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GAS HYDRATE DISTRIBUTION OFFSHORE SOUTHWESTERN TAIWAN AND ITS GEOPHYSICAL AND GEOCHEMICAL CHARACTERISTICS
Char-Shine Liu1,2, Philippe Schnurle1, Ta-Hen
Hsiuan3, Tsanyao Yang1, Saulwood Lin1, Yunshuen
Wang5, and San-Hsiung Chung5
1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC
2 National Center for Ocean Research, Taipei, Taiwan, ROC
3 Chinese Petroleum Corporation, Miao-Li, Taiwan, ROC
4 Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC
5 Central Geological Survey, Taipei, Taiwan, ROC
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 new BSR map was generated that shows the distribution of gas hydrate in the studied area. This map indicates that gas hydrates are widely presented in an area extending over 20,000 km2 offshore southwestern Taiwan. BSRs are observed in both the passive margin of the South China Sea continental slope and the submarine Luzon accretionary wedge, from water depths of 700 m to over 3500 m (Fig. 1). 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. The relationship of BSR depths below seafloor versus seafloor depths shows that BSR depth normally increases as seafloor depth increases. However, this trend becomes unclear or even reverse where water depths are greater than 2000 m. This study has found that gas hydrates are concentrated the most in the accretionary wedge north of 21°45’N offshore southwestern Taiwan, especially under anticlinal ridges and adjacent to thrust faults, and are strongly influenced by the structural control.
Preliminary results from a recent coring cruise in areas where BSR are highly concentrated show that some coring locations, especially those on top of anticlinal ridges, exhibit unusually high methane concentrations in the bottom water (the highest value measured is near 200,000 nl/liter). Methane concentrations in the core samples also give similar results. This suggests methane extrusion to the sea floor from dissociated gas hydrates below.
In order to better understand the gas hydrate distribution and its geological controls, the sources of gases, and their migration and accumulation in the area offshore southwestern Taiwan, the Central Geological Survey of Taiwan has funded a 4-year gas hydrate investigation program. A brief outline of the program will also be presented.
Figure 1. BSR distribution offshore southwestern Taiwan. Thin lines show ship tracks of various seismic cruises, thick lines indicate where BSR are observed.