(A) the use of amplitude information to detect oil and gas-bright spot technology
The so-called "bright spots" refer to some reflection "points" with relatively strong amplitude on the seismic reflection amplitude profile, also known as "hot spots". The "bright spot" of seismic profile may be caused by oil and gas reservoirs (or other factors). According to the existence and distribution of bright spots on the profile, the characteristics of reflected waves near bright spots are analyzed, and the existence of oil and gas can be directly judged by combining various formation parameter information. Therefore, "bright spot" has been widely used as a means to detect oil and gas. The physical basis of this technology is that the amplitude of seismic wave will change when it propagates in rock stratum to oil-bearing stratum.
1. Treatment of "bright spot" technology
The processing of maintaining amplitude (that is, true amplitude recovery processing) is the core of bright spot processing, and its purpose is to eliminate the influence of various factors unrelated to reflection coefficient, such as wavefront diffusion, earth absorption, the characteristics of seismograph itself, changes in excitation and reception conditions, scattering, transmission loss and so on. These are also called compensation processing.
In addition to the above compensation processing, bright spot processing also needs deconvolution operation, static and dynamic correction, horizontal superposition, filtering, migration and many other conventional processing. When doing these conventional processing, we must consider the problem of "fidelity" of waveform and amplitude, that is, to protect the original waveform and amplitude from distortion in conventional processing, which is high fidelity processing.
2. Dynamic characteristics of seismic waves in oil-bearing strata.
When the rock contains oil and gas, its physical properties such as velocity and density will change, which will lead to changes in dynamic characteristics such as amplitude, frequency and reflection polarity of seismic waves.
1) Abnormal strong reflection amplitude-bright spot
In the "bright spot" technology, the relative amplitude is used, which can be considered to be mainly related to the reflection coefficient. Assuming that the reservoir is sandstone and the caprock is shale, when sandstone contains oil and gas, its density and velocity are generally much lower than that of water and caprock, forming a strong wave impedance interface, and the reflection coefficient can be as high as 0.2 ~ 0.3, resulting in strong amplitude reflection. In general sedimentary rocks, the reflection coefficient is very small, mostly below 0. 1, or even smaller.
2) The polarity of the reflected wave is reversed
There is a negative value when calculating the reflection coefficient of oil-bearing sandstone and caprock shale.
3) There is a horizontal strong amplitude reflection segment-flat point.
Under the action of gravity, oil, gas and water in sandstone reservoir make the fluid contact surface between gas-bearing sandstone and underlying oil-bearing sandstone or water-bearing sandstone close to horizontal. Generally, the interfacial reflection coefficient between gas-bearing sandstone and oil-bearing sandstone or water-bearing sandstone is relatively large. Therefore, when the thickness of gas-bearing sandstone is large, there will be strong horizontal reflection at the bottom of gas-bearing sandstone, which is called "flat point". Its characteristics reflect the location of oil and gas accumulation in traps. If there is a flat point below the bright spot on the profile, the credibility of reservoir interpretation will be greatly improved.
4) The frequency of reflected wave is reduced.
When seismic waves pass through oil-bearing sandstone, their frequency will be obviously reduced; And the higher the incident wave frequency, the more obvious this phenomenon is. Therefore, when the frequency of reflected wave decreases significantly in the lateral direction, it may be a sign of oil and gas.
3. Features of wonderful records
Figure 6-4-7 is a typical synthetic "bright spot" record, and the corresponding oil and gas layer profile is on the left. Its records have the following characteristics: ① The records are shaped like "eyes" or "lenses". ② Outstanding strong amplitude. ③ Dipole phase characteristics. Because the reflection coefficients at the top and bottom of gas-bearing sandstone are negative and positive respectively, the polarity of the corresponding reflected waves is positive and negative, and the phase difference is 180, which constitutes dipole phase characteristics. ④ Horizontal reflection bends downward (trap caused by speed). ⑤ Weak oil-water interface reflection (commonly known as dark spots).
The 6-4-7 highlight record
Fig. 6-4-8 is a seismic profile covering 12 times in an oil and gas area. The relative amplitude of the profile is processed, and there is obvious strong amplitude at 1.3s on the profile.
4. Interpretation of "bright spot" profile
The profile obtained after high fidelity processing and compensation processing is called bright spot profile, also known as amplitude-preserving profile. The amplitude of this profile is basically only related to the reflection coefficient. At present, the "bright spot" technology is mainly used in natural gas exploration, especially offshore natural gas exploration, with good results. Practice has proved that when interpreting the bright spot profile, we can't just judge by the strong amplitude (bright spot), but we must comprehensively analyze the characteristics of various reflected waves that may appear in the bright spot profile of oil-bearing strata in order to get more accurate conclusions. Generally speaking, the strata containing oil and gas reservoirs have the above characteristics and anomalies in the bright spot records. It must be noted that the above anomalies are not only related to oil and gas, but also the existence of oil and gas reservoirs does not necessarily lead to the above anomalies. The situation is complex, and only by combining it with structural interpretation can we get more reliable results.
