Method for evaluating formations using neutron induced gamma ray measurements

1. A method for well logging, comprising:

moving a well logging instrument along a wellbore drilled through subsurface formations, the well logging instrument comprising a neutron source and at least one gamma ray detector spaced apart by a first distance from the neutron source;

imparting neutrons into the formation using the neutron source;

detecting capture gamma rays resulting from the imparting neutrons into a formation at the first detector;

characterizing energy of the detected capture gamma rays;

removing from the detected capture gamma rays those of the detected gamma rays attributable to neutron capture by hydrogen nuclei;

detecting capture gamma rays at a second detector disposed at a second distance from the neutron source;

removing gamma rays attributable to neutron capture by hydrogen nuclei from the capture gamma rays detected by the second detector;

calculating a ratio of the number of detected capture gamma rays having hydrogen neutron capture gamma rays removed therefrom of the first detector with respect to the second detector; and

using the ratio to calculate a neutron porosity of the formation.

2. The method of claim 1 wherein the neutrons are imparted by a pulsed neutron source for a first selected time interval.

3. The method of claim 2 wherein the capture gamma rays are detected for a second selected time interval after an end of the first time interval.

4. The method of claim 1 wherein the gamma rays are detected by a scintillation detector.

5. The method of claim 4 wherein the characterizing the energy level of the detected capture gamma rays is performed by transmitting detected capture gamma ray signals from a detector to a multichannel analyzer.

6. The method of claim 5 wherein the removing the gamma rays attributable to neutron capture by hydrogen nuclei comprises integrating an output of the multichannel analyzer beginning at an energy level at least at an energy level of gamma rays emitted by neutron capture by hydrogen nuclei.

7. The method of claim 1 wherein the removing the gamma rays attributable to neutron capture by hydrogen nuclei comprises, in the computer, spectrally analyzing an output of the multichannel analyzer to determined presence of selected elements in the formation, and removing therefrom effects of hydrogen from the spectrally analyzed output.