苗栗出磺坑背斜是台灣最早發現油氣之構造，直到現在仍被認為其深部地層之裂隙中蘊含大量油氣。出磺坑背斜出露地區一向為研究地質構造與探採資源的重要區域，因此擁有大量的地表、震測與鑽井資料，然而此區深部之震測接收訊號十分混亂，無法直接得知背斜深部之地層分布情況，因此岀磺坑背斜深部構造解釋仍有爭議。本研究利用三角剪切模式以解釋形成機制可能為斷層相關褶皺之出磺坑背斜構造發育，同時亦計算構造剖面中應變橢圓的分佈。至於裂隙分佈集中區域以及裂隙方向則可藉由三角剪切模式中的LNFE (Lines of No Finite Elongation) 得到。初步之參數敏感度分析結果顯示三角剪切模式中解釋斷層滑脫褶皺形貌的斷層傾角參數範圍，必須由以實際背斜構造之兩翼地層傾角決定；同樣以形成出磺坑背斜形貌為目的之情況下，背斜軸部高角度斷層傾角角度愈小則構造形成所需要之斷層滑移量愈大，而斷層傾角角度愈大則較少的斷層滑移量即可形成其高聳之幾何形貌；而三角剪切帶頂角角度愈大，則其褶皺兩翼愈寬廣且形貌對稱性愈低，角度愈小則褶皺兩翼愈緊閉，換句話說，近乎對稱的出磺坑背斜暗示著其形成過程中三角變形區之頂角角度局限在50至60度之範圍內。模擬成果更顯示，與地層方向平行之斷層其褶皺形貌無法符合出磺坑背斜之形貌，欲模擬如出磺坑背斜之斷層滑脫褶皺形貌，則必須在斷層末端假設一高角度斷層。本研究之三角剪切模式最佳模型顯示模擬出磺坑背斜之最佳參數值斷層傾角必須為50度，斷層滑移量3700公尺，三角剪切頂角角度60度，斷層前緣破裂距離與斷層滑移量之比值P/S為1，基底滑脫面為8.6公里深。然而，三角剪切模擬結果顯示，褶皺兩翼的地層傾角及厚度，會隨深度改變而明顯改變，此一結果與前人認為出磺坑背斜地層等厚且地表與深部地層傾角一致之看法並不一致；而利用三角剪切模式符合實際地表資料之模型雖然與前人對出磺坑背斜之幾何形貌略有差異，其分析之LNFE仍可符合實際裂隙之方向。 The Chuhuangkeng anticline is the first oil and gas producing structure in Taiwan. It is possible that considerable oil and gas is still stored in the fractures system of the strata. There is abundant geologic information in the Chuhuangkeng anticline. However, the quality of seismic reflection image in this area is extremely poor. Consequently, the geometry of the Chuhuangkeng anticline is mainly derived from surface geology indirectly and different geological models were proposed near the Chuhuangkeng anticline. It is hypothesized that the Chuhuangkeng anticline is a fault-related fold. Trishear model was utilized to simulate the geometry of the Chuhuangkeng anticline. The strained ellipse and directions of the fractures in Chuhuangkeng anticline (LNFE, Lines of No Finite Elongation) were also calculated. It is demonstrated that a steep fault under the Chuhuangkeng anticline is required in the trishear model to reproduce a nearly vertical and symmetrical fold. Based on the result of parametric study, the trishear angle of the Chuhuangkeng anticline is 50 to 60 degrees if one want to derive a nearly symmetrical fold. Besides, the ramp angle is 50 degrees, fault slip is 3700 meters, trishear angle is 60 degrees, P/S is 1, and the depth of the detachment is 8.6 kilometers for the best fitted model. Finally, the calculated LNFEs of trishear model have a similar dip with the actual dip of fractures observed in the well. Therefore, trishear model is potentially useful to evaluate the fracture system in an oil field.
