A method for distinguishing primary organic matter from secondary organic matter

Abstract

The present invention discloses a method for distinguishing primary organic matter from secondary organic matter. The method comprises: (1) obtaining scanning electron microscopic petrological characteristics and atomic composition characteristics of the organic matter of a rock sample; and (2) distinguishing primary organic matter from secondary organic matter based on the scanning electron microscopic petrological characteristics and atomic composition characteristics of the organic matter of the rock sample. The method of the present invention utilizes the excellent high-magnification micro-area observation technology of a scanning electron microscope, combined with real-time energy spectrum quantitative detection technology, to accurately distinguish primary organic matter from secondary organic matter.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas geological exploration, and more specifically, relates to a method for distinguishing primary organic matter from secondary organic matter. Background Art

[0002] High-maturity shales typically undergo lengthy and complex burial thermal alteration, resulting in significant changes in the morphology and chemical composition of the original sedimentary organic matter. Along with burial thermal alteration, the primary organic matter undergoes hydrocarbon generation, producing large amounts of secondary organic matter (oil and bitumen). The oil further undergoes cracking reactions to produce natural gas and pyrobitumen. Therefore, the organic matter in high-maturity shales can be broadly divided into two categories: primary and secondary organic matter. These two categories share similar morphology, distribution, and optical characteristics, leading to significant uncertainty in distinguishing between primary and secondary organic matter using organic petrography methods.

[0003] Organic petrology uses an optical microscope to observe the color, morphology, and structure of solid organic matter, and then determine the source and type of solid organic matter. It is widely used in the study of solid organic matter in sedimentary rocks (Suárez-Ruiz I., Flores D., Graciano Filho J., Hackley PC, 2012. Review and update of the applications of organic petrology: Part 1, geological applications. Int. J. Coal Geol. 99:54-112). Primary organic matter originates from organisms in the sedimentary environment, and its hydrocarbon generation and transformation produce secondary organic matter. Therefore, secondary organic matter does not have its own specific form and is produced from primary organic matter to fill the pores or cracks of the mineral matrix. For marine shales in the oil-generation window, identifying secondary organic matter based on its filling morphology and contact relationship with minerals is an effective method. As thermal alteration of organic matter intensifies, organic matter particles in high-maturity marine shales become smaller and smaller, and their petrological characteristics are basically similar. High-magnification scanning electron microscopy technology has begun to be widely used in organic matter research (Loucks RC, Reed RM, Ruppel SC, Jarvie DM, 2009. Morphology, genesis, and distribution of nanometer-scale pores in siliceous mudstones of the Mississippian Barnett Shale. J. Sediment. Res., 79:848-861). However, scanning electron microscopic petrological characteristics of organic matter cannot distinguish between primary and secondary organic matter (Hackley PC, Cardott B.J., 2016. Application of organic petrography in North American shale petroleum systems: A review. Int. J. Coal. Geo. 163:8-51). Therefore, the application of organic petrology methods to high-maturity marine shales is greatly limited, hindering in-depth research on the origin and evolution of organic matter in high-maturity marine shales and further affecting the evaluation of marine shale reservoir characteristics. Summary of the Invention

[0004] The purpose of the present invention is to provide a quantitative method for distinguishing primary organic matter from secondary organic matter in high-maturity mud shales. The method combines scanning electron microscopic observation with energy spectrum quantitative analysis to accurately distinguish primary organic matter from secondary organic matter based on both petrological and atomic composition characteristics, providing a basic technical method for solving the research on the origin and evolution of organic matter in high-maturity marine shales, as well as the formation and evolution of organic pores.

[0005] In order to achieve the above object, the present invention provides a method for distinguishing primary organic matter from secondary organic matter, which comprises:

[0006] (1) Obtain the scanning electron microscopic petrological characteristics and atomic composition characteristics of organic matter in rock samples;

[0007] (2) Distinguish primary organic matter from secondary organic matter based on the organic matter scanning electron microscopic petrological characteristics and organic matter atomic composition characteristics of the rock samples.

[0008] Preferably, the scanning electron microscopic petrological characteristics of organic matter include: organic matter morphology and structure, and / or the contact relationship between organic matter and minerals.

[0009] Preferably, the organic matter atomic composition characteristics include: the mass percentage of carbon atoms in the organic matter and the molar ratio of carbon atoms to oxygen atoms in the organic matter.

