An interpretation method of a seismic fault shadow zone structure

Abstract

The application discloses a kind of earthquake fault shadow area structure's interpretation method, comprising the following steps in turn: S1.determining T0 structure map compensation correction quantity calculation formula;S2.determining fault shadow area range;S3.calculating T0 compensation correction quantity value;S4.preparing after correction time domain T0 structure map;S5.preparing depth domain structure chart.The earthquake fault shadow area structure's interpretation method provided in the application, on time domain seismic profile by the application of fault shadow area structure interpretation device, significantly improve the fine degree of structure under fault, and further improve the fault shadow area structure interpretation precision.The application eliminates fault shadow in the area where there is fault shadow, and provides strong support for obtaining accurate depth domain structure chart and trap element under ground.

Description

Technical Field

[0001] This invention belongs to the field of geophysical exploration technology and relates to the interpretation technology and method of structural correction of fault shadow areas in seismic profiles, specifically a method for interpreting the structure of fault shadow areas in seismic profiles. Background Technology

[0002] As early as 1973, foreign scholars noted that in fault-developed areas, a triangular seismic imaging distortion zone usually exists on the footwall of the fault, which manifests as twisted and dislocated phase axes and weakened amplitude in time-migrated seismic profiles. This tectonic artifact of distortion was defined as fault shadowing. Due to the influence of faults, fault shadowing is prevalent in time-migrated seismic profiles within rift basins.

[0003] Due to the influence of fault shadowing, false faults and false structures may appear in time-migrated seismic data, severely affecting the accuracy of structural interpretation and drilling success rate in fault-shadowed areas. False faults occur when the seismic phase axis of the underlying strata shows vertical displacement, which may be interpreted as a small fault accompanying the main fault, thus affecting the accuracy of structural interpretation. False structures occur when the seismic phase axis of the underlying strata shows localized "downward" twisting deformation, forming structural illusions such as anticlines and fault noses, which can lead to drilling failures.

[0004] Domestic and international scholars have mainly focused on qualitative analysis and how to use pre-stack depth migration imaging to reduce or eliminate fault shadows. However, in some areas, due to low exploration levels, uneven well distribution, and cost constraints, relying solely on pre-stack depth migration to solve the fault shadow problem is impractical, and this method cannot be widely applied. Therefore, inventing a widely applicable method for eliminating fault shadows is particularly important. Summary of the Invention

[0005] The purpose of this invention is to provide a universally applicable method for interpreting earthquake fault shadow zone structures, thereby improving the accuracy of fault shadow zone structure interpretation.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for interpreting the structure of earthquake fault shadow zones, comprising the following steps performed sequentially:

[0008] S1. Determine the calculation formula for the compensation correction amount of the T0 construction diagram.

[0009] Using seismic forward modeling technology, the influencing factors that generate fault shadows are analyzed, and the calculation formula for the compensation correction amount ΔT0 of the T0 tectonic map in the study area is determined.

[0010] According to the results of seismic forward modeling, the main influencing factors for fault shadowing are changes in formation velocity difference, fault displacement, and fault dip angle. The changes in formation velocity difference and fault displacement cause an "upward" or "downward" pull on the time-domain seismic axis in the fault shadowing area of ​​the time-migrated seismic profile, severely affecting the accuracy of structural interpretation. Therefore, it is necessary to compensate for the T0 of the seismic axis in the fault shadowing area. Based on the relationship between the anomalous time difference caused by changes in formation velocity difference, the anomalous time difference caused by changes in fault displacement, and the T0 compensation correction, the T0 compensation correction can be expressed as follows:

[0011] ΔT0=A0×(ΔTv+ΔTf)

[0012] In the formula, A0 is a constant -1 or 1, which is related to whether the strata are low-velocity or high-velocity anomalies. When the study area is a low-velocity anomaly model with a set of low-velocity strata in the middle of normal-velocity strata, A0 takes the value of 1. When the study area is a high-velocity anomaly model with a set of high-velocity strata in the middle of normal-velocity strata, A0 takes the value of -1. ΔTv is the time difference value of the time-domain profile anomaly caused by the change in strata velocity difference. ΔTf is the time difference value of the time-domain profile anomaly caused by the change in fault displacement.

[0013] Based on the actual geological structure and stratigraphic velocity of the study area, a geological model is proposed, and seismic forward modeling is performed to determine the relationship between the change in stratigraphic velocity difference and the time-domain reflection layer pull-down (or pull-up) anomaly time difference. Under normal circumstances, in areas with lateral stability in tectonics and burial depth, the stratigraphic velocity is laterally stable, and the stratigraphic velocity difference is basically unchanged. The time-domain reflection pull-down (or pull-up) time difference caused by the change in velocity difference is a constant of 0, so this term can be ignored.

[0014] S2. Determine the extent of the fault shadow area.

