A method for identifying the best hydrocarbon generation window of low mature shale pyrolysis by using a laser confocal microscope

Abstract

The application discloses a method for identifying the best hydrocarbon generation window of low-mature shale by using a laser confocal microscope, belongs to the field of oil and gas development and organic geochemical testing technology, and solves the problems that open pyrolysis of crushed samples cannot reflect in-situ hydrocarbon generation and expulsion processes and cannot finely identify the best hydrocarbon generation and expulsion window of low-mature shale. The method comprises the following steps: selecting a block sample of low-mature shale rich in organic matter in a target layer section; selecting a plurality of pyrolysis temperature points according to a pre-estimated hydrocarbon generation window temperature range, and performing multi-temperature staged pyrolysis on the low-mature shale; simulating in-situ semi-closed multi-temperature pyrolysis; preparing low-temperature frozen thin sections of the samples pyrolyzed at different temperatures; quantitatively characterizing the spatial distribution and signal amount of light and heavy residual oil at different temperatures by using a laser confocal microscopic imaging technology; and identifying the best hydrocarbon generation and expulsion window of the low-mature shale. The application is suitable for identifying the best hydrocarbon generation and expulsion window of low-mature shale and evaluating the evolution law of pore connectivity and expulsion mechanism.

Description

Technical Field

[0001] This application relates to the fields of oil and gas development and organic geochemical testing technology, and in particular to the optimal hydrocarbon generation window of low-maturity shale pyrolysis. Background Technology

[0002] In unconventional oil and gas resource development, in-situ conversion of organic-rich, low-maturity shale (oil shale) is a crucial technology for achieving efficient and low-environmental-risk utilization, and is gradually becoming an effective way to improve the development efficiency of oil shale. Existing traditional source rock evaluation and hydrocarbon generation evolution studies generally employ techniques such as organic geochemical parameters and programmed temperature pyrolysis. In current engineering and scientific research practice, the most commonly used method for evaluating hydrocarbon generation potential and hydrocarbon generation windows is Rock-Eval programmed temperature pyrolysis. This method typically requires grinding source rock or oil shale samples into powder and performing programmed temperature pyrolysis in an open or semi-open system. Parameters such as S1, S2, and Tmax are obtained to characterize hydrocarbon generation potential and thermal evolution stages, and this method has been widely used in basin analysis and source rock evaluation. However, this type of pulverized sample pyrolysis method inevitably destroys the original pore and fracture structure and pore connectivity, making it difficult to truly reflect the coupling relationship between hydrocarbon generation, expulsion, and residual oil retention under in-situ underground conditions. Specifically:

[0003] 1. Crushing leads to a significant increase in specific surface area, making it easier for pyrolysis products to escape. The hydrocarbon generation temperature range and yield distribution show systematic deviations from the bulk sample and are significantly different from the in-situ underground conditions.

[0004] 2. Methods such as Rock-Eval only provide macroscopic parameters such as the overall S1 / S2 peak and Tmax, but cannot provide information on the spatial distribution of residual oil in pores at different evolution temperatures, and cannot distinguish between light and heavy component residual oil;

[0005] 3. For in-situ conversion processes, hydrocarbon removal efficiency and pore connectivity are equally critical. Traditional pyrolysis data are difficult to reveal at the microscale which temperature range is conducive to both sufficient hydrocarbon generation and timely hydrocarbon removal and reduction of residual oil, thus failing to identify the optimal hydrocarbon generation window.

[0006] Most existing research on in-situ refining techniques for oil shale involves pyrolysis experiments after crushing the oil shale, focusing primarily on macroscopic pyrolysis kinetics, product yield, and changes in physical properties. This leads to discrepancies between the obtained data and actual formation conditions, making it impossible to distinguish changes in organic matter within isolated pores. Furthermore, the specific surface area of ​​powdered samples is much larger than that of oil shale samples of the same mass, resulting in inaccurate test results. Quantitative characterization methods based on the microscopic scale are still lacking for questions such as "how pore connectivity affects hydrocarbon expulsion at different temperatures" and "how the microscopic occurrence of residual oil in pores changes with temperature after pyrolysis." Summary of the Invention

[0007] The purpose of this invention is to solve the technical problems in the prior art where open pyrolysis of pulverized samples is difficult to reflect the in-situ hydrocarbon generation-expulsion process and cannot accurately identify the optimal hydrocarbon generation-expulsion window of low-maturity shale. The invention provides a method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using a laser confocal microscope.

[0008] The technical solution adopted by this invention to solve the above problems is: a method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using laser confocal microscopy, the method comprising:

[0009] Step S1: Select a block sample of low-maturity shale rich in organic matter from the target section and determine the basic geochemical parameters of the sample to confirm that the sample is in a low-maturity stage.

