Page 1 - RFP_2013_Cilona

NATURE AND DISTRIBUTION OF STRIKE-SLIP FAULTS CROSSCUTTING
CARBONATE GRAINSTONES (FAVIGNANA, SOUTHERN ITALY) AND THEIR

IMPACT ON FLUID FLOW

Antonino Cilona1, Marco Antonellini2, Emanuele Tondi3 and Fabrizio Agosta4
1Department of Geological and Environmental Sciences, Stanford University, California, USA; 2Department of
Biological, Geological and Environmental Sciences, University of Bologna, Italy; 3School of Environmental
Sciences, University of Camerino, Italy; 4Department of Sciences, University of Basilicata, Potenza, Italy

e-mail: cilona@stanford.edu

Abstract these shear structures has been accompanied by a
statistical characterization of their dimensional
At Favignana Island (southern Italy), several parameters (i.e. length, thickness and displacement).
quarries provide an excellent 3D view of Lower-
Pleistocene grainstones crosscut by a strike-slip fault The power law distributions of the dimensional
system. This fault system is made up of two conjugate parameters from that study were inserted into the
sets of strike-slip structural features such as Fracture Modelling module of MOVETM in order to
Compactive Shear Bands, Zones of Compactive Shear generate our DFN model. Lastly fluid flow simulations
Bands and larger faults with discrete slip surfaces. This for two cases were carried out by means of the finite
contribution integrates structural analysis and differences software MODFLOW 2005.
numerical modeling to build up a 3D Discrete Fracture
Network model, which is used for fluid flow simulations Geological setting
of a carbonate reservoir analogue.
Favignana is the largest of the Egadi Islands, and is
This new workflow appears to be promising for located at northwest of Sicily along the southern edge
reservoir-scale assessment of sub-seismic structures of the Tyrrhenian Sea (Fig. 1 a and b). This area
and their impact on the bulk permeability of porous represents the westernmost, and most external, sector of
carbonate reservoirs. the Sicilian orogenic belt, which is mainly comprised of
southverging, Neogene fold-thrust tectonic elements
Keywords: (Scandone et al., 1974; Giunta et al., 2000). The most
recent faults of northwest Sicily (Nigro et al., 2000;
Compactive shear bands, Slip surfaces, Porous Renda et al., 2000; Gueguen et al., 2002; Giunta et al.,
carbonates, DFN model, Fluid flow simulation 2009) form a grid of high-angle strike-slip faults
roughly oriented either W-NW (right-lateral) or N-NE
Introduction (left-lateral). The kinematics of these two sets of strike-
slip faults is compatible to the current regional stress
In this contribution we present a novel modeling field, which is characterized by a NW-SE oriented,
route used to predict the fluid flow within faulted greatest horizontal compression direction (Giunta et al.,
porous carbonates. Statistical structural data, obtained 2004; Tondi et al., 2006 b). At Favignana Island (Fig. 1
from the same outcrop by Tondi et al. (2012), have b), deformed Triassic to Miocene platform carbonates
been used to build a Discrete Fracture Network (DFN) that pass upwards into deep-water marls and
model of a deformation band-based fault system. The limestones, from the basement units underlying Plio-
generated stochastic distribution of fractures has been Pleistocene marine deposits (Abate et al., 1995, 1997;
converted into a map of the hydraulic conductivity. Incandela, 1995; Tavarnelli et al., 2003). These marine
Eventually, the hydraulic conductivity map was used to deposits are comprised of Upper Pliocene shales
predict the fluid flow pathways in the studied outcrop. overlain by 20-25 m thick Lower Pleistocene carbonate
grainstones (Fig. 1 b).
Favignana Island is located 5 km west of the Island
of Sicily (southern Italy). There several quarries The carbonate grainstones of Favignana are Lower
provide an excellent 3D view of Lower-Pleistocene Pleistocenic in age. These grainstones are characterized
porous carbonate grainstones crosscut by a strike-slip by beds dipping 5-10 ESE, and a bed thickness ranging
fault system. This fault system was described by Tondi between 0.2 and 1 m (Tondi et al., 2012). The mean
et al. (2012) who identified two conjugate sets of strike- thickness of the whole Lower Pleistocene succession is
slip shear structures with hierarchical sub-classes as nearly 21 m at Cala San Nicola and 23 m at Cala Rossa
compactive shear bands (CSB), zones of compactive (Fig. 1 b). The maximum burial depth experienced by
shear bands (ZB) and faults with discrete slip surfaces the carbonate grainstones is estimated to be between 0
and cataclastic material. The qualitative description of

Stanford Rock Fracture Project Vol. 24, 2013 E-1