Pre-Stack Depth Imaging and Horizontal Velocity Analysis of Gas Hydrates off SW Taiwan

Yang, Ben Jhong ¹; Wang, Tan Kin ^1
1 Institute of Applied Geosciences, National Taiwan Ocean University, Keelung, Taiwan.

Pre-stack depth migration of seismic profiles collected by R/V Maurice Ewing in 1995 along the continental slope of the northern South China Sea (EW9509-34, EW9509-45) and across the accretionary prism off southwestern Taiwan (EW9509-35, EW9509-36, EW9509-46) are presented for exploring gas hydrates. Below the upper continental slope, a hydrate layer above the bottom simulated reflector (BSR) with a P-wave velocity of about 2000~2100 m/s and a thickness of about 100 m is imaged from the horizon velocity analysis and the pre-stack depth migration. Similarly, a free-gas layer below BSR with a velocity of about 1700~1900 m/s and a thickness of about 80 m is found. Highest hydrate velocity of 2100 m/s is observed NW of an erosion canyon in the upper continental slope whereas the velocity contrast along the BSR is decreasing southeastward toward the lower continental slope. The highest hydrate velocity near the erosion canyon may have resulted form gas migration along the boundary of the continental crust and the transitional crust and gas accumulation below the erosion canyon. However, in the accretionary prism off SW Taiwan, the BSR is not prevailed as that appeared in the continental margin. Near the trench and below the topographic ridges in the lower slope of the accretionary prism, P-wave velocities of hydrates and free gas are about 1900~2000 m/s and 1700~1900 m/s, respectively. The thickest portions of hydrate and free gas are both about 80~100 m thick. We suggest that continuity of BSR and velocity of hydrates in the accretionary prism less than those in the continental slope may be associated with severely deformation and fracture due to folds and thrusts. Nevertheless, folds and thrusts in the Manila subduction zone provide excellent conduits for gas migration and hydrate accumulation off SW Taiwan.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009