[0010] Step S2: Based on the estimated hydrocarbon generation window temperature range, select several pyrolysis temperature points to perform multi-temperature staged pyrolysis on the low-maturity shale.

[0011] Step S3: Simulate in-situ semi-closed multi-temperature pyrolysis;

[0012] Step S4: Prepare cryogenic thin films from samples pyrolyzed at various temperatures;

[0013] Step S5: Use a laser confocal microscope to identify the fluorescence or reflection signals and spatial distribution of light and heavy residual oils respectively, and obtain the total signal quantity or volume fraction of residual oil, the signal quantity or volume fraction of light residual oil and the signal quantity or volume fraction of heavy residual oil under each temperature condition.

[0014] Step S6: Correlate the quantitative data of residual oil obtained in Step S5 with the corresponding pyrolysis temperature to determine the quantitative relationship between residual oil volume and temperature or the quantitative relationship between residual oil signal quantity and temperature; combine the oil and gas yield data generated during the block pyrolysis process to identify the temperature range where the residual oil signal quantity reaches a trough or decreases significantly, and compare it with the high value range of oil and gas yield; determine the temperature range where the residual oil is minimized and the oil and gas yield and the proportion of light components are higher than the preset values ​​as the optimal hydrocarbon generation / expulsion window for low-maturity shale.

[0015] In step S1, the organic-rich, low-maturity shale or oil shale block sample is a undisturbed or nearly undisturbed columnar core; the basic geochemical parameters include organic carbon content (TOC), hydrogen index (HI), and vitrinite reflectance (Ro).

[0016] Furthermore, step S2 specifically includes:

[0017] Step S2.1: Process low-maturity shale samples with similar properties from the same layer or block into blocks or short columnar samples with regular geometric shapes;

[0018] Step S2.2: Based on the estimated hydrocarbon generation window temperature range, within 300 to 550 ℃, select several pyrolysis temperature points according to specific core parameters to perform multi-temperature staged pyrolysis on the low-maturity shale.

[0019] Furthermore, in step S2.1, the diameter of the regular geometric block or short columnar sample is 10–30 mm; and the length or thickness is 20–60 mm.

[0020] Furthermore, the plurality of pyrolysis temperature points mentioned in step S2.2 include: an initial temperature lower than the expected hydrocarbon generation window, a medium-low temperature close to the initial hydrocarbon generation temperature, one or more medium-high temperatures close to the peak hydrocarbon generation, and an overtemperature range higher than the hydrocarbon generation peak.

[0021] Furthermore, the plurality of pyrolysis temperature points are specifically 300 ℃, 400 ℃, 450 ℃, 500 ℃ or multiple temperature points near them, and the interval between adjacent temperature points is 25 to 100 ℃.

[0022] Furthermore, step S3 specifically includes:

[0023] Step S3.1: The low-maturity shale block processed in step S2 is loaded into a semi-enclosed high-temperature core clamping device or reactor, and axial and radial sealing is achieved through metal sleeves, high-temperature sealing rings and end caps;

[0024] Step S3.2: Apply confining pressure to the outside of the device and fill the inside with inert gas or other oxygen-free fluid medium to form a semi-closed pyrolysis environment simulating in-situ conditions; inject an appropriate amount of distilled water to ensure that each sample is uniformly wetted, and increase the confining pressure and axial pressure slowly and synchronously to near the in-situ underground conditions to avoid excessive pressure difference leading to core fragmentation; the oxygen content in the system should be controlled at an extremely low level to avoid significant oxidation reactions;

[0025] Step S3.3: Heat the sample according to the set heating program;

[0026] Step S3.4: Set the isothermal pyrolysis time to ensure that the organic matter fully generates and expels hydrocarbons at this temperature;

[0027] Step S3.5: Each target temperature point corresponds to one or more samples. After the isothermal period is over, the samples are allowed to cool naturally to room temperature, the pressure is slowly released, the block samples are taken out, and the corresponding pyrolysis temperatures are marked.

[0028] Step S3.6: Collect the sample products at different temperature points and calculate the oil and gas yield.

[0029] Furthermore, step S4 specifically includes:

[0030] Step S4.1: Quickly transfer the block samples after pyrolysis at each temperature to a low-temperature environment for pre-cooling, so that the residual oil in the sample is in a relatively stable state, reducing the re-migration of residual oil in subsequent film preparation;

[0031] Step S4.2: Under low temperature or near room temperature conditions, cut the pre-cooled sample along the direction perpendicular or parallel to the bedding plane to prepare rock slices, slices or thin sections.

[0032] Furthermore, step S5 specifically includes:

[0033] Step S5.1: The pyrolysis samples prepared in step S4 at different temperatures are placed sequentially under a laser confocal microscope for multi-layer scanning and three-dimensional imaging to obtain the fluorescence or reflection signals of organic matter and residual oil;

[0034] Step S5.2: By setting different excitation wavelengths, the light and heavy components of residual oil are distinguished and identified: the excitation wavelength of the laser confocal microscope is located in the 400-600 nm band; pseudo-color identification and three-dimensional laser imaging are performed on the light and heavy crude oils.

