Abstract: There are lots of igneous rocks developed in the west of Tiantai, Xihu Sag.They are generally characterized by high speed, high natural gamma and high density on the logging curves, and strong amplitude on the seismic sections. Some seismic technologies can be used to identify igneous rock, such as the seismic geometric attributes to depict the spatial distribution, coherent slice to roughly delineate the distribution range, the maximum similarity inclination attributes to locate the crater position and depict the lateral boundary of igneous intrusion. Based on the fine depiction of igneous rocks and the study of the relationship between igneous rocks and faults, the authors think that most of the traps in the study area are related with the igneous rocks and fault block.The forming time of traps was slightly later than the peak time of hydrocarbon expulsion.The activity of igneous rocks made changs to the traps, which resulted in the hydrocarbon accumulation in this area with the characteristics of multi stage charging and being adjusted in later period.