How to Calc How much Oil we have?

Was trying to extrapolate some kind of estimate of how much oil we may potentially have now without a 'stab in the dark' but rather via some kind of calculation and have come to the conclusion that I have no idea .... so if someone with more knowledge on this subject could help .....

Was thinking if I can get a rough idea of actual field size (was thinking 9km x 6km = 54km square but then I remembered they're now talking over 300km square so I think the 9km I'm getting confused with is just the distance between the last 2 wells) ... so assuming 300 km square = 300,000,000 square metres .... x average 20m net oil column (current averaging 30m but lets assume 20m as will thin on the edges) = 6 billion cubic metres .....

Was then thinking I would divide this by how many cubic metres would tend to be required to roughly equate to a barrel of oil .... Now this I have no idea about as guessing it is different for fields depending on porosity etc .... anyone in the know have a range where they would expect this to fit?

Thanks