宜蘭地熱區裂隙活化趨勢分析初探以芃芃溪、排谷溪及土場-仁澤為例

Abstract

為因應全球於2050年實現淨零排放的目標，臺灣政府積極推動地熱發電作為能源轉型的重要策略，因此極需大規模且精準的前期地熱探勘作業。前人研究指出，宜蘭地區具備多處溫泉與噴氣孔等地熱徵兆，配合鑽探資料推估的高地溫梯度，顯示其具有優良的地熱潛能。本研究以宜蘭地區芃芃溫泉、排谷溪、土場與仁澤等地為範例，評估裂隙作為地熱流體通道的潛力。首先利用高解析度數值高程模型判釋地表線型構造之特徵線段，並運用三角網格法進行裂隙位態計算，再結合球型變異分析與投影角篩選技術以提升資料準確度。配合震源機制反演所得應力場與野外地質調查，探討裂隙分布與地質構造及應力場之間的關係。最後根據裂隙活化潛勢統計製作裂隙密度分布圖與活化趨勢圖，推估熱液流出至地表之潛在區位，作為後續地熱探勘與開發的依據。數值高程模型與三角網格分析顯示，芃芃溪與排谷溪地區共判釋出252條裂隙，主要以近南北走向高角度裂隙為主；土場與仁澤地區共296條裂隙，走向以東北-西南與西北-東南為主。震源機制應力逆推結果顯示，芃芃溪與排谷溪區域應力為正斷層應力場， 水平最小擠壓方向為東-西向。土場-仁澤地區區域應力為正斷層應力場， 水平最小擠壓方向為東北東-西南西向。震源機制反演與裂隙活化分析指出，芃芃溪與排谷溪地區導水性佳的高活化裂隙主要為西北-東南走向之高角度裂隙；土場與仁澤地區則以西北-東南走向裂隙具導水性佳的高活化裂隙。參考地質調查現地熱水裂隙位態所訂定之活化趨勢閥值，結果顯示芃芃-排谷溪地區之擴張趨勢閾值為0.8、不安定性0.6；土場-仁澤地區之擴張趨勢閾值為0.8、不安定性0.7。綜合上述分析，芃芃、排谷溪與土場、仁澤地區皆具有良好的地熱裂隙活化潛勢與導水能力，可能為熱液上湧的主要通道。本研究建立之裂隙密度圖與活化潛勢圖可有效降低地熱探勘風險與不確定性，並為未來宜蘭地區地熱資源開發提供重要參考依據。To achieve the global goal of net-zero emissions by 2050, the Taiwanese government is actively promoting geothermal power generation as a key strategy for energy transition. As a result, large-scale and accurate preliminary geothermal exploration is urgently needed. Previous studies have indicated that the Yilan area possesses numerous geothermal indicators such as hot springs and fumaroles. Combined with drilling data that suggest a high geothermal gradient, it suggests that Ilan is a excellent geothermal potential.This study focuses on evaluating the potential of natural fractures as geothermal fluid conduits in selected areas of Ilan, including the BonBon Hot Spring, PaiGu River, Tuchang, and Renze. High-resolution Digital Elevation Models (DEMs) were used to interpret surface lineaments and analyze their structural orientations using Delaunay triangulation algorithm. The spatial distribution of fractures was then examined in relation to geological structures and stress fields, according to result from focal mechanism inversion and field geological surveys.Based on the statistical analysis of fracture activation potential, density distribution map and fracture potential map were generated to estimate potential zones where geothermal fluids may surface up, providing a basis for future geothermal exploration and development.DEM and Delaunay triangulation algorithm analysis identified of 252 fractures which domonately strike N-S oriention and dip subvertically in the BonBon and PaiGu River areas. There were 296 fracture identified in the Tuchang and Renze zones, predominantly striking NE-SW and NW-SE. The focal mechanism inversion revealed a normal faulting stress regime in the BonBon and PaiGu River, with E-W oriention of the minimum horizontal stress. The Tuchang-Renze area also exhibits a normal faulting regime, but with a minimum horizontal stress oriented NE-SW.Fracture activation analysis and focal mechanism results indicates that NW-SE trending fractures with steep dip in the BonBon and PaiGu River have high activation potential and perhaps good permeability. In the Tuchang and Renze areas, NW-SE fractures also show activation potential and good permeability. Based on the characteristics of geothermal conduits observed in the field, activation thresholds were established. For BonBon and PaiGu River, the dilation threshold value is 0.79, slip threshold 0.15, and instability is 0.65. For Tuchang-Renze, these values are 0.95, 0.14, and 0.68, respectively. Therefore, the threshold values were set at an dilation threshold of 0.8 and instability of 0.6 for BonBon and PaiGu River, and 0.8 and 0.7 for Tuchang-Renze.In summary, the BonBon and PaiGu River, Tuchang, and Renze areas exhibit significant geothermal fracture activation potential and permeability, suggesting they may serve as key conduits for upward hydrothermal flow. The fracture activation potential and density maps developed in this study can effectively reduce the risks and uncertainties associated with geothermal exploration and provide valuable references for future geothermal development in the Ilan area