台灣西南海域天然氣水合物賦存區 沉積物間隙水地球化學與數值模擬分析研究

Abstract

甲烷自海洋沉積物及土壤逸散至海水甚至大氣已被視為全球碳循環之重要傳輸過程。近年來地球物理研究顯示，「海床仿擬反射」（Bottom Simulating Reflection，簡稱BSR）廣布於台灣西南海域，顯示海床仿擬反射層該深度以上蘊可能含天然氣水合物，以下則有游離氣體(free gas)存在，甲烷的儲量與遷移過程影響天然氣水合物生成與游離氣的分布，其中甲烷的儲量主要受控於沉積物中有機物的保存與降解量，而甲烷在遷移過程中受微生物作用影響，常於沉積物淺層發生甲烷厭氧氧化反應並消耗表層硫酸鹽。由於大量甲烷儲存於天然氣水合物中，其重要性可見於相關能源、氣候變遷與地質災害研究。本研究主要目的在了解台灣西南海域海床上以及沉積物中甲烷濃度於空間中分布情形、沉積物中甲烷通量及其來源、甲烷逸散至海床之通量、甲烷厭氧氧化速率、硫酸根與有機物對甲烷通量之影響，最後並探討大地構造與甲烷來源、通量及淺層甲烷反應之關係。 對於台灣西南海域海床上以及沉積物中甲烷濃度於空間中分布情形，本研究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。即使無法避免甲烷在採樣過程因溫壓條件改變而造成逸氣情形，調查分析結果顯示，台灣西南海域仍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；MD05-2912；MD05-2913；MD05-2914；ORI-758航次之GH10、GH16；ORI-765航次A、C、D、H；ORI-792航次之GS5；ORI-835航次之GT39B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自沉積物深部向上遷移，並逸散至海床與海水中。綜合各航次調查結果顯示，主要高甲烷濃度的站位皆分佈於活動大陸邊緣(台南海脊、永安海脊、好景海脊、枋寮海脊)，而被動大陸邊緣顯著異常甲烷逸氣的站位主要出現於福爾摩沙海脊。 本研究部分站位沉積物之硫酸鹽-甲烷交界面（sulfate methane interface, SMI）非常淺，顯示沉積物深部具高甲烷通量，由於硫酸鹽梯度呈線性、總有機碳（TOC）含量低，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（AOM）所進行。根據AOM反應所消耗的甲烷及硫酸鹽的莫爾數比為1:1，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。由硫酸鹽濃度隨深度變化梯度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。 本研究亦利用數值模擬方法，模擬間隙水化學濃度隨深度變化情形((CH4, SO42–, I–, Cl–, TOC)，計算甲烷、硫酸鹽及有機物之通量與反應速率，結果顯示沉積物表層之硫酸鹽主要經AOM反應所消耗，有機物降解對本區硫酸鹽濃度變化影響較小。部分岩心分析結果顯示，表層硫酸鹽濃度不隨深度遞減反而維持海水值，甚至大於一米深之沉積物中亦可見此情形，其主要是因氣泡(甲烷)擾動影響所致，此外，由於地球化學資料無法顯示深部流體存在之訊號，硫酸鹽的消耗需由沉積物深部之氣體遷移(gas flow)支持，甲烷自氣相溶解至間隙水中以進一步行AOM反應。而各項反應速率與通量也受控於採樣地點之地質構造。 沉積物間隙氣體碳同位素分析結果顯示，甲烷之碳同位素值（δ13C）介於-28.3‰ ~ -95.0 ‰。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。整合甲烷與溶解無機碳(Dissolved Inorganic Carbon, DIC)之碳同素隨深度變化情形，可見兩者之碳同位素最小值皆出現於 SMI深度附近，主要是因AOM與二氧化碳還原反應在此深度附近形成碳循環，本研究發展之數值模擬方法亦證明碳循環與同位素交換等過程存在於此區沉積物中。 利用數值模擬方法可進一步推估天然氣水合物上界，該深度為甲烷濃度與天然氣水合物達飽和平衡，初步估算結果顯示天然氣水合物有機會於所採岩心中形成(< 40m)，且多位於SMI深度較淺的站位中，未來研究將配合沉積速率與其他地球化學濃度資料進行估算，以減少誤差並可進一步估算天然氣水合物儲量。