[0035] Step S5.3: For each temperature sample, take points on multiple representative fields of view or profiles and perform laser confocal scanning, recording the following information for each field of view or profile:

[0036] The overall signal intensity of residual oil, the signal intensity and spatial distribution of light residual oil, the signal intensity and spatial distribution of heavy residual oil, and the specific location and morphology of residual oil in pores, cracks and matrix;

[0037] Step S5.4: Through image processing and voxel statistics, the residual oil signal intensity of each field of view or profile is quantitatively converted to obtain the total residual oil signal quantity or volume fraction, light residual oil signal quantity or volume fraction, and heavy residual oil signal quantity or volume fraction of each field of view or profile; the average value of the signal quantity or volume fraction of multiple fields of view or profile is taken as the quantitative data of residual oil corresponding to the sample at that temperature.

[0038] Furthermore, the excitation wavelength described in step S5.2 is located in the 405–488 nm band.

[0039] The beneficial effects of this invention are:

[0040] The "method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using laser confocal microscopy" proposed in this invention, through staged pyrolysis of massive, organic-rich, low-maturity oil shale under near-in-situ semi-enclosed conditions, and combined with laser confocal microscopy identification of residual oil at multiple temperature points, achieves a comprehensive quantitative judgment of hydrocarbon generation efficiency, degree of depletion of hydrocarbon generation potential, and residual oil occurrence state. Compared with existing technologies, it has the following effects and advantages:

[0041] (1) Precisely pinpoint the optimal hydrocarbon generation-expulsion temperature window for low-maturity oil shale.

[0042] In the method of this invention, by setting multiple termination temperatures (such as 300, 350, 400, 425, 450, and 500 ℃) for the same block of oil shale in the range of 300 to 500 ℃, and statistically analyzing parameters such as oil and gas yield and weight loss rate, it is possible to observe that the oil yield shows a trend of "first increasing and then decreasing" with temperature to identify the effective cracking concentration range.

[0043] Building upon this foundation, the present invention further prepares cryogenic thin sections of samples after pyrolysis at various temperatures, and uses laser confocal microscopy to identify the fluorescence signals and spatial distribution of light and heavy residual oils. When the temperature rises to a certain level, the residual oil signal in connected pores and microcracks significantly decreases, while a certain amount of residual oil remains in isolated pores. As the temperature continues to rise, non-flowing phases such as coke and carbides appear in localized areas, causing the pores to become blocked again. By combining three indicators—oil yield, weight loss rate, and residual oil signal—the optimal hydrocarbon generation window can be refined from the traditionally broad range (approximately 430–460 °C) based solely on Tmax and yield ambiguity to a narrower range (425–450 °C in this embodiment) based on the residual oil valley value, significantly improving the accuracy and reliability of determining the optimal hydrocarbon generation window.

[0044] (2) It truly reflects the in-situ pore connectivity and residual oil occurrence mechanism.

[0045] Existing pyrolysis methods for pulverized samples damage the original pore-fracture structure, making it impossible to distinguish the differences in residual oil between connected and isolated pores. This invention uses centimeter-sized block samples for in-situ simulated pyrolysis under semi-enclosed conditions, preserving the true pore network and transport channels. Under the same temperature conditions, the three-dimensional spatial distribution of residual oil is imaged using a laser confocal microscope. This not only distinguishes the location of light and heavy components but also identifies whether the residual oil is mainly retained in isolated pores and micropores or blocked in connected pore throats and microfractures, thus confirming the synergistic completion of "efficient hydrocarbon generation and smooth hydrocarbon expulsion" at the microscopic level.

[0046] Therefore, this invention not only provides the macroscopic conclusion that "hydrocarbon generation efficiency is high in a certain temperature range", but also reveals key information such as changes in pore connectivity and whether hydrocarbon discharge channels are blocked by residual oil or coke at different temperatures. It provides visual evidence for judging whether the temperature is too low (insufficient hydrocarbon generation), moderate temperature (efficient hydrocarbon generation and smooth hydrocarbon discharge), or too high temperature (coking blockage and reduction of effective oil), and supports the optimized design of thermal recovery / in-situ reforming process windows from a mechanistic perspective.

[0047] This invention is applicable to the evaluation of identifying the optimal hydrocarbon generation-expulsion window, pore connectivity, and the evolution of hydrocarbon expulsion mechanisms in low-maturity shale. Attached Figure Description

[0048] To more clearly illustrate the technical solution of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 This is a schematic diagram illustrating the three-dimensional volume changes of residual oil content signals in light and heavy crude oils obtained using laser confocal microscopy at different pyrolysis temperatures.

