臺灣地層的岩石熱導係數研究

Abstract

熱導係數為計算熱流的重要參數之一，臺灣測量岩層熱導係數的研究很少，因此本研究於西部麓山帶、雪山山脈、脊樑山脈、海岸山脈和恆春半島等地質區主要地層進行岩層熱導係數量測。使用岩心取樣器，鑽取直徑6公分、長10~30公分的岩心或採集岩石樣本，共計於40個地層中取得101個岩石或岩心樣本及1個土壤樣本，樣本經磨平處理後，以熱導係數儀測量其熱導係數。測量結果發現，樣本中最高值為四稜砂岩的變質砂岩(~5.21 Wm-1K-1)，最低值為大平層的紅土(~0.33 Wm-1K-1)，平均值為2.25 Wm-1K-1，中位數為2.11 Wm-1K-1，標準差為0.93 Wm-1K-‍1。依照地質年代分類，成岩地層中熱導係數最大值為雪山山脈始新世的四稜砂岩(~5.21 Wm-1K-1)，最小值為恆春半島上新世馬鞍山層泥質砂岩(~0.39Wm-1K-1)，推測雪山山脈的四稜砂岩由於其礦物組成多為熱導係數較高的石英組成，因此熱導係數偏高，恆春半島的馬鞍山層泥質砂岩可能因孔隙率較高，因此熱導係數偏低。若是以地質分區分類，最高值為雪山山脈地質區(~2.77 Wm-1K-1)，其岩性主要由變質砂岩和硬頁岩組成，透過樣本觀察，推測其石英含量高且孔隙率低，所以其導熱能力最好，最低值為海岸山脈地質區(~1.52 Wm-1K-1)，其岩石組成主要為火成岩、泥岩、砂岩、礫岩，是其導熱能力偏弱的原因。以地理分類，可發現臺灣山區(雪山山脈、脊樑山脈)有相對較高的熱導係數。以三大岩性分類，熱導係數以變質岩最高(中位數為2.36 Wm-1K-1，標準差約0.76 Wm-1K-1)，沉積岩其次(中位數為1.95 Wm-1K-1，標準差約1.02 Wm-1K-1)，火成岩最低(中位數為1.52 Wm-1K-1，標準差約0.38 Wm-1K-1)。與過去研究岩性和熱導係數關係的文獻對比結果，層理發達的頁岩、硬頁岩、板岩、千枚岩、片岩可能受非均向性影響，測量面與層面角度不一樣而產生較大的差異性，泥質砂岩與石灰岩受低孔隙率的影響而有偏低的現象，玄武岩和安山岩則因岩石礦物組成類似，測量結果相近。另外，與區域性岩性組成的熱導係數文獻對比結果，由於查表屬區域性的代表值，跟局部岩層的實際數值可能有一定的差距，因此文獻中的熱導係數可能低估了四稜砂岩區域的熱流值，並高估其他六個地層的熱流值。本研究結果提供較完整的臺灣岩層熱導係數分佈情形，有助於未來臺灣熱流的研究，以及地熱潛能的評估。

Thermal conductivity was one of the important parameters for calculating heat flow. The detail thermal conductivity measurements of different stratigraphic and lithological features are insufficient in Taiwan. In this study, we measured the thermal conductivity systematically in the Western Foothills, Hsuehshan Range, Central Range, Coastal Range and Hengchun Peninsula. A total of 101 rock or core samples and one soil sample, including cores of 10-30 cm in length and rock blocks of 20×20×10 cm in size, were obtained from 40 different strata. We smoothed the samples and measured them with a thermal conductivity meter. The results show that the highest value is the metamorphic sandstone of Szuling sandstone (~5.21 Wm-1K-1), while the lowest value is the laterite (~0.33 Wm-1K-1) of the Daping layer. Statistically, the average value is 2.25 Wm-1K-1, the median is 2.11 Wm-1K-1, and the standard deviation is 0.93 Wm-1K-1. According to geological ages in different areas, the highest thermal conductivity (~5.21 Wm-1K-1) of the diagenetic strata is the Eocene Szuling sandstone in the Hsuehshan Range; in contrast, the lowest one (~0.39Wm-1K-1) is the argillaceous sandstone of the Pliocene Ma-an Shan layer in the Hengchun Peninsula. We speculate that the high thermal conductivity of Szuling sandstone is due to its primary composition of quartz, demonstrating a high thermal conductivity property. The low thermal conductivity of argillaceous sandstone can result from its higher porosity causing low thermal conductivity. Geologically, the highest thermal conductivity occurs in the Hsuehshan Range geologic province (~2.77 Wm-1K-1), mainly composed of metamorphic sandstone and argillite. We believe that the high thermal conductivity comes from the high quartz content and low porosity. The lowest thermal conductivity is discovered in the Coastal Range geologic province (~1.52 Wm-1K-1), mainly composed of igneous rock, mudstone, sandstone, and conglomerate. Geographically, it can be found that the thermal conductivity of inland Taiwan (Hsuehshan Range, Central Range) is higher than that of the inshore (Western Foothills, Coastal Range, Hengchun Peninsula). For the three major lithologies, including metamorphic, sedimentary, and igneous rocks, the median thermal conductivity of metamorphic rocks is 2.36 Wm-1K-1 with a standard deviation of about 0.76 Wm-1K-1 which is the highest among the three major lithologies. The median thermal conductivity of sedimentary rocks is 1.95 Wm-1K-1 with a standard deviation of about 1.02 Wm-1K-1. The standard deviation is the largest among the three, similar to previous studies. The median thermal conductivity of igneous rocks is 1.52 Wm-1K-1 with a standard deviation of about 0.38 Wm-1K-1. The median and standard deviations are the lowest of the three. Compared with the thermal conductivity of Wu et al. (2013), our results indicate that the heat flow in the Szuling sandstone area was underestimated, and the heat flow in the other six formations was overestimated. Comparing the results of rock types in thermal conductivity between the literature, strongly foliated rocks like shale, argillite, slate, phyllite, and schist are affected by anisotropy, which makes the thermal conductivity different from the literature. Besides, argillaceous sandstone and limestone have lower thermal conductivity, which is affected by low porosity. Basalt and andesite have similar thermal conductivity due to the similar mineral composition of the rock. The thermal conductivity measurement results in this study can provide a reference for future research on heat flow estimation in Taiwan.