Molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lacustrine facies

Abstract

The invention discloses a molecular geochemical method for distinguishing shallow lake and semi-deep lacustrine facies high-quality hydrocarbon source rocks. The method comprises the following steps: establishing a 2-methylhopane index for indicating cyanobacteria contribution in hydrocarbon source rock and a 4-methylstane index for indicating algae contribution; evaluating the contribution of cyanobacteria and algae to shallow lacustrine facies and semi-deep lacustrine facies hydrocarbon source rocks by using a 2-methylhopane index and a 4-methylstane index; and a high-quality hydrocarbon source rock developmental facies belt and strata series with great contribution of cyanobacteria and algae are disclosed. According to the method, shallow lake and semi-deep lacustrine facies high-quality hydrocarbon source rocks can be quickly distinguished, the development mechanism and the distribution rule of the high-quality lacustrine facies hydrocarbon source rocks are disclosed, and help is provided for next oil-gas exploration deployment and decision making.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas exploration and development, and particularly relates to a molecular geochemical method for distinguishing high-quality source rocks in shallow lake and semi-deep lake facies. Background Art

[0002] The Beibu Gulf Basin and the Pearl River Estuary Basin in the northern part of the South China Sea are important oil-rich basins in the offshore area of China. The crude oil in these basins is considered to mainly come from algae in the high-quality semi-deep lake - deep lake facies source rocks of the Eocene. Taking the Beibu Gulf Basin as an example, the significant molecular marker of the high-quality semi-deep lake - deep lake facies source rocks (TOC > 3%) is a high 4-methylsterane index indicating the contribution of algae. With the in-depth exploration and research, it is found that high-quality source rocks with high organic matter abundance, high type, and mainly oil generation also develop in the shallow lake facies of the Beibu Gulf Basin. However, the 4-methylsterane index indicating the contribution of algae in these shallow lake facies high-quality source rocks is relatively low. Therefore, the molecular geochemical index 4-methylsterane index cannot evaluate the high-quality source rocks in the shallow lake facies. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention provides a molecular geochemical method for distinguishing high-quality source rocks in shallow lake and semi-deep lake facies, aiming to evaluate the high-quality source rocks in different sedimentary facies zones, reveal the development mechanism and distribution law of high-quality source rocks, and serve for oil and gas exploration and development decision-making.

[0004] The present invention is achieved by adopting the following technical solutions.

[0005] A molecular geochemical method for distinguishing high-quality source rocks in shallow lake and semi-deep lake facies includes the following steps: S1. Establish the 2-methylhopane index indicating the contribution of cyanobacteria in the source rock and the 4-methylsterane index indicating the contribution of algae; S2. Use the 2-methylhopane index and the 4-methylsterane index to evaluate the contributions of cyanobacteria and algae to the source rocks in shallow lake and semi-deep lake facies; S3. Reveal the development facies zones and strata of high-quality source rocks with greater contributions from cyanobacteria and algae.

[0006] Further, in step S1, the calculation formula of the 2-methylhopane index indicating the contribution of cyanobacteria in the source rock is as follows: 2-methylhopane index = C 31 2-methylhopane / C 30 αβ-hopane.

[0007] Further, C 31 2-methylhopane, C 30The determination method of αβ-hopane is as follows: After crushing and extracting the hydrocarbon source rock to obtain chloroform bitumen, it is separated by alumina column chromatography; the total hydrocarbon is obtained by filtering the insoluble asphaltene from chloroform bitumen A or crude oil, and the saturated hydrocarbon is further eluted from the total hydrocarbon using n-hexane, and after concentration, it is tested on a gas chromatography-mass spectrometry (GC-MS) instrument; The chromatographic column used is a 30 m long HP-5MS fused silica capillary column, and the temperature programming adopts the following specific program: from 50 °C to 120 °C at a rate of 8 °C / min and hold for 0 min; from 120 °C to 200 °C at a rate of 4 °C / min and hold for 0 min; from 200 °C to 300 °C at a rate of 1.5 °C / min and hold for 5 min; helium is used as the carrier gas with a flow rate of 1.0 mL / min, and the injection mode adopts non-separating injection; C 31 2-Methylhopane, C 30 The identification of αβ-hopane uses m / z the retention time of the corresponding compound peak on the 205 mass chromatogram and the mass spectrum for determination, and the relative content is determined according to its peak area.

