Re: How to Calculate Vcarb and Vdol from well logs

The

GR log responds to radioactivity in the formation, mainly clay minerals. Both calcite and dolomite, if they are clean and does not contain any radioactive minerals, you will have low GR readings. Therefore GR log cannot differentiate between calcite and dolomite.

The formation resistivity

RT responds to formation water resistivity (or salinity), porosity and water saturation.

It does not respond to lithology. Therefore RT cannot be used to differentiate between calcite and dolomite.

The sonic log does respond to lithology, as well as formation porosity and the fluid inside the pore spaces.

If your carbonate formation does not have any porosity, in other words 100% tight carbonate, the sonic log may be used to compute the percentage composition of calcite and dolomite. The sonic log will be a proportional sum of the matrix transit times for calcite and dolomite. Even then, it is not going to be very accurate as the matrix transit times for calcite and dolomite are quite to close to each other.

Alternatively, if you know the formation porosity and transit time of the pore fluid, you can back calculate the matrix transit time using the sonic log and then derive the calcite and dolomite volumes from the formation matrix time.

Also, if your carbonate formation is 100% water bearing, you can back calculate the formation porosity from RT using an appropriate formation water resistivity. Once you get the formation porosity, you can compute the matrix transit time from the sonic log and then derive the volumes of calcite and dolomite from that.