Page 11 - Tondietall2015

Italian Journal of Geosciences Accepted manuscript

For both grainstones we documented a steep gradient of permeability reduction, generally two
orders of magnitude, moving from the host rocks toward the fault cores through the damage zones
(figs. 6 and 7). In particular, the air permeability measurements performed across the Favignana
fault zones fell within the following ranges of values: 38.3-119.4 Darcy (mean value 69.15) for the
host rock, 0.03-1.3 Darcy (mean value 0.51) for the fault cores (Zone I), and 7.8-41.6 Darcy (mean
value 19.22) for the fault damage zones (Zones II and III). Due to their narrow width, we were not
able to investigate separately Zones II and III.

Lower ranges of air permeability values were measured across the fault zones at Madonna della
Mazza quarry. For the host rock we measured 0.28-0.6 Darcy (mean value 0.45), for the fault cores
(Zone I) 0.004-0.015 Darcy (mean value 0.008), and for the fault damage zones (Zone II, here Zone
III is absent in the fault damage zones, see TONDI et alii, 2006) 0.08-0.26 Darcy (mean value 0.16).

We compared the air permeability values measured across two pairs of faults characterized by
different kinematics (i.e. strike-slip and dip-slip) but with the same amount of slip. In particular we
compared two faults from Favignana with one meter displacement and two from Madonna della
Mazza quarry with 0.2 m displacement. In both cases the permeability of the strike-slip faults was
higher than the permeability of the dip-slip ones (figs. 6 and 7).

DISCUSSION
In this study we documented remarkable difference in the air-permeability values obtained for
the two carbonate grainstones. The difference of about two orders of magnitude is documented for
the two host rocks: the grainstones of Favignana are more permeable than the Orfento ones, but also
for the respective fault cores and damage zones show consistent permeability contrasts (fig. 8). The
significant differences in permeability measured in the two study areas cannot be simply justified by
the initial difference in porosity of the two host rocks (around 15%). Indeed, Favignana grainstones
have, on average, a 3D porosity of 47% whereas Orfento grainstones have a 32% of average 3D
porosity. By integrating our thin section observations (fig. 2c-d) with data on the pore size