A method for correcting paleoproductivity of low matured shale in oxygen-rich environment

Abstract

The present application provides a kind of low mature shale in oxygen-rich environment for the paleoproductivity correction method, comprising: step 1. Determine the present residual organic carbon content;Step 2. Thermal maturity effect correction: step 3. Oxidative degradation effect correction: step 4. Land input rejection: step 5. Paleoproductivity calculation.The present application comprehensively considers the superimposed influence of redox degree, thermal evolution degree and land organic matter mixing on the quantitative recovery of paleoproductivity, and adopts organic-inorganic geochemistry multi-parameter system, links organic petrology, rock pyrolysis analysis and hydrocarbon generation thermal simulation experiment together, establishes the original organic recovery method, and corrects the loss amount of original organic matter under oxygen-rich background.Especially, the stable inert component in the stratum is used as internal scale, and the oxidative degradation intensity is directly calculated by using conventional experimental data, which solves the key defect that the traditional method has long ignored early oxidation, and realizes the quantitative characterization of the loss of organic matter caused by oxidation and degradation in the early stage of deposition-diagenesis.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas exploration technology, and in particular relates to a paleoproductivity correction method for low-maturity shale in oxygen-rich environments. Background Technology

[0002] Organic-rich shale is an important source of hydrocarbon-generating parent material for oil and gas resources, and research on its formation environment has significant guiding implications for global oil and gas exploration. Paleoproductivity refers to the rate at which organisms in oceanic or lacustrine basins fixed energy during energy cycling processes throughout geological history.

[0003] At present, a set of technical systems has been initially formed for the reconstruction and characterization of ancient productivity. The existing methods mainly rely on a variety of geological and geochemical data and can be divided into two major method systems: qualitative-semi-quantitative and quantitative. (1) Semi-quantitative methods include paleontological index method, isotope ratio method, trace element ratio method, carbon isotope method, etc. (2) Quantitative methods include organic carbon method, biogenic carbonate method, biogenic barium method, biogenic silicon method, etc.

[0004] Among these methods, the organic carbon method is the most direct and reliable quantitative research tool for characterizing paleoproductivity. The core idea of ​​the organic carbon method for reconstructing paleoproductivity is to further reconstruct the original organic carbon content based on the current residual organic carbon content, and then combine a series of geological parameters and empirical formulas to invert the paleoproductivity values ​​of the depositional period.

[0005] In reality, during sedimentation and burial, the organic carbon content in sediments decreases to varying degrees due to factors such as hydrocarbon generation and expulsion. Therefore, restoring the original organic carbon content is key to recovering paleoproductivity through organic carbon analysis. In existing methods, most scholars focus on the depletion of organic matter caused by thermal maturation. This depletion can be effectively recovered through artificial thermal simulation experiments and rock pyrolysis experiments. Recovery methods can be further categorized into pyrolysis simulation experiments, hydrogen index methods, and degradation rate methods. Among these, pyrolysis simulation experiments are the simplest and most practical. Through thermocompression simulation experiments, the original organic carbon content in the immature to low-maturity stage is used as a benchmark, and the residual organic carbon content in the high-maturity to over-maturity stage is compared. The recovery coefficient can be calculated by dividing the original organic carbon content by the residual organic carbon content. Furthermore, rock pyrolysis parameters also provide a convenient means to assess the organic carbon recovery coefficient. Application number CN201910276046.9, invention title: Method for restoring original hydrogen index and organic carbon in high-to-overmature saprophytic marine shale, which proposes to restore original hydrogen index and organic carbon by using existing TOC and organic phosphorus content, taking into account the influence of paleoproductivity and terrigenous input.

[0006] Existing technologies have proposed improved hydrogen index calculation models for shale with different parent material types and proposed formulas for calculating the original organic carbon content. =(kI H ) / (kI H0 )× ,in, This represents the initial organic carbon content. The current organic carbon content, k is the conversion factor, and I H I is the hydrogen index. H0 Using the original hydrogen index, this method yields a recovery coefficient between 1.3 and 2.9. Besides the methods mentioned above, other methods have also been proposed, such as the mass balance method and the degradation rate method.

