Structural and petrophysical evolution of extensional fault zones in low-porosity, poorly lithified sandstones of the Barreiras Formation, NE Brazil

Abstract

We describe the structural and petrophysical evolution of extensional fault zones developed in low porosity, poorly lithified, quartz-dominated sandstones from the Mio-Pliocene continental Barreiras Formation, NE Brazil. We studied eight fault zones developed as sands were lithified. Fault displacement ranges from a few centimetres to ∼50 m. A diagnostic feature of the studied fault zones is the lack of deformation bands, which typically develop in high porosity sand(stone)s. Structural and microstructural analyses, grain size and shape analyses, porosity and pore size analyses, and laboratory and in situ permeability measurements show relationships between deformation processes and hydrologic properties. Undeformed rocks are very poorly sorted, medium- to fine-grained, clay-rich sandstones with an average intergranular porosity of about 3%. Sandstones in damage zones record non-destructive dilatant granular flow and formation of opening-mode intergranular extensional fractures, which increase porosity, pore connectivity and permeability. Deformation in fault cores evolved from particulate flow to compactional cataclastic flow, with progressive grain size reduction increasing the amount of silt- and clay-size fractions. Porosity was dramatically reduced to an average value of 0.2% and permeability is generally lower than the related protoliths. All this evidence highlights a conduit/barrier behaviour of the studied fault zones, which significantly differs from the sealing behaviour of deformation band fault zones commonly observed in high-porosity sandstones.

Introduction

Porosity and its evolution through time are an important factor controlling what kind of mesoscopic deformation structures develop during rock failure (e.g. Dunn et al., 1973, Vernik et al., 1993, Kwon et al., 2005, Fossen et al., 2007). In the last four decades, considerable attention has been devoted to the understanding of fault zone evolution in high-porosity (>10–15%), lithified to loose granular material both in the field (Aydin, 1978, Aydin and Johnson, 1978, Pittman, 1981, Lucas and Moore, 1986, Antonellini and Aydin, 1994, Fowles and Burley, 1994, Fossen and Hesthammer, 1997, Heynekamp et al., 1999, Cashman and Cashman, 2000, Shipton and Cowie, 2001, Rawling and Goodwin, 2003, Flodin et al., 2003, Flodin et al., 2005, Johansen et al., 2005, Minor and Hudson, 2006) and in experimental studies (Borg et al., 1960, Mandl et al., 1977, Menéndez et al., 1996, Wong et al., 1997, Zhu and Wong, 1997, Mair et al., 2000). This because of their important influence on fluid flow in hydrocarbon reservoirs and groundwater aquifers (e.g. Haneberg, 1995, Walsh et al., 1998, Heynekamp et al., 1999, Aydin, 2000, Fisher and Knipe, 2001, Rawling et al., 2001, Manzocchi et al., 2002, Nelson et al., 2009). Deformation in high-porosity granular materials occurs by development of small displacement deformation structures comprehensively referred to as deformation bands, which evolve into zones of deformation bands and slip surfaces with increasing offset (e.g. Aydin and Johnson, 1978, Fowles and Burley, 1994, Shipton and Cowie, 2001, Fossen et al., 2007). A typical result of deformation band faulting in high-porosity sandstones is that their extensive development in fault damage zones may reduce fault transmissibility, thus providing an effective barrier to fluid flow (e.g. Antonellini et al., 1994, Antonellini et al., 1999, Sigda et al., 1999, Rotevatn et al., 2007). This hydraulic behaviour differs from the typical conduit behaviour of fault damage zones in low-porosity fully lithified rocks, where deformation is dominated by opening-mode fracturing (e.g. Caine et al., 1996, Billi et al., 2003, Kim et al., 2004).

The lower threshold porosity limit for deformation band development is at about 10–15% (e.g. Dunn et al., 1973, Flodin et al., 2003, Wong et al., 1997). Below this threshold limit, shear strength becomes a fundamental parameter controlling deformation mechanisms. Joints and slip surfaces are expected to develop in fully lithified sandstones (e.g. Johansen et al., 2005, Fossen et al., 2007). On the other hand, deformation in low-porosity poorly lithified sand(stone)s is still poorly understood. In this paper, we attempt to bridge the gap by describing the structural and petrophysical evolution of extensional fault zones developed in low-porosity, poorly lithified quartz-dominated sandstones of the Barreiras Formation, NE Brazil. The relative compositional maturity and homogeneity of the Barreiras sandstones allow us to discount the effects of clay smearing and tectonic mixing of strongly different sedimentary units within fault zones (e.g. Antonellini and Aydin, 1994, Gibson, 1998, Heynekamp et al., 1999, Caine and Minor, 2009). Results of structural, microstructural, grain size, grain shape, and porosity analyses and permeability measurements are described with the aim of (1) inferring the deformation mechanisms that governed the evolution of these extensional fault zones; (2) proposing an evolutionary model of grain size, grain shape and porosity changes during extensional faulting; and (3) assessing the influence of faulting on fluid flow by establishing a relationship between fault-related permeability variations and fault displacement. The latter provides a useful tool for predicting the expected permeability and transmissibility of sub-seismic and seismic fault zones in sand-dominated clastic reservoirs.