Hi Atonija...This is obviously to do with certain processes/stages in the refinery. The second part it is a bit confusing.
I'm not familiar with 'the short side chain' and also 'napthene ring' so I'm guessing they are stages or sections of the refining process and I've rearraged it accordingly.
I hope I haven't altered your original context.
Aromatic hydrocarbons are ring shaped. The aromatic content of the oil grows with a corresponding increase in the average molecular mass of the fraction.
In the lower fractions there are aromatics with a single nucleus which ('are sent through' or 'belong to') the short side chain (petrol). In the higher fractions there are condensed aromatics and other aromatics which ('are sent to'or 'belong to') the napthene ring (gas oils and fuel oils).