[0007] Regarding the impact of oxidative degradation on original organic matter, only a very few cases have been explored. The paper "Paleoproductivity of the Chang 7 unit in the OrdosBasin (North China) and its controlling factors," published in *Palaeogeography, Palaeoclimatology, Palaeoecology* in August 2020 by Guo Chen, Wenzhe Gang, Xiangchuan Chang, Ning Wang, Pengfei Zhang, Qingyun Cao, and Jianbin Xu, corrected for organic matter oxidation consumption by using pyrite content in shale samples. The underlying basis is that the formation of 1g of pyrite consumes 0.75g of organic matter. However, this quantitative relationship is unreliable and lacks universality across different sedimentary environments globally. Therefore, it is difficult to objectively calibrate the loss of organic matter caused by oxidative degradation using this method.

[0008] The organic carbon method proposed by the above scholars is mainly based on empirical formulas for modern sediments, using the original organic carbon content as the core parameter and combining it with indicators such as sedimentation rate, sediment density, and porosity to quantitatively estimate paleoproductivity. However, this method has significant limitations when applied to shale formations. Most studies only consider the loss of organic carbon due to thermal maturation, while the impact of terrigenous organic matter incorporation and oxidative degradation during sedimentation is seriously underestimated. On the one hand, the original organic carbon corresponding to the productivity during sedimentation refers to the organic matter formed by primitive aquatic organisms, not terrigenous organic matter. On the other hand, many shale formations are actually formed in oxygen-rich sedimentary bottom water environments. In this context, organic matter undergoes strong oxidative degradation during sedimentation and burial, resulting in severe loss of original organic matter, especially in the low-maturity stage, where this effect is more significant than that of thermal maturation.

[0009] Therefore, when reconstructing paleoproductivity, it is necessary not only to focus on the depletion effect of thermal evolution, but also to pay attention to the interference of terrestrial organic matter and the superimposed effect of microbial oxidative degradation on the original organic matter; otherwise, it will seriously affect the restoration results of paleoproductivity. Summary of the Invention

[0010] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a paleoproductivity correction method for low-maturity shale in oxygen-rich environments.

[0011] The present invention adopts the following technical solution: A paleoproductivity correction method for low-maturity shale under oxygen-rich environments includes: Step 1. Determine the current residual organic carbon content: The current residual organic carbon content of shale samples was obtained through organic carbon experiments, serving as baseline data for subsequent corrections. The current residual organic carbon content is expressed as... .

[0012] Step 2. Correction for thermal maturation effect: To determine the extent of organic matter loss caused by ultra-long-term geothermal baking on a million-year timescale, artificial hydrocarbon generation thermal simulation experiments were conducted on immature lacustrine shale. The organic carbon content of rock residues was tested at different template experimental temperatures, and the organic carbon content recovery coefficient under different thermal evolution backgrounds was calculated to restore the organic carbon content before thermal evolution. This was achieved using the formula... Obtain the corresponding thermal evolution stages The fitting formula is: .in, The organic carbon recovery coefficient. Represents the initial organic carbon content. This represents the current organic carbon content. Based on this, and according to the vitrinite reflectance of the shale... Calculate the organic carbon recovery coefficient Thus, the organic carbon content after correction for thermal maturation effect was obtained. : .

[0013] Step 3. Correction of oxidative degradation effect: The microscopic components of organic matter are divided into active and inert components. The active components are sapropelic and chalcanthitic groups. The inert components are vitrinite and inertinite groups. If no significant oxidative degradation has occurred, then... Oxidative degradation occurs. The percentage greater than that of the mirror inert group led to an increase in the ratio between the two, indicating the recovery of organic carbon content after paleooxidation. The calculation formula is: The meaning of vitrinite percentage: In the identification of organic matter microscopic components, vitrinite is related to... The sum of the percentages.

[0014] Step 4. Remove land-source input: Paleoproductivity calculations focus on the amount of organic matter formed by ancient aquatic organisms, requiring the removal of terrestrial inputs and retaining only aquatic organic matter. The proportion of sapropelic components in the microstructure of organic matter reflects the proportion of aquatic organic matter in the total organic matter. Therefore, the organic carbon content after removing terrestrial inputs... The calculation formula is: .

