A method of determining the age of a sedimentary formation

Abstract

This invention belongs to the field of sedimentary basin mineral exploration technology, specifically relating to a method for determining the age of sedimentary strata. The method includes: Step 1: core logging; Step 2: sample collection from the target layer; Step 3: analysis of the collected samples using trace elements and lead isotopes, and calculation of the age of each sample. 238 U / 204 Pb and 235 U / 204 Pb value; Step 4: Based on 238 U / 204 Pb and 235 U / 204 Pb value, to perform 238 U- 206 Pb, 235 U- 207 Pb isochron age fitting; Step 5: Based on 238 U- 206 Pb, 235 U- 207 Step 6: Determine the age of the early diagenetic stage of the mudstone section using Pb isochron dating; Step 7: Determine the stratigraphic age based on the age of the early diagenetic stage of the mudstone section. This invention avoids the influence of late diagenetic stages and post-diagenetic thermal events, accurately determining the age of the early diagenetic stage of the mudstone, and providing data support for determining the stratigraphic age.

Description

Technical Field

[0001] This invention belongs to the field of mineral exploration technology in sedimentary basins, and specifically relates to a method for determining the age of sedimentary strata. Background Technology

[0002] Determining the age of sedimentary strata has always been one of the difficulties in sedimentology and stratigraphy. However, determining the age of sedimentary strata is very important for dividing strata, identifying ore-forming strata, and determining the target strata in mineral exploration in sedimentary basins.

[0003] Currently, stratigraphic age determination mainly employs methods such as paleontology, paleomagnetics, and isotopes, with isotope methods being the most commonly used and accurate. The main isotopic geochronological methods currently used for determining sedimentary age include the K-Ar dilution method, the U-Pb method, and the Rb-Sr isochron method. 40 Ar / 39 Methods such as Ar (Ar) and fission track methods primarily select authigenic minerals (e.g., illite, dolomite, anhydrite, barite, celestite, etc.) that can indicate sedimentary age, or syn-sedimentary volcanic clastic minerals (e.g., interlayers of volcanic clastic rocks such as bentonite and olivine andesite). All these methods require that the sample was formed during the syn-sedimentary (or early diagenetic) period and remained in a closed system after sedimentary rock formation, without undergoing dissolution, recrystallization, or other processes. Since it is difficult to identify whether authigenic minerals achieved isotopic homogenization at the beginning of sample formation and whether they were significantly disturbed by later thermal events through compositional analysis and morphological identification, the measured isotopic ages need to be analyzed and interpreted to determine whether they represent sedimentary age, diagenetic age, or thermal event age. Volcanic clastic interlayers formed during volcanic eruptions often contain well-preserved weathering-resistant minerals such as zircon, apatite, and rutile. Theoretically, the isotopic ages of these minerals best represent the sedimentary age of the strata; however, this method is only applicable to areas with well-developed volcanic clastic rocks.

[0004] In summary, there are many methods for determining the age of sedimentary strata, with isotopic methods being the most accurate. However, when calculating isotopic ages using authigenic minerals, the formation time of the authigenic minerals and subsequent thermal alteration must be considered, while the calculation of isotopic ages using volcanic clastic minerals has geographical limitations.

[0005] In sedimentary basin strata, multiple layers of mudstone or coal seam uranium anomalies are often developed, mainly due to syn-sedimentary sedimentary origin. In the early B stage of mudstone formation, most of the internal water has been discharged, and the rock has changed from semi-consolidation to consolidation, with a sharp decrease in permeability. This keeps the remaining uranium in the mudstone in a relatively closed system. Since uranium decay is not affected by other physical factors, the influence of changes in mineral composition caused by late diagenetic stages and post-diagenetic thermal events can be ruled out.