[0010] Preferably, in step (1), a scanning electron microscope equipped with an energy dispersive spectrometer is used to obtain the organic matter scanning electron microscopic petrological characteristics and organic matter atomic composition characteristics of the rock sample.

[0011] Preferably, in step (1), the rock sample is subjected to microscopic observation and energy spectrum quantitative analysis in a scanning electron microscope equipped with an energy spectrometer to obtain the organic matter scanning electron microscopic petrological characteristics and organic matter atomic composition characteristics of the rock sample.

[0012] Preferably, in step (1), before the rock sample is subjected to microscopic observation and energy spectrum quantitative analysis in a scanning electron microscope equipped with an energy spectrometer, the rock sample is subjected to a pre-treatment comprising the following processing steps:

[0013] S1, cutting, grinding and first polishing the rock sample;

[0014] S2. Perform a second polishing treatment on the rock sample processed in step S1, wherein the second polishing treatment is an ion beam polishing treatment.

[0015] Preferably, in step S1, the cutting is to cut the rock sample into block rock samples of about 1 cm;

[0016] In step S2, the second polishing treatment is performed for 2.5-3.5 hours.

[0017] Preferably, the rock sample is microscopically observed in a scanning electron microscope equipped with an energy spectrometer at an observation magnification of 5000-50000 times.

[0018] Preferably, in step (2), when the rock sample's organic matter meets the requirements of the scanning electron microscopic petrological characteristics of being in the form of clumps or strips, with a particle diameter larger than or equal to that of surrounding mineral particles, and having a distinct boundary with the surrounding minerals; and the carbon atomic mass percentage of the organic matter in the rock sample is not less than 70% and the molar ratio of carbon atoms to oxygen atoms in the organic matter is not less than 4, it is considered primary organic matter;

[0019] When the scanning electron microscopic petrological characteristics of the organic matter of the rock sample meet the requirements of interstitial distribution, irregular shape, particle diameter smaller than the surrounding mineral particles, and smooth boundaries with the surrounding minerals; and the carbon atomic mass percentage of the organic matter of the rock sample is less than 70% and the molar ratio of carbon atoms to oxygen atoms of the organic matter is less than 4, it is secondary organic matter.

[0020] Preferably, the rock sample is a rock sample of high-maturity marine shale.

[0021] The technical features of the present invention have the following beneficial effects:

[0022] (1) The method of the present invention utilizes the excellent high-magnification micro-area observation technology of a scanning electron microscope, combined with real-time energy spectrum quantitative detection technology, to accurately distinguish primary organic matter from secondary organic matter.

[0023] (2) Compared with traditional optical microscopes, scanning electron microscopes can more clearly reflect the morphological and structural characteristics of fine organic matter and their relationship with minerals;

[0024] (3) The petrological characteristics of organic matter in high-maturity marine shales are similar. The atomic composition characteristics of organic matter are a powerful supplement to the petrological characteristic analysis results and are of great significance for correcting petrological analysis results and improving organic matter identification technology.

[0025] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above and other objects, features and advantages of the present invention will become more apparent through a more detailed description of exemplary embodiments of the present invention with reference to the accompanying drawings, wherein like reference numerals generally represent like components throughout the exemplary embodiments of the present invention.

[0027] Figure 1 The figure shows an electron microscope scanning image of a graptolite fragment under a scanning electron microscope according to an embodiment of the present invention.

[0028] Figure 2 An electron microscope scanning image of spore pollen under a scanning electron microscope according to an embodiment of the present invention is shown.

[0029] Figure 3An electron microscope scanning image of a bacterial fossil under a scanning electron microscope according to an embodiment of the present invention is shown.

[0030] Figure 4 The figure shows an electron microscope scanning image of secondary organic matter (solid asphalt) under a scanning electron microscope according to an embodiment of the present invention.

[0031] in, Figures 1-4 Spectrum1 or Spectrum2 shown in the figure refers to the analysis points corresponding to the energy spectrum tables 1-4 of the corresponding pictures. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. Instead, these embodiments are provided to make the present invention more thorough and complete and to fully convey the scope of the present invention to those skilled in the art.

[0033] The present invention provides a method for distinguishing primary organic matter from secondary organic matter, the method comprising:

[0034] (1) Obtain the scanning electron microscopic petrological characteristics and atomic composition characteristics of organic matter in rock samples;

[0035] (2) Distinguish primary organic matter from secondary organic matter based on the organic matter scanning electron microscopic petrological characteristics and organic matter atomic composition characteristics of the rock samples.

