A benchmark crude or marker crude is a crude oil that serves as a reference price for buyers and sellers of crude oil. There are three primary benchmarks, West Texas Intermediate (WTI), Brent Blend, and Dubai Crude. Other well-known blends include the OPEC Reference Basket used by OPEC, Tapis Crude which is traded in Singapore, Bonny Light used in Nigeria, Urals oil used in Russia and Mexico's Isthmus. Energy Intelligence Group publishes a handbook which identified 195 major crude streams or blends in its 2011 edition. Benchmarks are used because there are many different varieties and grades of crude oil. Using benchmarks makes referencing types of oil easier for sellers and buyers. There is always a spread between WTI, Brent and other blends due to the relative volatility (high API gravity is more valuable), sweetness/sourness (low sulfur is more valuable) and transportation cost. This is the price that controls world oil market price. West Texas Intermediate (WTI) West Texas Intermediate is used primarily in the U.S. It is light (API gravity) and sweet (low-sulfur) thus making it ideal for producing products like low-sulfur gasoline and low-sulfur diesel. Brent is not as light or as sweet as WTI but it is still a high-grade crude. The OPEC basket is slightly heavier and more sour than Brent. As a result of these gravity and sulfur differences, (at least before 2011) WTI is typically traded at a dollar or two premium to Brent and another dollar or two premium to the OPEC basket. Since 2011, WTI has traded at lower prices than Brent. Brent Blend Brent Crude is a mix of crude oil from 15 different oil fields in the North Sea. It is the benchmark used primarily in Europe though it is also mixed in with the OPEC reference basket which is used around the world. Dubai and Oman Dubai Crude, also known as Fateh, is a heavy sour crude oil extracted from Dubai. It is produced in the Emirate of Dubai, part of the United Arab Emirates. Dubai's only refinery, at Jebel Ali, takes condensates as feedstocks, and therefore all of Dubai's crude production is exported. For many years it was the only freely traded oil in the Middle East, but gradually a spot market has developed in Omani crude as well. For many years, most of the oil producers in the Middle East have taken the monthly spot price average