The following table was cut-and-pasted,
Hydrocarbon Average RangeAlkanes (paraffins) 30% 15 to 60%Naphthenes 49% 30 to 60%Aromatics 15% 3 to 30%Asphaltics 6% remainder
the Wikipedia article on Petroleum.
The actual composition varies widely depending on the character of the crude oil.
Also the refineries have some chemistry tricks they can use to make more of the kind of products they want to sell (at the expense of losing some other product, plus energy lost in the conversion). Broadly speaking, there are two techniques for this, cracking is used to make heavy molecules into lighter ones, and unification is a technique to combine light molecules into heavier ones.
So, to some extent, the exact composition of the refined products (i.e how much petrol, how much diesel, how much ethylene, how much benzene, etc) is whatever they want it to be.
The HowStuffWorks article on Oil Refining, sort of explains how this works,
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