Lake basin paleo-water depth calculation method based on sporopollen and algae content

Abstract

The invention relates to the technical field of sedimentary basin research, in particular to a lake basin paleo-water depth calculation method based on sporopollen and algae content, which comprises the following steps: S1, selecting a sampling lake basin, and analyzing the percentage contents of sporopollen and algae in effective sample points; s2, obtaining a lake plane period; s3, performing compaction correction and stratigraphic dip correction on the overlying strata of all the effective sample points to obtain the initial true thickness of the overlying strata of the effective sample points; s4, carrying out compaction correction and stratigraphic dip angle correction on the overlying strata of the ancient lake plane in each stage to obtain a corresponding lake plane height difference; s5, obtaining the paleo-water depth of the corresponding effective sample point; s6, performing formula fitting according to the percentage contents of sporopollen and algae and the corresponding paleo-water depth to obtain a relational expression; and S7, obtaining the percentage contents of sporopollen and algae at a plurality of positions through standard sampling, and obtaining a corresponding paleo-water depth range according to a relational expression in S6. The paleo-water depth calculation method can quickly and simply calculate the paleo-water depth, and is good in applicability and high in reliability.

Description

Technical Field

[0001] The invention relates to the technical field of sedimentary basin research, and more specifically, to a method for calculating the paleo-water depth of a lake basin based on spore pollen and algae content. Background Art

[0002] Paleo-water depth calculation has always been one of the important contents of sedimentary basin research, especially for continental lake basins. Paleo-water depth is crucial to the sedimentary evolution and source rock research of basins. At present, there are many methods for paleo-water depth research of lake basins, including paleontological paleo-water depth calculation method based on trace elements and ostracod differentiation, but because ostracods are less developed in most basins, their promotion and application are restricted; paleo-water depth calculation method based on progradational delta is also included, but this method depends on a specific "S" type progradational delta. Since this type of delta can only be formed under certain geological conditions, its scope of use is also restricted; paleo-water depth calculation method based on Fe / Co ratio in mudstone, relationship between Mn / Fe and RT, AC and Th / U ratio of lake basin is also included, but because the elements in sedimentary rocks are not only affected by the water depth environment, but also directly affected by the element characteristics of the parent rock in the source area, the reliability of paleo-water depth calculated by this method needs to be improved. Summary of the invention

[0003] The purpose of the present invention is to overcome the deficiency of limited application scope of paleo-depth calculation in the prior art, and to provide a method for calculating paleo-depth of lake basins based on spore pollen and algae content, which can quickly and easily calculate paleo-depth and has good applicability and high reliability.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0005] A method for calculating the paleo-water depth of a lake basin based on the content of pollen and algae is provided, comprising the following steps:

[0006] S1. Select a sampling lake basin, conduct standardized sampling in the sampling lake basin to obtain multiple valid sample points, and then analyze the percentage of spores, pollen and algae in the valid sample points;

[0007] S2. Perform well-seismic calibration on the sampling lake basin area to clarify the position of each valid sample point on the seismic profile, obtain the lake level phase and identify the position of each phase of the ancient lake level on the seismic profile;

[0008] S3. According to the effective sample point positions determined in step S2, compaction correction and formation dip correction are performed on the overlying strata of all effective sample points to obtain the initial true thickness of the overlying strata of the effective sample points;

[0009] S4. According to the lake level phases and the positions of the ancient lake planes of each phase identified in step S2, compaction correction and formation dip correction are performed on the overlying strata of each ancient lake plane phase to obtain the lake level elevation difference corresponding to each valid sample point corresponding to the phase;

[0010] S5. Obtaining the paleo-water depth corresponding to the effective sample point according to the initial true thickness of the overlying stratum and the elevation difference of the lake surface at the effective sample point;

[0011] S6. According to the percentage of pollen and algae in each valid sample point and the ancient water depth corresponding to each valid sample point, a formula is fitted to obtain the relationship between the percentage of pollen and algae and the ancient water depth;

[0012] S7. For the lake basin to be calculated for the paleo-water depth applicable to the relational expression in step S6, the percentage of pollen and algae at multiple locations is obtained through standardized sampling, and the corresponding paleo-water depth range is obtained according to the relational expression in step S6.