Figure 6-4-8 Highlighting Outline
In addition, in the interpretation of bright spot profile, special attention should be paid to identifying the so-called false "bright spots", that is, the "bright spots" are not produced by stratigraphic reservoirs.
1) factors that produce false "bright spots"
Not all "bright spots" are related to oil and gas reservoirs with industrial value, and there are three main factors that cause false bright spots on the profile.
(1) Hard stratum with high reflection coefficient. Hard strata with particularly high reflection coefficient, such as igneous rocks, conglomerate layers, hard siltstones, limestone and lignite layers, can also form false bright spots. This is because they are quite different from the upper and lower strata in density and velocity, and their reflection coefficient can reach above 0.2, which will lead to strong reflection amplitude and false "bright spots".
(2) Sandstone with low gas saturation. As long as sandstone contains gas, its velocity is obviously reduced, which makes the upper and lower interfaces become good reflection interfaces and abnormal amplitude appears. According to the actual data and theoretical research, as long as the sandstone contains 5% gas, it is similar to the amplitude anomaly caused by complete gas saturation.
(3) Adjust the amplitude of thin layer. The tuning effect of thin layer can also cause amplitude anomaly, and this anomaly is difficult to correct. Therefore, oil and gas are mixed with amplitude anomalies caused by thin layers, which undoubtedly makes it very difficult to detect oil and gas with "bright spots".
2) Identify true and false "bright spots"
At present, the only way to identify true and false "bright spots" is to comprehensively analyze seismic, geological and drilling data, that is, to comprehensively interpret them. At the same time, the bright spot record profile is combined with the general seismic record profile to analyze whether the position of "bright spot" wave is in a position conducive to the formation of gas field structural traps and whether abnormal bright spot sandstone is the main gas reservoir.
(1) comprehensive analysis. As far as the seismic method itself is concerned, information such as amplitude, velocity and frequency should be comprehensively utilized. For example, high-speed dikes and low-speed gas-bearing sandstone will produce strong reflection, the former is high-speed and the latter is low-speed. At the same time of amplitude analysis and velocity analysis, the causes of "bright spots" can be pointed out.
The interpretation of "bright spot" profile must also be combined with geological data such as drilling. Drilling and geological data can show whether the sand layer with "bright spot" anomaly is a low gas-bearing sand layer or a main gas reservoir, the former is a false "bright spot" and the latter is a true anomaly. When drilling data has confirmed that there are lignite beds or igneous rocks in a part of the profile, we will not interpret the bright spots in this part of the profile as gas fields.
(2) Calibration analysis. Calibration analysis is to draw the curve of reflection coefficient of oil-bearing sandstone with depth according to the exploration well and logging data in this area, and then analyze whether the bright spots, flat spots, dark spots and polarity conversion phenomena on the seismic profile conform to the reflection coefficient characteristics of oil-bearing strata in this area. What matches is a true anomaly, and what does not match is a false anomaly.
(3) Analog interpretation. According to the known geological and seismic data, the initial petroliferous lithologic model is assumed, and then the theoretical synthetic record is made and compared with the actual profile.
(4) Shear wave exploration. We know that the velocity of shear wave is only related to shear modulus and density, and liquids and gases have no shear modulus. Therefore, in sandstone with pores, the propagation speed of shear wave is only related to the skeleton in sandstone, but has nothing to do with the nature and content of fluid in pores. P-wave velocity is mainly related to bulk modulus. The filling of gas and liquid in rock pores will greatly change the bulk modulus of rock, which will lead to the great change of P-wave velocity. If the "bright spot" on the longitudinal wave profile is also the "bright spot" on the shear wave profile, it is a false "bright spot". For example, hard strata such as limestone with high reflection coefficient will produce longitudinal waves and shear waves at the same time. If the longitudinal wave is a "bright spot" and the transverse wave is not "bright", that is the real "bright spot".
In addition, practice shows that even if the same oil and gas reservoir is located in different areas and depths, its performance on the bright spot profile may be different. In the actual interpretation, the reflection coefficient map is made according to the specific conditions and actual data of the survey area to guide the analysis and interpretation. The so-called reflection coefficient diagram is the curve of the reflection coefficient of oil, gas and water layers with depth in the survey area. According to the data of wave velocity, density and porosity at different depths in the survey area.
