臺東鹿野溪下游地熱區古流體溫壓及三維應力場之探討

Abstract

臺灣位於歐亞板塊與菲律賓海板塊交界處，在板塊聚合作用的影響下，地殼持續碰撞、造山及快速掘升，由於火山活動與板塊交互作用，造就國內擁有豐富的地熱資源可供開發使用，地熱發電供電穩定且不易受天候影響，若善加利用不僅對於國內能源自主與多樣化有相當大的幫助，更可以減少我國對石化能源過度依賴的情形，因此臺灣的地熱資源探勘與地熱潛能評估是現今刻不容緩的重要工作之一。臺東縣延平鄉鹿野溪，地處中央山脈東翼的紅葉地區，近年來是國家發展地熱產業的重點區域之一，張郁敬(2022)綜合遙測判釋、地質調查以及裂隙活化趨勢分析之研究結果，研判現地導水裂隙位態以西北－東南方向高傾角為主。經本研究地質調查結果指出，鹿野溪沿岸有相當多溫泉，並且有正斷層構造出露，正斷層多伴隨石英脈發育與熱液流出，這些後期礦脈位態也以西北－東南方向為主，與現地導水裂隙方向一致，因此本研究延續前人結果，以臺東鹿野溪紅葉谷溫泉地區為例，結合礦脈位態與液包體（fluid inclusion）分析對鹿野溪進行地熱開發評估，評估鹿野地區溫度的增減。由於石英脈發育時，二氧化矽流體液壓至少要達到最小正應力，才能將岩體撐開，因此能利用礦脈位態統計進行古應力場評估進而解算其三維應力狀態與生成深度，並以石英脈中纖維狀液包體進行分析，瞭解礦脈形成時的溫度液壓資訊。經野外觀察及室內分析，最後一期西北－東南向礦脈發生於正斷層應力場，石英礦脈形成在地下五公里左右，並且包裹溫度達到407"°C" ，包裹壓力為109MPa，顯示鹿野溪有超額液壓的存在，然而現地鑽井卻沒有井噴的狀況，推測地下有類似蓋層的滲透屏障存在，導致地下儲集層富含高溫過壓流體，受到後期構造活動影響，鹿野地區發展出許多礦脈與斷層，並且形成大量高傾角的西北-東南向開口裂隙群，這些正斷層與裂隙很有機會成為高溫過壓流體通道，綜合液包的溫壓資料以及應力場的重建與評估，以及前人磷灰石與鋯石核飛跡定年資料及井下溫度數據，本研究推測，鹿野溪下游地熱區在未來4.5百萬年內，其地溫梯度仍可維持在47°C/km以上，具有良好的地熱潛力，若能將其地熱資源有效應用於地熱工業的發展，將具備極高的利用價值。Taiwan is located at the boundary between the Eurasian Plate and the Philippine Sea Plate. Due to the ongoing convergence of these plates, the region experiences continuous crustal collision, mountain building, and rapid uplift. The interaction between volcanic activity and tectonic processes has endowed Taiwan with abundant geothermal resources, which hold significant potential for development. Geothermal power generation offers stable energy output, unaffected by weather conditions, making it a valuable resource for enhancing Taiwan’s energy self-sufficiency and diversifying its energy portfolio. Additionally, leveraging geothermal energy can help reduce Taiwan’s reliance on fossil fuels. Consequently, geothermal resource exploration and potential assessment are critical and urgent tasks for Taiwan's energy development. In Yenping Township, Taitung County, the Luye River area, located on the eastern flank of the Central Range in the Hongye region, has recently become one of the key areas for developing the national geothermal industry. Chang (2022) integrated results from remote sensing interpretation, geological surveys, and fracture activation trend analysis, concluding that the dominant water-conducting fractures in this area are characterized by steep northwest-southeast orientations.This study’s geological surveys along the Luye River revealed numerous hot springs and the presence of normal fault structures. These normal faults are often associated with quartz vein development and hydrothermal outflows, with late-stage mineral veins predominantly aligned northwest-southeast, consistent with the water-conducting fracture trends.Building upon previous findings, this study focused on the Hongye Valley hot spring area along the Luye River, using mineral vein orientations and fluid inclusion analyses to evaluate the geothermal development potential of the Luye River region, particularly in terms of temperature fluctuations. During the formation of quartz veins, the silica-bearing fluid pressure must exceed the minimum principal stress to propagate fractures. This allows for paleo-stress field reconstruction through statistical analysis of mineral vein orientations, enabling the determination of three-dimensional stress states and formation depths. Fluid inclusions within fibrous quartz veins were analyzed to infer temperature and pressure conditions during mineralization.Field observations and laboratory analyses indicate that the latest northwest-southeast-oriented mineral veins formed within a normal fault stress regime. Fluid inclusions in quartz veins suggest formation at depths of approximately 5 kilometers, with inclusion temperatures reaching 407°C and pressures of 109 MPa, indicating the presence of overpressured fluids in the Luye River region. However, no blowouts were observed in current boreholes, suggesting the existence of a subsurface permeability barrier, akin to a caprock, trapping high-temperature overpressured fluids within the reservoir. Subsequent tectonic activity has produced numerous mineral veins and faults in the Luye area, forming dense, high-angle northwest-southeast-oriented open fracture networks. These normal faults and fractures likely serve as conduits for high-temperature overpressured fluids.By integrating fluid inclusion data with stress field reconstruction and geothermal assessments, this study concludes that the Luye River region holds substantial geothermal potential.Combined with apatite and zircon fission-track dating and downhole temperature data, it is estimated that the geothermal gradient in the downstream geothermal zone of the Luye River could still exceed 47°C/km after 4.5 million years. If these geothermal resources can be utilized for industrial development, the potential for sustainable exploitation appears promising.