[0008] Furthermore, in step S1, the calculation formula of the 4-methylsterane index indicating the contribution of algae in the hydrocarbon source rock is as follows: 4-Methylsterane index = C 30 4-Methylsterane / C 29 Regular sterane.

[0009] Furthermore, the determination method of C 30 4-Methylsterane, C 29 Regular sterane is as follows: After crushing and extracting the hydrocarbon source rock to obtain chloroform bitumen, it is separated by alumina column chromatography; the total hydrocarbon is obtained by filtering the insoluble asphaltene from chloroform bitumen A or crude oil, and the saturated hydrocarbon is further eluted from the total hydrocarbon using n-hexane, and after concentration, it is tested on a gas chromatography-mass spectrometry (GC-MS) instrument; The chromatographic column used is a 30 m long HP-5MS fused silica capillary column, and the temperature programming adopts the following specific program: from 50 °C to 120 °C at a rate of 8 °C / min and hold for 0 min; from 120 °C to 200 °C at a rate of 4 °C / min and hold for 0 min; from 200 °C to 300 °C at a rate of 1.5 °C / min and hold for 5 min; helium is used as the carrier gas with a flow rate of 1.0 mL / min, and the injection mode adopts non-separating injection; C 30 4-Methylsterane, C 29 The identification of regular sterane uses m / z the retention time of the corresponding compound peak on the 217 mass chromatogram and the mass spectrum for determination, or C 30 4-Methylsterane uses m / zDetermine the peak emergence time of the corresponding compound peak on the mass chromatogram of 231 and judge it by the mass spectrum, and determine the relative content according to its peak area.

[0010] Furthermore, in step S2, before using the 2-methylhopane index and 4-methylsterane index to evaluate the contributions of cyanobacteria and algae to the shallow lacustrine and semi-deep lacustrine source rocks, the sedimentary facies division of the source rocks and the determination of total organic carbon are carried out.

[0011] Furthermore, the method for sedimentary facies division and classification of source rocks refers to the industry standard SYT6314-1997.

[0012] Furthermore, the method for determining the total organic carbon of source rocks is as follows: Crush the source rock rock sample, soak it with dilute hydrochloric acid to remove carbonate minerals in the sample, then rinse it with deionized water to remove chloride ions in the sample, and test the processed source rock sample on a carbon-sulfur analyzer to obtain the TOC test value of the rock sample.

[0013] Furthermore, the high-quality source rock development facies belts and formations with relatively large contributions from cyanobacteria and algae revealed in step S3 include: a. Establish the relative contribution index of cyanobacteria and algae in the source rock; b. Analyze the contributions of cyanobacteria and algae to the source rock by using the relative contribution index of cyanobacteria and algae in different formations and facies belts.

[0014] Furthermore, the method for establishing the relative contribution index of cyanobacteria and algae in the source rock is as follows: Characterize the absolute abundances of 2-methylhopane and 4-methylsterane in the organic matter of the source rock according to the base peak ions (the ions with the highest abundance in the mass spectrum) of 2-methylhopane and 4-methylsterane. Among them, the absolute abundance of 2-methylhopane is m / z the peak area corresponding to it in the mass chromatogram of 205, and the absolute abundance of 4-methylsterane is m / z the peak area corresponding to it in the mass chromatogram of 231; The calculation formula for the relative contribution index of cyanobacteria and algae in the source rock is as follows: Relative contribution index of cyanobacteria and algae = C 28 -C 36 2-methylhopane ( m / z 205) / [C 28 -C 36 2-methylhopane ( m / z 205)+C 28 -C 32 Absolute abundance of 4-methylsterane ( m / z 231)].

[0015] Use m / z The 2-methylhopanes obtained from the 205 mass chromatogram and m / z The 4-methylsteranes obtained from the 231 mass chromatogram can more objectively reflect the relative abundances of these two types of compounds in the organic matter of source rocks.

[0016] The present application has the following beneficial effects.

