oil

Three conditions must be present for oil reservoirs to form: a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil; a porous and permeable reservoir rock for it to accumulate in; and a cap rock (seal) or other mechanism that prevents it from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs. Because most hydrocarbons are lighter than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.



The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where hydrocarbons are broken down to oil and natural gas by a set of parallel reactions, and oil eventually breaks down to natural gas by another set of reactions. The latter set is regularly used in petrochemical plants and oil refineries.
Crude oil varies greatly in appearance depending on its composition. It is usually black or dark brown (although it may be yellowish or even greenish). In the reservoir it is usually found in association with natural gas, which being lighter forms a gas cap over the petroleum, and saline water which, being heavier than most forms of crude oil, generally sinks beneath it. Crude oil may also be found in semi-solid form mixed with sand and water, as in the Athabasca oil sands in Canada, where it is usually referred to as crude bitumen. In Canada, bitumen is considered a sticky, tar-like form of crude oil which is so thick and heavy that it must be heated or diluted before it will flow.[5] Venezuela also has large amounts of oil in the Orinoco oil sands, although the hydrocarbons trapped in them are more fluid than in Canada and are usually called extra heavy oil. These oil sands resources are called unconventional oil to distinguish them from oil which can be extracted using traditional oil well methods. Between them, Canada and Venezuela contain an estimated 3.6 trillion barrels (570×10^9 m3) of bitumen and extra-heavy oil, about twice the volume of the world's reserves of conventional oil.
etroleum is found in porous rock formations in the upper strata of some areas of the Earth's crust. There is also petroleum in oil sands (tar sands). Known reserves of petroleum are typically estimated at around 190 km3 (1.2 trillion (short scale) barrels) without oil sands, or 595 km3 (3.74 trillion barrels) with oil sands.Consumption is currently around 84 million barrels (13.4×10^6 m3) per day, or 4.9 km3 per year. Because the energy return over energy invested (EROEI) ratio of oil is constantly falling (due to physical phenomena such as residual oil saturation, and the economic factor of rising marginal extraction costs), recoverable oil reserves are significantly less than total oil in place[citation needed] . At current consumption levels, and assuming that oil will be consumed only from reservoirs, known recoverable reserves would be gone around 2039 , potentially leading to a global energy crisis. However, to date discoveries of new oil reserves have more than matched increased usage. In addition, there are factors which may extend or reduce this estimate, including the increasing demand for petroleum in developing nations, particularly China and India; further new discoveries; increased economic viability of recoveries from more difficult to exploit sources; energy conservation and use of alternative energy sources; and new economically viable exploitation of unconventional oil sources.
Petroleum is used mostly, by volume, for producing fuel oil and gasoline (petrol), both important "primary energy" sources. 84% by volume of the hydrocarbons present in petroleum is converted into energy-rich fuels (petroleum-based fuels), including gasoline, diesel, jet, heating, and other fuel oils, and liquefied petroleum gas.[8] The lighter grades of crude oil produce the best yields of these products, but as the world's reserves of light and medium oil are depleted, oil refineries are increasingly having to process heavy oil and bitumen, and use more complex and expensive methods to produce the products required. Because heavier crude oils have too much carbon and not enough hydrogen, these processes generally involve removing carbon from or adding hydrogen to the molecules, and using fluid catalytic cracking to convert the longer, more complex molecules in the oil to the shorter, simpler ones in the fuels.

Due to its high energy density, easy transportability and relative abundance, oil has become the world's most important source of energy since the mid-1950s. Petroleum is also the raw material for many chemical products, including pharmaceuticals, solvents, fertilizers, pesticides, and plastics; the 16% not used for energy production is converted into these other materials.

Petroleum is found in porous rock formations in the upper strata of some areas of the Earth's crust. There is also petroleum in oil sands (tar sands). Known reserves of petroleum are typically estimated at around 190 km3 (1.2 trillion (short scale) barrels) without oil sands, or 595 km3 (3.74 trillion barrels) with oil sands.Consumption is currently around 84 million barrels (13.4×10^6 m3) per day, or 4.9 km3 per year. Because the energy return over energy invested (EROEI) ratio of oil is constantly falling (due to physical phenomena such as residual oil saturation, and the economic factor of rising marginal extraction costs), recoverable oil reserves are significantly less than total oil in place[citation needed] . At current consumption levels, and assuming that oil will be consumed only from reservoirs, known recoverable reserves would be gone around 2039 , potentially leading to a global energy crisis. However, to date discoveries of new oil reserves have more than matched increased usage. In addition, there are factors which may extend or reduce this estimate, including the increasing demand for petroleum in developing nations, particularly China and India; further new discoveries; increased economic viability of recoveries from more difficult to exploit sources; energy conservation and use of alternative energy sources; and new economically