A method for inverting seafloor deposition rate and formation porosity distribution and application thereof

Abstract

This application belongs to the field of marine geology, specifically disclosing an inversion method and application for seafloor sedimentation rate and stratigraphic porosity distribution. The method includes: obtaining the possible distribution range of stratigraphic porosity *n* with depth *z* within the proposed study area; obtaining the porosity of surface sediment samples; based on the distribution range and the porosity of the surface sediments, assuming *m* possible stratigraphic porosity variations with depth at the proposed study location; and for each assumed stratigraphic porosity variation with depth, calculating the corresponding average mass sedimentation rate *R*. si Then, the deposition rate R is simulated in numerical software. si The porosity distribution of the seafloor strata below; will be shown in each R si The simulated formation porosity distribution with depth is compared with the corresponding assumed distribution under the given conditions. If they match, the simulated formation porosity distribution with depth is the actual formation porosity distribution at that location. This application is low-cost, convenient, quick, and easy to promote.

Description

Technical Field

[0001] This application belongs to the field of marine geology, and more specifically, relates to an inversion method and application for seafloor sedimentation rate and stratigraphic porosity distribution. Background Technology

[0002] Accurately obtaining information on seabed sedimentation rates and formation porosity distribution is of great significance for seabed geological hazard risk assessment, marine oil and gas resource evaluation, and marine engineering construction.

[0003] Dating and geotechnical testing of seabed core samples are the main methods for obtaining seabed sedimentation rates and formation porosity distributions. Although this method yields relatively accurate sedimentation rates and formation porosity distributions, it has the following drawbacks: 1) Seabed core samples often need to be obtained by drilling with ocean drilling vessels. Currently, there are few ocean drilling vessels, and drilling costs increase exponentially with drilling depth and water depth, limiting the target audience; 2) After obtaining core samples, a large amount of sediment and geotechnical testing work needs to be carried out, which is time-consuming and labor-intensive.

[0004] Therefore, it is necessary to provide a method for calculating seabed sedimentation rate and formation porosity distribution that is low in implementation cost and easy to promote. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this application is to provide an inversion method and application for the seafloor sedimentation rate and formation porosity distribution, which is low in cost, convenient and fast, and easy to promote.

[0006] To achieve the above objectives, in a first aspect, this application provides an inversion method for seafloor sedimentation rate and formation porosity distribution, comprising the following steps:

[0007] S10: Collect and obtain the possible distribution range of formation porosity n with depth z in this area based on the test data of ocean drilling cores near the proposed research location;

[0008] S20, collect surface sediment samples from the proposed research location and conduct geotechnical tests to obtain the porosity n0 of the surface sediment samples;

[0009] S30, based on the distribution range of formation porosity n with depth z in this region, and the porosity n0, assume m possible relationships of formation porosity with depth at the proposed research location, n = n i (z), i∈[1:m];

[0010] S40, for each assumed formation porosity variation with depth n = n i (z), calculate the corresponding average mass deposition rate R siThen, the deposition rate R is simulated in numerical software. si The porosity distribution of the seafloor strata;

[0011] S50 will be available in various R si The simulated formation porosity distribution with depth is compared with the corresponding assumed formation porosity distribution with depth. If they match, the simulated formation porosity distribution with depth is the actual formation porosity distribution at that location; otherwise, different R values ​​are used. si The simulation was continued to obtain the distribution of formation porosity with depth and compared with the corresponding hypothetical distribution of formation porosity with depth.

[0012] The beneficial effects of this application are as follows: The inversion method for seafloor sedimentation rate and formation porosity distribution provided in this application can, based solely on existing core data in the vicinity of the study point, obtain average mass settling rate and formation porosity variation with depth by acquiring surface sediment samples and conducting a small amount of geotechnical testing to determine model calculation parameters. Compared to traditional technologies, this application does not require deep-sea core drilling by ocean drilling vessels or extensive geotechnical testing, resulting in lower costs and easier implementation.

[0013] As a further preferred embodiment, in step S40, the average mass deposition rate R si The calculation formula is:

[0014]

[0015] In the formula, d is the depth of the seafloor strata; T is the stratigraphic age at depth d; G s ρ represents the specific gravity of sediment particles. w This is the density of seawater.

[0016] As a further optimization, the stratigraphic age T at the seabed burial depth d is determined by collecting three-dimensional seismic profiles at the proposed research location and combining them with publicly available stratigraphic frameworks at similar locations.

[0017] As a further preferred embodiment, the stratigraphic framework is a framework structure that reflects the spatiotemporal distribution and interrelationships of strata, established by analyzing and integrating various information about seafloor strata.

