Introduction

Reservoir monitoring is a standard practice in oil and gas field management and is a     pre-requisite for all investment decisions. Carbon Oxygen logging is an effective instrument to this effect. The technique is used for identifying hydrocarbon bearing zones behind casing. It is an externally induced nuclear technique that records the spectral frame of gamma rays influenced by some of the principal elements sensitive to operative nuclear reaction. All principal elements in mud, casing, reservoir rocks and fluids present in the pore system contribute to this reaction.

The C/O measurement is based on C and O reactions with high energy (fast) neutrons producing characteristic gamma rays. From this measurement, distinction between hydrocarbon (C+H) and water (H+O) is made. A higher C/O is indicative of hydrocarbon bearing layers. The increase in C/O value could, however, be due to lime as well because lime comprises CaCO3. To resolve this ambiguity, a supplementary support becomes necessary which comes from Ca/Si curve.

The reaction being nuclear, the records are subjected to high degree of statistical variations. This is taken care by repeat measurements at a very slow speed and then merging and stacking data sets for a stabilized and workable database and adjusting the fringe sides of the spectrum to the set standard windows and focusing attention to CO statistical uncertainty.