The purpose of this paper is to (i) review field data on stress‐induced permeability changes in fractured rock; (ii) describe estimation of fractured rock stress‐permeability relationships through model calibration against such field data; and (iii) discuss observations of temperature and chemically mediated fracture closure and its effect on fractured rock permeability. The field data that are reviewed include in situ block experiments, excavation‐induced changes in permeability around tunnels, borehole injection experiments, depth (and stress) dependent permeability, and permeability changes associated with a large‐scale rock‐mass heating experiment. Data show how the stress‐permeability relationship of fractured rock very much depends on local in situ conditions, such as fracture shear offset and fracture infilling by mineral precipitation. Field and laboratory experiments involving temperature have shown significant temperature‐driven fracture closure even under constant stress. Such temperature‐driven fracture closure has been described as thermal overclosure and relates to better fitting of opposing fracture surfaces at high temperatures, or is attributed to chemically mediated fracture closure related to pressure solution (and compaction) of stressed fracture surface asperities. Back‐calculated stress‐permeability relationships from field data may implicitly account for such effects, but the relative contribution of purely thermal‐mechanical and chemically mediated changes is difficult to isolate. Therefore, it is concluded that further laboratory and in situ experiments are needed to increase the knowledge of the true mechanisms behind thermally driven fracture closure, and to further assess the importance of chemical‐mechanical coupling for the long‐term evolution of fractured rock permeability. This paper reviews stress‐induced permeability changes in fractured rock observed from field data, including effects of temperature and chemically mediated fracture closure. While the stress‐permeability relationship of a rock mass might be bounded from site specific field investigations, it is concluded that further laboratory and in situ experiments are needed to increase the knowledge of the true mechanisms underlying thermally driven fracture closure, and to further assess chemical‐mechanical coupling effects on the long‐term evolution of fractured rock permeability.
Study of the pore space in mudstones by mercury intrusion porosimetry is a common but indirect technique and it is not clear which part of the pore space is actually filled with mercury. We studied samples from the Opalinus Clay, Boom Clay, Haynesville Shale, and Bossier Shale Formations using Wood's metal injection at 316 MPa, followed by novel ion beam polishing and high-resolution scanning electron microscopy. This method allowed us to analyze at high resolution which parts of a rock are intruded by the liquid alloy at mm to cm scale. Results from the Opalinus Clay and Haynesville Shale show Wood's Metal in cracks, but the majority of the pore space is not filled although mercury intrusion data suggests that this is the case. In the silt-rich Boom Clay sample, the majority of the pore space was filled Wood's metal, with unfilled islands of smaller pores. Bossier Shale shows heterogeneous impregnation with local filling of pores as small as 10 nm. We infer that mercury intrusion data from these samples is partly due to crack filling and compression of the sample. This compaction is caused by effective stress developed by mercury pressure and capillary resistance; it can close small pore throats, prevent injection of the liquid metal, and indicate an apparent porosity. Our results suggest that many published MIP data on mudstones could contain serious artifacts and reliable metal intrusion porosimetry requires a demonstration that the metal has entered the pores, for example by Wood's metal injection, broad ion beam polishing, and scanning electron microscopy.
Magmatic‐hydrothermal ore deposits document the interplay between saline fluid flow and rock permeability. Numerical simulations of multiphase flow of variably miscible, compressible H 2 O–NaCl fluids in concert with a dynamic permeability model can reproduce characteristics of porphyry copper and epithermal gold systems. This dynamic permeability model uses values between 10 −22 and 10 −13 m 2 , incorporating depth‐dependent permeability profiles characteristic for tectonically active crust as well as pressure‐ and temperature‐dependent relationships describing hydraulic fracturing and the transition from brittle to ductile rock behavior. In response to focused expulsion of magmatic fluids from a crystallizing upper crustal magma chamber, the hydrothermal system self‐organizes into a hydrological divide, separating an inner part dominated by ascending magmatic fluids under near‐lithostatic pressures from a surrounding outer part dominated by convection of colder meteoric fluids under near‐hydrostatic pressures. This hydrological divide also provides a mechanism to transport magmatic salt through the crust. With a volcano at the surface above the hydrothermal system, topography‐driven flow reverses the direction of the meteoric convection as compared to a flat surface, leading to discharge at distances of up to 7 km from the volcanic center. The same physical processes at similar permeability ranges, crustal depths, and flow rates are relevant for a number of active systems, including geothermal resources and excess degassing at volcanos. The simulations further suggest that the described mechanism can separate the base of free convection in high‐enthalpy geothermal systems from the magma chamber as a driving heat source by several kilometers in the vertical direction in tectonic settings with hydrous magmatism. These root zones of high‐enthalpy systems may serve as so‐called super‐critical geothermal resources. This hydrology would be in contrast to settings with anhydrous magmatism, where the base of the geothermal systems may be closer to the magma chamber. Dynamic permeability changes in response to expulsion of magmatic fluids from an upper crustal magma chamber.
