A method for identifying fresh water leaching origin of bauxite karst hole

Abstract

This invention provides a method for identifying karst pores in bauxite formed by freshwater leaching. Porous bauxite samples are collected and thin sections are prepared by grinding. A polarizing microscope is used to search for suspected oolitic chlorite and karst pores in the thin sections, and the order of their formation is determined. Suspected oolitic chlorite formed after the karst pores is selected for electron probe microanalysis to identify the true oolitic chlorite. Karst pores formed before the true oolitic chlorite are of freshwater leaching origin. This method is simple and efficient, providing a technical approach to solving the problem of identifying the genesis of karst pores in bauxite, and has broad application prospects.

Description

Technical Field

[0001] This invention belongs to the technical field of oil and gas geological exploration, specifically relating to a method for identifying freshwater leaching-origin dissolution pores in bauxite using microscopic observation and scanning electron microscopy. Background Technology

[0002] Bauxite reservoirs are a new type of unconventional reservoir discovered in recent years, containing abundant natural gas resources. The reservoir space in bauxite is mainly composed of various dissolution pores. Freshwater leaching, mineral recrystallization, and organic acid fluid dissolution are considered the three main causes of bauxite dissolution pores (Nan Junxiang et al., 2021). However, due to the extremely small mineral grains that make up bauxite and the underdeveloped dissolution products, key evidence of bauxite dissolution is difficult to preserve. This makes the identification of the genesis of bauxite dissolution pores exceptionally difficult, thus restricting the successful exploration of bauxite oil and gas.

[0003] Currently, there is no quick and effective method to identify the formation of dissolution pores in bauxite. Summary of the Invention

[0004] To address the aforementioned problems in the existing technology, this invention provides a method for identifying karst pores in bauxite formed by freshwater leaching. The technical problem to be solved by this invention is achieved through the following technical solution:

[0005] A method for identifying karst pores in bauxite formed by freshwater leaching includes the following steps:

[0006] Step A: Collect porous bauxite samples from well cores or field outcrops;

[0007] Step B: Grind the bauxite sample into a thin cast sheet;

[0008] Step C: Use a polarizing microscope to search for solution pores in the thin section of the casting that are suspected to be filled with oolitic chlorite;

[0009] Step D: Use a condenser lens to illuminate the spot on the cast sheet to lock the field of view in step C. Use a ballpoint pen to circle the spot along the edge of the cast sheet and mark the circle number.

[0010] Step E: Carbon plating is applied to the upper surface of the cast sheet;

[0011] Step F: Move the cast thin section from Step E to the sample exchange chamber of the electron probe microanalysis system, and conduct backscattering mode observation and electron probe elemental analysis on the suspected oolitic chlorite in the circle.

[0012] Step G: Identify the true oolitic chlorite based on its crystal form and major elements, and record the circle number where the true oolitic chlorite is located;

[0013] Step H: The formation of the dissolution pores in the circle containing the true oolitic chlorite is determined to be due to freshwater leaching.

[0014] Furthermore, the cast sheet has a thickness of 0.03 mm, no cover glass, and a polished upper surface.

[0015] Furthermore, the carbon plating method on the upper surface of the cast sheet was carried out in accordance with the industry standard GB / T 17359-2012 "Quantitative Analysis by Microbeam Analysis and Energy Dispersive Spectroscopy".

[0016] Furthermore, the experimental conditions in step F are carried out in accordance with the national standard "General Rules for Electron Probe Quantitative Analysis Methods GB / T 15074-2008".

[0017] Furthermore, the criteria for identifying true oolitic chlorite in step G are (1) crystal morphology as single needle-like or rose-like, and (2) major elements as Si, Al, O, Fe, and Mg, with FeO content ranging from 34.3% to 42.3%.

[0018] The advantages of this invention are: the casting thin-section preparation, microscopic observation, and scanning electron microscopy testing used in this method are all conventional and mature methods, thus making the operation simple and quick. Given the current technological background where the genesis of bauxite karst pores is difficult to determine, this method provides a technical approach for identifying the genesis of bauxite karst pores, and has broad application prospects.

[0019] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. Attached Figure Description

[0020] Figure 1 These are freshwater leaching pores in bauxite identified by this method (microscopic photograph, the pores are filled with oolitic chlorite; the area within the dashed line is the pore area).

[0021] Figure 2 These are backscattered images of oolitic chlorite and electron probe microanalysis locations (oolitic chlorite crystals are monolithic needle-like).

