<P> Access to oil was and still is a major factor in several military conflicts of the twentieth century, including World War II, during which oil facilities were a major strategic asset and were extensively bombed . The German invasion of the Soviet Union included the goal to capture the Baku oilfields, as it would provide much needed oil - supplies for the German military which was suffering from blockades . Oil exploration in North America during the early 20th century later led to the US becoming the leading producer by mid-century . As petroleum production in the US peaked during the 1960s, however, the United States was surpassed by Saudi Arabia and the Soviet Union . </P> <P> Today, about 90 percent of vehicular fuel needs are met by oil . Petroleum also makes up 40 percent of total energy consumption in the United States, but is responsible for only 1 percent of electricity generation . Petroleum's worth as a portable, dense energy source powering the vast majority of vehicles and as the base of many industrial chemicals makes it one of the world's most important commodities . Viability of the oil commodity is controlled by several key parameters, number of vehicles in the world competing for fuel, quantity of oil exported to the world market (Export Land Model), Net Energy Gain (economically useful energy provided minus energy consumed), political stability of oil exporting nations and ability to defend oil supply lines . </P> <P> The top three oil producing countries are Russia, Saudi Arabia and the United States . About 80 percent of the world's readily accessible reserves are located in the Middle East, with 62.5 percent coming from the Arab 5: Saudi Arabia, UAE, Iraq, Qatar and Kuwait . A large portion of the world's total oil exists as unconventional sources, such as bitumen in Canada and extra heavy oil in Venezuela . While significant volumes of oil are extracted from oil sands, particularly in Canada, logistical and technical hurdles remain, as oil extraction requires large amounts of heat and water, making its net energy content quite low relative to conventional crude oil . Thus, Canada's oil sands are not expected to provide more than a few million barrels per day in the foreseeable future . </P> <P> In its strictest sense, petroleum includes only crude oil, but in common usage it includes all liquid, gaseous and solid hydrocarbons . Under surface pressure and temperature conditions, lighter hydrocarbons methane, ethane, propane and butane occur as gases, while pentane and heavier hydrocarbons are in the form of liquids or solids . However, in an underground oil reservoir the proportions of gas, liquid, and solid depend on subsurface conditions and on the phase diagram of the petroleum mixture . </P>

Why can crude oil not be used straight out of the ground