Crude oil is a mixture of large and small hydrocarbon molecules which, along with other factors, affect color, consistency, and thus, quality. Two of the most important quality characteristics are density (weight, viscosity) and sulfur content.
Weight: The most basic crude types are heavy and light. Crude “weight” is measured by “API” gravity (American Petroleum Institute). API greater than 10 is very light, and floats on water; Anything less than 10 is heavier, and sinks. Global values fall between 10 and 70 API gravity degrees.
Heavy crude is considered a lower grade as it is higher in metals and sulfur content and thus takes more work to process into finished products.
Content: Crude oil is further classified into sweet and sour types. Crude oil with less than 0.42% sulfur content is considered sweet. Anything above is classified as sour. (It’s called “sweet” because it acutall smealls, and tastes sweet.
Light sweet crude contains far less H2S (sulfur and hydrogen) and is thus easier to refine into products requiring low levels of these impurities (like gasoline).