Method and system are disclosed for determining hydrocarbon CDVs in a multiphase environment. The method and system involve calculating a total hydrocarbons volume from an apparent oil volume and an apparent gas volume. The apparent oil volume is determined using C/O data from a nuclear logging tool operating in inelastic neutron scattering mode. The apparent gas volume is determined using neutron porosity data from a nuclear logging tool operating in pulsed neutron capture ("PNC") mode. It is also possible to determine the total hydrocarbons volume using log data from formation thermal neutron capture cross section measurements, or a cased hole formation resistivity tool. The total hydrocarbons volume is then used to back-calculate the total hydrocarbons CDV at various formation depths using the logged C/O data. The resulting independently calculated total hydrocarbon CDV may be used for proper matching of log data and simulation results for history matching purposes.