A log of a response primarily related to hydrogen concentration but also affected by mineralogy and borehole effects. The neutron log does not distinguish between the hydrogen in the pore fluids (i.e., water, oil, gas), in water of crystallization, or water bound to solid surfaces. In clean oil-filled or water-filled formations the apparent porosity reading ot the neutron log reflects the amount of liquid-filled pore volume. Used with other porosity information. the neutron log is useful to ascertain the presence of gas and determine mineralogy and shaliness.

The tool contains a continuously emitting neutron source and either a neutron- (n-n tool) or a gamma-ray detector (n-g tool). High energy neutrons from the source are slowed down by collisions with atomic nuclei. The hydrogen atoms are by far the most effective in the slowing down process because their mass is nearly equal to that of the neutron. Thus, the distribution of the neutrons at the time of detection is primarily determined by the hydrogen concentration. Depending on the tool type, detection is made of either (1) thermal neutrons; (2) gamma rays, generated when thermal neutrons are captured by thermal-neutron absorbers in the formation (primarily chlorine); or (3) epithermal neutrons (neutrons having energies higher than thermal).

Neutron curves are scaled in API units or in terms of apparent porosity. The neutron log can be recorded in open or cased liquid-filled well bores. There is a maximum hole size limitation in empty holes for running tools in which the detector does not contact the formation wall. See also sidewall neutron log and compensated neutron log.