Reconciliation of Bulk and Local Analyses to Assess the Clay Content of a Sandstone

In order to make an exact estimation of the clay content of a sandstone containing less than 10% clays, three steps were followed :(a) Assessment of the consistency of experimental values from elemental analyses of separated fractions, taking into account standard deviations for each analysis. (b) Solution of a system of equations based on fixed stoichiometric coefficients for all minerals present in the rock. (c) Improvement of accuracy by taking into account the real composition of the clay as determined from nanometer level analysis, with standard deviations. This procedure allows determination of the clay content with the maximum precision and accuracy allowed by the quality of the raw data, and the soundness of the hypotheses, in sandstone containing less than 10% clays as well as in clay-rich fractions