Thermal springs in the Southern Alps, New Zealand, originate through penetration of fluids into a thermal anomaly generated by rapid uplift and exhumation on the Alpine Fault. Copland hot spring (43.629S, 169.946E) is one of the most vigorously flowing, hottest of the springs, discharging strongly effervescent CO 2 ‐rich 56–58°C water at 6 ± 1 l sec −1 . Shaking from the Mw7.8 Dusky Sound (Fiordland) 2009 and Mw7.1 Darfield (Canterbury) 2010 earthquakes, 350 and 180 km from the spring, respectively, resulted in a characteristic approximately 1°C delayed cooling over 5 days. A decrease in conductivity and increase in pH were measured following the Mw7.1 Darfield earthquake. Earthquake‐induced decreases in Cl, Li, B, Na, K, Sr and Ba concentrations and an increase in SO 4 concentration reflect higher proportions of shallow‐circulating meteoric fluid mixing in the subsurface. Shaking at amplitudes of approximately 0.5% g Peak Ground Acceleration (PGA) and/or 0.05–0.10 MPa dynamic stress influences Copland hot spring temperature, which did not respond during the Mw6.3 Christchurch 2011 aftershock or other minor earthquakes. Such thresholds should be exceeded every 1–10 years in the central Southern Alps. The characteristic cooling response at low shaking intensities (MM III–IV) and seismic energy densities (approximately 10 −1 J m −3 ) from intermediate‐field distances was independent of variations in spectral frequency, without the need for post‐seismic recovery. Observed temperature and fluid chemistry responses are inferred to reflect subtle changes in the fracture permeability of schist mountains adjacent to the spring. Permanent 10 −7 –10 −6 strains recorded by cGPS reflect opening or generation of fractures, allowing greater quantities of relatively cool near‐surface groundwater to mix with upwelling hot water. Active deformation, tectonic and topographic stress in the Alpine Fault hanging wall, where orographic rainfall, uplift and erosion are extreme, make the Southern Alps hydrothermal system particularly susceptible to earthquake‐induced transient permeability. In response to large distant earthquakes Copland hot spring cooled approximately 1°C and changed fluid chemistry. Relatively low intensity shaking induced small permanent strains across the Southern Alps – opening fractures which enhanced mixing of relatively cool near‐surface groundwater with upwelling hot water. Active deformation, tectonic and topographic stress in the Alpine Fault hanging wall makes the Southern Alps hydrothermal system particularly susceptible to earthquake‐induced transience.