[0036] Based on the scanning electron microscopic petrological characteristics and atomic composition characteristics of organic matter, the present invention can accurately distinguish primary organic matter from secondary organic matter, and provide accurate basic information for the study of hydrocarbon generation evolution and pore formation evolution of organic matter.

[0037] According to the present invention, preferably, the scanning electron microscopic petrological characteristics of organic matter include: organic matter morphology and structure, and / or the contact relationship between organic matter and minerals.

[0038] According to the present invention, preferably, the organic matter atomic composition characteristics include: the mass percentage of carbon atoms in the organic matter and the molar ratio of carbon atoms to oxygen atoms (C / O atomic molar ratio) in the organic matter.

[0039] According to the present invention, preferably, in step (1), a scanning electron microscope equipped with an energy dispersive spectrometer is used to obtain the organic matter scanning electron microscopic petrological characteristics and organic matter atomic composition characteristics of the rock sample.

[0040] According to the present invention, preferably, in step (1), the rock sample is subjected to microscopic observation and energy spectrum quantitative analysis in a scanning electron microscope equipped with an energy spectrometer to obtain the organic matter scanning electron microscopic petrological characteristics and organic matter atomic composition characteristics of the rock sample.

[0041] In the present invention, when performing microscopic observation on a rock sample, the observation surface is perpendicular to the rock layer; the observation surface must be ion beam polished and must not be subjected to coating treatment.

[0042] In the present invention, the rock sample needs to be placed in a scanning electron microscope for observation to meet the observation accuracy requirements. The scanning electron microscope is equipped with an energy spectrum analyzer, which can simultaneously realize in-situ micro-area elemental analysis of organic particles during microscopic observation, thereby obtaining the organic matter scanning electron microscope petrological characteristics and organic matter atomic composition characteristics of the rock sample.

[0043] According to the present invention, preferably, in step (1), before the rock sample is subjected to microscopic observation and energy spectrum quantitative analysis in a scanning electron microscope equipped with an energy spectrometer, the rock sample is subjected to a pre-treatment comprising the following processing steps:

[0044] S1, cutting, grinding and first polishing the rock sample;

[0045] S2. Perform a second polishing treatment on the rock sample processed in step S1, wherein the second polishing treatment is an ion beam polishing treatment.

[0046] According to the present invention, preferably, in step S1, the cutting is to cut the rock sample into block rock samples of about 1 cm;

[0047] In step S2, the second polishing treatment is performed for 2.5-3.5 hours.

[0048] According to the present invention, preferably, the rock sample is microscopically observed in a scanning electron microscope equipped with an energy spectrometer at an observation magnification of 5000-50000 times.

[0049] According to the present invention, preferably, in step (2), when the rock sample's organic matter meets the requirements of the scanning electron microscopic petrological characteristics of being in the form of clumps or strips, with a particle diameter larger than or equal to that of the surrounding mineral particles, and having a distinct boundary with the surrounding minerals; and the carbon atomic mass percentage of the organic matter in the rock sample is not less than 70% and the molar ratio of carbon atoms to oxygen atoms in the organic matter is not less than 4, the rock sample is considered to be primary organic matter;

[0050] When the scanning electron microscopic petrological characteristics of the organic matter of the rock sample meet the requirements of interstitial distribution, irregular shape, particle diameter smaller than the surrounding mineral particles, and smooth boundaries with the surrounding minerals; and the carbon atomic mass percentage of the organic matter of the rock sample is less than 70% and the molar ratio of carbon atoms to oxygen atoms of the organic matter is less than 4, it is secondary organic matter.

[0051] In the present invention, in step (2), when the rock sample's organic matter meets the following petrological characteristics as generally lumpy or strip-shaped, well retains some of the original organic matter's morphology, has a particle diameter larger than or equal to that of surrounding mineral particles, and has a distinct boundary with surrounding minerals; and the rock sample's organic matter has a carbon atomic mass percentage of not less than 70% and a molar ratio of carbon atoms to oxygen atoms of not less than 4, it is considered primary organic matter;

[0052] When the scanning electron microscopic petrological characteristics of the organic matter in the rock sample meet the general interstitial distribution, irregular shape, particle diameter smaller than the surrounding mineral particles, and smooth boundaries with the surrounding minerals; and the carbon atomic mass percentage of the organic matter in the rock sample is less than 70% and the molar ratio of carbon atoms to oxygen atoms in the organic matter is less than 4, it is secondary organic matter.

