The initial stage of producing oil from a reservoir.
Use Natural Forces such as: 1- Expansion of oil, gas or both.
2- Displacement by naturally pressurized water.
3- Drainage from a reservoir in high elevation to a well in lower elevation.
4- Artificial techniques (pumps).
Injection of fluids in a series of wells to force oil into another series of wells (essentially augmenting the natural forces used in primary methods).