Prestack earthquake-based ancient channel description method

Abstract

The invention relates to the technical field of oil-gas exploration, and discloses a prestack earthquake-based paleochannel depiction method, which comprises the following steps of: performing radon denoising on earthquake prestack gathers of all CMP points in a target area; carrying out different offset superposition on the de-noised trace gather; carrying out calculation and statistics on trough sample points of the superposition bodies with different offsets and the lower marker layer; carrying out plane distribution on trough sample points of the superposition bodies with different offset distances; and analyzing and comparing the physical attributes of the trough sample point diagrams of the offsets so as to describe and identify the ancient river channel. The method provided by the invention is stable and reliable, and effectively solves the technical problems of paleochannel distribution range, continuity degree, fine degree, channel bright spot intensity, channel hiding difficulty and the like. The method is suitable for identifying and depicting ancient meandering river channels and diversion river channels, can also be applied to groove depicting of ancient carbonate, and has a wide application field.

Description

Technical Field

[0001] The invention relates to a paleo-river channel characterization method based on pre-stack seismic, and belongs to the technical field of oil and gas exploration. Background Art

[0002] The ancient river channel is a geological unit of paleogeography. Its formation process is the natural subsidence of the river channel strata in the early geological movement, and the subsequent strata gradually deposited and covered the geological unit. When the river channel of the ancient strata well preserved by natural sedimentation is not eroded, fractured or dislocated in the subsequent geological movement, its geological form and sedimentary materials can be well preserved in the new strata.

[0003] In oil and gas exploration, the characterization of ancient river channels is a very important task, because the sedimentary lithology of ancient river channels is usually sandstone, which is an important gas-bearing reservoir. Due to the velocity difference between the lithology preserved in this ancient river channel and the surrounding rock, it usually appears as a very weak reflection structure in seismic. In the prior art, the identification and characterization of ancient river channels are basically based on the residual thickness method of the post-stack result body, that is, a simple stratigraphic time difference. Or it is based on the coherence and curvature method of the result post-stack body. However, in the process of processing conventional result post-stack bodies, the pursuit of signal-to-noise ratio is strengthened for the continuity of the strata, and the balance between the channel sets and the time window is often strengthened. The result of such processing is to cover up and ignore the microstructural changes in the geological body, making it difficult to realize the identification and characterization of the ancient river channel. For example, the use of the residual thickness method of the result data body to obtain the paleo-geomorphology does not show the shadow of the ancient river channel at all, see Figure 1 ; If the coherence method is used to identify the paleochannel using the post-stacked volume, the continuity is poor and the whole picture cannot be traced. Figure 2 .

[0004] The Chinese patent with publication number CN113945964A discloses a method for obtaining multi-band coherence values ​​of pre-stack seismic data and a method for predicting faults. The method is mainly used to predict faults. However, the paleo-river channel is not a fault in seismic structure, because the flow direction of the river channel is usually continuous, the lithology of the sedimentary riverbed is single and stable, and the riverbed of the paleo-river channel is naturally continuous with the river bank during the sedimentation process. It is not a physical up-and-down displacement fault. In addition, the method is essentially mainly to obtain the fault direction by the coherence change between the pre-stack traces of the same formation reflection point, without considering the velocity and physical property changes between the formation reflection point and other formation reflection points, and is not applicable to the description of paleo-river channels.

[0005] Therefore, the existing post-stack results such as residual thickness and coherence are not suitable for paleo-river channel characterization. Therefore, it is necessary to propose a technical solution for paleo-river channel characterization based on pre-stack seismic. Summary of the invention

[0006] The purpose of the present invention is to provide a paleo-river channel characterization method based on pre-stack seismic to solve the problems of paleo-river channel characterization failure, discontinuous characterization results, incomplete channel details, etc. due to the disappearance of velocity differences in post-stack bodies during paleo-river channel characterization.

