南海北部大陸邊緣斜坡海脊含天然氣水合物地層的構造與沉積特徵

Abstract

近年來，台灣西南海域的天然氣水合物研究已邁入精查階段，針對探勘好景區進行精密的震測調查有助於瞭解天然氣水合物在調查區域中之分布情形。本研究利用密集震測資料分析南海北部大陸邊緣天然氣水合物的分布與特性，研究區域主要在大陸斜坡上的一個斜坡海脊(侵蝕殘餘高區)，此海脊形貌寬長並且遍布BSR。本研究將2005年海研一號754、764航次收集的多頻道反射震測資料整編成一組測線間距約400公尺的2.5D震測資料，並對穿過研究地區的EW9509-34與MGL0905-22兩筆長支距震測剖面資料進行疊前深度移位處理，以利用密集震測剖面與精確速度分析資訊來探討斜坡海脊的地質架構與天然氣水合物系統。 震測剖面中可見海脊內部反射層序有切蝕填充的古河道、侵蝕與不整合面、沉積波的發育等等，顯示此處地質作用相當複雜。本研究將海脊地形分為上部海脊區與下部海脊區，各自有不同的沉積特徵，而BSR訊號又受到沈積與構造的影響，表現出不同之特徵。本研究將BSR為三類：1. 代表穩定富集的清晰連續型BSR，主要分布在地形平緩的上部海脊區。此處層序反射特徵平行，沉積環境屬穩定的加積作用，天然氣水合物可穩定存在於此；2. 振幅增強型BSR，遍布地形梯度大的下部海脊區。此處觀察到的波狀層序與侵蝕截切現象顯示該區域有快速的沈積與侵蝕作用，造成沉積環境的不穩定，可能影響天然氣水合物之穩定帶，造成BSR之動態移動；3. 較不易辨識的層間型BSR，主要分布在海脊尾端至大陸隆堆區。由於大陸隆堆為一穩定沈積環境，故此區之BSR訊號易與地層訊號平行而難以辨識。速度分析顯示在BSR上方有水合物存在孔隙中形成的高速層，而下方為富含游離氣體的低速層，此現象尤以海脊上兩處地形高區最為明顯，顯示應有豐富天然氣水合物賦存。

This study uses a dense seismic data set to analyze the distribution and characters of hydrate-bearing strata in the northern South China Sea (SCS) continental margin. The study area is a slope ridge formed as a remnant erosional topographic high where BSRs are abundant. The dense seismic data block consists of a series of 2D seismic profiles with 400-m line spacing and covers an area of 13-by-23 km. In addition, two large offset seismic profiles which run across the study area, EW9509-34 and MGL0905-22, have been processed through prestack depth migration to provide good velocity constraints and better deep structural images of the slope ridge. Numerous cut-and-fill channel deposits, erosional truncations and sediment waves observed on seismic sections indicate that geological processes in the slope ridge are active and complicated. Our study suggests that there are different sedimentary characters in the upper ridge and in the lower ridge, respectively. BSRs are affected by local sedimentary processes and can be categorized into 3 types: Type 1 is continuous and clear BSR, usually distributed in the upper ridge. This type of BSR suggests that gas hydrate is stable. Type 2 BSR is associated with enhanced reflectors, it is commonly observed in the lower ridge where rapid sedimentation and erosion have changed local temperature and pressure, result in adjusting of gas hydrate stability zone, therefore shifting the BSR. Type 3 BSR is intra-layer BSR. It mainly exists in distal lower ridge, and hard to be identified, because the seafloor is parallel to strata. High velocity zone above BSR may indicate the existence of gas hydrate and low velocity zone below BSR are interpreted to be free gases zone.