Method for determining characteristics of a gas-oil transition zone in an uncased well

A method for determining characteristics of a gas-oil transition zone in an uncased well drilled into a gas capped oil reservoir with a known mineralogical composition of constituent rocks, the method comprising:

• taking at least one sample of a reservoir fluid from a gas part of the gas capped oil reservoir and at least one sample of the reservoir fluid from an oil part of the gas capped oil reservoir;
• measuring reservoir temperature and pressure in places of the gas part and the oil part of the gas capped oil reservoir where the reservoir fluid samples are taken;
• determining densities and compositions of the reservoir fluid samples taken from the gas part and the oil part of the gas capped oil reservoir;
• using the determined densities and compositions of the reservoir fluid samples and the measured reservoir pressure and temperature to configure an equation of state of hydrocarbon mixtures;
• measuring porosity, water saturation and total hydrogen content of a saturated rock along a wellbore;
• computing a volume of hydrocarbon phases from the measured porosity and water saturation;
• determining a hydrogen content of the hydrocarbon phases from the measured total hydrogen content of the saturated rock;
• computing density and composition of the hydrocarbon phases along the wellbore based on gravitational and thermodynamic equilibrium of gas and oil phases and using the configured equation of state of hydrocarbon mixtures;
• determining specific hydrogen content in gas and oil along the wellbore from the computed density and composition of the hydrocarbon phases along the wellbore, and
• determining gas and oil saturation distribution along the wellbore based on the computed specific hydrogen content in gas and oil, the determined hydrogen content of the hydrocarbon phases and the measured porosity of the saturated rock.