Expulsion of petroleum from type III kerogen source rocks in gaseous solution: modelling of solubility fractionation

The composition of n-alkane mixtures expelled from type III kerogen-bearing source rocks at moderate maturity levels shows strong signs of fractionation according to molecular chain length. This effect is attributed to the process of expulsion in gaseous solution. The degree of fractionation varies with the pressure and temperature conditions of primary migration. Fractionation effects for C 15+ -n-alkanes have been simulated with the help of equations developed from experimental data of Price et al. (1983) for gaseous solution of oil. For known compositions of expelled n-alkanes our simulation model allows the estimation of pressure and temperature conditions during primary migration. Expulsion of petroleum in gaseous solution is a particularly effective migration mechanism for overpressured type III kerogen-bearing source rocks. Two geological case histories are presented which demonstrate the compositional effects of petroleum expulsion in gaseous solution from such source beds. The observed n-alkane distributions expelled can be closely matched by simulated distributions, which were calculated for P-T conditions estimated from reconstructions of the burial histories of both case histories.