Usually, the pore pressure which is the fluid pressure inside the pores of the reservoir is the hydrostatic pressure or in other words, the pressure that is exerted by the fluid column of water from the depth of the formation to the sea level. When the impermeable formation (rock) such as shale form as sediments are in compaction, the fluids inside the pores is able to escape and the overlying rock column must be supported, leading to anomalously high formation pressure. The pressure must be described as measured at a specific time because the pressure of the reservoir various with the production of the fluid from the reservoir.