Energy Costs and the Costs of Energy Exchanges
From technocrats, I often hear or read a claim that it would no longer make sense to extract petroleum when reserves were depleted to the point that extractions took more energy than could be got from the petroleum. What is unstated in the reasoning is that the value of an energy source is solely determined by the quantity of energy that it could yield; but that proposition is mistaken.
When petroleum is extracted, not only is one quantity of energy exchanged for another, but a quality of energy is exchanged for another. Petroleum is extracted by the use of mechanical or hydraulic energy. Those forms of energy might be derived ultimately from the burning of petroleum, but they might come from other sources instead.
When energy is converted from one form to another, as when a motor converts electrical energy into mechanical energy, that conversion too is an exchange, and as a practical matter those exchanges invariably involve a loss in the useable quantity of energy; that consistent loss is a matter of thermodynamics. Yet the change in form is implicitly deemd to be worth the loss in quantity.
A world in which the extraction of petroleum involved a net loss in the quantity of available energy would be very different from the present; if that state were somehow reached to-morrow, it would be catastrophic. Even just a decline in the net gain in the quantity of energy from extraction is note-worthy when it occurs (as is an increase). But, in a more gradually changing world, petroleum could and probably would continue to be produced even under circumstances that required a sacrifice in the net quantity of available energy.