HR: 1340h
AN: OS33B-1480    [Abstracts]
TI: Structural Controls on the BSR Distribution Offshore Southwestern Taiwan from a 2.5-D Seismic Reflection Survey
AU: * Liu, C
EM: csliu@ntu.edu.tw
AF: National Taiwan University, P. O. Box 23-13, Taipei, 106 Taiwan
AU: * Liu, C
EM: csliu@ntu.edu.tw
AF: National Center for Ocean Research, P. O. Box 23-13, Taipei, 106 Taiwan
AU: Schnurle, P
EM: schnurle@oc.ntu.edu.tw
AF: National Taiwan University, P. O. Box 23-13, Taipei, 106 Taiwan
AU: Wang, Y
EM: wangys@linx.moeacgs.gov.tw
AF: Central Geological Survey, P. O. Box 968, Chung-Ho, 235 Taiwan
AB: A 2.5-D seismic reflection survey with 400-m line spacing has been conducted in a 14 x 16 km wide area offshore southwestern Taiwan where BSRs are highly concentrated and geochemical signals for the presence of gas hydrate are strong. This survey is part of a 4-year gas hydrate investigation program supported by the Central Geological Survey of Taiwan. We have mapped the BSR distribution in the survey box, and computed the BSR arrival times, sub-bottom depths, and associated heat flow values. A NE-SW trending fault structure, the Yung-An lineament (YAL), runs through the survey area and divides the survey box into a southeastern half and a northwestern half. Clear BSRs are observed almost continuously in the southeastern half of the survey box, while BSRs are either appeared as small patches or are absent in the northwestern half. Southeast of YAL, our estimates of the local heat flow values from BSR sub-bottom depths range from 55 to 70§/Km. The estimated heat flow values drop to 45 to 55§/Km in the area northwest of YAL. These estimated values are consistent with the actual heat flow measurements using a heat probe. The average reflection coefficient of the BSR is -0.094, or 32% of the sea floor reflection value, suggesting a maximum of about 1.25% of free gas or 47% of gas hydrates floating in the pore space. Further more, local dips of BSR and sedimentary horizons that cross cut the BSR at the intersection of 2 seismic profiles have been computed in the survey box. A dominant N308§ up-dip direction characterizes 2 anticlinal ridges, named R1 and R2, respectively, in the central portion of the survey box. BSR dip directions fan out toward N285§ in the southern side and change to N330§ in the northern side of the ridges. East of R2 where abnormally high heat flow is observed, an elongated diapiric feature deforms the sedimentary layers and causes the shallowing of the BSR. Experimental results of methane-water, and hydrocarbon gas mixtures show that methane solubility decreases rapidly with respect to temperature, and up to 5-fold between 16 and 1 MPa. Thus, upward fluid fluxes with significant dissolved gas content provides a very efficient mechanism for gas moving into shallower zones. With detailed BSR distribution derived from this 2.5-D survey, we are able to outline the regional BSR variations and discuss the importance of structural controls to the gas hydrate emplacements.
DE: 3022 Marine sediments: processes and transport
DE: 3025 Marine seismics (0935, 7294)
DE: 3045 Seafloor morphology, geology, and geophysics
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005