Like light crude, the definition of heavy crude varies by region and by the organization making the determination. In general, if it has API gravity less than 20, it is considered heavy. At an API gravity of 10, crude oil will have the same density as water. Any API below 10 and the crude sinks in water rather than floating. Oils with an API in this range are often called extra heavy oils. It is estimated that there is twice the amount of heavy crude in reserves than light crude.
When it comes to general properties, heavy crude is thicker, more resistant to flow, and usually contains higher levels of sulfur and other contaminants than does light oil. For heavy oil to be made into gasoline, it has to be refined, cracked to make large hydrocarbons smaller, and treated to remove contaminants like sulfur. All of this extra refining requires more energy input for the same energy output, which reduces the energy returned on energy invested (EROEI) ratio. EROEI helps to determine how valuable a barrel of crude is because crude that requires more energy input is more expensive to refine, which reduces profit. Beyond the need for additional refinement, heavy crude also poses extraction and transportation issues that are not present with light crude.
Extraction of heavy crude requires higher energy input. Heavy crude does not flow like light crude. In fact, its consistency is often compared to that of molasses at room temperature and it is even occasionally solid if not heated. The field of petroleum chemistry has its origins in attempts to make heavy crude easy to extract and transport. Current methods of extraction include open-pit mining, steam stimulation (to make it less viscous), the addition of sand to the oil, and the injection of air into wells to create fires that burn heavier hydrocarbons and degrade them into lighter, more easily pumped varieties.
Transporting heavy crude often requires the addition of diluting agents, particularly in pipeline transport. These diluents are referred to as Heavy Oil Drag Reducing Agents or DRAs. Most pipelines were initially designed for light crude and thus cannot accommodate heavy crude unless it is modified. Often times, heavy and light crude are mixed to promote transport through pipeline. This, of course, results in contamination of the light crude and a reduction in its value.
The other major drawback to heavy crude is its environmental impact. Two specific aspects of heavy crude contribute to this. First, it is contaminated with sulfur and heavy metals, both of which must be removed. Heavy metals are often toxic and their removal from crude presents disposal issues. The sulfur content of heavy oil may be as high as 4.5%. Sulfur contributes to acid rain and in combination with hydrogen, produces hydrogen sulfide, which can be deadly. Sulfur is corrosive to pipeline metal and refinery components.
The other environmental impact of heavy crude is carbon dioxide output, which can be as much as 3 times that of light crude of the same quantity. There are two reasons for this. First, more energy must be input to generate the same quantity of useable material from heavy crude compared to light. This means more carbon dioxide is released for the same amount of useable energy produced. In addition, heavy crude has a higher carbon to hydrogen ratio than light crude. In other words, it contains less hydrogen per carbon than does light crude, which means that when it is burned, more carbon dioxide is created.