Abstract
Various seismic attributes of gas hydrate bearing sediments were analyzed in the accretionary prism offshore of southwestern Taiwan utilizing seismic imaging, velocity analysis, AVO analysis, and AVO inversion of large offset seismic data. A bottom-simulating reflector (BSR) is clearly observed on the seismic section with a reversed polarity compared to that of the seabed reflection. Instantaneous amplitude sectioning clearly shows lateral variations of the BSR. The zero-phase waveform of the BSR is distinct and the weak reflectors above the BSR can be observed on the instantaneous phase section. AVO analysis shows the absolute value of the negative BSR amplitude increasing with offset. A low P-wave interval velocity layer was found below the strong BSR by detailed velocity analysis. Both the P (normal incident P-wave reflection coefficient) and G (AVO gradient) values are highly negative for the strong BSR on the P and G sections, and they lie in the third quadrant of the P and G cross-plot section. The P+G (reflectivity of Poisson's ratio) value is also negative on the P+G section and the P-G (normal incident S-wave reflection coefficient) value is approximately zero on the P-G section along the same strong BSR. All the seismic characters described above suggest that a gas hydrate layer exists together with a free gas layer below it along strong BSRs in the area offshore southwestern Taiwan.
Highlights
INTRODUCTIONThe accretionary prism offshore of southwestern Taiwan has been suggested to be favorable and have bright prospects for the formation of gas hydrate (Chi et al 1998; Shyu et al 1998; McDonnell et al 2000; Chi et al 2006; Liu et al 2006)
Rifting and sea-floor spreading have led to the formation of the Cenozoic sedimentary basins and petroleum accumulation in the northern margin of the South China Sea
We describe the data acquisition first, and show different processing and analyses performed for various seismic attributes of bottomsimulating reflector (BSR), and the characters of the gas hydrate bearing zone are presented
Summary
The accretionary prism offshore of southwestern Taiwan has been suggested to be favorable and have bright prospects for the formation of gas hydrate (Chi et al 1998; Shyu et al 1998; McDonnell et al 2000; Chi et al 2006; Liu et al 2006). In 2001, seismic reflection data were acquired in the area offshore of southwestern Taiwan (line L, Fig. 1) using a long multi-channel streamer. This survey was supported by the Chinese Basic Research Priorities Program. The seismic data of line L have a maximum source-receiver offset of 3237.5 m, and present 30-fold with high signal-tonoise ratio This makes it possible to conduct extensive velocity analyses, seismic imaging and AVO (Amplitude-Versus-Offset) processing. We describe the data acquisition first, and show different processing and analyses performed for various seismic attributes of BSR, and the characters of the gas hydrate bearing zone are presented
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