[0053] In the present invention, the scanning electron microscopic petrological characteristics of organic matter include the morphology and structure of organic matter, and the contact relationship between organic matter and minerals. Primary organic matter generally has a specific morphology and structure because it inherits the characteristics of the parent organic matter. In high-maturity marine mud shale, it is generally in agglomerate form, with large particles, comparable to the surrounding mineral particles. Secondary organic matter originates from the hydrocarbon generation and transformation of primary organic matter. With the process of rock burial and compaction, secondary organic matter is discharged into the pores of the mineral matrix. Since the mineral particles that make up the marine mud shale are very small (concentrated in 10-30 microns), the secondary organic matter particles filling the pores of the mineral matrix are even smaller (less than 10 microns), with smooth boundaries, and the morphology is restricted by the surrounding minerals, generally harbor-shaped or irregular.

[0054] In the present invention, during the geological burial period, primary organic matter undergoes geopolymerization, gradually shedding side chains and heterofunctional groups, ultimately becoming high-carbon organic matter with no hydrocarbon generation potential. Therefore, primary organic matter and secondary organic matter exhibit significant differences in atomic composition. Scanning electron microscopic analysis of organic matter reveals that primary organic matter has a carbon atomic mass percentage greater than 70% and a C / O atomic molar ratio greater than 4, while secondary organic matter has a carbon atomic mass percentage less than 70% and a C / O atomic molar ratio less than 4.

[0055] According to the present invention, preferably, the rock sample is a rock sample of high-maturity marine shale.

[0056] The present invention is further described below by way of examples:

[0057] Example

[0058] This example provides a method for distinguishing primary organic matter from secondary organic matter in high-maturity mud shale. The Lower Silurian marine mud shale in the Jiaoshiba area of ​​the Sichuan Basin was selected to conduct a study on the distinction between primary organic matter and secondary organic matter. The specific steps are as follows:

[0059] (1) Pre-processing the rock sample

[0060] The drilling core samples were cut into block samples of about 1 cm square, fixed on a base, and then fixed on a Leica grinding machine for grinding and polishing (first polishing) in sequence. The samples were then transferred to a Leica ion thinning instrument and ion beam polished (second polishing) for 3 hours. After ion polishing, the samples were placed in a dryer for use.

[0061] (2) Obtain the SEM petrographic characteristics and atomic composition characteristics of the organic matter of the rock sample using a scanning electron microscope equipped with an energy dispersive spectrometer. (3) Distinguish primary organic matter from secondary organic matter based on the SEM petrographic characteristics and atomic composition characteristics of the organic matter of the rock sample.

[0062] Microscopic observation and energy spectrum quantitative analysis of ion-polished shale samples were performed under a field emission scanning electron microscope (a scanning electron microscope equipped with an energy spectrometer) (the observation surface of the rock sample was perpendicular to the rock layer during microscopic observation), generally with an observation magnification of 5000-50000 times;

[0063] SEM petrological characteristics of organic matter obtained from microscopic observation (see Figure 1-Figure 3 ) is primary organic matter, Figure 4 Secondary organic matter, Figure 1 It is a graptolite remnant with smooth outline, large individual body, lamellar shape, segmented features, and clear boundaries with surrounding minerals. Figure 2 It is pollen, and the vertical section is mostly flat ring-shaped, and becomes linear after being compressed; Figure 3 Bacterial fossils are mainly rod-shaped or short columnar, with flattened features, and are superimposed in granular form to form colonies. Figure 4 It is secondary organic matter (solid asphalt), fine-grained and dispersed, filling in mineral cracks. Under high-maturity conditions, the surface is rough and nanoporous.

[0064] The results of quantitative analysis of organic matter energy spectrum (Tables 1-4), combined with the statistics of a large number of previous experimental results, show that the carbon atomic mass percentage of larger agglomerated organic matter is between 70-98%, generally greater than 80%, the oxygen atomic mass percentage is between 0-20%, generally within 15%, and the C / O atomic molar ratio is greater than 4. The carbon atomic mass percentage of smaller filling-like organic matter is between 52-63%, the oxygen atomic mass percentage is between 12-31%, and the C / O atomic molar ratio is less than 4.

