Faults as conduit‐barrier systems to fluid flow in siliciclastic sedimentary aquifers

Abstract

[1] We argue that the observed conduit-barrier behavior of fault zones in siliciclastic sedimentary aquifer systems can be understood by considering a strongly anisotropic hydraulic structure in the fault. Hydraulic anisotropy in the fault is expected from a variety of mechanisms including clay-smearing, drag of sand, grain re-orientation and vertical segmentation of the fault plane. In this paper, we present an algorithm to predict fault zone width, lithological heterogeneity and hydraulic anisotropy. Estimation of these parameters is based upon the amount of fault throw and the clay-content of the lithologies flanking the fault zone. A suite of steady-state flow models are presented using an idealized stratigraphy consisting of alternating clay and sand-rich layers that are offset by a fault zone. These conceptual simulations show the impact of a fault zone on shallow (<500 m) fluid flow patterns and solute transport for different scenarios of fault throw. Fault width varies along the fault zone and increases from an average width of~2 m for a throw of 50 m to~8 m for a throw of 200 m. Hydraulic anisotropy in the fault zone in these scenarios is predicted to range between two to three orders of magnitude. Our results show that faults can form a preferential path way between aquifers at different depths over vertical distances of several hundreds of meters (that are otherwise separated by confining units) when fault permeability is strongly anisotropic. However, in the same scenario anomalously high hydraulic head gradients across the fault would still suggest that they act as an effective barrier to lateral groundwater flow. This has important implications for the assessment of the risk of a spread of contaminated groundwater or the reconstruction of hydrocarbon migration within sedimentary basins.

Geologic carbon storage is unlikely to trigger large earthquakes and reactivate faults through which CO2could leak

“…We contend that close analysis of fault zone architecture reveals that CO 2 will not easily penetrate into the portions of the fault contained within shale rocks (31). Fault permeability, which is highly variable in reservoir-caprock sequences (32,33), decreases several orders of magnitude for increasing clay content, leading to a much lower permeability in the caprocks than in the reservoirs (34,35). Rocks with low clay content, like reservoirs, tend to fracture, increasing the width of the damaged zone and usually increasing permeability in response to shear (34).…”

“…Fault permeability, which is highly variable in reservoir-caprock sequences (32,33), decreases several orders of magnitude for increasing clay content, leading to a much lower permeability in the caprocks than in the reservoirs (34,35). Rocks with low clay content, like reservoirs, tend to fracture, increasing the width of the damaged zone and usually increasing permeability in response to shear (34). However, clay-rich rocks, like caprocks, tend to concentrate shearing in the fault core, which reduces the grain size by friction, thus reducing fault permeability (34).…”

“…Rocks with low clay content, like reservoirs, tend to fracture, increasing the width of the damaged zone and usually increasing permeability in response to shear (34). However, clay-rich rocks, like caprocks, tend to concentrate shearing in the fault core, which reduces the grain size by friction, thus reducing fault permeability (34). Therefore, shear slip will usually increase fault permeability in the reservoir, but decrease it in the caprock, increasing the permeability contrast in the vertical direction (31,36).…”

Groundwater in the Tibet Plateau, western China

“…Faults can be conduits, barriers, or combined barrier-conduits to flow [Bredehoeft et al, 1992]. Quantifying fault permeability is still an area of active research [Caine et al, 1996;Fairley and Hinds, 2004;Bense and Person, 2006;Ball et al, 2007]. Field evidence suggests that faults in the study area actively channel water to the surface [Wu et al, 2005].…”

Hydrogeological properties of fault zones in a karstified carbonate aquifer (Northern Calcareous Alps, Austria)

“…Bense and Person 2006;Bense et al 2013 for full reviews; Faulkner et al 2008;Leray et al 2013;Faulkner 2008, 2009), since these rocks host some of the world's most active large faults or important hydrocarbon reservoirs. In crystalline rocks, fault cores contain breccias and gouges of strongly reduced permeability along the principal slip plane, whereas fractured rocks of the damage zones form permeable conduits oriented parallel to the fault plane (Caine et al 1996).…”