Oil initially in place (OIIP) is the amount of crude oil first estimated to be in a reservoir. Oil initially in place differs from oil reserves, as OIIP refers to the total amount of oil that is potentially in a reservoir and not the amount of oil that can be recovered. Calculating OIIP requires engineers to determine how porous the rock surrounding the oil is, how high water saturation might be and the net rock volume of the reservoir. The numbers for the aforementioned factors are established by conducting a series of test drills around the reservoir.