Gas hydrate drilling results show that gas hydrate has a close relationship with strong bottom-simulating reflectors (BSRs) identified from seismic data in the Baiyun sag, South China Sea. The BSRs observed on seismic profiles at the crests of submarine canyons indicate the likely existence of gas hydrate. We calculate the acoustic impedance using constrained sparse spike inversion (CSSI), the interval velocity, and the seismic reflection characteristics such as reflection strength, instantaneous frequency, blanking, and enhanced reflection to demonstrate the presence of gas hydrate. Higher acoustic impedance and P-wave velocity were identified above the BSR. A remarkable low impedance, low frequency, and acoustic blanking indicated the presence of gas below gas hydrate stability zone. The occurrence of gas hydrate at the crests of canyons suggests that the abundance of gas hydrate in Baiyun sag may be due to the migrating submarine canyons providing the structural reliefs and the topographic ridges.
Gas hydrates are ice-like crystalline solids and are composed of water molecules and hydrocarbon gas (usually methane). They are distributed worldwide in the continental margin sediments and beneath permafrost [
In China, eight sites were drilled in 2007 in Shenhu area, Pearl River Mouth basin (PRMB), the northern slope of South China Sea [
The main objective of this study is to use the P-impedance profile obtained from Constrained Sparse Spike Inversion (CSSI) and geophysical attributes computed from 3D seismic data to show the presence of gas hydrate.
The SCS is the largest marginal sea in the western Pacific and covers an area of
(a) Areas of gas hydrate exploration in the northern part of the South China Sea and the gas hydrate drilling zone of Shenhu area, Baiyun sag; (b) Bathymetric map of Baiyun sage that was interpolated by the 2D seismic data in this area. Modern canyons shown by Zhu et al. [
The SCS has the favorable temperature and pressure condition for the formation of gas hydrate. The water depth ranges from 150 to 3700 m. The QDN basin is characterized by the geothermal gradients with the values of 39–41°C/km and high sedimentation rates (up to 1.2 mm/yr). The average geothermal gradient in the PRM basin is about 36°C/km. However, it goes up to 67.7°C/km in the gas hydrate drilling zone from the in situ temperature measurement [
The seismic line A and line C were collected in 2006 with the following a 3000-m long streamer with 240 channels (trace interval 12.5 m) and a tuned airgun source with a total volume of
Acoustic impedance is obtained from the product of density and seismic velocity, so it can be defined as a layer property and not as an interface property. The acoustic impedance can be directly related to porosity, lithology, and permeability. Although seismic data can be interpreted on its own without inversion, but this does not provide the most detailed view of the subsurface and can be misleading under certain conditions.
Seismic inversion has been used for several decades in the petroleum industry. Seismic inversion methods have progressed from the initial recursive inversion to present software package available to transform band-limited seismic traces to impedance traces. Stochastic seismic inversion combined the seismic and log data to derive the vertical resolution from the log data [
(1) Wavelet is estimated by using seismic and pseudowell data at the arbitrary CMP. We first create the Ricker wavelet and then we make synthetic seismogram at different locations. The input wavelet was the scaled average wavelet based on different wells. (2) The low-frequency information is constructed by using the EarthModel model generator. The trace gate, solid model created using the interpreted horizons and structural information, and the interpolation method of natural neighbour were chosen to generate the low frequency impedance data. (3) The bandpass P-impedance data are obtained by choosing the sparse spike parameters such as weighting factor in the inversion process. (4) Last, the low-frequency model is merged with the band pass P-impedance.
CSSI determines the acoustic impedance within prescribed constraints by minimizing the objective function
Seismic attributes like instantaneous frequency, instantaneous phase, blanking, and reflection strength have been used to identify gas hydrate from seismic data [
The Variance Cube operation was used to highlight faults and subtle stratigraphic features in a 3D seismic volume. In the output cube, incoherent (high amplitude) areas are displayed in high contrast (black). In the Seis 3DV, the output variance cube can be shown the inline or crossline profile and compared it with the seismic profile. As the 3D seismic data is pseudo-3D seismic data, we calculated the coherence in larger window length and we compare it with the seismic profile. The output values vary between 0 indicating the worst, and 1 indicating the best.
