Abstract: Based upon the exploration data from the major Xijiang depression, we put forward in this paper the two concepts of effective primary migration and effective primary migration area.The effective primary migration of oil and gas depends upon the capability of remigration or secondary migration of oil and gas after charging into the sand body close to the source rocks.The effectiveness is depending on two main factors:one is the occurrence of large-scale sand body near the hydrocarbon source rock and the other is the availability of coarse pore throats within the sand body.The former will decide if there is enough oil and gas for effective primary migration, the latter determines the priority of filling direction of oil and gas.Based on this working hypothesis, we made a comparison between the Xijiang A and Xijiang B structural traps with the exploration results available and discussed the failure of exploration in the Xijiang A region.The conclusion may help further understanding the Paleogene hydrocarbon accumulation in this region and provide a case for the endeavor to look for similar reservoirs.