[0017] The present invention discovers that cyanobacteria are important contributors to oil generation in high-quality source rocks in the shallow lake facies of the Beibu Gulf Basin. Based on this, a 2-methylhopane index indicating the contribution of cyanobacteria is established as an evaluation index for high-quality source rocks in the shallow lake facies; the overall contribution of algae in the semi-deep lake facies source rocks is greater than that of cyanobacteria, and the traditional 4-methylsterane index can be used to evaluate high-quality source rocks in the semi-deep lake facies. Based on this, the present invention uses a new index system composed of the 2-methylhopane index indicating the contribution of cyanobacteria and the 4-methylsterane index indicating the contribution of algae, which can conveniently and quickly distinguish high-quality source rocks in the shallow lake and semi-deep lake facies, effectively solve the problem that the 4-methylsterane index is low in some oil-prone high-quality source rocks in the shallow lake facies and cannot comprehensively evaluate high-quality source rocks in the semi-deep lake facies and shallow lake facies, expand the crude oil exploration field in the northern South China Sea, and provide assistance for the next crude oil exploration deployment and decision-making in the northern South China Sea. Description of the Drawings

[0018] Figure 1 It is a result diagram for identifying 2-methylhopane and 4-methylsterane series compounds in a typical high-quality source rock sample (Well B, 3021.9 m) in the Weixinan Sag of the Beibu Gulf Basin of the present invention; Figure 2 It is a comparison result diagram of the 2-methylhopane index and the 4-methylsterane index of shallow lake facies and semi-deep lake facies source rock samples in the Weixinan Sag of the Beibu Gulf Basin of the present invention; Figure 3 It is a comparison result diagram of the relative contribution indices of typical cyanobacteria and algae in shallow lake facies and semi-deep lake facies source rock samples in the Weixinan Sag of the Beibu Gulf Basin of the present invention. Detailed Embodiments

[0019] The present invention provides a molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies. The main implementation steps include: 1. Establishing the 2-methylhopane index indicating the contribution of cyanobacteria in the hydrocarbon source rock and the 4-methylsterane index indicating the contribution of algae; 2. Using the 2-methylhopane index and the 4-methylsterane index to evaluate the contributions of cyanobacteria and algae to the hydrocarbon source rocks in shallow lake facies and semi-deep lake facies; 3. Revealing the development facies belts and strata of high-quality hydrocarbon source rocks with relatively large contributions from cyanobacteria and algae. The present invention can quickly distinguish hydrocarbon source rocks in shallow lake and semi-deep lake facies, reveal the development mechanism and distribution law of high-quality lake facies hydrocarbon source rocks, improve the exploration potential of hydrocarbon source rocks in shallow lake facies, serve for the decision-making of oil and gas exploration and development, and has great value for popularization and application.

[0020] Specifically, a molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies includes the following steps: Step 1): Establishing the 2-methylhopane index indicating the contribution of cyanobacteria in the hydrocarbon source rock and the 4-methylsterane index indicating the contribution of algae S1.1: 2-Methylhopane in the saturated hydrocarbons of the hydrocarbon source rock comes from cyanobacteria, which is an autotrophic photosynthetic microorganism living on the water surface. The 2-methylhopane index from cyanobacteria can be used to evaluate the contribution level of cyanobacteria to the hydrocarbon source rock. The higher the 2-methylhopane index, the greater the contribution of cyanobacteria. The calculation method of the 2-methylhopane index is as follows: 2-methylhopane index = C 31 2-methylhopane / C 30 αβ-hopane Among them, the 2-methylhopane index is a simple calculation method. After demonstration, it can replace the internationally common 2-methylhopane evaluation index; saturated hydrocarbons are components separated from hydrocarbon source rocks and crude oil / condensate through Soxhlet extraction and column chromatography. After the hydrocarbon source rock is crushed to 200 meshes and Soxhlet extracted with dichloromethane for 72h or the obtained chloroform bitumen is quickly extracted, it is separated by alumina column chromatography. Total hydrocarbons are obtained by filtering insoluble asphaltenes from chloroform bitumen A or crude oil with filter paper. The total hydrocarbons are further eluted with n-hexane to obtain saturated hydrocarbons, which are concentrated to 2 ml and then tested on a gas chromatography-mass spectrometry instrument; 2-Methylhopane in the saturated hydrocarbons of the hydrocarbon source rock is detected using a Thermo Fisher TSQ8000 Evo gas chromatography-mass spectrometry instrument. It is recommended to use a fused silica capillary gas chromatography column of HP-5MS with a length of 30m (inner diameter 0.25mm, thickness 0.25μm). The temperature programming adopts the following specific program: rising from 50°C to 120°C (8°C / min, holding for 0 min), rising from 120°C to 200°C (4°C / min, holding for 0 min), rising from 200°C to 300°C (1.5°C / min, holding for 5 min). Helium is used as the carrier gas with a flow rate of 1.0 mL / min; the injection method adopts non-separating injection. C31 2-Methylhopane, C 30 The identification of αβ-hopane uses m / z the retention time of the corresponding compound peak on the 205 mass chromatogram and the mass spectrum for determination, and the relative content is determined according to its peak area.

