Sweet crude oil contains small amounts of hydrogen sulfide (in contrast to sour crude oil). Low-sulfur crude oil is in high demand as it is used to produce high-quality gasoline without the need for expensive processes of removing sulfur impurities. The term "sweet" originates from the fact that a low level of sulfur provides the oil with a mildly sweet taste and pleasant smell. Nineteenth century prospectors would taste and smell small quantities of oil to determine its quality.