Abstract: A laboratory simulation of the migration process of the hydrocarbons through the overlying cap rock to the surface of the earth has been carried out by means of the self-developed physical simulation device.The results show that the hydrocarbons in the reservoir can permeate and diffuse due to the pressure difference and the concentration difference between the reservoir and its surrounding strata,which is described as micro-seepage,and the direction of this micro-seepage is mainly vertical.It is this vertical micro-seepage that drives the hydrocarbons from the reservoir to migrate through the cap rock and eventually reach the surface of the earth,and the migrated hydrocarbons can be detected by modern apparatus and equipment in the surface.The results also show that the vertical hydrocarbon micro-seepage is characterized by intermittent and episodic migration.A new theory of "gas phase permeation along fracture driven by pressure" has been proposed as the possible main formation mechanism of the near-surface geochemical anomaly based on these simulation results and the long-observed results.