Welcome to Petroleum Geology Forums

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

I would add the next question to the subject:
How do you determine the Vclay, if your main clay mineral is kaolinite and the GR values are very low (similar to the carbonates' values)?
Please share your ideas.

Indeed, it is difficult to determine Vclay reliably, using the GR log, if the main clay mineral is Kaolinite.
You may want to try Vclay estimation using the density and neutron porosity logs.
Another method is to use a multi-mineral model approach.
If you have a Sigma log, it may help as the Sigma for Kaolinite (around 14 c.u.) is significantly different from those for quartz, calcite and dolomite.