[0015] The influence range of the shadow area is determined based on the extent of dislocation and distortion of the phase axis in the time-domain seismic profile.

[0016] Since the fault dip angle is related to the distribution range of the shadow area, the larger the fault dip angle, the smaller the shadow area; and the smaller the fault dip angle, the larger the shadow area. The influence range of the shadow area is determined by the range of the fault dislocation and distortion of the same phase axis in the time domain seismic profile. If the fault dip angle is stable and unchanged, then the range of the fault shadow area is stable.

[0017] S3. Calculate the T0 compensation correction value.

[0018] Based on the sonic logging curves, the formation velocity difference ΔTv at the upper and lower interfaces at the velocity abrupt change point in the study area is obtained. Based on the seismic profile, the fault displacement ΔTf is obtained. Using forward modeling, the abnormal time difference values ​​caused by the change in formation velocity difference and the abnormal time difference values ​​caused by the change in fault displacement are obtained respectively. Then, these two abnormal time difference values ​​are substituted into the T0 structural map compensation correction formula to calculate the scattered data of the T0 structural map compensation correction ΔT0 in the time domain of the fault shadow area. Then, the plane distribution value of the correction is obtained through gridded calculation.

[0019] S4. Construct the corrected time-domain T0 plot.

[0020] The time-domain T0 construction diagram is corrected by using the plane distribution value of the ΔT0 correction amount, and the corrected time-domain T0 construction diagram is obtained.

[0021] S5. Compile the depth domain construction graph

[0022] The time-depth relationship of the study area was used to perform time-depth transformation on the corrected time-domain T0 tectonic map to obtain a depth-domain tectonic map that eliminates fault shadows.

[0023] As a limitation, in step S1, the influencing factors include changes in formation velocity difference, changes in fault displacement, and changes in fault dip angle.

[0024] By adopting the above technical solution, the technical progress achieved by this invention compared with the prior art is as follows:

[0025] This invention utilizes seismic forward modeling and proposes a technique and method for interpreting the structural features of fault shadow zones in time-migrated seismic profiles, based on the mechanism and influencing factors of fault shadow generation. The application of a fault shadow zone structural interpretation device on time-domain seismic profiles significantly improves the detail of the underlying structures, thereby enhancing the accuracy of fault shadow zone structural interpretation. This yields accurate subsurface depth-domain structural maps and trap elements, and is universally applicable in areas where fault shadows exist. Attached Figure Description

[0026] Figure 1 This is a flowchart of the workflow of the present invention;

[0027] Figure 2 This is a graph showing the relationship between abnormal time difference values ​​and fault displacement changes affected by fault displacement in this embodiment;

[0028] Figure 3 This is the time-lapsed seismic profile in this embodiment;

[0029] Figure 4 This is a contour map of the correction value of the compensation correction amount ΔT0 in this embodiment;

[0030] Figure 5 shows the construction diagram of the target layer time domain T0 before and after correction in this embodiment;

[0031] Figure 6 shows the construction diagram of the target layer depth domain before and after correction in this embodiment. Detailed Implementation

[0032] The present invention will be further described in detail below through specific embodiments. It should be understood that the described embodiments are only for explaining the present invention and do not limit the present invention.

[0033] Example: A method for interpreting the structure of earthquake fault shadow zones

[0034] This embodiment describes a method for interpreting the structure of earthquake fault shadow zones. This embodiment was implemented on the Trakes slope in Niger. The workflow diagram for this embodiment is shown below. Figure 1 As shown in the flowchart, the T0 structural map of the fault shadow area in the time-migrated seismic profile can be structurally corrected to obtain an accurate subsurface depth domain structural map. This method includes the following steps performed sequentially:

[0035] S1. Determine the calculation formula for the compensation correction amount ΔT0 of T0.

[0036] S11. Based on the formation velocity obtained from the sonic logging curve, determine the A0 value of the study area: The study area is a high-velocity anomaly model with a high-velocity formation in the middle of a normal velocity formation. The velocity difference between the normal velocity layer and the adjacent underlying high-velocity layer is 800 m / s, so the value of A0 is -1.

[0037] In this study area, due to the lateral stability of the structure and burial depth, the lateral stability of the strata velocity, and the strata velocity difference remains basically unchanged, the time difference of the time domain reflection pull-down (or pull-up) caused by the change of velocity difference is 0 and can be ignored. That is, in this embodiment, it can be further simplified to ΔT0=(-1)×ΔTf;

[0038] S12. Statistical analysis revealed that fault displacements in the study area are mainly distributed between 50 and 550 m. Using a fault displacement step size of 100 m, and based on the geological structure and strata velocity of the study area, normal fault geological models with displacements ranging from 100 m to 600 m were established. Seismic forward modeling technology was used to demonstrate a positive correlation between fault displacement and time-domain reflection pull-down time difference in the study area, exhibiting characteristics such as… Figure 2 The linear relationship shown;

[0039] That is, the T0 compensation correction amount for the study area is ΔT0=(-1)×(0.3534X-5.8667), where X is the fault displacement.