[0050] Figure 2 The graph shows the changes in oil and gas production of low-mature oil shale samples at different pyrolysis temperatures.

[0051] Figure 3 This is a schematic diagram of laser confocal image recognition for residual oil. (a: 25℃; b: thermal effect at 450℃; c: synergistic effect at 450℃) ;df: Three-dimensional image of residual oil under AC conditions, green represents light components and red represents heavy components). Detailed Implementation

[0052] This embodiment provides a method for identifying the optimal hydrocarbon generation-removal window in the pyrolysis of low-maturity shale using laser confocal microscopy. Low-temperature frozen sections are prepared from the initial oil shale sample (denoted as T0) and samples pyrolyzed at multiple temperatures (e.g., 300℃, 400℃, 450℃, 500℃, with adjustable and denser temperature points as needed). The characteristic signals of light / heavy components are spatially quantitatively measured under a confocal microscope. The signal increment or decrease of the sample relative to the initial sample T0 at each pyrolysis temperature is calculated. Based on this, the hydrocarbon generation peak (formation peak) and hydrocarbon removal peak (migration-precipitation peak) are identified, and the results are compared and verified with conventional pyrolysis parameters to determine the optimal hydrocarbon generation window. The process and conditions are as follows:

[0053] Step S1: Selection of low-maturity shale samples and characterization of basic parameters

[0054] 1.1 Select organic-rich, low-maturity shale (oil shale) block samples from the target stratigraphic section, preferably undisturbed or nearly undisturbed columnar cores, and avoid or minimize fragmentation.

[0055] 1.2 Determine basic geochemical parameters such as total organic carbon (TOC), hydrogen index (HI), and vitrinite reflectance (Ro) in the samples to confirm that the samples are in a low-maturity stage and to determine their development potential.

[0056] Step S2: Design of a multi-temperature staged pyrolysis scheme

[0057] 2.1 Process low-maturity shale samples with similar properties from the same layer or block into regularly shaped blocks or short columnar samples:

[0058] The diameter is preferably 10–30 mm; the length or thickness is preferably 20–60 mm.

[0059] 2.2 Based on the estimated hydrocarbon generation window temperature range, select several pyrolysis temperature points within the range of 300–550 ℃ (depending on specific core parameters) to perform multi-temperature staged pyrolysis of the low-maturity shale, including at least:

[0060] An initial temperature lower than the expected hydrocarbon generation window (e.g., 280–320 °C);

[0061] A medium-low temperature close to the initial hydrocarbon generation temperature (e.g., 350–400 °C).

[0062] One or more medium-high temperatures (e.g., 425–460 °C) close to the peak of hydrocarbon formation.

[0063] A supertemperature range above the hydrocarbon generation peak (e.g., 480–520 °C).

[0064] Preferably, the pyrolysis temperature points are 300 ℃, 400 ℃, 450 ℃, 500 ℃ or multiple temperature points near them, and the interval between adjacent temperature points is 25 to 100 ℃.

[0065] Step S3: Simulated in-situ semi-closed multi-temperature pyrolysis

[0066] 3.1 The low-maturity shale blocks processed in step S2 are loaded into a semi-enclosed high-temperature core clamping device or reactor, and axial and radial sealing is achieved through metal sleeves, high-temperature sealing rings and end caps.

[0067] 3.2 Apply confining pressure to the outside of the device and fill the inside with inert gas or other oxygen-free fluid media to create a simulated in-situ semi-closed pyrolysis environment. Inject an appropriate amount of distilled water to ensure that each sample is uniformly wetted. The confining pressure and axial pressure should be increased slowly and synchronously to near the in-situ underground conditions to avoid excessive pressure difference that could cause core fragmentation. The oxygen content in the system should be controlled at an extremely low level to avoid significant oxidation reactions.

[0068] 3.3 Heat the sample according to the set heating program:

[0069] The heating rate is preferably 1 to 10 °C / min, more preferably 3 to 7 °C / min; when the temperature reaches the target pyrolysis temperature, it is kept at a constant temperature for pyrolysis.

[0070] 3.4 The isothermal pyrolysis time is preferably 12 to 72 h, more preferably 24 to 48 h, to ensure that the organic matter fully generates and expels hydrocarbons at this temperature.

[0071] 3.5 Each target temperature point corresponds to one or more samples. After the isothermal period, the samples are allowed to cool naturally to room temperature, the pressure is slowly released, the block samples are removed, and the corresponding pyrolysis temperatures are marked.

[0072] 3.6 Collect the product samples from different temperature points and calculate the yield (for comparison with the hydrocarbon generation window).

