Have you heard of the concept of Energy Returned on Energy Invested? If not, you should familiarize itself as it will play a key role in future development patterns.
The answer is really simple. With suggestions that tar sands or oil shale or biofuel or other sources may replace conventional petroleum to fuel our society, the problem is that none of them make any sense unless the energy invested in converting them to fuel is less than the energy achieved.
Here's a good primer.