it is not easy to understand the chemical composition of sulfur at the molecular level because of the complex composition

Reduction of sulfur/compounds from oil products is one of the most important tasks in refining business. However, it is not easy to understand the chemical composition of sulfur at the molecular level because of the complex composition. 

Efforts have been made to develop methods for the separation and characterisation of sulfur compounds in order to understand more on their impact on refining and products. Sulfur compounds include hydrogen sulphide, mercaptans, sulfides, thiols, sulfoxides, and sulfones, etc. The composition of the sulfur compound in crude oil could be altered by thermal ageing, resulting in highly condensed aromatic sulfur compounds found in e.g. residual oil fractions.

Removing sulfur from crude oil is an essential task for the refinery as it will impact the refining upgrading units (e.g. catalysts) employed. Of course the oil products having sulfur compounds are also maximised on sulfur content all around the world for environmental reasons.