Petroleum consists of three main hydrocarbon groups:
Paraffins
These consist of straight or branched carbon rings saturated with hydrogen atoms, the simplest of which is methane (CH4) the main ingredient of natural gas. Others in this group include ethane (C2H6), and propane (C3H8).
Chemically paraffins are very stable compounds.
Naphthenes
Naphthenes consist of carbon rings, sometimes with side chains, saturated with hydrogen atoms. Naphthenes are chemically stable, they occur naturally in crude oil and have properties similar to paraffins.