Seismic identification marks of natural gas hydrate are of great significance to the exploration and research of marine natural gas hydrate. Based on foreign exploration research results, the seismic identification marks and characteristics of marine natural gas hydrate D, such as BSR, amplitude blank, negative polarity and abnormal high-speed zone, are analyzed in detail.

Keywords natural gas hydrate; Identification mark; Submarine-like reflection wave

1 preface

Natural gas hydrate has aroused great interest and concern because of its huge reserves and wide distribution, and is generally considered as a huge energy mineral with more potential than conventional oil, natural gas and coal. Natural gas hydrate is an ice-like mixture formed in a specific low temperature and high pressure environment, which is formed by complexing water molecules with gas molecules (mainly methane). Natural gas hydrates are distributed all over the world, but due to the pressure and temperature conditions and the limitation of gas content, they are mainly distributed in permafrost and deep-water oceans, especially in continental slopes and continental uplift sediments at the outer edge of the continental shelf with a water depth of more than 300 ~ 500 m [1]. According to statistics, natural gas hydrate has been found directly or indirectly in 52 sea areas around the world, among which 16 cores containing natural gas hydrate have been found [2]. The United States, Russia, Canada, Germany, Japan and other countries have done a lot of work in the investigation and research of natural gas hydrate, but China's work in this field has just started in recent years, and soon set off a wave of exploration and research of natural gas hydrate. At the end of 1999, the relevant departments conducted an experimental seismic survey of offshore natural gas hydrate. After data processing, interpretation and analysis, it is preliminarily confirmed that there is natural gas hydrate as a potential energy mineral in China sea area. Yao Bochu (1998) pointed out for the first time that there may be natural gas hydrate in the northern South China Sea through careful analysis of existing multi-channel seismic profiles and sonar buoy data [3]; Yang Muzhuang and others (1999) believe that the South China Sea has favorable conditions for the formation of natural gas hydrate, especially in the wide deep water and continental slopes and ridges on the north and south sides, which are likely to contain abundant natural gas hydrate [4]; Zhang Optics et al. (2000) analyzed the seismic data in the southern basin of Gaby, and found that this area has the seismic characteristics of natural gas hydrate [5]. So, what is the sign of gas hydrate in the ocean? How to use seismic exploration data to find natural gas hydrate? This is the primary problem in the investigation and study of natural gas hydrate. Therefore, according to the foreign exploration research results, the seismic identification marks and characteristics of marine natural gas hydrate are analyzed in detail, such as BSR, amplitude blank, polarity inversion and high-speed zone. I hope it can provide useful reference for the investigation and research of marine natural gas hydrate in China.

2 bottom analog reflectors) —BSR

A continuous reflection wave with strong amplitude often appears on the seismic reflection profile of gas hydrate-bearing strata, which is roughly parallel to the seabed reflection wave, so it is called submarine simulated reflector (BSR), which roughly represents the bottom boundary of hydrate stability zone. Foreign research results show that the bottom boundary of hydrate stability zone represents a specific pressure and temperature surface. Because the formation pressure under the seabed changes little, but the ground temperature changes greatly (there is a ground temperature gradient), the fluctuation of the seabed will cause the fluctuation of the formation isothermal surface, thus forming the bottom boundary of the hydrate stability zone. Therefore, BSR is generally parallel to the seabed topography, but inclined to the stratum plane (when the stratum plane is inclined to the seabed) or parallel (when the stratum plane is parallel to the seabed).

Regarding the formation and evolution of BSR, Kvenvolden( 1993) thinks that there are two modes. The first model: methane is generated by microorganisms in the hydrate stability zone (Claypool and Kaplan, 1974), and hydrate formation and deposition occur simultaneously. When the methane hydrate zone thickens and deepens, its bottom boundary eventually sinks to the temperature range that causes hydrate instability, and free gas can be generated in this range. If there are suitable migration channels, these gases will migrate back to the overlying Kvenvolden. As a result of the model, hydrate will be generated in the whole hydrate stability zone, and there may or may not be free gas below BSR. In the second mode, methane produced by microorganisms in the bottom pore fluid moves upward into the hydrate stability domain to form hydrate (Hyndman and Davis, 1992)[9]. The result of this model is that hydrate accumulates at the bottom of the stable region near BSR, and there is no free gas below BSR.

Although BSR is simply parallel to the seabed in shape, its amplitude and continuity are often variable, and various BSR reflections appear on seismic profiles. According to the intensity and continuity of reflected wave amplitude, BSR can be divided into three categories: S-BSR (strong BSR), W-BSR (weak BSR) and I-BSR (presumed BSR)(Tucholke et al.,1977; Kvenvolden， 1993).

