Worldwide proven oil reserves are estimated at around 1.35 trillion barrels, or 43 years production at current rates. However, that excludes Canadian and Venezuelan tar sands with 3.6 trillion barrels of recoverable oil, or 116 years production at current rates. Tar sands oil costs around $31 a barrel to extract and upgrade to the quality of conventional crude. So as long as the price of oil stays over $31 (currently $85.00), peak oil will likely be postponed until: (a) output in one of the major oil producing regions is terminated by a political or geological catastrophe, or (b) a convenient and cheaper alternative to oil becomes widely available.
Natural gas seems most likely to replace oil at some point this century. World reserves total about 190 trillion cubic meters or 1.1 trillion barrels of oil equivalent, enough to substitute for oil at present consumption rates for about 30 years. And that's not counting conventional gas reserves yet to be discovered, or shale gas, or deep water gas, or gas in tight sedimentary deposits that new technology is now making available, or methane hydrates, which exist in vast abundance on the seafloor throughout the temperate and polar regions, which might keep the World powered for another hundred years or so.
In fact, natural gas is already a lot cheaper than oil. At around $4.00 a thousand cubic feet, gas in North America is less than a third the price of oil on an energy equivalent basis (6000 cubic feet having the same energy content as a barrel of oil). Natural gas is bulky, however, which makes it expensive both to transport and to store, and unsuitable for some applications, e.g., as a motor or aviation fuel. Moreover, the major gas producing regions of the World are mostly remote from the densely populated and industrialized areas of high energy demand.
But as new methods are applied to the extraction of gas from shales and other unconventional sources, supplies will increasingly be developed close to places of use. In addition, the World's infrastructure for gas transportation and storage will continue to expand in range and capacity. We can expect, therefore, to see natural gas continually encroach on the market for oil.
In addition, there is solar power, a source of the highest quality form of energy: electricity. Both solar thermal and photoelectric generation can achieve efficiencies of more than 40%, which means that global energy demand could be met by solar installations covering only a tiny fraction of the World's land (desert) area. Cost, however, remains high, which means that solar power will continue to be of only marginal significance, without major cost reductions in photovoltaic cell manufacture, and major technical advances in battery technology and electrical transmission systems, including perhaps, the development of World-wide superconducting grids, ensuring the availability of solar power 24/7 during every season of the year.
With the disaster of Fukushima spreading poison around the World, let us hope that the nuclear option will be abandoned altogether.