Relationships Between Permeability, Clay Mineralogy and Electrical Conductivity in Icelandic Altered Volcanic Rocks
Lea LEVY, Benoit GIBERT, David ESCOBEDO, Patricia PATRIER, Bruno LANSON, Daniel BEAUFORT, Didier LOGGIA, Philippe PEZARD, Nicolas MARINO
[ISOR - Iceland GeoSurvey, Iceland]
Based on a set of 94 core samples extracted from the Krafla volcano (Iceland), we study the influence of primary lithology and hydrothermal activity on permeability. We also study the sensitivity of electrical parameters, which can be measured from geophysical profiles, to permeability changes in these altered volcanic rocks. We classify the samples into two main lithological types: hyaloclastite, including tuff and breccia, and lava, including vesicular and dense parts of lava flows as well as dykes. Scanning Electron Microscope (SEM), X-ray Diffraction (XRD) analyses allow identifying three main types of clay minerals: (i) tri-octahedral smectite (saponite), often occurring as replacement of glassy material in hyaloclastite samples, and sometimes replaced by corrensite/chlorite, (ii) chlorite/corrensite filling vesicles and other pore types of lava samples and (iii) illite/smectite mixed layers, kaolinite and aluminous smectite (montmorillonite) observed more locally, in samples relatively close to the surface. We show that the total quantity of secondary minerals is mostly controlled by the primary lithology and permeability. The most permeable samples are welded breccia (in the hyaloclastite group) and fractured basalts (in the lava group). The least permeable samples are felsic viscous lava flows and basaltic dykes, in which little clay content is usually found and coincides with low porosity. Hyaloclastite rocks generally have permeability higher than 10-17 m2 (10-2 mDa), regardless of alteration degree. The abundance of secondary minerals (mainly smectite, corrensite, chlorite and zeolites) generally increases with gas permeability in altered samples, regardless of original lithology. However, the permeability measured by water is significantly lower than gas permeability, especially in smectite-rich samples. Such a discrepancy can be explained by the fact that, due to a swelling behavior, smectite minerals can easily clog micro-fractures in saturated conditions. Regarding the sensitivity of electrical parameters to permeability changes in altered volcanic rocks, we observe that, at salinity higher than 40 g/L (5 S/m), the total conductivity is proportional to the permeability. At low salinity, it is proportional to the product of permeability and smectite content. As a consequence, fractured altered rocks containing swelling clay minerals can be very conductive although poorly permeable to water. The high electrical conductivity in low-permeability smectite-rich samples is attributed to conduction pathways through connected smectite particles, which seal the former fracture network. Our results illustrate the double influence of permeability on the electrical conductivity of volcanic rocks: (i) direct influence of water permeability on conductivity, as observed at high salinity and (ii) influence of primary permeability (“geological” permeability, before the formation of secondary minerals) on the abundance of electrically conductive connected smectite. Although in-situ hydrothermal flows are also affected by regional features, which cannot be reproduced at laboratory scale, our results provide additional guidance for the interpretation of geo-electrical measurements, in terms of past and present day fluid flow.
|        Topic: Exploration||Paper Number: 11093|