[0006] Therefore, by measuring the U-Pb isochron age of the whole mudstone, the early diagenetic stage age of the mudstone can be determined. This age is smaller than the stratigraphic deposition age, so this diagenetic age can approximately represent the stratigraphic age. Summary of the Invention

[0007] The purpose of this invention is to provide a method for determining the age of sedimentary strata. Based on borehole cores, a systematic method is used to collect radioactive mudstone samples for trace and Pb isotope testing. U-Pb isochron ages are then calculated to determine the stratigraphic age and stratigraphic era. This method can avoid the influence of late diagenetic stages and post-diagenetic thermal events, accurately determine the age of mudstone in the early diagenetic stage, and provide data support for determining stratigraphic era.

[0008] Technical solution to achieve the purpose of this invention: A method for determining the age of sedimentary strata, the method comprising: Step 1: Core logging; Step 2: Sample collection from the target layer; Step 3: Perform trace rock and lead isotope analysis on the collected samples, and calculate the values ​​of each sample. 238 U / 204 Pb and 235 U / 204 Pb value; Step Four: According to 238 U / 204 Pb and 235 U / 204 Pb value, to perform 238 U- 206 Pb, 235 U- 207 Pb isochron age fitting; Step 5: According to 238 U- 206 Pb, 235 U- 207 Pb isochron dating determines the age of the early diagenetic stage of the mudstone section; Step Six: Determine the stratigraphic age based on the age of the early diagenetic stage of the mudstone section.

[0009] Step one specifically involves: selecting a borehole containing radioactive mudstone; using a core logger to perform radioactive logging on the radioactive mudstone of the target layer and measuring the radioactive data of the mudstone.

[0010] Step two specifically involves: based on the radioactivity data of the mudstone obtained in step one, collecting mudstone samples with different radioactivity intensities, removing the mud slurry from the surface of the mudstone and the surface layer of the core, and using these samples as mud slurry samples.

[0011] In step two, the sampling interval is controlled at 5-10 cm, the weight of a single sample is not less than 50g, and the number of radioactive mudstone samples collected from each research target layer is not less than 5, with uniform distribution of radioactivity intensity.

[0012] Step three specifically involves: pulverizing the samples collected in step two, performing trace element and lead isotope tests, and calculating the values ​​for each sample. 238 U / 204 Pb and 235 U / 204 Pb value; 238 U / 204 The formula for calculating Pb is: ; 235 U / 204 The formula for calculating Pb is: ; In the formula, 137.88 is 238 U / 235 The value of U, m u represents the uranium element content; Mu is the average atomic weight of uranium, with a value of 238.028; m Pb Lead content; M Pb This represents the average atomic weight of lead.

[0013] The formula for calculating the average atomic weight of lead in step three is as follows: ; In the formula, M Pb N is the average atomic weight of lead. 208 N 207 N 206 N 204 They are respectively 208 Pb, 207 Pb, 206 Pb, 204 The proportion of Pb.

[0014] Step four specifically involves: calculating the values ​​for each sample based on the results from step three. 238 U / 204 Pb and 235 U / 204 Pb values ​​were fitted using Isoplot software. 238 U- 206 Pb, 235 U- 207 Pb isochron age.

[0015] Step five specifically involves: fitting the result from step four... 238 U- 206Pb isochron age and 235 U- 207 Compare the Pb isochron ages. If the error between the two isochron ages is within 5%, and the initial age calculated by fitting is... 206 Pb / 204 Pb value and initial 207 Pb / 204 The Pb values ​​were compared with those of other low-uranium-content samples within the basin. 206 Pb / 204 Pb value and 207 Pb / 204 If the Pb values ​​differ by ±0.2, then 238 U- 206 The Pb isochron age can be used to determine the age of the early diagenetic stage of this mudstone segment; if two isochron ages are similar, but the initial age calculated by fitting is... 206 Pb / 204 Pb value and initial 207 Pb / 204 The Pb values ​​were higher than those of other low-uranium-content samples within the basin. 206 Pb / 204 Pb value and 207 Pb / 204 If the Pb value is above 0.2, it means that the mudstone section was not in a closed state after its formation, and changes in uranium content occurred during its formation. 238 U- 206 Pb isochron ages represent the ages at which uranium changed after diagenesis; if two isochron ages differ significantly, then the age has no geological significance.

