Carbon-Oxygen (C/O) Log: Use and Interpretation
Authors: Lock G.A. , Hoyer W.A. 
Published: December 2007

Where other commercial logs fail or are marginally acceptable, the C/O log should be especially valuable. It has proved a highly reliable indicator of hydrocarbon in sandstone formations, independent of formation water salinity, and usually independent of formation shaliness.

Introduction
A carbon-oxygen (C/O) log, which measures the carbon content of a formation and its contained fluids, has been field tested in 23 wells. The C/O ratio bas proved to be a reliable indicator of hydrocarbons in proved to be a reliable indicator of hydrocarbons in sandstone formations, independent of formation water salinity and, in most instances, independent of shaliness. The C/O log should find its primary application or are marginally acceptable. The results discussed in this paper are based on laboratory data and on more than 1,800 field measurements using the Dresser Atlas Carbon-Oxygen Log, which is patterned after one previously developed by Exxon Production Research Co. The field data were collected in Exxon Company, U.S.A., wells having known well conditions (to confirm laboratory data) and in workover wells where the predictions of the C/O log were often subsequently checked against production data. All the data resulted from 5-minute production data. All the data resulted from 5-minute stationary measurements. Using this procedure, about 20 to 40 zones can be tested in a well in an 8-hour period under normal well conditions - i.e., moderate period under normal well conditions - i.e., moderate depth and reasonably consistent lithology. This time can be reduced, as desired, by isolating the zones of interest and by ignoring intervals of known saturation. Although the commercialization of the C/O log has just begun and the eventual form of the log is still somewhat uncertain, a great deal of flexibility of operation is built into the current tool, which permits several other types of nuclear logs to be run permits several other types of nuclear logs to be run with the same instrument. In addition to the C/O log, a continuous log similar to an ordinary neutron log can be run. Also, by selective timing within the tool, gamma ray responses indicative of silicon and calcium can be recorded to aid in interpreting the C/O log in areas of unknown lithology or of mixed lithology. In a practical sense, this tool has the capability to obtain information on the hydrocarbon content and porosity of a formation, as well as to differentiate porosity of a formation, as well as to differentiate sandstone from carbonates and clean sands from limy sands. The purpose of this paper is to demonstrate the usefulness of the C/O and silicon-calcium (Si/Ca) ratios for hydrocarbon detection and to summarize field experience using these techniques.

Oil Detection Using the C/O Log C/O Ratio
The operation of the C/O logging tool and the physical principles upon which it relies for results have been principles upon which it relies for results have been previously reviewed in the literature. Only a few previously reviewed in the literature. Only a few comments regarding the basis for the measurement will be included here to make the remainder of the paper more comprehensible. A gamma ray spectrum, such as shown in Fig. 1, is the basic data from which the oil content of a formation is determined. These data are composed of measured gamma rays induced by high-energy neutrons in the elements oxygen, carbon, silicon, iron, and possibly other less important elements.
JPT
P. 1044