Metal‐catalysed CO 2 hydrogenation is considered a source of methane in serpentinized (hydrated) igneous rocks and a fundamental abiotic process germane to the origin of life. Iron, nickel, chromium and cobalt are the catalysts typically employed in hydrothermal simulation experiments to obtain methane at temperatures >200°C. However, land‐based present‐day serpentinization and abiotic gas apparently develop below 100°C, down to approximately 40–50°C. Here, we document considerable methane production in thirteen CO 2 hydrogenation experiments performed in a closed dry system, from 20 to 90°C and atmospheric pressure, over 0.9–122 days, using concentrations of non‐pretreated ruthenium equivalent to those occurring in chromitites in ophiolites or igneous complexes (from 0.4 to 76 mg of Ru, equivalent to the amount occurring approximately in 0.4–760 kg of chromitite). Methane production increased with time and temperature, reaching approximately 87 mg CH 4 per gram of Ru after 30 days (2.9 mg CH 4 /g ru /day) at 90°C. At room temperature, CH 4 production rate was approximately three orders of magnitude lower (0.003 mg CH 4 /g ru /day). We report the first stable carbon and hydrogen isotope ratios of abiotic CH 4 generated below 100°C. Using initial δ 13 C CO 2 of ‐40‰, we obtained room temperature δ 13 C CH 4 values as 13 C depleted as −142‰. With time and temperature, the C‐isotope separation between CO 2 and CH 4 decreased significantly and the final δ 13 C CH 4 values approached that of initial δ 13 C CO 2 . The presence of minor amounts of C 2 ‐C 6 hydrocarbons is consistent with observations in natural settings. Comparative experiments at the same temperatures with iron and nichel catalysts did not generate CH 4 . Ru‐enriched chromitites could potentially generate methane at low temperatures on Earth and on other planets. Methane was abiotically produced in CO2 hydrogenation experiments at temperatures from 20 to 90° using concentrations of non‐pretreated ruthenium equivalent to those occurring in chromitites in ophiolites or igneous complexes. Stable C and H isotope ratios of abiotic CH4 generated below 100°C is reported for the first time. Comparative experiments at the same temperatures with iron and nichel catalysts did not generate CH4. Ru‐enriched chromitites can potentially generate methane at low temperatures on Earth and on other planets.
The permeability structure resulting from high fluid pressure stimulation of a geothermal resource is the most important parameter controlling the feasibility and the viability of enhanced geothermal systems ( EGS ), yet is the most elusive to constrain. Linear diffusion models do a reasonably good job of constraining the front of the stimulated region because of the t 1/2 dependence of the perturbation length, but triggering pressures resulting from such models, and the permeability inferred using the diffusivity parameter, drastically underestimate both permeability and pressure changes. This leads to incorrect interpretations about the nature of the system, including the degree of fluid pressures needed to induce seismicity required to enhance the system. Here, I use a minimalist approach to modeling and show that all of the observations from Basel (Switzerland) fluid injection experiment are well matched by a simple model where the dominant control on the system is a large‐scale change in permeability at the onset of slip. The excellent agreement between observations and these simplest of models indicates that these systems may be less complicated than envisaged, thus offering strategies for more sophisticated future modeling to help constrain and exploit these systems. The evolution of the permeability field in the Basel enhanced geothermal system was modelled using a simple non‐linear diffusion model with a step‐wise increase in permeability when the failure condition is reached. This simple model reproduces all the observations obtained during that experiment.
After the occurrence of the 2011 magnitude 9 Tohoku earthquake, the seismicity in the overriding plate changed. The seismicity appears to form distinct belts. From the spatiotemporal distribution of hypocenters, we can quantify the evolution of seismicity after the 2011 Tohoku earthquake. In some earthquake swarms near Sendai (Nagamachi‐Rifu fault), Moriyoshi‐zan volcano, Senya fault, and the Yamagata–Fukushima border (Aizu‐Kitakata area, west of Azuma volcano), we can observe temporal expansion of the focal area. This temporal expansion is attributed to fluid diffusion. Observed diffusivity would correspond to the permeability of about 10 −15 (m 2 ). We can detect the area from which fluid migrates as a seismic low‐velocity area. In the lower crust, we found seismic low‐velocity areas, which appear to be elongated along N–S or NE–SW, the strike of the island arc. These seismic low‐velocity areas are located not only beneath the volcanic front but also beneath the fore‐arc region. Seismic activity in the upper crust tends to be high above these low‐velocity areas in the lower crust. Most of the shallow earthquakes after the 2011 Tohoku earthquake are located above the seismic low‐velocity areas. We thus suggest fluid pressure changes are responsible for the belts of seismicity. Temporal expansion of the focal area in some earthquake swarms near S endai ( N agamachi‐ R ifu fault), M oriyoshi‐zan volcano, S enya fault, and the Y amagata‐ F ukushima border ( A izu‐ K itakata area, west of A zuma volcano) induced by the 2011 T ohoku‐ O ki earthquake was observed. This temporal expansion can be explained by fluid diffusion. We found seismic low‐velocity areas, which is the possible areas with fluid, beneath the swarms. From the results, the induced earthquakes are thought to be affected by the possible fluid pressure change.
