Templates for Quantifying Clay Type and Clay Content from Magnetic Susceptibility and Standard Borehole Geophysical Measurements

Abstract

This paper proposes a rapid means of identifying clay type and quantifying clay content from new template crossplots that compare magnetic susceptibility measurements with standard borehole well log data. The templates are similar in format to standard industry charts, but have a number of advantages over the commonly used charts. Laboratory measurements of magnetic susceptibility on core samples and drill cuttings have recently shown strong correlations with key petrophysical parameters, particularly clay content and fluid permeability [1] [2]. A new template crossplot between magnetic susceptibility and borehole spectral gamma ray log data can firstly help to quickly identify the types of clay present in the formation. Additional new template crossplots between magnetic susceptibility and borehole bulk density data allow the mineral contents and porosities of binary mixtures of clay minerals and matrix minerals (such as illite clay + quartz) to be rapidly quantified. The templates can use ambient (room temperature) magnetic susceptibility data from measurements on core samples or drill cuttings in the laboratory or at the wellsite. Furthermore, the paper shows how the templates can potentially be extended to utilize borehole magnetic susceptibility data for in situ estimations of the type and content of clay. This requires accounting for the temperature dependence of the magnetic susceptibility of paramagnetic minerals (such as illite clay), which varies with depth in a borehole. Whilst borehole magnetic susceptibility measurements are rarely part of standard well logging operations, they could be a potentially useful tool for in situ clay type and content quantification, which in turn can help predict fluid permeability.