Method for judging source, forming time and cause of dolomite fluid

Abstract

The invention provides a method for judging the source, formation time and cause of dolomite fluid, which comprises the following steps: determining age data of a dolomite sample, and judging the formation time of dolomite; respectively carrying out carbon and oxygen isotope determination and rare earth element determination on the dolomite sample, comparing the determination result with seawater data in the same period to obtain an analysis result, and judging the source of the diagenesis fluid by combining the analysis result of the carbon and oxygen isotope and the analysis result of the rare earth element; and determining the dolomite formation cause by combining the dolomite formation time and the diagenetic fluid source result. According to the judgment method provided by the invention, the dolomite cause can be quickly judged, the process is simplified, few testing means are used, time, manpower and material resources are greatly saved, and the problems of conclusion contradiction and uncertainty caused by adopting multiple judgment standards at the same time in the prior art are solved.

Description

Technical Field

[0001] This invention belongs to the field of geological exploration technology, specifically relating to a method for determining the source, formation time, and origin of dolomite fluids. Background Technology

[0002] Hydrochloric acid rock reservoirs possess abundant oil and gas resources, accounting for more than half of the global oil and gas reserves and production, making them an important strategic resource. Among these, dolomite is the largest reservoir type. Dolomite is mainly composed of dolomite, often mixed with minor minerals such as quartz, feldspar, calcite, and clay. It belongs to sedimentary carbonate rocks and is an important reservoir rock for oil and natural gas. Under deep burial conditions, dolomite has stronger resistance to compaction and pressure solution than limestone, thus possessing higher porosity, making it a crucial deep-seated oil and gas reservoir.

[0003] The vast majority of dolomite in geological history was formed by the alteration of limestone by magnesium-rich dolomitizing fluids, a process known as dolomitization. Dolomitization is generally considered to occur in two main stages: the quasi-syngenetic stage and the burial stage. Quasi-syngenetic dolomitization is primarily associated with arid climates, resulting in dolomite that often retains the original rock structure. Burial-stage dolomitization is mainly related to deep, magnesium-rich diagenetic media, leading to dolomite that is often fine-grained or finer-grained. Dolomite is composed of dolomite, and the genesis and controlling factors of dolomite determine its overall distribution pattern. Therefore, understanding the genesis of different types, occurrences, and stages of dolomite formation is crucial. Among these, the source of the diagenetic fluids and the formation time of the dolomite are the most critical factors in determining its formation. In the long-term high-temperature and high-pressure burial environment, the exchange of ions in dolomite is very frequent. Mineral components migrate and recrystallize, or undergo neomorphic deformation, which can cause dolomite of the same origin and period to exhibit different states with varying burial depths. Existing methods, on the one hand, often rely on the crystal form of dolomite to determine its formation time and stage, which is far from sufficient; on the other hand, although there are many geochemical experimental analysis methods, numerous geochemical data indicators may contradict each other to some extent. Therefore, traditional methods for determining the origin of dolomite are prone to subjective misguidance.

[0004] Therefore, there is a need to provide a simple and effective analytical method and process to quickly determine the fluid source, formation time, and cause of dolomite. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining the fluid source, formation time, and origin of dolomite, and to achieve rapid determination of the origin of dolomite by combining dating and geochemical analysis methods.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] This invention provides a method for determining the source, formation time, and origin of dolomite fluid, the method comprising the following steps:

[0008] (1) Determine the formation time of dolomite by measuring the age data of the dolomite sample;

[0009] (2) Carbon and oxygen isotope analysis and rare earth element analysis were performed on the dolomite samples respectively. The analysis results were compared with the seawater data of the same period to obtain the analysis results. The source of the diagenetic fluid was determined by combining the analysis results of carbon and oxygen isotope analysis and rare earth element analysis.

[0010] (3) Based on the results of the formation time of dolomite and the source of diagenetic fluid, the genesis of dolomite was determined.

[0011] Steps (1) and (2) are not in any particular order.