[0018] As a further preferred embodiment, in step S40, the deposition rate R is simulated in numerical software. si The steps for determining the porosity distribution of the seabed strata are as follows:

[0019] A geometric model of the sedimentary layer was established, and then the seabed was divided into layers.

[0020] Based on the sedimentary layer thickness h0 and mass deposition rate R defined in the sedimentary layer geometric model s The analysis step time is calculated.

[0021] Based on the geotechnical test results of the surface sediment samples taken in step S20, values ​​are assigned to the seepage parameters and modified Cambridge model parameters of each layer of material in the sedimentary geometric model.

[0022] In the initial analysis step, the birth and death cell command in the numerical software is used to remove all sedimentary layers except the first sedimentary layer. Then, in subsequent analysis steps, the birth and death cell command is used to activate each sedimentary layer from bottom to top and apply the corresponding gravity load to achieve the layer-by-layer deposition process.

[0023] In the initial analysis step, the displacement of the seafloor slope base is fixed at 0 in both the horizontal and vertical directions, the displacement of the sedimentary layer side boundary is fixed at 0 in the horizontal direction, and in each subsequent analysis step, the surface sediment is set as a permeable layer, and its excess pore water pressure is kept at 0 in the corresponding analysis step.

[0024] A four-node plane strain element was used for the base portion, and a quadratic reduced integral element was used for the deposition layer, resulting in the deposition rate R. si The distribution of formation porosity with depth obtained under simulated conditions.

[0025] As a further preferred embodiment, the modified Cambridge model parameters include the slope M of the critical state line of the surface sediments in the p′-q plane, the slope λ of the isotropic consolidation curve of the surface sediments in the e-lnp′ plane, and the slope k of the rebound curve of the surface sediments in the e-lnp′ plane.

[0026] As a further preferred embodiment, the modified Cambridge model parameters are determined by triaxial testing.

[0027] As a further preferred embodiment, the analysis step employs a soil analysis step that simulates fluid-structure interaction in porous media.

[0028] As a further preferred option, in step S40, the numerical software is Abaqus numerical software.

[0029] Secondly, this application provides an application of the inversion method for seafloor sedimentation rate and formation porosity distribution as described above, which can be applied to seafloor geological hazard risk assessment, marine oil and gas resource assessment, and marine engineering construction.

[0030] It is understandable that the beneficial effects of the second aspect mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here. Attached Figure Description

[0031] Figure 1This is a flowchart of the inversion method for seafloor sedimentation rate and formation porosity distribution provided in the embodiments of this application;

[0032] Figure 2 This is a diagram showing the porosity distribution of seafloor strata with depth near the research point provided in this application embodiment;

[0033] Figure 3 This is a diagram showing the hypothetical formation porosity distribution and the simulated formation porosity distribution provided in the embodiments of this application. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0035] It should be understood that, in the description of this application, the term "several" means at least one, such as one, two, etc., unless otherwise expressly and specifically defined; the term "multiple" means two or more, unless otherwise expressly and specifically defined; the terms "first" and "second," etc., are used to distinguish different objects, not to describe a specific order of objects; the term "and / or" includes any and all combinations of one or more of the related listed items.

[0036] Furthermore, throughout this specification, references to "an embodiment"; "an embodiment," "an example," or similar language indicate that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. Therefore, the appearance of the phrase "in one embodiment;" throughout this specification, and similar language, may, but not necessarily, refer to the same embodiment.

[0037] It should be noted that the sedimentation rate is the mass of sediment particles deposited per unit time and per unit area. The ratio of pore volume to total volume in seafloor strata is called formation porosity, usually expressed as a percentage, and its calculation formula is: V v V is the pore volume, and V is the total volume of the formation (including the volume occupied by solid particles and pores).

[0038] This application analyzes the porosity data of sediment borehole samples from four stations in the northern South China Sea and concludes two objective facts: (1) the sediments gradually become denser with increasing burial depth, and the porosity of the seabed strata changes logarithmically with the stratum depth; (2) although the porosity of the strata at different locations within a certain area varies with depth, they are all distributed within a certain range.

[0039] Based on the above two points of understanding, this application proposes an inversion method for seafloor sedimentation rate and stratigraphic porosity distribution, specifically including steps S10 to S50, which are detailed below:

[0040] S10: Collect and obtain the possible distribution range of formation porosity n with depth z in this area based on ocean drilling core test data near the proposed research location.

[0041] S20. Collect surface sediment samples from the proposed research location and conduct geotechnical tests to obtain the porosity n0 of the surface sediment samples.

