Back in the days when we were first learning that the future availability of oil just might be limited, the term Peak Oil came into use. It was a recognition that the new sources of oil being developed would be supplying oil at a rate slower than the rate at which the supply from older sources was diminishing, so that the rate of production of oil would go through a peak and then begin a long term drop-off. The current consensus is that this peak occurred several years ago.
More helpful conceptually was the introduction of the concept of Peak Net Oil. Here the energy needed to produce the oil is estimated and then converted into the equivalent barrels of oil, which is then subtracted from the total rate of production of oil. This energy of production varies widely between sources. For example, the oil extracted from a field past its peak is a lower quality crude which requires more intensive refining, and the energy required to get a deep sea well into operation and then to keep it flowing is much greater than historically necessary for the traditional well. This energy needed for production is increasing with time; estimates are that in the early days of the oil era it took about 1 barrel of oil equivalent to produce 100 barrels of oil, where currently it takes about 15. Since the production of oil has peaked and the energy used to produce that oil has been steadily rising, then the Net Oil is clearly past its peak.
Further, if my concern is how available will oil be to me, then I also need to guess how many others are competing with me for the Net Oil that is being produced. The global population began a markedly more rapid increase in the early days of readily available oil and continues to increase, although now at a reduced rate. So even if the Net Oil were a constant, my fraction of it, the Net Oil per Person, would in principle be shrinking. And it is shrinking even more as many of these other Persons are found in industrializing societies where the styles of living and transporting are rapidly becoming more energy intensive, i.e., where demand/use per Person is increasing.
While useful for a global perspective, this last concept is difficult to apply to a particular economy. For example, one that is an exporter of oil can always increase the supply available to its own people, at a cost of course. And population is not a good measure of demand, as some economies are industrializing much more rapidly than others, with their citizens living increasingly more energy-intensive life styles, e.g., India and China.
It is difficult to apply these concepts to other sources of energy, either exhaustible or alternative, since the numbers are more uncertain. Also the net per person concept is little discussed because of the obvious suggestion that population control is one approach to amelioration of the problem, and this is a political no–no. However, oil is our dominant source, it will remain so, and its availability is clearly and irreversibly diminishing.