[0012] The method provided by this invention combines the dating data of the sample with geochemical analysis. It comprehensively determines the origin of the diagenetic fluid of dolomite by analyzing the carbon and oxygen isotopes and rare earth element data of the dolomite. By combining age data with fluid origin, it can quickly determine the genesis of dolomite. Compared with existing methods, the process is simplified, fewer testing methods are used, and the accuracy is high, which greatly saves time, manpower and material resources. It also solves the problem of contradictory and uncertain conclusions caused by the simultaneous use of multiple judgment criteria in existing technologies.

[0013] Preferably, the method for determining age data in step (1) includes: measuring U and Pb isotopes using laser ablation inductively coupled plasma mass spectrometry, and calculating... 207 Pb / 206 Pb and 238 U / 206 The age of dolomite is obtained by harmonic mapping of the ratio of Pb to Pb.

[0014] Preferably, the method for determining the formation time of dolomite in step (1) includes: comparing the age data of the dolomite sample with the age of the same strata, and determining the formation time of the dolomite as the quasi-syngenetic period or the burial period based on the difference between the age data of the dolomite and the age of the same strata.

[0015] Preferably, the process of determining the source of diagenetic fluid in step (2) includes:

[0016] (2.1) Record whether the carbon and oxygen isotope values ​​of the dolomite sample are within the range of the carbon and oxygen isotope values ​​of seawater during the same period.

[0017] The rare earth element values ​​of the dolomite sample were PAAS-normalized with the rare earth element values ​​of seawater during the same period to form an elemental distribution map, and the normal or abnormal results of the Eu and Ce element values ​​were recorded.

[0018] (2.2) Combining the carbon and oxygen isotope results obtained in step (2.1) with the results of Eu and Ce elements, the source of the diagenetic fluid of dolomite was determined.

[0019] In this invention, the method for determining whether the rare earth element values ​​are normal or abnormal is as follows:

[0020] The calculation method for Ce anomalies is: Ce / Ce*=2Ce N / (La N +Pr N The calculation method for Eu anomalies is: Eu / Eu*=2Eu N / (Sm N +Gd N In the formula, N represents the PAAS standardized result. If the abnormal value of Ce or Eu is in the range of 0.001-0.1, it is determined to be a normal result; otherwise, it is an abnormal result.

[0021] In this invention, the carbon and oxygen isotope values ​​or rare earth element values ​​of the seawater of the same period are obtained by performing carbon and oxygen isotope analysis or rare earth element analysis on micritic carbonate rocks or fossil samples that were not affected by diagenesis at different times, and the numerical results represent the values ​​of the seawater of the same period.

[0022] Preferably, the carbon and oxygen isotope values ​​obtained in step (2.1) are divided into: within the range of carbon and oxygen isotope values ​​of seawater during the same period, or outside the range of carbon and oxygen isotope values ​​of seawater during the same period.

[0023] The results of the values ​​of Eu and Ce in step (2.1) are divided into: the values ​​of Eu and Ce are both normal, or one or both of the values ​​of Eu and Ce are abnormal.

[0024] Preferably, the diagenetic fluid source in step (2.2) includes marine fluids or deep hydrothermal fluids.

[0025] Preferably, the process of determining the source of the diagenetic fluid for dolomite in step (2.2) includes:

[0026] When the carbon and oxygen isotope values ​​of the dolomite sample are within the range of the carbon and oxygen isotope values ​​of seawater of the same period, and the values ​​of Eu and Ce are both normal, it is determined that the diagenetic fluid originated from marine fluid.

[0027] When the carbon and oxygen isotope values ​​of the dolomite sample are outside the range of carbon and oxygen isotope values ​​of seawater of the same period, and one or both of the Eu and Ce values ​​are abnormal, it is determined that the diagenetic fluid originated from deep hydrothermal fluids.

[0028] When the carbon and oxygen isotope values ​​of the dolomite sample are outside the range of carbon and oxygen isotope values ​​of seawater during the same period, and the values ​​of Eu and Ce are normal, it is determined whether there is a regional thermal event in the studied area based on the geological background information of the dolomite. If there is a regional thermal event, it is determined that the diagenetic fluid source of the dolomite is marine fluid.