The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 10 −17 –10 −15 m 2 at targeted depths for enhanced geothermal systems ( EGS ) applications, that is, >3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability. In the mining sector, fluid injection methods are being investigated to increase rock fragmentation and mitigate high‐stress hazards due to operations moving to unprecedented depths. Here as well, detailed understanding of permeability and its enhancement is required. This paper reports findings from a series of hydromechanically coupled distinct‐element models developed in support of a hydraulic fracture experiment testing hypotheses related to enhanced permeability, increased fragmentation, and modified stress fields. Two principal injection designs are tested as follows: injection of a high flow rate through a narrow‐packed interval and injection of a low flow rate across a wider packed interval. Results show that the development of connected permeability is almost exclusively orthogonal to the minimum principal stress, leading to strongly anisotropic flow. This is because of the stress transfer associated with opening of tensile fractures, which increases the confining stress acting across neighboring natural fractures. This limits the hydraulic response of fractures and the capacity to create symmetric isotropic permeability relative to the injection wellbore. These findings suggest that the development of permeability at depth can be improved by targeting a set of fluid injections through smaller packed intervals instead of a single longer injection in open boreholes. In this paper a set of distinct‐element models illustrates key factors limiting the development of connected rock mass permeability by fluid injection. Stress transfer accompanying the opening of a pressurized fracture confines nearby natural and incipient fractures limiting their response. This promotes a limited development of permeability focused to a relatively thin layer of rock instead of across a large volume.
Porosity waves are a mechanism by which fluid generated by devolatilization and melting, or trapped during sedimentation, may be expelled from ductile rocks. The waves correspond to a steady‐state solution to the coupled hydraulic and rheologic equations that govern flow of the fluid through the matrix and matrix deformation. This work presents an intuitive analytical formulation of this solution in one dimension that is general with respect to the constitutive relations used to define the viscous matrix rheology and permeability. This generality allows for the effects of nonlinear viscous matrix rheology and disaggregation. The solution combines the porosity dependence of the rheology and permeability in a single hydromechanical potential as a function of material properties and wave velocity. With the ansatz that there is a local balance between fluid production and transport, the solution permits prediction of the dynamic variations in permeability and pressure necessary to accommodate fluid production. The solution is used to construct a phase diagram that defines the conditions for smooth pervasive flow, wave‐propagated flow, and matrix fluidization (disaggregation). The viscous porosity wave mechanism requires negative effective pressure to open the porosity in the leading half of a wave. In nature, negative effective pressure may induce hydrofracture, resulting in a viscoplastic compaction rheology. The tubelike porosity waves that form in such a rheology channelize fluid expulsion and are predicted by geometric argumentation from the one‐dimensional viscous solitary wave solution. Porosity waves are the steady‐state response to flow perturbations such as fluid production in ductile rocks. This study develops an analytical solution for these waves that is general with respect to the constitutive relations that define the viscous matrix rheology and permeability. The analytical solution permits anticipation of the scales of lower crustal fluid pressure and permeability variations caused by flow perturbations. The figure shows a numerical simulation of a porosity wave in a viscoplastic matrix.
There is considerable interest in the use of thick argillaceous geologic formations to contain nuclear waste. Here, we show that diffusion can be the controlling transport process in these formations and diffusional time scales for δ 18 O and δ 2 H in water, dissolved He, and Cl transport in shale‐dominated aquitards are typically over 10 6 years, well exceeding the regulatory requirements for isolation in most countries. Our scientific understanding of diffusive solute transport processes through argillaceous formations would benefit from the application of additional isotopic tracers (e.g., using new 4 He sampling technology), multidimensional diffusive‐dispersive modeling of groundwater flow and diffusive‐dispersive solute transport over long geologic time scales, and an improved understanding of spatial heterogeneity as well as time‐dependent changes in the subsurface conditions and properties of argillaceous formations in response to events such as glaciation. Based on our current isotopic and geochemical understanding of transport, we argue that argillaceous formations can provide favorable long‐term conditions for isolating nuclear wastes.
