Primarily a hydrogen log, useful for the following purposes: (1) provides valuable correlating curve to replace the S.P. in holes containing oil or invert muds; (2) provides a means of qualitatively distinguishing zones containing hydrocarbons from zones containing only water; (3) provides a means of measuring quantitatively what proportion of the total fluid-filled porosity in a formation is sufficiently free from the influence of 2145 chemical binding forces to be considered mobile and thus potentially recoverable; and (4) provides a means of estimating the permeabilities of formations.