West Texas Intermediate at Cushing, Oklahoma (WTI Cushing), a light, sweet crude grade, is North America’s most closely observed crude oil price benchmark and the underlying commodity of the NYMEX crude futures contract. Until 2008, all North American crude grades broadly tracked fluctuations in WTI Cushing prices and were clustered within about $8 per barrel of the WTI Cushing price. Pricing differences between crude grades were largely explained by the different quality characteristics of the crude oil in each location and transportation costs to Cushing, the delivery point of the NYMEX contract.

Since 2008, however, the price differences between WTI Cushing and other North American crude oil benchmarks have increased sharply (see chart above). In addition to WTI, other crude grades have emerged as alternative benchmarks. In particular, the Argus Sour Crude Price Index (ASCI), a weighted average of prices for several offshore Gulf of Mexico sour crude grades, has become the benchmark or reference used for assessing the price of several imported grades sold on a long-term contract basis, including Saudi Arabian and Kuwaiti crude grades.

via North American spot crude oil benchmarks likely diverging due to bottlenecks – Today in Energy – U.S. Energy Information Administration (EIA).