Method for identifying biological siliceous shale

Abstract

The invention discloses a method for identifying biological siliceous shale, which comprises the following steps of: firstly, identifying a bright stripe layer in the biological siliceous shale as a diatom, calcium algae and radiation insect concentrated development section by utilizing an optical microscope, then quantitatively identifying the total siliceous mineral content of a sample by utilizing energy spectrum minerals, and measuring organic matters and porosity by utilizing a field emission scanning electron microscope. According to the method, the biological siliceous shale is identified from different microscales by adopting three methods, the results are mutually verified, and the method provides effective technical support for evaluating favorable layer sections of the shale and is of great significance for improving the exploration and development effect of shale gas.

Description

Technical Field

[0001] This invention relates to the field of oil and gas and shale gas exploration technology, and in particular to a method for identifying biogenic siliceous shale. Background Technology

[0002] With advancements in unconventional oil and gas exploration and evaluation technologies, resource assessment and exploration of source rock strata, especially shale itself, have become a hot topic in the current oil and gas exploration field. Multiple marine organic-rich shale strata are widely developed in the middle and upper Yangtze region of southern my country, particularly the Wufeng Formation-Longmaxi Formation of the Ordovician-Silurian system, which has attracted widespread attention due to breakthroughs and successful development of shale gas in this stratum. The lower section of the Wufeng Formation-Longmaxi Formation, characterized by high organic matter and high silica content, is known as biogenic siliceous shale. Its fracturability and several key factors for evaluating shale gas as a "sweet spot" make it a preferred target stratum for shale gas development. Accurate identification of biogenic siliceous shale has become a crucial factor for target evaluation and efficient development of the Wufeng Formation-Longmaxi Formation shale in the Sichuan Basin.

[0003] Currently, methods for identifying biogenic siliceous shale mainly fall into two categories: ① whole-rock optical microscopy observation and environmental scanning electron microscopy analysis of microscopic features; ② X-ray diffraction and energy dispersive spectroscopy analysis of shale mineral composition. These two methods can only determine the silica content and the characteristics of coarse-grained siliceous minerals. They cannot achieve the observation of nanoscale siliceous mineral particles, the determination of organic matter and porosity in biogenic siliceous shale, or the identification of the connectivity of organic matter and porosity. Therefore, a comprehensive approach using multiple microscopic features is necessary to identify biogenic siliceous shale. Summary of the Invention

[0004] The purpose of this invention is to provide a method for identifying biogenic siliceous shale, which measures shale samples based on the identification of laminae and mineral structures under an optical microscope, quantitative identification of minerals by energy dispersive spectroscopy, and identification of organic matter and pores by field emission scanning electron microscopy, thereby identifying biogenic siliceous shale.

[0005] To solve the above-mentioned technical problems, the present invention is implemented using the following technical solution: This invention improves a method for identifying biogenic siliceous shale, comprising: Optical methods were used to identify the structure of shale samples and determine whether they conformed to the characteristics of biogenic siliceous shale; The shale samples were subjected to energy dispersive spectroscopy (EDS) for mineral quantitative identification to determine whether they conformed to the characteristics of biogenic siliceous shale. The shale samples were subjected to organic matter and porosity identification to determine whether they conformed to the characteristics of biogenic siliceous shale; If the above judgment results all meet the characteristics of biogenic siliceous shale, then it is identified as biogenic siliceous shale.

[0006] Preferably, the step of using optical methods to identify the structure of shale samples and determine whether they conform to the characteristics of biogenic siliceous shale includes: Prepare thin sections of samples for observation under an optical microscope to obtain two-dimensional images of shale under an optical microscope; Based on the acquired two-dimensional optical microscope images of shale, the mineral laminae and mineral composition characteristics of the shale samples were identified. If the bright laminae in a shale sample are concentrated segments of phanerocrystalline quartz formed by the diagenesis of diatoms, calcareous algae, and radiolarian shells, with a laminae thickness of 0.1–3.0 mm, and the dark laminae are relatively undeveloped segments of diatoms, calcareous algae, and radiolarians, with a content of >48% of nanoscale cryptocrystalline quartz minerals, and the dark laminae and bright laminae have an interlocking structure, then it is judged to be consistent with the characteristics of biogenic siliceous shale.

