台灣西南海域天然氣水合物賦存區之氣體地球化學研究

Abstract

由台灣西南海域廣佈的「海床仿擬反射」（bottom simulating reflection，簡稱BSR），顯示此區域沉積物中蘊含大量的天然氣水合物。受限於溫度、壓力等條件，天然氣水合物的穩定範圍有其界線，一旦超出此範圍，則原已生成的天然氣水合物將會解離產生水和甲烷。故沉積物中若含有天然氣水合物，其解離產生的水和氣體混入沉積物中，將會使海水中甲烷濃度增加及沉積物間隙氣體的甲烷濃度增高。因此海水以及沉積物間隙氣體中的甲烷濃度，可作為探勘此區域天然氣水合物存在潛力的重要指標之ㄧ。 本研究自2004~2005年共經歷了八個航次（ORI-697、ORI-718、ORII-1207、ORII-1230、ORI-732、Marion Dufresne (MD)、ORI-758及ORI-765等航次）採集海水及沉積物樣品，分析其中海水溶解甲烷氣濃度及沉積物間隙氣體甲烷濃度。調查分析結果顯示，台灣西南海域海床上的確有許多異常高濃度含量甲烷氣的位置被發現（ORI-697航次之G23、ORI-718航次之N8、ORI-732航次之G96、MD05-2911(G22)、MD05-2912(G24)、MD05-2913(G5A)、MD05-2914(732-5)、ORI-758航次之GH10、GH16、ORI-765航次A、C、D、H等站位），且甲烷濃度有隨沉積物深度增加而逐漸增高的趨勢。此外，ORI-765航次A、H兩站位海水溶解氣體也出現異常高之甲烷氣濃度，也有隨深度加深而逐漸增高的趨勢，對應兩站位沉積物間隙氣體的甲烷氣濃度也異常高，顯示該站位有大量甲烷氣正由海床逸散之地表。綜合兩年之調查結果顯示，主要異常甲烷氣高濃度的站位皆分佈於活動大陸邊緣，而被動大陸邊緣初步調查結果，海床淺處則尚未有顯著異常甲烷逸氣的站位發現。 配合沉積物間隙氣體甲烷濃度及間隙水硫酸鹽濃度剖面，可觀察到本研究區有些站位沉積物之硫酸鹽-甲烷交界面（sulfate methane interface, SMI）非常淺。另外由線性的硫酸鹽梯度、總有機碳（TOC）含量低及高甲烷濃度等證據，顯示沉積物中的硫酸鹽還原反應主要是經甲烷厭氧氧化反應（AMO）所進行。由於AMO反應所消耗的甲烷及硫酸鹽的莫爾數比為1:1，因此可經由擴散定律公式先計算出硫酸鹽通量用以代表甲烷通量。計算結果顯示台灣西南海域普遍都具有高的甲烷通量，尤其是ORI-697航次之G23（4.12×10-2 mmol cm-2yr-1）及ORI-718航次之N8（2.11×10-2 mmol cm-2yr-1）兩站位更有異常高的甲烷通量出現於沉積物中。此結果顯示海床底部有一很高的甲烷逸氣來源，最有可能為天然氣水合物解離釋出大量的甲烷所致。 本研究也選取了部分沉積物間隙氣體樣品（ORI-718航次之N6; N8; G22; ORI-732之G96及MD航次等站位）分析碳同位素值以探討氣體來源。分析結果顯示，甲烷之碳同位素值（d13C）介於-28.3~-95.0 ‰，二氧化碳之碳同位素值介於-11.6~-31.4 ‰。由甲烷之碳同位素分析結果，可推論本研究區沉積物淺處之氣體成份以生物來源為主，越往深處則可能有熱分解來源之氣體加入。

It has been found that Bottom Simulating Reflections (BSRs) widely distribute in offshore southwestern Taiwan which infer the existence of potential gas hydrates underneath the seafloor sediments. Fluids and gases derived from dissociation of gas hydrates which usually enriched in methane concentrations would affect the compositions of sea water and sediments nearby the venting areas. Hence, methane concentrations of sea waters and sediments become useful proxies for exploration of potential gas hydrates in one area. We systematically collected sea waters and cored sediments for dissolved and pore-space gas analysis through eight cruises: ORI-697, ORI-718, ORII-1207, ORII-1230, ORI-732, Marion Dufresne (MD) cruise, ORI-758 and ORI-765 from 2003 to 2005 in this study. Some unusual high methane concentrations can be found in offshore southwestern Taiwan, e.g., sites G23 of ORI-697, N8 of ORI-718, G96 of ORI-732, MD05-2911(G22), MD05-2912(G24), MD052913(G5A) and MD05-2914(73205) of MD cruise, GH10 and GH16 of ORI-758 cruise, sites A, C, D, and H of ORI-765 cruise. The methane concentrations of cored sediments display an increasing trend with depth. Meanwhile, the water column also showed unusual high dissolve methane concentrations at sites A and H of cruise ORI-765. It indicates that gases are venting actively from seafloor in this region. Compiling the available data, high CH4 concentrations are mainly distributed in active margin SW offshore Taiwan; nevertheless, no abnormally high concentrations were found in passive margin so far. In addition, the profiles of methane and sulfate of cored sediments reveal very shallow depths of sulfate methane interface (SMI) at some sites in this study. The linear sulfate gradients, low total organic carbon (TOC) and high methane concentrations imply that sulfate reduction is mainly driven by the process of anaerobic methane oxidation (AMO) in sediments. Thus, the methane fluxes can be determined through the gradients of sulfate reduction and steady state solutions to diffusion equations and show that the methane fluxes are very high, especially at sites G23 of ORI-697 (4.12×10-2 mmol cm-2yr-1) and N8 of ORI-718 (2.11×10-2 mmol cm-2yr-1) in offshore southwestern Taiwan. It indicates that there is a methane-enriched venting source, which may be the product of dissociation of gas hydrates, in this area. Some selected gas samples from ORI-718 (N6； N8； G22); ORI-732 (G96) and MD cruise have also been analyzed for carbon isotopic compositions. The δ13C data of CH4 gases range from -28.3~ -95.0 o/oo and -11.6 ~ -31.6 o/oo for CO2. The carbon isotopic compositions of methane show that organic gas source is dominated at shallower depth, however, some thermogenic gases might be introduced from deeper source in this region.