[0029] Preferably, when the carbon and oxygen isotope values ​​of the dolomite sample are outside the range of carbon and oxygen isotope values ​​of seawater during the same period, and the values ​​of Eu and Ce are normal and there are no regional thermal events, or when the carbon and oxygen isotope values ​​of the dolomite sample are within the range of carbon and oxygen isotope values ​​of seawater during the same period and one or both of the values ​​of Eu and Ce are abnormal, the judgment method further includes: measuring the trace elements V and Cr of the sample to analyze the diagenetic environment.

[0030] Preferably, the analyzed diagenetic environment includes:

[0031] Calculate the V / Cr ratio;

[0032] If the V / Cr ratio is <4, the diagenetic environment is an oxidizing environment, and the diagenetic fluid is determined to be marine fluid.

[0033] If the V / Cr ratio is greater than 4, the diagenetic environment is a reducing environment, and the diagenetic fluid is determined to be a deep hydrothermal fluid.

[0034] As a preferred embodiment of the determination method provided by the present invention, the determination method includes the following steps:

[0035] (1) Select dolomite samples from field outcrops and / or drill cores and record geological background information, including stratigraphic occurrence, thickness, color and crystal size. Prepare thin slices with a thickness of more than 50 μm and powder samples with a particle size of less than 200 mesh from the dolomite samples for isotope dating and geochemical experimental analysis.

[0036] (2) The thin-film sample was subjected to U and Pb isotope analysis using laser ablation inductively coupled plasma mass spectrometry, and the values ​​at each measurement point were calculated. 207 Pb / 206 Pb and 238 U / 206The Pb ratio is used to generate dolomite age data through Tera-Wasserburg plotting. This dolomite age data is then compared with the ages of contemporaneous stratigraphic layers to determine the formation time of the dolomite. Specifically: if the difference between the dolomite age data and the contemporaneous stratigraphic age is within ±10%, the formation time of the dolomite is considered a quasi-syngenetic period; if the dolomite age data is less than the contemporaneous stratigraphic age and the difference is >10%, the formation time of the dolomite is considered a burial period; if the dolomite age data is greater than the contemporaneous stratigraphic age and the difference is >10%, the dolomite age data is invalid.

[0037] (3) The powder samples were subjected to carbon and oxygen isotope testing and rare earth element testing, respectively. The test results were compared with the values ​​of seawater at the same time to obtain the comparison results. The comparison results of carbon and oxygen isotopes and rare earth elements were combined to determine the source of the diagenetic fluid of dolomite, specifically:

[0038] The carbon and oxygen isotope values ​​of the dolomite sample are compared with the carbon and oxygen isotope values ​​of the seawater during the same period. If the value of the dolomite sample is within the range of the seawater during the same period, it is recorded as CO(IN). If the value of the dolomite sample is outside the range of the seawater during the same period, it is recorded as CO(OUT).

[0039] The rare earth element (REE) values ​​of Eu and Ce in the dolomite sample were compared with those of Eu and Ce in seawater during the same period after PAAS standardization. If the results of Eu and Ce in the REE distribution map are both within the normal range, it is recorded as REE(N). If one or both of the results of Eu and Ce are positively abnormal, it is recorded as REE(PA).

[0040] Combining the comparison results of carbon and oxygen isotopes and rare earth elements, the following four situations exist:

[0041] If the diagenetic fluids are CO(IN) and REE(N), then the diagenetic fluid source of the dolomite is determined to be marine fluid.

[0042] If the results are CO(OUT) and REE(PA), then the diagenetic fluid source of the dolomite is determined to be deep hydrothermal fluid.

[0043] If the dolomite is CO(OUT) and REE(N), determine whether there are regional thermal events in the study area based on the geological background information of the dolomite. The regional thermal events include orogenic movements and / or volcanic activity. If there are regional thermal events, the source of the diagenetic fluid of the dolomite is determined to be marine fluid. If there are no regional thermal events, continue to test the trace elements V and Cr in the sample.

