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.
Changes in water level are commonly reported in regions struck by a seismic event. The sign and amplitude of such changes depend on the relative position of measuring points with respect to the hypocenter, and on the poroelastic properties of the rock. We apply a porous media flow model (TOUGH2) to describe groundwater flow and water‐level changes associated with the first M L 5.9 mainshock of the 2012 seismic sequence in Emilia (Italy). We represent the earthquake as an instantaneous pressure step, whose amplitude was inferred from the properties of the seismic source inverted from geodetic data. The results are consistent with the evolution recorded in both deep and shallow water wells in the area and suggest that our description of the seismic event is suitable to capture both timing and magnitude of water‐level changes. We draw some conclusions about the influence of material heterogeneity on the pore pressure evolution, and we show that to reproduce the observed maximum amplitude it is necessary to take into account compaction in the shallow layer.
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 (R 2 ≥ 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 (R 2 = 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. We quantify how well existing and new equations predict permeability using a compilation of permeability data of pure sands, clays, and mixed siliciclastic sediments. Equations that calculate permeability as the power mean of the pure clay and sand components provided much better estimates of sediment permeability than ideal packing models. Combining the power mean equation with well‐log data to estimate porosity, clay content, and permeability provides opportunities to characterize sediment permeability at larger scales.
The boron stable isotope ratio δ 11 B of 12 water samples representative of three chemical facies (fresh N a‐bicarbonate, brackish N a‐chloride, saline, and brine C a‐chloride) has been analyzed. Interpretation of the δ 11 B data, along with the chemical compositions, reveals that N a‐carbonate waters from the N orthern A pennine are of meteoric origin, with boron contributions from clay desorption and mixing with seawater‐derived fluids of N a‐chloride or C a‐chloride compositions. The comparison of our new results with the literature data on other sedimentary basins of M editerranean, and worldwide, confirms the contribution of N a‐bicarbonate waters to the genesis of mud volcano fluids. The N a‐chloride sample of S alvarola ( SAL ), which may represent the end‐member of the mud volcanoes, and the C a‐chloride brine water from S alsomaggiore ( SM ) indicate boron release from clays compatible with the diagenetic process. The empirical equation: δ 11 B = [ 5.1364 × ln(1/B) mg l − 1 ] + 44.601 relating boron concentration and the stable isotope composition of the fluids observed in this study and the literature is proposed to trace the effect of diagenesis in sedimentary basins. A geothermometer associated to the diagenetic equation is also proposed: T ∘ C = [ δ 11 B − 38.873 ( ± 1.180 ) ] / [ − 0.164 ( ± 0.012 ) ] The application of this equation to obtain reservoir temperatures from δ 11 B compositions of waters should be carefully evaluated against the results obtained from other chemical and isotopic geothermometers from other basins around the world.
Capillary trapping is a physical mechanism by which carbon dioxide ( CO 2 ) is naturally immobilized in the pore spaces of aquifer rocks during geologic carbon sequestration operations, and thus a key aspect of estimating geologic storage potential. Here, we studied capillary trapping of supercritical carbon dioxide (sc CO 2 ), and the effect of initial sc CO 2 saturation and flow rate on the storage/trapping potential of Berea sandstone. We performed two‐phase, sc CO 2 ‐brine displacements in two samples, each subject to four sequential drainage–imbibition core‐flooding cycles to quantify end‐point saturations of sc CO 2 with the aid of micro‐ and macro‐computed tomography imaging. From these experiments, we found that between 51% and 75% of the initial CO 2 injected can be left behind after the brine injection. We also observed that the initial sc CO 2 saturation influenced the residual sc CO 2 saturation to a greater extent than the rate of brine injection under the experimental conditions examined. In spite of differences in the experimental conditions tested, as well as those reported in the literature, initial and residual saturations were found to follow a consistent relationship. Retention of CO 2 by capillary trapping in sandstone samples can reach about 38% of residual CO 2 saturation according to empirical model fitting to the experimental data presented in this work. In spite of relative differences in permeability and experimental conditions tested in this study, and those reported in the literature, initial and residual saturations follow a consistent relationship. Empirical modeling of this relationship proved to be sound and of practical use in estimating capillary trapping in sandstones.
