Abstract: Based on a plenty of exploration achievements in Xihu sag, it is found that the oil reservoir is distributed in the upper stratum, while the gas is distributed in the lower stratum, with the pattern of more resources in the southern fields than those in northern ones. Through the analysis of the geochemical parameters of the condensate and condensate gas in Xihu sag, it is concluded that the condensate and condensate gas are the same for their genetic type and maturity, and are generated from the coal measures source rocks of Pinghu Formation. Based on the research results of reservoir type, oil and gas accumulation time,and trap forming time in the western slope and central inversion belts, it is believed that the oil and gas filled in Pinghu Formation in the western slope belt, and then migrated to Huagang Formation due to the late fault opening. Because the pressure was reduced in Huagang Formation, gas was lost, and finally the condensate reservoir was formed in Huagang Formation. In the central inversion zone, due to the late trap, oil and gas underwent one time reservoir accumulation, and then migrated to Huagang Formation through faults. Because the low pressure in the upper part of Huagang Formation, oil and gas were separated with gas dispersing, thus the pure condensate reservoir was formed. According to the research on the attributes of oil/gas accumulation and distribution, we can get the conclusion as follows:the reservoir distribution is controlled by the gas-generation center, and the abundance is controlled by faults; the distributing interface between condensate oil reservoir and condensate gas reservoir is controlled by overpressure in the west slope belt.

Key words: Xihu sag, condensate reservoir, accumulation feature, hydrocarbon generation center, fault distribution, accumulation feature