[0013] Furthermore, in step S1, the sampling lake basin meets the conditions of developing strata since the late Paleozoic, having physical data of drilling cores and rock cuttings, and developing spores, pollen, and algae fossils.

[0014] Furthermore, step S2 includes: performing well-seismic calibration on the sampling lake basin to clarify the position of each effective sample point on the seismic profile; at the same time, clarifying the anti-cyclic sedimentary characteristics between the lake planes of each period in the foreset body based on drilling, and obtaining the ancient lake plane stage according to the anti-cycle; and tracing the position of the top overlap feature according to the seismic in-phase axial provenance area corresponding to the top of the anti-cycle to identify the position of the ancient lake plane of each period on the seismic profile.

[0015] Further, step S3 includes the following steps:

[0016] S31. Perform compaction correction to obtain the initial thickness of the overlying stratum at the effective sample point;

[0017] S32. Perform formation dip correction, and obtain the initial true thickness of the formation overlying the effective sample point in combination with the initial thickness of the formation overlying the effective sample point.

[0018] Furthermore, in step S31, the calculation formula for the initial thickness of the overlying stratum at the effective sample point is:

[0019]

[0020] Where H represents the initial thickness of the overlying stratum at the effective sample point, h s Indicates the current thickness of the sandstone overlying the effective sample point, represents the initial porosity of the overlying sandstone at the effective sample point, represents the current average porosity of the sandstone overlying the effective sample point, h nIndicates the current thickness of the mudstone overlying the effective sample point, represents the initial porosity of the overlying mudstone at the effective sample point, Indicates the current average porosity of the overlying mudstone at the effective sample point.

[0021] Furthermore, in step S32, the calculation formula of the initial true thickness of the overlying stratum at the effective sample point is:

[0022] D=H×Cosα

[0023] Where D represents the initial true thickness of the overlying stratum at the effective sample point, H represents the initial thickness of the overlying stratum at the effective sample point, and α represents the current dip of the stratum at the effective sample point.

[0024] Further, step S4 includes the following steps:

[0025] S41. Perform compaction correction to obtain the initial thickness of the overlying strata on the ancient lake plane at each stage;

[0026] S42. Performing formation dip correction, and combining the initial thickness of the formation overlying the ancient lake plane of each period to obtain the initial true thickness of the formation overlying the ancient lake plane of each period;

[0027] S43. Determine the lake level difference of the i-th period from top to bottom at the corresponding position according to the location of the valid sample point;

[0028] S44. Calculate the height difference of the lake surface from top to bottom in the i-th period:

[0029] H Li =D i -D 1

[0030] In the formula, H Li represents the lake level difference from top to bottom in the i-th period, D i represents the initial true thickness of the strata overlying the i-th stage paleo-lake plane from top to bottom, D 1 It indicates the initial true thickness of the strata overlying the first stage of the paleo-lake plane from top to bottom.

[0031] Furthermore, in step S5, the calculation formula of the ancient water depth is:

[0032] S=DH Li

[0033] Where S represents the paleo-water depth, D represents the initial true thickness of the overlying strata at the effective sample point, and H Li Represents the height difference of the lake level in the i-th period from top to bottom.

[0034] Further, in step S6, when the sampling lake basin is selected as a lake basin with a progradational delta development, and the pollen is selected as a fern, the relationship between the percentage of pollen and algae and the ancient water depth can be obtained by fitting:

[0035] If 1.5<S<290m, the relationship between the shallow lake and semi-deep lake area is:

[0036]

[0037] If 290≤S≤595m, the deep lake area relationship is:

[0038] S=595.02e -0.00723fx

[0039] Where S represents the paleo-water depth, f x Indicates the percentage of algae, f y Indicates the percentage of fern.

