[en] Five-group, time-dependent diffusion theory has been used to compute the apparent formation lifetime of the pulsed neutron thermal decay tool in the presence of invasion. The computer code was benchmarked against Monte Carlo calculations for computing formation lifetime, with very good results. Two cases are treated, one in which a weak brine invades a strong brine in a 30-p.u. sandstone, and one in which a strong brine invades fresh water in a 20-p.u. sandstone. The depth of investigation at the 50% point is about 5'' in the first case and 10'' in the second. The effect of ramp transition zones as wide as 4'' at an invasion depth of 8'' does not exceed 0.1 capture unit, showing that the assumption of a sharp invasion interface is not seriously in error. (Author)