S-BSR has strong amplitude and is easy to identify on seismic profile. Most S-BSR is a strong amplitude valley-peak combination (bimodal, appearing in pairs), rather than isolated peaks and valleys. Bimodal waveform is a typical seismic response of low impedance thin layer in high impedance layer. W-BSR is characterized by a valley-peak with weak amplitude. Because of its low amplitude, the West BSR is usually difficult to identify in seismic profiles unless it is adjacent to the South BSR. However, W-BSR is widespread.

Figure 1 is a 6-channel superimposed shallow seismic profile of the black ridge along the southeast coast of the United States. S-BSR is shown between B and C, which is a typical BSR in deep-sea sediments (Shipley et al.,1979; Tommy Tam and Paul, 1983)[8]. In the figure, W-BSR is between A and B, adjacent to S-BSR, showing weak amplitude valley-peak, and the apparent reflection coefficient of valley is less than -0.05.

It is inferred that BSR(I-BSR) is a discontinuous reflection interface, which is located near the theoretical bottom boundary of hydrate stability zone, usually the bottom boundary of blank zone. Fig. 2 is also a seismic profile from the Black Ridge on the southeast coast of the United States, showing the predicted BSR(I-BSR), that is, the relationship between D and E. Because the apparent reflection coefficient of I-BSR is usually less than -0.05, it is difficult to directly identify I-BSR. The possibility of the existence of I-BSR is mainly inferred from the event axis of abnormal strong amplitude reflection under I-BSR. On seismic profile, I-BSR is usually interpreted as a line (as shown by the connecting line between D and E in Figure 3), which is connected with the strong reflection rising and falling in the hydrate zone, and the strong reflection is inclined in phase. These inclined strong reflection events are caused by gas filling the formation, and the upward inclined end of the formation is blocked by hydrate cemented sediments (Lee et al., l993)[7].

Fig.1seismic reflection profile of south and west BSR (Blackridge, USA)1seismic profile of south and west BSR (according to Lee et al., 1993).

Fig. 2 I-BSR seismic reflection profile (American black ridge) and fig. 2i-BSR seismic profile (according to Lee et al., 1993).

3 amplitude blank

In hydrate-bearing strata, due to the increase of seismic wave velocity, the reflection coefficient between them and the underlying strata increases, and a corresponding strong reflection interface appears on the seismic profile, while the hydrate-bearing strata above them become "uniform" because the sediment gap is cemented by hydrate, and the wave impedance difference decreases, so the seismic reflection profile usually presents a weak amplitude or an amplitude blank area. Void is proportional to the amount of cemented hydrate in pores. The higher the hydrate content, the weaker the amplitude and the higher the blank degree [8]. There are weak amplitude or amplitude blank bands in all reflection layers, which is interpreted as above BSR in the study area. Therefore, the amplitude blank phenomenon over BSR is to detect hydrate deposition, especially in the southern BSR area, where there is no obvious seismic indication of hydrate deposition. At the same time, the amplitude information of hydrate deposition on seismic profile can provide a method to estimate the amount of natural gas hydrate (Lee, 1993).

It should be noted that the amplitude blank area on seismic profile is not always related to hydrate. There are many factors that cause the amplitude blank zone on the seismic profile, such as sedimentary environment. However, most seismic profiles showing the existence of BSR also show amplitude blank zones in different degrees, and the blank zones are concentrated above BSR.

Fig. 3 is a single channel seismic reflection profile across the top of the Black Sea platform, showing clear S-BSR reflection wave and W-BSR reflection wave. There is a large reflection blank or weak reflection area above BSR, and the blank area is considered as a hydrate cemented sediment with good continuity. Blank area basically represents the thickness of hydrate stability zone (Dillon, 1993).

Fig. 3 shows a seismic profile showing blank reflection (according to Dillon, 1993).

4 negative polarity

The wavelet seismic reflection characteristics of hydrate zone usually show negative polarity, that is, the so-called polarity inversion (opposite to seabed reflection), which has a large reflection coefficient (Shipley et al.,1979; Li et al., 1993). Fig. 4 is a wavelet waveform with a clear BSR display in a section of the Black Ridge off the southeast coast of the United States, which has a typical polarity reversal feature, that is, the polarity of the BSR reflection wave is just opposite to that of the seabed reflection wave (the seabed waveform is right and the BSR waveform is left). In order to analyze the reflection characteristics conveniently, the maximum amplitude on the right side is regarded as a peak (a reflection interface from low resistance layer to high resistance layer), and the maximum amplitude on the left side is a trough (a reflection interface from high resistance layer to low resistance layer). This abnormal reflection interface shows a pair of valley-peak waves with strong amplitude, and the apparent reflection coefficient of the valley is greater than -0. 1. This pair of waveforms is a typical low-impedance thin-layer top-down high-impedance seismic response, which is probably the free gas layer below the hydrate layer. The reflectivity of the whole wavelet is-0.12 0.04, and the negative reflectivity indicates that the high-speed layer covers the reflection interface of the low-speed layer.

