PULSED NEUTRON COMBINATION WELL LOGGING SYSTEM

A pulsed neutron combination well logging system for performing simultaneous logs of hydrogen/oxygen ratio, chlorine log, and thermal neutron decay time is disclosed. A downhole pulsed neutron source repetitively irradiates the borehole environs with 14 MEV neutron pulses. Some of the fast neutrons are captured by oxygen. The remaining neutrons are rapidly slowed to thermal energy and captured by other materials in the borehole vicinity. Voltage pulses representative of the capture gamma rays are sent to the surface and input to a multichannel pulse height analyzer by three time gates. Output signals from the analyzer in three different energy regions corresponding to oxygen, chlorine and hydrogen are provided. These signals are used to determine the neutron thermal decay time, the hydrogen to oxygen ratio and the chlorine content of the earth formations, thus providing a direct indication of hydrocarbon contained in the pore spaces therein.