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Highly viscous oil that cannot easily flow to production wells under normal reservoir conditions
Heavy crude oil (or extra heavy crude oil) is highly viscous oil that cannot easily flow from production wells under normal reservoir conditions. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°. Physical properties that differ between heavy crude oils and lighter grades include higher viscosity and specific gravity, as well as higher molecular weight hydrocarbon composition. In 2010, the World Energy Council(WEC) defined extra heavy oil as crude oil having a gravity of less than 10° and a reservoir viscosity of more than 10,000 centipoises. In other words, oil with a density greater than 1000 kg/m3 (or a specific gravity greater than 1) and a reservoir viscosity of more than 10,000 centipoises. Heavy oils and asphalt are dense nonaqueous phase liquids (DNAPLs). They have a low solubility and a viscosity greater than, and density higher than, water. Large spills of DNAPL will quickly penetrate the full depth of the aquifer and accumulate at the bottom.

Heavy crude oil is closely related to natural bitumen from oil sands. Petroleum geologists categorize bitumen from oil sands as 'extra heavy oil' due to its density of less than 10° API. Bitumen is the heaviest, thickest form of petroleum. According to the World Resources Institute, concentrations of remarkable quantities of heavy oil and oil sands are found in Canada and Venezuela. The U. S. Energy Information Administration (EIA) reported in 2001 that the largest reserves of heavy crude oil in the world were located north of the Orinoco river, in the 270 mile long by 40 mile wide Orinoco Belt, in eastern Venezuela. At that time Venezuela began authorizing "joint ventures to upgrade the extra heavy crude resources." the same amount as the conventional oil reserves of Saudi Arabia. The Orinoco Belt in Venezuela is sometimes described as oil sands, but these deposits are non bituminous, falling instead into the category of heavy or extra heavy oil due to their lower viscosity. Natural bitumen and extra heavy oil differ in the degree by which they have been degraded from the original conventional oils by bacteria. Thirty or more countries are known to have reserves. Production, transportation, and refining of heavy crude oil present special challenges compared to light crude oil. Generally, a diluent is added at regular distances in a pipeline carrying heavy crude to facilitate its flow. Dilbit (diluted bitumen) is a means of transporting highly viscous hydrocarbons. Per the Alberta Oil Sands Bitumen Valuation Methodology, "Dilbit Blends" means "Blends made from heavy crudes and/or bitumens and a diluent usually condensate, for the purpose of meeting pipeline viscosity and density specifications, where the density of the diluent included in the blend is less than 800 kg/m3."

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 those of conventional light crude oil. In October 2009, the United States Geological Survey updated the Orinoco deposits (Venezuela) recoverable value to , making this area one of the world's largest recoverable oil deposits. However, recovery rates for heavy oil are often limited from 5 30% of oil in place. The chemical makeup is often the defining variable in recovery rates. New technology utilized for the recovery of heavy oil is constantly increasing recovery rates. On one hand, due to increased refining costs and high sulfur content for some sources, 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 California. The relatively shallow depth of heavy oil fields (often less than 914.40 m) can contribute to lower production costs; however, these are offset by the difficulties of production and transport that render conventional production methods ineffective. However, because crude oil is refined before use, generating specific alkanes via cracking and fractional distillation, this comparison is not valid in a practical sense. Heavy crude refining techniques may require more energy input though, so its environmental impact is presently more significant than that of lighter crude if the intended final products are light hydrocarbons (gasoline motor fuels). On the other hand, heavy crude is a better source for road asphalt mixes than light crude. With present technology, the extraction and refining of heavy oils and oil sands generates as much as three times the total CO2 emissions when compared with conventional oil, primarily driven by the extra energy consumption of the extraction process (which may include burning natural gas to heat and pressurize the reservoir to stimulate flow). Current research into better production methods seek to reduce this environmental impact. In a 2009 report, the National Toxics Network, citing data provided by the Carbon Dioxide Information Analysis Center of the government of the United States and the Canadian Association of Petroleum Producers (CAPP), stated that emissions of CO2 per unit of energy produced were ~84% of those for coal (0.078/0.093) higher than CO2 emissions of conventional oil. Environmental Research Web has reported that "because of the energy needed for extraction and processing, petroleum from Canadian oil sands has higher life cycle emission" compared with conventional fossil fuels; "up to 25% more."