Method for recovering oxygen isotope of ancient lake by using cluster isotope of lacustrine carbonate rock

Abstract

The invention discloses a method for recovering oxygen isotopes of an ancient lake by using cluster isotopes of lacustrine carbonate rocks. Comprising the following steps: selecting a sample which is not influenced by diagenesis alteration; performing oxygen isotope and cluster isotope analysis on the sample to obtain delta < 18 > O and cluster isotope temperature of the sample; determining the maximum burying temperature experienced by the hydrocarbon source rock of the strata series where the sample is located; determining the initial equilibrium temperature of the target strata series according to the maximum burying temperature and the solid-state reset kinetic parameters of the cluster isotopes; and obtaining the oxygen isotope of the lake water in the target area by using an oxygen isotope thermometer according to the initial equilibrium temperature and delta < 18 > O. According to the method, the maximum paleo-burial temperature and the initial equilibrium temperature of the shell limestone are restrained by using the delta 47 of the shell limestone which is not obviously altered; according to the method, the initial equilibrium temperature of shell limestone is restrained by independently utilizing a solid reset model aiming at the maximum paleo-burial temperature of different intervals, and then the oxygen isotope of lake water of a target strata is recovered by utilizing a traditional oxygen isotope thermometer.

Description

Technical Field

[0001] The invention relates to a method for restoring oxygen isotopes of ancient lakes by utilizing cluster isotopes of lake-phase carbonate rocks, and belongs to the technical field of petroleum geological exploration. Background Art

[0002] The δ 18 The determination of the δ O (such as seawater, lake water, etc.) is crucial for determining the formation temperature of carbonates using traditional oxygen isotope thermometers. However, to determine the oxygen isotopes of seawater or lake water in ancient times, it is necessary to use fossils or carbonate minerals that have not undergone alteration and determine the formation temperature of the minerals. The formation temperature of fossils is often unknown and difficult to determine. In comparison, the δ 18 O has been widely studied and its evolution curve has been established. 18 There is no unified evolution curve for O. On the one hand, there are relatively few carbonate fossils in the lacustrine environment, which makes it difficult to use fossils to determine δ 18 O makes it difficult to calculate the freshwater inflow and evaporation conditions between lakes in the same period. 18 O also has large differences, making it difficult to form a unified evolution curve. 47 ) thermometer, it is not necessary to know the δ 18 O, can determine the formation temperature of carbonates alone, and combined with the δ 18 O, which can also be used to determine the δ 18 O, which provides a new solution for determining the diagenetic fluid of carbonates.

[0003] Cluster isotopes of carbonate rocks in ancient strata (Δ 47 ) often changes under the influence of later burial diagenesis and relatively high temperature, resulting in the Δ 47 The determined temperature does not represent the initial temperature of carbonate formation. According to previous studies, the Δ 47 It is mainly affected by diagenetic alteration (recrystallization) and high-temperature solid-state reset. Assuming that no alteration occurs, it is necessary to consider the effect of solid-state reset on Δ 47 The process can be restored using a kinetic model. The reset model needs to be constrained by the thermal evolution history of the mineral, especially the maximum paleotemperature. 47 The maximum burial temperature can be constrained. Summary of the invention

[0004] The purpose of the present invention is to provide a method for restoring the oxygen isotopes of ancient lakes using cluster isotopes of lacustrine carbonate rocks. The present invention uses Δ47 of shell limestone that has not undergone significant alteration to constrain the maximum ancient burial temperature of the shell limestone, and finally uses a traditional oxygen isotope thermometer to restore the oxygen isotopes of lake water in the target stratum.

[0005] The method provided by the present invention for restoring oxygen isotopes of ancient lakes by using cluster isotopes of lacustrine carbonate rocks comprises the following steps:

[0006] S1. Collect samples for petrological analysis and select samples that have not been affected by diagenetic alteration;

[0007] S2, the sample is subjected to δ 18 O and cluster isotope analysis to obtain the δ 18 O and cluster isotope temperatures;

[0008] S3. Determine the maximum burial temperature experienced by the source rock to which the sample belongs;

[0009] S4, determining the initial equilibrium temperature of the target layer system according to the maximum burial temperature;

[0010] S5, according to the initial equilibrium temperature and the δ 18 O, using an oxygen isotope thermometer, obtain the oxygen isotope of the lake water in the target area.

[0011] In the method for restoring oxygen isotopes of ancient lakes of the present invention, in step S1, XRD analysis is performed on the sample to determine the mineral composition;

[0012] Collect or prepare cast thin sections of the reservoir core section of the target layer of a single well, and conduct petrological observations on the shell limestone samples using a microscope and a cathode ray luminometer to determine whether the target layer samples are affected by diagenetic alteration (recrystallization).

