A method for evaluating the maturity of organic matter in source rocks.

Abstract

This invention discloses a method for evaluating the organic matter maturity of source rocks. The method involves extracting pollen organic matter through a hydrochloric acid-hydrofluoric acid-sieving process, preparing optical thin sections, capturing images under transmitted light using a polarizing microscope, extracting the chromaticity values ​​of the pollen organic matter, and calculating the grayscale index (GI). The calculated GI value is compared with a pre-established standard chart to directly determine the organic matter maturity level of the source rock sample. Compared with traditional qualitative methods, this invention is less affected by subjective factors, uses simpler equipment, is lower in cost, is suitable for batch sample screening, and has good applicability in marine sedimentary strata lacking vitrinite and in early Paleozoic ancient strata.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas geological exploration technology, specifically relating to a method for evaluating the maturity of organic matter in source rocks. Background Technology

[0002] The maturity of source rock organic matter is a key indicator for evaluating the hydrocarbon generation potential and exploration prospects of a basin. Accurately determining the degree of thermal evolution of organic matter is crucial for hydrocarbon resource assessment. Currently, the most commonly used parameters for organic matter maturity in the industry include vitrinite reflectance (R0). o ), peak temperature of rock pyrolysis (T) max ) and thermal variation index (TAI), etc. Among them, vitrinite reflectance is a commonly used benchmark for the maturity of organic matter in source rocks in the industry, and is widely used in the study of the thermal evolution of oil and gas basins and oil and gas exploration projects.

[0003] However, the vitrinite reflectance method has significant limitations. Vitrinite primarily originates from the xylem tissue of higher plants. In marine sedimentary strata lacking terrestrial organic matter and in ancient Early Paleozoic strata, vitrinite content is often extremely low or even absent, making it impossible to accurately measure R. o This is a long-standing technical challenge in evaluating the maturity of organic matter in source rocks of hydrocarbons in ancient strata and marine sedimentary rocks, which has limited the in-depth development of oil and gas exploration.

[0004] Besides vitrinite reflectance, the peak temperature of rock pyrolysis is also constrained by the type of source rock and the abundance of organic matter, and is easily affected by mineral matrix effects and soluble organic matter contamination, resulting in often poor stability. Traditional pollen color indices and thermal discoloration indices mainly rely on manual comparison with reference color charts under a microscope, which is greatly influenced by the operator's subjective experience. These are qualitative or semi-quantitative methods, resulting in poor comparability between results from different laboratories or by different personnel, making it difficult to meet the needs of precise evaluation.

[0005] Therefore, there is an urgent need to develop a new method for evaluating the maturity of organic matter in areas lacking vitrinite samples, which requires low equipment specifications, is easy to operate, is quantitative, is less affected by subjective factors, and is suitable for such applications. This invention, based on pollen organic matter extraction and combined with the correlation between the gray index (GI) calculated from RGB color values ​​and the organic matter maturity of source rocks, provides a low-cost, easily scalable, and alternative method for assessing the organic matter maturity of source rocks, effectively overcoming the shortcomings of existing technologies. Summary of the Invention

[0006] This invention provides a method for evaluating the maturity of organic matter in source rocks, comprising: extracting pollen organic matter from source rock samples and preparing optical thin sections; capturing images under transmitted light using a polarizing microscope and extracting red (R), green (G), and blue (B) chromaticity values; calculating the grayscale index (GI) based on the RGB values; and comparing the GI with a pre-established standard plate to determine the maturity level of the source rock organic matter. This method eliminates the need to measure vitrinite reflectance or the peak temperature of rock pyrolysis.

[0007] This invention provides a method for evaluating the maturity of organic matter in source rocks, employing the following technical solution: A method for evaluating the maturity of organic matter in source rocks includes the following steps: S1. Extract pollen organic matter from source rock samples and prepare optical thin sections; S2. Take images of pollen organic matter particles in thin sections under transmitted light using a polarizing microscope, and extract the red (R), green (G), and blue (B) chromaticity values ​​of the images; S3. Calculate the gray index (GI) based on the chromaticity value; S4. Compare the calculated gray index with the pre-established standard chart of source rock organic matter maturity to determine the organic matter maturity level of the source rock sample.

