In order to determine characteristics of a gas-oil transition zone in a gas capped oil reservoir, samples of a reservoir fluid from a gas and an oil part of the reservoir are taken. To configure an equation of state of hydrocarbon mixtures, reservoir temperature and pressure are measured in places where the samples of the reservoir fluid are taken and densities and compositions of the samples are determined. Porosity, water saturation and total hydrogen content of a saturated rock are measured along the wellbore and a volume and a hydrogen content of hydrocarbon phases are determined. Using the equation of state, density and composition of the hydrocarbon phases are computed and a specific hydrogen content in gas and oil along the wellbore is determined. Gas and oil saturation distribution along the wellbore is determined from the determined specific hydrogen content, determined hydrogen content of the hydrocarbon phases and the measured porosity.