Abstract: The volcanic eruption was intensive and frequent in the Nanpu Sag during the period of Neogene when the Guantao Formation was deposited. Through logging and seismic interpretation, it is revealed that the volcanic rocks were mainly basalt in lithology formed in the early Neogene. The volcanic sequence is thicker in the southwest and thinner in the northeast, and formed by multi-stage eruption. The comprehensive study suggests that the eruption of huge amount of volcanic materials changed the geomorphology of the Sag, that influenced the spatial distribution of the fluvial strata deposited later on. Many stratigraphic traps were formed in the west of the Sag. The volcanic activities also brought changes in the geothermal field in the Sag, which led to the abnormal evolution of source rocks, and greatly accelerated the hydrocarbon generation process within the Sag. Acting as an effective sealing, volcanic bodies could be the cover of many types of traps. The significance of the volcanic rocks should never be underestimated in hydrocarbon migration, accumulation, and distribution.