Crude oil that does not easily flow at room temperature is called heavy crude oil. This differs from light crude oil, which flows freely at the same temperatures because of its lower density. Crude oil with an American Petroleum Institute gravity of less than 20 degrees is classified as either heavy crude oil or extra heavy crude oil. It is speculated that the total amount of heavy crude oil worldwide is more than double that of the more traditionally used light crude oil.
Deposits of heavy crude oil are often found near tar sands, or oil sands, which are mix of sand, clay, water and extremely dense petroleum. Occasionally, the term "bitumen," which is the sticky, semi-solid, tar-like form of petroleum found in tar sands, will be used interchangeably with the extra heavy crude oil found in such deposits. Some geologists contend that bitumen is merely a form of extra heavy crude oil that is so dense that it does not flow at all at room temperature.
Transporting and refining heavy crude oil poses greater environmental risks than transporting and refining the more traditional light crude oil. Heavy oil contains more carbon, along with many other impurities, so it releases more carbon dioxide during production and during use, which can negatively affect the Earth's ozone layer. The production process of heavy oil can generate as much as three times the amount of carbon dioxide that light oil production generates.
Economically, heavy crude oil is often priced lower than light crude oil because it is considered more difficult and more expensive to work with. Although heavy oil can often be found at far shallower depths than light oil — a trait that can significantly reduce the cost of extraction — the production of heavy crude oil presents additional expenses. Many of the traditional methods in place for the transportation and extraction of light crude oil prove ineffective when dealing with the increased density of heavy crude, necessitating expensive infrastructure alterations.
As the easily accessible deposits of light crude oil diminish, the production of heavy oil has become more common. Although it is found in more than 30 countries, the largest deposits can be found in Venezuela near the Orinoco River. These deposits are estimated to hold more than 500 billion barrels of oil, making them the largest deposits of recoverable oil in the world.