[0040] S2. Based on the extent of faulting and tortuosity of the in-phase axis in the time-domain seismic profile, the distance between the fault shadow area and the main fault in the study area remains stable, and the two are basically parallel. Figure 3As shown, Figure 3 This is a time-shifted seismic profile; the area between the black main fault and the white pseudo-fault is the fault shadow zone.

[0041] Depend on Figure 3 It can be seen that the area between the black main fault and the white false fault is the fault shadow zone, which is the range where the earthquake axis of the target layer is distorted and needs to be corrected.

[0042] S3. Using sonic logging curves, the formation velocity difference between the upper and lower interfaces at a local velocity abrupt change is calculated. The fault displacement is calculated from the seismic profile. The T0 compensation correction formula is used to calculate the scatter plot data of the time-domain structural T0 compensation correction ΔT0 in the fault shadow area. A contour map of the correction grid values ​​is then calculated. Figure 4 As shown;

[0043] Depend on Figure 4 It can be seen that the contour map represents the correction value of the compensation correction amount ΔT0 for T0.

[0044] S4. The time-domain T0 structural map is corrected using the contour map of the ΔT0 correction amount to obtain the corrected time-domain structural map, as shown in Figure 5. Figure 5a is the time-domain T0 structural map of the target layer before correction, and Figure 5b is the time-domain T0 structural map of the target layer after correction.

[0045] As shown in Figure 5, a false fault is developed on the T0 time domain structural map before correction in Figure 5a, and the fault block trap area is small. After correction in Figure 5b, there is no false fault on the T0 time domain structural map, and the fault block trap area is large and complete.

[0046] S5. The time-depth relationship is used to perform time-depth transformation on the corrected time-domain structural map, and finally the depth-domain structural map after the fault shadow is eliminated is obtained, as shown in Figure 6. Figure 6a is the depth-domain structural map of the target layer before correction, and Figure 6b is the depth-domain structural map of the target layer after correction.

[0047] As shown in Figure 6, the area of ​​the trap before correction is 0.11 km². 2 The trap closure degree is 5m, and the corrected trap area is 1.02km². 2 The trap closure degree is 40m. By applying the time-migrated seismic fault shadow zone structural interpretation device to the study area, accurate subsurface depth domain structural maps and trap elements were obtained.

Claims

1. A method for interpreting the structure of earthquake fault shadow zones, characterized in that, The method includes the following steps performed sequentially: S1. Determine the calculation formula for the compensation correction amount of the T0 construction diagram. Using seismic forward modeling technology, the influencing factors of fault shadows were analyzed, and the calculation formula for the compensation correction ΔT0 of the T0 structural map in the study area was determined: ΔT0 = A0 × (ΔTv + ΔTf) In the formula, A0 is a constant -1 or 1. When the study area is a low-velocity anomaly model with a set of low-velocity strata in the middle of normal velocity strata, A0 takes the value of 1. When the study area is a high-velocity anomaly model with a set of high-velocity strata in the middle of normal velocity strata, A0 takes the value of -1. ΔTv is the time difference value of the time-domain profile anomaly caused by the change in stratum velocity difference. ΔTf is the time difference value of the time-domain profile anomaly caused by the change in fault displacement. S2. Determine the extent of the fault shadow area. The influence range of the shadow area is determined based on the extent of dislocation and distortion of the phase axis in the time-domain seismic profile. S3. Calculate the T0 compensation correction value. Based on the sonic logging curves, the formation velocity difference at the upper and lower interfaces at the velocity abrupt change point in the study area is obtained. The fault displacement is obtained from the seismic profile. Using forward modeling, the abnormal time difference values ​​caused by the change in formation velocity difference and the abnormal time difference values ​​caused by the change in fault displacement are obtained respectively. Then, these two abnormal time difference values ​​are substituted into the T0 structural map compensation correction formula to calculate the scattered data of the T0 structural map compensation correction ΔT0 in the time domain of the fault shadow area. Then, the plane distribution value of the correction is obtained through gridded calculation. S4. Construct the corrected time-domain T0 plot. The time-domain T0 construction diagram is corrected by using the plane distribution value of the ΔT0 correction amount, and the corrected time-domain T0 construction diagram is obtained. S5. Compile the depth domain construction graph The time-depth relationship of the study area was used to perform time-depth transformation on the corrected time-domain T0 tectonic map to obtain a depth-domain tectonic map that eliminates fault shadows. In step S1, the influencing factors include changes in formation velocity difference, changes in fault displacement, and fault dip angle.

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