[0073] Step S4: Low-temperature freezing and slide preparation of the pyrolysis sample

[0074] 4.1 After pyrolysis at various temperatures, the bulk samples are quickly transferred to a low-temperature environment for pre-cooling, preferably at −20 to −40 °C or under liquid nitrogen conditions, so that the residual oil in the samples is in a relatively stable state and the re-migration of residual oil in subsequent film preparation is reduced.

[0075] 4.2 Under low temperature or near room temperature conditions, pre-cooled samples are cut along the direction perpendicular or parallel to the bedding plane to prepare representative rock slices, sections or thin sections: the thickness of the rock slices is preferably 0.5 to 5 mm; the thickness of the thin sections or frozen sections is preferably 30 to 200 μm.

[0076] 4.3 If necessary, polish or perform simple flattening treatment on the sample surface to ensure that the sample surface is flat and the image is clear during laser confocal microscopy.

[0077] Step S5: Laser confocal microscopy and quantitative identification of residual oil

[0078] 5.1 The pyrolysis samples prepared in step S4 at different temperatures were sequentially placed under a laser confocal microscope for multi-layer scanning and three-dimensional imaging to obtain the fluorescence or reflection signals of organic matter and residual oil.

[0079] 5.2 By setting different excitation wavelengths, the residual oil of light and heavy components can be distinguished and identified:

[0080] The excitation wavelength of the laser confocal microscope is located in the 400–600 nm band, preferably 405–488 nm, to excite the fluorescence signal of residual oil after pyrolysis of oil shale. False-color identification and three-dimensional laser imaging are then performed on light and heavy crude oils.

[0081] 5.3 For each temperature sample, scan points on multiple representative fields of view or cross sections and record the average values: the overall fluorescence / signal intensity of the residual oil; the signal intensity and spatial distribution of the light residual oil; the signal intensity and spatial distribution of the heavy residual oil; and the specific location and morphology of the residual oil in pores, cracks and matrix.

[0082] 5.4 Through image processing and voxel statistics, the residual oil signals of samples at different temperatures were quantitatively analyzed to obtain the following signals or volume fractions of residual oil under each temperature condition: total residual oil signal quantity or volume fraction; light residual oil signal quantity or volume fraction; heavy residual oil signal quantity or volume fraction.

[0083] Step S6: Determining the optimal hydrocarbon generation-expulsion window

[0084] 6.1 Correlate the quantitative data of residual oil (total amount and light and heavy components) obtained in step S5 with the corresponding pyrolysis temperature to determine the quantitative relationship between residual oil volume and temperature or the quantitative relationship between residual oil signal quantity and temperature (the signal quantity is proportional to the three-dimensional volume).

[0085] 6.2 Combine the oil and gas yield data generated during the block pyrolysis process (such as the amount of oil and gas collected during the pyrolysis process or measured afterward) to identify the temperature range where the residual oil signal quantity shows a trough or a significant decrease, and compare it with the high value range of oil and gas yield.

[0086] 6.3 The temperature range with minimal residual oil, high oil and gas yield, and high proportion of light components is determined as the optimal hydrocarbon generation-expulsion window for low-maturity shale.

[0087] 6.4 Preferably, the hydrocarbon generation-expulsion window determined by laser confocal microscopy can be further compared and verified with the hydrocarbon generation window (Tmax, S1+S2 peak value, etc.) indicated by conventional rock pyrolysis (Rock-Eval) parameters to verify the reliability and precision of this method.

[0088] Through the above-mentioned process and conditions, this invention combines simulated in-situ semi-enclosed multi-temperature pyrolysis with laser confocal microscopy quantitative characterization. It can not only obtain the microscopic occurrence state and signal changes of light and heavy components of residual oil at different temperatures, but also use the temperature range with the minimum residual oil and the most sufficient hydrocarbon expulsion as the criterion to accurately identify the optimal hydrocarbon generation-expulsion window of organic-rich low-maturity shale, and simultaneously evaluate the evolution of pore connectivity and hydrocarbon expulsion mechanism.

[0089] Example: Identifying the optimal hydrocarbon generation-expulsion window of a low-maturity oil shale using laser confocal microscopy.

[0090] 1. Sample selection and basic characterization

[0091] (1) Select a section of low-maturity oil shale rich in organic matter from the Jiufotang Formation of the Cretaceous in a certain basin, and cut six columnar blocks with a diameter of about 25 mm and a length of about 40 mm along the bedding direction, numbered A1 to A6.

[0092] (2) Organic geochemical analysis of representative samples: Total organic carbon (TOC) content was 5.0%–6.0%; hydrogen index (HI) was 450–550 mg HC / g TOC; vitrinite reflectance (Ro) was approximately 0.60%–0.70%. This confirmed that the samples were organic-rich, low-maturity type II oil shale.

[0093] 2. Multi-temperature staged pyrolysis and product collection

[0094] (1) A1 to A6 are respectively placed into a high temperature and high pressure semi-closed core clamping device. A confining pressure of 15 MPa is applied to the outside of the device, and 5 MPa of inert gas is filled inside to remove oxygen, forming a near-in-situ semi-closed environment.

