台灣西南部活動斷層帶之二氧化碳逸氣量與天然儲氣量之估算

Abstract

二氧化碳是現今溫室效應貢獻最大的溫室氣體成份，約占所有溫室氣體之貢獻之60%以上。盡管二氧化碳對於氣候之影響相當大，但是科學家對於自然界二氧化碳通量的研究(尤其是斷層帶上的逸氣通量估算)卻非常少。因此，有系統地估算二氧化碳的天然逸氣量為一重要課題，可以提供估計自然界對於溫室氣體的供應量，並可對人為二氧化碳之排放量進行比較。為了因應全球暖化之問題，地質封存被視為能有效降低大氣中之二氧化碳濃度的方法之一，但是目前對於地下二氧化碳儲氣層之瞭解卻仍然有限;而對於天然類比之研究能進一步讓吾人瞭解二氧化碳儲藏層於地質尺度下之狀況，增加對於二氧化碳儲氣層之瞭解。故而本研究將選擇台灣西南部活斷層區進行二氧化碳逸氣通量之量測，並對於該地區之二氧化碳儲量進一步予以估算。 中崙背斜位於台灣西南部麓山帶，經前人研究瞭解此區域有高壓天然二氧化碳儲氣層出現。而背斜東翼為觸口斷層所截切，在此區域形成多處劇烈逸氣地點。其中，二氧化碳為地區逸氣之主要成份，占氣體成份75%以上，而甲烷為第二大之氣體組成成份，可占氣體成份10%以上。 本研究使用土壤氣體地球化學探勘法(包括密閉式氣罩法及土壤氣採樣分析）於台灣西南部主要的斷層帶區域進行二氧化碳氣體通量及氣體來源之調查；同時使用大型漏斗(1m x 1m)倒置於嘉義中崙濁水潭中，設置了氣體流量連續監測系統，連續監測並估算該地區之逸氣通量。 連續監測氣結果顯示，中崙濁水潭之二氧化碳通量約114 t/yr。根據本地區斷層活動性與逸氣量推估，本區域地底下之氣體儲集層至少曾有900 Mt以上之二氧化碳儲藏量。 本研究調查顯示，台灣西南部斷層帶之二氧化碳通量約為17 g/m2/day，而中崙濁水潭周圍之二氧化碳氣體通量更可高達29 g/m2/day，而於背景值區域之二氧化碳通量則約為7 g/m2/day，顯示台灣西南部活動斷層帶為一高量二氧化碳逸氣之區域。而進一步分析結果，顯示本研究區域土壤氣中之二氧化碳濃度與碳同位素比值有密切之關係；因此推測本地區之二氧化碳可能為深處來源之二氧化碳與淺層之二氧化碳依不同比例混合而造成。 依據不同分區之二氧化碳逸氣量之結果，可以進一步估算台灣地區土壤二氧化碳逸氣總量，初步計算之高估值約為132 Mt/yr。而此高估值仍小於台灣人為二氧化碳之排放量253 Mt/yr，而台灣人為二氧化碳釋放量更以極快之速度持續攀升，推估於2010年更可能達到366 Mt/yr(APEC,2005)。

CO2 is well-known as a greenhouse gas, which is identified as a species affecting the Earth climate. CO2 could account for more than 60% of all greenhouse gas generated globally. Although CO2 is very important to the climate system, its nature flux at fault zone was seldom quantified. Therefore, systematic estimation of the nature CO2 emission is important to better understand the global emission amount of natural greenhouse gases and to compare with the amount of anthropogenic emission. To cope with global warming, CO2 geological storage is identified as an effective method to cut down CO2 concentration in atmosphere. However, the understanding of CO2 behavior in the reservoir is still rare. To understand the behavior of CO2 in the reservoir, study the natural analogue is a proper method to increase the understanding of CO2 reservoir in geological timescale. Therefore, we will choose the active fault zone area in SW Taiwan to systematically study the soil CO2 flux and estimate the gas reservoir in study area. Chunglun mud pool is located in the southwest Taiwan and is cut by the active Chukuo Fault. The main component of its bubbling gas is CO2 (>75%) with minor CH4 (10%). The CO2 is considered to be the eruption products of Miocene magmatism in West Taiwan and might be stored in the reservoir and started degassing due to recent fault activity. A general survey of soil CO2 flux and soil gas composition along the fault zone was conducted using the closed chamber method. Meanwhile, a continuous monitoring system was setup to record the gas flow at Chunglun mud pool, using a big funnel (1m x 1m) to cover the bubbling gas of the mud pool. The average CO2 flux from mud pool is ca. 114 t/yr. Considered the CO2 flux and the activity of the fault, more than 900 Mt CO2 could be stored in the reservoir. The results show that the soil CO2 emission from the fault zone and near the mud pool is ca. 17 and 29 g/m2/day, respectively. Meanwhile, the CO2 flux in background area is ca. 7 g/m2/day. It reveals that active fault zone, at least in study area, exhibits relatively high CO2 efflux. The results of soil gas survey show that the soil CO2 concentration is closely related to its 13C value. Therefore, the soil CO2 in active fault zone of SW Taiwan is considered as the mixture of different components from deep and shallow CO2 sources. Taiwan can be divided into several domains to estimate the amount of natural CO2 emission. An overestimated value of soil CO2 emission in Taiwan can be obtained ca. 132 Mt/yr. Although it is an overestimated value, it is still much less than the anthropogenic CO2 amount in Taiwan (ca. 253 Mt/yr) and the value could be up to 366 Mt/yr in 2010 (APEC, 2005).