Dual Seal-Conduit Behaviour of Extensional Fault Zones in Poorly Lithified, Low-Porosity Sandstones of the Barreiras Formation, NE Brazil
Faults developed in poorly lithified sandstones are typically characterised by deformation band faulting. In particular, faulting in high-porosity (>10-15%) sandstones and loose sediments typically starts with progressive development of deformation bands, zones of deformation band and, with increasing offset, discrete slip surfaces. Extensive development of low permeability cataclastic deformation bands in fault damage zones significantly reduces fault transmissibility, thus providing an effective barrier to fluid flow. Conversely to this, extensional fault zones developed in low-porosity, poorly lithified quartz-dominated sandstones from the Mio-Pliocene continental Barreiras Formation, NE Brazil, show a quit different structural and hydraulic behaviour. We studied eight fault zones having displacement between few centimetres up to about 50 meters and developed from soft-sediment up to more brittle conditions during progressive burial and Fe-oxide cementation. We collected structural and microstructural data, grain size and shape, porosity and pore size data, and laboratory and in situ permeability data. Undeformed sandstones are very poorly sorted with an average intergranular porosity of about 3 %. Sandstones in damage zones are characterised by non destructive dilatant granular flow and opening-mode intergranular tensional fractures, which increase bulk porosity up to about 10%, pore size and permeability. Deformation in fault cores evolved from particulate flow to compactional cataclastic flow with progressive grain size and porosity reduction up about 0.1%. We observed permeability reductions of 2-3 orders of magnitude with respect to the undeformed protoliths. Our data highlight a seal-conduit behaviour of the studied fault zones, which scales with fault displacement and significantly differs from the sealing behaviour of deformation band fault zones commonly observed in high-porosity sandstones.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009