| 摘要: | 由反射震測剖面所顯現的”海底仿擬反射”(BSR)可指示天然氣水合物穩定帶的底部，以及游離氣存在的指標。台灣西南海域於震測剖面中發現大量的BSR，推測可能有大量的天然氣水合物賦存於此區域。根據觀察BSR與地形和構造之間的關係，本研究辨識出四個主要的BSR類型包含海脊型、盆地型、峽谷型和大陸斜坡型。此外，基於構造沉積特徵、BSR分布、天然氣來源、流體移棲管道、地質地化異常、水合物賦存層特性以及封閉構造等因素，本研究於增積岩體區與大陸斜坡區，建立了三個大尺度的水合物系統模式以及分出了六個主要的水合物賦存區。位於各水合物賦存區中的高潛力探勘好景區，歸納其水合物賦存模式可再細分為八種成藏類型。結果顯示若探勘好景區位於具有混合的天然氣來源、多重流體移棲通道、大範圍面積的高孔隙賦存層以及良好封阻的構造上，該好景區可能有較高的水合物賦存潛能，例如增積岩體最前緣褶皺(R1.1構造)、高屏峽谷下游附近的古深海扇(KP-12)和掩埋泥貫入體(KP-14)。 增積岩體最前緣褶皺(R1.1構造)為一高潛能的探勘好景區。廣泛的BSR分布顯示可能有水合物賦存於此區。在四面圈合的構造下，震測剖面顯示具有油氣徵兆的”平點”特徵以及BSR下方地層反射有下拉的現象，推測可能有大量游離氣賦存於構造下。這些游離氣的主要來源除了淺層的生物氣之外，可能來自於深部含天然氣流體沿著滑脫面、逆衝斷層或高孔隙傾斜地層向上移棲至此。此外，根據震測相可辨識出六期的古水道充填特徵，推測可能是過去古澎湖海底峽谷因構造抬升而向北移棲所遺留下來的特徵。數條古峽谷的沉積物可能含有高孔隙濁流砂層，或可成為良好的天然氣水合物賦存層。 本研究利用體積法，估算研究區域的天然氣水合物資源量。所使用的參數有水合物可能賦存面積、水合物可能存在的總厚度、淨厚對總厚的比率、隨深度增加的孔隙率、水合物於孔隙中的飽和度、轉換成天然氣的體積比以及甲烷在水合物中的晶格佔有率。利用蒙地卡羅統計模擬法估算每個參數與最後水合物的資源量，結果顯示研究區域內的天然氣水合物資源量高達2.7兆立方公尺；此外，具有最高水合物資源量的探勘好景區為增積岩體最前緣褶皺，約為5900億立方公尺的天然氣。此結果仍需要未來的深海鑽探加以驗證，以求得更精準的結果。Bottom simulating reflectors (BSRs) observed on seismic sections are often considered as indicators for the existence of free gas, delineating the base of the gas hydrate stability zone. Abundant BSRs seen on seismic sections acquired off the SW coast of Taiwan indicate the likely and prevalent existence of gas hydrates in the study area. Four major occurrences of BSR types, including ridge type, basin type, submarine-canyon type, and continental-slope type, are recognized on the basis of the relationship of BSRs to topographic and structural features. On the basis of tectonic and sedimentary features, BSR spatial distribution, inferred gas sources, migration pathways of gas-bearing fluids, geological and geochemical anomalies, characteristics of reservoir for hydrate occurrences, trap structures, this study established 3 large-scale gas-hydrate system models and 6 gas hydrate-bearing provinces in both the accretionary wedge and the South China Sea continental slope. The mechanisms of gas hydrate occurrence of the high-potential prospects situated in gas-bearing provinces can be categorized into 8 major gas hydrate play types. The results show that a prospect, which has higher potential for gas hydrate occurrence, features the combination of mixed gas sources, multiple migration conduits, large area of high-porosity reservoir, and well-developed trap structure. For examples, the frontal fold of accretionary wedge (R1.1 structure), ancient submarine fan (KP-12), buried mud diapir (KP-14) in the lower reach of the Kaoping canyon among others, are three prospects inferred to host a large amount of gas hydrates. There is a widespread occurrence of BSRs beneath the frontal fold (R1.1 structure) suggesting a promising gas hydrate existence. A seismic flat spot and a few push-down reflectors below BSR found lying underneath the anticlinal axis with bathymetric four-way dip closure also reveal abundant free gas accumulation beneath the structure. The free gas may derive from deep-seated gas-bearing fluids which migrate upward along multiple fault zones or permeable beds in addition to shallow-seated biogenic gas. The northward migrating of paleo-Penghu canyons driven by tectonic uplift of frontal fold have six stages of canyon/channel incisions marked by distinct channel infills. The multiple paleo-canyon infills seen along the frontal fold and above BSRs may provide thick porous sands to host a large amount of gas hydrate beneath the frontal fold. This study employed volumetric method to calculate the gas-hydrate resource for each prospect in the study area. The approach of Monte Carlo simulation was used to evaluate the probabilistic distributions for each parameter and gas hydrate resource. The parameters involved in the calculation are gas hydrate-bearing area, gross thickness, net-to-gross ratio, porosity, gas-hydrate pore saturation, volume ratio, and cage occupancy. The results show that the total resource of gas hydrates in the study area is 2.7 trillion cubic meters taking the computed value at 50% probability of occurrence. The prospect with the largest resource potential is the frontal fold of accretionary wedge, amounting to 21 tcf (~5.9×1011 cubic meters) of natural gas. The results indicate that future deep-sea drilling is essential for exploring and confirming the gas-hydrate resource off SW Taiwan. |
