  Among the numerous questions that arise concerning the exploitation of
petroleum from unconventional reservoirs, lie the questions of the composition
of hydrocarbons present in deep seated HP-HT reservoirs or produced during
in-situ upgrading steps of heavy oils and oil shales. Our research shows that
experimental hydrocarbon cracking results obtained in the laboratory cannot be
extrapolated to geological reservoir conditions in a simple manner. Our
demonstration is based on two examples: 1) the role of the hydrocarbon mixture
composition on reaction kinetics (the "mixing effect") and the effects of
pressure (both in relationship to temperature and time). The extrapolation of
experimental data to geological conditions requires investigation of the
free-radical reaction mechanisms through a computed kinetic model. We propose a
model that takes into account 52 reactants as of today, and which can be
continuously improved by addition of new reactants as research proceeds. This
model is complete and detailed enough to be simulated in large ranges of
temperature (150-500\degree C) and pressures (1-1500 bar). It is thus adapted
to predict the hydrocarbons evolution from upgrading conditions to geological
reservoirs.
