The price of oil, or the oil price, generally refers to the spot price of a barrel of benchmark crude oil—a reference price for buyers and sellers of crude oil such as West Texas Intermediate (WTI), Brent ICE, Dubai Crude, OPEC Reference Basket, Tapis Crude, Bonny Light, Urals oil, Isthmus and Western Canadian Select (WCS). There is a differential in the price of a barrel of oil based on its grade—determined by factors such as its specific gravity or API and its sulphur content—and its location—for example, its proximity to tidewater and/or refineries. Heavier, sour crude oils lacking in tidewater access—such as Western Canadian Select— are less expensive than lighter, sweeter oil—such as WTI.