[0016] Step six specifically involves: based on the determination of isochronous age in step five, if... 238 U- 206 If the Pb isochron age is determined to be the age of the early diagenetic stage of the mudstone section, then it can be approximately considered as the depositional age of the mudstone section strata. Based on the age of the regional stratigraphic timescale stratigraphic boundary, the stratigraphic age of the target stratum can be determined.

[0017] The beneficial technical effects of this invention are as follows: 1. The present invention provides a method for determining the age of sedimentary strata. Using radioactive mudstone cores from boreholes as the test object, U-Pb isochron age calculations are performed. This method can avoid the influence of late diagenetic stages and post-diagenetic thermal events, accurately determine the age of mudstone in the early diagenetic stage, and provide data support for the determination of stratigraphic age.

[0018] 2. The present invention provides a method for determining the age of sedimentary strata, which uses whole-rock samples as the U-Pb isotope test object. Compared with traditional methods, it does not require the selection of minerals and determination of mineral genesis, and avoids the influence of minerals after the diagenetic period on the dating data. It has the advantages of simple operation and small influencing factors.

[0019] 3. The method for determining the age of sedimentary strata provided by this invention, through step five, [determines the age of the sedimentary strata]. 238 U- 206 Pb, 235 U- 207 By comparing the Pb isochron ages and combining the initial lead isotope composition calculated by fitting with the initial lead isotope composition of other low-uranium-content samples in the basin, the accuracy of age determination was effectively improved. Attached Figure Description

[0020] Figure 1 A flowchart illustrating a method for determining the age of sedimentary strata provided by this invention; Figure 2 This refers to a certain region in Embodiment 1 of the present invention. 238 U- 206 Pb isochron age diagram; Figure 3 This refers to a certain region in Embodiment 1 of the present invention. 235 U- 207 Pb isochron age diagram. Detailed Implementation

[0021] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0022] like Figure 1 As shown, the present invention provides a method for determining the age of sedimentary strata, which specifically includes the following steps: Step 1: Borehole Core Logging Select a borehole containing radioactive mudstone; perform geological logging on the target layer, and observe in detail the integrity, permeability, fracture development, and post-alteration of the core. If the radioactive mudstone has developed fractures and post-alteration phenomena, this method is not applicable; use a core logger to perform radioactive logging on the radioactive mudstone of the target layer, with a measurement interval of 5 cm.

[0023] Step 2: Sample collection of the target layer Based on the radioactivity data of mudstone obtained in step one, mudstone samples with different radioactivity intensities were collected. The mud and the surface layer of the core were scraped off to remove the influence of drilling fluid. The sampling interval was controlled at 5-10 cm, and the weight of a single sample was not less than 50g. The number of radioactive mudstone samples collected from each research target layer was not less than 5, and the radioactivity intensity was evenly distributed.

[0024] Step 3: Perform trace rock and lead isotope analysis on the collected samples. The samples collected in step two were sent to the laboratory and crushed to 200 mesh for trace rock analysis and lead isotope testing. Lead isotope data are ratios of lead atoms; the average lead atomic weight of each sample needs to be recalculated based on the proportion of each lead isotope, as shown in Formula 1. , formula 1; In Formula 1, M Pb N is the average atomic weight of lead. 208 N 207 N 206 N 204 They are respectively 208 Pb, 207 Pb, 206 Pb, 204 The proportion of Pb.