(2) Detecting oil and gas by using "three instants" information of earthquake-multi-trace analysis technology.
Complex trace analysis technology is a processing technology developed since 1970s to separate the characteristic parameters (usually called three instantaneous parameters) such as instantaneous amplitude, instantaneous frequency and instantaneous phase of waves from seismic traces. Because these parameters are related to lithology, lithofacies changes and the properties of fluids contained in rock pores, it is very helpful to study them for seismic lithology interpretation and oil and gas detection.
The concept and establishment of 1. complex path
The usual actual seismic trace x(t) is a real continuous function of the time variable t, which is called the actual seismic trace. If x(t) is regarded as the real part of complex seismic trace u(t), but there is no restriction on its imaginary part R(t), such a complex trace
Principle and data interpretation of reflected wave seismic exploration
There may be countless. At present, the purpose of constructing complex seismic trace is to extract information about amplitude, frequency and phase, and complex seismic trace can not be constructed at will, that is, the imaginary part R(t) should be limited to some extent.
Because seismic records can be regarded as infinite simple harmonic vibrations with different amplitudes, periods and phases, any simple harmonic vibration can be expressed as
Principle and data interpretation of reflected wave seismic exploration
Where A(t) is the amplitude (or envelope) varying with time, ω0 is the angular frequency and φ0 is the initial phase. How to separate A(t), ω0 and φ0 from x(t)? Naturally, it is conceivable to form such a complex number u(t), the real part of which is x(t) in the formula (6-4- 15) and the imaginary part is
Principle and data interpretation of reflected wave seismic exploration
At this point, the modulus of complex trace is the amplitude of real trace:
Principle and data interpretation of reflected wave seismic exploration
Its amplitude angle is the phase of the real trajectory:
Principle and data interpretation of reflected wave seismic exploration
Thus, the frequency and initial phase can be obtained:
Principle and data interpretation of reflected wave seismic exploration
It can be seen that although we construct the real trace into a complex trace according to certain laws, we have not changed the physical properties of the problem, but only made a formal transformation. After this transformation, we can easily separate the parameters we need from the complex trajectory.
Comparing the real part formula (6-4- 15) and the imaginary part formula (6-4- 16) of this complex, one is a cosine function and the other is a sine function, and their amplitudes are the same, only the phase difference is 90. It can be seen that the imaginary part of the complex trace is obtained from the actual seismic record (real seismic trace) after 90 phase shift, and the real trace is a Hilbert transform in mathematics. The original seismic trace x(t) is called real seismic trace, and its Hilbert transform R(t) is called virtual seismic trace. Because the phase angles of x(t) and R(t) are orthogonal, the virtual seismic trace is also called orthogonal trace.
The relationship among complex seismic trace, real seismic trace and virtual seismic trace is shown in Figure 6-4-9.
Figure 6-4-9 Complex Seismic Trace Map
2. Calculation of three instantaneous parameters
Figure 6-4- 10 Complex Representation
After the complex seismic trace u(t) is established from the real seismic trace x(t) and its Hilbert transform R(t), it is easy to calculate three instantaneous parameters. According to the analysis of complex numbers, any complex number can be expressed as a vector on the complex plane, as shown in Figure 6-4- 10. In the figure, the horizontal axis represents the real part of the trace, and the vertical axis represents the imaginary part of the trace. If the angle between the vector u(t) of the complex trace and the real axis is θ(t), the real part and imaginary part of the complex trace can be expressed as formula (6-4- 15) and formula (6-4- 16) respectively. Instantaneous amplitude is defined as
Principle and data interpretation of reflected wave seismic exploration
Or reflection intensity, which is the envelope of real seismic trace x(t).
Instantaneous phase is defined as
Principle and data interpretation of reflected wave seismic exploration
The rate of change of instantaneous phase over time is defined as instantaneous frequency, i.e.
Principle and data interpretation of reflected wave seismic exploration
Instantaneous amplitude and instantaneous phase are two independent quantities, which are only related to time t.
The extraction results of three instantaneous parameters can be displayed in many ways, and the most convenient and intuitive is color display.
3. Using three instantaneous parameters to detect oil and gas.
Three instantaneous parameters, also known as earthquake instantaneous information, correspond to the geological characteristics of the underground. Therefore, they can be used to achieve the purpose of lithology and oil and gas detection.
