Hydrocarbon accumulation dynamics can be described with excess differential pressure and buoyancy.In normal-pressure basins

buoyancy is the major force for hydrocarbon accumulation.While in over-pressured basins

the resultant force of excess pressure differential and buoyancy is the force for oil migration and the minimum force for gas accumulation.An analysis of the contributions of excess differential pressure and buoyancy to the total accumulation dynamics in different pooling processes indicates that the excess differential pressure dominates both the vertical and lateral gas migration and accumulation thus it is the major force for gas accumulation.Dissections of 60 or so gas reservoirs in China also point out that the excess differential pressure controls the efficiency of gas accumulation.Mid-or highly-efficient gas pools will not form in places where low excess differential pressure exists;and high excess differential pressure is a necessary condition for highly efficient gas accumulations.