Water saturation determination using an oxygen estimate and formation lithology

1. A method for estimating at least one parameter of interest of a formation, comprising:

conveying a carrier in a borehole intersecting the formation, the carrier having disposed thereon a pulsed neutron source and a radiation detector;

activating the pulsed neutron source;

detecting gamma radiation in the borehole from at least the formation, the gamma radiation responsive to the pulsed neutron source and resulting from at least one of: (i) decay of nitrogen-16 formed by activation of oxygen-16, and ii) inelastic scattering of neutrons from oxygen;

estimating, by an information processing device including at least one processor and a processor memory for executing programmed instructions, a water saturation of the formation from the gamma radiation by using the at least one processor to:

obtain a gamma ray count measurement, representing gamma rays from one of: (i) the decay of nitrogen-16 (ii) and the inelastic scattering, via spectral stripping of gamma rays from a measured gamma ray spectrum of the detected gamma radiation, and

estimate the water saturation using the gamma ray count measurement and a model comprising a relationship between measured gamma ray counts and modeled gamma ray counts from each of a fully water saturated formation and a minimally water saturated formation.

2. The method of claim 1 , comprising using the at least one processor to:

model nitrogen-16 decay gamma rays with respect to fully water saturated and minimally water saturated formations by forming curves for a predicted maximum value of nitrogen-16 gamma ray counts and a predicted minimum value for nitrogen-16 gamma ray counts; and

identify a corresponding water saturation value by comparing the gamma ray count measurement to the predicted maximum value and the predicted minimum value.

3. The method of claim 1 , comprising using the at least one processor to:

model oxygen inelastic scattering gamma rays with respect to fully water saturated and minimally water saturated formations by forming curves for a predicted maximum value of oxygen inelastic scattering gamma counts and a predicted minimum value for oxygen inelastic scattering gamma counts; and

identify a corresponding water saturation value by comparing the gamma ray count measurement to the predicted maximum value and the predicted minimum value.

4. The method of claim 2 or 3 , wherein the predicted maximum value and the predicted minimum value are determined using at least one of: i) direct measurement, ii) indirect measurement, iii) historical analysis, iv) interpolation, v) professional expertise, and vi) modeling.

5. The method of claim 1 , wherein:

the carrier has disposed thereon an oxygen estimator including the pulsed neutron generator, at least a portion of the at least one processor, and the radiation detector.

6. The method of claim 1 , further comprising using the water saturation to evaluate the earth formation for production of at least one of: i) oil; and ii) natural gas.

7. A method for estimating at least one parameter of interest of a formation, comprising:

conveying a carrier in a borehole intersecting the formation, the carrier having disposed thereon a pulsed neutron source and a radiation detector;

activating the pulsed neutron source;

detecting gamma radiation in the borehole from at least the formation, the gamma radiation responsive to the pulsed neutron source and resulting from at least one of: (i) decay of nitrogen-16 formed by activation of oxygen-16, and ii) inelastic scattering of neutrons from oxygen;

estimating, by an information processing device including at least one processor and a processor memory for executing programmed instructions, a parameter δf interest of the formation from the gamma radiation by using at least one processor to:

obtain an oxygen estimate comprising an estimate of elemental content of the formation for oxygen using a spectrum of the detected radiation; and

estimate the at least one parameter of interest using at least a formation lithology and the oxygen estimate, wherein the oxygen estimate is the sole estimate of elemental content of the formation used in estimating the at least one parameter of interest, and wherein the formation lithology is at least a characterization of a combination of minerals in the formation, and at least one of the minerals contains oxygen.

8. The method of claim 7 , wherein the at least one parameter of interest comprises water saturation.

9. The method of claim 7 , wherein the formation lithology comprises at least one of: i) a sandstone; ii) a limestone; iii) a dolomite; iv) a carbonate.

10. The method of claim 7 , wherein the estimation of the at least one parameter of interest comprises:

comparing the oxygen estimate with a model based on the formation lithology.

11. The method of claim 10 , wherein the model comprises:

at least one predicted value for oxygen based on the formation lithology.

12. The method of claim 11 , wherein the at least one predicted value comprises at least one of: (i) a predicted maximum value of oxygen and (ii) a predicted minimum value for oxygen.

13. The method of claim 12 , wherein the predicted maximum value for oxygen and the predicted minimum value for oxygen are determined using at least one of: i) direct measurement, ii) indirect measurement, iii) historical analysis, iv) interpolation, and v) modeling.

14. The method of claim 7 , further comprising using the parameter of interest to evaluate the earth formation for production of at least one of: i) oil; and ii) natural gas.