[0015] Step 5. Calculation of ancient productivity: Will Geological parameters such as sedimentation rate, porosity, and density are input into the formula, and the paleoproductivity of the shale deposition period is calculated using the formula. The formula is as follows:

[0016] For shale density, For sediment porosity, For deposition rate; final C represents organic carbon; a represents year; m represents meter.

[0017] The beneficial effects of this invention are: This invention achieves quantitative correction of the superimposed effect of thermal maturation, oxidative degradation, and terrigenous interference on the original organic matter content, and constructs a multi-method synergistic inversion technical process with good universality and scalability. It not only enriches the scientific connotation of paleoproductivity restoration but also significantly improves the reliability and objectivity of quantitative evaluation of paleoproductivity, providing a more reasonable and objective basis for assessing shale hydrocarbon generation potential. Attached Figure Description

[0018] Figure 1 This is a flowchart of the steps of the present invention.

[0019] Figure 2 This is a calculation diagram of the paleoproductivity of a sample from a certain location. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0021] This invention focuses on the quantitative recovery of paleoproductivity of low-maturity shale under oxygen-rich environments. Addressing the shortcomings of existing technologies, it solves three main problems in sequence: (1) How to correct the loss effect of thermal evolution on organic matter. (2) How to assess the degree of impact of oxidative degradation on organic matter loss. (3) How to eliminate the influence of terrigenous organic matter.

[0022] To address the first problem, this invention obtained the organic carbon recovery coefficient of Type II lacustrine shale by conducting hydrocarbon generation simulation experiments on lacustrine shale. This parameter is related to the reflectance of vitrinite. It exhibits a significant logarithmic relationship. This method can correct the loss of organic matter due to thermal degradation caused by high burial temperature and solve the problem of organic matter loss caused by thermal evolution.

[0023] To address the second problem, this invention utilizes organic geochemistry and organic petrology. Based on the characteristics of nearly constant organic matter micro-components and significant changes in rock pyrolysis parameters during low-temperature evolution, the degree of oxidative degradation of the original organic matter is determined by the change in the proportion of inert organic matter. By dividing the proportion of residual carbon obtained from rock pyrolysis by the proportions of vitrinite and inertite, the degree of oxidative degradation can be assessed, and the original organic carbon content can be restored. This effectively compensates for the underestimation of paleoproductivity caused by the lack of consideration of the oxidative background.

[0024] To address the third issue, this invention obtains the proportions of various microscopic components through organic petrology and then locks in the content of saprophytic strata to determine the organic carbon content contributed by aquatic organisms, thus resolving the interference of confused organic matter sources on paleoproductivity. Addressing the shortcomings of existing organic carbon methods in oxygen-rich, low-maturity environments, this invention integrates three major technologies: organic petrology, rock pyrolysis, and hydrocarbon generation simulation. It constructs a multi-system correction process, filling the technical gap in the quantitative restoration of paleoproductivity under this special geological scenario. This objectively restores the original paleoproductivity of mature shale under oxygen-rich conditions, providing a reliable basis for assessing shale hydrocarbon generation potential.

[0025] like Figure 1 As shown, the present invention provides a paleoproductivity correction method for low-maturity shale in an oxygen-rich environment, comprising: Step 1. Determine the current residual organic carbon content: The current residual organic carbon content of shale samples was obtained through organic carbon experiments, serving as baseline data for subsequent corrections. The current residual organic carbon content is expressed as... For example, sample 1: =1.23%.

[0026] Step 2. Correction for thermal maturation effect: To determine the extent of organic matter loss caused by ultra-long-term geothermal baking on a million-year timescale, artificial hydrocarbon generation thermal simulation experiments were conducted on immature lacustrine shale. The organic carbon content of rock residues was tested at different template experimental temperatures, and the organic carbon content recovery coefficient under different thermal evolution backgrounds was calculated to restore the organic carbon content before thermal evolution. This was achieved using the formula... Obtain the corresponding thermal evolution stages The corresponding fitting formula is: .in, The organic carbon recovery coefficient. This represents the initial organic carbon content. This represents the current organic carbon content. Based on this, and according to the vitrinite reflectance of the shale... Calculate the organic carbon recovery coefficient Thus, the organic carbon content after correction for thermal maturation effect was obtained. : .

