Here is a basic article from Scientific American on how oil reserves are interpreted. This article concludes with the statement that the world is not runningout of oil but that oil will become much much more expensive fairly soon. The issue isn't the total amount of oil but our ability to extract it out of the ground at the rate which is high than the demand for that oil. The latter is what has kept oil cheap. There is lots of oil left but its getting harder to find and even harder to get. It is inevitable that as oil costs more to get oil prices will go up and world competition for the same amount of oil will increase as we consume more and more. Even with increasing technology demand is increasing at faster clip than ever predicted.
The End of Cheap Oil, by Colin J. Campbell, Jean Laherrere