[0007] To achieve the above object, the solution of the present invention includes:

[0008] A technical solution of a paleo-river channel characterization method based on pre-stack seismic analysis of the present invention comprises the following steps:

[0009] 1) Obtain pre-stack seismic data for each CMP point in the target area;

[0010] 2) Perform Radon denoising on the pre-stack seismic gathers in the target area;

[0011] 3) Superimposing the radon denoised data at different offset ranges to obtain superimposed data volumes at different offset ranges;

[0012] 4) Calculate and count the trough sample points between the stratum and the underlying marker horizon in the stacked data volume with different offset distances and spread them into a plane map;

[0013] 5) The difference characteristics of the ancient river channel are determined based on the trough sample point plan formed by superimposing offset distances in different ranges, so as to realize the depiction of the ancient river channel on the plane.

[0014] This method is stable and reliable, and effectively solves technical problems such as the distribution range, continuity, precision, intensity of bright spots in the ancient river channel, and the difficulty of discovering the hidden river channel. It is suitable for the identification and depiction of ancient meandering river channels and diversion river channels, and can also be used for the groove depiction of ancient carbonates, and has a wide range of applications.

[0015] Furthermore, in step 1), the pre-stack seismic data include seismic values ​​of different offset distances, as well as stratigraphic time data determined by the post-stack volume at the point and marker stratigraphic time data below the stratigraphic layer.

[0016] Furthermore, in step 3), the denoising parameters of the radon denoising adopt the default denoising parameters:

[0017] Low Delta-T: -10ms, Hight Delta-T: 100ms, At Max Offset: 3500m.

[0018] Conventional pre-stack denoising methods are mainly bandpass and radon denoising. Figure 8(a) shows the original CRP gather and Figure 8(b) shows the radon denoised gather. Compared with the bandpass denoised gather shown in Figure 8(c), the linear noise of the gather after radon denoising is significantly suppressed, the validity of the gather offset is significantly confirmed, and the overall effect is better than the bandpass denoising method.

[0019] Further, in step 3), there are three types of offsets in different ranges, namely small offset, medium offset and large offset.

[0020] Furthermore, in step 5), the difference characteristics include: fracture accuracy, boundary range, continuity, river channel bright spot intensity, and hidden river channel.

[0021] Furthermore, in step 5), the difference characteristics of the ancient river channel are judged according to the trough sample point plan map of different offset distances, including using the small offset trough sample point distribution map to determine the fineness and boundary range of the river channel, using the medium offset trough distribution map to determine the bright spot intensity and the degree of continuity of the river channel, and using the large offset trough distribution map to determine the hidden river channel.

[0022] Furthermore, in step 5), the small offset, the medium offset, and the large offset are re-superimposed, and the trough sample points are calculated to obtain the ancient river channel distribution map.

[0023] Furthermore, the small offset is 1000-2000m, the medium offset is 2000-3000m, and the large offset is 3000-4000m.

[0024] The offset distance (Offset) selected for stacking in the present invention avoids 0-1000m and above 4000m, as shown in Figure 9(a) and Figure 9(b). The high-frequency interference of the seismic signal of the near-offset distance of 0-1000m shown in Part A of the figure is relatively large, and the waveform distortion of the far-offset distance shown in Part B of the figure is relatively large.

[0025] For example, if the research layer is the gather corresponding to line a in the figure, the trough time that needs to be calculated is the time between the gather corresponding to the layer a and the gather corresponding to the layer b. The high-frequency interference here refers to the near-offset gather corresponding to the layer b, and the low-frequency waveform distortion refers to the waveform distortion at the far-offset position of the layer b.

[0026] The present invention avoids the gathers of near and far offset distances, and avoids the gather errors after superposition affecting the trough time difference, thereby avoiding causing inaccurate subsequent velocity time difference and affecting the accuracy of paleo-river channel depiction.

[0027] The beneficial effects of the present invention are:

[0028] 1. From the trough sample point diagram at small offset as shown in Figure 6(a), it can be clearly found that at the lower right of point A, a slight breakpoint appears in the meandering river in the work area, and the boundary range of the distributary channel at the end of the meandering river can be clearly seen. However, no breakpoint is found at medium and large offsets, and the boundary is blurred.

