Architecture and fluid flow properties of faults and associated structures in different rock types: examples from outcrops

Rock type strongly influences the way in which faults and other fractures develop and their resulting geometrical patterns. Consequently faults in sandstone have a different influence on fluid flow than faults in dolomite or carbonates. Knowing the mechanics of fault development helps to predict the distribution of faults and associated fractures in an aquifer, especially where geophysical or other data are lacking. We compare the geometry, architecture and petrophysical properties of faults and other fractures documented in outcrops of porous sandstone (Utah, USA and Italy), tight sandstones (Bolivia), dolomite (Northern Italy) and carbonates (Central Italy). We consider the fractured surface outcrops as a natural analog for faulted and fractured aquifers. We characterized the faults and fractures assemblages by type (fault, joint, compaction band, stylolite, and vein), length, structural position, stress state and their relationship to nearby structures. Faults in porous sandstone typically consist of a zone of compacted, cataclastic material called deformation bands that have been shown to inhibit fluid flow. Some faults have a slip plane and open joints at their tips (tail cracks), especially when they are along bedding planes. Compaction bands occur in porous sandstone and are equivalent to stylolites in carbonates. Faults in tight sandstones are associated with joint clusters, breccias zones and cataclasis and they form typically along preexisting discontinuities. Faults in Dolomite typically consist of breccias zones that formed in zones of joint localization. The Dolomite host rock is characterized by a very high density of joints. Carbonate outcrops show a variety of faulting mechanisms with many different types of fractures involved such as faults, veins and stylolites. Typically there are different generations of faults and fractures whereby faulting along pre-existing discontinuities such as older veins or older stylolite play an important role. Another important characteristic is the fact that the secondary fractures that typically form at the tip of faults play an important role in fragmenting the rock. Whether faults are barriers or pathways for groundwater flow in an aquifer depends on their architecture, size and connectivity. Assuming that joints enhance fluid flow whereas cataclastic bands, cemented breccia zones and slip planes inhibit fluid flow we evaluate and quantify the effect of all structures observed in the outcrops in terms of fluid flow properties.