Hi rcml,
Not all the volume will be oil. It is made up of rock water and oil. so conservatively we say that only 20% volume is oil
This gives the oil in place.
Not all the oil in place can be recovered. the recovery factor varies between 20 to 50% so say again 20%
Thus you have to multiply the volume by 0.2 * 0.2 to get to the recoverable oil.