[0008] Preferably, in step S1, the extraction of pollen organic matter specifically includes: (1) Crushing: Crushing the source rock sample into granular form; (2) Hydrochloric acid treatment: The source rock sample treated in step (1) was soaked in 8% and 13% hydrochloric acid solutions in turn to remove carbonate rock cement and wash until neutral; (3) Hydrofluoric acid treatment: Soak the source rock sample treated in step (2) in 40% hydrofluoric acid and heat it in a 40°C water bath to remove silicate components and wash until neutral; (4) Sieving: After the source rock sample processed in step (3) is subjected to ultrasonic vibration, it is filtered with a 500-mesh nylon sieve to collect pollen organic matter and seal it with glycerol and deionized water.

[0009] Preferably, in step S1, the optical thin film is prepared by: adding glycerol to the center of a glass slide, transferring spore pollen organic matter to the glass slide, mixing it thoroughly with glycerol, covering it with a coverslip and pressing it lightly to allow the spore pollen organic matter to diffuse evenly and expel air bubbles.

[0010] Preferably, in step S2, at least 50 effective pollen organic matter particles are observed and photographed for each sample, and the red, green, and blue chromaticity values ​​of each particle are extracted using image processing software. The average value of all pollen organic matter particles is taken as the representative chromaticity value of the sample.

[0011] Preferably, in step S3, the grayscale index is calculated according to the following formula: GI (%) = 100 - (100 Z / 255) Z=0.299×R×255 / Rc+0.587×G×255 / Gc+0.114×B×255 / Bc Where R, G, and B are the red, green, and blue chromaticity values ​​measured on the sample, respectively; Rc, Gc, and Bc are the corrected chromaticity values ​​against a white background under the same observation conditions; and Z is the weighted corrected composite chromaticity value.

[0012] Preferably, by comparing the calculated grayscale index with the standard chart, the organic matter maturity level of the source rock sample can be directly determined, specifically as follows: By comparing the measured color of pollen organic matter with the standard pollen color comparison chart, the organic matter maturity level of the source rock sample can be obtained. Simultaneously, the calculated GI value of the source rock sample is substituted into the GI-organic matter maturity relationship chart to read the corresponding organic matter maturity level of the source rock sample.

[0013] Preferably, the method is applicable to the determination of organic matter maturity in source rock samples from marine sedimentary strata lacking vitrinite and ancient strata from the Early Paleozoic era.

[0014] In summary, the beneficial effects of the present invention are as follows: On the one hand, compared with traditional qualitative methods such as pollen color index, this invention calculates the grayscale index (GI) using RGB color values, eliminating subjective errors from human visual comparison and resulting in highly repeatable results. More importantly, by establishing a standard chart of grayscale index and the evolutionary level of source rock organic matter maturity, this invention achieves a direct and quantitative evaluation of source rock maturity without relying on auxiliary indicators such as vitrinite reflectance or pyrolysis parameters, providing a new path for source rock maturity evaluation independent of existing technologies.

[0015] On the other hand, this invention only requires a common optical microscope and a digital camera, eliminating the need for expensive microphotometers or rock pyrolysis analyzers. The operation is simple, and the cost per sample is low, making it suitable for large-scale sample screening. Most importantly, this invention can still effectively evaluate the organic matter maturity of source rocks through the grayscale index of pollen organic matter in samples where traditional methods are inapplicable, such as marine sedimentary strata lacking vitrinite and early Paleozoic ancient strata. This fills a technological gap in traditional evaluation methods and has broad prospects for promotion and application. Attached Figure Description

[0016] Figure 1 A chart for evaluating the maturity of source rock organic matter based on pollen organic matter extraction and color gray index (GI); Figure 2The gray index (GI) of pollen organic matter and the peak temperature of rock pyrolysis (T) max A diagram showing the correspondence between the two relationships. Figure 3 The gray index (GI) of pollen organic matter and the reflectance of vitrinite (R) o The correspondence diagram of ). Detailed Implementation

[0017] The present invention will be further described in detail below with reference to the embodiments.

