Abstract: Jilong sag is the main sedimentary sag in the east China sea shelf basin. The sag has undergone four phases of tec-tonic evolution: fault depression,fault sag,depression and coating. The sag is mainly filled with Cenozoic sediments and composed by fault terrace belt in western gentle slope,hollow zone in center and abrupt slope belt in the east. Four suits of source rocks were developed from Eocene to Miocene formation in which the hydrocarbon was mainly reserved. There are four favorable petroleum systems. According to the hydrocarbon evaluation report in 2005,Xihu sag has pro-duction capacity of 1. 56 billion squares BOE. Xihu sag has similar oil geology condition as Jilong sag. They were con-sidered as unitary sags which were temporal separated in late Oligocene to early Miocene. Hydrocarbon geological analysis shows that Jilong sag is a favorable exploration area where the fault terrace belt in western gentle slope has the largest exploration potential capacity.