I'm not sure if that's a safe assumption. Do we know what the sources we are exploiting right now are going to look like in tens of millions of years? Our models of the evolution of hydrocarbon reserves are based on those we've found, and we simply assume the differences between different deposits are natural in origin. It's easily possible that places where we didn't find hydrocarbons did in fact have them at a previous point in time, or that there was more in a given reserve in the past.