Induced polarization of rocks containing pyrite: Interpretation based on X-ray computed tomography

In this paper, we compared Time Domain Induced Polarization (IP) data obtained from a collection of natural samples that contained pyrite with X-ray micro-Computed Tomography (CT) data. Induced polarization allowed us to obtain the relaxation time distribution and the total chargeability. Computed Tomography enabled us to assess the volumetric content of pyrite and the distribution of the pyrite grains. Comparison of the total chargeability and the volumetric pyrite content revealed a non-linear relationship between these parameters, which we initially attributed to the chargeability of the rock matrix and to anisotropy of some of the samples. Further data analysis, based on a comparison of the Relaxation Time Distributions (RTD) obtained from IP data with those computed based on CT data, revealed the following: (a) for the samples containing pyrite veinlets, the characteristic particle size that allowed the best agreement between the IP-based and synthetic RTDs was 1/2 of the veinlet length; (b) for the samples containing rounded pyrite particles, characteristic size was the Sauter radius. In some of the samples, the CT data show larger pyrite content compared to the IP-derived data. We believe that part of pyrite particles enclosed in carbonate or quartz shells were fully or partially isolated from the pore water, which precluded charge transfer between the pore water and the pyrite particles, thus reducing the overall polarization effect.