[0025] because 238 U / 235 The ratio of U in nature is relatively close ( 238 U / 235 (U=137.88), the values ​​of each sample can be calculated using the following formula. 238 U / 204 Pb and 235 U / 204 The ratio of Pb , Formula 2; , Formula 3; In Formulas 2 and 3, m u represents the uranium element content; Mu is the average atomic weight of uranium, with a value of 238.028; m Pb Lead content; M Pb This is the average atomic weight of lead, which is calculated using Formula 1; Step 4: U-Pb isochron age fitting Based on the values ​​calculated in step three, use Isoplot software to fit the values ​​respectively. 238 U- 206 Pb, 235 U- 207 Pb isochron age. During the fitting process... 206 Pb / 204 Pb, 206 Pb / 204 The error of the Pb value is expressed as the measured value; 238 U / 204 Pb, 235 U / 204 The Pb value is calculated based on trace elements and lead isotopes. When determining trace elements, if the trace element content is >1×10⁻⁶, the result is different. -6 The time error is less than 5%, therefore238 U / 204 Pb, 235 U / 204 The error of the Pb value is calculated as 5% of the numerical value; if a sample data point shows significant drift, it indicates that the uranium content at that point has changed in the later stage and should be removed during the fitting process.

[0026] Step 5: According to 238 U- 206 Pb, 235 U- 207 Pb isochron dating determines the age of the early diagenetic stage of the mudstone segment. The fitted in step four 238 U- 206 Pb isochron age and 235 U- 207 Compare the Pb isochron ages. If the error between the two isochron ages is within 5%, and the initial age calculated by fitting is... 206 Pb / 204 Pb value and initial 207 Pb / 204 The Pb values ​​were compared with those of other low-uranium-content samples within the basin. 206 Pb / 204 Pb value and 207 Pb / 204 If the Pb values ​​differ by ±0.2, then 238 U- 206 The Pb isochron age can be used to determine the age of the early diagenetic stage of this mudstone section (although the uranium content of radioactive mudstone is higher than that of non-radioactive rocks, it is still lower than that of uranium ore). 235 U decay products 207 Pb is still insufficient to make the overall 207 Pb undergoes a significant change, at which point 238 U- 206 Pb isochron age is relatively 235 U- 207 (Pb isochron ages are more accurate); if two isochron ages are similar, but the initial values ​​obtained from the fitting calculation are different... 206 Pb / 204 Pb value and initial 207 Pb / 204 The Pb values ​​were higher than those of other low-uranium-content samples within the basin. 206 Pb / 204 Pb value and 207 Pb / 204 If the Pb value is above 0.2, it means that the mudstone section was not in a closed state after its formation, and changes in uranium content occurred during its formation. 238 U- 206Pb isochron ages represent the ages at which uranium changed after diagenesis; if two isochron ages differ significantly, then the age has no geological significance.

[0027] Step Six: Determine the stratigraphic age Based on the determination of isochronous age in step five, if 238 U- 206 If the Pb isochron age is determined to be the age of the early diagenetic stage of the mudstone section, then it can be approximately considered as the depositional age of the mudstone section strata. Based on the age of the regional stratigraphic timescale stratigraphic boundary, the stratigraphic age of the target stratum can be determined.

[0028] Example 1: Determination of Stratigraphic Age in a Certain Region Step 1: Borehole Core Logging The study area contains multiple uranium anomalies, representing potential target layers for uranium exploration. However, their age cannot be determined based on well logging curves and lithological variations. A borehole containing radioactive mudstone in this area was selected as the research object. Geological logging was performed on the target layer, and detailed observations were conducted on core integrity, permeability, fracture development, and post-alteration. The radioactive mudstone section in this borehole had a complete core, significant thickness, and minimal fractures and post-alteration, making it suitable for this method. A core logger was used to perform radiometric logging on the radioactive mudstone of the target layer, with measurements spaced 5 cm apart.

[0029] Step 2: Sample collection of the target layer Based on the radioactivity data of mudstone obtained in step one, mudstone samples with different radioactivity intensities were collected. The sampling interval was controlled at 8 cm, and a total of 6 samples were collected. For each mudstone sample, the mud and the top layer of the core, about 0.5 cm thick, were scraped off to remove the influence of drilling fluid. The weight of each sample was greater than 50 g, and the radioactivity intensity of all samples was evenly distributed.