[0018] Preparation Example The evaluation standard chart for the organic matter maturity of source rocks, and the specific steps are as follows: S1, fragmented sample Before processing, prepare sample bags, labels, and other necessary equipment. Select large and relatively intact source rock samples (covering source rocks from different regions and stratigraphic ages; standard samples with a GI value ≤ 40 are taken from Jurassic terrestrial sedimentary strata in Haixi Prefecture, Qinghai Province (95.42°E, 37.84°N); standard samples with a GI value ≤ 70 are taken from Triassic marine strata in Luzhou City, Sichuan Province (106.15°E, 28.01°N); and standard samples with a GI value > 70 are Paleozoic marine strata samples from Dengfeng City, Henan Province (113.05°E, 34.47°N)). Scrape off the surface layer with a knife and wash with deionized water to prevent external contamination. After drying, crush the sample into granular form (less than 2 mm) with a hammer. Generally, take 10 g of sample, but adjust the weight according to lithological differences. After each weighing, clean the relevant equipment with alcohol before continuing with subsequent steps to prevent cross-contamination.

[0019] S2, hydrochloric acid (cold treatment) Place the weighed source rock particles from the previous step into a 250 mL PTFE beaker and arrange them at appropriate intervals in a fume hood at room temperature (approximately 20°C). First, slowly pour in an approximately 8% hydrochloric acid solution, with the volume of hydrochloric acid being at least three times the sample volume to ensure a complete reaction. During the reaction, observe and stir as needed, recording the bubble formation, acid color changes, and sample dissolution status. After the reaction has stabilized, let it stand for 12 hours, then remove the supernatant using a dropper. Next, add 13% hydrochloric acid, stir, and let it stand for 4-5 hours. Remove the supernatant using a dropper, add deionized water, stir, and let it stand until the supernatant becomes clear. Repeat this step 4-5 times until the sample is neutral. This step aims to remove carbonate rock cement and allow the sample to fully dissociate.

[0020] S3, hydrofluoric acid (thermal treatment) Add 40% hydrofluoric acid slowly in multiple portions to the sample treated in the previous step, stirring continuously with a plastic rod. After the reaction gradually slows down, let it stand for 48 hours, stirring every 8 hours and recording the bubble formation, acid color change, and sample dissolution status. If dissolution is incomplete, add an appropriate amount of hydrofluoric acid. Then, transfer the beaker to a 40°C water bath and heat for 2 hours, observing and stirring continuously during the process. After the water bath, let it stand for 24 hours. Once the upper acid layer is clear, remove it with a pipette. Then, add deionized water, stir, and let it stand. Once the upper liquid is clear, remove it with a pipette. Repeat this step 5 to 6 times until the sample is neutral. This step aims to remove the silicate-based components from the sample, thereby obtaining pollen organic particles from the source rock.

[0021] S4. Sieving The treated sample was placed in an ultrasonic cleaner and vibrated for 1-2 minutes. The instrument parameters were set to 30℃, 50 W, and 40 kHz. After vibration, the sample was poured through a 500-mesh nylon sieve for filtration. The filtered sample was then transferred to a 10 mL centrifuge tube and sealed with glycerol and deionized water.

[0022] S5, Vitreous reflectance (R) o Determination: Take an appropriate amount of pollen organic matter extract, prepare a source rock slide according to the industry standard "Method for Determination of Vitrinite Reflectance in Sedimentary Rocks (SY / T 5124-2012)", and measure R under a polarizing microscope equipped with a photometer. o value.

[0023] S6, Peak temperature of rock pyrolysis (T) max Determination: Take an appropriate amount of the corresponding source rock sample, and perform standard analysis using a Rock-Eval 6 rock pyrolysis analyzer according to the national standard "Rock Pyrolysis Analysis (GB / T 18602-2012)" to determine T. max value.

[0024] S7. Fabrication of optical thin films First, place a small amount of glycerol in the center of a glass slide. Use a pipette to draw an appropriate amount of the extracted pollen organic matter particles and place it on the glycerol. Use a dissecting needle to thoroughly mix the glycerol and pollen organic matter particles. Then, cover with a coverslip, gently press one side of the coverslip with tweezers to allow the sample to spread outwards, and gently tap the coverslip to remove air bubbles. After the sample diffusion stops, attach a label and number it.