Co‐seismic groundwater‐level changes induced by earthquakes have been reported for thousands of years. The M8.0 Wenchuan earthquake caused co‐seismic groundwater‐level responses across the Chinese mainland. Three types of changes were recorded in 197 monitoring wells: co‐seismic oscillations ranging in amplitude from 0.004 to 1.1 m, immediate co‐seismic step changes ranging from 0.0039 to 9.188 m, and more gradual postseismic changes ranging from 0.014 to 1.087 m. We find that the co‐seismic groundwater‐level response is complex. There is neither a clear relationship between the response amplitude and the distance from the epicenter, nor a clear relationship between the groundwater response and lithology at the continental scale. Both the sign and amplitude of water‐level changes are random at the continental scale, and a poroelastic response to the co‐seismic static strain cannot explain most of the co‐seismic changes. However, wells located near the edges of tectonically active blocks have larger response amplitudes than those in the middle of these ‘stable’ blocks. Considered together, these observations indicate that permeability enhancement caused by the earthquake is a significant or dominant mechanism causing water‐level changes. These data indicate that large earthquakes can cause the widespread permeability changes in the shallow crust although the magnitude of permeability change is uncertain. We report the co‐seismic groundwater level response to the M8.0 earthquake across the Chinese mainland. There is great variability in the relationship between water level changes, and epicentral distance or static strain. Permeability enhancement in the crust caused by the earthquake is a significant or dominant mechanism in causing water level changes.
Fault stepovers are features where the main trace of a fault steps from one segment to the next in either an underlapping or overlapping manner. Stepovers exert a critical influence on crustal permeability and are known to control phenomena such as the migration of hydrocarbons and the location of geothermal fields. In the Kalgoorlie‐Ora Banda greenstone district, Western Australia, we demonstrate a spatial association between stepovers and gold deposits. It is shown that although underlapping stepover geometries are typically rare in fault systems, they are anomalously associated with gold deposits. Further, the along‐strike and across‐strike dimensions of both underlapping and overlapping fault stepovers fit, to a first‐order approximation, the same self‐similar trend. Boundary element modelling of Coulomb failure stress changes is used to explain these observations in terms of damage generated by rupture events on the bounding fault segments and associated aftershock sequences. Our models indicate that a larger region of damage and permeability enhancement is created around underlapping stepovers than around overlapping stepovers. By taking into account both the enhancement and decay of permeability during the seismic cycle, it is estimated that a 5 Moz goldfield could feasibly form in 1–16 earthquake‐aftershock sequences, potentially representing durations of just 10–8000 years. The existence of supergiant gold deposits is evidence that crustal permeability attains transiently high values on the order of 10 −12 m 2 . It should be expected that transient and time‐integrated permeability values have a distinct three‐dimensional structure in continental crust due to stepover‐related channels.
We assembled a data set of permeability measurements from 317 subduction zone and reference site samples worldwide made over nearly 25 years of scientific drilling. This data set allowed us to examine the influence of grain size, structural domain, and measurement type on permeabilities ranging from 10 −21 to 10 −14 m 2 . We found that porosity–permeability behavior is a function of clay‐size fraction, which is consistent with previous work. Sediments within the slope, accretionary prism, and fault‐zone structural domains are strongly affected by shearing, which alters the permeability behavior with burial. Consolidation, flow‐through, and transient pulse decay measurements all provide comparable results. Measurements of horizontal and vertical permeability show significant cm‐scale permeability anisotropy (ratio of horizontal to vertical permeability >10) in the slope and accretionary prism structural domains, further indicating shear deformation in these domains. Laboratory consolidation trends match large‐scale (10 2 m) field trends in structural domains with negligible shear, but tend to underestimate the rate of permeability reduction with porosity loss where shear is significant. Comparison with downhole measurements shows that permeability is controlled by higher‐permeability (>10 −15 m 2 ) layers at the meter to tens of meters scale, while wireline formation tester measurements closely match laboratory results. Sediments from the underthrust and reference structural domains exhibit similar porosity–permeability trends, which suggests that shallow subduction (total burial <1 km) does not significantly alter the porosity–permeability behavior of incoming sediments. Comparison with measurements of deeper analog data from 14 passive‐margin samples show that porosity–permeability trends are maintained through burial and diagenesis to porosities <10%, suggesting that behavior observed in shallow samples is informative for predicting behavior at depth following subduction. We assembled a data set of 317 permeability measurements from subduction zones and reference sites worldwide. We found that permeability–porosity behavior is a function of clay‐size fraction, and that structural domain is a secondary influence, while measurement type has little effect on the results. Comparison with measurements of deeper analog data show that porosity–permeability trends are maintained through burial and diagenesis to porosities <10%, suggesting that behavior observed in shallow samples is informative for predicting behavior at depth.
