The viewpoint that oil and gas in rocks can migrate only when they reach certain threshold is not correct.The primary condition for oil and gas migration(secondary migration)is that the dynamic force must be greater than capillary resistance and fractional resistance.The dynamic force includes hydrodynamic force and buoyancy

both of which are related to the length of oil and gas phases.That is to say

oil and gas can migrate so long as the length of oil or gas phase is sufficient.This suggests that there is no inevitable relationship betwee oil and gas migration and saturation.It is pointed out that differental pressure greater than 10 MPa between the two ends of the oil or gas phase is needed to cause the migration.Only sudden event can produce such differential pressure but not formation residual pressure.When dynamic differential pressure is certain

the reservoir's pore texture and wettability remain invariable

water driving oil may reach stable residual oil saturation.But the residual saturation would change with the variation of water driving condition.So it is considered that oil and gas can migrate or not is determined by the lengh of continuous oil(gas)phase but not the oil saturation.