There is a general agreement that most of the oil there is to be found has now been found, and the world is using it up at an inexorable rate. My understanding is there are around one trillion (one thousand billion) barrels remaining in known geological holdings, and we get through just over 30 billion barrels of oil each year: thus, simple arithmetic suggests there are about 33 years worth left. Growth in demand is relentless, not only from the West but in the form of ever more aggressive efforts from China and India, who are undergoing an unprecedented phase of industrial expansion. To maintain the pace of this will to convert a dominantly agrarian culture into a technologically underpinned (and hugely populous) society demands oil, and in colossal quantities, almost certainly for longer than 3 decades. Clearly, supplying greater quantities of oil, year on year to meet rising demand, will exhaust its finite supply more quickly, and most likely there are rather less than 30 years of it in hand. As I stress, even that figure is based on simple arithmetic and may prove reassuringly misleading.
An oil-well is not like a water-well, and it cannot similarly and simply be drained to bottom. How much oil can actually be extracted depends on many factors, especially the local geology which dictates the nature, and permeability of the rock that contains it. I have mentioned previously some speculations that less oil will be finally obtained even from the giant fields in Saudi, because the enhanced recovery methods used to match the enormous world thirst for oil have probably damaged the rock rendering it less permeable, and in consequence less of the total oil will finally flow out through it, toward mostly Western oil refineries. It is noteworthy too that something like 9 million barrels of oil are extracted per day from Saudi, and the resulting empty-volume is filled by 9 million barrels of water. Thus a considerable demand is placed on a secondary resource - water - in order to extract oil.
Modern exploration for petroleum is an accomplished technical procedure. Seismic measurements shot in a set of 3-D grid-lines will reveal structural differences within particular strata, or geologic/sedimentary basins, and hence show any significant oil prospects where drilling might prove worthwhile. Today, it is fair to say that there are precious few areas where exploration for petroleum cannot be performed with success, if regional seismic studies indicate there is a good chance of finding substantial petroleum fields, i.e. those which might yield 100 million barrels of oil or more. However, in spite of assiduous efforts by the oil companies of the world, only a few of the major fields promised by geologists were actually found. Those accessible oil reserves had all been recognised previously, and most of their major fields already located. The upshot is that while many major finds were made in the late 1960's, which new off-shore production technology had brought on-stream to bring the OPEC producers to heel in the mid-1970's, no new major oil provinces (producing between 7 and 35 billion barrels) have been discovered since 1980.
Advances in 3-D seismic measurements and horizontal-drilling techniques have enhanced the rate of oil-recovery from known fields, but they have not proved the existence of major new fields, and so the known oil reserves of the planet remain finite. There persist contradictions over figures for resources versus reserves, and the picture looks misguidedly rosy when all forms of oil are accounted for on a single balance sheet - namely all the oil in the ground, oil from tar sands, and even from putative (i.e. as yet non-existent) coal liquefaction plants. Use of either term depends on whose pocket is being dipped into: "resources" means "your" money, while "reserves" is "my" money. Hence oil "reserves" are those that are profitable, while "resources" (which include everything that might be flagged-up, even on theoretical grounds) may be less readily obtained at any sensible fiscal or energy costs. Put another way, conservative bankers will not lend money on "resources" - "reserves" yes!
There are various estimates as to when the peak in production ("Peak Oil") will come. Some are of the opinion that it has already happened, but other than optimistic projections from the oil industry itself, all predictions are that it will be around 2010 (i.e. in a couple of years from now). Beyond the peak, production will fall. The world currently runs mainly on the "sweet", "light crude" oil which is readily refined into fuel, and as the wells are drained further down, and the barrel (literally and metaphorically) is scraped to yield further supplies over time, the quality of that oil will fall too, meaning that the oil provided is of a dirtier, "heavier" kind that is more intensive in effort and energy to purify and refine - hence it will become more expensive, both on account of its quantity and its quality. Expressed alternatively its EROEI (energy returned on energy invested), which currently stands at about 8, will fall. Oil from tar sands has an EROEI of about 3.
The result may be a collapse of the world economy, after which the societies of many Western countries will begin to look like that in Russia. The West (United States especially) will find itself competing with China for every tanker of oil. This places the fulcrum of world stability firmly in the sands of the Middle East. The New York Times printed an article in their March 5, 2007 edition, "Oil innovations pump new life into old wells,"which essentially sent the message that new technology casts doubt on the threat or practical reality of peak oil. In support of this view, they note that production is "up" from the Kern River Field in California and Duri Field in Indonesia, and that the Means Field in Texas is expected to yield double the original estimates, all thanks to innovations in technology. However, what was not mentioned is the rapid decline in production rates of an ever increasing number of large fields. Overall production from California and Texas has fallen markedly, in spite of all the technology that has been thrown there to try and avert this outcome.
The broad picture is that supplies of oil worldwide from large conventional fields are falling. If we were to believe that there is 4.7 trillion barrels available as a resource - that would suggest there is around 150 years worth left to us. However, it is unlikely that most of that will ever become a "reserve", because the EROEI or simply the requisite quantities of gas and water etc. required to produce it cannot be met. The truth is that the world is beginning to run out of oil, an eventuality we must now confront and plan-for.
(1) Energy Bulletin, "Peak Oil: What the media don't want you to know," and references and links appended there. http://www.energybulletin.net/newswire.php?id=27170
(2) L.F. Ivanhoe, "Get Ready for Another Oil Shock", printed in The Futurist, January/February 1997. It is interesting to look back 10 years.
(3) C.J.Rhodes, previous articles in http://ergobalance.blogspot.com