[0029] 2. Through the trough sample map at large offset as shown in Figure 6(c), it can be clearly found that a group of horizontal narrow river channel bright spots appear below point B, which is suspected to be the appearance of another group of east-west hidden narrow river channels, but they are not found or difficult to identify at small and medium offsets.

[0030] 3. Through the mid-offset distance, it can be seen that the river body of the depicted ancient river channel has strong continuity, the bright spots of the river channel have a strong contrast with the surrounding lithology, and are strongly correlated with the gas-bearing bright spots of the reservoir, which has important significance for oil and gas exploration.

[0031] Therefore, by superimposing the sample point maps with different offset distances, the ancient river channel can be portrayed continuously and in complete detail. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is the result post-stack stratigraphic trough count map;

[0033] Figure 2 It is the post-stack seismic coherence map of the results;

[0034] Figure 3 This is a flowchart of paleochannel characterization based on three different offset pre-stack gathers according to an embodiment of the present invention;

[0035] FIG4( a ) is a diagram of seismic data before radon denoising according to the present invention;

[0036] FIG4( b ) is a diagram of seismic data after radon denoising according to the present invention;

[0037] FIG5( a ) is a post-stack volume image of a small offset stack according to the present invention;

[0038] FIG5( b ) is a post-stack volume image of offset stacking in the present invention;

[0039] FIG5(c) is a post-stack volume image of a large offset stack according to the present invention;

[0040] FIG6( a ) is a trough counting diagram of small offset seismic horizons according to the present invention;

[0041] FIG6( b ) is a diagram of offset seismic horizon trough counts in the present invention;

[0042] FIG6( c ) is a trough counting diagram of a large offset seismic horizon according to the present invention;

[0043] Figure 7 is a schematic diagram of the ancient river channel identified by the present invention;

[0044] Figure 8(a) is a schematic diagram of the original CRP gather used to compare the effects of bandpass denoising and radon denoising;

[0045] FIG8( b ) is a schematic diagram of a radon denoising gather used to compare the effects of bandpass denoising and radon denoising;

[0046] FIG8(c) is a schematic diagram of a bandpass denoising gather for comparing the effects of bandpass denoising and radon denoising;

[0047] Figure 9(a) shows the different offset gathers Figure 1 ;

[0048] Figure 9(b) shows the different offset gathers Figure 2 . DETAILED DESCRIPTION

[0049] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0050] The main idea of ​​the present invention is that, at a same CMP reflection point, seismic waves retain the velocity difference between the ancient river channel and the surrounding rock through radon denoising and superposition, thereby ensuring the prominence of the ancient river channel in the plane, that is, based on the anomaly of the stratum trough caused by the velocity anomaly, the ancient river channel is found according to the anomaly; at the same time, the reflection of the ancient river channel by earthquakes of different offset distances will produce different physical characteristics, and these differences are reflected in the river channel in the form of fineness, intensity of bright spots in the river channel, continuity, boundary range of the river channel and concealed river channel, that is, different offset distances characterize different properties of the river channel, and the ancient river channel is detected by different offset distances; the present invention realizes the accurate characterization of the ancient river channel through these two main methods.

[0051] Specifically, the process of the ancient river channel depiction method of the present invention is as follows: Figure 3 As shown, the following steps are included:

[0052] 1) Perform Radon denoising on the pre-stack seismic gathers in the target area;

[0053] 2) Superimpose the radon denoised data at different offset ranges;

[0054] 3) Calculate and count the trough sample points between the stratigraphic layer and the underlying marker layer in the three different offset data volumes and spread them out as a plane map;

[0055] 4) By comparing the plan views of trough sample points at three different offset distances, the distribution and direction of the ancient river channel on the plane can be depicted.

[0056] The technical solution of the present invention is further described below in conjunction with specific examples.

