QUANTIFYING THE RELATIVE ROLES OF ILLITE AND HEMATITE ON PERMEABILITY IN RED AND WHITE SANDSTONES USING LOW AND HIGH FIELD MAGNETIC SUSCEPTIBILITY

ABSTRACT Recent work has demonstrated how rapid, non-destructive magnetic susceptibility measurements in clastic reservoir samples (Potter et al, 2004; Potter, 2005, 2007; Ivakhnenko, 2006; Ivakhnenko and Potter, 2004, 2006a, 2006b, 2008; Potter and Ivakhnenko 2007, 2008) and carbonate reservoir samples (AlGhamdi, 2006; Potter et al 2008) correlate with several key petrophysical parameters. The present paper details a methodology using low and high field magnetic susceptibility measurements to quantify the relative roles of illite clay and hematite (iron oxide) on the permeability of red and white reservoir sandstone samples. The motivation for the present study was the observation that for various North Sea clastic tight gas and unconventional reservoirs, containing both red and white sandstone formations, the permeability is generally lower in the red sandstones compared to adjacent white sandstones. We show how magnetic hysteresis measurements are able to quantify both the illite and hematite content in red sandstones. Hematite can be identified and quantified from its characteristic hysteresis loop behaviour over a range of low to high applied magnetic fields. The magnetic technique is very sensitive and can identify and quantify extremely small concentrations of hematite that would not be seen by X-ray diffraction (XRD). Illite can be quantified from the slope of the hysteresis curve at high fields (the high field magnetic susceptibility). The influence of each mineral on permeability in red sandstones can be estimated with the help of comparative studies in white sandstones, where the permeability correlates primarily with the magnetically derived illite content alone. Significantly, we found that permeability is systematically lower in red sandstones compared to white sandstones containing similar magnetically derived illite contents. The implication is that there is a further factor causing a reduction in permeability in red sandstones, and our work suggests that this is due to the presence of the fine-grained hematite.