以反射震測資料探討臺灣西南海域南海大陸斜坡之天然氣水合物儲集系統

Abstract

過去研究指出臺灣西南海域不論是活動或被動大陸邊緣區均有相當高的天然氣水合物賦存潛能。本研究分析近年來收集的高品質反射震測資料，探討南海大陸斜坡各探勘好景區，包括了九龍海脊、馬蹄鐵海脊、指標海脊與福爾摩沙海脊，以及位於變形前緣區的手掌海脊等5個天然氣水合物探勘好景區的天然氣水合物儲集系統，其次深入分析手掌海脊（橫跨變形前緣）與指標海脊（南海大陸斜坡）區的3維震測影像，探討大地構造與沉積環境對本區水合物儲集系統的影響，最後彙整分析結果，建立區域天然氣水合物儲集模式並探討其天然氣水合物的賦存潛能。 在技術發展方面，本研究利用浮纜定位資訊，開發真實3維震測資料處理技術，改善了資料的品質與解析度，有效提升後續資料分析以及地質解釋的可信度。由於古水道濁流砂層被認為是最佳的天然氣水合物儲集層，而斷層與氣囪構造更常扮演重要的地下流體移棲管道，詳細地辨識其空間位置並分析其震波特性，對於天然氣水合物儲集系統的研究至關重要。因此在研究方法方面，為期能從震測資料中擷取利用最大限度地相關地質資訊，本研究除了依循傳統的震測解釋方法，更應用震測屬性分析與類神經網路技術，幫助偵測如古水道濁流砂層、斷層以及氣囪構造等重要的地下地質目標。 橫跨變形前緣的震測剖面顯示手掌海脊區由西而東可分為正斷層區、原逆衝斷層區以及逆衝斷層區等三個主要的構造單元。由於變形前緣的定義為一聚合板塊邊界最前緣的壓縮構造，而位於前緣逆衝斷層西側的原逆衝斷層顯示由板塊聚合所造成的擠壓應力已經向西傳遞至此區域，本研究建議將變形前緣的位置西移至原逆衝斷層帶的西緣。藉由分析變形前緣區的構造與沉積特徵，本研究觀察到增積岩體向西發育的過程，並發現古水道（斜切陸坡走向）和現生海底峽谷（順坡方向）流向的顯著差異，因此本研究認為本區海底峽谷的演化亦受到板塊聚合的影響。從天然氣水合物儲集系統的角度而言，正斷層與逆衝斷層為手掌海脊區主要的流體移棲管道，這些流體管道使得深部的含天然氣流體得以向淺部傳輸，最後進入天然氣水合物穩定帶中，形成天然氣水合物，本區雖有背斜構造和古水道濁流砂層等良好的儲氣層，但因其位於天然氣水合物穩定帶以下，故不能成為天然氣水合物儲集層。 指標海脊區有一條東北-西南走向的主要正斷層構造，該斷層（稱為PR斷層）之上、下盤可分別觀察到沉積物波堆積與水道侵蝕特徵。震測屬性分析結果指出本區域主要的地下流體管道為PR斷層與其下盤之氣囪構造，這些地下流體管道位置和漁探儀在水層中觀測到的冒氣位置對應良好，指示了活躍的流體移棲現象；而PR斷層上盤的氣囪構造則多被沉積物深埋，可能較不活躍或已停止活動。震測資料顯示指標海脊區有許多古水道切蝕填充的痕跡，不論在PR斷層上、下盤均有古水道濁流砂層分布，特別是PR斷層下盤的古水道堆積物均位於天然氣水合物穩定帶中，是相當具潛能的天然氣水合物儲集層。 活躍的流體滲漏徵兆顯示南海大陸斜坡區的4個探勘好景區均有滲漏型探勘好景區的特性，以氣囪和正斷層構造為本區主要的地下流體管道，其中尤以福爾摩沙海脊的滲漏徵兆最為明顯，可期待在近海床的地層裂隙中取得天然氣水合物標本；而指標海脊區的砂質水合物儲集層則是研究區域中最富水合物探勘潛能的場址。本研究發現在南海大陸斜坡的活躍氣囪構造多出現在侵蝕特徵較明顯的環境，在沉積作用較穩定的環境則較少觀察到發育至近海床的氣囪構造，另在手掌海脊區則沒有發現氣囪構造，本研究認為此現象這可能是由於氣囪構造的發育受到了沉積環境與大地應力的影響而導致。

