The oxygen within the oil, present at higher levels than in crude oil, lends itself to the formation of destructive free radicals.[3] Hydrodesulfurization and hydrodenitrogenation can address these problems and result in a product comparable to benchmark crude oil.[4][31][39][40] Phenols can be first be removed by water extraction.[40] Upgrading shale oil into transport fuels requires adjusting hydrogen–carbon ratios by adding hydrogen (hydrocracking) or removing carbon (coking).[39][40]