[0042] S30, based on the distribution range of formation porosity n with depth z in this region, and the porosity n0, assume m (m≥1) possible formation porosity variation relationships with depth at the proposed study location, n=n i (z), i∈[1:m].

[0043] S40, for each assumed formation porosity variation with depth n = n i (z), calculate the corresponding average mass deposition rate R si Then, the deposition rate R is simulated in numerical software. si The porosity distribution of the seabed strata.

[0044] S50 will be available in various R si The simulated formation porosity distribution with depth is compared with the corresponding hypothesized formation porosity distribution with depth. If they match, it indicates that the sedimentation rate of this mass is the average sedimentation rate since time T at the proposed study location, and the simulated formation porosity distribution with depth is the actual formation porosity distribution at that location; otherwise, different R values ​​are used. si The simulation was continued to obtain the distribution of formation porosity with depth and compared with the corresponding hypothetical distribution of formation porosity with depth.

[0045] The inversion method for seafloor sedimentation rate and formation porosity distribution provided in this embodiment only requires existing core data in the vicinity of the study point. It only needs to acquire surface sediment samples and conduct a small amount of geotechnical testing to determine the model calculation parameters to invert the relationship between the average mass subsidence rate and formation porosity with depth at the study point. Compared with traditional techniques, this application does not require deep-sea core drilling by ocean drilling vessels or extensive geotechnical testing, resulting in lower costs and easier implementation.

[0046] In a specific embodiment, the technical solution to achieve the above objective is: an inversion method for seafloor sedimentation rate and formation porosity distribution, specifically including the following steps:

[0047] Step 1: Collect publicly available ocean drilling core test data near the proposed research location to obtain the possible distribution range of formation porosity n with depth z in this area.

[0048] Step 2: Collect surface sediment samples from the proposed study location and conduct geotechnical tests to obtain the dry density ρ of the surface sediment samples. d0 Porosity n0, corrected Cambridge model parameters, the relationship between permeability coefficient and porosity (i.e., permeability parameter) k = k(e).

[0049] In this embodiment, the modified Cambridge model is used as the constitutive model for sediments. Based on the Cambridge model, the modified Cambridge model adopts the phase-adaptive flow law, with the total volumetric strain increment dε of the element being... v Equal to the strain increment of the single elastic body With the strain increment of the unit plastic body The sum can be calculated using the following formula.

[0050]

[0051] In the formula, p′ is the average effective value; q is the deviatoric stress; M is the slope of the critical state line of the surface sediment on the p′-q plane; e is the porosity ratio; λ is the slope of the isotropic consolidation curve of the surface sediment on the e-lnp′ plane; k is the slope of the rebound curve of the surface sediment on the e-lnp′ plane; and η is the stress ratio.

[0052] In this embodiment, the main parameters of the modified Cambridge model include λ, k, and M, which can be determined through conventional triaxial tests. By determining the model parameters, the sedimentary constitutive model can be used to invert the relationship between the settling rate and formation porosity as a function of depth at the study point. Under sedimentary loading, as the sedimentary layer increases, and given a relatively uniform sedimentation rate, the main factor affecting the depth-dependent distribution of formation porosity is sediment compressibility. Sediment compressibility is related to the permeability coefficient; the smaller the permeability coefficient, the greater the compressibility, and vice versa.

[0053] Step 3: Collect three-dimensional seismic profiles at the proposed research location, and combine them with publicly available stratigraphic frameworks from nearby locations (a framework structure that reflects the spatiotemporal distribution and interrelationships of strata by analyzing and integrating various information on seafloor strata) to determine the stratigraphic age T at the seafloor burial depth d.

[0054] Step 4: Based on the measured porosity n0 of the surface sediment samples and the distribution range of formation porosity n with depth z in this area, reasonably assume m possible relationships of formation porosity variation with depth at the proposed study location, n = n i (z), i∈[1:m].

[0055] Step 5: For each assumed formation porosity variation with depth n = n i (z), the corresponding average mass deposition rate R can be calculated by the following formula (2). si (Mass of sediment particles deposited per unit time and per unit area):

[0056]

[0057] In the formula, G s ρ represents the specific gravity of sediment particles. w This is the density of seawater.

[0058] Step 6: Simulate the deposition rate R in Abaqus numerical software. si The porosity distribution of seafloor strata under certain conditions.

[0059] In this embodiment, step 6 can specifically be:

[0060] Establish a geometric model of the sedimentary layer. Based on the dry density ρ of the surface sediments... d0 Calculate the unconsolidated formation thickness H. A geometric model of the sedimentary layer is established, and then the layer is divided into sections. Considering that the surface sediments of the seabed are relatively loose and there is no excess pore pressure accumulation, the excess pore pressure of the surface sediments should be set to 0 in each analysis step when setting the seepage boundary conditions. The thickness g0 is determined according to the actual situation and is used as the layer thickness of the geometric model.