The instantaneous frequency is a time or depth derivative of the instantaneous phase and a measure of the frequency of the waveform at every sample. It is independent of reflection magnitude, weak events, and noise, which are all equally weighted in display. The minimum and maximum frequencies can be given in the output. The instantaneous frequency attribute is considered a good tool for lateral seismic character correlation. Oil and gas may preferentially attenuate higher frequencies so a low instantaneous frequency anomaly has been used to predict conventional hydrocarbons accumulations in the oil industry.
The reflection strength is simply an expression of the amplitude envelope of the seismic trace and is independent of phase. It shows the total energy of the seismic signal and has the maximum value at point other than the peak or trough of the real trace, especially when the event is the composite of several reflections [
One reflector is located at about 280 ms of two-way travel time below seafloor, which has the reversed polarity compared with the seafloor in seismic line A (Figure
In Baiyun sag. (a) The variable-density display of part profile of seismic line A; A strong bottom-simulating reflector and enhanced reflections can be easily identified. (b) The acoustic impedance estimation obtained by CSSI. High acoustic impedance above the BSR is present and low impedance is shown below the BSR.
Seismic line B is one of the inline of 3D seismic data which is parallel to seismic line A (Figure
(a) Part of seismic line B that is parallel to seismic line A, which can show the continuous reflections of the sediments. Enhanced reflections are identified at the similar locations to seismic line A; (b) Coherence profile for the seismic line B showing the faults and subtle stratigraphic change. (c) Reflection strength profile for the line, showing loss of amplitude above the BSR and the enhanced reflection. (d) Instantaneous frequency profile shows high frequency above the interpreted BSR caused by the presence of gas hydrate and low frequency zone below the interpreted BSR caused by the presence of free gas.
Geophysical attributes show an indictor of the presence of gas hydrate in seismic lines A and B. The direct indicator of gas hydrate is the study of the anomalies in the P-wave velocity above and below BSR. Higher velocities should be associated with gas hydrate and lower velocities should be associated with free gas. Figure
CMP gather shows how interval velocities were obtained. (a) Normal-moveout-corrected CMP gather shown in Figure
Seismic line C passed through site SH2 (Figure
Part of seismic line C profile (a) and interpreted profile showing BSR, gas chimneys, and migrating submarine canyons, Baiyun Sag, Pearl River Mouth Basin, South China Sea (b).
The submarine canyon near site SH2 was interpreted to show the migration of the canyon. It has been referred that the thalwegs of the submarine canyons are progressively offset toward the northeast from the middle Miocene to present [
Gas hydrates have been identified by the gas hydrate drilling exploration at the Shenhu area, Baiyun sag, Pearl River Mouth basin, South China Sea. Another gas hydrate province was identified at the adjacent canyon by using new seismic data and seismic attributes. Amplitude blanking, high acoustic impedance, high frequency and enhanced reflections identified from the 2D and 3D seismic profiles at about 280 ms of two-way travel time below seafloor were caused by the presence of gas hydrate. The reflector that is subparallel to the seafloor and cuts across the strata is the BSR. The acoustic impedance profile has lower impedance below it. A strong reflection at the BSR was observed in the reflection strength profile, and low frequencies were seen below the BSR from the instantaneous frequency profile. These anomalies suggest the presence of free gas below the BSR. The distribution of BSRs has a close relationship with the submarine canyons and gas chimneys. Prominent and strong BSRs exist in the crests of the migrating submarine canyons in the middle of Baiyun sag. Few BSRs are found in the axes of these canyons, which are controlled by the geological setting in this basin.
The authors would like to thank the team of Guangzhou Marine Geological Survey for shooting the seismic data used in this paper. Our research is supported by the International Science and Technology Cooperation Program of China (2010DFA21740), National Basic Research Program (2009CB219505), and Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources (MRE201105).