Formation pressure p_i at which the field has been formed before anthropogenic invasion, either directly or indirectly through the connected reservoir(s).
Usually picked up from the first exploration well but can be adjusted if their is indication that it's been affected by the connected reservoir(s).
As the pressure may change at different field areas due to formation top variation the initial formation pressure is recalculated to some datum thus making p_i a one number for the whole connected reservoir.
See also
[Reservoir pressure] [Formation pressure, Pe] [Drilled formation pressure, Pd] [Startup formation pressure, P0]