應用震測屬性探討台灣西南海域永安海脊地區天然氣水合物之分布與特性

Applying seismic attribute analysis for gas hydrate distribution and characterization in the Yung-An Ridge area offshore southwestern Taiwan

李佳芳(Jia-Fang Lee)

指導教授 ： 劉家瑄

國立臺灣大學/理學院/海洋研究所/碩士(2012年)

https://doi.org/10.6342/NTU.2012.00171

摘要

永安海脊為台灣西南海域呂宋隱沒碰撞系統中的一個構造高區。過去於永安海脊附近辨識出廣泛的海底仿擬反射(Bottom Simulating Reflector, BSR)，且根據過去地球物理及地球化學調查顯示此區域於海脊下方及兩側的斜坡盆地中可能有大量天然氣水合物賦存及游離天然氣體聚集，故此區域被認為是天然氣水合物優先探勘好景區，因此探討永安海脊區域之天然氣水合物特性及其分布範圍有其必要性。本研究應用震測屬性分析技術(seismic sttribute analysis)至天然氣水合物研究上，重新檢視台灣西南海域永安海脊附近的一組多頻道反射震測三維資料與三條長支距多頻道反射震測資料。屬性分析最常用的方法之一為利用複波線原理進行資料屬性的轉換，以求解震波反射強度、瞬時相位和瞬時頻率。研究結果顯示澎湖海底峽谷下方因為過去切蝕與填充作用造成海床溫壓條件改變，天然氣水合物不易聚集於此，游離天然氣也可能因切蝕作用而逸散，故未觀察到明顯的天然氣水合物賦存徵兆；永安海脊西側之斜坡盆地則表現出具掩埋峽谷特徵之強振幅反射信號，掩埋峽谷中多有高孔隙率之濁流砂堆積，極適合天然氣水合物賦存及游離天然氣聚集；永安海脊頂部無明顯振幅異常，亦未觀察到瞬時頻率異常；海脊東側之斜坡盆地內強反射信號範圍廣，顯示有大量游離天然氣聚集於此，高振幅反射信號向東於一個逆衝斷層帶中斷，而此斷層帶之震測屬性分析顯示明顯振幅反白及異常低頻之煙囪狀構造，推測有大量游離天然氣沿著此斷層向海床滲漏。本研究顯示透過屬性分析技術有助於在震測剖面上辨識、評估研究區域內流體移棲及天然氣水合物之空間分布及潛能，以利未來天然氣水合物之鑽探調查。

關鍵字

台灣西南海域 ； 天然氣水合物 ； 海底仿擬反射 ； 震測屬性分析 ； 含天然氣水合物之沉積物

並列摘要

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 region, and combined geophysical and geochemical investigations have revealed that large amount of gas hydrates and free gases are accumulated beneath the ridge and adjacent slope basins on both sides of the Yung-An Ridge where high priority drilling sites for gas hydrate investigation are proposed. It is thus important to have a good understanding of the potential gas hydrate reservoir characters there. This study applies seismic attribute analysis (SAA) technique to analyse a 3D seismic volume data and three large offset multi-channel seismic reflection data in the Yung-An Ridge area. Seismic attribute analyses of the 3D seismic data help us to better constrain the geological features identified in the 3D seismic volume. The reflection strength analyses show that in addition to the high amplitude character of BSRs, we have also observed some very strong reflection signals below BSRs in the slope basins on both sides of the Yung-An Ridge. Instantaneous frequency images show that there are low frequency patches beneath BSRs which may reflect gas-bearing strata. The analyses of instantaneous phase images help to identify faults easily. The SAA results suggest that there is little or no gas hydrate presence beneath the Penghu Canyon, while in the slope basin to the west of the Yung-An Ridge, characters of buried channel deposits are clearly visible. There are widely distributed high amplitude reflections in the slope basin to the east of the Yung-An Ridge suggesting the presence of large amount of free gases. A fault structure has been identified at the eastern edge of the slope basin which terminates the high amplitude reflections. The gas chimney character of the fault feature suggest that this fault forms a good conduit for gas migration.

並列關鍵字

Offshore Southwestern Taiwan ； Gas Hydrate ； Bottom simulating reflector ； Multichannel Seismic Reflection Data ； Seismic Attribute Analysis

參考文獻

塗書琳 (2010) 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。 國立台灣大學海洋研究所碩士論文。共80頁。

廖士瑋 (2010) 應用三維反射震測技術調查台灣西南海域天然氣水合物系統。國立台灣大學海洋研究所碩士論文。共84頁。

Castagna, J. P., Swan, H. W., and Foster, D. (1998) Framework for AVO gradient and intercept interpretation. Geophysics, 63, 948-956.

Chi, W. C., Reed, D. L., Liu, C. S., Lundberg, N. (1998) Distribution of the bottonm-simulating reflection in the offshore Taiwan collision zone. Terrestrial Atmosphere and Oceanic Science, 9, 779-794.

Chopra, S., and Marfurt, K. J. (2005) Seismic attributes - a historical perspective. Geophysics, 70, 3SO–28SO.