Geochemical Study on Hydrocarbon Gases in Seafloor Sediments, Southwestern Offshore Taiwan - Implications in the Potential Occurrence of Gas Hydrates

ABSTRACT1 Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC 2 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC 3 Central Geological Survey, MOEA, Taipei, Taiwan, ROC * Corresponding author address: Prof. Tsanyao Frank Yang, Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC; E-mail: tyyang@ntu.edu.tw It has been found that Bottom Simulating Reflections (BSRs), which infer the existence of potential gas hydrates underneath seafloor sediments, are widely distributed in offshore southwestern Taiwan. Fluids and gases derived from dissociation of gas hydrates, which are typically methane enriched, affect the composition of seawater and sediments near venting areas. Hence, methane concentration of seawater and sediments become useful proxies for exploration of potential gas hydrates in a given area. We systematically collected bottom waters and sedimentary core samples for dissolved and pore-space gas analyses through five cruises: ORI-697, ORI-718, ORII-1207, ORII-1230, and ORI-732 from 2003 to 2005 in this study. Some sites with extremely high methane concentrations have been found in offshore southwestern Taiwan, e.g., sites G23 of ORI-697, N8 of ORI-718, and G96 of ORI-732. The methane concentrations of cored sediments display an increasing trend with depth. Furthermore, the down-core profiles of methane and sulfate reveal very shallow depths of sulfate methane interface (SMI) at some sites in this study. It implies sulfate reduction being mainly driven by the process of anaerobic methane oxidation (AMO) in sediments; thus indicating that there is a methane-enriched venting source, which may be the product of dissociation of gas hydrates in this area.

“…The results are consistent with recent work of Oung et al (2006) in this area. Table 2 shows both methane and ethane concentrations of all cored samples of each cruise.…”

Section: Methane and Ethane Concentrations Of Cored Sedimentssupporting

confidence: 93%

Extremely High Methane Concentration in Bottom Water and Cored Sediments from Offshore Southwestern Taiwan

Chuang¹,

Yang²,

Lin³

et al. 2006

“…This indicates that abundant gas hydrate deposits may exist in the sediments of both the passive and active continental margins offshore of SW Taiwan. In addition to the BSRs, geological and geochemical data also support the interpretation that gas hydrates may exist in southwestern Taiwan continental margin sediments (Chao and You 2006;Chuang et al 2006;Horng and Chen 2006;Huang et al 2006;Jiang et al 2006;Lin et al 2006;Oung et al 2006;Shyu et al 2006). Hence, the region has been considered a gas hydrate potential area.…”

Section: Introductionmentioning

confidence: 58%

Methane Venting in Gas Hydrate Potential Area Offshore of SW Taiwan: Evidence of Gas Analysis of Water Column Samples

Yang¹,

Chuang²,

Lin³

et al. 2006

“…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 compare the sources, formation, migration paths and accumulation of gas hydrate in an accretionary wedge and a passive continental slope environment.…”

Section: Discussion and Summarymentioning

confidence: 99%

Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

Liu¹,

Schnürle²,

Wang³

et al. 2006

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ABSTRACT1 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC 2 Central Geologic Survey, Taipei, Taiwan, ROC 3 Exploration Department, Chinese Petroleum Company, Miaoli, Taiwan, ROC * Corresponding author address: Prof. Char-Shine Liu, Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC; E-mail: csliu@ntu.edu.tw Bottom simulating reflector (BSR) is a key indicator for the presence of gas hydrate beneath the sea floor. Widely distributed BSRs have been observed in the area offshore of southwestern Taiwan where the active accretionary complex meets with the passive China continental margin. In order to better understand the distribution and characters of the gas hydrate in the region, closely spaced (1.86-km line spacing) multichannel seismic reflection surveys have been conducted in recent years under the support of the Central Geological Survey, ROC. Over 10000 km of multichannel seismic reflection profiles have been collected that cover an area of about 10000 km 2 offshore of southwestern Taiwan. BSRs can be identified along 50% of the seismic profiles that we collected. A newly compiled BSR distribution map suggests that gas hydrates are distributed both in the passive margin of the China continental slope as well as in the submarine Taiwan accretionary wedge, from water depths of 500 to over 3000 m. Gas hydrates are most concentrated underneath anticlinal ridges in the accretionary wedge, and underneath the slope ridges of the passive continental margin that were formed due to sedimentary processes. Active fluid activities are evident from various features observed on seismic reflection and chirp sonar profiles, such as mud volcanoes, gas plumes and gas charged shallow sedimentary layers. Fluid migration model has been established from a set of pseudo 3D seismic reflection data. The predicted locations of high fluid Terr. Atmos. Ocean. Sci., Vol. 17, No. 4, December 2006 616 flux correlate well with those interpreted from geochemical analyses that show very high methane concentrations and very shallow sulfate-methane interfaces (SMI). This demonstrates the importance of structural control over gas hydrate emplacement. From the observed gas hydrate distribution and characters, the area offshore of southwestern Taiwan provides an ideal place to study and compare the formation and migration of gas hydrates under different tectonic settings.