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 (<2.5 km). Large data sets from Sweden, Germany and Switzerland provide new opportunities to characterize the permeability of crystalline rocks in the shallow crust. Here we compile in situ permeability measurements ( n = 973) and quantitatively test potential relationships between permeability, depth (0–2.5 km), lithology (intrusive and metamorphic) and tectonic setting (active and inactive). Higher permeabilities are more common at shallow depths (<1 km), but trend analysis does not support a consistently applicable and generalizable relationship between permeability and depth in crystalline rock in the shallow crust. Results suggest lithology has a weak control on permeability–depth relations in the near surface (<0.1 km), regardless of tectonic setting, but may be a more important control at depth. Tectonic setting appears to be a stronger control on permeability–depth relations in the near surface. Permeability values in the tectonically active Molasse basin are scattered with a very weak relationship between permeability and depth. While results indicate that there is no consistently applicable relationship between permeability and depth for crystalline rock in the shallow crust, some specific lithologies and tectonic settings display a statistically significant decrease of permeability with depth, with greater predictive power than a generalized relationship, that could be useful for hydrologic and earth system models. The authors present a compilation of 973 in‐situ crystalline rock permeability measurements and quantitatively test potential relationships between permeability, depth, lithology, and tectonic setting in the shallow crust (<2.5 km). Results indicate that there is no consistently applicable relationship between permeability and depth in crystalline rock. Tectonic setting appears to be an important control on permeability‐depth relations in the upper 100 metres of the subsurface, while lithology may be a more important control at depth.
Heat‐flow mapping of the western USA has identified an apparent low‐heat‐flow anomaly coincident with the Columbia Plateau Regional Aquifer System, a thick sequence of basalt aquifers within the Columbia River Basalt Group ( CRBG ). A heat and mass transport model ( SUTRA ) was used to evaluate the potential impact of groundwater flow on heat flow along two different regional groundwater flow paths. Limited in situ permeability ( k ) data from the CRBG are compatible with a steep permeability decrease (approximately 3.5 orders of magnitude) at 600–900 m depth and approximately 40°C. Numerical simulations incorporating this permeability decrease demonstrate that regional groundwater flow can explain lower‐than‐expected heat flow in these highly anisotropic ( k x / k z ~ 10 4 ) continental flood basalts. Simulation results indicate that the abrupt reduction in permeability at approximately 600 m depth results in an equivalently abrupt transition from a shallow region where heat flow is affected by groundwater flow to a deeper region of conduction‐dominated heat flow. Most existing heat‐flow measurements within the CRBG are from shallower than 600 m depth or near regional groundwater discharge zones, so that heat‐flow maps generated using these data are likely influenced by groundwater flow. Substantial k decreases at similar temperatures have also been observed in the volcanic rocks of the adjacent Cascade Range volcanic arc and at Kilauea Volcano, Hawaii, where they result from low‐temperature hydrothermal alteration. Regional groundwater flow can explain lower‐than‐expected heat flow in a thick sequence of highly anisotropic (k x /k z ~10 4 ) continental flood basalts (Columbia River Basalt Group). A steep permeability decrease (approximately 3.5 orders of magnitude) is observed at 600–900 m depth and approximately 40°C, possibly a result of low‐temperature hydrothermal alteration. Substantial k decreases at similar temperatures have also been observed in the volcanic rocks of the Cascade Range and at Kilauea Volcano, Hawaii
Well TS 1 reveals many uncemented pores and vugs at depths of more than 8000 m in a deep Cambrian dolomite reservoir in the Tarim Basin, northwestern China. The fluid environment and mechanism required for the preservation of reservoir spaces have yet not been well constrained. Carbon, oxygen, and strontium isotope compositions and fluid inclusion data suggest two types of fluids, meteoric water and hydrothermal fluid, affecting the Lower Paleozoic carbonate reservoirs in the Tarim Basin. Based on simulation using a thermodynamic model for H 2 O‐ CO 2 ‐NaCl‐Ca CO 3 system, meteoric water has the ability to continuously dissolve carbonate minerals during downward migration from the surface to deep strata until it reaches a transition depth, below which it will begin to precipitate carbonate minerals to fill preexisting pore spaces. In contrast, hydrothermal fluid has the ability to dissolve carbonate in deep strata and precipitate carbonate in shallow strata during upward migration. Based on the dissolution–precipitation characteristics of the two types of fluids, the ideal fluid environment for the preservation of preexisting reservoir spaces occurs when carbonate reservoir is neither in the Ca CO 3 precipitation domain of meteoric water nor in the Ca CO 3 precipitation domain of hydrothermal fluid. Taking the Lower Paleozoic carbonate reservoirs in the north uplift area as an example, the spaces in the deep Cambrian dolomite reservoir near well TS 1 were seldom filled because thick Ordovician deposits blocked meteoric water from migrating downward into the Cambrian dolomite reservoir and because the Cambrian dolomite reservoir has been in the domain of hydrothermal dissolution since the Permian. The deep carbonate layers in basins elsewhere with a similar fluid environment may have high uncemented porosity and consequently have good hydrocarbon exploration potential. Both meteoric water and hydrothermal fluid affected the deep Cambrian dolomite reservoir in the Tarim Basin. The fluid environment for preservation of preexisting pore spaces occurs when the deep reservoir is in CaCO 3 dissolution domain of hydrothermal fluid but not in precipitation domain of meteoric water.
