台灣西南海域福爾摩沙海脊之構造及沉積特徵

The Structural and Sedimentary Characteristics of the Formosa Ridge Offshore Southwestern Taiwan

廖奕翔(Yi-Hsiang Liao)

指導教授 ： 劉家瑄

國立臺灣大學/理學院/海洋研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.02879

摘要

福爾摩沙海脊位於台灣西南海域變形前緣以西，南中國海北緣非活動型大陸邊緣的大陸坡處，屬於大陸地殼過渡到海洋地殼的張裂地帶。此地區之構造特徵主要為地壘-地塹構造（horst-and-graben structures），且大陸坡上發育了許多切蝕作用造成的海底峽谷及海底山脊。震測資料顯示整個區域海底仿擬反射（Bottom Simulating Reflector，簡稱BSR）廣佈，此區域可能有大量的天然氣水合物賦存。在福爾摩沙海脊上，不論是採到的岩心樣本中有異常高的甲烷氣濃度，直接證實了海床表面甲烷氣的逸散，或是在海底攝影中觀察到大規模海底生物群落並直接採集到甲烷氣體，因為這些徵候我們推測福爾摩沙海脊為一特殊的海底冷泉區。本研究欲根據水深及震測資料，來瞭解福爾摩沙海脊的構造及沉積等特徵，並期望可解析地層中從深部到淺部游離氣的來源及流體在地層中移棲可能的影響因素。
本研究使用資料為水深資料及三種不同解析範圍的震測資料，包括1995中美合作航次所蒐集之EW9509-45測線可提供地層較深部的資訊，配合國內天然氣水合物計畫所蒐集之MCS719與MCS754兩個航次的多頻道反射震測資料對陸坡區與本區域相似環境提供良好的對比，及日本JAMSTEC研究船Natsushima號所收集的單頻道反射震測資料，提供淺部地層特徵的解析。福爾摩沙海脊位於台南盆地東部，在接近棚裂（shelf break）處有一向東南傾斜的大型斷層，造成上盤地層大規模傾斜，若斷層以下油氣供應充沛，此大型斷層破裂面可成為油氣移棲的重要管道之一。另外，研究區域附近屬非活動型大陸邊緣陸坡區，切蝕作用旺盛，造成許多切蝕溝（gully），也會影響天然氣水合物的溫壓平衡。在陸坡區的震測剖面資料顯示有BSR的出現，尤其是在福爾摩沙海脊。而在福爾摩沙海脊的高解析度震測剖面上除可以辨識出有明顯連續的BSR，證實此處應有相當的游離氣聚集，並且觀察到有一垂直向上通道為甲烷氣上升到海床的跡象。
綜合本研究，台灣西南海域福爾摩沙海脊區域在其上方接近陸棚坡折處有一大型生長正斷層的構造，所造成的傾斜地層可作為流體從深部移棲至接近地表的管道，在福爾摩沙海脊可觀察到連續的BSR信號表示有流體富集；並在震測剖面中，可看到流體垂直移棲通道，使流體可從地層中向上移動至地表溢散，另外我們還觀測到地層中小型沉積波在縱向上的分布不甚相同及放大來看可看到地層有些裂隙出現，可能是流體在山脊內向上溢出的管道之一。藉由對福爾摩沙海脊的構造及沉積特徵的辨識，我們可瞭解此福爾摩沙海脊的細部構造及山脊下方甲烷氣逸散的來源及分布概況。

關鍵字

福爾摩沙海脊 ； 天然氣水合物 ； 海底仿擬反射

並列摘要

The Formosa Ridge is located in the passive South China Sea continental margin, to the west of the deformation front offshore southwestern Taiwan. The structural character is mainly extensional horst-and-graben structure. On the continental slope region, there are numerous submarine canyons and slope ridges formed by submarine erosion mechanism. The seismic data show clear BSR（Bottom Simulating Reflector）distributed widely in the study area. High concentrations of methane are also revealed from the core samples collected at the crest of the Formosa Ridge. It indicates that there are venting activities of methane gases in the area. From seafloor photographic images, bacterial mats and carbonate mounds were observed. Massive chemosynthetic colonies have also been observed from the video clips taken by the ROV Hyper Dolphin. Therefore, we consider that the Formosa Ridge has an active cold seep site which is worth further studying.
The data we used include a deep seismic reflection profile EW9509-45, the MCS719 and MCS754 multichannel seismic reflection data collected for the gas hydrate investigation, and the high-resolution single channel seismic reflection data of the JAMSTEC R/V Natsushima cruise NT07-05. The Formosa Ridge is located east of the Tainan basin. There is a large growth fault dipping toward SE, and this fault may provide a convenient path for the deeper gases flow up to the shallow reservoir. Gas vents could be identified on the high resolution seismic profile. This suggests possible fluid paths. This thesis summarizes some characters of the Formosa Ridge and proposes a simple model trying to explain the fluid circulation at the observed cold seep site. In the Formosa Ridge, sediment waves or polygonal faulting system could be the major factors controlling the fluid migrating upward from BSR to the seafloor.

並列關鍵字

Formosa Ridge ； gas hydrate ； Bottom Simulating Reflector (BSR)

參考文獻

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