Classifications of Crude Oil

Crude oil otherwise known as petroleum (from Greek: petra (rock) + Latin: oleum (oil)) is a composition of hydrocarbons (chemicals composed solely of hydrogen and carbon in various molecular arrangements) and other compounds which is usually brown or black in color.

It is extracted from the earth, formed naturally from the fossil of animals and plants. The viscosity and relative weight of crude oil varies and it can exist in either liquid or solid state.

The different types of crude oil are classified based on the American Petroleum Gravity (API) gravity and viscosity. The properties may vary in terms of proportion of hydrocarbon elements, sulfur content etc as it is extracted from different geographical locations all over the world.

Light crude oil is defined as having an API gravity higher than 31.1 °API
Medium oil is defined as having an API gravity between 22.3 °API and 31.1 °API
Heavy oil is defined as having an API gravity below 22.3 °API.

Classifications are made based on the sulfur content as well. Crude oil with low content of sulfur means ‘sweet’ and the presence of high content sulfur is known as ‘sour’.

One of the largest and major Classifications of Crude oil is Brent Blend, which is found in the North Sea. With an API gravity of 38.3 degrees and 0.37% of sulfur, this blend of crude oil comes from 15 various oil fields in the North Sea.

West Texas Intermediate (WTI) otherwise known as Texas Light Sweet, OPEC Reference Basket (ORB) and Dubai Crude are other major benchmarks or references. The deposits for West Texas Intermediate are found in Texas and Mexico whereas for OPEC Reference Basket oil is sourced from Bonny light (Nigeria), Arab light (Saudi Arabia), Basra light (Iraq), Saharan blend (Algeria) and Minas (Indonesia). Although, Brent blend is graded as a light crude oil, it is not as light as WTI.

Russian Export Blend, the Russian benchmark crude, is a mixture of several crude grades used domestically or sent for export. Russian Export Blend is a medium, sour crude oil with an API gravity of approximately 32 and a sulfur content of approximately 1.2%. Its spot price is reported at Augusta, Italy, and Rotterdam, the Netherlands, which act as the two primary delivery points.

Light, sweet crude is more expensive than heavier, sourer crude because it requires less processing and produces a slate of products with a greater percentage of value-added products, such as gasoline, diesel, and aviation fuel. Heavier, sourer crude typically sells at a discount to lighter, sweeter grades because it produces a greater percentage of lower value-added products with simple distillation and requires additional processing to produce lighter products.

Sources:
http://enggstudent.com/forum/viewtopic.php?id=977
http://en.wikipedia.org/wiki/API_gravit 
http://www.liveoilprices.co.uk/crude_oil/types_of_crude_oil.html
http://www.nesteoil.com/default.asp?path=1,41,538,2035,5196,5199
http://en.wikipedia.org/wiki/Petroleum
http://www.oilprices.org/types-of-crude-oil.html