[0007] Preferably, the shale sample is subjected to energy dispersive spectroscopy (EDS) for quantitative mineral identification to determine whether it conforms to the characteristics of biogenic siliceous shale, including: Thin sections of shale samples were prepared, and energy dispersive spectroscopy (EDS) was used to perform two-dimensional mineral scanning on the thin sections of the shale samples using an automated mineral analysis electron microscope to obtain two-dimensional images of shale with micron-level resolution. Based on the micron-resolution two-dimensional image of shale, the percentage of the area of ​​each mineral in the shale relative to the sample area is extracted; the minerals in the shale include: quartz, plagioclase, potassium feldspar, calcite, dolomite, illite, chlorite, and pyrite. The determination is based on the percentage of the area of ​​each mineral in the shale relative to the sample area. If the following conditions are met: quartz content > 50%, clay mineral content < 30%, feldspar content < 10%, and calcium content < 15%, then it is determined to meet the characteristics of biogenic siliceous shale. The clay includes illite and chlorite, the feldspar includes plagioclase and potassium feldspar, and the calcium includes calcite and dolomite.

[0008] Preferably, the automated mineral analysis electron microscope is Maipscan 2.0.

[0009] Preferably, the shale sample is subjected to organic matter and porosity identification to determine whether it conforms to the characteristics of biogenic siliceous shale, including: The shale sample was subjected to field emission scanning electron microscopy (FEM) two-dimensional scanning imaging to obtain a two-dimensional image of the shale with nanometer-scale resolution; Based on the nanometer-resolution two-dimensional image of shale, the percentage S of shale organic matter area to sample area is extracted. 有机质 And, the percentage of pore area to sample area, S 孔隙度 , If the percentage of organic matter area and total porosity of a shale sample are both within the preset range, it is judged to conform to the characteristics of biogenic siliceous shale.

[0010] Preferably, a field emission scanning electron microscope with a resolution of 4 nm is used for two-dimensional scanning.

[0011] Preferably, the preset range of the percentage of organic matter area is 5.02%-13.38%, with an average of 8.52%.

[0012] Preferably, the total porosity is within a preset range of 1.60%-5.88%, with an average of 2.86%, and the proportion of organic pores is 90%.

[0013] The beneficial effects of this invention are as follows: This invention proposes three methods to identify and characterize biogenic siliceous shale at different microscales, allowing the results to corroborate each other. This method provides effective technical support for evaluating favorable shale formations and is of great significance for improving the effectiveness of shale gas exploration and development.

[0014] The method of this invention is applicable to the marine shale development areas in the Sichuan Basin and its surrounding areas in China, including shallow shale development areas in mountainous regions. It can identify rich and high-yield sections of shale in a relatively economical and effective manner, and identify the electrical characteristics of shale. It can also use well logging curves to quickly identify and classify biogenic siliceous shale sections in other wells in the region, thereby improving the exploration and development efficiency of shale target areas. Attached Figure Description

[0015] Figure 1 This is a flowchart of a method for identifying biogenic siliceous shale provided in Embodiment 1 of the present invention; Figure 2 The image shows the radial striations and mineral structure features of a radiolarian under an optical microscope in Embodiment 2 of the present invention. Figure 3 The image shows the lamellar and mineral structure features of calcareous algae under an optical microscope in Embodiment 2 of the present invention. Figure 4 In Embodiment 2 of the present invention, a two-dimensional image for quantitative identification of minerals using energy dispersive spectroscopy (MaipSCAN) is used. Figure 5 This is a two-dimensional field emission scanning electron microscope (SEM) image from Embodiment 2 of the present invention. Detailed Implementation

[0016] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The embodiments described below with reference to the accompanying drawings are illustrative and intended to explain the present invention, and should not be construed as limiting the present invention.

[0017] It should be noted that, as used in this invention, "an embodiment" or "embodiment" refers to a specific feature, structure, or characteristic that can be included in at least one implementation of this invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that excludes other embodiments.

[0018] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention. Example 1

[0019] Embodiment 1 of the present invention provides a method for identifying biogenic siliceous shale, such as... Figure 1 As shown, it specifically includes: Optical methods were used to identify the structure of shale samples and determine whether they conformed to the characteristics of biogenic siliceous shale; Energy dispersive spectroscopy (EDS) was used to quantitatively identify minerals in shale samples to determine whether they conformed to the characteristics of biogenic siliceous shale. Organic matter and porosity were identified in shale samples to determine whether they conformed to the characteristics of biogenic siliceous shale; If the above judgment results all meet the characteristics of biogenic siliceous shale, then it is identified as biogenic siliceous shale.

