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Hello, I'm wondering if there's a way to differentiate gas-bearing and oil-bearing zones from resistivity log alone? Some of my seniors said that normally gas zone would have higher resistivity compared to the oil zone in the same formation. But if we look at the Archie's equation, the Rt is affected by only the Sw, which means that the hydrocarbon phase (gas or oil) does not have any effect on the Rt.

To make my question clear, let's say we have a gas-bearing formation and oil-bearing formation, both having the same Sw. Will the resistivity shows higher readings in gas zone? If so, what are the reasons for this?

Hope someone can enlighten me. TQ.

Indeed, I don't think gas will cause higher resistivity.

I believe what could work here is that gas usually has lower Sw due to wettability and capillary pressures, but there are many other factors that might influence this. Neutron-Density is the best method to identify Gas from Oil I think.

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.

Thanks for confirming my suspicion Kokokyi!

I slightly disagree with your views, Guys. We need to note, that Archie Law is purely empirical, derived in certain conditions, with certain assumptions. The fact, that resistivity of pore fluids other than water doesn't appear in Archie equation doesn't mean that they are not influencing resistivity. It means that Archie assumed their effect is negligible - or more accurately, assumed that they are ideal insulators. Archie assumed more things, like for example, that the rock frame is non-conductive. Which is obviously not the case also, especially in case of structural shale in matrix. In fact, resistivity of fluids other than water certainly influences the resistivity of the rock itself - it is just negligible in practical considerations.

i just start with finding I AND Vsh only and i stopped !