Method for determining oil and water saturation in earth formation surrounding a borehole

This paper describes a method for determining the hydrocarbon saturation S0 in an earth formation surrounding a borehole. It comprises: irradiating the formation with a source of neutrons of sufficient energy to interact with atoms of the formation in the borehole, according to inelastic neutron reactions; detecting and counting the gamma rays resulting from the reactions at least one location longitudinally spaced from the source; forming gamma ray spectrum signals from the gamma rays and deriving from the signals the carbon and oxygen yields; expressing the measured carbon (C) and oxygen (O) yields respectively as a first and second/linear function of S0 Sw the water saturation in the formation, Cb the hydrocarbon saturation in the borehole, and/or C0 the water saturation in the borehole; combining the respective measured carbon and oxygen yields (C, O) so as to form the difference C-O; and solving for the unknown So from the difference C-O