FINITE-TIME THERMODYNAMICS AND THE QUASI-STABILITY OF CLOSED-SYSTEMS OF NATURAL HYDROCARBON MIXTURES

The isothermal pyrolysis at 372 degrees C, between 400 and 500 bars, o f a paraffinic liquid hydrocar bon (natural physical conditions: 195 d egrees C, 1000 bars) has been performed over 3 months in order to obse rve composition changes and to calculate the total Gibbs energy of the fluid hydrocarbon mixture G(t). The approach of a G minimum correspon ding to a reversible equilibrium of the composition has been detected. This is consistent with the observation of a significant C-11+ paraff in neo-formation flux after 2 months pyrolysis, and the overall stabil ization trend for the fluid composition. The calculated stable composi tion of the saturates family is consistent with the one asymptotically reached after 1000 h of pyrolysis. This stable composition contains s ignificant amounts of C-6+ paraffins. Assuming the functionality of G in the time-composition space to be conserved when changing temperatur e from pyrolysis back to the initial fluid natural condition, the stab le composition extrapolated at 195 degrees C is that of a Liquid hydro carbon, very close to the natural oil used in the pyrolysis experiment s. The observed concentration of most of molecular components of matur e oils would thus be controlled by the effective equilibrium of a reve rsible chemical network. The reversibility of the oil saturates to gas + aromatics conversion is most probably the reason why C-11+ paraffin s may survive for as long as 100 Ma in the range 300 to 350 degrees C as Literature shows for hyper-mature rock extracts.