[0025] S8, Photography and Color Extraction The observation and photography of pollen organic matter were conducted under transmitted light using a crossed polarized microscope. Each source rock sample had at least 50 valid observation and photography points. Simultaneously, images with a white background under the same observation conditions were captured, and RGB color values ​​were extracted from the images. Image processing software (such as ImageJ, Photoshop, or a self-written script) was used to extract the red (R), green (G), and blue (B) chromaticity values ​​for each grain and the white background under the same observation conditions. The average value was taken as the representative RGB value for the source rock sample, and the RGB chromaticity values ​​of the white background images under the same observation conditions were used as correction chromaticity values.

[0026] S9. Gray Index (GI) Calculation Calculate the gray index of pollen organic matter using the following formula: GI(%) = 100 - (100Z / 255) Formula (1) Z = (0.299R*255 / Rc) + (0.587G*255 / Gc) + (0.114B*255Bc) Formula (2) In the formula: R, G, B — Red, green, and blue chromaticity values ​​measured in the sample; Rc, Gc, Bc — Corrected chromaticity values ​​for a white background under the same observation conditions; Z—The weighted and corrected composite chromaticity value; GI—Gray Index (%), the value increases with the increase of the organic matter maturity of the source rock.

[0027] S10. Establishment of a standard chart for evaluating the maturity of source rock organic matter based on pollen organic matter extraction and color grayscale index. Using the measured grayscale index of organic matter color in source rock pollen and its known organic matter maturity level indicators, a system was constructed through comparison and Monte Carlo correction analysis. Figure 1 The chart shown is a standard for evaluating the maturity of source rock organic matter based on pollen organic matter extraction and color grayscale index (GI).

[0028] Among them, the gray index GI value of source rock pollen organic matter calculated from step S9 and the traditional source rock organic matter maturity evaluation index R obtained from steps S5 and S6 are compared. o and T max Correlation analysis revealed that the maturity of source rock organic matter determined based on the gray index (GI) of pollen organic matter was correlated with the maturity determined based on the peak temperature (T) of rock pyrolysis. max The results of the determined source rock organic matter maturity are consistent. For example... Figure 2 As shown, the gray index (GI) of pollen organic matter is related to the peak temperature of rock pyrolysis (T). maxThe graph shows the correspondence between GI and T, with the horizontal axis representing the GI value (%) and the vertical axis representing T. max (°C), the positive correlation between the two was shown through scatter plots and fitted curves, that is, as T... max The GI value increases monotonically in an exponential function manner (reflecting an increase in the maturity of organic matter in the source rock).

[0029] like Figure 3 As shown, the gray index (GI) of pollen organic matter and the reflectance of vitrinite (R) are related. o The graph shows the correspondence between GI (%) and R. The horizontal axis represents GI (%), and the vertical axis represents R. o (%) shows a strong exponential positive correlation between the two: R o The value gradually increases from immature (<0.5%) to overmature (>2.0%), corresponding to a monotonically increasing GI value from low (<20%) to high (>80%).

[0030] The above GI values ​​and R o and T max The positive correlation between the values ​​further verifies the objective effectiveness of the gray index as an independent scale for the maturity of organic matter in source rocks, and it has good consistency with the results of traditional source rock organic matter maturity index measurements.

[0031] like Figure 1 As shown, this standard chart contains a one-to-one correspondence between the grayscale index (GI) range, pollen color, organic matter maturity level, and hydrocarbon generation stage, specifically: Immature stage: GI value ≤21%, pollen color is pale yellow to yellow-orange, organic matter begins to degrade thermally, generating a small amount of oil and gas; Mature stage: GI value is 21%~73%, pollen color is bright orange to dark yellowish brown, organic matter undergoes large-scale thermal degradation to generate oil and gas, mainly liquid hydrocarbons; High maturity stage: GI value is 73%~80%, pollen color is dark yellowish brown to very dark grayish brown, kerogen and the generated liquid hydrocarbons begin to crack in large quantities, mainly condensate oil and moisture; Over-mature stage: GI value ≥80%, pollen color is very dark gray to dark black, residual kerogen and liquid hydrocarbons continue to crack, and heavy hydrocarbon gases (such as ethane and propane) also crack in succession, with dry gas being the main component.