[0061] Material parameter assignment. Based on the geotechnical test results of the surface sediments, the seepage parameters k = k(e) and the modified Cambridge model parameters for each layer of material in the model are assigned values.

[0062] Analysis step settings. The analysis step adopts the Soil analysis step, which simulates fluid-structure interaction in porous media, based on the sedimentary layer thickness h0 and mass deposition rate R defined in the sedimentary layer geometry model. s The analysis step duration Δt = g0ρ was calculated. d0 / R s .

[0063] Deposition layer removal and activation. First, in the initial analysis step, the birth and death cell command provided by Abaqus' Interaction module is used to remove all sedimentary layers except the first sedimentary layer. Then, in subsequent analysis steps, the birth and death cell command is used to activate each sedimentary layer from bottom to top and apply the corresponding gravity load to achieve a layer-by-layer deposition process.

[0064] Boundary condition settings: In the initial analysis step, the displacement of the seafloor slope base is fixed to 0 in both the horizontal and vertical directions, and the displacement of the sedimentary layer side boundary is fixed to 0 in the horizontal direction. In each subsequent analysis step, the surface sediments are set as a permeable layer, and their excess pore water pressure is maintained at 0 in the corresponding analysis step.

[0065] Mesh generation and calculation. A four-node plane strain element (CPE4) was used for the base portion, and a quadratic reduced integration element (CPE8RP) was used for the deposition layer, resulting in the mesh generation and calculations in R... si The distribution of formation porosity with depth obtained under simulated conditions.

[0066] Step 7: In each R si The simulated formation porosity distribution with depth is compared with the corresponding hypothesized formation porosity distribution with depth. If they are consistent, it indicates that the sedimentation rate of this mass is the average sedimentation rate since time T at the proposed study location, and the simulated formation porosity distribution with depth is the actual formation porosity distribution at that location.

[0067] The following section uses the seafloor strata at a certain location on the continental shelf break zone of the northern South China Sea as an example to specifically illustrate the inversion method for seafloor sedimentation rate and stratum porosity distribution provided in this embodiment.

[0068] An inversion method for seafloor sedimentation rate and formation porosity distribution includes the following steps:

[0069] Step S1: Publicly available ocean drilling core test data were collected near this location to obtain the distribution range of formation porosity n with depth z in this area, see [link to relevant documentation]. Figure 2 .

[0070] Step S2: Dry density ρ of surface sediment samples at the study point d0 Porosity n0, modified Cambridge model parameters, the relationship between permeability coefficient and porosity k=k(e) is shown in Table 1.

[0071] Table 1

[0072] Calculation parameters value unit <![CDATA[ρ d0 ]]> 1.028 <![CDATA[g / cm 3 ]]> <![CDATA[n0]]> 0.62 / Correcting the Cambridge model parameter λ 0.148 / Correcting the Cambridge model parameter κ 0.030 / Correcting the Cambridge model parameter M 0.734 / k = k(e) <![CDATA[k=4.3×10 -8 ×exp[(e-1.63) / 0.27]]]> m / s

[0073] Step S3: The stratigraphic age at the seabed burial depth of 101.9m at the study point is 306ka.

[0074] Step S4: Four possible relationships between formation porosity and depth were assumed at the proposed research location, as shown in Table 2.

[0075] Table 2

[0076] Assume relation 1 n=0.62-0.0230ln[(z+0.92417) / 0.92417] Assume relation 2 n=0.62-0.0365ln[(z+0.92417) / 0.92417] Assume relation 3 n=0.62-0.0495ln[(z+0.92417) / 0.92417] Assumption Relation 4 n=0.62-0.0565ln[(z+0.92417) / 0.92417]

[0077] Step S5: For each assumed relationship between formation porosity and depth, the corresponding average mass deposition rate R can be calculated using equation (2). si See Table 3.

[0078] Table 3

[0079]

[0080] Step S6: Simulate the deposition rate R in Abaqus numerical software. si The simulation results show the porosity distribution of the seafloor strata under the given conditions. Figure 3 .

[0081] Step S7: By comparing the simulated formation porosity distribution with the corresponding hypothesized formation porosity distribution with depth, it can be considered that the average mass deposition rate at the study site since 306 ka is approximately 52.70 g / (cm³). 2 The porosity of the seafloor strata along the depth direction can be expressed as n=n0-0.0495·ln((h+0.92417) / 0.92417).

