There are some estimates that the world has 4.6 trillion barrels of oil left, which equates to about 150 years worth at current demand. However, this is misleading since 3.6 trillion barrels of that are "locked-up" in unconventional sources such as oil sands and will prove hugely expensive in terms of the energy required to extract it and damaging to the environment. Hence the conventional (real) oil is probably enough for 30 years (1 trillion barrels) , and that is if all of it can be winkled-out from the geology that contains it. According to a new report from Wood Mackenzie, who are a consultancy based in Edinburgh, within 15 years, any extra oil supply will come from highly energy demanding and polluting sources such as the oil sands of Canada and the Orinoco tar belt in Venezuela. The oil sands of Alberta do not contain oil as such, but bitumen which must be cracked into oil, and the same goes for the Orinoco reserve. The United States has very large deposits of oil-shale etc. and of course coal, believed to contain more carbon than the whole of the Middle East oil fields, but processing it into oil will return a very poor EROEI (Energy Returned on Energy Invested) and so it might not be worthwhile beyond the production of oil for niche applications - certainly not for running today's enormously inefficient fleet of cars in their presently vast and growing numbers. In addition to the demand from extractive energy and pollution, very large quantities of clean water are required for these processes, hence imposing pressure on another resource. It is sometimes said that "it takes energy to extract energy" but more literally producing one kind of resource always consumes another one (or more than one, e.g. gas and water).
To date, the development of only 8% of the 3.6 trillion barrel "reserve" of unconventional oil has started, for the simple reason that the world has relied on those far more readily available conventional resources of oil and gas we are familiar with. A mere 15% of those putative 3.6 trillion barrels are actually oil at all, even being of the heavy and extra-heavy kind that is far more intensive in its processing than the light crude that surges through the veins of the modern world, being relatively easily processed into gasoline and similar fuels. Wood Mackenzie were sanguine that some big fields will still be increasing their production by 2020, but this will not offset the decline of many (most?) of the others, and accordingly these sources of unconventional oil are the only way to prevent the world from running out of it altogether. Natural gas products such as liquids and condensate are also predicted to become important growth commodities but there is a limit to how much gas can be extracted from the Earth, with some estimates predicting that "peak gas" will happen just a decade or so after "peak oil" and clearly sooner if more of it is turned into synthetic crude.
Major oil companies, Royal Dutch Shell, Total (of Europe), ExxonMobil and Chevron (in the U.S.) have begun to invest substantially in Canada and Venezuela, while others (including Chinese energy groups) are evaluating the possibility of extracting heavy oil from Madagascar. In regard to gas, Devon Energy spent $2.2 billion in 2006 on expanding its already substantial holding in the Texas Barnett shale by acquiring Chief Oil and Gas. It is anticipated that the development of shale deposits of this kind will allow the U.S. to obtain 40% of its gas from unconventional sources by 2020.
Matthew Simmons, an industry backer who shook the oil world by questioning how much oil Saudi really has it its holdings and whether it can realistically continue to expand production to meet rising global demand, has metaphorically poured water on the truth of the massive "unconventional" reserve saying that "the ability to extract heavy oil in significant volumes is still non-existent. Worse, it takes vast quantities of scarce and valuable potable water and natural gas to turn unusable oil into heavy low-quality oil."
He concluded: "In a sense, this exercise is like turning gold into lead."