Generally speaking, instantaneous amplitude (also called reflection intensity and amplitude envelope) reflects the instantaneous change of seismic wave energy; It has nothing to do with the phase of seismic wave, but its intensity is closely related to the reflection coefficient of stratum interface. When the interlayer lithology on the geological profile changes strongly or contains gas, it has obvious strong amplitude characteristics on the instantaneous amplitude profile. When the rock thickness or lithology changes laterally, amplitude anomalies will appear in the instantaneous amplitude profile. In the fault or gas-bearing marginal zone, the instantaneous amplitude suddenly changes. Therefore, it can be used to infer geological bodies related to lithology. Such as unconformity, local faults, thin beds, oil and gas-rich zones, etc.
Instantaneous frequency anomaly reflects the thickness or lithology change of strata. The change of the thickness, lithology and lithofacies of rock stratum will lead to the gradual change of instantaneous frequency information along the lateral direction. Faults, unconformity or the edge of gas-water and oil-water interface will cause sudden changes in instantaneous frequency information. Porous or gas-bearing rocks will cause the instantaneous frequency to decrease. Generally speaking, the instantaneous frequency should decrease with the increase of time, but there may also be local low-frequency anomalies in the middle, which may be the performance of composite pulses composed of adjacent interfaces.
The consistency of instantaneous phase is consistent with the continuity of formation. In a geological section, the instantaneous phase can reflect the shape and continuity of the reflection interface, no matter whether the physical differences between rock layers are large or small, and whether the reflection amplitude is strong or weak. In the case of fault, unconformity or overlap, the instantaneous phase characteristics are consistent with the formation structure. Therefore, instantaneous phase information is very useful for dividing the boundary of earthquake sequence and determining the contact relationship between layers.
For reservoirs, the characteristics of seismic instantaneous information are obvious in oil-bearing or gas-bearing layers. Its performance is that there is a relatively stable low-frequency strong amplitude anomaly under the background of high-frequency weak amplitude, and its edge is accompanied by polarity inversion. These characteristics of instantaneous information can be used as a sign to detect oil and gas.
(c) Oil and gas velocity analysis using multi-layer velocity information to detect different layers.
DIVA (micro-layered velocity analysis) technology is a direct method to find oil and gas proposed by American geophysicist Neidell in 1985. The basic idea of DIVA technology to judge lithology and oil and gas is: In general, the stacking speed increases with the increase of the depth of reflection layer. If the porosity of deep sandstone or carbonate rocks increases and there are oil and gas, the velocity of the lower layer will be lower than that of the upper layer, resulting in abnormal velocity. Based on this, low velocity anomaly can be used to judge lithology and oil and gas situation. The specific method is as follows: firstly, wavelet shaping and amplitude processing are carried out on seismic records, and then the velocity values of main reflection layers are accurately extracted from each * * * central point trace set, and the velocity variation curves of each reflection layer in the lateral direction are made. As shown in Figure 6-4- 1 1, the transverse velocity curves of two layers to be compared are drawn on the same coordinate map (the lower part of Figure 6-4- 1 1), and the deep velocity is lower than the shallow velocity, which is an anomaly. In order to avoid accidental error interference, multi-layer comparison should be carried out. If there are four layers: A, B, C and D, we can compare them in the following order: A/B, A/C, A/D, B/C, B/D and C/D. Draw a comparison curve on the diagram, check the abnormal law and make a judgment, as shown in Figure 6-4- 12.
Fig. 6-4- 1 1 Velocity Contrast Curve of Different Layers
This technique has been successfully applied to the judgment of reef and tower reef in the United States.
(d) Using the synthesis of various seismic parameters to determine the oil and gas enrichment zone-HCI technology.
HCI (Hydrocarbon Indicator) technology is a method to determine the hydrocarbon-rich zone by comprehensively using various seismic characteristic parameters indicating the existence of hydrocarbons. This technology is to extract a series of seismic characteristic parameters from a certain target layer one by one on the seismic profile, find its residual curve and comprehensive curve, and interpret these curves to directly detect oil and gas.
There are many characteristic parameters related to oil and gas, such as vibration energy in time domain, total energy in frequency domain, energy in low frequency band, energy ratio between low frequency band and wide frequency band, central frequency (dividing the region under the energy spectrum curve into two equal parts), peak frequency (similar to the main frequency), velocity (mainly layer velocity), absorption coefficient, instantaneous parameters and so on. The extracted parameters are best uncorrelated, so the statistical effect is the best. The extraction methods of some parameters are given below.
Figure 6-4- 12 DIVA analysis diagram example
1. feature parameter extraction
There are many characteristic parameters related to oil and gas, which are as follows.
1) vibration energy
As shown in figure 6-4- 13, select a time window t at the destination layer and press the following formula to calculate:
Principle and data interpretation of reflected wave seismic exploration
Where I is the track number, tm is the center time, xi(t) is the sample value of the ith track at time t, and t is generally taken as100 ms.
