A log which presents a measure of the relative abundance of carbon to oxygen derived from the detection of the gamma rays produced from both elements by the inelastic scattering of 14-Mev neutrons. The gamma rays are measured within energy spectrum windows representing the gamma-ray escape peaks of carbon and oxygen. The ratio of counting rates provides a means of predicting the relative amounts of hydrocarbons and water. The log is an alternate means for detecting hydrocarbons (particularly oil) behind casing in formations not subject to flushing or reinvasion by borehole fluids. The C/O ratio is relatively independent of formation water salinity and shaliness. In order to differentiate carbon in hydrocarbon molecules from that in the rock framework (i.e., carbonate solid matter), a Si/Ca ratio is also determined.

The carbon-oxygen log can be put to nearly the same uses as the pulsed neutron capture logs, but has proved to be useful under some conditions where pulsed neutron capture logs have shown decreased effectiveness; e.g., in rocks where formation water cannot easily be distinguished from oil because of the lack of sufficient contrast in their neutron capture cross sections. Carbon/Oxygen Log is a Dresser Atlas trademark.