Abstract: A method for determining a characteristic of an earth formation traversed by a borehole precisely monitors the hydrocarbon saturation change during production of an oil field. For an initial logging pass, inelastic gamma ray spectra are detected in response to neutrons irradiating the formation and materials in the borehole. An energy window is selected to provide a carbon/oxygen ratio (COW) for determining a volume of oil. A carbon/oxygen ratio is also extracted from the inelastic energy spectra (COR) for determining a volume of oil. The linear correlation between the COW determined volume of oil and the COR determined volume of oil is used to derive a baseline .alpha.. For a subsequent logging pass, inelastic gamma ray spectra are detected and an energy window is selected to provide a subsequent carbon/oxygen ratio (COW') for determining a subsequent volume of oil. Baseline .alpha. is added to the subsequent volume of oil to improve the precision of the measurement.