台灣西南海域天然氣水合物潛藏區甲烷通量與流體來源探討

Abstract

台灣西南海域位於歐亞板塊及菲律賓板塊聚合隱沒交界處，地物資料顯示在台灣西南海域或深或淺的分布著「海床仿擬反射」（bottom simulating reflection, BSR），而這些「海床仿擬反射」指示天然氣水合可能分布在700-3000 公尺水深處，地球化學方面的資料也顯示，不論是底部海水或沉積物中的甲烷濃度都相當高；另外硫酸甲烷過度帶（sulfate-methane transition zone, SMTZ）所在深度很淺，皆指出在台灣西南的海床底下可能有很大的甲烷儲藏庫資源，也相當可能有天然氣水合物的存在。本研究利用2010年Marion Dufresne 178 航次中，在台灣西南海域採得的11根長岩心，計算出台灣西南海域活動大陸邊緣與被動大陸邊緣，不同的地質背景區域的甲烷通量，並發現溶解無機碳DIC總通量，可作為更精確推算甲烷通量的工具，可區分出硫酸鹽通量中進行甲烷厭氧氧化反應與有機物氧化反應的比例。結果發現，台灣西南海海域在活動大陸邊緣的甲烷通量，高於被動大陸邊緣的甲烷通量，但所有計算所得的甲烷通量，都高於一些有水合物或湧升流存在的研究區域。 沉積物孔隙水中Cl-、I-與Br-離子濃度隨深度的變化，常用來指示水合物的存在，將本研究的所有孔隙水中Cl-、I-與Br-離子濃度隨深度的變化，與前人研究在存在水合物的研究區域，所得到的結果數據作比較，發現本研究區域沒有明顯記錄到水合物形成或解離，及側向流體進入的現象，且I-與Br-離子濃度的關係圖也指出，沉積物孔隙水中的離子濃度變化主要受控於有機物降解作用。自沉積物孔隙水中離子濃度的變化，本研究結果（< 30 mbsf）無法明確指出水合物存在的證據，分布在台灣西南海域不同地質背景區域的高甲烷通量，可能來自更深處水合物解離出的甲烷，或底下甲烷蘊藏庫以氣體形式往上遷移。

Previous studies reported that the widely distribution of BSRs (Bottom Simulating Reflection) and the geophysical modeling indicating a large amount of methane hydrates in offshore Southwestern Taiwan. In this study, we estimate the methane fluxes along passive and active margins from the measured data of the cored sediments collected during Marion Dufresne 178 cruise in 2010. Total changes of the dissolved inorganic carbon (DIC) fluxes (ΔDIC-Prod ) can be used to estimate the methane flux quantitatively, and we confirm that the sulfate depletion is mainly controlled by the anaerobic oxidation of methane (AOM) reaction and/or the sedimentary organic matter. Although BSRs are widely distributed both in the active margin and in the passive margin, the methane fluxes in active margin are greater than in passive margin of the coring sites. All the estimated methane fluxes in offshore SW Taiwan are higher than other gas hydrate and upwelling area. Cl-, I- and Br- concentrations of pore water in cored sediments may be useful to indicate the existence of potential gas hydrates in study area. Compared the data with previous three studied cores in this area, we found that there is no evidence to indicate gas hydrate dissociation, formation or lateral fluid flow in studied area. The results shown in the I- vs. Br- diagrams, which can reflect these dissolved species in pore water, indicate the variations are mainly controlled by marine organic matter degradation. Since dissolved species in pore water can’t indicate the existence of gas hydrate in the studied samples (< 30 m), the high methane flux may result from deep gas hydrate reservoirs and/or free gas zones.