[0044] If the sample contains CO(IN) and REE(PA), the trace element test for V and Cr is performed. The specific results of the trace element test for V and Cr are as follows: calculate the V / Cr ratio. If the V / Cr ratio is <4, the source of the diagenetic fluid of the dolomite is determined to be marine fluid. If the V / Cr ratio is >4, the source of the diagenetic fluid of the dolomite is determined to be deep hydrothermal fluid.

[0045] (4) Combining the formation time of dolomite with the results of the diagenetic fluid source, the genesis of dolomite was determined, specifically as follows:

[0046] If the formation time is determined to be quasi-syngenetic and the source of the diagenetic fluid is determined to be marine fluid, then the genesis is determined to be sedimentary marine fluid dolomite with minimal alteration during the burial period.

[0047] If the formation time is determined to be the burial period and the source of the diagenetic fluid is determined to be marine fluid, then the genesis is determined to be marine fluid dolomite formed during the sedimentary period of drastic alteration during the burial period.

[0048] If the formation time is determined to be quasi-syngenetic and the source of the diagenetic fluid is determined to be deep hydrothermal fluid, then the genesis is determined to be early-buried deep hydrothermal dolomite.

[0049] If the formation time is determined to be the burial period and the source of the diagenetic fluid is determined to be deep hydrothermal fluid, then the genesis is determined to be late-stage deep hydrothermal dolomite.

[0050] Steps (2) and (3) are not in any particular order.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] The judgment method provided by this invention enables rapid determination of the formation of dolomite, simplifies the process, requires fewer testing methods, and significantly saves time, manpower, and material resources. It also solves the problem of contradictory and uncertain conclusions caused by the simultaneous use of multiple judgment criteria in the prior art. Attached Figure Description

[0053] Figure 1 This is a flowchart of the method for determining the source, formation time, and origin of dolomite fluid provided in Example 1;

[0054] Figure 2 This is a picture of the dolomite sample from Example 2;

[0055] Figure 3 This is a microscope image of dolomite sample 1 from Example 2;

[0056] Figure 4 This is a microscope image of dolomite sample 2 from Example 2;

[0057] Figure 5 This is an age diagram of dolomite sample 1 from Example 2;

[0058] Figure 6 This is the age diagram of dolomite sample 2 from Example 2;

[0059] Figure 7 This is a graph showing the carbon and oxygen isotope test results of the dolomite sample from Example 2;

[0060] Figure 8 This is a graph showing the rare earth element test results of the dolomite sample from Example 2. Detailed Implementation

[0061] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0062] Example 1

[0063] This embodiment provides a method such as Figure 1 The method for determining the source, formation time, and origin of dolomite fluid, as shown, includes the following steps:

[0064] (1) Select dolomite samples from the outcrop profile and / or drill core of the rock in the area to be tested, and record the geological background information. Prepare thin slices with a thickness of more than 50 μm and powder samples with a particle size of 200 mesh from the dolomite samples respectively.

[0065] (2) The thin section samples were subjected to U and Pb isotope dating using laser ablation inductively coupled plasma mass spectrometry (LC-MS). The testing method was in accordance with the standard (ZL201910635554.1) to obtain the approximate age of the sample formation. The sample age was compared with the age of the strata of the same period. If the difference between the sample age and the age of the strata of the same period was within ±10%, the formation time of the dolomite was identified as the quasi-cogenetic period (PD). If the sample age was less than the age of the strata of the same period and the difference was >10%, the formation time of the dolomite was identified as the burial period (DB). If the sample age was greater than the age of the strata of the same period and the difference was >10%, the dolomite age data was invalid (NULL).

[0066] (3) The powder sample was subjected to carbon and oxygen isotope determination. The determination method was in accordance with the implementation standard (SY / T5238-2019). The carbon and oxygen isotope test data were compared with the carbon and oxygen isotope values ​​of seawater during the same period. If the carbon and oxygen isotope values ​​of the sample were within the range of seawater during the same period, they were recorded as CO(IN). If the carbon and oxygen isotope values ​​of the sample were outside the range of seawater during the same period, they were recorded as CO(OUT).