[0040] Further, for the lake basin to be calculated paleo-water depth applicable to the relational expression of step S6, step S7 includes: if the lake basin to be calculated paleo-water depth shows red oxidized exposed shallow water signs, the paleo-water depth is calculated by using the percentage of pollen and algae obtained by standard sampling and the relational expression of coastal shallow lake-semi-deep lake area;

[0041] For shallow water areas without red oxidation exposure signs, if the basin characteristics meet the following conditions at the same time: the mudstone and shale content of the basin excluding special sediments exceeds 90%; the fern content in the mudstone and shale of the basin is less than 10%; and the basin is in the stage of intense faulting in terms of tectonic evolution; then the percentage of pollen and algae obtained through standardized sampling is used to calculate the ancient water depth using the deep lake area relationship; otherwise, the ancient water depth is calculated using the shallow lake-semi-deep lake area relationship.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] The present invention is a method for calculating the paleo-water depth of lake basins based on the content of pollen and algae. Pollen and algae that are widely developed in sedimentary basins and sensitive to the water depth environment are selected, and a relationship between the percentage of pollen and algae and the paleo-water depth is established by analyzing the relationship between the content of pollen and algae and the water depth. The paleo-water depth can be calculated by using the widely available pollen and algae. The method is simple to operate and has wide applicability. It can also lay a solid foundation for the study of lake basin sedimentary filling evolution and source rocks in oil and gas exploration, and has high reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a flow chart of a method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to the present invention;

[0045] Figure 2 To carry out well-seismic calibration of the sampled lake basin and identification of the paleo-lake plane stages;

[0046] Figure 3 A schematic diagram of performing formation dip correction in step S32;

[0047] Figure 4 This is a schematic diagram of calculating paleo-water depth of effective sample points in step S5;

[0048] Figure 5 This is a curve diagram showing the relationship between the percentage of ferns and paleo-water depth;

[0049] Figure 6 This is a curve diagram showing the relationship between the percentage of algae and the ancient water depth;

[0050] Figure 7 A schematic diagram of the results of paleo-water depth calculation for Well W in Shunde Sag for the implementation of the present invention. DETAILED DESCRIPTION

[0051] The present invention is further described below in conjunction with specific implementation methods. The accompanying drawings are only used for exemplary descriptions and are only schematic diagrams, not actual drawings, and cannot be understood as limiting this patent; in order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the accompanying drawings may be omitted.

[0052] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right" and the like indicate directions or positional relationships based on the directions or positional relationships shown in the drawings, it is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limitations on this patent. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0053] Example 1

[0054] like Figure 1 The first embodiment of the method for calculating the ancient water depth of a lake basin based on the content of spores, pollen and algae of the present invention is shown, and includes the following steps:

[0055] S1. Select a sampling lake basin, conduct standardized sampling in the sampling lake basin to obtain multiple valid sample points, and then analyze the percentage of spores, pollen and algae in the valid sample points;

[0056] S2. Carry out well-seismic calibration on the sampling lake basin area, clarify the position of each valid sample point on the seismic profile, obtain the lake level phase and identify the position of each phase of the paleo-lake level on the seismic profile;

[0057] S3. According to the effective sample point positions determined in step S2, compaction correction and formation dip correction are performed on the overlying strata of all effective sample points to obtain the initial true thickness of the overlying strata of the effective sample points;

[0058] S4. According to the lake level phases and the positions of the ancient lake planes of each phase identified in step S2, compaction correction and formation dip correction are performed on the overlying strata of each ancient lake plane phase to obtain the lake level elevation difference corresponding to each valid sample point corresponding to the phase;

[0059] S5. Obtain the paleo-water depth of the corresponding effective sample point according to the initial true thickness of the overlying strata and the elevation difference of the lake surface;

[0060] S6. According to the percentage of pollen and algae in each valid sample point and the ancient water depth corresponding to each valid sample point, a formula is fitted to obtain the relationship between the percentage of pollen and algae and the ancient water depth;

[0061] S7. For the lake basin to be calculated for the paleo-water depth applicable to the relational expression in step S6, the percentage of pollen and algae at multiple locations is obtained through standardized sampling, and the corresponding paleo-water depth range is obtained according to the relational expression in step S6.

