利用三維震測影像探討臺灣西南海域永安海脊區之天然氣水合物儲集層特性

Abstract

天然氣水合物的探勘與研究近年來一直是各國尋找替代能源的重點項目。永安海脊位於台灣西南海域高屏下部陸坡區，為呂宋隱沒-碰撞系統所擠壓、抬升出的構造高區。多頻道反射震測資料顯示，海底仿擬反射訊號(BSR)廣泛分布於海脊區及其兩側陸坡盆地內，表示此區域可能賦存大量天然氣水合物，是極具潛力的天然氣水合物極富開採潛力的好景區之一，所以尋找良好的天然氣水合物儲集層成為重要的研究工作。一般認為海域濁流砂層(turbidite)和水道砂(channel sands)是水合物賦存的良好儲集層，永安海脊西側斜坡盆地內，可見明顯海底峽谷切填構造(cut and fill structure)與掩埋水道(buried channels)的存在，視為擁有良好砂體存在的地質條件；好景海脊西側則有明顯逆衝斷層出露，成為流體向永安海脊移棲的重要管道，所以永安海脊區是研究天然氣水合物儲集層特性與濁流水道系統關係的重要區域。 本研究利用91條密集的二維震測測線建構出的三維震測影像方塊，透過視覺化震測特徵與空間上的構造分布，來討論永安海脊區的澎湖海底峽谷切填作用與深海濁流系統和永安線形構造對於儲集層特性的影響。根據區域內不同的構造關係與沉積作用，將永安海脊區分為六個區:澎湖海底峽谷切填區、西側斜坡盆地溢堤區、西南側濁流水道區、永安海脊區、東側斜坡盆地區及好景海脊斷層區。本研究認為永安海脊西側盆地中的濁流水道堆積區為整個永安海脊好景區內較佳的天然氣水合物賦存儲集層位置，而峽谷溢堤堆積層則為次佳地點。永安海脊則為逆衝斷層海脊型的BSR型態，東側斜坡盆地的東傾層序及MTD侵蝕面可能成為來自好景海脊斷層區的良好流體移棲管道。

Gas hydrate has been regarded as potential future energy resources, thus many countries have actively engaged in gas hydrate investigations. Yung-An Ridge is a structural high located in the accretionary complex of the Luzon subduction-collision system offshore southwestern Taiwan. Bottom simulating reflectors (BSRs) are widespread in the Yung-An Ridge region and adjacent slope basins, other geological and geochemical investigations have also indicated that the region may possess high potential for gas hydrate prospect, gas hydrate reservoirs characterization thus is an important task in the region. In general, submarine turbidite sands and channel sands are considered to be good reservoir rocks. Seismic data has revealed that many cut and fill features interpreted as submarine canyons, and buried channels are present in the western slope basin of the Yung-An Ridge. The steeply chipping strata and thrust fault in eastern slope basin may provide good fluid migration paths. The Yung-An Ridge area thus provides good study ground to investigate gas hydrate reservoir characteristics, and their relationships with the turbidity channel system. In this study, we use dense 2D seismic lines constructed 3D seismic volume data to visualize and discuss gas hydrate reservoir characteristic and then spatial distribution. From 3D seismic image, and the identified different characteristics and sedimentary features, we divide the study area into 6 regions: the Penghu canyon cut and fill region, the canyon levee deposition region, the turbidity channels deposition region, Yung-An Ridge, the eastern slope basin and the Good Weather Ridge thrust fault zone, respectively. We suggest that the turbidity channels deposition region has better gas hydrate reservoirs in Yung-An Ridge and adjacent region.