[0013] In the method for restoring the oxygen isotope of ancient lakes of the present invention, in step S2, δ 18 O and cluster isotope analysis:

[0014] The sample is crushed to less than 100 mesh; then extracted with dichloromethane to obtain carbonate powder (to remove the residual soluble organic matter in the target layer sample); the carbonate powder is reacted with phosphoric acid, and the generated CO2 is measured by isotope mass spectrometer to obtain the δ 18 O and cluster isotopes (Δ 47 ), which can be obtained by the method provided by Huntington et al. (2009);

[0015] Before reacting with the phosphoric acid, the method further comprises a step of reacting with 3% H2O2.

[0016] Based on the sample obtained by the above treatment, the powder was reacted with 3% H2O2, and the reaction was repeated several times until no bubbles were generated to remove the residual organic matter.

[0017] In the method for restoring oxygen isotopes of ancient lakes of the present invention, solid carbonate standards ETH1 to ETH4 and InerCarb (NB4 and P1) are used to convert the cluster isotopes to an absolute reference system (Δ 47 , CDES 90°C), and then the cluster isotope temperature is obtained according to the calibration equation, which can be the calibration equation established by Dennis et al. (2011).

[0018] In the method for restoring oxygen isotopes of ancient lakes of the present invention, in step S3, the maximum burial temperature is obtained according to the following steps:

[0019] Collect or prepare casting thin sections of shale in the same formation as the sample, observe the casting thin sections microscopically, and perform a vitrinite reflectance test to obtain the vitrinite reflectance (Ro) of the shale sample, and then use the EasyRo model to obtain the maximum burial temperature.

[0020] In the method for restoring oxygen isotopes of ancient lakes of the present invention, in step S4, the initial equilibrium temperature is determined according to the following 1)-3):

[0021] 1) When the maximum burial temperature is less than 100°C, it indicates that the target layer is not affected by the solid-state reset effect, and the cluster isotope temperature is the initial temperature;

[0022] 2) When the maximum burial temperature is between 100°C and 175°C, the solid state reset model is used to restore the initial temperature of the target stratum mineral formation;

[0023] 3) When the maximum burial temperature is >175°C, it indicates that the target layer system is completely reset in the solid state and oxygen isotope calculations cannot be performed.

[0024] In the method for restoring oxygen isotopes of ancient lakes of the present invention, when the maximum burial temperature is between 100°C and 175°C, the initial equilibrium temperature is determined according to the following steps:

[0025] Use U-Pb dating or Re-Os dating to determine the geological age of the target strata;

[0026] Based on the maximum burial temperature, a kinetic solid-state reset model is used to obtain the initial equilibrium temperature of the target layer system.

[0027] In the method for restoring the oxygen isotopes of ancient lakes of the present invention, a three-stage temperature decrease method is used to verify whether the initial equilibrium temperature is consistent with the cluster isotopes.

[0028] The present invention is based on the initial equilibrium temperature after the solid-state reset recovery, combined with the oxygen isotope of the mineral, and uses a traditional oxygen isotope thermometer to obtain the oxygen isotope of the lake water in the target area.

[0029] The key to the method for restoring the oxygen isotope of ancient lakes by using the cluster isotope of lacustrine carbonate rock provided by the present invention is to use the cluster isotope (Δ 47 ) to constrain the maximum ancient burial temperature of the shell limestone; different from the traditional method, the present invention uses the solid-state reset model to independently constrain the initial equilibrium temperature of the shell limestone for the maximum ancient burial temperature in different intervals, and then uses the traditional oxygen isotope thermometer to restore the oxygen isotope of the lake water in the target stratum. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the maximum burial temperature map of the target layer obtained based on the EasyRo model in Example 1 of the present invention.

[0031] Figure 2 The isotope temperature (T Δ47 ) constrained initial temperature map.

[0032] Figure 3 is the cluster isotope temperature (T Δ47 )Evolution diagram.

[0033] Figure 4 is the δ of the target layer lake water obtained in Example 1 of the present invention 18 OResult graph.

[0034] Figure 5 The present invention is a flow chart of a method for restoring oxygen isotopes of ancient lakes using cluster isotopes of lacustrine carbonate rocks. DETAILED DESCRIPTION

[0035] In order to have a clearer understanding of the technical features and beneficial effects of the present invention, preferred examples of the present invention are now described in conjunction with the accompanying drawings. It should be understood that the preferred examples described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0036] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0037] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0038] Example 1: Restoration of oxygen isotopes of ancient lakes using cluster isotopes of lacustrine carbonate rocks

[0039] The flow chart of the method of the present invention is as follows Figure 5 shown.

[0040] Step 1: Perform petrological analysis on the samples.

[0041] Step 1: Collect nine shell limestone samples from a certain area in a certain basin, and conduct X-ray diffraction experiment (XRD) on the nine shell limestone samples. It is found that the main component of shell limestone is calcite (generally 82.7-97.6%), containing a small amount of quartz, hematite and clay (generally less than 5%).

