Formation water salinity from borehole measurements

A method for determining salinity of water in a borehole penetrating a formation and water in the formation, comprising:

• lowering a pulsed neutron logging (PNL) tool with a far gamma ray detector and a near gamma ray detector into a borehole penetrating a formation;
• positioning the near gamma ray detector to detect gamma rays predominantly from the borehole and capture a near detector gamma-ray spectra set including a near inelastic spectrum and a near capture spectrum;
• positioning the far gamma ray detector to detect gamma rays predominantly from the formation and capture a far detector gamma-ray spectra set including a far inelastic spectrum and a far capture spectrum;
• receiving, by a computer, a pulsed neutron logging (PNL) tool data set including the near detector gamma-ray spectra set and the far detector gamma-ray spectra set;
• calculating, by the computer using a standard elements database and a data regression method, a first set of coefficients of elements for the near inelastic spectrum, a second set of coefficients of elements for the near capture spectrum, a third set of coefficients for the far inelastic spectrum, and a fourth set of coefficients for the far capture spectrum, wherein the first, second, third, and fourth sets of coefficients of the elements indicate relative amounts of the elements contributing to the corresponding spectrum;
• calculating, by the computer, a near detector salinity ratio (SRATNear) and a far detector salinity ratio (SRATFar), wherein the near detector salinity ratio is equal to a ratio of a coefficient for chlorine from the second set of coefficients to a coefficient for oxygen from the first set of coefficients, and the far detector salinity ratio is equal to a ratio of a coefficient for chlorine from the fourth set of coefficients to a coefficient for oxygen from the third set of coefficients;
• calculating, by the computer, using a PNL tool characterization database, a near detector apparent salinity ratio using the near detector salinity ratio, and a far detector apparent salinity ratio using the far detector salinity ratio;
• and
• calculating, by the computer, a borehole salinity and a formation salinity using the near detector apparent salinity ratio and the far detector apparent salinity ratio and the PNL tool characterization database, and without using a water saturation value of the formation.