High-resolution seismic reflection data is well suited for mapping BSR. With the recon-naissance surveys conducted using a high-resolution MCS system, places where BSRs are densely distributed can be easily identified. However, the short-offset high-resolution MCS system cannot provide constraints on gas hydrate concentration in the sedimentary strata, large-offset seismic reflection data and/or OBS data are needed to estimate the amount of gas hy-drate or free gas in the sedimentary strata. Seismic velocity analyses from a large-offset seis-mic reflection dataset show that there are low velocity zones beneath BSRs under anticlinal ridges, suggesting free gases exist beneath hydrate-bearing sedimentary strata. There are ac-tive fluid activities in the investigation area. 2.5-D seismic data volume can provide details of the structure, stratigraphy and BSR variations, and use for fluid migration model construction.
With detailed mapping of BSRs and their sub-bottom depths, we have estimated the heat flow values in the investigation area. The regional heat flow value estimated from water depth versus BSR sub-bottom depth relationship is about 50 mW m-2, with lower regional values in the China continental margin (near 40 mW m-2) and higher regional values in the accretionary
Fig. 15. Estimated preferential fluid migration paths across the BSR in one of the 2.5-D survey area (see box in the index map for location). The length of each arrow indicates the flow rate at that point. The meaning of different symbols are given in the small box above this figure. YAL (the dark blue dash line) is the Yung-An lineament which is a linear topographic feature and could be the surface expression of a transpressional fault (Liu et al. 2004).
wedge province, reaching 60 mW m-2 near the Manila trench. These values are lower than that derived from seafloor heat flow measurements of Shyu et al. (2006) who used a different stability curve, and made corrections for sedimentation.
Gas hydrate has been found in both the convergent accretionary wedges and the exten-sional passive continental margins around the world. Offshore of SW Taiwan, these two dis-tinctive tectonic provinces connect together, and gas hydrate spreads across both tectonic provinces. Most of the sediment in the accretionary wedge is derived from the Taiwan mountain belt, while most of the sediment in the China continental margin came from China. Both biogenic and thermogenic gases have been detected in the accretionary wedge province (Oung et al. 2006) off SW Taiwan, but up to now only biogenic gases have been found in the passive China continental margin province (T. F. Yang, personal communication). So with different tectonic settings, different sedimentary sources, different structural developments, yet continuous BSR distribution, the gas hydrate field off SW Taiwan will provide us a good opportunity to com-pare the sources, formation, migration paths and accumulation of gas hydrate in an accretion-ary wedge and a passive continental slope environment.
“Petroleum system” approach has been widely accepted as an essential procedure in gas hydrate investigation for energy resources. The gas hydrate field off SW Taiwan could be associated with known gas fields on the continental shelf (the F-field, presently under devel-opment by the Chinese Petroleum Company of Taiwan) and on land of southern Taiwan (the PCC and HSY fields). The F- and PCC-fields are normal fault-block traps that host ther-mogenic gas in Oligocene to Miocene sandstones (Lee et al. 1995), while the HSY field is a shallow, combined structural and stratigraphic trap that contains biogenic gas in Pleistocene sandstones (Fuh et al. 2005). The gas hydrate in the passive China continental slope may have the same petroleum system as the adjacent F-field (Fig. 1), and the shallow HSY field can provide an analog for the establishment of a gas hydrate field in the accretionary wedge.
In summary, MCS surveys reveal that BSRs are widely as well as densely distributed in both the accretionary wedge province and the passive China continental margin province off-shore of SW Taiwan. Active fluid activities and venting systems in the investigation area have been observed using geophysical techniques (Chiu et al. 2006). Further evidence of gas vents came from seafloor photographs taken by a deep-tow camera system that show authigenic carbonates, carbonate mounds, chimney structures, and clam communities associated with bacterial mats on the sea floor where geochemical proxies for methane flux are high (S. Lin, personal communication). With all the geophysical data presented in this study, together with many geological observations (e.g., Jiang et al. 2006; Huang et al. 2006; Horng and Chen 2006) and geochemical analyses results (e.g., Lin et al. 2006; Chuang et al. 2006; Yang et al.
2006), we suggest that enormous amounts of gas hydrate should exist beneath the seafloor off SW Taiwan.
Acknowledgments We would like to thank Director C. C. Lin of the Central Geological Survey for the initiation and strong support of this gas hydrate investigation program. Cap-tains and crew of the R/V Ocean Researcher I and Mr. S. D. Chiou of the Ocean Researcher I Precious Instrument Center are thanked for helping to collect the marine geophysical data. S.
Y. Liu and K. R. Lai of the National Center for Ocean Research, J. K. Chiu, C. S. Chen, H. L.
Chang, H. J. Chuang, W. H. Tseng, and many graduate students in the Exploration
Seismol-ogy Lab of the Institute of Oceanography, National Taiwan University have participated in seismic cruises, and have prepared the bathymetry, sub-bottom profile and seismic reflection data and drafted many figures used in this paper. Discussions among gas hydrate team mem-bers are greatly appreciated, they being J. C. Chen, S. Lin, T. F. Yang, A. T. Lin, C. Y. Huang, C. S. Lee, T. K. Wang, W. B. Cheng, C. T. Shyu, H. S. Yu, S. K. Hsu, C. S. Horng, C. F. You, and W. T. Chiang. Comments and suggestions made by L. S. Teng and A. T. Lin greatly improve this paper. This study is supported by the Central Geological Survey grants 5226902000-06-93-01, 5226902000-05-94-01, and 5226902000-05-95-01.
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