Amid all the recent speculation about the point at which oil supplies peak and prices begin to soar, it is often forgotten that the exact peak oil date depends on a number of practical commercial factors, not just how much oil is left in the ground.

Recently, a growing number of oil analysts have begun to accept this truism and warned that logistical constraints could soon cause production to slow, bringing about a practical peak in oil supplies well ahead of schedule and regardless of how much crude remains to be extracted.

Oil consumption in 2007 is expected to average 86 million barrels a day (mmb/d). Opec held its third ever summit this month and announced a review of measures to increase capacity and stabilise prices. Consequently, hundreds of billions of dollars' worth of new investment in wells and refineries are planned to meet estimated global oil demand growth of 2.3 per cent a year.

However, despite this predicted increase in capacity, concerns remain. In July, an International Energy Agency (IEA) report caused panic with its projection that Opec spare capacity would decline to minimal levels by 2012. Pressurising factors include Venezuela's resource protectionism and continued instability in Africa and the Middle East.

Werner Zittel, author of a recently published report from the Energy Watch Group think-tank, believes the global oil peak was passed last year. His report claims there are only 854 billion barrels of remaining global oil reserves, much lower than the 1.3 trillion reported by oil companies.

But if reserves are running low, Zittel warns that more immediate constraints on production will further limit the amount of available oil well ahead of industry projections.

"If you believe the research data, Iraq could increase its output to 3-4 million barrels a day in the next five years, but there's no sign these fields will be developed," says Zittel. "In addition to the security risk, some of the fields were damaged in the past. Their development was rushed and areas of the fields were bypassed. Those reserves are not recoverable."

The oil industry, however, has not succumbed to this gloomy outlook just yet. As conventional oil fields grow more mature, it is pinning its hopes on new techniques such as Enhanced Oil Recovery (EOR) to give production a boost.

Horizontal drilling and water injection are two such approaches, but Zittel remains sceptical about their long-term effectiveness, pointing to experiences at the largest Saudi field, Ghawar, where EOR techniques have been used for a number of years. "Now the water level is rising," he says. "It is up to 45 to 50 per cent in some areas of the field. This is a clear indicator of decline [in production]."

In a presentation at the London Oil & Money conference in October, Sadad al-Husseini, former head of exploration at oil giant Saudi Aramco, said the giant oilfields of the Arabian Gulf are now 41 per cent depleted on average and warned that as the fields approach peak production, costs will rise. "Oil production has reached a structural ceiling," he said in an interview. "The technical floor for the oil price will rise by $12 annually for the next four to five years."

Al-Husseini also claims that estimates of global oil reserves are inflated by 300 gigabarrels, implying that supply will actually plateau in the next few years. "Many of the so-called reserves should be re-categorised as speculative resources," he said. "They're not accessible. They're not available for production."

A major hope for those industries still dependent on oil is that as supplies of crude dwindle, new, "unconventional" sources will be developed as replacements. The thick, bituminous tar sands in Canada, for example, contain 15 billion barrels of proven reserves and although the oil is difficult to extract, experts claim economies of scale could make it competitive with increasingly expensive crude.

Since 2000, production of synthetic crude at Canada's Alberta tar sands has doubled to 1.2 mmb/d and the Canadian Association of Petroleum Producers have said output will triple by 2020. That would more than compensate for the decline in Canada's conventional oil production; however, as with conventional oil, there are reasons to doubt the Canadian producers' growth projections.

"It takes large amounts of natural gas to melt the oil out of the tar sands" said Stephen Andrews of the Association for the Study of Peak Oil and Gas (ASPO). "North American gas has also peaked, and the amount of local water available isn't likely to grow, so supply will probably not meet projections."

Similarly, Venezuela also plans to raise extra-heavy oil output from its Orinoco tar belt to 1 mmb/d by 2010, but national production as a whole will fall slightly to 2.6 mmb/d by 2012. Andrews said the decline in Venezuelan production is partly due to geology and partly geo-politics. "Conventional production is not increasing, and with the reduced role of the international oil companies, the prospects of an increase from unconventional sources have slipped," he said.

The net result is that the proportion of our energy supplied by non-crude oil is increasing steadily at present, but if conventional oil started to decline from 2007, the growth in other fuels would struggle to fill the supply gap for more than a couple of years.

An influential 2005 US Department of Energy publication on peak oil made the seemingly obvious point that to mitigate the economic risk of peaking crude supply, a number of " wedges", including new fuel sources and increased vehicle efficiency, will be required. However, it warned that even on a "crash programme rate", each of the wedges could take up to 20 years to develop.

Somewhat inevitably, the oil industry has sought to downplay many of these concerns, insisting that capacity will continue to grow, current supply constraints can be resolved and that plenty of oil remains in the ground.

But given the uncertainty surrounding the date of both the actual and practical peak in oil supply and the problem many oil firms are currently facing extracting the oil from the ground, growing numbers of firms are realising they need to reduce their dependence on oil.

Buying fuel efficient vehicles, minimising travel miles, increasing use of public transport and investing in energy efficiency measures all make sense as a means for businesses to help tackle climate change. But with oil prices hovering around the $100 a barrel mark and seemingly no one certain about when oil supplies will peak and prices will soar, they also make good commercial sense as well. The sooner we adapt to a less oil-intensive economy, the lower the costs will be in future.