It's complicated... crude oil is a mix of hydrocarbons with carbon chain lengths between say 5 and 40+, plus various impurities such as sulfur. It is refined in

It's complicated... crude oil is a mix of hydrocarbons with carbon chain lengths between say 5 and 40+, plus various impurities such as sulfur. It is refined into gasoline (~7-12 carbon atoms per molecule), diesel (~12-18) etc, with bunker fuel containing the heaviest fractions. It is possible to crack the heavy fractions into lighter ones, and this is commonly done at most refineries. However, what to do with the impurities? You can take them out, but then disposal becomes a problem (see image below for example). So, if possible, refiners tend to leave them in the product. http://www.petroleumjournal.kz/imgdb/article_3/article_3_pre_777_2349.jpg http://www.petroleumjournal.kz/imgdb/article_3/article_3_pre... That yellow stuff is elemental sulfur.