It may help to understand basic distillation/fractionation columns as in,what they have in oil
refineries to answer your question? Imagine a kettle of water
,you slowly heat it up on a stove,until the water reaches boiling point (ie 100 Celsius) then you get steam,as the steam cools it turns back into water!So now, imagine a kettle full of crude oil
,crude oil being a mixture of many different hydrocarbons,which all have their own different boiling points,so basically you heat up the crude oil and as the temperture rises,the hydrocarbons are released in order from lowest to highest of their given boiling points, so the first to go are the light gases ie ethane/methane,then the heavier gas,propane/butane,then the distillates,gasoline/diesoline,until your basically left with bitumen,which has the highest boiling point!...Jeeeezzz,i must be bored!! Anyway,hope this helps?