[0042] Step 2: Petrological analysis of the shell limestone samples was conducted. Based on polarized observation of rock thin sections, it was concluded that the morphology of biological shells can be divided into two categories: one is that the morphology of the biological shells is well preserved (SFP), and the other is that the morphology of the shells is partially destroyed and is in a fragmentary state (SPP). The cathode luminescence result is dark and basically non-luminescent, and only dark organic matter between the shells shows bright yellow or blue cathode luminescence.

[0043] Step 3: Determine that the sample is not substantially affected by diagenetic alteration (recrystallization).

[0044] Step 2: δ 18 O and cluster isotopes (Δ 47 ) analysis, specifically including the following steps:

[0045] Step 1: Crush all shell limestone samples to less than 100 mesh.

[0046] Step 2: The carbonate powder was then extracted with dichloromethane (DCM) for 24 hours to remove the residual soluble organic matter in the target layer sample.

[0047] Step 3: After the above treatment, the powder is reacted with 3% H2O2, and the process is repeated several times until no bubbles are generated and the residual organic matter is removed.

[0048] Step 4: After removing the organic matter, about 10 mg of carbonate powder in each sample was reacted with 103% phosphoric acid (Guo et al., 2009; Wacker et al., 2013) at 90°C, and the generated CO2 was measured using a Thermo MAT253 isotope mass spectrometer. Each sample was measured three times to ensure that the sample error was within 0.02 thousandths.

[0049] Step 5: The δ 18 O and original Δ 47 The values ​​were calculated according to the method provided by Huntington et al. (2009). 18O is between -14.820±0.009‰ and -10.231±0.006‰. See Table 1 for specific results.

[0050] Table 1 δ 18 O. Correction Δ 47 (CDES90) and T Δ47 (℃), and the reconstructed Early Jurassic lake water δ 18 O data.

[0051]

[0052] Step 6: Use the solid carbonate standards (ETH1 to ETH4) and InerCarb (NB4 and P1) method (Bernasconi et al., 2018) to convert the original Δ 47 The value is converted to an absolute reference system (Δ 47 , CDES 90℃), this example obtains the ETH-corrected Δ 47 Between 0.448±0.005 and 0.463±0.006.

[0053] Step 7: Calculate the cluster isotope temperature (T) using the calibration equation developed by Dennis et al. (2011). Δ47 ), and finally the cluster isotope temperature of shell limestone was obtained to be between 64.4±0.8℃ and 69.7±1.4℃, with an average of 67.2±1.4℃.

[0054] Step 3: Based on the cluster isotope temperature obtained above, the kinetic model is combined to constrain the solid-state reset process, and then the maximum burial temperature experienced by the source rock is constrained, which specifically includes the following steps:

[0055] Step 1: Collect cast thin sections of shale in the same stratum of 17 target strata, observe the thin sections microscopically, and perform vitrinite reflectance test to obtain the vitrinite reflectance (Ro) of the shale samples. Ro is mainly between 1.28-1.47%, with an average of 1.36%, indicating that the organic matter maturity of the shale is at a high maturity stage, which is consistent with the production of a small amount of condensate oil in the shale strata.

[0056] Step 2: Based on the above vitrinite reflectance results, the maximum burial temperature constraint obtained by the EasyRo model is 167.3℃ (163.5-173.5℃). The results are as follows Figure 1 As shown, the Δ 47The temperature is 67.3℃, which is much higher than the equilibrium temperature of shell limestone (i.e., the ambient temperature). The temperature of the shell of an organism is generally in equilibrium with the temperature of the surrounding environment. The high cluster isotope temperature of shell limestone indicates that it has undergone solid state reset or recrystallization during burial. Based on the observation of microscopic thin sections and cathode luminescence, both samples showed no significant recrystallization and late alteration. The analysis showed that the change in cluster isotope temperature was mainly affected by solid state reset.

[0057] Step 4: Based on the obtained maximum burial temperature, determine the initial temperature of the target stratum mineral formation. The maximum burial temperature of the sample provided in this example is between 163.5-173.5℃, which belongs to the second partial solid reset situation. Specifically, the following steps are included:

[0058] Step 1: The initial formation temperature and time of the calcite sample are important parameters for running the solid-state reset model. Collect and process the sample, and use U-Pb dating or Re-Os dating to determine the geological age of the target layer, and the geological age of the sample is 180.3±3.2Ma.

[0059] Step 2: Based on the above analysis results, using the widely used calcite diffusion model, assuming that the initial equilibrium temperature of shell limestone is 20, 25, 28, 30, 35 ° C, the maximum burial temperature obtained based on the solid reset model is 120, 140, 168, 188, 196 ° C. Compared with the maximum burial depth of 167.3 ° C obtained by Ro, the initial equilibrium temperature of shell limestone should be around 28 ° C.