[0032] Example S1, fragmented sample Before processing, prepare sample bags, labels, and other necessary equipment. Select large and relatively intact source rock samples, scrape off the surface layer with a knife, and wash with deionized water to prevent external contamination. After drying, crush the sample into granules (less than 2 mm) with a hammer. Generally, take 10 g of sample, but adjust the weight according to lithological differences. After each weighing, clean the relevant equipment with alcohol before continuing with subsequent steps to prevent cross-contamination.

[0033] S2, hydrochloric acid (cold treatment) Place the weighed source rock particles in a 250 mL PTFE beaker and arrange them at appropriate intervals in a fume hood at room temperature (approximately 20°C). First, slowly pour in an 8% hydrochloric acid solution, with the volume of the hydrochloric acid being at least three times the sample volume to ensure a complete reaction. During the reaction, observe and stir as needed, recording the bubble formation, acid color changes, and sample dissolution status. After the reaction has stabilized, let it stand for 12 hours, then remove the supernatant using a dropper. Add 13% hydrochloric acid, stir, and let it stand for 4-5 hours. Remove the supernatant using a dropper, add deionized water, stir, and let it stand until the supernatant becomes clear. Repeat this step 4-5 times until the sample is neutral. This step aims to remove carbonate rock cement and allow the sample to fully dissociate.

[0034] S3, hydrofluoric acid (thermal treatment) Add 40% hydrofluoric acid slowly in multiple portions to the sample treated in the previous step, stirring continuously with a plastic rod. After the reaction gradually slows down, let it stand for 48 hours, stirring every 8 hours and recording the bubble formation, acid color change, and sample dissolution status. If dissolution is incomplete, add an appropriate amount of hydrofluoric acid. Then, transfer the beaker to a 40°C water bath and heat for 2 hours, observing and stirring continuously during the process. After the water bath, let it stand for 24 hours. Once the upper acid layer becomes clear, remove it with a pipette. Then, add deionized water, stir, and let it stand. Once the upper liquid becomes clear again, remove it with a pipette. Repeat this step 5 to 6 times until the sample is neutral.

[0035] S4. Sieving The treated sample was placed in an ultrasonic cleaner and vibrated for 1-2 minutes. The instrument parameters were set to 30℃, 50 W, and 40 kHz. After vibration, the sample was poured through a 500-mesh nylon sieve for filtration. The filtered sample was then transferred to a 10 mL centrifuge tube and sealed with glycerol and deionized water.

[0036] S5. Fabrication of optical thin films Add a small amount of glycerol to the center of a glass slide. Use a pipette to draw an appropriate amount of the well-mixed extracted pollen organic matter particles and place it on the glycerol. Use a dissecting needle to thoroughly mix the glycerol and pollen organic matter particles. Then, cover with a coverslip, gently press one side of the coverslip with tweezers to allow the sample to spread outwards, and gently tap the coverslip to remove air bubbles. After the sample diffusion stops, attach a label and number it.

[0037] S6. Photography and Color Extraction The observation and photography of pollen organic matter were conducted under transmitted light using a crossed polarized microscope. Each source rock sample had at least 50 valid observation and photography points. Simultaneously, images with a white background under the same observation conditions were captured, and RGB color values ​​were extracted from the images. Image processing software (such as ImageJ, Photoshop, or a self-written script) was used to extract the red (R), green (G), and blue (B) chromaticity values ​​for each grain and the white background under the same observation conditions. The average value was taken as the representative RGB value for the source rock sample, and the RGB chromaticity values ​​of the white background images under the same observation conditions were used as correction chromaticity values.

