Like robotobo above said, it depends a lot on the field.

From the perspective of petroleum engineering, many of our principles aren't proved. They're good enough for the work, they seem to hold up to real world results. But proved? No.

We can try to recreate conditions of an oil field. We can simulate it in hypermesh using Finite Element Analysis, we can do lab tests to determine all the properties of the oil and rock... But all of those are our best guess at what the real world conditions are like. Because our actual test subject came from a 7" diameter wellbore that was drilled to 3800 meters depth, 200 deg. C, and 3500 psi. And that single point down there in the rock may not be anything like what the oil field is like a half mile away. It's akin to having an olympic size swimming pool filled with marbles of all random colors and sizes. Then sticking a 20 foot straw anywhere down in the pool and sucking up one single marble. We try to make our best guess about the whole pool on that one sample. So, obviously, the more samples we can get (more wellbores drilled) the better our guesses can get. But we still can't prove what the rest of the pool is like.

Nobody's actually been down there to do tests in the real world. So 'proven?' Nope.

And even mathematically two different engineers could come up with widely different estimates of how much oil is in a reservoir. Because those calculations are made on interpretations of the data. An engineer who works for a company trying to sell the oil field will use the range of numbers that give him more oil in the ground. Another engineer who works for a bank who is being asked to give a loan will use the conservative numbers. That way they are more sure of the collateral against the loan.