[0020] In this embodiment, optical methods are used to identify the structure of shale samples and determine whether they conform to the characteristics of biogenic siliceous shale, specifically including: 1.1 Prepare thin sections of the sample for observation under an optical microscope, measuring 2cm x 3cm. Use a microscope (Leica, DM4500P optical microscope) to photograph the shale sample and obtain high-resolution two-dimensional images of the shale. 1.2. Based on the acquired two-dimensional images of shale, identify the mineral laminae and mineral composition characteristics of the shale samples. If the bright laminae in a shale sample are concentrated sections of phanerocrystalline quartz formed from the diagenesis of diatoms, calcareous algae, and radiolarian shells, with a laminae thickness of 0.1–3.0 mm; and the dark laminae are relatively less developed sections of diatoms, calcareous algae, and radiolarians, mainly composed of nanoscale cryptocrystalline quartz minerals, with occasional white bright spots representing scattered phanerocrystalline quartz, calcite, and dolomite; and there is a distinct mosaic structure between the thick dark laminae and the thin bright laminae, then it is judged to be consistent with the characteristics of biogenic siliceous shale.

[0021] In this embodiment, energy dispersive spectroscopy (EDS) is performed on shale samples to quantitatively identify minerals and determine whether they conform to the characteristics of biogenic siliceous shale. Specifically, this includes: 2.1. Shale samples were prepared into thin sections with a diameter of 1 cm. Energy dispersive spectroscopy (EDS) was performed using an automated mineral analysis electron microscope (Maipscan 2.0) to obtain two-dimensional images of shale with micron-level resolution. 2.2 Using image processing technology, extract the percentage of the area of ​​each mineral (quartz, plagioclase, potassium feldspar, calcite, dolomite, illite, chlorite, pyrite, etc.) in the shale sample.

[0022] If a shale sample contains more than 50% quartz, low feldspar content, moderate clay (illite, chlorite) content, and low calcium (calcite, dolomite) content, it is judged to be consistent with the characteristics of biogenic siliceous shale.

[0023] In this embodiment, the organic matter and porosity of the shale sample are identified to determine whether it conforms to the characteristics of biogenic siliceous shale, specifically including: 3.1. Perform 4nm resolution SEM (field emission scanning electron microscope) two-dimensional scanning imaging on the shale sample to obtain a nanometer-resolution two-dimensional image of the shale with a field of view of 400μm*150μm. 3.2 Using image processing techniques, extract the percentage S of shale organic matter area to the sample area. 有机质 The percentage of pore area to sample area, S 孔隙度 , If the organic matter volume content (area percentage) of a shale sample is between 5.02% and 13.38%, with an average of 8.52% (mass content between 2.51% and 6.69%, with an average of 4.26%), and the total porosity is between 1.60% and 5.88%, with an average of 2.86%, and the proportion of organic pores is around 90% with a small proportion of inorganic pores, then it is judged to be consistent with the characteristics of biogenic siliceous shale.

[0024] This embodiment uses multiple methods for observation and measurement to comprehensively identify biogenic siliceous shale. Since the petroleum geological conditions of each region have their own universality and particularity, when different regions refer to these technical methods, they need to conduct comparative analysis, studying not only their similarities but also their detailed differences. Example 2

[0025] Based on the same inventive concept, this embodiment 2 uses the method of embodiment 1 to identify biogenic siliceous shale, and the specific implementation process is as follows: Step 1: Identify the structure of the shale sample using optical methods, specifically including: 1.1 Prepare thin sections of the sample for observation under an optical microscope, measuring 2cm x 3cm. Use a microscope (Leica, DM4500P optical microscope) to photograph the shale sample, obtaining high-resolution two-dimensional images of the shale, such as... Figure 2 and Figure 3 As shown; 1.2. Based on the acquired two-dimensional images of shale, identify the mineral laminae and mineral composition characteristics of the shale samples. Microscopic observation under an optical microscope revealed that the bright laminae in biogenic siliceous shale are concentrated areas of diatoms, calcareous algae, and radiolarians, with a laminae thickness generally ranging from 0.1 to 3.0 mm. The dark laminae in biogenic siliceous shale are relatively undeveloped areas of diatoms, calcareous algae, and radiolarians, mainly composed of nanoscale cryptocrystalline quartz minerals (>48%), micron-sized quartz minerals (about 2%), and the remainder consisting of clay, calcareous minerals, and pyrite.

