Log analysis techniques used to quantify clay volume generally rely upon log response within shale for calibration. This paper critically looks at the validity of using shales for the calibration of log clay response. Several assumptions are examined including the clay fraction content of shale, clay mineral variation between sandstone and shale and thermal neutron absorber associations. Several equations for calculations of clay volume from gamma ray, neutron, and density logs are presented. Techniques used to refine constants within the equations are explained. Problems with the calibration of clay on the low end are examined, especially when there are no clean sands within the section. A technique is presented to deal with this problem in gas reservoirs which are not thoroughly flushed.