Unravelling Mangoʹs mysteries: a kinetic scheme describing the diagenetic fate of C7-alkanes in petroleum systems

Analyses of the composition of the light (gasoline range) hydrocarbons in oils are widely used to assess oil maturity or to track petroleum alteration. As Mango observed, the concentration ratios of some isomeric alkanes in the gasoline range do not match their relative thermodynamic stabilities, although these compounds are probably derived from the same alkene precursors. This indicates that light hydrocarbon isomerisation is proceeding via a kinetically rather than an energetically controlled reaction mechanism. We present here a reaction scheme for the formation of isomeric C7-heptanes from heptenes and kinetic calculations based on this scheme, that result in 2-methylhexane/3-methylhexane ratios very similar to those observed in sediments. As such, these results provide a useful theoretical backbone for the utilisation of the gasoline range hydrocarbon compounds in petroleum geochemistry.