SENSITIVE CARBONATE RESERVOIR ROCK CHARACTERISATION FROM MAGNETIC SUSCEPTIBILITY: MINERAL QUANTIFICATION, CORRELATION WITH PETROPHYSICAL PROPERTIES, AND ANISOTROPY

摘要

Recent work(Potter et al,2004;Potter,2005;Ivakhnenko,2006;Ivakhnenko and Potter, 2006a,b;Potter,2007;Ivakhnenko and Potter,2008;Potter and Ivakhnenko,2008) has demonstrated how rapid,non-destructive magnetic susceptibility measurements in clastic reservoir samples correlate with several key petrophysical parameters.The present paper shows how such measurements can also be a potentially important tool for characterising carbonate samples.Typical carbonate minerals such as calcite or dolomite are diamagnetic (negative magnetic susceptibility),whilst some other carbonates such as siderite(an iron carbonate) are paramagnetic(positive magnetic susceptibility).Carbonate rock typing can be achieved via high field susceptibility measurements which indicate the diamagnetic plus paramagnetic content.Small concentrations of ferrimagnetic impurities(such as magnetite) can significantly increase the low field magnetic susceptibility values,providing a further sensitive means of distinguishing between different carbonate samples and identifying samples that may give anomalous nuclear magnetic resonance(NMR) results. Experimental magnetic susceptibility measurements demonstrated subtle differences between samples in a suite of Middle East carbonates.Low field magnetic susceptibility values from hysteresis curves indicated small concentrations of ferrimagnetic impurities in some samples.The low field values did not correlate well with key petrophysical parameters.Significantly,however,high field measurements exhibited extremely good correlations with permeability and porosity.The high field results reflected the diamagnetic and paramagnetic minerals(comprising the main rock volume and which are the main controls on the petrophysical properties),were not affected by ferrimagnetic impurities since they saturated at lower fields.Results for some North Sea carbonates were very different to the Middle East samples.Magnetic susceptibility values were substantially higher(both low field and high field),indicating increased ferrimagnetic and paramagnetic(mainly clays) concentrations in the North Sea carbonates.This generally meant that the reservoir quality of the North Sea carbonates was poorer. The North Sea carbonates also exhibited higher values of magnetic anisotropy(which can be determined very rapidly),due mainly to their increased paramagnetic clay content.