Monte Carlo simulation for determining formation lithology and influential factors with pulsed neutron gamma spectroscopy

The Inelastic scattering gamma and thermal neutron capture gamma spectroscopy may be measured in the different time range by using Reservoir Performance Monitor (RPM). The C/O ratio and formation macroscopic absorption cross section are obtained by means of the processing of the spectroscopy, but also the lithology would be determined by the crossplot Si/O Vs Ca/O or Si/Fe Vs Ca/Fe from the spectroscopy. The different inelastic scattering and capture gamma spectrum was simulated by using Monte Carlo method under the condition of different formation and borehole. therefore, the relations between determination of lithology by crossplot and saturation, porosity, salinity, holdup, casing, shale content, and so on, were studied respectively. This paper proposes a method of formation lithology definition and provides theory support these influential factors of this method by means of inelastic scattering and thermal neutron capture gamma spectroscopy. (authors)