Abstract: According to the theory of mechanics and physical chemnistry, a quantitative study is conducted on the mechanism of primary migration of oil and gas. It shows that oil and gas moves fom source rock to reservoir mnck in continuous phase. The motivating farce of primary migation i the coalition of buoyancy and capillary pressure. The hydrocarbon expulsion efficiency of source nckprimary migation is usually high because of its propenty of natural drive. However, the hydrocarbon accumulation or reservoir formation requires some thickness of source rock.

Key words: crude oil, natural gas, primary migration, mechanism