臺灣西南海域永安海脊熱流變化對天然氣水合物穩定帶的影響

Abstract

許多地球物理、地質與地球化學的海域調查結果顯示台灣西南海域應有廣泛的天然氣水合物蘊藏。由於天然氣水合物的形成必須滿足一定的溫壓條件，相較於壓力的影響而言，溫度的影響顯然大得多，即溫度的變化成為控制天然氣水合物分布的一個主要因素。本研究分析台灣西南海域永安海脊地區的熱流變化，並透過震測剖面探討影響熱流變化的可能因素。我們根據地貌特徵及地質構造，將研究地區自東向西分成五個區帶: (1)東側斷層區 (2)東側斜坡盆地 (3)海脊區 (4)西側斜坡盆地 (5)古海底峽谷堆積區。研究地區中熱流值最高的區域位在永安海脊東側斜坡盆地東緣的逆衝斷層出露附近，熱流高值僅次於東側斷層區的為海脊區，而斜坡盆地之熱流值則一般偏低。本研究認為永安海脊區域熱流值高低主要受控於地層中是否有提供流體移棲的管道，使深部的高溫流體向上移棲，如斷層構造與傾斜角度大的地層可能易使高溫流體向上移棲，造成高熱流值，而高溫也使得該處之BSR深度明顯變淺或是消失；沉積作用則會降低表層地溫梯度造成低熱流值，說明了構造與沉積環境均會影響熱流值高低。本研究亦比較海底仿擬反射(BSR)與天然氣水合物穩定帶底部(BGHS)深度的差異，以及BSR深度估算出的熱流值與實際量測熱流值的差異，結果顯示兩者差異大的地區均是流體移棲活躍的地區，這可能是因表層沉積物受高溫流體擾動或是沉積、侵蝕作用的影響，導致深部與淺層地溫梯度尚未達到一致的結果。由震測剖面影像與構造解釋，再配合BSR的深度變化與熱流值分布，本研究證實在永安海脊區域由於受到流體移棲的影響，導致局部的天然氣水合物穩定帶狀態改變，進而觀察到BSR、BGHS深度及熱流值的變化。整體來說，永安海脊區域有許多測站量測到高熱流值，高熱流現象扮演破壞天然氣水合物穩定帶的角色，造成BSR深度變淺。因此透過分區的熱流解釋與震測剖面上所顯示的地下構造變化，便能清楚了解永安海脊天然氣水合物穩定帶的分布與變化。

Geophysical, geological and geochemical investigation results indicate that gas hydrates are widely distributed in the area offshore southwestern Taiwan. The formation of gas hydrate is controlled by temperature and pressure conditions. In the offshore environment, pressure normally follow water depths, thus gas hydrate stability is more susceptible to changes in temperature then in pressure. In this study, we analyze the variation of heat flow values in the Yung-An Ridge area, and compare them with the seismic profile to discuss the possible reasons. We divide the study area into five zones based on morpho-structure and sedimentary characteristics, from east to west, they are 1. the fault located at the eastern edge of the slope basin; 2. the slope basin east of the Yung-An Ridge; 3. the Yung-An ridge;4.the slope basin west of the Yung-An Ridge, and 5. the paleo-canyon cut and fill zone. Our study indicates that fluid migration is a major factor controlling heat flow variations in this area. The highest heat flow values are observed near fault outcrops located at eastern edge of the slope basin, thus are caused by fluid migration through faults. Another zone has high heat flow values is the Yung-An Ridge where steeply dipping strata provide good fluid migration paths. In both of the above zones, we can see the BSR depths become shallower or even disappear. We suggest that high values of heat flow are primarily caused by warm fluids flowing upward along faults or dipping porous strata, while low heat flow values could be caused by rapid sedimentation. It appears that the heat flow values are influenced by structural and sedimentary processes. Furthermore, we compare the depths of BSR and BGHS at heat flow stations, calculate heat flow values based on BSR depths and compare them with the measured heat flow values. We found that discrepancies are large in the locations where active fluid migration occur. So the thermal gradient may not be a constant from shallow to deep. In summary, based on multichannel seismic reflection and, chirp sonar profile data, structural interpretation, the distribution of BSR depths and measured heat flow values, we suggest that the base of gas hydrate stability zone could be changed by high temperature fluids from deep, high heat flow may shift BSR to be shallow, and produce measured high heat flow values. Through interpretation of the observed heat flow values and seismic data in the Yung-An ridge, we could explain the distribution and variation of gas hydrate stability zone.