苗栗地區錦水背斜地下構造演育與裂縫發育

Abstract

新竹-苗栗地區是臺灣主要的油氣生產區域，此處的油氣開發及生產歷史可追溯至西元1861年，最古老的出磺坑油田歷經了百年歷史，至今仍在生產中；而同樣作為此處重要油氣田之一的錦水油氣田，為達生產與研究需求，百年之間開鑿了82口井，然而其地下構造型態的解讀至今仍具爭議，得以長時間生產的因素亦未明，地層中的裂縫儲集可能為重要因素之一，故背斜內部的裂縫分布形態則有待釐清。因此本研究首先根據地表地質、井下與震測資料重新建立由苗栗外海延伸至內麓山帶的地下構造剖面，並使用平衡剖面法將已變形的地層恢復至原始樣貌，推估該區域每個時期的構造型態，得到錦水背斜與其鄰近構造之間的演育歷史，最後建立出錦水背斜之三維構造模型，利用地層面回復的方式，推估背斜中裂縫發育可能性較高的位置，作為未來油氣田再開發的參考依據之一。 有別於早期剖面多以薄皮構造理論建立，本研究在五指山層底部的滑脫面之下另添加一深部滑脫面，兩滑脫面之間這些老於五指山層的地層以逆衝楔形體的方式堆疊，不僅造就錦水、出磺坑以及八卦力背斜的形成，亦使竹苗地區八卦力背斜南北延伸的區域有抬升兩公里的情形，內麓山帶普遍出露較老地層的現象也同樣來自其影響，而這些逆衝楔形體的形態繪製在地震資料上皆可有良好的對應。在錦水背斜的部分，由於切穿其西翼的逆斷層未對此構造的形成有明顯的貢獻，再加上其西北翼緩、東南翼陡的形貌，因此本研究認為錦水背斜的形成應主要受控於此深部構造的活動。 透過剖面回復的結果顯示大窩山斷層、碧靈斷層、鹿場斷層與紅毛館斷層依序發育，形成後又受到沿著大東河斷層爬升的逆衝楔形體所影響，接著小東勢斷層活動，並且另一個逆衝楔形體產生將其褶皺變形，最後藤坪斷層與竹南斷層發生逆斷層活動，最年輕的逆衝楔形體隨後發育；而各個構造活動時期所造成的水平縮短量分別為: 內麓山帶的逆衝楔形體5.718公里、八卦力背斜下的逆衝楔形體8.228公里、藤坪斷層166公尺、竹南斷層77公尺以及錦水背斜下方的逆衝楔形體2.617公里，總和以上構造所導致的剖面縮短量為16.806公里。 接著本研究利用GOCAD軟體搭配依據井下資料所繪製出的地層頂部等深線圖，以錦水背斜內部東坑層、打鹿頁岩、出磺坑層、木山層以及五指山層中的五個地層面建立起此構造的三維模型，並針對任一地層面，透過KINE3D-2進行2.5維的回復，去除變形地層面中褶皺以及斷層作用的影響，同時以回復前後面上區域面積的改變，推估地層褶皺時所受的應變量多寡，搭配伸張變形區域的分布情形，即可得知該層中於背斜何處可能有最高的裂縫密度；而根據本研究的結果顯示錦水背斜內部木山層中，以最北端的A地塊與最南端的C地塊頂部可能具有最高的裂縫密度，五指山層中則為中央的B地塊頂部，且透過地層面曲率變化與高擴張量區域分布情形的比較，本研究發現曲率與擴張量有頗高的相關性，當局部的曲率越高，該區域應變量也越高，而當地層面整體曲率變小時，該面上的應變量也減少。

Hsinchu-Miaoli area is the major hydrocarbon producing fields in Taiwan. Oil and gas production in this area has been explored and produced since 1861, and the oldest gas field is still producing gas until now. To understand the nature and the geometry of the reservoirs in this area, 82 wells were drilled in the Chinshui Field, which is one of the important gas fields in the Hsinchu-Miaoli area. However, the subsurface structures and fracture distribution of these fields are still unclear, and the reason for long duration in producing is also unknown. Fracture distribution in the oil-bearing reservoir might be one of the important factors of long time gas producing, but the fracture reservoirs attaining hydrocarbons associated with fault-related folding need to be further clarified. In this study, we first combine surface geological data, seismic profile and well logs to represent a new structural interpretation of Chinshui anticline and adjacent structures by a geological cross section from Miaoli offshore to inner western Foothills. After conducting 2D restoration with 2DMove, we could test whether our interpretation is reasonable and clarify the evolution history of Chinshui anticline and adjacent structures. We further construct a 3D structural model of Chinshui anticline. By using surface restoration, the location with higher fracture density could be inferred and the results could become a consideration of reproduction. Instead of basing on thin-skinned model, we add a deeper detachment underneath the shallow detachment beneath Wuchihshan formation. The old strata between these two detachments develop three thrust wedges and deform upper strata to form Chinshui, Chuhuangkeng and Pakuali anticlines. The areas near Pakuali anticline have been uplifted two kilometers at maximum and the old strata outcrop at the ground surface in inner Foothills also contributes to these thrust wedges. Furthermore, we have no direct evidence to confirm the existence of the bedding fault in the upper part of Chinshui anticline and the fault cutting through the west limb does not contribute to the formation of this anticline either. Therefore, we conclude that Chinshui anticline is mainly formed by the movement of the deep structure. According to the restoration, the Tawoshan, Piling, Luchang and Hungmaokuan fault are an in-sequence fault system. They were all influenced later by the thrust wedge which climbed along the Tatungho fault. And then another thrust wedge formed following the Hsiaotungshih fault. The Tengping and Chunan fault developed as reverse faults in later stage and deformed by the youngest thrust wedge again at last. The shorting caused by the thrust wedges beneath the inner Foothills, Pakuali and Chinshui anticline is 5.718, 8.228 and 2.617 km respectively. The Tengping and Chunan faults also lead to 166 and 77 m shorting. And the total shorting of our cross section is 16.806 km. Finally, we use subsurface structure maps gathering from well data to build five top bedding surfaces of the strata in Talu shale, Tungkeng, Chuhaungkeng, Mushan and Wuchihshan Formation of Chinshui anticline. By conducting 2.5D restoration with KINE3D-2, we could obtain strain fields and the extension areas during the deformation to infer the location with higher possibilities of developing fractures within this structure. The results reveal that the highest fracture density might lie in the hinge of A and C blocks in Mushan Formation as well as the hinge of B block in Wuchihshan Formation. After comparing the curvature and strain fields of these surfaces, we also find out that the strain field is highly relevant to the curvature of anticline.