Re: Differentiating gas/oil-bearing zones from Resistivity
Yes, you are right. Rt is a function of formation porosity, formation water resistivity, water saturation and Archie's parameters, namely a, m and n. If all of these are the same, Rt should be the same regardless of the hydrocarbon type, either gas or oil.
However, usually in the same reservoir, due to its higher mobility and capillary effects, gas saturation tends to be higher than oil saturation. Therefore, Sw will be lower if the formation is gas bearing compared to being oil bearing. This will result in higher Rt if it is gas bearing.
That is probably the reason why we tend to say that Rt in a gas bearing zone is higher than in an oil bearing zone.