2) Total broadband energy in frequency domain
As shown in figure 6-4- 14, select a time window t at the destination layer. After Fourier transform, find a broad band (fWL-fWH) in frequency domain to sum the energy, that is, calculate the area under the energy spectrum curve in this band. The calculation formula is
Principle and data interpretation of reflected wave seismic exploration
Figure 6-4- 13 Time Domain Energy
Fig. 6-4- 14 total energy in frequency domain
In ...
Ai (female) =Xi (female) 2
I is the channel number, and Xi(f) is the frequency spectrum of the i-th recorded signal segment. (fWL-fWH) is generally 5hz ~ 100hz.
3) Energy in low frequency band.
In frequency domain, the calculation is carried out with a lower frequency fL and a narrower width (fLL-fLH).
Principle and data interpretation of reflected wave seismic exploration
As shown in figure 6-4- 15. FL is generally 10Hz, 12Hz or 15Hz, while (fLL~fLH) is generally 5Hz.
4) Energy ratio of low frequency band to wide frequency band
Principle and data interpretation of reflected wave seismic exploration
As can be seen from figure 6-4- 15, this ratio is equal to the ratio of the area b under the energy spectrum curve to the total area (A+B+C). This ratio reflects the number of low-frequency components, and its increase indicates the decrease of high-frequency components. The rapid decrease of high frequency components is related to the strong absorption of gas-bearing sandstone.
Figure 6-4- 15 frequency domain energy relationship
5) Center frequency
The frequency at which the area under the energy spectrum curve is divided into two equal parts is called the center frequency fM(i) (Figure 6-4- 16).
Figure 6-4- 16 center frequency
Principle and data interpretation of reflected wave seismic exploration
6) Peak frequency
The frequency corresponding to the maximum value of Ai(f) is the peak frequency fH(i), as shown in Figure 6-4- 17.
Figure 6-4- 17 peak frequency
7) Speed
The velocity closely related to oil and gas is mainly interval velocity vT(i). Because HCI requires high speed accuracy, it must be extracted by high precision method. For example, the influence of reflection point dispersion and the modification of Dix formula when the interface is tilted and bent are considered in the calculation.
2. Making residual curve and composite curve
After various characteristic parameters are extracted by different methods, the statistical average value of a parameter on a measuring line (or measuring area) is calculated first, and then the values of each point are subtracted from the average value to get the residual curve. These residual values are equivalent to abnormal values, so it is more meaningful to analyze them.
Let,,, and be the average of each characteristic parameter, then the average residual curve of each HCI is
Principle and data interpretation of reflected wave seismic exploration
After calculating the average residual curve, the comprehensive curve reflecting oil and gas anomalies can be obtained by adding them.
Principle and data interpretation of reflected wave seismic exploration
A survey line (or a survey area) cannot only use a statistical average to reflect the regional variation of parameters. This regional change is caused by non-oil and gas factors and should be eliminated. Therefore, the more accurate method is to find the trend curve or graph of a parameter through trend analysis, and then subtract the calculated values of each point from the values obtained by trend analysis to get the abnormal values.
Figure 6-4- 18 is the average residual curve and trend residual curve of the calculated amplitude energy in time domain and total energy in frequency domain. As can be seen from the figure, where the trend changes greatly, the regional change is removed from the trend residual curve, which not only improves the resolution, but also highlights the oil and gas anomalies. Where the trend changes little, there is little difference between the two.
In addition, weighted addition can be used instead of simple addition when calculating HCI comprehensive curve. The relationship between weight data and different characteristic parameters of oil and gas is determined by the compactness and accuracy of calculation. This comprehensive result is more reliable.
3. Interpretation of abnormal phenomena
When explaining anomalies, we can't rely solely on a residual curve or a comprehensive curve. Comprehensive analysis and repeated comparison are needed. Generally speaking, when oil and gas are reflected in the comprehensive curve and more than five remaining curves, the interpretation results are more reliable. In addition, it should be noted that the importance of each parameter is not equal in all exceptions. Generally speaking, the lateral variation of amplitude is the most important, followed by the lateral variation of interval velocity and frequency. The analysis must be considered comprehensively according to its importance, and a more reliable conclusion can be drawn.
Figure 6-4- 18 Amplitude Energy and Frequency Domain Total Energy Curve
In addition, anomaly interpretation must follow a very important principle, that is, from known to unknown, from point to surface step by step. Find out the relationship between various anomalies and oil and gas distribution from the known oil and gas reservoir area or the location of wells, and then extend it to the unknown area to predict by point-to-area.