[0067] (4) The powder sample was tested for rare earth elements. The test method was in accordance with the standard (GB / T14352.20-2021). The rare earth element values ​​were compared with the rare earth element values ​​of seawater at the same time after PAAS standardization. If the results of Eu and Ce are both within the normal range, it is recorded as REE(N). If one or both of the results of Eu and Ce are abnormal, it is recorded as REE(PA).

[0068] (5) Combining the comparison results of carbon and oxygen isotopes and rare earth elements, the following four situations exist:

[0069] If the diagenetic fluids are CO(IN) and REE(N), then the diagenetic fluid source of the dolomite is determined to be marine fluid.

[0070] If the results are CO(OUT) and REE(PA), then the diagenetic fluid source of the dolomite is determined to be deep hydrothermal fluid.

[0071] If the values ​​are CO(OUT) and REE(N), the geological background information of the dolomite determines whether there are regional thermal events in the studied area. These regional thermal events include orogenic movements and / or volcanic activity. Oxygen isotopes will be significantly negative due to the baking effect of thermal events, but since no thermal fluids are involved, the rare earth element values ​​will not be abnormal. Therefore, if regional thermal events exist, the diagenetic fluid source of the dolomite is determined to be marine fluid. If no regional thermal events exist, the trace element V and Cr tests on the sample will continue.

[0072] If the result is CO(IN) and REE(PA), continue to test the sample for trace elements V and Cr.

[0073] The testing methods for trace elements V and Cr were performed in accordance with the standard (GB / T19143-2017). After obtaining the test results, the V / Cr ratio was calculated. The diagenetic environment was analyzed by the V / Cr ratio. If V / Cr < 4, it indicates that the diagenetic environment was an oxidizing environment, which is an open system and formed relatively early. Therefore, the diagenetic fluid was identified as marine fluid (M). If V / Cr > 4, it indicates that the diagenetic environment was mainly a reducing environment, which is a closed system and formed relatively late. Therefore, the diagenetic fluid was identified as deep hydrothermal fluid (H).

[0074] (6) Combining the formation time of dolomite with the source of diagenetic fluids to determine its genesis, the following four situations exist:

[0075] If the formation time is determined to be quasi-syngenetic (PD) and the source of the diagenetic fluid is determined to be marine fluid (M), then the genesis is determined to be sedimentary marine fluid dolomite with minimal alteration during the burial period;

[0076] If the formation time is determined to be the burial period (DB) and the source of the diagenetic fluid is determined to be marine fluid (M), then the genesis is determined to be marine fluid dolomite from the sedimentary period that underwent drastic alteration during the burial period.

[0077] If the formation time is determined to be quasi-syngenetic (PD) and the source of the diagenetic fluid is determined to be deep hydrothermal (H), then the genesis is determined to be early-buried deep hydrothermal dolomite.

[0078] If the formation time is determined to be the burial period (DB) and the source of the diagenetic fluid is determined to be deep hydrothermal fluid (H), then the genesis is determined to be late-stage deep hydrothermal dolomite.

[0079] Example 2

[0080] This embodiment provides a method for determining the fluid source, formation time, and origin of dolomite. Based on the determination method provided in Embodiment 1, this embodiment uses a Permian Maokou Formation dolomite sample from the central Sichuan Basin as the specific research object to illustrate the technical solution of the present invention. The determination method includes the following steps:

[0081] (1) Select a dolomite sample from the drill core, such as Figure 2 As shown, two different types of dolomite are present in this sample. Samples of each type were taken separately and designated as Sample 1 and Sample 2. Sample 1 is dark gray dolomite, as shown... Figure 3 As shown, under the microscope, the dolomite is mainly composed of powder crystals, exhibiting a subhedral to anhedral shape. Sample 2 is white dolomite, as... Figure 4 As shown, under the microscope, dolomite is mainly composed of fine to medium crystal structure, exhibiting a semi-euhedral to euhedral shape. Thin sheet samples with a thickness of 50 μm and polished samples and powder samples with a particle size of 200 mesh were prepared for the two types of samples respectively.

