Structural Evolution and Fracture Development of Chinshui Anticline in Miaoli Area, Taiwan

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.