Formation pressure is the pressure of fluid contained in pore space of rock and there are 3 categories of the formation pressure which are normal pressure, abnormal pressure and subnormal pressure.
1. Normal Pressure: Normal pressure is the hydrostatic of water column from the surface to the subsurface formation. The concentration of salt in water affects the normal pressure. Higher salt concentration in water, higher specific gravity of water will be. Therefore, the normal pressure can vary from slightly salt 0.433 psi/ft (8.33 PPG) to highly concentrated salt 0.478 psi/ft (9.2 PPG) based on salt concentration in water.
2. Abnormal Pressure: The abnormal pressure is the pressure greater than the pressure column of water. Generally, the abnormal pressure zones are good reservoir which oil companies are looking for. This kind of pressure can create well control problem.
3. Subnormal Pressure: The subnormal pressure is the pressure that is less than normal pressure and it possibly causes lost circulation problems.
Looking at the drawing below, it demonstrates the comparison of formation pressure when drilling into each pressure regime. At the same True Vertical Depth (TVD), subnormal pressure shows least pressure in comparison to others. However, abnormal pressure gives the highest pressure at the same level of TVD.
Reference book: Well Control Books