Clay typing - Sensitive quantification and anisotropy in synthetic and natural reservoir samples using low- and high-field magnetic susceptibility for improved petrophysical appraisals

This paper describes a systematic magnetic analysis of different clay types for reservoir characterization. Most clay minerals (for example, illite or chlorite) are paramagnetic (positive magnetic susceptibility), whereas some clay minerals (e.g. kaolinite) and matrix minerals such as quartz and calcite are diamagnetic (negative magnetic susceptibility). Model magnetic hysteresis plots for various concentrations of different clays in a quartz matrix were first produced. Experimental measurements were then undertaken for comparison on a series of synthetic reservoir samples. The experimental results showed substantial agreement with the model values, and with magnetic susceptibility estimates based on XRD-derived mineral contents. Results for natural reservoir samples (turbidites) also showed a strong correspondence between the measured high field magnetic susceptibility, and that estimated from XRD-derived mineral contents. The magnetic measurements potentially provide a sensitive, rapid, quantitative technique to complement XRD measurements. We demonstrate improved correlations between permeability and high field magnetic susceptibility, compared to low-field magnetic susceptibility, since the latter can be influenced by small amounts of ferrimagnetic impurities. Furthermore, magnetic susceptibility measurements before and after core cleaning suggest that some paramagnetic clay is removed during hot soxhlet cleaning particularly in medium to high permeability samples. The results are consistent with permeability measurements undertaken before and after core cleaning. In addition, 3D anisotropy of magnetic susceptibility (AMS) measurements, which can be determined very rapidly on single plugs, correlate with the total magnetically-derived illite content. © 2008 Society of Petrophysicists and Well Log Analysts. All rights reserved.