[0060] Step 3: Apply the three-stage temperature reduction method of the solid reset model again, and the final equilibrium temperature of the shell limestone is 67.3℃ when the maximum burial temperature is 170℃. The results are as follows: Figure 2 As shown, it is highly consistent with the measured cluster isotopes.

[0061] Step 5: Based on the initial equilibrium temperature of about 28°C after solid state reset, the final equilibrium temperature of the shell limestone obtained above is 67.3°C, such as Figure 3 As shown in the figure, the solid reset effect increases the cluster isotope temperature of the shell limestone by about 39.3°C. The initial equilibrium temperature of each shell limestone can be obtained by subtracting 39.3°C from the cluster temperature of the shell limestone. Combined with the oxygen isotope of the mineral, the oxygen isotope of the lake water in the area can be obtained using the traditional oxygen isotope thermometer (Formula 1).

[0062] 1000lnα calcite-fuid =18.03×10 3 / T 2 -32.42 (1)

[0063] Where T is the temperature at which calcite forms;

[0064] α acalcite.fiuid is the oxygen isotope fractionation coefficient between calcite and fluid.

[0065] like Figure 4 As shown, the oxygen isotope of the lake water fluctuates between -12.55‰ and -8.48‰.

Claims

1. A method for restoring oxygen isotopes of ancient lakes using cluster isotopes of lacustrine carbonate rocks, comprising the following steps: S1. Collect samples for petrological analysis and select samples that have not been affected by diagenetic alteration; S2, the sample is subjected to δ 18 O and cluster isotope analysis to obtain the δ 18 O and cluster isotope temperatures; S3. Determine the maximum burial temperature experienced by the source rock to which the sample belongs; S4, determining the initial equilibrium temperature of the target layer system according to the maximum burial temperature; S5, according to the initial equilibrium temperature and the δ 18 O, using an oxygen isotope thermometer, obtain the oxygen isotope of the lake water in the target area.

2. The method for restoring oxygen isotopes of ancient lakes according to claim 1, characterized in that: In step S1, performing XRD analysis on the sample to determine the mineral composition; Collect or prepare cast thin sections of the reservoir core section of the target layer of a single well, and conduct petrological observations on the shell limestone samples using a microscope and a cathode ray luminometer to determine whether the target layer samples have been diagenetically altered.

3. The method for restoring oxygen isotopes of ancient lakes according to claim 1 or 2, characterized in that: In step S2, the following method is used to perform δ 18 O and cluster isotope analysis: The sample is crushed to less than 100 mesh; then extracted with dichloromethane to obtain carbonate powder; the carbonate powder is reacted with phosphoric acid, and the generated CO2 is measured by isotope mass spectrometer to obtain the δ 18 O and cluster isotopes.

4. The method for restoring oxygen isotopes of ancient lakes according to claim 3, characterized in that: The cluster isotope is converted to an absolute reference system using solid carbonate standards ETH1-ETH4 and InerCarb (NB4 and P1), and the cluster isotope temperature is obtained according to a calibration equation.

5. The method for restoring oxygen isotopes of ancient lakes according to any one of claims 1 to 4, characterized in that: In step S3, the maximum burial temperature is obtained according to the following steps: Collect or prepare casting thin sections of shale in the same stratum as the sample, observe the casting thin sections microscopically, and perform a vitrinite reflectance test to obtain the vitrinite reflectance of the shale sample, and then use the EasyRo model to obtain the maximum burial temperature.

6. The method for restoring oxygen isotopes of ancient lakes according to any one of claims 1 to 5, characterized in that: In step S4, the initial equilibrium temperature is determined according to the following 1)-3): 1) When the maximum burial temperature is less than 100°C, it indicates that the target layer is not affected by the solid-state reset effect, and the cluster isotope temperature is the initial temperature; 2) When the maximum burial temperature is between 100°C and 175°C, the solid state reset model is used to restore the initial temperature of the target stratum mineral formation; 3) When the maximum burial temperature is >175°C, it indicates that the target layer system is completely reset in the solid state and oxygen isotope calculations cannot be performed.

7. The method for restoring oxygen isotopes of ancient lakes according to claim 6, characterized in that: When the maximum burial temperature is between 100°C and 175°C, the initial equilibrium temperature is determined according to the following steps: Use U-Pb dating or Re-Os dating to determine the geological age of the target strata; Based on the maximum burial temperature, a kinetic solid-state reset model is used to obtain the initial equilibrium temperature of the target layer system.

8. The method for restoring oxygen isotopes of ancient lakes according to claim 7, characterized in that: A three-stage temperature reduction method is used to verify whether the initial equilibrium temperature is consistent with the cluster isotope.

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