[0082] (2) The thin-section samples were subjected to U and Pb isotope analysis using laser ablation inductively coupled plasma mass spectrometry, and the values ​​at each measurement point were calculated. 207 Pb / 206 Pb and 238 U / 206 Pb ratios were then plotted onto a Tera-Wasserburg plot. The Tera-Wasserburg age maps for samples 1 and 2 are shown below. Figure 5 and Figure 6 As shown, the calculated dolomite age of sample 1 is 256.9±2.7 Ma, and the dolomite age of sample 2 is 241.4±2 Ma. The stratigraphic age of the middle and late Maokou Formation is approximately 259 Ma. The difference between the dolomite age and the stratigraphic age of samples 1 and 2 is less than 10% (less than 25.9 Ma). Therefore, the formation time of samples 1 and 2 is the quasi-syngenetic period (PD).

[0083] (3) Carbon and oxygen isotope determination was performed on two types of samples, each weighing 300 μg. The δ of sample 1 was measured. 13 The C‰ (VPDB) value is 2.4, δ 18 The VPDB value was -8.51, and the δ of sample 2 was... 13 The C‰ (VPDB) value is 3.38, δ 18 The VPDB value was -7.63. The carbon and oxygen isotope values ​​were compared with the carbon and oxygen isotope ranges of Middle Permian seawater, and the results are as follows: Figure 7 As shown, the values ​​of both Sample 1 and Sample 2 are within the range of seawater values ​​during the same period. The carbon and oxygen isotope analysis results of Sample 1 and Sample 2 are CO(IN).

[0084] (4) Rare earth element (REE) values ​​were determined in 200 mg samples of each type. The REE results were compared with those of seawater (Chongqing seawater) from the same period after PAAS standardization. Figure 8 As shown, the rare earth element distribution pattern of sample 1 is similar to that of seawater, reflecting that the diagenetic fluid is similar to that of seawater. Its rare earth element analysis result is REE(N). Compared with seawater of the same period, the rare earth element distribution pattern of sample 2 shows a small positive anomaly of Eu (ordinate greater than 0.1). Its rare earth element analysis result is REE(PA).

[0085] (5) Combine carbon and oxygen isotope analysis results with rare earth element analysis results to analyze the source of diagenetic fluids:

[0086] For sample 1, the analysis results of sample 1 are CO(IN) and REE(N), and its diagenetic fluid source is determined to be marine fluid (M);

[0087] For sample 2, the analysis results of sample 2 are CO(IN) and REE(PA). Further trace element testing is required. Take 200mg of sample powder for V and Cr trace element determination. The V / Cr ratio is 0.44, which is less than 4. Therefore, the source of its diagenetic fluid is determined to be marine fluid (M).

[0088] (6) The genesis was determined by combining the formation time and the source of the diagenetic fluid. The analysis results of Sample 1 and Sample 2 were both quasi-syngenetic (PD) and marine fluid (M), that is, the genetic analysis conclusion of Sample 1 and Sample 2 is that they are marine fluid dolomite from the sedimentary period with little alteration during the burial period. This result indicates that although the Maokou Formation dolomite in this area has some differences in color and dolomite crystal form, they are all products formed during the quasi-syngenetic period, with only slight adjustments and alterations during the burial period, resulting in some differences in crystals. This result also means that this type of dolomite has the characteristics of quasi-syngenetic layered distribution, which may be controlled by the distribution of sedimentary facies zones.

[0089] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for determining the source, formation time, and origin of dolomite fluid, characterized in that, The determination method includes the following steps: (1) Determine the formation time of dolomite by measuring the age data of the dolomite samples; (2) Carbon and oxygen isotope analysis and rare earth element analysis were performed on the dolomite samples respectively. The analysis results were compared with the seawater data of the same period to obtain the analysis results. The source of the diagenetic fluid was determined by combining the analysis results of carbon and oxygen isotope analysis and rare earth element analysis. (3) Determine the genesis of dolomite by combining the results of the formation time of dolomite and the source of diagenetic fluid; Steps (1) and (2) are not in any particular order.