In this paper, we attempt to differentiate hydrocarbon‐bearing reservoir horizons of the Junggar Basin of NW China based on the characteristics of diagenesis and associated elemental geochemistry. Reservoirs at this site have varying levels of oil saturation that correlate with the degree of dissolution in minerals (e.g., calcite and feldspar). Four different horizons with varying diagenetic mineral assemblages were observed, including (i) kaolinite‐rich, oil‐dominated horizons, (ii) kaolinite–pyrite–hematite‐rich, oil–water‐dominated horizons, (iii) siderite–chlorite‐rich, water‐dominated horizons, and (iv) chlorite‐rich horizons with negligible hydrocarbon production. The mean MnO content of the representative diagenetic mineral (e.g., calcite) in each of the above horizons is >2.5, 2.0–2.5, 1.5–2.0, and <1.0 wt%, respectively. We propose that the above methodology can be used for the identification of reservoir hydrocarbon‐bearing horizons. We argue that the indicators presented here can be applied in oil exploration across the Junggar Basin. We attempted to differentiate hydrocarbon‐bearing reservoir horizons of the Junggar Basin of NW China based on the characteristics of diagenesis and associated elemental geochemistry. Reservoirs have varying levels of oil saturation that correlate with the degree of dissolution in minerals (e.g., calcite and feldspar) and different diagenetic mineral assemblages. MnO content of the representative diagenetic mineral (e.g., calcite) is indicative of reservoir oil saturation.
Hot springs can occur in amagmatic settings, but the mechanisms of heating are often obscure. We have investigated the origin of the Truth or Consequences, New Mexico low‐temperature (approximately 41°C) hot springs in the southern Rio Grande Rift. We tested two hypotheses that could account for this amagmatic geothermal anomaly: lateral forced convection in a gently dipping carbonate aquifer and circulation through high‐permeability crystalline basement rocks to depths of 8 km that is then focused through an overlying faulted hydrologic window. These hypotheses were tested using a regional two‐dimensional hydrothermal model. Model parameters were constrained by calibrating to measured temperatures, specific discharge rates, and groundwater residence times. We collected 16 temperature profiles, 11 geochemistry samples, and 6 carbon‐14 samples within the study area. The geothermal waters are Na + /Cl − ‐dominated and have apparent groundwater ages ranging from 5500 to 11 500 years. Hot‐spring geochemistry is consistent with water/rock interaction in a silicate geothermal reservoir, rather than a carbonate system. Peclet number analysis of temperature profiles suggests that specific discharge rates beneath Truth or Consequences range from 2 to 4 m year −1 . Geothermometry indicates maximum reservoir temperatures are around 170°C. Observed measurements were reasonably reproduced using the deep circulation permeable‐basement modeling scenario (10 −12 m 2 ) but not the lateral forced‐convection carbonate‐aquifer scenario. Focused geothermal discharge is the result of localized faulting, which has created a hydrologic window through a regional confining unit. In tectonically active areas, such as the Rio Grande Rift, deep groundwater circulation within fractured crystalline basement may play a more prominent role in the formation of geothermal systems than has generally been acknowledged. Geochemical analysis and two‐dimensional numerical modeling results strongly suggest that the Truth or Consequences, New Mexico geothermal system is the result of deep (2–8 km) groundwater circulation within highly permeable (10 −12 m 2 ) crystalline basement rocks. Geothermal waters discharge at the surface only through a hydrologic window in a regional overlying confining unit. This study provides additional evidence that the crystalline basement rocks beneath the Rio Grande Rift can be remarkably permeable in some places.