Fig. 4 bsr waveform (seismic profile in Figure 1 ranges from CMP1000 to1200) The waveform of Figure 4 BSR (according to Lee et al., 1993).

5 Abnormal high-speed zone

The formation speed of hydrate-bearing layer is often higher than that of general formation, and its speed is related to hydrate content. The higher the content, the higher the flow rate [9]. As far as velocity is concerned, BSR is the interface between hydrate cemented sediments with high sound velocity and loose sediments with low speed. The seismic velocity of P-wave in shallow marine sediments is generally1.6 ~1.8km/s. If there is hydrate, the seismic velocity will be greatly increased to1.85 ~ 2.5km/s. If there is free gas layer below the hydrate layer, the seismic velocity can be sharply reduced to 0.5 ~ 0.2km/s.

Table 1 is the velocity data of shallow sea bottom (Yao Bochu, 1998) calculated based on the data of nine sonar buoy stations in the northern South China Sea, which is 1.95 ~ 2.45 km/s, higher than the normal velocity of marine sediments (1.6 ~/kloc-0)

6 discussion

To sum up, marine natural gas hydrate has seismic identification marks such as BSR, amplitude blank, negative polarity and abnormal high-speed zone, especially BSR and amplitude blank, and is considered as a sign of natural gas hydrate. However, it should be noted that BSR is like a "bright spot" technology in oil exploration, and hydrate does not necessarily coexist with BSR; At the same time, many hydrate-bearing formations do not necessarily have BSR. To judge whether natural gas hydrate develops in a region, it is necessary to comprehensively analyze various factors. In addition to the above seismic signs, the important elastic and physical characteristics of hydrate-bearing strata, such as positive AVO anomaly, high S/P velocity ratio and light carbon isotope value (δ 13C, usually less than -60‰), can be used for comprehensive judgment. In addition, geophysical information obtained from electrical logging curves is also useful information for detecting and evaluating gas hydrate intervals (Kvenvolden and Grantz, 1990), including borehole diameter, gamma, spontaneous potential, resistivity, sound velocity and neutron porosity logging (Goodman, 1980). It is believed that in the near future, with the deepening of exploration and research, we will get clear and effective identification marks of natural gas hydrate, thus uncovering the mystery of natural gas hydrate.

Table 1 Table of velocity, depth and thickness of shallow sediments at nine sonar buoy stations in the northern South China Sea 1 Velocity and thickness of shallow sediments are calculated according to the data of sonar buoys in the northern margin of the South China Sea.

refer to

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2.Gornitz, V. et al., 1994, Potential distribution of methane hydrate in the world's oceans. Global Biochemistry Cycle, Volume 8, No.3, pp. 335-346.

3. Yao Bochu, 1998, Preliminary Study on Natural Gas Hydrates in the Northern Continental Margin of the South China Sea, Marine Geology and Quaternary Geology, 1998, Vol. 18, No.4,1~18.

4. Yang Muzhuang et al .. 1999, Exploration prospect of natural gas hydrate in South China Sea. The 4th International Conference on Marine Geology in Asia.

5. Zhang Guang, Wen Pengfei, 2000, Study on Seismic Characteristics of Methane Hydrate in the South China Sea, Proceedings of the First Guangdong Young Scientists Forum, China Science and Technology Press.

6. J.J. Miller and others .. 199 1, seismic reflection analysis of the bottom of natural gas hydrate zone off the coast of Peru. Bulletin of the United States atomic energy agency, vol. 75, No.5, 9 10~924.

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Clear evidence of marine natural gas hydrate

Yangmuzhuang

abstract

Identifying seismic evidence of natural gas hydrate on seismic reflection profile is of great significance to the preliminary exploration and research of marine natural gas hydrate. On the basis of foreign exploration and research results, combined with the data collected from the first investigation of natural gas hydrate in China, this paper looks for seismic evidence, analyzes its characteristics, and points out the existence of natural gas hydrate, such as BSR, amplitude blank, polarity inversion and high-speed anomaly.

Keywords: natural gas hydrate, discernible evidence, BSR