Methane emission’ from ocean sediments or soils to the water column or atmosphere is an important process for global carbon cycling. According to the geophysical study, the widely distributed Bottom Simulating Reflectors (BSRs) imply the potential existence of gas hydrates above them in offshore southwestern Taiwan. Free gas can be trapped below BSRs. Except temperature and pressure conditions, the existences of gas hydrate and free gas are controlled by the amount of methane and methane migration paths in marine sediments. Organic matter perseveration and degradation determine the size of methane reservoir. When methane migrates to the surface, methane would involve the surface biogeochemical diagenetic reactions. Through the Gas hydrate exploration program conducted by the Central Geological Survey in Taiwan, we have systematically collected sea waters and cored sediments for the dissolved and pore-space gas composition and isotope analysis in offshore southwestern Taiwan. The main purpose of this study is to understand the sources and sinks for methane, since large amount of methane trapped in gas hydrates plays important roles on some issues, e.g., a potential energy source, a factor in global climate change and a submarine geohazard. Some sites with extremely high methane concentrations have been found, e.g., sites G23 of ORI-697, N8 of ORI-718, G96 of ORI-732; MD05-2911; MD05-2912; MD05-2913; MD05-2914; GH10, GH16 of ORI-758; A, C,D, H of ORI-765; GS5 of ORI-792 and GT39B of ORI-835. The methane concentrations of cored sediments display an increasing trend with depth. In addition, sites with high dissolved methane concentrations in the water column have been found at sites A and H of ORI-765 which indicates the high gas venting activities in this region. Data reveal that high CH4 concentrations are mainly distributed in the active margin, e.g. Yuan-An Ridge, Tai-Nan Ridge, Good Weather Ridge and Fangliao Ridge. The high gas venting sites found in the passive South China Sea continental margin are mainly around the Formosa Ridge. In addition, the depth profiles of methane and sulfate contain very shallow depths of sulfate methane interface (SMI) at some sites. The linear sulfate gradients, low total organic carbon (TOC) and high methane concentrations imply that anaerobic oxidation of methane (AOM) is the main process for sulfate consumption. Thus, the diffusive methane fluxes can be determined through the gradients of sulfate by Fick’s first law. The methane fluxes are very high in comparison with other gas hydrate study areas, 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). A numerical transport-reaction model applied to the data (CH4, SO42–, I–, Cl–, TOC) can quantify the diagenesis processes. Results show that AOM is the major reaction for sulfate reduction rather than organic matter degradation. Some porewater sulfate concentrations show bottom-water like signatures deeper than 1 meter which may be induced by bubble irrigation. Due to there is no evidence for upward fluid flows, methane consumed by sulfate needs to be dissolved from gas phase indicating the migration of methane through gas flows. The modeled results also demonstrate that the geochemical trends in the sediments offshore SW Taiwan can be explained by the different geological settings. The δ13C-CH4 values range from -28.3‰ ~ -95.0‰. Except sites in the upper slope domain (site GT39B or G96), bacterial methane is dominated in the surface sediments. The 13C-depletion around the SMI can be observed both from δ13C-CH4 and δ13C-DIC profiles. Hence, δ13C-CH4 and δ13C-DIC data coupled with the particulate organic carbon content (POC) less than 1% can confirm again that sulfate reduction is mainly driven by the process of anaerobic methane oxidation in this study area. Furthermore, the depths of 13C-depletion in methane and DIC are discordant. The lightest δ13C-CH4 values are slightly deeper than the lightest δ13C-DIC values among seven cores. This may infer the carbon cycling around the depths of SMI through AOM and CO2 reduction. A reaction-transport model developed to fit carbon isotopic data in this study can proof carbon cycling and isotopic mixing in the surface sediment. Finally, the upper boundary of gas hydrate can be predicted by numerical simulations according to the methane saturation depth. The preliminary results show that gas hydrate can form within the cored sediments (<40 m) are coexist with the shallow depth of the SMI. However, to get reliable results for the upper boundary of gas hydrate, sedimentation rates and more completed geochemical profiles are needed to apply in the model. Afterward, we can estimate the amount of gas hydrates stored in offshore SW Taiwan.