Re: Clay Volume from Den- Neutron crossplot
Generally, a carbonate reservoir would be clean, with no or very little clays. A good clay indicator for carbonates (limestone and dolomite) is the
CGR (corrected
GR) log from a Spectral GR log. With the uranium component removed, a CGR log tends to respond to the presence of clays better than the standard GR log. Using the density and neutron logs or N-D crossplot to determine clay volume in a limestone reservoir with dolomite inside will be tricky. The presence of dolomite makes the limestone reservoir appear shaly due to the higher density of the dolomite. Therefore, it will not be straightforward to decide where to put the clean matrix line on the N-D crossplot, if the exact matrix density is not known. On one hand, if the limestone matrix line is used, the dolomitic intervals will appear as shaly zones. On the other hand, if the dolomite matrix line is used, the clean pure limestone intervals will appear as gas bearing points. Furthermore, in clean carbonates, it will be difficult to identify the 100% clay point. You will probably have to take it from the overlying shale bed, if there is one. A suitable approach will be to identify the cleanest carbonate (non-gas bearing) intervals on the GR log. Use these intervals on the N-D crossplot as a baseline for the clean matrix line. It should fall between the limestone and dolomite matrix lines, parallel to the limestone line
