The low-hanging fruit has been picked. Most of the world's giant, dirt-cheap oil reserves are controlled by state-run oil companies within Organization of the Petroleum Exporting Countries (Opec) member states. Major non-Opec oil provinces like Alaska, the Gulf of Mexico and the North Sea have already been heavily exploited. Meanwhile, demand continues to rise inexorably, leading some to speculate that the world has already passed its natural peak capacity for oil production. So where will the oil of the future come from?
Not where it's come from in the past. New and different thinking is needed, along with new technology and massive investment. Welcome to the world of unconventional hydrocarbons, where oil is mined from giant pits and gas released from coal seams. Once on the periphery of the industry, regarded as too complex and too expensive, unconventional oil and gas is becoming increasingly mainstream.
Witness oil giant ExxonMobil's $41bn (£27bn) acquisition of XTO Energy last December. That huge deal was widely viewed as a belated attempt by the world's largest energy company to catch up lost ground on rivals in unconventional resources, most notably shale gas. Exxon's transaction is just one of many in which energy majors have been snapping up smaller firms or grabbing acreage holding the promise of unconventional resources.
"It's not as if the world is going to run out of reserves," says Patrick Gibson of the global oil supply team at energy consultants Wood Mackenzie. BP's statistics support this view, showing global reserves sufficient for over 40 years' oil consumption and over 60 years' gas consumption at current rates – and that assumes the industry never finds another barrel of oil or another cubic metre of gas.
"The majority of production is being replaced by exploration and reserves upgrades," explains Mr Gibson, "but conventionals are a finite resource."
