Heavy crude oil or extra heavy crude oil is any type of crude oil which does not flow easily. 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 crudes lighter grades include higher viscosity and specific gravity, as well as heavier molecular composition. Extra heavy oil is defined with a gravity of less than 10° API (i.e. with density greater than 1000 kg/m³ or, equivalently, a specific gravity greater than 1) and a reservoir viscosity of no more than 10,000 centipoises. With a specific gravity of greater than 1, extra heavy crude is present as a dense non-aqueous phase liquid in ambient conditions.

Heavy crude oil is closely related to natural bitumen from oil sands. Some petroleum geologists categorize bitumen from oil sands as extra heavy crude oil due to the density of less than 10 °API. Other classifications label this as bitumen differing it from extra-heavy oil. They differ in the degree by which they have been degraded from the original crude oil by bacteria and erosion. Often, bitumen is present as a solid and does not flow at ambient conditions.

Petroleum(L. petroleum, from Latin: petra (rock) + Latin: oleum (oil)). The Latin word petra is a loanword from Greek πέτρα.

Petroleum or crude oil, is a naturally occurring flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. A fossil fuel, it is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock and undergo intense heat and pressure.

Petroleum is recovered mostly through oil drilling. This comes after the studies of structural geology (at the reservoir scale), sedimentary basin analysis, reservoir characterization (mainly in terms of porosity and permeable structures). It is refined and separated, most easily by boiling point, into a large number of consumer products, from petrol (or gasoline) and kerosene to asphalt and chemical reagents used to make plastics and pharmaceuticals. Petroleum is used in manufacturing a wide variety of materials, and it is estimated that the world consumes about 88 million barrels each day.