[0026] Biogenic laminae (bright laminae) are mainly caused by the vigorous development of organisms and are unrelated to changes in terrestrial resource supply. They belong to endoclasts and are mainly composed of large-grained authigenic quartz, calcite, and dolomite formed by radiolarians, calcareous algae, and graptolites during diagenesis. Radiolarian laminae and calcareous algae laminae can be identified among them.

[0027] Radiolarians are a type of planktonic protozoan with a siliceous skeleton. See also Figure 2 Through identification and observation of ordinary thin sections, radiolarians were found scattered in the siliceous rock in a star-like pattern, with a small and sporadic presence. The siliceous rock matrix mainly consisted of cryptocrystalline chalcedony and clay-grade quartz. The radiolarian silicification (authigenic quartz) laminae lacked grain order, with distinct angular grains that were not rounded and distributed without grain order, and a thickness of 1–3 mm. The radiolarians were round or elliptical, generally less than 0.2 mm in diameter, and many had been dissolved to form molded pores, with a clear overall outline. The presence of radiolarians indicates that the sedimentary area was located in a deep-water region within a semi-restricted shallow sea, and that the quartz was of authigenic origin.

[0028] Calcareous algal laminae: In the anaerobic-oxygen-deficient environment of deep-sea continental shelves, phytoplankton, during burial and diagenesis, gradually form aggregates of cryptocrystalline and microcrystalline quartz, calcite, and dolomite with high hardness structures due to the effects of temperature and pressure. See also Figure 3 The grain distribution lacks grain order characteristics, and high-powered microscopes show a "sparkling" feature. Sparkling agglomerates are "enriched" to form a bright lamellar layer. The thickness of the lamellar layer is generally 0.1-2.0 mm. The thinnest layer contains only a single calcite mineral, which is spheroidal, with a small lateral extension range and discontinuous distribution.

[0029] Step 2: Quantitative identification of shale samples using energy dispersive spectroscopy (MaipSCAN), specifically including: 2.1. Shale samples were prepared into thin sections with a diameter of 1 cm. Energy dispersive spectroscopy (EDS) was performed using a Maipscan 2.0 automated mineral analysis microscope to obtain two-dimensional images of the shale at micrometer-level resolution. Figure 4 As shown; 2.2 Using image processing technology, extract the percentage of the area of ​​each mineral (quartz, plagioclase, potassium feldspar, calcite, dolomite, illite, chlorite, pyrite, etc.) in the shale sample.

[0030] The mineral characteristics that identify biogenic siliceous shale are high quartz content (generally >50%), clay mineral content (generally <30%), low feldspar content (generally <10%), and low calcium content (generally <15%).

[0031] Step 3: Identify organic matter and porosity in shale samples, specifically including: 3.1. Shale samples were subjected to 4nm resolution SEM (Field Emission Scanning Electron Microscopy) two-dimensional scanning imaging to obtain nanometer-resolution two-dimensional images of the shale, with a field of view of 400μm*150μm. Figure 5 As shown; from Figure 5 The high-resolution images show that organic matter (displayed as black in the image) fills the gaps between the matrix minerals, i.e., the intergranular or intragranular spaces. The magnified two-dimensional image on the right reveals well-developed organic pores within the organic matter.

[0032] 3.2 Using image processing techniques, extract the percentage S of shale organic matter area to the sample area. 有机质 The percentage of pore area to sample area, S 孔隙度 , Field emission scanning electron microscopy (FESEM) identifies biogenic siliceous shale, with organic matter volume content (area percentage) ranging from 5.02% to 13.38%, averaging 8.52% (mass content ranging from 2.51% to 6.69%, averaging 4.26%); total porosity ranging from 1.60% to 5.88%, averaging approximately 2.86%, with organic pores accounting for about 90%. Biogenic siliceous shale exhibits higher organic matter content and porosity than other types of shale, such as clayey shale, silty shale, and calcareous shale, which are key characteristics for its identification.