[0057] This example provides a paleo-river channel characterization method based on pre-stack seismic data, which specifically includes the following steps:

[0058] 1) Perform Radon denoising on the pre-stack seismic gathers in the target area;

[0059] According to the needs, the specific parameters of Radon denoising are Low Delta-T: -10ms, Hight Delta-T: 100ms, At Max Offset: 3500m. The comparison before and after denoising is shown in Figure 4(a) and Figure 4(b).

[0060] 2) Superimpose the radon denoised data at different offset ranges;

[0061] According to the situation in this area, the pre-stack data are superimposed at 1000-2000m, 2000-3000m, and 3000-4000m. In this way, three superimposed data bodies with different offset distances, small, medium, and large, will appear, as shown in Figure 5(a), Figure 5(b), and Figure 5(c).

[0062] 3) Calculate and count the trough sample points of the stratigraphic layer in the three different offset data volumes and spread them out as a plane diagram, as shown in Figure 6(a), Figure 6(b) and Figure 6(c).

[0063] 4) Comparing the three different offset distances of the trough sample point plane map can depict the distribution and direction of the ancient river channel on the plane. The specific method is to first determine the distribution area and refinement of a meandering river channel and a diversion river channel through the small offset distance sample point map, then determine the continuity of the two rivers through the medium offset distance sample point map, and finally find the hidden bright spots of the river channel through the large offset distance sample point map. Finally, the three small, medium and large offset distances can be re-superimposed as needed, and then the trough sample points can be calculated to obtain the following Figure 7 The final paleochannel distribution map is shown.

Claims

1. A paleo-river channel characterization method based on pre-stack seismic, characterized in that: The following steps are involved: 1) Obtain pre-stack seismic data for each CMP point in the target area; 2) Perform Radon denoising on the pre-stack seismic gathers in the target area; 3) Superimposing the radon denoised data at different offset ranges to obtain superimposed data volumes at different offset ranges; 4) Calculate and count the trough sample points between the stratum and the underlying marker horizon in the stacked data volume with different offset distances and spread them into a plane map; 5) The difference characteristics of the ancient river channel are determined based on the trough sample point plan formed by superimposing offset distances in different ranges, so as to realize the depiction of the ancient river channel on the plane.

2. The paleo-river channel characterization method based on pre-stack seismic according to claim 1, characterized in that: In step 1), the pre-stack seismic data include seismic values ​​of different offset distances, as well as stratigraphic time data determined by the post-stack volume at the point and marker stratigraphic time data below the stratigraphic layer.

3. The paleo-river channel characterization method based on pre-stack seismic according to claim 1, characterized in that: In step 3), the denoising parameters of the radon denoising are: Low Delta-T: -10ms, Hight Delta-T: 100ms, At Max Offset: 3500m.

4. The method for paleo-river channel characterization based on pre-stack seismic according to claim 1, characterized in that: In step 3), there are three types of offsets in different ranges, namely small offset, medium offset and large offset.

5. The method for paleo-river channel characterization based on pre-stack seismic according to claim 4, characterized in that: In step 5), the difference features include: fracture accuracy, boundary range, continuity, intensity of river channel highlights, and hidden river channels.

6. The method for paleo-river channel characterization based on pre-stack seismic according to claim 5, characterized in that: In step 5), the difference characteristics of the ancient river channel are judged according to the trough sample point plan map of different offset distances, including using the small offset trough sample point distribution map to determine the refinement and boundary range of the river channel, using the medium offset trough distribution map to determine the bright spot intensity and the degree of continuity of the river channel, and using the large offset trough distribution map to determine the hidden river channel.

7. The method for paleo-river channel characterization based on pre-stack seismic according to claim 6, characterized in that: In step 5), the small offset, medium offset, and large offset are re-superimposed, and then the trough sample points are calculated to obtain the ancient river channel distribution map.

8. The method for paleo-river channel characterization based on pre-stack seismic according to claim 7, characterized in that: The small offset distance is 1000-2000m, the medium offset distance is 2000-3000m, and the large offset distance is 3000-4000m.

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