[0027] Sample 1: , .

[0028] Step 3. Correction of oxidative degradation effect: Organic matter microstructures are divided into active and inert components. Active components include sapropelic and crustal groups, which are easily oxidized and degraded. Inert components include vitrinite and inertinite, collectively referred to as vitrin-inert groups, which have strong antioxidant properties. Even when organic matter microstructures are oxidized and degraded, the relative content of each fibrous component remains almost unchanged, while organic geochemical parameters, such as the hydrogen index and residual carbon percentage, will change significantly. Based on this concept, this invention proposes obtaining the residual organic carbon percentage through rock pyrolysis. To represent the current proportion of inert carbon, the sum of the inert and vitrinite contents obtained from organic matter microscopic component analysis reflects the proportion of inert carbon in the original organic matter. Dividing the two yields the recovery coefficient of oxidative degradation. If no oxidative degradation has occurred, then... If oxidative degradation occurs, The percentage was greater than that of the mirror inert group, leading to an increase in the ratio between the two. The organic carbon content after the restoration of the paleooxidation effect. The calculation formula is: .

[0029] Sample 1: The formula is referenced from "Rock-Eval 6 Technology: Performances and Developments" published in Oil & Gas Science and Technology in 2001 by F. Behar, V. Beaumont and HL De B. Penteado.

[0030] The formula is referenced from "Rock-Eval 6 Technology: Performances and Developments" published in Oil & Gas Science and Technology in 2001 by F. Behar, V. Beaumont and HL De B. Penteado.

[0031] The proportion of inert groups is 12%. =6.66%.

[0032] It is pyrolytic organic carbon. Residual organic carbon, The percentage of residual organic carbon This represents the total organic carbon content from rock pyrolysis. Among them, Free hydrocarbons Oil production potential Pyrolysis of organic CO2 is the source of CO2. CO: Organic source from pyrolysis CO: CO from the pyrolysis of organic and mineral sources. CO: Oxidized organic source CO, CO2: Oxidized organic source CO2.

[0033] Step 4. Remove land-source input: The core object of paleoproductivity calculation is the amount of organic matter formed by ancient aquatic organisms. This requires removing terrestrial organic matter and retaining only aquatic organic matter. The saprophytic group in the microstructure is a specific marker of aquatic organic matter: formed by algae or plankton in ancient lake / sea basins, it is clearly distinguishable from terrestrial organic matter, and its proportion directly reflects the proportion of aquatic organic matter. Therefore, the organic carbon content after removing terrestrial inputs... The calculation formula is: .

[0034] Sample 1: Saprolegnia group accounted for 80%, =6.66%×80%=5.33%.

[0035] Step 5. Calculation of ancient productivity: Will Using a series of geological parameters such as sedimentation rate, porosity, and density as inputs, and employing the classic formula proposed by PJ Müller and E. Suess in "Productivity, sedimentation rate, and sedimentary organic matter in the oceans—I. Organic carbon preservation" published in Deep Sea Research Part A. Oceanographic Research Papers in 1979, the paleoproductivity of shale deposition can be quantitatively calculated. The formula is as follows:

[0036] Sample 1: The sample used was lacustrine shale from a certain location. For shale density, refer to the article "Main Controlling Factors of High-Yield Concentrate Shale Gas in Da'anzhai Member of Yuanba Area" published in Geology in China in 2014 by Wei Xiangfeng, Huang Jing, Li Yuping, Wang Qingbo, Liu Ruobing, and Wen Zhidong. Take 2.58 g / cm 3 ; The porosity of the sediment is referenced to "Geological Characteristics of Continental Shale Gas Reservoirs in the Da'anzhai Section of Northeast Sichuan" published in China Petroleum Exploration in 2020 by Zhou Dehua, Sun Chuanxiang, Liu Zhongbao, and Nie Haikuan. Take 3.9%; For the deposition rate, refer to Li Xujie's 2016 master's thesis published at Southwest Petroleum University, "Study on the Cyclic Stratigraphy of the Da'anse Member in the Gong 39 Well Area of ​​Central Sichuan," and take 14.9 cm / kyr; finally... C represents organic carbon; a represents year; m represents meter.

