Abstract: Under the guidance of modern petroleum geological and hydrocarbon accumulation theory, we summarized the distribution law of discovered oil and gas in Xihu Sag, and holds that the sag is characteristic of "large in deep and small in shallow" on scale, "upper oil and lower gas" in type, and oil and gas enrichment "near source and near fault". The main controlling factors of oil and gas accumulation were analyzed, and the characteristic "tower-shaped" reservoir forming model of the Xihu Sag was established. Result shows that the conditions of oil and gas accumulation in the middle and shallow Huagang Formation are not very favorable due to the relatively poor mudstone capping and the transformation and destruction of early oil and gas reservoirs by later near-EW trending faults. The deep to ultra-deep layer have favorable geological conditions for the formation of large oil and gas fields. Continuous hydrocarbon generation from source rocks provides sufficient oil and gas sources. The reservoir performance of deep to ultra-deep sandstone remains good. The abnormal high pressure improved the reservoir physical properties and provides a driving force for oil and gas migration; and the lasagna-like intercalation of reservoir and cap and early faulting across the source area controlled the vertical distribution of oil and gas. Therefore, the deep and ultra-deep sandstone gas resources in the Xihu Sag have great potential and good exploration prospects.