利用疊前深度移位處理技術探討台灣西南海域天然氣水合物之分布特性與飽和度

Abstract

台灣西南海域是弧陸碰撞入侵非活動性大陸邊緣的區域，其形貌特徵以背斜與海脊峽谷為主。海域震測調查結果顯示台灣西南海域的地層中可能蘊藏有豐富的天然氣水合物，而BSR信號不只在活動性增積岩體上有大量的分布，在非活動性中國大陸邊緣上亦有相當廣泛地分布，此結果意味著台灣西南海域是個相當好的研究區域，能同時探討在兩個不同地體架構下天然氣水合物的分布特徵、可能的形成機制與流體移棲模式的異同。本研究的主要重點區域分別為位在活動性增積岩體的永安海脊區域與位在非活動性大陸邊緣的福爾摩沙海脊區域，在此兩重點區域內不僅地球物理資料指出有大量的天然氣水合物賦存，地球化學資料亦顯示有異常高的甲烷濃度與相當快的硫酸還原速率，以及硫酸鹽－甲烷界面深度淺等特徵，推測研究區內流體活動相當頻繁。由於震波速度對天然氣水合物飽和度的估算相當重要，為提升速度演算上的精確性，我們利用疊前深度移位處理技術(pre-stack depth migration, PSDM)及疊代移位速度分析方法(iterative migration velocity analysis, MVA)對2009年春季TAIGER計畫所收集之長支距多頻道震測資料進行速度場的反演，進而求得精確的地層速度構造與良好的震測剖面影像。研究結果顯示，在永安海脊與福爾摩沙海脊區域BSR信號明顯的地層中有強烈的速度阻抗差異，在BSR上方有一高速層，P波速度值介於1750至2000公尺/秒間，判斷此速度高值帶的地層中應有豐富的天然氣水合物賦存；而在BSR信號下方有一低速層，P波速度值介於1450至1550公尺/秒間，推測應是天然氣穩定帶下方有游離氣體聚集而造成速度的驟降。透過岩石物理模型的建立與聲波速度的對比，我們推估永安海脊區域之天然氣水合物的飽和度含量高可達50 %(佔孔隙之百分比)；而在福爾摩沙海脊區域之天然氣水合物的飽和度含量可達35 %。最後，藉由震測剖面的影像辨識與構造解釋，配合所建立之速度構造模型，我們認為位在活動性大陸邊緣的永安海脊區域，其天然氣水合物之形成與分布主要受控於褶皺逆衝斷層帶的構造作用及濁流攜入之高孔隙率的砂岩層所影響；而位在非活動性大陸邊緣的福爾摩沙海脊，其天然氣水合物的形成與分布則受海底的侵蝕與崩塌作用及海脊內的斷層或裂隙發育所影響。

The area offshore southwestern Taiwan is the place where active Luzon accretionary wedge province meets the passive China continental margin province. Not only marine seismic reflection data indicate that large amount of gas hydrates and free gases may exist beneath the seafloor sediments, but geochemical data also reveal high methane flux and shallow sulfide-methane interface (SMI) at many cored locations. In this study, we intend to investigate detail distribution and possible formation and migration of gas hydrates in both active and passive continental margins. We implement pre-stack depth migration technique to derive depth sections with accurate velocity information from large-offset multichannel seismic data collected during the 2009 TAIGER survey. We focus our study on the Yung-An Ridge and the Formosa Ridge areas in active and passive margin settings, respectively. Large variations in acoustic velocities are observed in both areas where BSRs are prominent. The high velocity values derived in shallow sedimentary layers above BSR range from 1750 to 2000 m/s, and this high velocity zone may indicate the existence of gas hydrates. The low velocity values ranging from 1450 to 1550 m/s below BSR are interpreted to be the free gases zone. Through rock physics modeling, we establish the relationships between velocity and gas hydrate saturation values. Estimated gas hydrate saturation values suggest that gas hydrate could occupy up to 50 % of pore space in the Yung-An Ridge area and 35 % of pore space in the Formosa Ridge area. Comparing seismic images of the constructed depth sections with corresponding velocity models, we suggest that the distribution of gas hydrates and free gases in the Yung-An Ridge area are controlled by complex fault systems and porous sand layers. Distribution of gas hydrates and free gases in the Formosa Ridge area are controlled by submarine erosion and mass movement processes and development of faults and fractures in the Formosa Ridge area.