利用孔隙率-有效應力關係曲線評估海岸山脈南段弧前盆地之地層侵蝕量;Using porosity-effective stress relationship curve to evaluate the erosion amount of the fore-arc basin in the southern part of Coastal Range

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Title:	利用孔隙率-有效應力關係曲線評估海岸山脈南段弧前盆地之地層侵蝕量;Using porosity-effective stress relationship curve to evaluate the erosion amount of the fore-arc basin in the southern part of Coastal Range
Authors:	洪世明;Hong, Shi-Ming
Contributors:	應用地質研究所
Keywords:	海岸山脈;侵蝕厚度;孔隙率;鏡煤素反射率;Taiwan Coastal Range;Erosion Amount;Porosity;Vitrinite Reflectance
Date:	2020-07-30
Issue Date:	2020-09-02 19:18:00 (UTC+8)
Publisher:	國立中央大學
Abstract:	地層侵蝕量對於盆地演化歷史推測、造山帶演化推估、油氣成熟與遷徙的過程都是一個重要的參數。地層侵蝕量估計多使用地球物理、地球化學、地層對比等方法進行，但在海岸山脈弧前盆地(泰源與樂合盆地)，因(1)地物井測與震測資料欠缺；(2)沉積物來源主要來自中央山脈，其帶來的高變質度沉積物嚴重干擾地球化學指標之測量；(3)地層受到大範圍的抬升與侵蝕，地層對比不易等三個限制，造成單一方法推估地層侵蝕量不確定性高。本研究於泰源盆地之馬達吉達溪剖面採集五個砂岩樣本、於樂合盆地之樂合溪剖面採集三個砂岩樣本，進行孔隙率-有效應力分析得出預壓密應力，並藉此計算地層侵蝕量。孔隙率-有效應力分析的原理基於顆粒沉積物在力學壓密過程中孔隙率的變化，若樣本受化學壓密則視為無效樣本，為增加成果的可信度，本研究將孔隙率實驗之砂岩樣本製作薄片進行觀察、並引用前人在此二研究區之鏡煤素反射率資料進行古地溫分析，確立孔隙率實驗之砂岩樣本的壓密型態。孔隙率-有效應力分析所得之地層侵蝕量計算成果將以地層對比與鏡煤素反射率分析進行限縮。成果顯示，馬達吉達溪剖面利用五個有效樣本獲得了709~2,248公尺的地層侵蝕量分析成果；地層對比成果顯示此研究區之地層侵蝕量下限為562公尺；鏡煤素反射率分析顯示，在無地溫異常之條件下，地層侵蝕量為3,688公尺。樂合溪剖面僅有一個有效樣本進行孔隙率-有效應力分析，獲得了1,186~2,162公尺的地層侵蝕量分析成果，而地層對比與鏡煤素反射率分析皆無法為此研究區限縮成果，使得本研究於樂合溪剖面獲得的成果仍有不確定性。;Erosion amount is an important parameter for prediction of evolution history of sedimentary basin, estimation of orogenic belt growth, and the process of organic material maturation and migration. Traditionally, the estimation of erosion amount is mostly performed by geophysical, geochemical, and stratigraphic contrast. However, in the fore-arc basins of the Coastal Range (Taiyuan and Loho Basin), due to (1) lack of logging and seismic data by geophysics; (2) the source of sediments is mainly from Central Mountain Range, the highly metamorphic sediments seriously interfere the measurement of geochemical indicators. (3) The area has large-scale uplift and erosion, stratigraphic contrast is hardly to be used. These three factors result in high uncertainty in the estimation of erosion amount from any single method. In this study, five sandstone samples were collected from Madagida section of the Taiyuan Basin and three sandstone samples were collected from Loho section of the Loho Basin. By attempting porosity-effective stress analysis, we can get preconsolidation pressure of each samples and use it to calculate their erosion amount. The principle of porosity-effective stress analysis is based on mechanical compaction, if the sample was compacted by chemical reaction, it will be regarded as an invalid sample. In order to increase the credibility of the results, this study use two method to judge the compaction style of the sandstone samples: (1) thin section observation; (2) paleotemperature analysis by vitrinite reflectance. The result of calculated erosion amount will then be compared by two method: (1) stratigraphic contrast; (2) vitrinite reflectance analysis. The results show that, all five sandstone samples in Madagida section are available to attempt porosity-effective stress analysis and derive the erosion amount between 709~2,248 meters; the result of stratigraphic comparison suggest that the lower boundary of erosion amount in Madagida section is 562 meters; whereas the result of vitrinite reflectance analysis shows that, in the condition of normal geothermal condition, the erosion amount is 3,688 meters. Furthermore, in Loho section, only one sandstone sample can be used for porosity-effective stress analysis and erosion amount is estimated between 1,186~2,162 m. However, neither stratigraphic contrast nor vitrinite reflectance analysis can constrain the result from porosity-effective stress analysis, causing the result of Loho section have higher uncertainty.
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