Specific type of Lift mechanism where bottom-hole pressure is reduced naturally by the light fluid column in the tubing due to gas liberation from oil in the lifting up fluid column.

Many petroleum fluids contain enough solution gas to support the fluid lifting all the way to the surface due to the growing gas liberation along the fluid column which reduces its weight and the bottom-hole pressure thus causing the inflow from perforated reservoir into the wellbore.

This is obviously the simplest and cheapest Lift mechanism of all. 

See Also

Petroleum Industry / Upstream /  Production / Subsurface Production Operations / Well Control / Lift Mechanism