[0021] S1.2: 4-Methylsteranes in the saturated hydrocarbons of source rocks come from algae dominated by dinoflagellates, and algae are considered an important source of organic matter for most high-quality lacustrine source rocks. The 4-methylsterane index can be used to evaluate the contribution of algae to source rocks. The higher the 4-methylsterane index, the greater the contribution of algae to source rocks. The calculation method of the industry-standard 4-methylsterane index is as follows: 4-Methylsterane index = C 30 4-Methylsterane / C 29 Regular steranes Among them, the detection of 4-methylsteranes can be carried out using various models of gas chromatography-mass spectrometry. 4-Methylsteranes and C 29 regular steranes can be detected with different lengths of chromatographic columns and most temperature programming procedures; C 30 4-Methylsteranes, C 29 The identification of regular steranes usually uses m / z the retention time of the corresponding compound peak on the 217 mass chromatogram and the mass spectrum for determination, and C 30 4-Methylsteranes can also be determined by using m / z the retention time of the corresponding compound peak on the 231 mass chromatogram and the mass spectrum for determination, and the relative content is determined according to its peak area.

[0022] Step 2): Use the 2-methylhopane index and 4-methylsterane index to evaluate the contributions of cyanobacteria and algae to shallow lacustrine and semi-deep lacustrine source rocks S2.1: Division of sedimentary facies of source rocks and determination of total organic carbon (TOC) The method for dividing and classifying the sedimentary facies of source rocks refers to the industry standard SYT6314-1997.

[0023] The determination process of the total organic carbon of source rocks refers to the national standard GB / T 19145-2022. The source rock rock samples will be crushed to 200 mesh, soaked in 5% dilute hydrochloric acid for 12 hours (water bath heating at 60-80 °C) to remove carbonate minerals in the samples. Then, the samples will be rinsed with deionized water in a permeable crucible more than 50 times to remove chloride ions in the samples. The processed source rock samples will be tested on a Leco CS-344 carbon-sulfur analyzer to obtain the TOC test values of the rock samples.

[0024] S2.2: Evaluating the contributions of cyanobacteria and algae to the source rocks in shallow lacustrine and semi-deep lacustrine facies using the 2-methylhopane index and 4-methylsterane index Comparatively analyzing the 2-methylhopane index of the source rocks in shallow lacustrine and semi-deep lacustrine facies to reveal the high or low contributions of cyanobacteria to the source rocks in shallow lacustrine and semi-deep lacustrine facies.

[0025] Comparatively analyzing the 4-methylsterane index of the source rocks in shallow lacustrine and semi-deep lacustrine facies to reveal the high or low contributions of algae to the source rocks in shallow lacustrine and semi-deep lacustrine facies.

