(Guangzhou Marine Geological Survey Guangzhou 5 10760)
Introduction to the first author: Sha Zhibin (1972.4—), male, senior engineer, mainly engaged in petroleum geology and gas hydrate research.
In the process of gas hydrate seismic data interpretation, it is difficult to determine the positions of BSR, hydrate metallogenic belt and free gas belt on conventional (superimposed and migrated) seismic profiles. Through years of practice, the author thinks that AVO (amplitude and offset) inversion, wave impedance inversion, seismic instantaneous attribute and energy half-life profile can better reveal the geophysical anomaly characteristics of hydrate, thus providing a strong basis for identifying hydrate and dividing its existing area.
AVO inversion of natural gas hydrate; Wave impedance; Instantaneous profile inversion; Energy half-life profile
1 preface
Since 1999, when China first discovered the seismic sign of natural gas hydrate on the north slope of Nanhai -BSR, China has strengthened the research on hydrate resources. At the same time, Guangzhou Marine Geological Survey conducted a geological and geophysical survey on the northern slope of the South China Sea for the purpose of finding hydrate resources, and found a large number of important geophysical and geochemical evidences, such as BSR, abnormal methane content, abnormal chloride and sulfate ion concentrations, carbonate crusts and methane reefs, with obvious characteristics. In 2004, through the cooperation with the German ship sousaphone, the authigenic carbonate area closely related to hydrate-"Jiulong Methane Reef" was discovered for the first time on the northern slope of the South China Sea. The above evidence shows that there is a great possibility of hydrate in this area, and the reserves are considerable. With the deepening of hydrate research, many problems have been encountered, such as how to determine the BSR with unclear characteristics, how to determine the hydrate metallogenic belt, how to determine the location of free gas belt and so on.
According to the experience of hydrate seismic data interpretation in recent years, the author thinks that AVO inversion, wave impedance inversion, seismic instantaneous attribute and energy semi-attenuation profile can better reveal the comprehensive seismic anomaly characteristics of hydrate when it is difficult to distinguish the abnormal characteristics of hydrate by using conventional seismic profiles (Li Zhengwen et al., 1988). In the process of detecting hydrate by analyzing various seismic information, the existing characteristics of hydrate and the possible location of mineral deposits can be accurately determined by comprehensively utilizing various seismic profiles (Yang Muzhuang, 2000). Taking the A line in the southern continental slope area as an example, the application of various seismic attribute profiles in gas hydrate identification is illustrated.
2 AVO inversion
AVO (amplitude varying with offset) is a seismic exploration technique that uses amplitude varying with offset to analyze and identify lithology and oil and gas reservoirs. The analysis method is to analyze the variation characteristics of seismic reflection amplitude with offset before stack, so as to infer the properties and lithology of pore fluid in rock. The purpose of AVO data processing is to provide reliable and sufficient data for interpreters to observe and measure the variation of amplitude with offset or incident angle (Lei Huaiyan et al., 2002; Song Haibin et al., 2003; Sha Zhibin et al., 2004). The core of processing quality requirements is to recover and protect amplitude information as much as possible.
The test shows that the intercept attribute (AVO 1) profile, gradient and intercept profile, correlation coefficient product attribute (AVO4) profile, gradient and intercept sign product attribute (AVO6) profile and fluid factor attribute (AVO9) profile have obvious recognition effects on BSR, hydrate and its free gas, which can reflect the enrichment degree and distribution of hydrate in hydrate metallogenic belt. Therefore, it is necessary to explain these four AVO attribute profiles.
2. 1 intercept attribute (AVO 1) configuration file
Intercept profile (P wave superposition profile). Compared with the conventional stacking profile, the P-wave profile is closer to the zero offset profile, which reflects the amplitude stacking of seismic waves at normal incidence. A large value of I means that the velocity difference between upper and lower P waves is large, and vice versa. It can be clearly seen from the 5438+0 section in Figure 65 that the polarity of BSR is opposite to that of the seabed, so it is mainly used to identify BSR.
Profile characteristics of AVO 1 A line in figure1a
Profile characteristics of AVO 1 A line in figure1a
2.2 Product attributes of gradient, intercept and correlation coefficient (AVO4) profile
Product profile of gradient, intercept and correlation coefficient (I * G * correlation coefficient). It can be seen from this attribute profile that the strong reflection is the reflection of the free gas cap (Figure 2), and there is obvious gas-bearing anomaly under BSR, so this profile can be mainly used to detect the free gas layer.
