Distribution and Seismic Characteristics of Gas Hydrate Offshore Southwestern Taiwan

This study compiled and analyzed all the available seismic reflection data in the area offshore southwestern Taiwan for the identification and characterization of BSR (bottom simulating reflection). A BSR map has been generated that shows the distribution of gas hydrate in the studied area. This map indicates that gas hydrates are widely presented beneath the sea floor over an area of 20,000 km2, from the passive margin of the South China Sea continental slope and the near shore accretionary Kaoping Slope, to the toe of the accretionary wedge adjacent to the Manila Trench in 3500 m water depth. Seismic characteristics typically associated with the presence of gas hydrate, such as blanking zones above BSR, high amplitude reflections beneath BSR, negative polarity and amplitude increases with offset for BSR reflections, etc., have all been observed. Velocity structures derived from pre-stack depth migration and from analyzing the wide-angle reflection and fraction data collected by the ocean bottom seismometers show that the hydrate-bearing sediments generally have velocity ranges from 1750 to 2000 m/s, with most values around 1900 m/s. Low velocity zones observed beneath the gas hydrate bearing sediments clearly indicate the presence of free gas below. This study has found that gas hydrates are concentrated the most in the accretionary wedge north of 21o45'N offshore southwestern Taiwan, especially under anticlinal ridges and adjacent to thrust faults. This clearly demonstrates the structural control of gas hydrate distribution within the accretionary wedge environment.