Most of the compounds in crude oil are hydrocarbons. This means they only contain hydrogen and carbon atoms, joined together by covalent bonds. Remember that a covalent bond is a shared pair of electrons.ydrocarbons are compounds made from carbon and hydrogen atoms joined by covalent bonds. Alkanes are saturated-they have only single bonds. Alkenes have a double bond-they are unsaturated. Alkenes react with brown bromine water and decolourise it, but alkanes do not. Alkenes can act as monomers.