Figures 20a - 20c Illustrate the behavior of two indices of compositional change attributable to evporative fractionation, increase in C1/C2 and concomitant decrease in P4/P8. Increase in C1 is due to its principal role in advection, and decrease in P4 at a greater rate than that of P8 is due to the greater fugacity of the lighter compounds. These reservoir fluids from western Canada illustrate the initial stages of evaporative fractionation, the secondary maximum migrating from P6 to P8.