2.3 Gradient and Intercept Symbol Product Attribute (AVO6) Profile
Product introduction of gradient and intercept symbols (symbol (I)*G). In the profile of Figure 3, the peak represents the gas cap, obvious gas-bearing anomalies can be seen below BSR, and the development thickness of strong reflection represents the development thickness of free gas, so this profile is mainly used to detect free gas layers.
Fig. 2 A AVO4 profile characteristics of survey line
Fig. 2 A AVO4 profile characteristics
Fig. 3 A profile characteristics of survey line AVO6
Fig. 3 A section characteristics of AVO6
2.4 fluid factor attribute (AVO9) profile
AVO9 profile is a fluid factor profile. Because a large amount of gas combines with a small amount of water to form hydrate, a large amount of water is adsorbed in the sedimentary layer. Compared with the sedimentary layer without hydrate, the gas content is less, while the latter has more gas content. In the profile of Figure 4, the hydrate-bearing metallogenic belt is basically a zero-value belt, so it can be mainly used to determine the hydrate metallogenic belt.
Fig. 4 A AVO9 profile characteristics of survey line
Fig. 4 characteristics of AVO9 section on line 4 A
Three-wave impedance inversion
At present, there are two main wave impedance inversion methods used at home and abroad, one is sparse pulse inversion, and the other is model constrained inversion. Sparse pulse inversion can directly extract reflection information from seismic information, and the reliability of inversion depends entirely on the quality of seismic data itself, so the seismic data used for inversion should have wider frequency band, lower noise, relative amplitude preservation and accurate imaging. The main advantage of this method is that it can obtain the reflection coefficient of broadband, solve the calibration problem of seismic records well, and be faithful to seismic data in the inversion process, thus making the wave impedance model obtained by inversion more realistic. In the case of no well, this method can establish a pseudo well through velocity information and carry out no well inversion.
Because there is no drilling data in the investigation area, sparse pulse wave impedance inversion method is adopted in the processing based on the special properties of hydrate. The impedance value shown in the relative wave impedance profile is relative, and the seabed interface shows strong reflection and high impedance; The weak reflection above BSR shows that the wave impedance difference between hydrate and surrounding rock is small; No reflection indicates that the purer the hydrate, the richer it is. Because the relative wave impedance obviously reflects the hydrate, the results obtained are more in line with the actual geological conditions. In the profile of fig. 5, the conversion surface between weak wave impedance and strong wave impedance is BSR position, and the top boundary of weak wave impedance is the top of hydrate metallogenic belt. Therefore, this profile can be mainly used to determine BSR and hydrate metallogenic belt.
4 seismic instantaneous attribute profile
The attributes of multi-channel superimposed data include geometry, dynamics, kinematics and statistical characteristics. Some properties are sensitive to rock reservoir environment, while others are sensitive to fluids in reservoir pores. In practical application, the most important thing is to master the hydrate sensitive profile. Calculating complex trace attributes is basically a transformation, which decomposes amplitude and angle information (frequency and phase) and displays them independently. This information on seismic profile is generated by mathematical calculation, which is a display method of highlighting amplitude or angle at the expense of ignoring other parts. The profile produced by complex seismic trace analysis is the well-known instantaneous attribute profile.
Fig. 5 A wave impedance profile characteristics of survey line
Fig. 5 A characteristics of line wave impedance profile
4. 1 instantaneous amplitude curve
Instantaneous amplitude profile is also called reflection intensity profile, and its amplitude is the amplitude envelope of reflection, which makes strong reflection stronger and weak reflection weaker, reflecting the instantaneous change of seismic wave energy, thus highlighting the strong reflection on BSR surface and the strong reflection amplitude where hydrate develops. In the part of fig. 6, the boundary of color conversion is the BSR position, so the BSR can be mainly determined by this part.
Fig. 6 A instantaneous amplitude profile characteristics of survey line
Fig. 6 characteristics of instantaneous amplitude curve of line a
4.2 instantaneous frequency curve
Instantaneous frequency is a measure of the center frequency (average value) at the initial time corresponding to the complex energy density function (i.e. power) of the signal at a given time. It can clearly reflect the range of free gas enrichment area. When the free gas develops to a certain thickness and range, due to the existence of the free gas, the high-frequency components of the reflected wave are absorbed in large quantities, so there is obviously a low-frequency phenomenon in the free gas distribution area. According to this principle, it is easier to divide the distribution range of free gas. In the cross section of fig. 7, the phenomenon of high-frequency strong absorption indicates the existence of free gas, so the cross section can be mainly used to determine the free gas zone.