[0082] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for inverting seafloor sedimentation rate and stratigraphic porosity distribution, characterized in that, Includes the following steps: S10: Collect and obtain the possible distribution range of formation porosity n with depth z in this area based on the test data of ocean drilling cores near the proposed research location; S20, collect surface sediment samples from the proposed research location and conduct geotechnical tests to obtain the porosity n0 of the surface sediment samples; S30, based on the distribution range of formation porosity n with depth z in this region, and the porosity n0, assume m possible relationships of formation porosity with depth at the proposed research location, n = n i (z), i∈[1:m]; S40, for each assumed formation porosity variation with depth n = n i (z), calculate the corresponding average mass deposition rate R si Then, the deposition rate R is simulated in numerical software. si The porosity distribution of the seafloor strata; S50 will be available in various R si The simulated formation porosity distribution with depth is compared with the corresponding assumed formation porosity distribution with depth. If the two match, the simulated formation porosity distribution with depth is the actual formation porosity distribution at that location; otherwise, different R values ​​are used. si The simulation was continued to obtain the distribution of formation porosity with depth and compared with the corresponding hypothetical distribution of formation porosity with depth.

2. The inversion method for seafloor sedimentation rate and formation porosity distribution as described in claim 1, characterized in that, In step S40, the average mass deposition rate R si The calculation formula is: In the formula, d is the depth of the seafloor strata; T is the stratigraphic age at depth d; G s ρ represents the specific gravity of sediment particles. w This is the density of seawater.

3. The inversion method for seafloor sedimentation rate and formation porosity distribution as described in claim 2, characterized in that, The stratigraphic age T at a depth d on the seabed was determined by collecting three-dimensional seismic profiles at the proposed research location and combining them with publicly available stratigraphic frameworks at similar locations.

4. The inversion method for seafloor sedimentation rate and formation porosity distribution as described in claim 3, characterized in that, The stratigraphic framework is a structure that reflects the spatial and temporal distribution and interrelationships of strata by analyzing and integrating various information about seafloor strata.

5. The inversion method for seafloor sedimentation rate and formation porosity distribution as described in claim 1, characterized in that, In step S40, the deposition rate R is simulated in numerical software. si The steps for determining the porosity distribution of the seabed strata are as follows: A geometric model of the sedimentary layer was established, and then the seabed was divided into layers. Based on the sedimentary layer thickness h0 and mass deposition rate R defined in the sedimentary layer geometric model s The analysis step time is calculated. Based on the geotechnical test results of the surface sediment samples taken in step S20, values ​​are assigned to the seepage parameters and modified Cambridge model parameters of each layer of material in the sedimentary geometric model. In the initial analysis step, the birth and death cell command in the numerical software is used to remove all sedimentary layers except the first sedimentary layer. Then, in subsequent analysis steps, the birth and death cell command is used to activate each sedimentary layer from bottom to top and apply the corresponding gravity load to achieve the layer-by-layer deposition process. In the initial analysis step, the displacement of the seafloor slope base is fixed at 0 in both the horizontal and vertical directions, the displacement of the sedimentary layer side boundary is fixed at 0 in the horizontal direction, and in each subsequent analysis step, the surface sediment is set as a permeable layer, and its excess pore water pressure is kept at 0 in the corresponding analysis step. A four-node plane strain element was used for the base portion, and a quadratic reduced integral element was used for the deposition layer, resulting in the deposition rate R. si The distribution of formation porosity with depth obtained under simulated conditions.

6. The inversion method for seafloor sedimentation rate and formation porosity distribution as described in claim 5, characterized in that, The modified Cambridge model parameters include the slope M of the critical state line of the surface sediment sample in the p′-q plane, the slope λ of the isotropic consolidation curve of the surface sediment sample in the e-lnp′ plane, and the slope k of the rebound curve of the surface sediment sample in the e-lnp′ plane.

7. The inversion method for seafloor sedimentation rate and formation porosity distribution as described in claim 5, characterized in that, The modified Cambridge model parameters were determined through triaxial testing.

8. The inversion method for seafloor sedimentation rate and formation porosity distribution as described in claim 5, characterized in that, The analysis step employs a soil analysis step that simulates fluid-structure interaction in porous media.

9. The inversion method for seafloor sedimentation rate and stratigraphic porosity distribution as described in claim 1, characterized in that, In step S40, the numerical software is Abaqus numerical software.

10. An application of the inversion method for seafloor sedimentation rate and stratigraphic porosity distribution as described in any one of claims 1 to 9, characterized in that, It is applied to seabed geological hazard risk assessment, marine oil and gas resource assessment, and marine engineering construction.

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