There is also natural gas, which can be associated with oil or found alone. Crude oil comes in many forms. Usually it is black, but green, red or brown oils are not uncommon. These oils float easily on water. By contrast, some heavy oils have an API gravity of less than 12 degrees and are so dense that they sink, rather than float, in water.
Most oils are mixtures of many different compounds, most of which are hydrocarbons.
There are four main hydrocarbon groups in petroleum. The saturates are hydrocarbons consisting of straight chains of carbon atoms. Aromatics are hydrocarbons consisting of rings of carbon. Asphlatenes are complex polycylic hydrocarbons that contain many complicated carbon rings, and NSO compounds are mostly nitrogen, sulfur, and oxygen. In most oils, the saturate fraction is the largest and is made up of two subgroups called paraffins and isoprenoids.