Crude Oil
The petroleum industry often characterizes crude oils according to their geographical source. Oils from different geographical areas have unique properties; they can vary in consistency from a light volatile fluid to a semi-solid. We would recommend the use of the classification scheme below, which we have found to be more useful in defining requirements.
Class A: Light, Volatile Oils
These oils are highly fluid, often clear, spread rapidly on solid or water surfaces, have a strong odour, a high evaporation rate, and are usually flammable. They penetrate porous surfaces such as dirt and sand, and may be persistent in such a matrix. They do not tend to adhere to surfaces; flushing with water generally disperses them. Class A oils may be highly toxic to humans, fish, and other biota. Most refined products and many of the highest quality light crudes can be included in this class.
Class B: Non-Sticky Oils
These oils have a waxy or oily feel. Class B oils are less toxic and adhere more firmly to surfaces than Class A oils, although they can be removed from surfaces by vigorous flushing. As temperatures rise, their tendency to penetrate porous substrates increases and they can be persistent. Evaporation of volatiles may lead to a Class C or D residue. Medium to heavy paraffin-based oils fall into this class.
Class C: Heavy, Sticky Oils.
Class C oils are characteristically viscous, sticky or tarry, and brown or black. Flushing with water will not readily remove this material from surfaces; however the oil does not readily penetrate porous surfaces. The density of Class C oils may be near or heavier that of water and they often sink. Weathering or evaporation of volatiles may produce solid or tarry Class D oil. Toxicity is low, but wildlife can be smothered or drowned when contaminated. This class includes residual fuel oils and medium to heavy crudes.
Class D: Nonfluid Oils.
Class D oils are relatively non-toxic, do not penetrate porous substrates, and are usually black or dark brown in colour. When heated, Class D oils may melt and coat surfaces, making cleanup very difficult. Residual oils, heavy crude oils, some high paraffin oils, and some weathered oils fall into this class.
The oil industry classifies "crude" by the location of its origin (e.g., "West Texas Intermediate, WTI" or "Brent") and often by its relative weight (API gravity ) or viscosity ("light", "intermediate" or "heavy"); refiners may also refer to it as "sweet", which means it contains relatively little sulphur , or as "sour", which means it contains substantial amounts of sulphur and requires more refining in order to meet current product specifications.