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Dear All,
When I calculate Vclay ( I use IP software), I wonder how I can calculate Vclay exactly. I know that it is depend on geologic characteristic, some sand horizons contain radium mineral causing high GR index.As you know, It is very inportant to calculate Vclay due to we can lost out reservoir. If you have experience, please share some idea.
Thank so much.

Yes, I agree that Vclay computation is the most important step in Sw calculation. In case ur sand reservoir contains radioactive minerals then you can use CGR (computed GR - Th + K - U) derived from spectral GR. There are techniques to derive Vclay from Sigma log and even Neutron-Density logs gives you Vclay. So use a combination of these techniques and arrive at a rational conclusion.

I agree with deepak's points
thanks