The paper examines the influence of axial stress‐induced closure of a fracture on its permeability. The experiments were conducted on a cylinder of Barre Granite measuring 457 mm in diameter and 510 mm in height, containing a central cylindrical cavity of diameter 75 mm. Radial flow hydraulic pulse tests were conducted in a previous research investigation (Selvadurai et al ., PAGEOPH, 2005) to determine the permeability characteristics of the intact granite. In the continuation of the research, a fracture was introduced in the cylinder with its nominal plane normal to the axis of the cylinder. Axial compressive stress was applied normal to the plane of the fracture. An increase in the compressive normal stress acting on the fracture caused a reduction in the aperture of the fracture, which resulted in the reduction in its permeability. Steady state radial flow tests were conducted on the fractured axially stressed sample to determine the variation of fracture permeability with axial normal stress. The analytical developments also take into account flow through the matrix region as the normal stress increases. The results of the experimental investigations indicate that the complete stress relief of a fracture previously subjected to a normal stress of 7.5 MPa can result in a permeability increase of approximately three orders of magnitude. These findings are relevant to shallow depth geotechnical construction activities where enhanced fluid flow can be activated by stress relief. As the fracture aperture closes with high normal stress, the flow through the matrix can be appreciable and if this factor is not taken into consideration the interpretation of fracture permeability can be open to error. This factor can be of interest to the interpretation of permeability of fractures in deep crustal settings where the stresses acting normal to the fracture surface can inhibit flow in the fracture. Fluid flow in fractures can be significantly influenced by the stresses that are acting both normal to and in the plane of the fracture. This experimental research illustrates the influence of normal stress‐induced hydraulic closure of the fracture on the evolution of fracture permeability. Experiments conducted on a 457 mm diameter cylinder containing a planar fracture show that the permeability can exhibit a three orders of magnitude decrease as the normal stresses are increased from zero to 7.5 MPa. This change can occur without the development of gouge during the application of axial stresses.
Accurate simulation of multiphase flow in fractured porous media remains a challenge. An important problem is the representation of the discontinuous or near discontinuous behaviour of saturation in real geological formations. In the classical continuum approach, a refined mesh is required at the interface between fracture and porous media to capture the steep gradients in saturation and saturation‐dependent transport properties. This dramatically increases the computational load when large numbers of fractures are present in the numerical model. A discontinuous finite element method is reported here to model flow in fractured porous media. The governing multiphase porous media flow equations are solved in the adaptive mesh computational fluid dynamics code IC ‐ FERST on unstructured meshes. The method is based on a mixed control volume – discontinuous finite element formulation. This is combined with the P N +1 DG ‐ P N DG element pair, which has discontinuous (order N +1) representation for velocity and discontinuous (order N ) representation for pressure. A number of test cases are used to evaluate the method's ability to model fracture flow. The first is used to verify the performance of the element pair on structured and unstructured meshes of different resolution. Multiphase flow is then modelled in a range of idealised and simple fracture patterns. Solutions with sharp saturation fronts and computational economy in terms of mesh size are illustrated.
