A petroleum reservoir is not a pool of oil floating in an...

A petroleum reservoir is not a pool of oil floating in an underground cave. The reservoir consists of "solid" rock; well nearly solid. A reservoir rock has porosity (small holes within the rock that are usually not visible with the naked eye) and the hydrocarbons are contained within these pores in the rock. There are three substances found in reservoirs (not including the rock itself): Oil (liquid hydrocarbons), gas (gaseous hydrocarbons), and water. Hydrocarbons are lighter than water, so they tend to "float" and rise to the top of the reservoir. Gas is lighter than oil, so it wants to be on the top of the hydrocarbon mixture.
My first 3 calculations are to find the oil in place. I used 10% porosity since I don't have logs to calculate the actual figures. The 50% oil/water saturation is also just a guess but it would not be out of the question to be near that figure. Higher saturations at the top of the reservoir and lower at the base. Once these calculations are made (oil in place), the recovery factor is applied. There are other complex factors if you were doing a complete analysis; compressibility, reservoir temperature, pressure, etc. It does not make a difference to this type of discussion.