[0095] (2) The heating rate was set to 5 °C / min. The six columnar samples were heated to 300 °C, 350 °C, 400 °C, 425 °C, 450 °C and 500 °C respectively. After reaching the target temperature, the temperature was kept constant for 48 h. During the constant temperature period, the oil production (mg / g) and gas production (mL / g) were recorded through the connected oil and gas collection system.

[0096] (3) Experimental results show that: with increasing temperature, oil production first increases and then decreases in the range of 300-500 ℃, reaching a peak at 450 ℃; gas production increases monotonically with temperature, reaching a maximum at 500 ℃; oil production at 450 ℃ is approximately 12 mg / g, significantly higher than at 300 ℃ (<2 mg / g) and 500 ℃ (approximately 7 mg / g). A graph showing the relationship between oil production and gas production as a function of temperature was constructed.

[0097] 3. Frozen preparation of pyrolysis samples followed by laser confocal microscopy analysis

[0098] (1) After pyrolysis at each temperature, allow the sample to cool naturally to room temperature and slowly release the pressure to remove the block sample. Immediately place the sample in a −30 ℃ freezer for at least 2 h to stabilize the residual oil state.

[0099] (2) Cut along the core axis under low temperature conditions to prepare rock slices with a thickness of about 1.0 mm, and then cut them into frozen slices with a thickness of about 80 μm to ensure that the slices cover representative bedding and pore development areas.

[0100] (3) Using a confocal laser scanning microscope (such as Leica SP8), select a laser line suitable for oil fluorescence excitation, and perform three-dimensional scanning of at least 5 fields of view for each temperature sample, such as... Figure 3 As shown, the following images were acquired: fluorescence channel images of light residual oil; fluorescence channel images of heavy residual oil; and corresponding three-dimensional voxel data.

[0101] (4) Using image analysis software, voxel integration is performed on the residual oil signal in each field of view to obtain the volume values ​​of light residual oil signal, heavy residual oil signal, and total residual oil signal, respectively. The average value of the results of each field of view is taken as the representative value of the sample at that temperature.

[0102] (5) Plot the light residual oil signal V_L and the heavy residual oil signal V_H at temperatures from 25 ℃ (unheated original sample) to 500 ℃ to obtain the results. Figure 1 The "Three-Dimensional Volume Values ​​of Light and Heavy Crude Oil as a Perspective of Temperature" curves are shown. The horizontal axis represents the pyrolysis temperature (25℃, 300℃, 350℃, 400℃, 425℃, 450℃, 500℃), and the vertical axis represents the three-dimensional volume values ​​of the oil-bearing signal. The solid line represents the light crude oil signal variation curve, and the dashed line represents the heavy crude oil signal variation curve. This curve is used to characterize the quantitative changes in the light and heavy components of residual oil at different temperatures and their corresponding hydrocarbon generation and expulsion characteristics. The temperature corresponding to the lowest value of the residual oil variation with temperature is the optimal hydrocarbon generation temperature window. It can be seen that the light residual oil signal generally decreases slightly in the range of 300–400℃, reaching its lowest point in the range of 425–450℃. The heavy residual oil signal increases slightly in the intermediate temperature range, shows a significant decrease in the range of 425–450℃, and under high temperature conditions of 500℃, local areas show the accumulation of heavy residual oil and coke, resulting in a signal rebound.

[0103] 4. Determination of the hydrocarbon generation-expulsion window

[0104] (1) Figure 2 Oil and gas production rates in China Figure 1 A comprehensive comparison of residual oil signal changes: In the temperature range of 425-450 ℃, the oil production is close to or reaches its peak, and the gas production increases significantly; In the same temperature range, the total residual oil signal V_T and the heavy residual oil signal V_H both drop to low values, indicating that most of the hydrocarbon-generating organic matter has been converted and successfully discharged, and the residual oil in pores and cracks is minimal.

[0105] Figure 2 In the figure, the horizontal axis represents the pyrolysis temperature (300–500 °C), and the vertical axis represents the production. The solid square line represents the gas production (mL / g), and the dashed triangular line represents the oil production (mg / g). This figure is used to reflect the variation of oil and gas production with temperature and the peak hydrocarbon generation range under staged pyrolysis conditions, providing a macroscopic yield basis for determining the optimal hydrocarbon generation-expulsion window.

[0106] (2) When the temperature continues to rise to 500 ℃, although the gas production continues to increase, the oil production decreases significantly. The laser confocal image shows that some pores and fissures are blocked by coke and heavy residues, indicating that the process has entered the over-mature stage. Further heating is not conducive to the production and migration of liquid hydrocarbons.