[0026] Step 3): Revealing the favorable facies belts and formations of high-quality source rocks with relatively large contributions from cyanobacteria and algae S3.1: Establishing the relative contribution indices of cyanobacteria and algae in source rocks. Characterizing the absolute abundances of 2-methylhopane and 4-methylsterane in the organic matter of source rocks based on the base peak ions (the ions with the highest abundances in mass spectrometry) of 2-methylhopane and 4-methylsterane. Among them, the absolute abundance of 2-methylhopane is m / z the peak area corresponding to it in the 205 mass chromatogram, and the absolute abundance of 4-methylsterane is m / z the peak area corresponding to it in the 231 mass chromatogram. The calculation method of the relative contribution indices of cyanobacteria and algae in source rocks is as follows: Relative contribution index of cyanobacteria and algae = C 28 -C 36 2-methylhopane ( m / z 205) / [C 28 -C 36 2-methylhopane ( m / z 205)+C 28 -C 32 Absolute abundance of 4-methylsterane ( m / z 231)] Using m / z 2-methylhopane obtained from the 205 mass chromatogram and m / z 4-methylsterane obtained from the 231 mass chromatogram can more objectively reflect the relative abundances of these two types of compounds in the organic matter of source rocks; the relative contribution index of cyanobacteria and algae in source rocks can indicate the high or low contributions of cyanobacteria and algae to source rocks, that is, the higher this index, the greater the contribution of cyanobacteria compared to algae; the lower this index, the greater the contribution of algae compared to algae.

[0027] S3.2: Analyzing the high or low contributions of cyanobacteria and algae to source rocks by using the relative contribution indices of cyanobacteria and algae in different formations and facies belts.

[0028] Comparatively analyze the relative contribution indices of cyanobacteria and algae in shallow lacustrine and semi-deep lacustrine source rocks to reveal the high and low contributions of cyanobacteria and algae to shallow lacustrine and semi-deep lacustrine source rocks.

[0029] Comparatively analyze the 4-methylsterane index of source rocks in different formations to reveal the high and low contributions of cyanobacteria and algae to source rocks in different formations.

[0030] The following will describe in detail the embodiments of the present invention in the Weixinan Sag of the Beibu Gulf Basin in conjunction with the accompanying drawings. In the following embodiments, the materials used in the preparation process are not specially described and are all obtained through commercial channels without further treatment.

[0031] Embodiment

[0032] A molecular geochemical method for distinguishing high-quality source rocks in shallow lacustrine and semi-deep lacustrine facies, the main implementation steps are as follows: 1. Establish the 2-methylhopane index indicating the contribution of cyanobacteria in the source rocks of the Weixinan Sag of the Beibu Gulf Basin and the 4-methylsterane index indicating the contribution of algae (Table 1; Figure 1 ); 2. Use the 2-methylhopane index and the 4-methylsterane index to evaluate the contributions of cyanobacteria and algae to shallow lacustrine and semi-deep lacustrine source rocks ( Figure 2 ); 3. Reveal the facies belts and formations where high-quality source rocks with large contributions from cyanobacteria and algae are developed ( Figure 3 ). The specific method is as follows: Step 1): Establish the 2-methylhopane index indicating the contribution of cyanobacteria in the source rocks and the 4-methylsterane index indicating the contribution of algae S1.1: The 2-methylhopane in the saturated hydrocarbons of the source rocks is detected using a Thermo Fisher TSQ8000 Evo gas chromatography-mass spectrometry instrument. The chromatographic column uses a fused silica capillary chromatographic column of HP-5MS with a length of 30 m (inner diameter 0.25 mm, thickness 0.25 μm). The temperature programming adopts the following specific program: rising from 50 °C to 120 °C (8 °C / min, holding for 0 min), rising from 120 °C to 200 °C (4 °C / min, holding for 0 min), rising from 200 °C to 300 °C (1.5 °C / min, holding for 5 min). Helium is used as the carrier gas with a flow rate of 1.0 mL / min; the injection method adopts non-separating injection. C 31 2-methylhopane, C 30 The identification of αβ-hopane uses m / z the retention time of the compound peak corresponding to the 205 mass chromatogram and the mass spectrometry diagram for determination Figure 1 (a)], and determine the relative content according to its peak area. The calculation method of the 2-methylhopane index is as follows: 2-methylhopane index = C 31 2-methylhopane / C30 αβ-hopane Among them, the saturated hydrocarbons are the components separated from the source rocks and crude oil / condensate by column chromatography after Soxhlet extraction. After the source rocks are crushed to 200 mesh and the chloroform bitumen is obtained by Soxhlet extraction with dichloromethane for 72 h or rapid extraction, it is separated by alumina column chromatography. The total hydrocarbons are obtained by filtering the insoluble asphaltenes from chloroform bitumen A or crude oil with filter paper. The saturated hydrocarbons are further eluted with n-hexane, concentrated to 2 ml, and then tested on a gas chromatography-mass spectrometry (GC-MS) instrument.