Fig. 7 A instantaneous frequency profile characteristics of survey line.
Fig. 7 instantaneous frequency distribution characteristics of line a
4.3 Energy Half-life Curve
The energy semi-attenuation curve is a measure of the degree of energy attenuation after the reflected wave passes through the stratum. This profile highlights strong reflection and high frequency absorption, which is intuitive for studying BSR and free gas (Figure 8). In the profile of Figure 8, the phenomenon of high-frequency strong absorption clearly indicates the existence of free gas, and the boundary of color conversion is the location of BSR, so the profile can be mainly used to determine BSR and free gas zone.
Application of seismic attribute profile
Due to the nature of natural gas hydrate and the particularity of mineralization, important identification marks will be produced in various seismic profiles. Through the comparative interpretation of the survey area's routine, intercept attribute (AVO 1) profile, gradient and intercept, correlation coefficient product attribute (AVO4) profile, gradient and intercept sign product attribute (AVO6) profile and fluid factor attribute (AVO9) profile, wave impedance, instantaneous frequency, instantaneous amplitude and energy half-life, etc. Taking Line A in the southern continental slope area as an example, the attached figure 1 ~ 8 illustrates the response characteristics of various attribute profiles in hydrate seismic detection. In the process of interpretation, through the comprehensive analysis and study of various seismic attributes, BSR, hydrate metallogenic belt and free gas layer under BSR can be well identified (Zhang Optics et al., 2003).
Fig. 8 A Profile characteristics of half attenuation of line energy.
Fig. 8 characteristics of energy half-life of line a
5. Identification of1bsr
In the conventional profile, when the BSR occurrence and formation occurrence are oblique at a certain angle, the BSR reflection is easy to identify. When the occurrence of BSR is parallel to the formation occurrence, it is not easy to judge. Because the intercept attribute (AVO 1), wave impedance, instantaneous amplitude and energy half-life are sensitive to BSR response, the BSR on the intercept attribute (AVO 1) profile is obviously opposite to the seabed polarity; The transition surface between weak wave impedance and strong wave impedance on wave impedance profile is the development position of BSR. On the instantaneous amplitude profile, the position of BSR is at the obvious transition junction of two colors; When the energy is semi-decayed, the junction of color conversion on the profile is also BSR position. Therefore, these attribute profiles can be mainly used to identify BSR, and it is easier to identify BSR.
5.2 Identification of hydrate metallogenic belt
Hydrate metallogenic belt is usually a geological body with relatively uniform physical properties, and it shows a weak amplitude reflection belt on seismic profile, which is called blank zone. Generally speaking, the amplitude blank zone is related to BSR, which gradually transits vertically with seabed sediments and suddenly contacts with the free gas zone bounded by BSR below. Generally speaking, the reflection amplitude is related to the hydrate content. The higher the hydrate content, the weaker the amplitude and the higher the blank degree. On the other hand, if the formation contains only a small amount of hydrate, it only shows a decrease in amplitude (Ecker et al., 2000; Miller et al., 199 1).
1) because the elastic parameters of hydrate are greater than those of water and gas. When the pores of sediments are filled with hydrates, their physical properties are obviously different from those of surrounding rocks. Therefore, for seismic technology, the hydrate metallogenic belt is a physical belt with obvious seismic characteristics, which can be revealed to some extent in various seismic attribute profiles.
2) Due to the filling and cementation of hydrate, the hydrate-bearing zone will be a relatively uniform geological body. This filling and cementation reduces the difference of wave impedance between layers in the metallogenic belt, thus weakening the reflection in the metallogenic belt and forming an amplitude gap.
3) In hydrate-bearing sedimentary strata, the reflection frequency of seismic wave has the characteristics of relatively high frequency, but it is strongly attenuated in free gas-bearing area.
4) In the wave reflection characteristics of the hydrate metallogenic belt, because the seabed surface and BSR interface are both strong wave impedance surfaces, it can be inferred theoretically that the top boundary of the hydrate layer is a weak wave impedance interface relative to BSR.
Comprehensive analysis shows that fluid factor and wave impedance are sensitive to hydrate response: on the profile of fluid factor (AVO9), the hydrate-bearing metallogenic belt is basically a zero value zone, that is, a blank zone; On the wave impedance profile, the top boundary of weak wave impedance is the top of hydrate metallogenic belt. Therefore, these two attribute profiles can be mainly used to determine the hydrate metallogenic belt.