There is a great contrast in geochemical and hydrogeologic estimates of the residence times of pore fluids in sedimentary basins. This contrast is particularly evident in the Alberta Basin, Canada, which has served as the study area for important studies of long‐term fluid flow and transport. To address these differences, we developed two‐dimensional simulations of groundwater age, constrained by both hydrogeologic and geochemical observations, to estimate the residence time of fluids and the amount and timing of flushing by meteoric waters in the Alberta Basin. Results suggest that old, residual brines have been retained in the deepest parts of the basin since their formation ca. 400 Ma, but significant dilution by younger waters has reduced the age of these pore waters to no more than approximately 200 My. Shallower formations have been flushed extensively by fresh, young waters. Loss of brines and dilution of older pore waters occurred primarily after the uplift of the Rockies with the introduction of the gravity‐driven flow regime. Despite these large changes in flow regime, solute exchange between deep saline aquifers and the overlying vigorous freshwater flow system was found to be consistent with long‐term dispersive mixing across subhorizontal concentration gradients rather than by direct flushing. Sensitivity studies using an analytic solution supported the use of 100 m for transverse dispersivity in large‐scale numerical models. These simulations confirm that the age and origin of brines are in many cases poor indicators of long‐term solute transport rates in sedimentary basins, but the geochemical indicators that are used to determine the origin of brines can provide useful constraints for calculating groundwater age and are far more commonly available than isotopic groundwater age tracers.
The pollen 14 C age and oxygen isotopic composition of siliceous sinter deposits from the former Beowawe geyser field reveal evidence of two hydrothermal discharge events that followed relatively low‐magnitude (10 −11 m 2 ) following each earthquake. However, the timescale for onset of thermal convection implied by an overturned temperature profile in a geothermal well 300 m from the Malpais fault is much shorter: 200–1000 years. We speculate that individual segments of the Malpais fault become clogged on shorter timescales and that upward flow of groundwater subsequently follows new routes to the surface.
Flow of high‐concentration suspensions through fractures is important to a range of natural and induced subsurface processes where fractures provide the primary permeability (e.g., mud volcanoes, sand intrusion, and hydraulic fracturing). For these flows, the simple linear relationship between pressure gradient and flow rate, which applies for viscous‐dominated flows of Newtonian fluids, breaks down. We present results from experiments in which a high concentration (50% by volume) of granular solids suspended in a non‐Newtonian carrier fluid (0.75% guar gum in water) flowed through a parallel‐plate fracture. Digital imaging and particle‐image‐velocimetry analysis provided detailed two‐dimensional maps of velocities within the fracture. Results demonstrate development of a strongly heterogeneous velocity field within the fracture. Surprisingly, we observed the highest velocities along the no‐flow boundaries of the fracture and the lowest velocities along the centerline of the fracture. Depth‐averaged simulations using a recently developed model of the rheology of concentrated suspensions of monodisperse solids in Newtonian carrier fluids reproduced experimental observations of pressure gradient versus flow rate. Results from additional simulations suggest that small (3%) variations in solid concentration within the fracture can lead to significant (factor of two) velocity variations within the fracture yet negligible changes in observed pressure gradients. Furthermore, the variations in solid concentration persist over the length of the fracture, suggesting that such heterogeneities may play a significant role in the transport of concentrated suspensions. Our results suggest that a simple fracture‐averaged conductivity does not adequately represent the transport of suspended solids through fractures, which has direct implications for subsurface suspension flows where small concentration variations are likely. Experimental observations of flow through a parallel‐plate fracture of solids (50% by volume) suspended in a non‐Newtonian fluid demonstrate strongly heterogeneous velocity fields. We observed the highest velocities along the no‐flow boundaries (top and bottom). Simulations using a rheological model for concentrated suspensions suggest that factor‐of‐two velocity variations were caused by small variations (~3%) in solid concentration. These results have significant implications for the transport of suspended sediments in natural settings where small variations in solid concentrations are likely.
Fluid circulation in the Earth's crust plays an essential role in surface, near surface, and deep crustal processes. Flow pathways are driven by hydraulic gradients but controlled by material permeability, which varies over many orders of magnitude and changes over time. Although millions of measurements of crustal properties have been made, including geophysical imaging and borehole tests, this vast amount of data and information has not been integrated into a comprehensive knowledge system. A community data infrastructure is needed to improve data access, enable large‐scale synthetic analyses, and support representations of the subsurface in Earth system models. Here, we describe the motivation, vision, challenges, and an action plan for a community‐governed, four‐dimensional data system of the Earth's crustal structure, composition, and material properties from the surface down to the brittle–ductile transition. Such a system must not only be sufficiently flexible to support inquiries in many different domains of Earth science, but it must also be focused on characterizing the physical crustal properties of permeability and porosity, which have not yet been synthesized at a large scale. The DigitalCrust is envisioned as an interactive virtual exploration laboratory where models can be calibrated with empirical data and alternative hypotheses can be tested at a range of spatial scales. It must also support a community process for compiling and harmonizing models into regional syntheses of crustal properties. Sustained peer review from multiple disciplines will allow constant refinement in the ability of the system to inform science questions and societal challenges and to function as a dynamic library of our knowledge of Earth's crust. We describe the motivation, vision, challenges and an action plan for a community‐governed, four‐dimensional data system of the Earth's crustal structure, composition and material properties from the surface down to the brittle‐ductile transition.