[0033] It is calculated that the 2-methylhopane index in the source rock samples of the Weixinan Sag in the Beibu Gulf Basin is shown in Table 1.

[0034] S1.2: The detection of 4-methylsteranes in the saturated hydrocarbons of the source rock samples is carried out using a Thermo Fisher TSQ8000 Evo gas chromatography-mass spectrometry instrument. The chromatographic column model and temperature programming used are the same as those in S1.1. C 30 4-methylsteranes, C 29 The identification of regular steranes uses m / z the retention time and mass spectrum of the corresponding compound peaks on the 217 mass chromatogram Figure 1 (b)], and the relative content is determined according to its peak area. The calculation method of the 4-methylsterane index of the source rock samples is as follows: 4-methylsterane index = C 30 4-methylsteranes / C 29 regular steranes It is calculated that the 4-methylsterane index in the source rock samples of the Weixinan Sag in the Beibu Gulf Basin is shown in Table 1.

[0035] Step 2) Use the 2-methylhopane index and 4-methylsterane index to evaluate the contributions of cyanobacteria and algae to the shallow lacustrine and semi-deep lacustrine source rocks S2.1: Sedimentary facies division and determination of total organic carbon (TOC) of source rocks The methods for sedimentary facies division and classification of source rocks refer to the industry standard SYT6314-1997. The sedimentary facies division results of the source rocks in the Weixinan Sag of the Beibu Gulf Basin are shown in Table 1.

[0036] The determination process of total organic carbon in source rocks refers to the national standard GB / T 19145-2022. Rock samples of source rocks will be crushed to 200 mesh and soaked in 5% dilute hydrochloric acid for 12 hours (water bath heating at 60-80 °C) to remove carbonate minerals in the samples. Then, the samples are rinsed with deionized water in a permeable crucible more than 50 times to remove chloride ions in the samples. The processed source rock samples are tested on a Leco CS-344 carbon-sulfur analyzer to obtain the TOC test values of the rock samples. The TOC determination results of the source rocks in the Weixinan Sag of the Beibu Gulf Basin are shown in Table 1.

[0037] S2.2: Use the 2-methylhopane index and 4-methylsterane index to evaluate the contributions of cyanobacteria and algae to the shallow lacustrine and semi-deep lacustrine source rocks The comparative analysis of the 2-methylhopane index of the shallow lacustrine and semi-deep lacustrine source rocks in the Weixinan Sag of the Beibu Gulf Basin is shown in Figure 2 (a), revealing that the overall contribution of cyanobacteria to the shallow lacustrine source rocks is greater than that of the semi-deep lacustrine.

[0038] The comparative analysis of the 4-methylsterane index of the shallow lacustrine and semi-deep lacustrine source rocks in the Weixinan Sag of the Beibu Gulf Basin is shown in Figure 2 (b), revealing that the overall contribution of algae to the semi-deep lacustrine source rocks is greater than that of the shallow lacustrine.

[0039] Step 3): Reveal the favorable facies belts and strata where high-quality source rocks with greater contributions from cyanobacteria and algae develop S3.1: Establish the relative contribution index of cyanobacteria and algae in the source rocks of the Weixinan Sag of the Beibu Gulf Basin.

[0040] According to the base peak ions of 2-methylhopane and 4-methylsterane (the ions with the highest abundance in the mass spectrum) to characterize the absolute abundances of 2-methylhopane and 4-methylsterane in the organic matter of source rocks, the relative contribution index of cyanobacteria and algae in the source rocks of the study area is shown in Table 1. Among them, the absolute abundance of 2-methylhopane is m / z the peak area corresponding to the 205 mass chromatogram, and the absolute abundance of 4-methylsterane is m / z the peak area corresponding to the 231 mass chromatogram. The calculation method of the relative contribution index of cyanobacteria and algae in source rocks is as follows: Relative contribution index of cyanobacteria and algae = C 28 -C 36 2-methylhopane ( m / z 205) / [C 28 -C 36 2-methylhopane ( m / z 205)+C 28 -C 32Absolute abundance of 4-methylsteranes ( m / z 231)) Using the m / z 205 mass chromatogram, the relative abundances of 2-methylhopanes and m / z 4-methylsteranes obtained from the 231 mass chromatogram can more objectively reflect the relative abundances of these two types of compounds in the organic matter of source rocks.

