Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

Abstract

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.

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

“…Geophysical surveys show that intensive Bottom Simulating Reflectors (BSRs), that are considered to be one of the most effective indicators of existing gas hydrate in a given region, are distributed in a wide area of offshore southwestern Taiwan (Chi et al 1998;Schnürle 1999Schnürle , 2002Liu et al 2004Liu 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.…”

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

“…Based on seismic reflection profiles, they found many Bottom Simulating Reflections (BSRs) and concluded that gas hydrates are widely distributed under the seafloor of depths of 700 to 3000 m in this region (Chi et al 1998;Chow et al 2000;Schnürle et al 1999Schnürle et al , 2002Schnürle et al , 2006Liu et al 2006). The study presents the first systematic survey based on gas geochemical techniques performed in the gas hydrate area in offshore SW Taiwan.…”

Discovery of native aluminum and its possible origin from prospective gas hydrate areas in the South China Sea

“…Sur- [28,30], seep community [31] and carbonate precipitation [26,29,32], high level of CH4 and H2S in sediments [31], mud diapir structures [33], seep carbonate precipitations and musselshell in site ROV-G Carbonate precipitations in core of ODP184-1146 [34], BSR in core's adjacent area [28], nature gas hydrate [35] and seep precipitations [27], high level of CH 4 in water column [36] Strike-slip faults, exploited oil/gas fields and cold seeps [37,38], oil/gas seeps [39], hydrocarbon anomaly [39,40], pyrite in sediments [41] Fold-thrust structures, BSR [42], pyrite and gypsum [41,43], methane-concentration enhancement beneath the seafloor [44], hydrocarbon anomaly [39,44] ; 8. Other prospective area in the SCS [39,40] face sediment samples were hermetically sealed in polythene bags after separation on board of the ship, and core sediment samples were preserved under room temperature.…”

“…Since the Mesozoic, the SCS went through a succession of geotectonic regimes and multiple faulted terraces and basins were formed with abundant organic matter. Under these temperature-pressure conditions, the SCS geological settings are in favor of gas hydrate formation and conservation [28][29][30]. Recently, a great deal of work about gas hydrate reservoirs has been done in the northern continental slope of SCS, and subsequently multiple lines of evidence related to gas hydrate were identified [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] (Table 1) and several prospective areas for gas hydrate occurrence were discovered and outlined (Figure 1).…”