Sensitivity study of C/O logging measurements by Monte Carlo method.

Carbon oxygen ratio (C/O) logging has great importance in the accurate determination of hydrocarbon saturation in the reservoir region. This measurement is independent of the salinity of the formation water, unlike alternative logging methods. Analysis of the measurement requires modelling of the time-dependent coupled neutron-gamma field produced by the tool, which is most efficiently done by the Monte Carlo (MC) method. MC simulation can be used to generate the gamma spectrum at the detectors of the probe for a variety of rock physics conditions and borehole environments and thus the C/O can be determined by the processing of the simulated gamma spectrum. The simulation results are used to derive the interpretation diagrams for the basic petrophysical effects and to investigate the role of the side effects. Considering the industrial practice of log evaluation, the resolution and limitations of the method is quantified in the measurement space by defining a goodness factor based on the area of the interpretation chart. The focus of this paper is on detector arrangement, but it also covers the effect of porosity, lithology, and the casing.