[0062] The present invention selects pollen and algae that are widely developed in sedimentary basins and are sensitive to water depth environment, and establishes a relationship between the percentage of pollen and algae and the ancient water depth through the analysis of the relationship between the content of pollen and algae and the water depth, so that the ancient water depth can be calculated through the widely available pollen and algae. The operation is simple and the applicability is wide. It can also lay a solid foundation for the study of lake basin sedimentary filling evolution and source rocks in oil and gas exploration, and has high reliability.

[0063] Example 2

[0064] This embodiment is similar to Embodiment 1, except that, in this embodiment, step S1 includes the following steps:

[0065] S11. Select sampling lake basin;

[0066] Among them, the sampling lake basin meets the conditions of developing strata since the late Paleozoic, having drilling core and rock cuttings physical data, and developing pollen and algae fossils; in this embodiment, the sampling lake basin is selected as a lake basin with developed progradational delta, which can obtain reliable paleowater depth data of different phase zones of the lake basin.

[0067] S12. Collect multiple sample points according to the specifications;

[0068] Among them, cores or well cores are selected. When physical data is lacking, cuttings data can also be selected; the sample obtained during sampling should be no less than 100 grams, and the distance between adjacent sample points meets the research accuracy requirements, and the maximum distance does not exceed 30m;

[0069] S13. Screening valid sample points; among them, the sample points that meet the requirements of spore pollen and algae abundance greater than or equal to 65 are valid sample points;

[0070] S14. Calculate the percentage of spores, pollen and algae in the effective sample points;

[0071] Among them, the calculation formula for the percentage of spores, pollen and algae in the effective sample points is:

[0072]

[0073] In the formula, f x Indicates the percentage of algae at the effective sample point, n x represents the number of algae at the sample point, f y Indicates the percentage content of a certain type of pollen at the effective sample point, n y It represents the number of a certain type of pollen at the effective sample point, and N represents the total number of algae and pollen at the sample point.

[0074] It should be noted that pollen can be divided into fern spores, gymnosperm pollen and angiosperm pollen according to vegetation evolution. Ferns usually develop in lakeside humid areas, while gymnosperm and angiosperm pollen usually develop in plains and high mountains far away from lake basins. Therefore, the content of fern spores is closely related to the depth of lake basins. Based on this, the pollen selected in this embodiment is fern.

[0075] In this embodiment, step S2 includes: performing well seismic calibration on the sampling lake basin to obtain Figure 2 As shown, from Figure 2 The position of each effective sample point on the seismic profile can be clearly identified; at the same time, the anti-cyclic sedimentary characteristics between the lake levels in the foreset body are clarified based on drilling, and the paleo-lake level stages are obtained according to the anti-cycles; and the position of the paleo-lake levels of each period on the seismic profile is identified by tracing the seismic in-phase axial provenance area corresponding to the top of the anti-cycle to the position of the top overlap feature.

[0076] In this embodiment, step S3 includes the following steps:

[0077] S31. Compact and correct the overlying strata of all valid sample points to obtain the initial thickness of the overlying strata of each valid sample point; wherein the calculation formula for the initial thickness of the overlying strata of the valid sample point is:

[0078]

[0079] Where H represents the initial thickness of the overlying stratum at the effective sample point, h s Indicates the current thickness of the sandstone overlying the effective sample point, represents the initial porosity of the overlying sandstone at the effective sample point, represents the current average porosity of the sandstone overlying the effective sample point, h n Indicates the current thickness of the mudstone overlying the effective sample point, represents the initial porosity of the overlying mudstone at the effective sample point, Indicates the current average porosity of the overlying mudstone at the effective sample point.

