Abstract
Recent work has shown how magnetic susceptibility and hysteresis measurements correlate with several petrophysical parameters in clastic reservoir samples. The present paper applies these techniques to carbonate samples. Carbonate rock typing can be achieved from high field magnetic susceptibility, which indicates a sample's diamagnetic plus paramagnetic mineral content. High field measurements are very sensitive and can quantify small differences in paramagnetic clay content that X-ray diffraction (XRD) or scanning electron microscopy (SEM) cannot. Temperature dependent hysteresis measurements can also identify and quantify small concentrations of paramagnetic minerals.
Experimental magnetic hysteresis curves demonstrated subtle differences between samples in a suite of Middle East carbonates. Significantly, the high field magnetic susceptibility values from the hysteresis curves exhibited extremely good correlations with permeability (small variations in paramagnetic clay content seem responsible for this) and porosity. The low field magnetic susceptibility values, however, did not correlate well with these petrophysical parameters merely because some samples contained small concentrations of ferrimagnetic impurities that contributed to the low field signal. The low field part of a hysteresis curve provides a further sensitive means of characterizing carbonate samples, and can be used to quantify these extremely small concentrations of ferrimagnetic material (down to a few parts per million) that XRD cannot.
Magnetic susceptibility values (both low and high field) for some US and North Sea carbonates were generally higher than the Middle East samples, indicating increased ferrimagnetic and paramagnetic (mainly clays) content. This suggested that the reservoir quality of the Middle East carbonates studied was generally better.