[0030] Step 3: Trace and lead isotope analysis of the collected samples. The samples collected in Step 2 were sent to the laboratory and ground to 200 mesh for trace and lead isotope analysis. The results are shown in Table 1. The average atomic weight of lead for each sample was calculated based on the lead isotope composition, U content, and Pb content. 238 U / 204 Pb and 235 U / 204 The values ​​of Pb are shown in Table 2.

[0031] Table 1. U and Pb contents and Pb isotope analysis results

[0032] Table 2 Data Processing Calculation Results

[0033] Step 4: U-Pb isochron age fitting Based on the calculation in step three 238 U / 204 Pb and 235 U / 204 Pb data and lead isotope test data were fitted using Isoplot software. 238 U- 206 Pb, 235 U- 207 Pb isochron age. During the fitting process... 206 Pb / 204 Pb, 206 Pb / 204 The error of the Pb value is expressed as the measured value; 238 U / 204 Pb, 235 U / 204 The error of the Pb value is calculated as 5% of the value. The data point of sample 3102 shows a significant drift in the distance between the initial fitted curve and other points in the scatter plot, indicating that the uranium content at this point has changed in the later stage. It should be removed and fitted again. 238 U- 206 Pb isochron age chart, 235 U- 207 Pb isochron age chart, such as Figure 2 , Figure 3 As shown.

[0034] Depend on Figure 2 , Figure 3 It can be seen that, 238 U- 206 The Pb isochron age is 83.2 ± 2.6 Ma, initially... 206 Pb / 204 Pb=17.913±0.031, MSWD=0.78; 235 U- 207 The Pb isochron age is 85±17 Ma, initially... 207 Pb / 204 Pb=15.61±0.03, MSWD=2.7.

[0035] Step 5: According to 238 U- 206 Pb, 235 U- 207 Pb isochron dating determines the age of the early diagenetic stage of the mudstone segment. The fitted in step four 238 U- 206 The Pb isochron age is 83.2 ± 2.6 Ma. 235 U- 207The Pb isochron ages of 85±17 Ma differ by 1.8 Ma, with a relative error of 2.1%, which is quite similar. Furthermore, the initial ages calculated through fitting are also similar. 206 Pb / 204 The Pb value was 17.913 ± 0.031 and the initial value was... 207 Pb / 204 The Pb value of 15.61 ± 0.03 was different from that of other low-uranium-content samples in the basin. 206 Pb / 204 Pb value 17.933 and 207 Pb / 204 The Pb value is within 0.2, which is 15.601. 238 U- 206 The Pb isochron age of 83.2 ± 2.6 Ma can be identified as the age of the early diagenetic stage of this mudstone section.

[0036] Step Six: Determine the stratigraphic age Based on the determination of isochronous age in step five, determine... 238 U- 206 The Pb isochron age of 83.2±2.6 Ma represents the early diagenetic stage of the mudstone section, and this age can be approximately considered as the depositional age of the mudstone section strata. According to the regional stratigraphic timescale, the revised age of the bottom boundary of the Nenjiang Formation is 86.3 Ma, and the revised age of the top boundary is 79.0 Ma. Therefore, the stratigraphic age of the target stratum can be determined to be the Nenjiang Formation.

[0037] The present invention has been described in detail above with reference to the accompanying drawings and embodiments. However, the present invention is not limited to the above embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention. All contents not described in detail in the present invention can be derived from existing technologies.

