Benzene is a hydrocarbon with six carbon atoms in an unsaturated aromatic ring structure. Benzene is the most fundamental of the aromatic hydrocarbons.
In gasoline, benzene content is limited because it is a known carcinogen and a contributor to smog formation.
However, benzene is an important feedstock to the petrochemicals industry, so many refiners produce and extract benzene to sell as a finished product.
The primary source of benzene in a refinery is from the reformate produced in the reformer.