2. The judgment method according to claim 1, characterized in that, The method for determining age data in step (1) includes: measuring U and Pb isotopes using laser ablation inductively coupled plasma mass spectrometry, and calculating... 207 Pb / 206 Pb and 238 U / 206 The age of dolomite is obtained by harmonic mapping of the ratio of Pb to Pb.

3. The determination method according to claim 1 or 2, characterized in that, The method for determining the formation time of dolomite in step (1) includes: comparing the age data of the dolomite sample with the age of the strata of the same period, and determining the formation time of the dolomite as the quasi-syngenetic period or the burial period based on the difference between the age data of the dolomite and the age of the strata of the same period.

4. The determination method according to any one of claims 1-3, characterized in that, Step (2) involves determining the source of the diagenetic fluid, which includes: (2.1) Record whether the carbon and oxygen isotope values ​​of the dolomite sample are within the range of carbon and oxygen isotope values ​​of seawater during the same period. The rare earth element values ​​of the dolomite sample and the rare earth element values ​​of the seawater at the same time were PAAS-normalized to obtain the rare earth element distribution map. The normal or abnormal results of Eu and Ce elements in the rare earth element distribution map of the sample were recorded. (2.2) Combining the carbon and oxygen isotope results obtained in step (2.1) with the results of Eu and Ce elements, the source of the diagenetic fluid of dolomite was determined.

5. The judgment method according to claim 4, characterized in that, The results of the carbon and oxygen isotope values ​​in step (2.1) are divided into: within the range of carbon and oxygen isotope values ​​of seawater during the same period, or outside the range of carbon and oxygen isotope values ​​of seawater during the same period. The results of the values ​​of Eu and Ce in step (2.1) are divided into: the values ​​of Eu and Ce are both normal, or one or both of the values ​​of Eu and Ce are abnormal.

6. The determination method according to claim 4 or 5, characterized in that, The diagenetic fluid sources mentioned in step (2.2) include marine fluids or deep hydrothermal fluids.

7. The determination method according to claim 5 or 6, characterized in that, Step (2.2) describes the process of determining the source of the diagenetic fluids for dolomite, which includes: When the carbon and oxygen isotope values ​​of the dolomite sample are within the range of the carbon and oxygen isotope values ​​of seawater of the same period, and the values ​​of Eu and Ce are both normal, it is determined that the diagenetic fluid originated from marine fluid. When the carbon and oxygen isotope values ​​of the dolomite sample are outside the range of carbon and oxygen isotope values ​​of seawater of the same period, and one or both of the Eu and Ce values ​​are abnormal, it is determined that the diagenetic fluid originated from deep hydrothermal fluids. When the carbon and oxygen isotope values ​​of the dolomite sample are outside the range of carbon and oxygen isotope values ​​of seawater during the same period, and the values ​​of Eu and Ce are normal, it is determined whether there is a regional thermal event in the studied area based on the geological background information of the dolomite. If there is a regional thermal event, it is determined that the diagenetic fluid source of the dolomite is marine fluid.

8. The determination method according to any one of claims 5-7, characterized in that, When the carbon and oxygen isotope values ​​of the dolomite sample are outside the range of carbon and oxygen isotope values ​​of seawater during the same period, and the values ​​of Eu and Ce are normal and there are no regional thermal events, or when the carbon and oxygen isotope values ​​of the dolomite sample are within the range of carbon and oxygen isotope values ​​of seawater during the same period and one or both of the values ​​of Eu and Ce are abnormal, the judgment method further includes: measuring the trace elements V and Cr of the sample to analyze the diagenetic environment.

9. The judgment method according to claim 8, characterized in that, The analyzed diagenetic environments include: Calculate the V / Cr ratio; If the V / Cr ratio is <4, the diagenetic environment is an oxidizing environment, and the diagenetic fluid is determined to be marine fluid. If the V / Cr ratio is greater than 4, the diagenetic environment is a reducing environment, and the diagenetic fluid is determined to be a deep hydrothermal fluid.