[0033] Based on the above identification analysis, the sampled shale specimen meets the characteristics of biogenic siliceous shale and is identified as biogenic siliceous shale.

[0034] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for identifying biogenic siliceous shale, characterized in that, include: Optical methods were used to identify the structure of shale samples and determine whether they conformed to the characteristics of biogenic siliceous shale; The shale samples were subjected to energy dispersive spectroscopy (EDS) for mineral quantitative identification to determine whether they conformed to the characteristics of biogenic siliceous shale. The shale samples were subjected to organic matter and porosity identification to determine whether they conformed to the characteristics of biogenic siliceous shale; If the above judgment results all meet the characteristics of biogenic siliceous shale, then it is identified as biogenic siliceous shale.

2. The method for identifying biogenic siliceous shale according to claim 1, characterized in that, The process of identifying the structure of shale samples using optical methods to determine whether they conform to the characteristics of biogenic siliceous shale includes: Prepare thin sections of samples for observation under an optical microscope to obtain two-dimensional images of shale under an optical microscope; Based on the acquired two-dimensional optical microscope images of shale, the mineral laminae and mineral composition characteristics of the shale samples were identified. If the bright laminae in a shale sample are concentrated segments of phanerocrystalline quartz formed by the diagenesis of diatoms, calcareous algae, and radiolarian shells, with a laminae thickness of 0.1–3.0 mm, and the dark laminae are relatively undeveloped segments of diatoms, calcareous algae, and radiolarians, with a content of >48% of nanoscale cryptocrystalline quartz minerals, and the dark laminae and bright laminae have an interlocking structure, then it is judged to be consistent with the characteristics of biogenic siliceous shale.

3. The method for identifying biogenic siliceous shale according to claim 1, characterized in that, Energy dispersive spectroscopy (EDS) was performed on the shale samples to quantitatively identify minerals and determine whether they conformed to the characteristics of biogenic siliceous shale, including: Thin sections of shale samples were prepared, and energy dispersive spectroscopy (EDS) was used to perform two-dimensional mineral scanning on the thin sections of the shale samples using an automated mineral analysis electron microscope to obtain two-dimensional images of shale with micron-level resolution. Based on the micron-resolution two-dimensional image of shale, the percentage of the area of ​​each mineral in the shale relative to the sample area is extracted; the minerals in the shale include: quartz, plagioclase, potassium feldspar, calcite, dolomite, illite, chlorite, and pyrite. The determination is based on the percentage of the area of ​​each mineral in the shale relative to the sample area. If the following conditions are met: quartz content > 50%, clay mineral content < 30%, feldspar content < 10%, and calcium content < 15%, then it is determined to meet the characteristics of biogenic siliceous shale. The clay includes illite and chlorite, the feldspar includes plagioclase and potassium feldspar, and the calcium includes calcite and dolomite.

4. The method for identifying biogenic siliceous shale according to claim 3, characterized in that, The automated mineral analysis electron microscope used is Maipscan 2.

0.

5. The method for identifying biogenic siliceous shale according to claim 1, characterized in that, The shale samples were subjected to organic matter and porosity identification to determine whether they conformed to the characteristics of biogenic siliceous shale, including: The shale sample was subjected to field emission scanning electron microscopy (FEM) two-dimensional scanning imaging to obtain a two-dimensional image of the shale with nanometer-scale resolution; Based on the nanometer-resolution two-dimensional image of shale, the percentage S of shale organic matter area to sample area is extracted. 有机质 And, the percentage of pore area to sample area, S 孔隙度 , If the percentage of organic matter area and total porosity of a shale sample are both within the preset range, it is judged to conform to the characteristics of biogenic siliceous shale.

6. The method for identifying biogenic siliceous shale according to claim 5, characterized in that, Two-dimensional scanning was performed using a field emission scanning electron microscope with a resolution of 4 nm.

7. The method for identifying biogenic siliceous shale according to claim 5, characterized in that, The preset range for the percentage of organic matter area is 5.02%-13.38%, with an average of 8.52%.

8. The method for identifying biogenic siliceous shale according to claim 5, characterized in that, The preset range of total porosity is 1.60%-5.88%, with an average of 2.86%, and the proportion of organic pores is 90%.

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