Claims

1. A method for determining the age of sedimentary strata, characterized in that, The method includes: Step 1: Borehole core logging; Step 2: Sample collection from the target layer; Step 3: Perform trace rock and lead isotope analysis on the collected samples, and calculate the values ​​of each sample. 238 U / 204 Pb and 235 U / 204 Pb value; Step Four: According to 238 U / 204 Pb and 235 U / 204 Pb value, to perform 238 U- 206 Pb, 235 U- 207 Pb isochron age fitting; Step 5: According to 238 U- 206 Pb, 235 U- 207 Pb isochron dating determines the age of the early diagenetic stage of the mudstone segment. Step Six: Determine the stratigraphic age based on the age of the early diagenetic stage of the mudstone section; Step one specifically involves: selecting a borehole containing radioactive mudstone; using a core logger to perform radioactive logging on the radioactive mudstone of the target layer and measuring the radioactive data of the mudstone. The second step is specifically as follows: based on the radioactivity data of the mudstone obtained in the first step, mudstones with different radioactivity intensities are collected, and the mud slurry on the surface of the mudstone and the surface layer of the core are removed to obtain mud slurry samples. Step four specifically involves: calculating the values ​​for each sample based on the results from step three. 238 U / 204 Pb and 235 U / 204 Pb values ​​were fitted using Isoplot software. 238 U- 206 Pb, 235 U- 207 Pb isochron age; Step five specifically involves: fitting the result from step four... 238 U- 206 Pb isochron age and 235 U- 207 Compare the Pb isochron ages. If the error between the two isochron ages is within 5%, and the initial age calculated by fitting is... 206 Pb / 204 Pb value and initial 207 Pb / 204 The Pb values ​​were compared with those of other low-uranium-content samples within the basin. 206 Pb / 204 Pb value and 207 Pb / 204 If the Pb values ​​differ by ±0.2, then 238 U- 206 The Pb isochron age can be used to determine the age of the early diagenetic stage of this mudstone segment; if two isochron ages are similar, but the initial age calculated by fitting is... 206 Pb / 204 Pb value and initial 207 Pb / 204 The Pb values ​​were higher than those of other low-uranium-content samples within the basin. 206 Pb / 204 Pb value and 207 Pb / 204 If the Pb value is above 0.2, it means that the mudstone section was not in a closed state after its formation, and changes in uranium content occurred during its formation. 238 U- 206 Pb isochron ages represent the ages at which uranium changed after diagenesis; if two isochron ages differ significantly, then the age has no geological significance.

2. The method for determining the age of sedimentary strata according to claim 1, characterized in that, In step two, the sampling interval is controlled at 5-10 cm, the weight of a single sample is not less than 50g, and the number of radioactive mudstone samples collected from each research target layer is not less than 5, with uniform distribution of radioactivity intensity.

3. The method for determining the age of sedimentary strata according to claim 1, characterized in that, Step three specifically involves: pulverizing the samples collected in step two, performing trace element and lead isotope tests, and calculating the values ​​for each sample. 238 U / 204 Pb and 235 U / 204 Pb value; 238 U / 204 The formula for calculating Pb is: ； 235 U / 204 The formula for calculating Pb is: ； In the formula, 137.88 is 238 U / 235 The value of U, m u represents the uranium element content; Mu is the average atomic weight of uranium, with a value of 238.028; m Pb Lead content; M Pb This represents the average atomic weight of lead.

4. The method for determining the age of sedimentary strata according to claim 3, characterized in that, The formula for calculating the average atomic weight of lead in step three is as follows: ； In the formula, M Pb N is the average atomic weight of lead. 208 N 207 N 206 N 204 They are respectively 208 Pb, 207 Pb, 206 Pb, 204 The proportion of Pb.

5. The method for determining the age of sedimentary strata according to claim 4, characterized in that, Step six specifically involves: based on the determination of isochronous age in step five, if... 238 U- 206 If the Pb isochron age is determined to be the age of the early diagenetic stage of the mudstone section, then it can be approximately considered as the depositional age of the mudstone section strata. Based on the age of the regional stratigraphic timescale stratigraphic boundary, the stratigraphic age of the target stratum can be determined.

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