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The relationship between the electromotive force (E.M.F.) across a shalebarrier and the concentrations of sodium chloride solutions on either side hasbeen investigated. It is shown that the action of a shale barrier is analogous to a glass membraneseparating two acid solutions of different hydrogen ion concentrations. Theshale behaves as a sodium electrode and is responsive to the activities of thesodium ions in the two solutions in such a way that the potential can becalculated by means of the Nernst equation. This conclusion is confirmed bylaboratory experiments. In a borehole the total E.M.F. of a shale cell is the algebraic sum of thepotential across the shale and a boundary potential. The relationship betweentotal E.M.F. and the resistivity ratio of two sodium chloride solutions isindicated for a number of formation temperatures. The E.M.F. thus predicted isthen compared with the self potential read from an electric log and goodagreement is demonstrated. Based on both the self potential and resistivity curves of the electrical log.a method is given for calculating connate water content in a bed havingintergranular porosity and containing both connate water and hydrocarbons. Introduction The first paper on electrical well logging by C. and M. Schlumberger and E.G.Leonardon in 1934 attributed the self potential curve principally to streamingpotentials, i.e. to electrokinetic effects. Almost immediately greatdifficulties were encountered in reconciling many of the curves they obtainedwith this interpretation, and a second paper by the same authors soon appeared.In this second paper self potentials were attributed to the combined effects ofstreaming potentials and electrochemical potentials, the electrochemicalpotential being considered the result mainly of the interaction of fluids ofdiffering salt concentrations, i.e. a boundary potential, and partly ofpotentials set up at the faces of impermeable materials. Some experimentsinvolving a gray clay for the impermeable material were quated. T.P. 2511

A Quantitative Analysis of the Electrochemical Component of the S.P. Curve