[0037] like Figure 2 The image shows some parameters of shale in a certain area. , Corresponding to The calculated paleoproductivity. Kelts, in his 1988 paper "Environments of deposition of lacustrine petroleum source rocks: an introduction" published in *Geological Society Special Publications*, classified modern lakes into oligotrophic lakes, with a productivity <200 gC / m³. 2 a. Mesotrophic lake, 200gC / m2 a < Productivity < 350 gC / m 2 a. Eutrophic lake, productivity 350gC / m³ 2 a≤Productivity<1000gC / m 2 a) and hypereutrophic lakes, with productivity >1000gC / m³ 2 a. By Figure 2 The calculation results show that, according to the traditional calculation method The productivity of shale in a certain area generally corresponds to a eutrophic lake basin, but according to the method provided in this invention, the productivity of shale in that area actually corresponds to a hypereutrophic lake basin. This means that traditional methods significantly underestimate the productivity level of ancient lake basins. Therefore, the combined effects of thermal maturation, organic matter oxidation and degradation, and the input of terrestrial organic matter, which cannot be ignored, must be systematically considered in the calculation of paleoproductivity to more accurately reflect the paleoproductivity of the original lakes.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A paleoproductivity correction method for low-maturity shale under oxygen-rich environments, characterized in that... ,include: Step 1. Determine the current residual organic carbon content: The residual organic carbon content of shale samples was obtained through organic carbon experiments and used as a baseline data for calibration. Step 2. Correction for thermal maturation effect: To determine the extent of organic matter loss caused by ultra-long-term geothermal baking on a million-year timescale, an artificial hydrocarbon generation thermal simulation experiment was conducted on immature lacustrine shale. The organic carbon content of rock residues under different template experimental temperatures was tested, and the organic carbon content recovery coefficient under different thermal evolution backgrounds was calculated to restore the organic carbon content before thermal evolution. Step 3. Correction of oxidative degradation effect: The organic matter microstructure is divided into active and inert components, and the proportion of residual organic carbon obtained from rock pyrolysis is... Representing the current proportion of inert carbon, the sum of the contents of inert group and vitrinite obtained by the identification of organic matter microstructure reflects the proportion of inert carbon in the original organic matter. Dividing the two yields the recovery coefficient of oxidative degradation effect. If no significant oxidative degradation has occurred, then Oxidative degradation occurs. Greater than Organic carbon content after paleooxidation effect recovery The calculation formula is: The meaning of vitrinite percentage in the identification of organic matter microscopic components is the relationship between vitrinite and microstructure. The sum of their proportions; Step 4. Remove land-source input: The object of paleoproductivity calculation is the amount of organic matter formed by ancient aquatic organisms. Terrestrial inputs need to be removed, and only aquatic organic matter needs to be retained. The proportion of saprophytic components in the microscopic components of organic matter reflects the proportion of aquatic organic matter in the total organic matter. Step 5. Calculation of ancient productivity: Will Geological parameters such as sedimentation rate, porosity, and density are input into the formula, and the paleoproductivity of the shale deposition period is calculated using the formula. ; The formula is as follows: ， For shale density, For sediment porosity, For deposition rate; final Where C represents organic carbon; a represents the year; and m represents the meter, the organic carbon content after removing land-based inputs. .

2. The method according to claim 1, characterized in that, Step 1 includes the current residual organic carbon content, expressed as .

3. The method according to claim 1, characterized in that, Step 2 includes using the formula Obtain the corresponding thermal evolution stages The fitting formula is: ; in, The organic carbon recovery coefficient. Represents the initial organic carbon content. This represents the current organic carbon content; based on the vitrinite reflectance of shale. Calculate the organic carbon recovery coefficient The organic carbon content after thermal maturation effect correction was obtained. : .

4. The method according to claim 1, characterized in that, Step 4 includes the organic carbon content after removing terrestrial inputs. The calculation formula is: .

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