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Heavy crude oil or Extra Heavy oil is any type of crude oil which does not flow easily. It is a relative term, compared to light crude oil, but relates to specific technical issues of its own on production, transportation, and refining. Physical properties that distinguish heavy crudes from lighter ones include higher viscosity and specific gravity, as well as heavier molecular composition. Extra heavy oil from the area north of the Orinoco river in Venezuela, which has by far the largest volume of the 30 or more countries with known reserves[1], has a viscosity of over 10,000 centipoise and 10° API gravity.[2] Generally a diluent is added at regular distances in a pipeline carrying heavy crude to facilitate its flow.

Some petroleum geologists categorize bitumen from tar sands as extra heavy oil although bitumen does not flow at ambient conditions.

Heavy crude oils provide an interesting situation for the economics of petroleum development. The Resources of Heavy oil in the world are more than twice of those conventional light crude oil. On one hand, due to increased refining costs and high sulfur content, heavy crudes are often priced at a discount to lighter ones. The increased viscosity and density also makes production more difficult (see: reservoir engineering). On the other hand, large quantities of heavy crudes have been discovered in the Americas including Canada, Venezuela and Northern California. The relatively shallow depth of heavy oil fields (often less than 3000 feet) contributes to lower drilling costs.

Heavy oil is asphaltic. It is "heavy" (dense and viscous) due to the high presence of naphthenes and paraffins. Heavy oil has over 60 carbon atoms and hence a high boiling point and molecular weight. For example, the viscosity of Venezuela's Orinoco extra-heavy crude oil lies in the range 1000-5000 cP, while Canadian extra-heavy crude has a viscosity in the range 5000-10,000 cP, about the same as molasses, and higher (up to 100,000 cP for the most viscous commercially exploitable deposits).^[1]

As a rule, heavy crudes have a more severe environmental impact than light ones. With more difficult production comes the employment of a variety of enhanced oil recovery techniques, including steam flooding and tighter well spacing, often as close as one well per acre. Heavy crudes also carry contaminants. For example, Orinoco extra heavy oil contains 3.5% sulfur as well as vanadium and nickel.[3] Heavy crude oils contain more carbon in relation to hydrogen, thus releasing more carbon dioxide (believed to be responsible for climate change) per amount of usable energy when burned.

Advanced technologies are mitigating the environmental impact via horizontal wells and increased energy efficiency, but barrel per barrel, heavy crudes will likely always be more environmentally damaging than light crudes.

Most geologists agree that crude becomes "heavy" as a result of biodegradation, in which lighter ends are preferentially consumed by bacterial activity in the reservoir, leaving heavier hydrocarbons behind. This hypothesis leans heavily on the techniques of petroleum geochemistry.

1. ^ Dusseault, M.B. (2001) "Comparing Venezuelan and Canadian Heavy Oil and Tar Sands", Paper 2001-061 of the Canadian International Petroleum Conference, Calgary, Alberta.

• Bitumen
• Oil shale
• Tar sands
• Mazut
• Steam injection (oil industry)

• OTS Heavy Oil Science Centre
• Schlumberger map of global heavy oil resources
• Molecular Basis of Heavy Organics in Petroleum and Heavy Oil

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