[0080] Specifically, as shown in the following table: Figure 2 Table of compaction correction parameters for three valid sample points in Well X1.

[0081]

[0082] According to the above table and calculation formula, the initial thickness of the overlying strata corresponding to the three effective sample points are 180.8m, 207.8m, and 360.0m.

[0083] S32. Perform formation dip correction on the overlying strata of all valid sample points, and obtain the initial true thickness of the overlying strata of the valid sample points in combination with the initial thickness of the overlying strata of the corresponding valid sample points; wherein the calculation formula of the initial true thickness of the overlying strata of the valid sample points is:

[0084] D=H×Cosα

[0085] In the formula, D represents the initial true thickness of the overlying stratum at the effective sample point, H represents the initial thickness of the overlying stratum at the effective sample point, and α represents the current dip of the stratum at the effective sample point. Figure 3 As shown, in Figure 2 At the three effective sample points in Well X1, the current dip angle of the nearby strata is about 10.8°. After structural correction, the initial true thickness of the overlying strata corresponding to the effective sample points with sample depths of 1940m, 1960m and 2000m are 177.6m, 266.0m and 353.7m.

[0086] In this embodiment, step S4 includes the following steps:

[0087] S41. Perform compaction correction to obtain the initial thickness of the overlying strata of each period of the ancient lake plane; among them, the calculation method of the initial thickness of the overlying strata of the ancient lake plane is similar to the calculation method of the initial thickness of the overlying strata of the effective sample point, and is calculated through the current stratum thickness of the overlying sandstone on the ancient lake plane, the initial porosity of the overlying sandstone on the ancient lake plane, the current average porosity of the overlying sandstone on the ancient lake plane, the current stratum thickness of the overlying mudstone on the ancient lake plane, the initial porosity of the overlying mudstone on the ancient lake plane, and the current average porosity of the overlying mudstone on the ancient lake plane.

[0088] S42. Perform formation dip correction, and combine the initial thickness of the formation overlying the ancient lake plane in each period to obtain the initial true thickness of the formation overlying the ancient lake plane in each period; wherein, the calculation method of the initial true thickness of the formation overlying the ancient lake plane is similar to the calculation method of the initial true thickness of the formation overlying the effective sample point, and is calculated by the current dip of the formation overlying the ancient lake plane and the initial thickness of the formation overlying the ancient lake plane obtained in step S41.

[0089] S43. Determine the lake level difference of the i-th period from top to bottom at the corresponding position according to the location of the valid sample point; wherein, Figure 2 As shown, in this embodiment, a total of 2 phases of lake levels were identified, and in Table 1, the effective sample points with sample depths of 1940m and 1960m are located in the late delta foreset, so the lake level height difference is 0, and the initial true thickness of the overlying strata is the paleo-water depth when the effective sample points were deposited; and the effective sample point with a sample depth of 2000m is located in the early delta foreset, with a lake level height difference;

[0090] S44. Calculate the height difference of the lake surface from top to bottom in the i-th period:

[0091] H Li =D i -D 1

[0092] In the formula, H Li represents the lake level difference from top to bottom in the i-th period, D i represents the initial true thickness of the strata overlying the i-th stage paleo-lake plane from top to bottom, D 1 It indicates the initial true thickness of the strata overlying the first stage of the paleo-lake plane from top to bottom. Figure 4 As shown, for the effective sample point with a sample depth of 2000m, the lake level difference is the difference between the initial true thickness of the overlying strata of the second period of the ancient lake plane and the initial true thickness of the overlying strata of the first period of the ancient lake plane, which is 61m.