At subduction zones, continuous influx of fluids drives a dynamic system in which fault slip, fluid flow, and advective transport are tightly coupled. Field and numerical modeling studies have provided insight into the nature and rates of flow in these systems and illustrate that active subduction faults, including the master décollement and splay faults cutting the upper plate, are important conduits. Observations of in situ fracture dilation, modeling studies, and direct measurements documenting strong pressure dependence of fault permeability collectively suggest that permeability varies in time, perhaps due to pore pressure cycling. However, mechanical and fluid budget considerations dictate that increased fault permeability cannot be sustained, nor can it be present across the entire fault surface at a given time. The emerging conceptual model is that permeable patches or channels occupy only a fraction of the fault surface and shift transiently. Fault zone permeabilities obtained by several approaches are consistent between margins, with time‐averaged values of approximately 10 −15 to 10 −14 m 2 , several orders of magnitude higher than for the sediment matrix. Higher, transiently increased values of approximately 10 −13 to 10 −11 m 2 are required to explain geochemical and thermal signals and observed focused flow rates. Although faults accommodate significant fluid fluxes from dewatering of the surrounding sediment, they have little effect on pore pressures within the wall rock, where drainage is limited by low matrix permeability. However, fault permeability is a key control on the transport and preservation of localized geochemical and thermal anomalies from depths where temperatures are higher and low‐temperature metamorphic reactions are underway. Despite significant recent progress, several key aspects of hydrologic behavior in these active faults remain incompletely understood, including the nature and timescale of transience, the causes of permeability enhancement and its relationship to fault slip and pore pressure fluctuations, and the depths and distances from which deeply sourced fluids are captured, mixed, and transported up‐dip. Subduction zone faults are key conduits for fluid flow and advection of heat and solutes. Drilling, numerical modeling, and laboratory studies all suggest that their permeability also varies through time, likely due to pore pressure cycling and dilation of fracture networks. Detailed studies of several active subduction zones illustrate that both time‐averaged and transiently elevated fault permeabilities are remarkably consistent between margins, ranging from approximately 10 −15 to 10 −14 m 2 and approximately 10 −11 to 10 −13 m 2 , respectively, and are up to six orders of magnitude higher than that of the surrounding sediment matrix.
The permeability (κ[m 2 ]) of fractured crystalline basement of the upper continental crust is an intrinsic property of a complex system of rocks and fractures that characterizes the flow properties of a representative volume of that system. Permeability decreases with depth. Permeability can be derived from hydraulic well test data in deep boreholes. Only a handful of such deep wells exist on a worldwide basis. Consequently, few data from hydraulically tested wells in crystalline basement are available to the depth of 4–5 km. The permeability of upper crust varies over a very large range depending on the predominant rock type at the studied site and the geological history of the drilled crystalline basement. Hydraulic tests in deep boreholes in the continental crystalline basement revealed permeability (κ) values ranging over nine log‐units from 10 − 21 to 10 − 12 m 2 . This large variance also decreases with depth, and at 4 km depth, a characteristic value for the permeability κ is 10 −15 m 2 . The permeability varies with time due to deformation‐related changes of fracture aperture and fracture geometry and as a result of chemical reaction of flowing fluids with the solids exposed along the fractures. Dissolution and precipitation of minerals contribute to the variation of the permeability with time. The time dependence of κ is difficult to measure directly, and it has not been observed in hydraulic well tests. At depths below the deepest wells down to the brittle ductile transition zone, evidence of permeability variation with time can be found in surface exposures of rocks originally from this depth. Exposed hydrothermal reaction veins are very common in continental crustal rocks and witness fossil permeability and its variation with time. The transient evolution of permeability can be predicted from models using fictive and simple starting conditions. However, a geologically meaningful quantitative description of permeability variation with time in the deeper parts of the brittle continental crust resulting from combined fracturing and chemical reaction appears very difficult. The permeability of continental crust varies with depth and time. A few deep boreholes drilled to 4–5 km depth provided transmissivity data from hydraulic well tests. We discuss the surprisingly complex conversion of transmissivity to permeability. We also present unique permeability data at different depths from a single 4 km deep borehole. We use exposed hydrothermal reaction veins to assess the permeability structure of the crust at depths >5 km and its variation with time.
