清水地熱區的熱源與流體演化

宜蘭清水地熱曾經建立台灣第一座地熱發電廠，從1960年代以來鑽鑿超過21口井，因而累積許多地質、地球物理探測和地球化學的資料。然而由於全區皆是厚層的廬山層板岩，缺乏地層對比的依據加上構造複雜，詳細的熱源以及主要的導水裂隙方向至今仍有爭議。利用清水地熱區的野外斷層帶附近和岩芯的方解石脈以及井內的結垢，以分子同位素溫度計和液包體溫度計獲得結晶溫度，結合碳氧同位素值來計算原水並推測清水地熱熱源的來源。清水地熱的井內結垢其原水的氧同位素值為-5.8±0.8 ‰ VSMOW是天水成分；然而清水野外斷層帶附近的方解石脈計算後的原水氧同位素值介在-1.0±1.6 ‰ 到10.0±1.3 ‰ VSMOW是岩漿水或變質水成分。因此根據同位素地球化學資料、大地電磁探測法和微震資料，我們認為清水地熱有兩個熱水儲集層，斷層帶的方解石脈來自深部的高溫儲集層，為岩漿水或變質水成分並有大理岩脫碳作用；而地熱電廠所抽取的熱水則來自淺部的儲集層，為天水被間接加熱形成。清水第21號井內岩芯內方解石晶型有刀刃狀、菱面晶體以及塊狀。刀刃狀的方解石指示其生長於約165℃ 快速逸氣沸騰的環境，計算後原水的氧同位素值為-6.8 ‰ 至 -10.2 ‰ VSMOW；菱面晶體方解石生長在約180℃ 離子濃度低的天水；塊狀方解石常常和石英交錯生長，同位素顯示其原水是天水和岩漿水以不同比例混合，其計算後的原水氧同位素值可達1.5±0.7 ‰ VSMOW。由這些資料可以發現刀刃狀方解石和菱面晶體方解石的原水與地熱電廠使用的熱水氧同位素值接近，這些岩脈的主要方向為北偏東10度向東傾70度，也就可能是現今主要的清水地熱裂隙導水的方向。

關鍵字

清水地熱；分子同位素；岩漿源；液包體；方解石晶型

並列摘要

The Chingshui geothermal field, a moderate-temperature and water-dominated hydrothermal system, was the site of the first geothermal power plant in Taiwan. Many geological, geophysical and geochemical studies with more than 21 drilling wells have been performed since 1960s. However, there are still controversies regarding the heat and fluid sources and the main conduits due to the tectonically complicated geological setting. To clarify the heat and fluid sources, the calcite scaling from production wells and veins from outcrops and cores were collected to analyze clumped isotopes and fluid inclusions for temperature measurements and carbon and oxygen isotopic compositions, then combined both data to calculate the 18O values of the source fluids. Two populations of 18O values were recognized: -5.8±0.8 ‰ VSMOW from scaling in the wells and -1.0±1.6 ‰ to 10.0±1.3 ‰ VSMOW from the calcite veins of outcrops, which are indicative of meteoric and magmatic fluid sources, respectively. Meanwhile, two hydrothermal reservoirs at different depths have been identified by magnetotelluric (MT) imaging with micro-seismicity underneath this area. As a result, two-reservoir model has been proposed: One is the shallow reservoir with fluids from meteoric water to provide the thermal water for scaling depositions inside the production wells, while the deep one supplies magmatic fluids mixing with deep marble decarbonization to precipitate the calcite veins near fault zones. Three types of calcite crystal morphologies have been identified in the veins of the cores of well IC-21: bladed, rhombic and massive crystals. Bladed calcites are generated via degassing under boiling conditions with a precipitation temperature of ~165℃ and calculated δ18O value of -6.8 ‰ to -10.2 ‰ VSMOW for the thermal water. Rhombic calcites grow in low concentration Ca2+ and CO32- meteoric fluids and precipitate at approximately ~180℃. Finally, massive calcites are characterized by co-precipitation with quartz in the mixing zone of meteoric water and magmatic or metamorphic fluids with calculated δ18O value of up to 1.5±0.7 ‰ VSMOW. Furthermore, the scaling and hot fluids at a nearby pilot geothermal power plant confirm a meteoric origin. Based on these observations, we propose that the current orientations of the main conduits for geothermal fluids are oriented at N10°E with a dip of 70°E. This result provides the basic information needed for deploying production and injection wells in future developments of the geothermal power plant in this region.

並列關鍵字

Chingshui geothermal field ； clumped isotopes ； magmatic fluid ； fluid inclusions ； calcite crystal morphologies

參考文獻

Lin, J.J., 2000. Study on the Geothermal Power Development of Ching-shui at Ilan Prefecture. J. Pet. Mar. 36, 29–37. (Chinese content with English abstract)

Chiang, C.W., Hsu, H.L., Chen, C.C., 2014. An Investigation of the 3D Electrical Resistivity Structure in the Chingshui Geothermal Area, NE Taiwan. Terr. Atmos. Ocean. Sci. 26, 269–281. doi:10.3319/TAO.2014.12.09.01(T)

Ho, G.R., Chang, P.Y., Lo, W., Liu, C.M., Song, S.R., 2014. New evidence of regional geological structures inferred from reprocessing and resistivity data interpretation in the Chingshui-Sanshing-Hanchi area of Southwestern Ilan County, NE Taiwan. Terr. Atmos. Ocean. Sci. 25, 491–504. doi:10.3319/TAO.2014.01.24.01(TT)

Yeh, E.C., Sun, T.H., Lin, S.T., Lee, W.C., Lin, W., Wu, Y.M., Wang, T.T., Song, S.R., Lin, W., 2013. Investigation of Relationship between In-Situ Stress and Fluid Conduits from Chinshui Geothermal Area, NE Taiwan, in: AGU Fall Meeting. p. H51D–1218.

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國際替代計量

清水地熱區的熱源與流體演化

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