[0022] Figure 3 The results are from an electron probe microanalysis (major elements are Si, Al, O, Fe, and Mg, with FeO content ranging from 34.3% to 42.3%). Detailed Implementation

[0023] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0024] A method for identifying karst pores in bauxite formed by freshwater leaching includes the following steps:

[0025] Step A: Collect porous bauxite samples from well cores or field outcrops;

[0026] Step B: Grind the bauxite sample into a thin cast sheet;

[0027] Step C: Use a polarizing microscope to search for solution pores in the thin section of the casting that are suspected to be filled with oolitic chlorite;

[0028] Step D: Use a condenser lens to illuminate the spot on the cast sheet to lock the field of view in step C. Use a ballpoint pen to circle the spot along the edge of the cast sheet and mark the circle number.

[0029] Step E: Carbon plating is applied to the upper surface of the cast sheet;

[0030] Step F: Move the cast thin section from Step E to the sample exchange chamber of the electron probe microanalysis system, and conduct backscattering mode observation and electron probe elemental analysis on the suspected oolitic chlorite in the circle.

[0031] Step G: Identify the true oolitic chlorite based on its crystal form and major elements, and record the circle number where the true oolitic chlorite is located;

[0032] Step H: The formation of the dissolution pores in the circle containing the true oolitic chlorite is determined to be due to freshwater leaching.

[0033] The cast sheet is 0.03 mm thick, without a cover glass, and has a polished upper surface.

[0034] The carbon plating method on the upper surface of the cast sheet was carried out in accordance with the industry standard GB / T 17359-2012 "Quantitative Analysis by Microbeam Analysis and Energy Dispersive Spectroscopy".

[0035] The experimental conditions for step F were carried out in accordance with the national standard GB / T 15074-2008, "General Rules for Electron Probe Quantitative Analysis".

[0036] The criteria for identifying genuine oolitic chlorite are (1) crystal morphology of single needle-like or rose-like, (2) major elements of Si, Al, O, Fe, and Mg, with FeO being approximately 34.3% to 42.3%.

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] The Taiyuan Formation bauxite in the Longdong area of ​​the Ordos Basin has a burial depth greater than 4000m and contains natural gas resources exceeding 400 billion cubic meters. The reservoir space of the bauxite is mainly composed of dissolution pores. Freshwater leaching, mineral recrystallization, and organic acid dissolution are considered potential causes of dissolution pores in the Longdong bauxite (Nan Junxiang et al., 2021), but conclusive evidence has been lacking. The process of identifying freshwater leaching-induced dissolution pores in bauxite according to the method provided in this invention is as follows: Step A involves collecting porous bauxite samples from bauxite drilling cores in the Longdong area; Step B involves grinding the bauxite samples into thin, uncovered glass slides with a polished upper surface, with a thickness of 0.03mm; Steps C-D involve outlining the areas of view in the thin slides where dissolution pores are suspected to be filled with oolitic chlorite (e.g., Figure 1 (As shown); Carbon plating is performed on the upper surface of the casting sheet according to step E; the true oolitic chlorite is determined according to steps F~G (e.g. Figure 2 and Figure 3 As shown in the figure, bauxite karst pores of freshwater leaching origin were identified.

[0040] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for identifying karst pores in bauxite formed by freshwater leaching, characterized in that: Includes the following steps: Step A: Collect porous bauxite samples from well cores or field outcrops; Step B: Grind the bauxite sample into a thin cast sheet; Step C: Use a polarizing microscope to search for solution pores in the thin section of the casting that are suspected to be filled with oolitic chlorite; Step D: Use a condenser lens to illuminate the spot on the cast sheet to lock the field of view in step C. Use a ballpoint pen to circle the spot along the edge of the cast sheet and mark the circle number. Step E: Carbon plating is applied to the upper surface of the cast sheet; Step F: Move the cast thin section from Step E to the sample exchange chamber of the electron probe microanalysis system, and conduct backscattering mode observation and electron probe elemental analysis on the suspected oolitic chlorite in the circle. Step G: Identify the true oolitic chlorite based on its crystal form and major elements, and record the circle number where the true oolitic chlorite is located; Step H: The formation of the dissolution pores in the circle where the true oolitic chlorite is located is determined to be due to freshwater leaching. The cast sheet is 0.03 mm thick, has no cover glass, and has a polished upper surface; The criteria for identifying genuine oolitic chlorite in step G are: (1) the crystal morphology is single needle-like or rose-like, and (2) the major elements are Si, Al, O, Fe, and Mg, with the FeO content ranging from 34.3% to 42.3%.

2. The method for identifying karst pores in bauxite formed by freshwater leaching as described in claim 1, characterized in that: The carbon plating method on the upper surface of the cast sheet is carried out in accordance with the national standard GB / T 17359-2012 "Quantitative Analysis by Microbeam Analysis and Energy Dispersive Spectroscopy".

3. The method for identifying karst pores in bauxite formed by freshwater leaching as described in claim 1, characterized in that: The experimental conditions in step F are in accordance with the national standard GB / T 15074-2008, "General Rules for Electron Probe Quantitative Analysis".

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