10. The determination method according to any one of claims 1-9, characterized in that, The determination method includes the following steps: (1) Select dolomite samples from field outcrops and / or drill cores and record geological background information, including stratigraphic occurrence, thickness, color and crystal size. Prepare thin slices with a thickness of more than 50 μm and powder samples with a particle size of less than 200 mesh from the dolomite samples for isotope dating and geochemical experimental analysis. (2) The thin-film sample was subjected to U and Pb isotope analysis using laser ablation inductively coupled plasma mass spectrometry, and the values ​​at each measurement point were calculated. 207 Pb / 206 Pb and 238 U / 206 The Pb ratio is used to generate dolomite age data through Tera-Wasserburg plotting. This dolomite age data is then compared with the ages of contemporaneous stratigraphic layers to determine the formation time of the dolomite. Specifically: if the difference between the dolomite age data and the contemporaneous stratigraphic age is within ±10%, the formation time of the dolomite is considered a quasi-syngenetic period; if the dolomite age data is less than the contemporaneous stratigraphic age and the difference is >10%, the formation time of the dolomite is considered a burial period; if the dolomite age data is greater than the contemporaneous stratigraphic age and the difference is >10%, the dolomite age data is invalid. (3) The powder samples were subjected to carbon and oxygen isotope testing and rare earth element testing, respectively. The test results were compared with the values ​​of seawater at the same time to obtain the comparison results. The comparison results of carbon and oxygen isotopes and rare earth elements were combined to determine the source of the diagenetic fluid of dolomite, specifically: The carbon and oxygen isotope values ​​of the dolomite sample are compared with the carbon and oxygen isotope values ​​of the seawater during the same period. If the value of the dolomite sample is within the range of the seawater during the same period, it is recorded as CO(IN). If the value of the dolomite sample is outside the range of the seawater during the same period, it is recorded as CO(OUT). The rare earth element (REE) values ​​of Eu and Ce in the dolomite sample were compared with those of Eu and Ce in seawater during the same period after PAAS standardization. If the results of Eu and Ce in the REE distribution map are both normal, it is recorded as REE(N). If one or both of the results of Eu and Ce are abnormal, it is recorded as REE(PA). Combining the comparison results of carbon and oxygen isotopes and rare earth elements, the following four situations exist: If the diagenetic fluids are CO(IN) and REE(N), then the diagenetic fluid source of the dolomite is determined to be marine fluid. If the results are CO(OUT) and REE(PA), then the diagenetic fluid source of the dolomite is determined to be deep hydrothermal fluid. If the dolomite is CO(OUT) and REE(N), determine whether there are regional thermal events in the study area based on the geological background information of the dolomite. The regional thermal events include orogenic movements and / or volcanic activity. If there are regional thermal events, determine that the diagenetic fluid source of the dolomite is marine fluid. If there are no regional thermal events, continue to test the trace elements V and Cr in the sample. If the result is CO(IN) and REE(PA), continue to perform trace element tests on the sample for V and Cr; The specific results of the trace element tests for V and Cr are as follows: calculate the V / Cr ratio. If the V / Cr ratio is <4, it is determined that the dolomite was formed in an oxidizing environment of an open system, and its diagenetic fluid source is marine fluid. If the V / Cr ratio is >4, it is determined that the dolomite was formed in a reducing environment of a closed system, and its diagenetic fluid source is deep hydrothermal fluid. (4) Combining the formation time of dolomite with the results of the diagenetic fluid source, the genesis of dolomite was determined, specifically as follows: If the formation time is determined to be quasi-syngenetic and the source of the diagenetic fluid is determined to be marine fluid, then the genesis is determined to be sedimentary marine fluid dolomite with minimal alteration during the burial period. If the formation time is determined to be the burial period and the source of the diagenetic fluid is determined to be marine fluid, then the genesis is determined to be marine fluid dolomite formed during the sedimentary period of drastic alteration during the burial period. If the formation time is determined to be quasi-syngenetic and the source of the diagenetic fluid is determined to be deep hydrothermal fluid, then the genesis is determined to be early-buried deep hydrothermal dolomite. If the formation time is determined to be the burial period and the source of the diagenetic fluid is determined to be deep hydrothermal fluid, then the genesis is determined to be late-stage deep hydrothermal dolomite. Steps (2) and (3) are not in any particular order.

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