Heavy crude oil (or extra heavy crude oil) is highly-viscous oil that cannot easily flow from production wells under normal reservoir conditions.. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil.Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°. Physical properties that differ between heavy ...

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 …