[0107] (3) Therefore, based on the comprehensive criteria of “high oil and gas yield, lowest residual oil signal, and good pore connectivity”, this embodiment determines the range of 425 to 450 ℃ as the optimal hydrocarbon generation-expulsion temperature window for this low-maturity oil shale.

[0108] As can be seen from the above embodiments, the method provided by the present invention can perform laser confocal microscopy quantitative analysis on blocky oil shale samples after pyrolysis at different temperatures under simulated in-situ semi-closed conditions. Combined with data such as oil production and gas production, it can accurately identify the optimal hydrocarbon generation-expulsion window of low-maturity oil shale and reveal the differences in the occurrence of residual oil in the pore system at different temperatures and its relationship with hydrocarbon expulsion efficiency. It has good repeatability and application value.

[0109] This application provides an innovative evaluation method for staged pyrolysis of organic-rich, low-maturity shale (oil shale) under simulated in-situ semi-closed pyrolysis conditions. After pyrolysis, thin sections or frozen sections of the rock are prepared, and laser confocal microscopy is used to quantitatively characterize the spatial distribution and signal intensity of light and heavy residual oil at different temperatures. This method identifies the optimal hydrocarbon generation-expulsion window, pore connectivity, and the evolution of hydrocarbon expulsion mechanisms in low-maturity shale. The method is highly quantitative, reproducible, and widely applicable.

[0110] The method described in this application has the following characteristics:

[0111] Highly quantifiable: This invention combines oil and gas yield, weight loss rate, organic geochemical parameters (TOC, S1+S2, HI, etc.) with laser confocal residual oil signal quantity and spatial occurrence morphology to construct a comprehensive evaluation system of "macro-yield - hydrocarbon generation potential - micro-residual oil". It can perform semi-quantitative or even quantitative evaluation of hydrocarbon generation efficiency and residual oil occurrence under different temperatures and pyrolysis environments. Compared with single Tmax or powder pyrolysis parameters, it provides more information and more objective results.

[0112] The operation is repeatable: the equipment used includes conventional rock pyrolysis / in-situ thermal simulation devices and commercially available laser confocal microscopes. The sample preparation process (block sampling, cryo-slide preparation, etc.) is standardized and can be standardized. The experimental parameters (temperature points, isothermal time, pressure, etc.) are easy to record and reproduce, and are suitable for repeated verification under different laboratory conditions.

[0113] Wide range of applications: The method of this invention is applicable to the identification of hydrocarbon generation windows in medium- to low-maturity oil shale, low-maturity shale, and even other organic-rich mudstones and shales. It can also be extended to evaluate different injected fluids. , The in-situ regeneration effect of steam (and other substances) has broad application prospects for in-situ development of unconventional oil and gas reservoirs, design of oil shale thermal recovery processes, and evaluation of residual oil utilization.

[0114] In summary, this invention, through the technical route of "in-situ block pyrolysis + multi-temperature point laser confocal residual oil identification," outperforms existing technologies in terms of accuracy in identifying hydrocarbon generation windows, analytical capabilities for pore connectivity and hydrocarbon expulsion mechanisms, quantitative evaluation of synergistic fluid effects, and method versatility. It has significant technological advancements and application value, providing a new methodological system for the efficient development and refined evaluation of low-maturity oil shale and similar source rock formations.

