Obs Imaging of Gas Hydrates in the Northernmost South China Sea
About 130 ocean-bottom seismometers (OBS), with a spacing of about 500-2000 m, have been deployed and recovered for exploring gas hydrates on the accretionary wedge and the rifted continental margin in the northernmost South China Sea since 2004. Travel times of head wave, refraction, reflection and converted shear wave identified from the hydrophone, vertical and horizontal components of OBS data were applied for imaging P-wave velocity and Poisson's ratio of hydrate-bearing sediments. In the orogenic wedge off SW Taiwan, we found hydrates, with a thickness of about 100-200 m, a relatively high P-wave velocity of 1.7-2.04 km/s and a relatively low Poisson’s ratio of 0.44-0.45, below anticlinal ridges near imbricate emergent thrusts in the drainage system of the Penghu and Kaoping Canyons. Free Gas, with a thickness of about 100 m, a relatively low P-wave velocity of 1.4-1.8 km/s and a relatively high Poisson’s ratio (0.485-0.5), was also observed below most of the bottom-simulating reflectors (BSR). Subsequently, based on rock physics of the three-phase effective medium, we evaluated the hydrate saturation of about 15-25% and the free-gas saturation of about 1-3%. The highest saturation (25% and 3%) of gas hydrates is found below anticlines due to N-S trending thrust-bounded folds and NE-SW thrusts and strike-slip ramps in the lower slope of the accretionary wedge. We suggest that gas hydrates may have migrated through the relay fault array due to decollement folding and have been trapped in anticlines formed by the basement rises along the thrust faults. On the other hand, in the rifted continental margin of the South China Sea, P-wave velocities of 1.9-2.2 km/s and 1.3-1.6 km/s, Poisson’s ratios of 0.445-0.45 and 0.48, and thicknesses of about 50-150 m and 100 m, respectively, for a hydrate layer and a free-gas layer were imaged below the remnant and erosion ridges in the upper continental slope. High P-wave velocity and low Poisson’s ratio of hydrates below erosion ridges may also indicate high saturation (30%) of hydrates there. Normal faults due to rifting of the South China continental crust may have provided a conduit for gas migration below the erosion ridges where saturation of hydrates in the passive continental margin of the South China Sea is greater than that in the active orogenic wedge off SW Taiwan.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009