the surface to be observed of the ion-polished sample. The horizontal and vertical lines are respectively parallel to two adjacent sides of the sample, and numbers 1, 2, 3... are engraved at the left end of the horizontal line and the upper end of the vertical line respectively as the straight line numbers.

[0012] Step S5: Observe the polished sample with an optical microscope, identify and photograph the microconstituents of interest, and record the magnification of the photo and the serial number of the horizontal or vertical line closest to each microconstituent.

[0013] Step S6: Load the sample after optical microscope observation into the sample chamber of the scanning electron microscope. At a magnification similar to that of the optical microscope in the environmental scanning mode, reposition the microscopic components identified under the optical microscope according to the horizontal or vertical line numbers recorded in Step S5, and perform nano-level observation and photography on them.

[0014] In Step S1, take a shale sample to be observed using the following two methods:

[0015] The first method: Cut the original shale sample with a cutting machine to obtain a sample with the required position and specifications.

[0016] The second method: Strike the original sample with a geological hammer to obtain shale sample fragments, and polish the fragments with sandpaper of 300 - 600 meshes to obtain a sample with the required specifications.

[0017] In Step S3, the argon ion polishing parameters are: acceleration voltage, 3 - 5 kV; polishing time, 3 - 6 hours; incident angle, 8° - 10°; sample stage rotation speed, 3 - 8 revolutions per minute.

[0018] In Step S5, the observation with the optical microscope can be carried out using a dry objective lens or an oil immersion objective lens. If observed with an oil immersion objective lens, immersion oil needs to be dropped on the surface to be observed. In this case, after the optical microscope observation is completed, the immersion oil needs to be wiped clean with a lens paper soaked in anhydrous ethanol before subsequent scanning electron microscopy observation can be carried out.

[0019] The beneficial effects of the present invention are as follows: The method provided by the present invention can accurately reposition the microscopic components observed under the optical microscope under the scanning electron microscope in a relatively short time; at the same time, the method provided by the present invention only adds a laser engraving operation with relatively low cost, significantly saving the equipment purchase cost or test cost compared with commercial optical - electron combined devices; in short, the present invention provides a method for efficiently and economically evaluating the pore development degree of microscopic components in shale. Description of the Drawings

[0020] Figure 1 It is a flowchart of the operation of the present invention.

[0021] Figure 2 It is a top - view schematic diagram of the laser engraving lines of the shale sample to be observed in the present invention.

[0023] Figure 3 It is an optical microscope photograph of the laser engraving lines of a certain shale sample to be observed.

[0024] Figure 4 They are optical microscope and scanning electron microscope photographs of Example 1 of the present invention. A is the optical microscope photograph of solid bitumen, and B is the scanning electron microscope photograph of the solid bitumen shown in the square of A.

[0025] Figure 5 Optical microscope and scanning electron microscope photos of Example 2 of the present invention. A is the optical microscope photo of the alginite, B is the scanning electron microscope photo of the alginite shown in the box of A, and C is the scanning electron microscope photo of the alginite shown in the box of B.

[0026] Figure 6 Optical microscope and scanning electron microscope photos of Example 3 of the present invention. A is the optical microscope photo of the semifusinite, B is the scanning electron microscope photo of the semifusinite shown in the box of A, and C is the scanning electron microscope photo of the semifusinite shown in the box of B.

[0027] In the figure:

[0028] a. Sample boundary; b. Laser engraving line; c. Laser engraving line number; d. Target microscopic component. Detailed implementation manners

[0029] The technical solution of the present invention will be specifically described below with reference to the accompanying drawings. The accompanying drawings form a part of the present invention and are used together with the present invention to explain the methods and steps of the present invention, and are not used to limit the scope of the present invention.

[0030] The implementation of the present invention includes the following steps, and the flowchart is as Figure 1 shown:

[0031] Step S1: Use a cutting machine to cut a shale sample to be observed with a specification of 1 cm × 1 cm × 1 cm perpendicular to the bedding direction from the core sample.

[0032] Step S2: Use 400-mesh sandpaper to polish the upper and lower surfaces of the sample to be observed to a thickness of 5 mm. The surface to be observed needs to be polished with 2000-mesh, 3000-mesh, and 5000-mesh sandpapers for 3 minutes each in sequence.

[0033] Step S3: Polish the surface to be observed with an argon ion polisher. The polishing parameters are: acceleration voltage, 4.5 kV; polishing time, 4 hours; incident angle, 9°; sample stage rotation speed, 6 revolutions per minute.

[0034] Step S4: Use a laser engraving machine to engrave perpendicular horizontal and vertical lines with a spacing of 1 mm from the upper left corner of the surface to be observed of the ion-polished sample. The horizontal and vertical lines are respectively parallel to two adjacent sides of the sample, and numbers 1, 2, 3... are respectively engraved at the left end of the horizontal line and the upper end of the vertical line as the line numbers. The engraving schematic diagram is as Figure 2 shown, and the actual engraving effect is as Figure 3 shown.

[0035] Step S5: Observe the sample after polishing and engraving the straight line under the dry objective lens of an optical microscope, identify and photograph the microscopic components of interest, and record the magnification of the photo and the serial number of the nearest horizontal or vertical line to the microscopic component of interest.

[0036] Step S6: Load the sample after optical microscope observation into the sample chamber of a scanning electron microscope. At a magnification similar to that of the optical microscope in the environmental scanning mode, reposition the microscopic components identified under the optical microscope according to the horizontal or vertical line numbers recorded in Step S5, and conduct nanoscale observation and photography of them.