[0041] Calculated, the relative contribution indices of cyanobacteria and algae in the source rocks of the Weixinan Sag in the Beibu Gulf Basin are shown in Table 1.

[0042] S3.2: Apply the relative contribution indices of cyanobacteria and algae by stratigraphic series and facies belt to evaluate the contributions of cyanobacteria and algae to source rocks.

[0043] By comparing and analyzing the relative contribution indices of cyanobacteria and algae in the source rocks of the shallow lake facies and semi-deep lake facies, it is revealed that the overall relative contribution indices of cyanobacteria and algae in the high-quality source rocks of the shallow lake facies in the Weixinan Sag of the Beibu Gulf Basin are greater than those of the semi-deep lake facies ( Figure 3 ), indicating that the overall contribution of cyanobacteria in the high-quality source rocks of the shallow lake facies is greater than that of algae; the overall contribution of algae in the high-quality source rocks of the semi-deep lake facies is greater than that of cyanobacteria, and in some semi-deep lake source rocks, algae and cyanobacteria coexist symbiotically.

[0044] By comparing and analyzing the relative contribution indices of cyanobacteria and algae in the source rocks of the upper sequence of the third member of the Liushagang Formation and the lower sequence of the second member of the Liushagang Formation, it is revealed that the overall contribution of cyanobacteria in the source rocks of the upper sequence of the third member of the Liushagang Formation in the Weixinan Sag of the Beibu Gulf Basin is greater than that of algae; the overall contribution of algae in the source rocks of the lower sequence of the second member of the Liushagang Formation is greater than that of cyanobacteria, and in some source rocks, algae and cyanobacteria coexist symbiotically ( Figure 3 ).

[0045] Table 1 Stratigraphic series, sedimentary facies, TOC, 2-methylhopane index, 4-methylsterane index, relative contribution indices of cyanobacteria and algae of source rock samples in the Weixinan Sag of the Beibu Gulf Basin

[0046] The embodiments of the specific implementation manners are all preferred embodiments of the present invention, and do not limit the protection scope of the present invention accordingly. Therefore, all equivalent changes made according to the structure, shape, and principle of the present invention shall be covered within the protection scope of the present invention.

Claims

1. A molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies, characterized in that: It includes the following steps: S1. Establish the 2-methylhopane index indicating the contribution of cyanobacteria and the 4-methylsterane index indicating the contribution of algae in the source rock; S2. Evaluate the contributions of cyanobacteria and algae to the shallow lacustrine and semi-deep lacustrine source rocks using the 2-methylhopane index and the 4-methylsterane index; S3. Reveal the favorable facies belts and formations where high-quality source rocks with large contributions from cyanobacteria and algae are developed.

2. The molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 1, wherein: In step S1, the calculation formula for the 2-methylhopane index indicating the contribution of cyanobacteria in the source rock is as follows: 2-Methylhopane index = C 31 2-Methylhopane / C 30 αβ-hopane.

3. The molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 2, characterized in that: C 31 2-Methylhopane, C 30 The determination method of αβ-hopane is as follows: After crushing the source rock and extracting to obtain chloroform bitumen, it is separated by alumina column chromatography; the total hydrocarbons are obtained by filtering the insoluble asphaltenes from chloroform bitumen A or crude oil, and the saturated hydrocarbons are further eluted from the total hydrocarbons using n-hexane. After concentration, it is tested on a gas chromatography-mass spectrometry (GC-MS) instrument; The HP-5MS fused silica capillary column with a length of 30 m was used for the chromatographic column, and the following specific temperature programming procedure was adopted: from 50 °C to 120 °C at a rate of 8 °C / min and held for 0 min; from 120 °C to 200 °C at a rate of 4 °C / min and held for 0 min; from 200 °C to 300 °C at a rate of 1.5 °C / min and held for 5 min; helium was used as the carrier gas with a flow rate of 1.0 mL / min, and the injection method was non-separating injection; C 31 2-methylhopane, C 30 The identification of αβ-hopane was carried out using m / z the retention time of the corresponding compound peak on the 205 mass chromatogram and the mass spectrometry diagram for determination, and the relative content was determined according to its peak area.