[0093] In this embodiment, in step S5, the calculation formula of the ancient water depth is:

[0094] S=DH Li

[0095] Where S represents the paleo-water depth, D represents the initial true thickness of the overlying strata at the effective sample point, and H Li It represents the height difference of lake level from top to bottom in the i-th period. Figure 5 As shown, in this embodiment, the paleo-water depths corresponding to the valid sample points with sample depths of 1940m, 1960m, and 2000m are 177.6m, 266.0m, and 292.7m.

[0096] According to the above steps, the percentage of pollen and algae and the ancient water depth corresponding to all valid sample points can be obtained, and the following can be obtained: Figure 5 The relationship between the percentage of ferns and paleo-water depth is shown in the figure, and the Figure 6 The relationship between algae percentage and paleo-water depth is shown in Figure 2. Figure 5 and Figure 6 It can be obtained that when the water depth is less than 290m, the percentage of ferns tends to decrease with increasing water depth, and the percentage of algae tends to increase with increasing water depth; when the water depth is greater than 290m, the change in the percentage of ferns is not obviously related to the water depth, and the percentage of algae tends to decrease with increasing water depth.

[0097] In step S6, through fitting, when the water depth is between 1.5 and 290 m, the percentage of algae and the ancient water depth satisfy:

[0098] S=20.505f x 0.608

[0099] The percentage of ferns and ancient water depth meet the following relationship:

[0100]

[0101] Combining the above two equations, we can get that when 1.5<S<290m, there is a relationship between the percentage of ferns and algae and the ancient water depth in the shallow lake-semi-deep lake area:

[0102]

[0103] When 290≤S≤595m, it has the deep lake area relationship:

[0104]

[0105] In the above formula, S represents the paleo-water depth, f x Indicates the percentage of algae, f y Indicates the percentage of fern.

[0106] It should be noted that according to the fitting formula, when the water depth approaches 595m, the percentage of algae approaches 0, so the maximum applicable water depth is 595m. However, since the water depth of most lake basins is within 595m, the above formula can calculate the ancient water depth of most lake basins. It should be supplemented that since ferns and algae are widely developed in sedimentary basins and are sensitive to the water depth environment, the above deep lake area relationship and shore-shallow lake-semi-deep lake area relationship are not only applicable to lake basins with progradational delta development, but can also be applied to the calculation of ancient water depths of other lake basins.

[0107] Example 3

[0108] This embodiment is similar to Embodiment 1 or 2, except that, in this embodiment, step S7 includes the following steps:

[0109] S71. For the lake basin whose paleo-water depth is to be calculated, during the period of deep lake phase development considered in sedimentary phase research, the algae content shows a trend of first increasing and then decreasing from the edge of the lake basin to the central area, and the total abundance of algae and ferns in the central area is less than 65, and after sampling in the lake basin, the total abundance of ferns and algae is less than 65. The number of sample points is greater than 90%; or, when the paleo-water depth of the lake basin obtained by the existing paleo-water depth calculation method is greater than 595m, the relational expression of step S6 is not applicable, otherwise, the lake basin is applicable to the relational expression of step S6, and then step S72 is executed;

[0110] S72. Determine whether the lake basin has red oxidized exposed shallow water signs. If so, calculate the paleo-water depth using the relationship between shallow lake and semi-deep lake area by using the percentage of pollen and algae obtained through standard sampling. Otherwise, execute step S73;

[0111] S73. Determine whether the characteristics of the lake basin meet the following requirements at the same time: the content of mudstone and shale in the lake basin excluding special sediments exceeds 90%; the content of ferns in the mudstone and shale in the lake basin is less than 10%; and the lake basin is in the stage of intense faulting in terms of tectonic evolution. If so, calculate the paleo-water depth using the percentage of pollen and algae obtained through standard sampling and the deep lake area relationship; otherwise, calculate the paleo-water depth using the shallow lake-semi-deep lake area relationship.

