Methods are disclosed herein for determining the point of entry of water cut into a cased well borehole. A well logging sonde is moved past a perforated interval in a cased well borehole. The borehole environment is irradiated with 14 MeV neutrons from a neutron source and measurement of movement of oxygen activated borehole fluid past two gamma ray detectors spaced from the neutron source is combined with the speed of movement of the logging sonde to derive a signal representative of the entry location and flow rate of the oxygen activated fluid. At least one borehole fluid sample is taken in the interval between the point of neutron irradiation and the detection of oxygen activation gamma rays to resolve any ambiguity in the total flow rate of borehole fluid in the interval.