[0037] Example 1

[0038] For a shale sample at 2361 m in the Qingshankou Formation of Well Gu-12 in the Songliao Basin, operate according to the above steps; obtain an optical microscope photo of a piece of solid bitumen in this sample as shown in Figure 4 Figure A. Record the laser engraving line numbers nearest to this piece of solid bitumen, horizontal line 3 and vertical line 8; under the scanning electron microscope, quickly locate this piece of solid bitumen according to the laser engraving line numbers, indicating that the method provided by the present invention has high efficiency, and obtain its scanning electron microscope photo, as shown in Figure 4 Figure B, thereby realizing the microscopic structure characterization of the solid bitumen microscopic components in this shale sample. It can be seen that there are no pores on this piece of solid bitumen.

[0039] Example 2

[0040] For a shale sample at 2384 m in the Qingshankou Formation of Well Gu-12 in the Songliao Basin, operate according to the above steps; obtain an optical microscope photo of a piece of alginite in this sample as shown in Figure 5 Figure A. Record the laser engraving line number nearest to this piece of alginite, horizontal line 6; under the scanning electron microscope, quickly locate this piece of alginite according to the laser engraving line number, further demonstrating the high efficiency of the method provided by the present invention, and obtain its scanning electron microscope photo, as shown in Figure 5 Figure B. Further magnify this piece of alginite and obtain its scanning electron microscope photo, as shown in Figure 5 Figure C, thereby realizing the microscopic structure characterization of the alginite microscopic components in this shale sample. It can be seen that a large number of pores are developed in this piece of alginite.

[0041] Example 3

[0042] For a shale sample at 2368 m in the Qingshankou Formation of Well Gu-12 in the Songliao Basin, operate according to the above steps; obtain an optical microscope photo of a piece of semifusinite in this sample as shown in Figure 6As shown in A, record the laser engraving line number closest to this semi - vitrinite, which is vertical 5; under a scanning electron microscope, according to the laser engraving line number, quickly locate this semi - vitrinite, which also reflects the effectiveness of the present invention, and obtain its scanning electron microscope photo, such as Figure 6 As shown in B, further magnify this semi - vitrinite and obtain its scanning electron microscope photo, such as Figure 6 As shown in C, thereby realizing the microscopic structure characterization of the semi - vitrinite maceral in this shale sample. It can be seen that there are sporadic isolated pores in this semi - vitrinite.

Claims

1. A method for characterizing the microstructure of microscopic components in shale by combining light microscopy and electron microscopy, characterized in that: The following steps are involved: Step S1: Take a shale sample to be observed. The specifications of the sample are determined by the observer according to the research purpose and the requirements of the ion sputtering instrument, optical microscope, scanning electron microscope and other instruments for the sample; Step S2: Use 300-600 mesh sandpaper to grind the upper and lower surfaces of the sample to be observed to a thickness of 3-5 mm. The surface to be observed needs to be grinded for 3 minutes each with 2000 mesh, 3000 mesh and 5000 mesh sandpaper respectively; Step S3: performing argon ion polishing on the surface to be observed using an ion sputtering apparatus; Step S4: using a laser engraving machine to carve mutually perpendicular horizontal and vertical straight lines with a spacing of 1 mm on the observation surface of the sample after ion polishing, wherein the horizontal line and the vertical line are respectively parallel to the two adjacent edges of the sample, and numbers 1, 2, 3, etc. are respectively engraved on the left end of the horizontal line and the upper end of the vertical line as the straight line numbers; Step S5: observing the polished sample with an optical microscope, identifying and photographing the microscopic components of interest, and recording the magnification of the photograph and the number of the horizontal or vertical line closest to each microscopic component; Step S6: Load the sample observed under the optical microscope into the sample chamber of the scanning electron microscope, and relocate the microscopic components identified under the optical microscope according to the horizontal or vertical line numbers recorded in step S5 under the environmental scanning mode with a magnification similar to that of the optical microscope, and observe and photograph them at the nanoscale.

2. The method for characterizing the microstructure of microscopic components in shale by combining light microscopy and electron microscopy as claimed in claim 1, characterized in that: In step S1, a shale sample to be observed is taken using the following two schemes: The first solution: Use a cutting machine to cut the original shale sample to obtain samples with the required position and specifications; The second option is to hit the original sample with a geological hammer to obtain shale sample fragments, and then grind the fragments with 300-600 mesh sandpaper to obtain samples of the required specifications.

3. The method for characterizing the microstructure of microscopic components in shale by combining light microscopy and electron microscopy as claimed in claim 1, characterized in that: In step S3, the argon ion polishing parameters are: acceleration voltage, 3-5 kV, polishing time, 3-6 hours, incident angle, 8°-10°, and sample stage rotation speed, 3-8 rpm.

4. The method for characterizing the microstructure of microscopic components in shale by combining light microscopy and electron microscopy as claimed in claim 1, characterized in that: In step S5, optical microscope observation can be performed with a dry objective lens or an oil-immersion objective lens; if an oil-immersion objective lens is used for observation, immersion oil needs to be dripped on the surface to be observed. In this case, after the optical microscope observation is completed, the immersion oil needs to be wiped clean with lens paper soaked in anhydrous ethanol before subsequent scanning electron microscope observation can be performed.

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