This paper presents log examples and describes the use of Cation/Oxygen ratio (CATO), a Carbon/Oxygen type measurement, to discriminate fresh formation water or water of varying salinity from oil. Instead of using the ratio of carbon to oxygen (C/O), CATO is the ratio of all inelastic gamma rays except those from oxygen, to the oxygen inelastic gamma rays. CATO responds to the presence of oil similarly to C/O, but has several advantages:
(1) CATO can be recorded using a combination 1⅝" Pulsed Neutron Decay-Inelastic gamma ray tool that uses Sodium Iodide crystals, (2) Variations in lithology have much less effect on water saturations, (3) Statistical fluctuations are significantly reduced, (4) The sensitivity to oil is an order of magnitude greater, and (5) Logging speeds are increased to 8 to 12 feet per minute.
Log examples are from sandstones and carbonates and include CATO responses in water floods with unknown water salinity and from formations with low salinity connate waters. Production and test results are also presented. Operating limitations of the technique are given.