[0112] The application of the method for calculating the ancient water depth of lake basins based on the content of spore pollen and algae in the present invention is as follows:

[0113] For Well W in Shunde Sag, which is applicable to this method, Well W in Shunde Sag reveals that the area is rich in Paleogene pollen and algae, but the evolution law of paleowater depth in the lake basin is still unclear. Therefore, it is necessary to first determine the applicable calculation relationship for different layers. Specifically, the second section of Wenchang Formation in Shunde Sag is a period of strong faulting, with low fern content and deep lake sedimentary signs such as oil shale and shale. Therefore, this section is applicable to the deep lake area relationship for paleowater depth calculation; the remaining layers do not have deep lake signs such as shale, and brown-red mudstone is seen locally, so it is applicable to the coastal shallow lake-semi-deep lake area relationship for paleowater depth calculation, especially the abundance of pollen and algae in multiple sections of Wenchang Formation 1 is less than 65, and brown-red mudstone is seen on the well. It is speculated that the water body is shallow and exposed in some periods. Based on the above understanding, calculations show that the water depth at the well point location during the deposition period of the third member of the Wenchang Formation in the Eocene in the Shunde Sag was 61-134m, the water depth at the well point location during the deposition period of the second member of the Wenchang Formation was 461-550m, and the maximum water depth at the well point location during the deposition period of the first member of the Wenchang Formation was about 150m, but there may be exposure in some periods; the maximum water depth at the well point location during the deposition period of the second member of the Enping Formation in the Oligocene was about 72m, and the water depth at the well point location during the deposition period of the first member of the Enping Formation was about 56-107m. Figure 7 As shown, the ancient water depth evolution process of the area can be quantitatively restored, laying a solid foundation for the study of regional sedimentary filling evolution and source rocks.

[0114] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae, characterized in that: The steps include: S1. Select a sampling lake basin, conduct standardized sampling in the sampling lake basin to obtain multiple valid sample points, and then analyze the percentage of spores, pollen and algae in the valid sample points; S2. Perform well-seismic calibration on the sampling lake basin area to clarify the position of each valid sample point on the seismic profile, obtain the lake level phase and identify the position of each phase of the ancient lake level on the seismic profile; S3. According to the effective sample point positions determined in step S2, compaction correction and formation dip correction are performed on the overlying strata of all effective sample points to obtain the initial true thickness of the overlying strata of the effective sample points; S4. According to the lake level phases and the positions of the ancient lake planes of each phase identified in step S2, compaction correction and formation dip correction are performed on the overlying strata of each ancient lake plane phase to obtain the lake level elevation difference corresponding to each valid sample point corresponding to the phase; S5. Obtaining the paleo-water depth corresponding to the effective sample point according to the initial true thickness of the overlying stratum and the elevation difference of the lake surface at the effective sample point; S6. According to the percentage of pollen and algae in each valid sample point and the ancient water depth corresponding to each valid sample point, a formula is fitted to obtain the relationship between the percentage of pollen and algae and the ancient water depth; S7. For the lake basin to be calculated for the paleo-water depth applicable to the relational expression in step S6, the percentage of pollen and algae at multiple locations is obtained through standardized sampling, and the corresponding paleo-water depth range is obtained according to the relational expression in step S6.

2. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 1 is characterized in that: In step S1, the sampling lake basin meets the conditions of developing strata since the late Paleozoic, having drilling cores and rock cuttings physical data, and developing spores, pollen, and algae fossils.

3. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 1 is characterized in that: Step S2 includes: performing well-seismic calibration on the sampling lake basin to clarify the position of each effective sample point on the seismic profile; at the same time, clarifying the anti-cyclic sedimentary characteristics between the lake planes of each period in the foreset based on drilling, and obtaining the paleo-lake plane stages according to the anti-cycles; and tracing the position of the top-overlap feature according to the seismic in-phase axial provenance area corresponding to the top of the anti-cycle to identify the position of the paleo-lake plane of each period on the seismic profile.

4. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 1, characterized in that: Step S3 includes the following steps: S31. Perform compaction correction to obtain the initial thickness of the overlying stratum at the effective sample point; S32. Perform formation dip correction, and obtain the initial true thickness of the formation overlying the effective sample point in combination with the initial thickness of the formation overlying the effective sample point.

5. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 4 is characterized in that: In step S31, the calculation formula for the initial thickness of the overlying stratum at the effective sample point is: Where H represents the initial thickness of the overlying stratum at the effective sample point, h s Indicates the current thickness of the sandstone overlying the effective sample point, represents the initial porosity of the sandstone overlying the effective sample point, represents the current average porosity of the sandstone overlying the effective sample point, h n Indicates the current thickness of the mudstone overlying the effective sample point, represents the initial porosity of the overlying mudstone at the effective sample point, Indicates the current average porosity of the overlying mudstone at the effective sample point.

6. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 4 is characterized in that: In step S32, the calculation formula for the initial true thickness of the overlying stratum at the effective sample point is: D=H×Cosα Where D represents the initial true thickness of the overlying stratum at the effective sample point, H represents the initial thickness of the overlying stratum at the effective sample point, and α represents the current dip of the stratum at the effective sample point.

7. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 1, characterized in that: Step S4 includes the following steps: S41. Perform compaction correction to obtain the initial thickness of the overlying strata on the ancient lake plane at each stage; S42. Performing formation dip correction, and combining the initial thickness of the formation overlying the ancient lake plane of each period to obtain the initial true thickness of the formation overlying the ancient lake plane of each period; S43. Determine the lake level difference of the i-th period from top to bottom at the corresponding position according to the location of the valid sample point; S44. Calculate the height difference of the lake surface from top to bottom in the i-th period: H Li =D i -D1 In the formula, H Li represents the lake level difference from top to bottom in the i-th period, D i D1 represents the initial true thickness of the overlying strata of the i-th period paleo-lake plane from top to bottom, and D2 represents the initial true thickness of the overlying strata of the 1st period paleo-lake plane from top to bottom.

8. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 1, characterized in that: In step S5, the calculation formula of the paleo-water depth is: S=D-H Li Where S represents the paleo-water depth, D represents the initial true thickness of the overlying strata at the effective sample point, and H Li Represents the height difference of the lake level in the i-th period from top to bottom.

9. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 1, characterized in that: In step S6, when the sampling lake basin is a lake basin with a progradational delta, and the pollen is selected as a fern, the relationship between the percentage of pollen and algae and the ancient water depth can be obtained by fitting: If 1.5<S<290m, the relationship between the shallow lake and semi-deep lake area is: If 290≤S≤595m, the deep lake area relationship is: Where S represents the paleo-water depth, f x Indicates the percentage of algae, f y Indicates the percentage of fern.

10. The method for calculating the ancient water depth of a lake basin based on the content of spore pollen and algae according to claim 1, characterized in that: For the lake basin to be calculated paleo-water depth applicable to the relational expression in step S6, step S7 includes: If the lake basin where the paleo-water depth is to be calculated has red oxidized exposed shallow water signs, the paleo-water depth is calculated by using the percentage of pollen and algae obtained by standard sampling and the relationship between the coastal shallow lake and semi-deep lake area; For shallow water areas without red oxidation exposure signs, if the basin characteristics meet the following conditions at the same time: the mudstone and shale content of the basin excluding special sediments exceeds 90%; the fern content in the mudstone and shale of the basin is less than 10%; and the basin is in the stage of intense faulting in terms of tectonic evolution; then the percentage of pollen and algae obtained through standardized sampling is used to calculate the ancient water depth using the deep lake area relationship; otherwise, the ancient water depth is calculated using the shallow lake-semi-deep lake area relationship.

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