碩士 / 臺灣大學 / 海洋研究所 / 98 / The area offshore southwestern Taiwan is the place where active Luzon accretionary wedge province meets the passive China continental margin province. Not only marine seismic reflection data indicate that large amount of gas hydrates and free gases may exist beneath the seafloor sediments, but geochemical data also reveal high methane flux and shallow sulfide-methane interface (SMI) at many cored locations. In this study, we intend to investigate detail distribution and possible formation and migration of gas hydrates in both active and passive continental margins. We implement pre-stack depth migration technique to derive depth sections with accurate velocity information from large-offset multichannel seismic data collected during the 2009 TAIGER survey. We focus our study on the Yung-An Ridge and the Formosa Ridge areas in active and passive margin settings, respectively. Large variations in acoustic velocities are observed in both areas where BSRs are prominent. The high velocity values derived in shallow sedimentary layers above BSR range from 1750 to 2000 m/s, and this high velocity zone may indicate the existence of gas hydrates. The low velocity values ranging from 1450 to 1550 m/s below BSR are interpreted to be the free gases zone. Through rock physics modeling, we establish the relationships between velocity and gas hydrate saturation values. Estimated gas hydrate saturation values suggest that gas hydrate could occupy up to 50 % of pore space in the Yung-An Ridge area and 35 % of pore space in the Formosa Ridge area. Comparing seismic images of the constructed depth sections with corresponding velocity models, we suggest that the distribution of gas hydrates and free gases in the Yung-An Ridge area are controlled by complex fault systems and porous sand layers. Distribution of gas hydrates and free gases in the Formosa Ridge area are controlled by submarine erosion and mass movement processes and development of faults and fractures in the Formosa Ridge area.
| Identifer | oai:union.ndltd.org:TW/098NTU05279007 |
| Date | January 2010 |
| Creators | Shu-Lin Tu, 塗書琳 |
| Contributors | Char-Shine Liu, 劉家瑄 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 80 |
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