4. The molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 1, wherein: In step S1, the calculation formula for the 4-methylsterane index indicating the contribution of algae in the source rock is as follows: 4-Methylsterane index = C 30 4-Methylsterane / C 29 Regular sterane.

5. The molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 4, wherein: C 30 4-Methylsterane, C 29 The determination method of regular sterane is as follows: After crushing the source rock and extracting to obtain chloroform bitumen, it is separated by alumina column chromatography; the total hydrocarbons are obtained by filtering the insoluble asphaltenes from chloroform bitumen A or crude oil, and the saturated hydrocarbons are further eluted from the total hydrocarbons using n-hexane, and after concentration, it is tested on a gas chromatography-mass spectrometry (GC-MS) instrument; The chromatographic column used is an HP-5MS fused silica capillary column with a length of 30 m. The temperature programming adopts the following specific program: from 50 °C to 120 °C at a rate of 8 °C / min and held for 0 min; from 120 °C to 200 °C at a rate of 4 °C / min and held for 0 min; from 200 °C to 300 °C at a rate of 1.5 °C / min and held for 5 min. Helium is used as the carrier gas with a flow rate of 1.0 mL / min. The injection method is non-separating injection; C 30 4-methylsterane, C 29 The identification of regular steranes uses m / z the retention time of the corresponding compound peak on the 217 mass chromatogram and the mass spectrometry determination, or C 30 4-methylsterane uses m / z the retention time of the corresponding compound peak on the 231 mass chromatogram and the mass spectrometry determination, and the relative content is determined according to its peak area.

6. The molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 1, characterized in that: In step S2, before evaluating the contributions of cyanobacteria and algae to the shallow lacustrine and semi-deep lacustrine source rocks using the 2-methylhopane index and the 4-methylsterane index, the sedimentary facies of the source rock are divided and the total organic carbon is determined.

7. The molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 6, wherein: The method for dividing and classifying the sedimentary facies of the source rock refers to the industry standard SYT6314-1997.

8. The molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 6, characterized in that: The method for determining the total organic carbon of the source rock is as follows: Crush the source rock rock sample, soak it in dilute hydrochloric acid to remove carbonate minerals in the sample, then rinse it with deionized water to remove chloride ions in the sample, and test the processed source rock sample on a carbon-sulfur analyzer to obtain the TOC test value of the rock sample.

9. The molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 1, characterized in that: The favorable facies belts and formations where high-quality source rocks with large contributions from cyanobacteria and algae are developed revealed in step S3 include: a. Establish the relative contribution index of cyanobacteria and algae in the source rock; b. Analyze the high and low contributions of cyanobacteria and algae to the source rock using the relative contribution index of cyanobacteria and algae by formation and facies belt.

10. A molecular geochemical method for distinguishing high-quality hydrocarbon source rocks in shallow lake and semi-deep lake facies according to claim 9, characterized in that: The method for establishing the relative contribution index of cyanobacteria and algae in source rocks is as follows: The absolute abundances of 2-methylhopane and 4-methylsterane in the organic matter of source rocks are characterized by the base peak ions of 2-methylhopane and 4-methylsterane. Among them, the absolute abundance of 2-methylhopane is m / z the peak area corresponding to the 205 mass chromatogram, and the absolute abundance of 4-methylsterane is m / z the peak area corresponding to the 231 mass chromatogram; The calculation formula for the relative contribution index of cyanobacteria and algae in source rocks is as follows: Relative contribution index of cyanobacteria and algae = C 28 -C 36 2-Methylhopane ( m / z 205) / [C 28 -C 36 2-Methylhopane ( m / z 205)+C 28 -C 32 Absolute abundance of 4-methylsterane ( m / z 231)].

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