[0065] Table 1 Energy spectrum analysis data of graptolite fragments

[0066]

[0067]

[0068] Table 2 Energy spectrum analysis data of pollen

[0069]

[0070] Table 3 Energy spectrum analysis data of bacterial fossils

[0071]

[0072] Table 4 Energy spectrum analysis data of secondary organic matter (solid asphalt)

[0073]

[0074] Combining the scanning electron microscopic petrological characteristics and atomic composition of the two types of organic matter reveals distinct differences between primary and secondary organic matter. Primary organic matter is lumpy, with particle diameters larger than or equal to those of surrounding minerals, and distinct boundaries with surrounding minerals. Its carbon content is greater than 70%, its oxygen content is very low, and its C / O atomic molar ratio is greater than 4. Secondary organic matter, on the other hand, is interstitially distributed and irregular, with particle diameters smaller than those of surrounding minerals, and its boundaries with surrounding minerals are smooth. Its carbon content is less than 70%, its oxygen content is high, and its C / O atomic molar ratio is less than 4.

[0075] Based on the above characteristics, primary organic matter and secondary organic matter can be accurately distinguished under a scanning electron microscope, providing clear information on the origin and composition of organic matter for further research on the relationship between organic matter hydrocarbon generation, organic matter pore formation and evolution, and organic matter types.

[0076] While various embodiments of the present invention have been described above, the above description is intended to be illustrative, not exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for distinguishing primary organic matter from secondary organic matter, characterized in that: The distinction includes: (1) Obtain the scanning electron microscopic petrological characteristics and atomic composition characteristics of organic matter in rock samples; (2) distinguishing primary organic matter from secondary organic matter based on the scanning electron microscopic petrological characteristics and atomic composition characteristics of the organic matter in the rock sample; Wherein, the said organic matter SEM petrological characteristics include: organic matter morphology and structure, and / or contact relationship between organic matter and minerals; The organic matter atomic composition characteristics include: the mass percentage of carbon atoms in the organic matter and the molar ratio of carbon atoms to oxygen atoms in the organic matter; In step (2), when the rock sample's organic matter meets the requirements of the scanning electron microscopic petrological characteristics, namely, being in the form of lumps or strips, with a particle diameter larger than or equal to that of surrounding mineral particles, and having a distinct boundary with the surrounding minerals; and the carbon atomic mass percentage of the organic matter in the rock sample is not less than 70% and the molar ratio of carbon atoms to oxygen atoms in the organic matter is not less than 4, the rock sample is considered to be primary organic matter; When the scanning electron microscopic petrological characteristics of the organic matter of the rock sample meet the requirements of interstitial distribution, irregular shape, particle diameter smaller than the surrounding mineral particles, and smooth boundaries with the surrounding minerals; and the carbon atomic mass percentage of the organic matter of the rock sample is less than 70% and the molar ratio of carbon atoms to oxygen atoms of the organic matter is less than 4, it is secondary organic matter.

2. The method of claim 1, wherein: In step (1), a scanning electron microscope equipped with an energy dispersive spectrometer is used to obtain the organic matter scanning electron microscopic petrological characteristics and organic matter atomic composition characteristics of the rock sample.

3. The method of claim 2, wherein: In step (1), the rock sample is subjected to microscopic observation and energy spectrum quantitative analysis in a scanning electron microscope equipped with an energy spectrometer to obtain the organic matter scanning electron microscopic petrological characteristics and organic matter atomic composition characteristics of the rock sample.

4. The method of claim 3, wherein: In step (1), before the rock sample is subjected to microscopic observation and energy spectrum quantitative analysis in a scanning electron microscope equipped with an energy spectrometer, the rock sample is subjected to a pre-treatment including the following processing steps: S1, cutting, grinding and first polishing the rock sample; S2. Perform a second polishing treatment on the rock sample processed in step S1, wherein the second polishing treatment is an ion beam polishing treatment.

5. The method of claim 4, wherein: In step S1, the cutting is to cut the rock sample into block rock samples of about 1 cm; In step S2, the second polishing treatment is performed for 2.5-3.5 hours.

6. The method of claim 3, wherein: The rock samples were microscopically observed in a scanning electron microscope equipped with an energy spectrometer at a magnification of 5000-50000 times.

7. The method of claim 1, wherein: The rock sample is a rock sample of high-maturity marine mud shale.

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