High potential gas hydrate reservoirs have been proposed in both active and passive margins offshore SW Taiwan. This study analyzes high quality seismic data for understanding the gas hydrate systems in gas hydrate prospects situated in the South China Sea continental slope and the deformation front. From west to east, they are Jiulong Ridge, Horseshoe Ridge, Pointer Ridge, Formosa Ridge and Palm Ridge. 3D seismic data images in Palm Ridge and Pointer Ridge prospects show the detail of the gas hydrate reservoir characters and reveal the relation between the geological processes and gas hydrate systems. Conceptual models of gas hydrate systems are constructed and then gas hydrate systems at different geological settings are compared. To improve the quality and resolution of the 3D seismic images, this study has developed a true 3D seismic data processing technique by utilizing the streamer feathering information from closely spaced 2D seismic profiles. To best extract the information from seismic data, seismic attribute analyses and neural network techniques are applied to enhance the seismic characters of potential reservoirs and fluid conduits in subsurface, such as sandy paleo-channel deposits, faults and chimneys. Three structural domains can be distinguished from seismic profiles that run across the deformation front from west to east in Palm Ridge: the normal fault zone, the proto-thrust zone, and the thrust fault zone. The presence of proto-thrusts that are located west of the frontal thrust implies that the compressional stress field has advanced westward due to the convergence of the Philippine Sea Plate and Eurasian Plate. Since the deformation front is defined as the location of the most frontal contractional structure, no significant contractional structure should appear west of it. We thus suggest moving the location of the previously mapped deformation front farther west to where the westernmost proto-thrust lies. High-resolution seismic and bathymetric data reveal that the directions of the paleo-submarine canyons run transverse to the present slope dip, while the present submarine canyons head down slope in the study area. We propose that this might be the result of westward migration of the deformation front that changed the paleo-bathymetry and thus the canyon path directions. The interactions of down-slope processes and active tectonics control the canyon paths in our study area. In terms of the gas hydrate system in the Palm Ridge prospect, normal faults and thrusts are major conduits for focused fluid flow. Although structural traps and turbidite channel sands are observed in Palm Ridge, none of them can be gas hydrate reservoirs because they all lie below the gas hydrate stability zone (GHSZ). A NE-SW striking normal fault which is called the PR fault acts as a major structure in Pointer Ridge. Sediment wave deposition and canyon erosion features are observed on the hanging wall and the footwall of the PR fault, respectively. Results from seismic attribute analyses show major fluid conduits are the PR fault and the gas chimney features in the footwall of the PR fault. The locations of these fluid conduits correlate well with the gas plume sites observed from water column images, indicating the active fluid flow processes in Pointer Ridge. The chimneys in the hanging wall of the PR fault may not be active conduits for fluid migration since they are all buried by sediments. Paleo-channel cut and fill features indicate that turbidite sands are distributed on both sides of the PR fault. Overall, the most potential gas hydrate reservoirs are the paleo-channel deposits in the footwall of the PR fault which are situated in the GHSZ. Our study results show that the four gas hydrate prospects located on the SCS continental slope have leakage-related structures such as chimneys and normal faults characterized by numerous seepage features, and gas hydrate samples might be recovered on or underneath the seafloor at shallow depth. This study proposes that the Formosa Ridge is the primary gas hydrate sampling site because of its strongest evidences of methane seepages, while the Pointer Ridge as the highest potential site for future gas hydrate development because the turbidite sands could trap significant amount of gases. Based on our observations, gas chimneys are likely to develop in erosion-dominated environments rather than deposition-dominated ones in the South China Sea slope, whereas there is no evidence of gas chimneys in the Palm Ridge area. These phenomena suggest the development of gas chimneys may be controlled by both of tectonic and sedimentary processes.