Claims

1. A method for identifying the optimal hydrocarbon generation window in the pyrolysis of low-maturity shale using laser confocal microscopy, characterized in that, The method includes: Step S1: Select a block sample of low-maturity shale rich in organic matter from the target section and determine the basic geochemical parameters of the sample to confirm that the sample is in a low-maturity stage. Step S2: Based on the estimated hydrocarbon generation window temperature range, select several pyrolysis temperature points to perform multi-temperature staged pyrolysis on the low-maturity shale. Step S3: Simulate in-situ semi-closed multi-temperature pyrolysis; Step S4: Prepare cryogenic thin films from samples pyrolyzed at various temperatures; Step S5: Use a laser confocal microscope to identify the fluorescence or reflection signals and spatial distribution of light and heavy residual oils respectively, and obtain the total signal quantity or volume fraction of residual oil, the signal quantity or volume fraction of light residual oil and the signal quantity or volume fraction of heavy residual oil under each temperature condition. Step S6: Correlate the quantitative data of residual oil obtained in Step S5 with the corresponding pyrolysis temperature to determine the quantitative relationship between residual oil volume and temperature or the quantitative relationship between residual oil signal quantity and temperature; combine the oil and gas yield data generated during the block pyrolysis process to identify the temperature range where the residual oil signal quantity reaches a trough or decreases significantly, and compare it with the high value range of oil and gas yield; determine the temperature range where the residual oil is minimized and the oil and gas yield and the proportion of light components are higher than the preset values ​​as the optimal hydrocarbon generation / expulsion window for low-maturity shale; Step S3 specifically includes: Step S3.1: The low-maturity shale block processed in step S2 is loaded into a semi-enclosed high-temperature core clamping device or reactor, and axial and radial sealing is achieved through metal sleeves, high-temperature sealing rings and end caps; Step S3.2: Apply confining pressure to the outside of the device and fill the inside with inert gas or other oxygen-free fluid medium to form a semi-closed pyrolysis environment simulating in-situ conditions; inject an appropriate amount of distilled water to ensure that each sample is uniformly wetted, and increase the confining pressure and axial pressure slowly and synchronously to near the in-situ underground conditions to avoid excessive pressure difference leading to core fragmentation; the oxygen content in the system should be controlled at an extremely low level to avoid significant oxidation reactions; Step S3.3: Heat the sample according to the set heating program; Step S3.4: Set the isothermal pyrolysis time to ensure that the organic matter fully generates and expels hydrocarbons at this temperature; Step S3.5: Each target temperature point corresponds to one or more samples. After the isothermal period is over, the samples are allowed to cool naturally to room temperature, the pressure is slowly released, the block samples are taken out, and the corresponding pyrolysis temperatures are marked. Step S3.6: Collect the sample products at different temperature points and calculate the oil and gas yield; Step S4 specifically includes: Step S4.1: Quickly transfer the block samples after pyrolysis at each temperature to a low-temperature environment for pre-cooling, so that the residual oil in the sample is in a relatively stable state, reducing the re-migration of residual oil in subsequent film preparation; Step S4.2: Under low temperature or near room temperature conditions, cut the pre-cooled sample along the direction perpendicular or parallel to the bedding plane to prepare rock slices, slices or thin sections; Step S5 specifically includes: Step S5.1: The pyrolysis samples prepared in step S4 at different temperatures are placed sequentially under a laser confocal microscope for multi-layer scanning and three-dimensional imaging to obtain the fluorescence or reflection signals of organic matter and residual oil; Step S5.2: By setting different excitation wavelengths, the light and heavy components of residual oil are distinguished and identified: the excitation wavelength of the laser confocal microscope is located in the 400-600 nm band; pseudo-color identification and three-dimensional laser imaging are performed on the light and heavy crude oils. Step S5.3: For each temperature sample, take points on multiple representative fields of view or profiles and perform laser confocal scanning, recording the following information for each field of view or profile: The overall signal intensity of residual oil, the signal intensity and spatial distribution of light residual oil, the signal intensity and spatial distribution of heavy residual oil, and the specific location and morphology of residual oil in pores, cracks and matrix; Step S5.4: Through image processing and voxel statistics, the residual oil signal intensity of each field of view or profile is quantitatively converted to obtain the total residual oil signal quantity or volume fraction, light residual oil signal quantity or volume fraction, and heavy residual oil signal quantity or volume fraction of each field of view or profile; the average value of the signal quantity or volume fraction of multiple fields of view or profile is taken as the quantitative data of residual oil corresponding to the sample at that temperature.

2. The method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using laser confocal microscopy according to claim 1, characterized in that: In step S1, the organic-rich, low-maturity shale or oil shale block sample is a undisturbed or nearly undisturbed columnar core; the basic geochemical parameters include organic carbon content (TOC), hydrogen index (HI), and vitrinite reflectance (Ro).

3. The method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using laser confocal microscopy according to claim 1, characterized in that: Step S2 specifically includes: Step S2.1: Process low-maturity shale samples with similar properties from the same layer or block into blocks or short columnar samples with regular geometric shapes; Step S2.2: Based on the estimated hydrocarbon generation window temperature range, within 300 to 550 ℃, select several pyrolysis temperature points according to specific core parameters to perform multi-temperature staged pyrolysis on the low-maturity shale.

4. The method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using laser confocal microscopy according to claim 3, characterized in that: The diameter of the regular geometric block or short columnar sample in step S2.1 is 10–30 mm; the length or thickness is 20–60 mm.

5. The method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using laser confocal microscopy according to claim 3, characterized in that: The pyrolysis temperature points mentioned in step S2.2 include: an initial temperature lower than the expected hydrocarbon generation window, a medium-low temperature close to the initial hydrocarbon generation temperature, one or more medium-high temperatures close to the peak hydrocarbon generation, and an overtemperature range higher than the hydrocarbon generation peak.

6. The method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using laser confocal microscopy according to claim 5, characterized in that: The aforementioned pyrolysis temperature points specifically refer to multiple temperature points at or near 300 ℃, 400 ℃, 450 ℃, 500 ℃, and the interval between adjacent temperature points is 25 to 100 ℃.

7. The method for identifying the optimal hydrocarbon generation window of low-maturity shale pyrolysis using laser confocal microscopy according to claim 1, characterized in that: The excitation wavelength described in step S5.2 is in the 405–488 nm band.

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