In short, the end of oil signals the end of civilization, as we know it. Oil's many advantages provide 1.3 to 2.45 times more economic value per kilocalorie than coal. No other energy source equals oil's intrinsic qualities of extractability, transportability, versatility and cost. Energy quality is by far the dominant factor. A BTU of sweet oil is fundamentally different than a BTU of sour oil.
Sour oil is contaminated with sulfur and requires special refineries with higher energy costs.
Once high quality reserves are depleted, however, producers will need to implement cost-effective methods for bringing greater volumes of subquality gas to the marketplace. A BTU of coal is fundamentally different than a BTU of wood. Coal contains more energy per pound than wood, which makes coal more efficient to store and transport than wood. Solar radiation is fundamentally different than natural gas. Natural gas is fundamentally different than oil shale, etc.