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 For-mation, 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 anti-cline traps are empty too. Therefore, the preservation condition is the key factor controlling hydrocarbon ac-cumulation in Xihu Depression. The areas with good preservation conditions are the favorable exploration targets in the future.