Permian hydrothermal activity in the Tarim Basin may have been responsible for the invasion of hot brines into Ordovician carbonate reservoirs. Studies have been undertaken to explain the origin and geochemical characteristics of the diagenetic fluid present during this hydrothermal event although there is no consensus on it. We present a genetic model resulting from the study of δ 13 C, δ 18 O, δ 34 S, and 87 Sr/ 86 Sr isotope values and fluid inclusions ( FI s) from fracture‐ and vug‐filling calcite, saddle dolomite, fluorite, barite, quartz, and anhydrite from Ordovician outcrops in northwest ( NW ) Tarim Basin and subsurface cores in Central Tarim Basin. The presence of hydrothermal fluid was confirmed by minerals with fluid inclusion homogenization temperatures being >10°C higher than the paleo‐formation burial temperatures both in the NW Tarim and in the Central Tarim areas. The mixing of hot (>200°C), high‐salinity (>24 wt% NaCl), 87 Sr‐rich (up to 0.7104) hydrothermal fluid with cool (60–100°C), low‐salinity (0 to 3.5 wt% NaCl), also 87 Sr‐rich (up to 0.7010) meteoric water in the Ordovician unit was supported by the salinity of fluid inclusions, and δ 13 C, δ 18 O, and 87 Sr/ 86 Sr isotopic values of the diagenetic minerals. Up‐migrated hydrothermal fluids from the deeper Cambrian strata may have contributed to the hot brine with high sulfate concentrations which promoted thermochemical sulfate reduction ( TSR ) in the Ordovician, resulting in the formation of 12 C‐rich (δ 13 C as low as −13.8‰) calcite and 34 S‐rich (δ 34 S values from 21.4‰ to 29.7‰) H 2 S, pyrite, and elemental sulfur. Hydrothermal fluid mixing with fresh water in Ordovician strata in Tarim Basin was facilitated by deep‐seated faults and up‐reaching faults due to the pervasive Permian magmatic activity. Collectively, fluid mixing, hydrothermal dolomitization, TSR , and faulting may have locally dissolved the host carbonates and increased the reservoir porosity and permeability, which has significant implications for hydrocarbon exploration.
Petrographic features, C , O , S , and S r isotopes were determined, and fluid inclusions ( FI ) were analyzed on various stages of vug‐ and fracture‐fillings from the C ambrian and L ower O rdovician reservoirs in the T azhong area, T arim basin, NW C hina. The aim was to assess the origin of pyrite and anhydrite and the processes affecting sulfur during diagenesis of the carbonates. Pyrite from seven wells has δ 34 S values from −22‰ to +31‰. The pyrites with low δ 34 S values from −21.8‰ to −12.3‰ were found close to fracture‐filling calcites with vapor‐liquid double‐phase aqueous fluid inclusions homogenization temperatures ( FI ‐ T h) from 55.7 to 73.2°C, salinities from 1.4wt% to 6.59wt% N a C l equiv and δ 13 C values from −2.3‰ to −14.2‰, indicating an origin from bacterial sulfate reduction by organic matter. Other sulfides with heavier δ 34 S values may have formed by thermochemical sulfate reduction ( TSR ) during two episodes. The earlier TSR in the M iddle and L ower C ambrian resulted in pyrites and H 2 S having δ 34 S values from 30 to 33‰, close to those of bedded anhydrite and oilfield water (approximately 34‰). The later TSR is represented by calcites with δ 13 C values as light as −17.7‰ and FI ‐ T h of about 120–145°C, and pyrite and H 2 S with δ 34 S values close to those of the U pper C ambrian burial‐diagenetic anhydrite (between +14.8‰ and +22.6‰). The values of the anhydrite are significantly lighter than contemporary seawater sulfates. This together with 87 S r/ 86 S r values of anhydrite and TSR calcites from 0.7091 to 0.7125 suggests a source from the underlying E diacaran seawater sulfate and detrital S r contribution. Pyrites were originated from BSR and two periods of TSR with different δ 34 S values. Free H 2 S and pyrite in the Ordovician with δ 34 S values from 15 to 23% may have been generated from the later TSR of burial‐diagenetic anhydrite by petroleum.