Search

The project benefited from funding by the Canadian Foundation for Innovation and McGill University.

To investigate the biases and trends in observations of the permeability structures of fault zones in various geoscience disciplines, we review and compile a database of published studies and reports containing more than 900 references. The global data are categorized, mapped, and described statistically. We use the chi-square test for the dependency of categorical variables to show that the simplified fault permeability structure (barrier, conduit, barrier–conduit) depends on the observation method, geoscience discipline, and lithology. In the crystalline rocks, the in situ test methods (boreholes or tunnels) favor the detection of permeable fault conduits, in contrast to the outcrop-based measurements that favor a combined barrier–conduit conceptual models. These differences also occur, to a lesser extent, in sedimentary rocks. We provide an estimate of the occurrence of fault conduits and barriers in the brittle crust. Faults behave as conduits at 70% of sites, regardless of their barrier behavior that may also occur. Faults behave as barriers at at least 50% of the sites, in addition to often being conduits. Our review of published data from long tunnels suggests that in crystalline rocks, 40–80% (median about 60%) of faults are highly permeable conduits, and 30–70% in sedimentary rocks. The trends with depth are not clear, but there are less fault conduits counted in tunnels at the shallowest depths. The barrier hydraulic behavior of faults is more uncertain and difficult to observe than the conduit.

The topic of crustal permeability is of broad interest in light of the controlling effect of permeability on diverse geologic processes and also timely in light of the practical challenges associated with emerging technologies such as hydraulic fracturing for oil and gas production (‘fracking’), enhanced geothermal systems, and geologic carbon sequestration. This special issue of Geofluids is also motivated by the historical dichotomy between the hydrogeologic concept of permeability as a static material property that exerts control on fluid flow and the perspective of economic geologists, geophysicists, and crustal petrologists who have long recognized permeability as a dynamic parameter that changes in response to tectonism, fluid production, and geochemical reactions. Issues associated with fracking, enhanced geothermal systems, and geologic carbon sequestration have already begun to promote a constructive dialog between the static and dynamic views of permeability, and here we have made a conscious effort to include both viewpoints. This special issue also focuses on the quantification of permeability, encompassing both direct measurement of permeability in the uppermost crust and inferential permeability estimates, mainly for the deeper crust. The directly measured permeability (k) of common geologic media varies by approximately 16 orders of magnitude, from values as low as 10 m in intact crystalline rock, intact shales, and fault gouge, to values as high as 10 m in well-sorted gravels. The permeability of Earth’s upper crust can be regarded as a process-limiting parameter, in that it largely determines the feasibility of advective solute transport (k ~ >10 20 m), advective heat transport (k ~ ≥10 16 m), and the generation of elevated fluid pressures (k ~ ≤10 17 m) – processes which in turn are essential to ore deposition, hydrocarbon migration, metamorphism, tectonism, and many other fundamental geologic phenomena. The hydrodynamics of fluids in the brittle upper crust, where topography and magmatic heat sources dominate patterns of flow and externally derived (meteoric) fluids are common (e.g. Howald et al. 2015) are distinct from the ductile lower crust, dominated by devolatilization reactions and internally derived fluids (e.g. Connolly & Podladchikov, 2015). The brittle–ductile transition between these regimes occurs at 10–15 km depth in typical continental crust. Permeability below the brittle–ductile transition is non-negligible, at least in active orogenic belts (equivalent to mean bulk k of order 10 19 to 10 18 m) so that the underlying ductile regime can be an important fluid source to the brittle regime (e.g. Ingebritsen & Manning 2002). The overall objective of this special issue is to synthesize current understanding of static and dynamic permeability through representative publications from multiple disciplines. The objective of this introduction to the special issue is to define crucial nomenclature and the ‘static’ and ‘dynamic’ permeability perspectives and to briefly summarize the contents of this special issue, which is divided into the following sections: the physics of permeability, static permeability, and dynamic permeability.

This project is supported by the joint NSF-USGS John Wesley Powell Center for Earth System Analysis and Synthesis working group and an NSF EarthCube Geo-Domain Community Workshop grant (EAR-1251557).

The permeability of sediments is a major control on groundwater flow and the associated redistribution of heat and solutes in sedimentary basins. While porosity–permeability relationships of pure clays and pure sands have been relatively well established at the laboratory scale, the permeability of natural sediments remains highly uncertain. Here we quantify how well existing and new porosity–permeability equations can explain the permeability of noncemented siliciclastic sediments. We have compiled grain size, clay mineralogy, porosity, and permeability data on pure sand and silt (n = 126), pure clay (n = 148), and natural mixtures of sand, silt and clay (n = 92). The permeability of pure sand and clay can be predicted with high confidence (R2 ≥ 0.9) using the Kozeny–Carman equation and empirical power law equations, respectively. The permeability of natural sediments is much higher than predicted by experimental binary mixtures and ideal packing models. Permeability can be predicted with moderate confidence (R2 = 0.26– 0.48) and a mean error of 0.6 orders of magnitude as either the geometric mean or arithmetic mean of the permeability of the pure clay and sand components, with the geometric mean providing the best measure of the variability of permeability. We test the new set of equations on detailed well-log and permeability data from deltaic sediments in the southern Netherlands, showing that permeability can be predicted with a mean error of 0.7 orders of magnitude using clay content and porosity derived from neutron and density logs.

The permeability of crystalline rocks is generally assumed to decrease with depth due to increasing overburden stress. While experiments have confirmed the dependence of permeability on stress, field measurements of crystalline permeability have not previously yielded an unambiguous and universal relation between permeability and depth in the shallow crust (

In polyorogenic regions, the superposition of structures during a protracted tectonic history produces complex fractured bedrock aquifers. Thrust-faulted regions, in particular, have complicated permeability patterns that affect groundwater flow paths, quantity, and quality. In the Appalachian foreland of northwestern Vermont, numerous bedrock wells that are spatially related to the Paleozoic Hinesburg thrust have elevated naturally occurring radioactivity and/or low yields. The association of groundwater quality and quantity issues with this thrust was a unique opportunity to investigate its structural and hydrogeologic framework. The Hinesburg thrust juxtaposed metamorphic rocks of the hanging wall with sedimentary rocks of the footwall during the Ordovician. It was then deformed by two orthogonal Devonian fold sets and was fractured during the Cretaceous. Median well yields in the hanging wall aquifer are significantly lower than those of the footwall aquifer, consistent with the respective permeability contrast between metamorphic and carbonate rocks. For wells drilled through the Hinesburg thrust, those completed closest (vertically) to the thrust have the highest median yields, whereas others completed farther below have yields in the footwall range. The geochemical signature of the hanging wall and footwall aquifers correlates with their whole-rock geochemistry. The hanging wall aquifer is enriched in alpha radiation, Na+K-Cl, Ba, and Sr, whereas the footwall aquifer is enriched in Ca-Mg-HCO3 and alkalinity. Wells that penetrated the Hinesburg thrust generally have hanging wall geochemical signatures. A simple hydrogeologic model for the permeability evolution of the Hinesburg thrust involves the ductile emplacement of a low-K hanging wall onto a high-K footwall, with subsequent modification by fractures.