地質作用對台灣西南海域地熱構造的影響

Abstract

地質作用之於熱流模式的影響是天然氣水合物能源探勘中相當重要的研究。但海域相較於陸地更難直接測量到地熱資料，因此我們將過去幾年台灣西南海域量測到的海床地熱資訊，結合震測調查顯示所得到的密集的海底仿擬反射(BSR)分布，運用天然氣水合物本身的物理特性，取得海床下數百米深的溫度場作為本研究之重要參考數據。本研究中，我們首先建構起兩種熱流模擬的相關技術，讓我們應用在區域性的溫度場上，進行一維流體速率以及二維剖面上、三維空間上溫度場差異的分析，以瞭解其與各項地質作用之間的關聯性。 首先，我們利用BSR深度轉換所得的溫度場，以及量測所得的海床溫度作佩克萊數（Peclet numbers） 的分析而推導出區域性的一維流體移棲模式。我們發現到研究區域內的垂直向上流體速率範圍為每年6到43公分，當中更不乏許多天然氣水合物的精查重點區域。本項研究的結果發現流體移棲速率由非主動板塊區域逐漸向主動板塊區域漸增，可能與增積岩體的脫水現象有關；而沿著海溝向北更可發現流速在前緣逆衝攀附上非主動大陸板塊時達最高。此外，考量到地形效應可能會對溫度場量測所造成的影響，我們採用與海床地形變化有關的BSR溫度場，去扣除我們熱傳導模型（finite element code; Pecube）所取得的差異溫度場，作為討論分析地質作用之於溫度場變化之依據。 因此，我們依照研究的結果提出了幾個可能影響差異溫度場分布的地質作用來討論：（1）我們發現到位於上部斜坡之下枋寮峽谷處的泥貫入體構造，較兩翼 的沉積構造高出38%的熱效應；（2）相對地，在下部斜坡區的永安逆斷層處，我們認為其具有斷斷續續的熱流自斷層面向上供給影響區域溫度場；（3）而前緣海脊處，我們則是觀察到增積岩體前緣處的許多斷層裂隙處以及趾部受壓熱流上升的現象；（4）我們還提出了關於澎湖峽谷上游區的塊體滑移構造，其對於上坡處溫度場的影響是加溫；而對於下坡處則是降溫的解釋模型；（5）除此之外，我們發現在非主動板塊處的福爾摩沙海脊，擁有冷水吸入脊部加壓、以及脊部有通道向上洩壓的冷泉循環，並加以平行脊部方向的強烈擴散作用，方才導致今日我們所觀察到的許多降溫現象，或是冷泉生物存在的證明。 我們的研究乃為初次結合震測、地熱作為區域性的熱構造做模擬分析，將來有待於鑽井之後取得更精確的物理參數，讓我們原本定性的趨勢性研究成果，有機會可以推廣到定量而取得甲烷通量等十分重要的數據，藉時將有助於我們台灣西南海域的能源探勘研究。

Geological processes on thermal patterns are important for understanding the energy potential for hydrocarbon exploration; however, thermal patterns below the seafloor are always difficult to derive. So we combined the geophysical in-situ thermal measurements and the wide spreading bottom-simulating-reflectors (BSR; bottom of the gas hydrate stability zone identified from the seismic data) collected recent years, to model the temperature fields beneath the seafloor. Using the advantage of the physical property of gas hydrates, we proposed two methods for analyzing the regional thermal patterns offshore SW Taiwan, and successfully derived several geological processes related to seafloor thermal structures according to our study results. First, we used BSR-based geothermal gradient patterns to derive vertical fluid flow models from Peclet Number analyses. We found the regional 1-D fluid flow rates are ranging from 6 cm/yr to 43 cm/yr, including several prospect sites for gas hydrate exploration. From passive margin to active margin, the increasing of fluid flow rate might be related to more active dewatering near the toe of the trench; in the active margin, there are higher fluid flow rates when the frontal thrust climbs onto the continental slope of the passive margin in the collision zone. Then, considering about the topography effect, we used temperature discrepancies between BSR-based and modeled temperatures, which are derived from the finite element code – Pecube, to reflect the regional cooling and heating effects. Therefore we could further analyze and compare the regional geological processes based on our study results. Finally, we proposed several geological processes that can affect regional thermal patterns: (1) From studying the 3-D cube in lower Fangliao Canyon of the upper slope domain, we discovered the regional heating effect from the diapir was reduced 38 % by recent sedimentation; (2) relatively, we found intermittent upward fluid migrations along the fault planes brought the heating effects in Yung-An Ridge of the lower slope domain; (3) as well as in Frontal Ridge, we found possible extensive fluid migration from the faulting pathways to cause heating effects at the toe of the accretionary wedge. (4) Related to the MTD (mass-transport deposits) effect occurred in upper reach of the Penghu Canyon, it caused the heating and cooling effects at the uphill and downhill respectively. (5) Beside, in the canyon incision region of Formosa Ridge, we found the cold seawater had been laterally siphoned into the ridge in the traverse direction, and diffused along the ridge strike to disturb the temperature fields. In this study, our contribution is developing the simulation methods for better using our current data to analyze thermal structures; in the future, we could improve our simulation results to get more quantifiable flux information for the energy research offshore SW Taiwan.