竹東重力低區成因之探討

The Study of Gravity Low in Chutung Area

林書帆(Shu-fan Lin)

指導教授 ： 顏宏元

國立中央大學/地球科學學院/地球物理研究所/碩士(2007年)

摘要

在台灣地區布蓋重力異常圖中(Yen et al., 1995)，除了中部、南部兩處明顯重力異常低區外，在新竹地區也可以發現一個規模較小的布蓋重力異常低區(稱為竹東重力低區)。在本研究區裡，除中油早期做過油氣探勘外，亦有諸多地球物理、地質、地形等方面研究。該區內存有青草湖油氣田、寶山油氣田、竹東油氣田等等重要資源。從前人文獻中可發現，此處為內、外麓山帶的分區，構造非常複雜，斷層與褶皺也相當發達，該區斷層和褶皺，往往相互截切出現，褶皺的發育多為斷層作用上抬所成，逆斷層的發育多為早期正斷層多次活化所致，從中油公司的震測剖面看來，許多斷層構造底下早期即出現過滑移的痕跡，如新竹斷層、新城斷層，所以在此區底下的岩層不但有出現凹陷也有較為破碎的現象。
本研究利用重力資料與地質模型並結合Talwani演算法，將觀測重力值與利用理論密度所獲得之計算重力值比對發現：重力等值線分佈為東北-西南向和構造走向幾乎一致。低區分布可分為兩部分，一為在北埔斷層以西到寶山背斜為止，為位在該區底下凹陷盆地處，厚達1.5公里低密度的頭嵙山層所致。北埔斷層以東到軟橋斷層處，在模型中密度平均需降低0.18g/cm3，才能使計算重力值與觀測重力值達到吻合。因此，本研究認為使密度降低可能原因為：底下岩層破碎、可能含有水、油、氣等低密度物質存在。如於北埔斷層以東，內麓山帶區能有更多的相關資訊，相信能更進一步找到其他可能性，對本研究也會有更大助益。

關鍵字

竹東 ； 新城斷層 ； 新竹斷層 ； 頭嵙山層 ； 重力低區 ； 布蓋重力異常

並列摘要

The Chutung gravity low is the northwest in the Taiwan anomaly map (Yen et al., 1995). We can exclude artificial observation error form the Hsinchu Bouguer anomaly map(Chang, 1972) which is the result of gravity surveyed by Chinese Petroleum Corporation (CPC) in 1960s. The area is the boundary of inter and outer foothills belt, there are many complex structures in this area. The oil and gas had found in the Chingtsaohu anticline, Chutung anticline. There are many folds and faults structures in Hsinchu area, they are Hsinchu fault, Hsincheng fault, Chutung fault, Peipu fault, Juanchiao fault, Chingtsaohu anticline, Kantzechi syncline, Paoshan anticline, Chutung anticline, Taping anticline. The folds and faults have very close relationship. The gravity low is relation with the subsurface formation density and location between Kuanhsi and Chutung area. The study selects the geology models from Yang et al., 2002 to build the initiate models. With the gravity data, the low area exist the west Paoshan anticline to the east Juanchiao fault. In the gravity low area, the Paoshan anticline to the Peipu fault gravity low area arises from the Toukoshan formation, but the Peipu fault to the Juanchiao fault gravity low area could be arose by the complex structures in this area. The Toukoshan formation is about depth 1.5 km, and the gravity curve is great fitting that have a lot of logging and seismic data in the west part of the gravity low. The broken formation could be the cause of the east part of the gravity low but the gravity curve is bad fitting. The east part of the gravity low, the initiate model could be correction. If we can have the others data from seismic and logging in the east of the Peipu fault, we can find the better result in this study.

並列關鍵字

Hsincheng fault ； Bouguer gravity anomaly map ； gravity low ； Toukoshan formation

參考文獻

Yang, K. M., Wu, J. C., Ting, H. H., Wang, J. B. and Chi, W.R. and Kuo, C. L., 1994, Sequential deformation in foothills belt, Hsinchu and Miaoli areas: Implications in hydrocarbon accumulation: Petrol. Geol. Taiwan, no. 29, p. 47-74.

Yang, K. M., Wu, J. C., Wickham, J. C., Ting, H. H., Wang, J. B. and Chi, W. R., 1996, Transverse structures in Hsinchu and Miaoli areas: Structural mode and evolution in foothills belt, northwestern Taiwan: Petrol. Geol. Taiwan, no. 30, p. 111-150.

Hammer S., 1939. Terrain corrections for gravity stations, Geophysics, 4,184-194.

Talwani, M., J. L. Worzel and M. Landisman, 1959. Rapid gravity computations for two-dimensional bodies with application to the Mendocino Submarine Feacture Zone, J. Geophys. Res., 64, 49-59.

Yen, H. Y., Y. H. Yeh, C. H. Lin, 1990. Free-air gravity map of Taiwan and its appliciations, TAO, vol.1, 2, 143-155.