Pulsed neutron combination well logging system

The patent relates to a method for logging a well borehole to obtain direct indications of the hydrocarbon bearing earth formations surrounding the borehole. A well logging tool having a pulsed source of fast neutrons and a radiation counter are passed through the borehole, and the earth in the vacinity of the borehole is repetitively irradiated with bursts of fast neutrons. Capture gamma rays from elemental hydrogen are detected along with the activation capture gamma rays from the elemental oxygen. As the hydrogen to oxygen ratio is substantially different for hydrocarbons than water, a well log which is indicative of the hydrocarbon in the pore spaces of earth surrounding the well borehole may be obtained. (J.R.)