Heavy and 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 U.S. Geological Survey, bitumen is further distinguished as extra-heavy oil with a higher (i.e., resistance to flow): “Natural bitumen, also called tar sands or oil sands, shares the attributes of heavy oil but is yet more dense and viscous. Natural bitumen is oil having a viscosity greater than 10,000 cP.“Natural bitumen (often called tar sands or oil sands) and heavy oil differ from light oils by their high viscosity (resistance to flow) at reservoir temperatures, high density (low API gravity), and significant contents of nitrogen, oxygen, and sulfur compounds and heavy-metal contaminants. They resemble the residuum from the refining of light oil.
Most heavy oil is found at the margins of geologic basins and is thought to be the residue of formerly light oil that has lost its light-molecular-weight components through degradation by bacteria, water-washing, and evaporation. Conventional heavy oil and bitumens differ in the degree by which they have been degraded from the original crude oil by bacteria and erosion.Often, bitumen is more viscous than cold molasses and does not flow at ambient conditions.