西湖凹陷油气成藏的主控因素

西湖凹陷位于东海大陆架边缘，地壳为相对软弱的过渡壳。西湖凹陷中新世末以前属于萎缩型盆地，上新世伴随冲绳海槽的形成才成为扩张性盆地。受构造控制盆地萎缩期沉积有以下3个特点：第一，整体表现为海退；第二，沉积范围逐次减小；第三，泥岩百分比越来越低，盖层条件越来越差。平湖组存在区域盖层，钻遇断块气藏；花港组存在局部盖层，背斜充满度高，断背斜充满度低，断块未成藏；新近系盖层条件更差，断背斜亦未能成藏。由此可见保存条件是西湖凹陷油气成藏的主控因素，保存条件好的领域是西湖凹陷下一步勘探的方向。

The key factor controlling hydrocarbon accumulation in Xihu Depression

Xihu Depression is located at the margin of the East China Sea＇s continental shelf where the crust is relatively weak. Xihu Depression was a shrunk basin before the end of Miocene, and transitioned into an extensional basin in Pliocene along with the forming of Okinawa Trench. Controlled by tectonic movements,the depositional process in the shrinking phase has 3 characteristics： firstly, the depositional process is in total a marine regression process; secondly, the sedimentary range decreased gradually upward; thirdly, the percentage of mudstone decreased upward and the sealing conditions became poorer. In Pinghu Formation, regional cap-rocks existed, so faulted block gas reservoirs have been found here, while in Huagang Formation, only local cap-rocks existed, and fullness coefficient is high in anticline traps but low in faulted anticline, so faulted block traps are empty. The sealing conditions for Neogene are poorer, and faulted anticline traps are empty too. Therefore, the preservation condition is the key factor controlling hydrocarbon accumulation in Xihu Depression. The areas with good preservation conditions are the favorable exploration targets in the future.