[0038] S7. Gray Index (GI) Calculation Calculate the gray index of pollen organic matter using the following formula: GI (%) = 100 - (100 Z / 255) Z=(0.299R*255 / Rc)+(0.587G*255 / Gc)+(0.114B*255Bc) In the formula: R, G, B — Red, green, and blue chromaticity values ​​measured in the sample; Rc, Gc, Bc — Corrected chromaticity values ​​for a white background under the same observation conditions; Z—The weighted and corrected composite chromaticity value; GI—Gray Index (%), the value increases with the increase of the organic matter maturity of the source rock.

[0039] S8. Maturity Assessment By comparing the measured color of pollen organic matter extracted from source rocks with a standard color chart, the organic matter maturity level and range of source rock samples can be obtained. Simultaneously, by substituting the calculated GI value of the source rock sample into the GI-organic matter maturity chart, the corresponding organic matter maturity level of the source rock sample can be read. For example, if the calculated GI value of a certain source rock sample is 45.2%, the corresponding organic matter maturity level is "mature," and the main hydrocarbon generation products are liquid hydrocarbons.

[0040] The above are all preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for evaluating the maturity of organic matter in source rocks, characterized in that, Includes the following steps: S1. Extract pollen organic matter from source rock samples and prepare optical thin sections; S2. Take images of pollen organic matter particles in thin sections under transmitted light using a polarizing microscope, and extract the red, green, and blue values ​​of the images; S3. Calculate the grayscale index based on the above chromaticity values; S4. Compare the calculated gray index with the pre-established standard chart of source rock organic matter maturity to determine the organic matter maturity level of the source rock sample.

2. The method for evaluating the maturity of organic matter in source rocks according to claim 1, characterized in that, In step S1, the extraction of pollen organic matter specifically includes: (1) Crushing: Crushing the source rock sample into granular form; (2) Hydrochloric acid treatment: The source rock sample treated in step (1) was soaked in 8% and 13% hydrochloric acid solutions in turn to remove carbonate rock cement and wash until neutral. (3) Hydrofluoric acid treatment: Soak the source rock sample treated in step (2) in 40% hydrofluoric acid and heat it in a 40°C water bath to remove silicate components and wash until neutral; (4) Sieving: After the source rock sample processed in step (3) is subjected to ultrasonic vibration, it is filtered with a 500-mesh nylon sieve to collect pollen organic matter and seal it with glycerol and deionized water.

3. The method for evaluating the maturity of organic matter in source rocks according to claim 1, characterized in that, In step S1, the optical thin film is prepared by: adding glycerol to the center of a glass slide, transferring spore pollen organic matter to the glass slide, mixing it thoroughly with glycerol, covering it with a coverslip and pressing lightly to allow the spore pollen organic matter to diffuse evenly and expel air bubbles.

4. The method for evaluating the maturity of organic matter in source rocks according to claim 1, characterized in that, In step S2, at least 50 effective pollen organic matter particles are observed and photographed for each sample. The red, green, and blue chromaticity values ​​of each particle are extracted using image processing software, and the average value of all pollen organic matter particles is taken as the representative chromaticity value of the sample.

5. The method for evaluating the maturity of organic matter in source rocks according to claim 1, characterized in that, In step S3, the grayscale index is calculated using the following formula: GI (%) = 100 - (100 Z / 255) Z=0.299×R×255 / Rc+0.587×G×255 / Gc+0.114×B×255 / Bc Where R, G, and B are the red, green, and blue chromaticity values ​​measured on the sample, respectively; Rc, Gc, and Bc are the corrected chromaticity values ​​against a white background under the same observation conditions; and Z is the weighted corrected composite chromaticity value.

6. The method for evaluating the maturity of organic matter in source rocks according to claim 1, characterized in that, The calculation of the grayscale index is compared with the standard chart to directly determine the organic matter maturity level of the source rock sample. Specifically: By comparing the measured color of pollen organic matter with the standard pollen color comparison chart, the organic matter maturity level of the source rock sample can be obtained. Simultaneously, the calculated GI value of the source rock sample is substituted into the GI-organic matter maturity relationship chart to read the corresponding organic matter maturity level of the source rock sample.

7. The method for evaluating the maturity of organic matter in source rocks according to claim 1, characterized in that, The method is applicable to the determination of organic matter maturity in source rock samples from marine sedimentary strata lacking vitrinite and from ancient Early Paleozoic strata.

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