5.3 Identification of Free Gas Layer
Because gradient and intercept, correlation coefficient product (AVO4), gradient and intercept sign product (AVO6), instantaneous frequency and energy half attenuation are sensitive to the response of free gas, strong reflection can be regarded as the reflection of free gas cap in the profile of gradient and intercept and correlation coefficient product property (AVO4), and there are obvious gas-bearing anomalies under BSR. It can be seen from the section of AVO6 that the peak represents the gas cap, there is obvious gas-bearing anomaly below BSR, and the thickness of strong reflection represents the development thickness of free gas. The phenomenon of high-frequency strong absorption can be seen in both instantaneous frequency and energy semi-attenuation profiles. Frequency information is related to strata, sedimentation, lithology and fluid, and the strong absorption of high-frequency information is related to the abundance of gas reservoirs. The strong absorption of high frequency at BSR often indicates the existence of free gas. The phenomenon of high-frequency strong absorption is obvious, and the estimated free gas abundance is high, which provides sufficient gas source guarantee for the formation of hydrate. Therefore, these four attribute profiles can be mainly used to detect the free gas layer in the lower part of BSR.
6 know
To sum up this article, the main understandings are as follows:
1) In order to accurately identify the abnormal characteristics of hydrate, the profile must be specially processed on the basis of conventional seismic data processing, and its various attribute profiles can be comprehensively used to better identify BSR, hydrate metallogenic zone and free gas zone;
2) Four attribute profiles (AVO 1), wave impedance, instantaneous amplitude and energy half-life are sensitive to BSR response, which are mainly used to identify BSR;
3) Fluid factor (AVO9) and wave impedance are sensitive to hydrate response, and the hydrate metallogenic belt is mainly determined by using these two attribute profiles;
4) Four attributes such as gradient and intercept, correlation coefficient product (AVO4), gradient and intercept sign product (AVO6), and attribute profile when instantaneous frequency and energy are half decayed are sensitive to the response of free gas. These four attribute profiles are mainly used to detect the free gas layer in the lower part of BSR.
5) There are many seismic attribute profiles of hydrate, and how to use them to explain the seismic characteristics of hydrate needs further research and verification.
refer to
Lei Huaiyan, Zheng Yanhong and Wu Baoxiang. 2002. Natural gas hydrate exploration method-BSR applicability analysis [J]. Offshore oil, 1 14, 1 ~ 8.
Li Zhengwen, Zhao Zhichao. 1988. seismic exploration data interpretation [M]. Beijing: Geological Publishing House.
Sha Zhibin, Yang Muzhuang, Liang Jinqiang, et al. 2004. Characteristics of BSR reflection wave and its application in gas hydrate identification [J]. Geological research of South China Sea (15). Beijing: Geological Publishing House, 55 ~ 6 1.
Song Haibin, Zhang Ling, Jiang Weiwei, et al. 2003. Geophysical research on marine natural gas hydrate (Ⅲ): seafloor-like reflection [J]. Geophysical progress,18 (2):182 ~187.
Yangmuzhuang 2000. Seismic identification mark of marine gas hydrate [J]. Geological research of South China Sea (12). Beijing: Geological Publishing House, 12, 1 ~ 7.
Zhang Optics, Huang Yongyang, Chen Bangyan, et al. 2003. Seismology of natural gas hydrate in sea area [M]. Beijing: Ocean Press.
Chen Jianmin, Chen Jianmin. 2000. Quantitative study of natural gas hydrate and free gas [J]. China Geological Publishing House, 2000. Geophysics, 65,565 ~ 573.
Miller J J, Ming W L, Feng Huene R. 199 1. Analysis of bottom reflection wave in Peru gas hydrate zone [J]. Acta Geologica Sinica, 75,910 ~ 924
How to judge the seismic characteristics of natural gas hydrate from various attribute profiles
Sha Zhibin Gong Yuehua Liang Jinqiang
(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)
Abstract: In the interpretation of gas hydrate profile, it is difficult to distinguish BSR, gas hydrate layer and free gas layer from superposition and migration profile. Through the practice in recent years, we think that AVO inversion profile, wave impedance inversion profile, instantaneous profile and half-time energy profile can better show the physical and geographical characteristics of anomalies. Therefore, we can use these profiles to judge the seismic characteristics of natural gas hydrate and its existing area.
Keywords: AVO inversion wave impedance inversion instantaneous profile energy half-life profile of natural gas hydrate