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Geological Society, London, Special Publications; 1992; v. 65; p. 361-368;
DOI: 10.1144/GSL.SP.1992.065.01.28
© 1992 Geological Society of London

Mineralogy and Geochemistry

Downhole mineralogy logs: mineral inversion methods and the problem of compositional colinearity

P. K. Harvey & M. A. Lovell

Borehole Research, Department of Geology, University of Leicester, Leicester, LE1 7RH, UK

The inversion of chemical data derived from geochemical and related nuclear spectroscopy tools to provide mineralogy logs is now a widespread approach to the interpretation of these data. The most valuable of such transforms attempt to provide the percentages of actual phases present (minerals and fluids) at each depth interval rather than the ideal minerals occurring in simple theoretical models. Of the numerous problems involved in the inversion for a particular phase assemblage, the most serious is probably that of compositional colinearity in which three or more of the phases sought lie on, or close to, the same compositional plane. Depending upon the algorithm used for the inversion the effects of such compositional constraints may vary between a failure to find any (numerical) solution, failure to find a unique solution, or a solution which may be significantly in error for only a very small deviation in the chemical log or phase compositions. These effects are illustrated using geochemical logs from sedimentary environments together with examples from laboratory derived and numerically simulated geochemical data.