CO 2 injected into rock formations for deep geological storage must not leak to surface, since this would be economically and environmentally unfavourable, and could present a human health hazard. In I taly natural CO 2 degassing to the surface via seeps is widespread, providing an insight into the various styles of subsurface ‘plumbing’ as well as surface expression of CO 2 fluids. Here we investigate surface controls on the distribution of CO 2 seep characteristics (type, flux and temperature) using a large geographical and historical data set. When the locations of documented seeps are compared to a synthetic statistically random data set, we find that the nature of the CO 2 seeps is most strongly governed by the flow properties of the outcropping rocks, and local topography. Where low‐permeability rocks outcrop, numerous dry seeps occur and have a range of fluxes. Aqueous fluid flow will be limited in these low‐permeability rocks, and so relative permeability effects may enable preferential CO 2 flow. CO 2 vents typically occur along faults in rocks that are located above the water table or are low permeability. Diffuse seeps develop where CO 2 (laterally supplied by these faults) emerges from the vadose zone and where CO 2 degassing from groundwater follows a different flow path due to flow differences for water and CO 2 gas. Bubbling water seeps (characterized by water bubbling with CO 2 ) arise where CO 2 supply enters the phreatic zone or an aquifer. CO 2 ‐rich springs often emerge where valleys erode into CO 2 aquifers, and these are typically high flux seeps. Seep type is known to influence human health risk at CO 2 seeps in Italy, as well as the topography surrounding the seep which affects the rate of gas dispersion by wind. Identifying the physical controls on potential seep locations and seep type above engineered CO 2 storage operations is therefore crucial to targeted site monitoring strategy and risk assessment. The surface geology and topography above a CO 2 store must therefore be characterized in order to design the most effective monitoring strategy.
The pollen C-14 age and oxygen isotopic composition of siliceous sinter deposits from the former Beowawe geyser field reveal evidence of two hydrothermal discharge events that followed relatively low-magnitude (10(-11)m(2)) following each earthquake. However, the timescale for onset of thermal convection implied by an overturned temperature profile in a geothermal well 300m from the Malpais fault is much shorter: 200-1000years. We speculate that individual segments of the Malpais fault become clogged on shorter timescales and that upward flow of groundwater subsequently follows new routes to the surface.
Understanding the effect of changing stress conditions on multiphase flow in porous media is of fundamental importance for many subsurface activities including enhanced oil recovery, water drawdown from aquifers, soil confinement, and geologic carbon storage. Geomechanical properties of complex porous systems are dynamically linked to flow conditions, but their feedback relationship is often oversimplified due to the difficulty of representing pore‐scale stress deformation and multiphase flow characteristics in high fidelity. In this work, we performed pore‐scale experiments of single‐ and multiphase flow through bead packs at different confining pressure conditions to elucidate compaction‐dependent characteristics of granular packs and their impact on fluid flow. A series of drainage and imbibition cycles were conducted on a water‐wet, soda‐lime glass bead pack under varying confining stress conditions. Simultaneously, X‐ray micro‐ CT was used to visualize and quantify the degree of deformation and fluid distribution corresponding with each stress condition and injection cycle. Micro‐ CT images were segmented using a gradient‐based method to identify fluids (e.g., oil and water), and solid phase redistribution throughout the different experimental stages. Changes in porosity, tortuosity, and specific surface area were quantified as a function of applied confining pressure. Results demonstrate varying degrees of sensitivity of these properties to confining pressure, which suggests that caution must be taken when considering scalability of these properties for practical modeling purposes. Changes in capillary number with confining pressure are attributed to the increase in pore velocity as a result of pore contraction. However, this increase in pore velocity was found to have a marginal impact on average phase trapping at different confining pressures.
The deep levels of former black smoker hydrothermal systems are widespread in the T roodos ophiolite in C yprus. They are marked by zones of hydrothermal reaction in the sheeted dyke unit close to the underlying gabbros. These zones are characterized by the presence of epidosite (epidote–quartz rock). In the reaction zones, the dykes are altered to a range of greenschist facies mineral assemblages, from a low degree of alteration with a five to seven phase metabasaltic assemblage to a high degree of alteration with a two to three phase epidosite assemblage. Individual dykes may contain the full range, with the epidosites forming yellow–green stripes within a darker background, often extending for more than several metres, parallel to the dyke margins. Field relations show that the alteration took place on a dyke‐by‐dyke basis and was not a regional process. SEM petrography reveals that the epidosites contain millimetre scale pores. The minerals surrounding the pores show euhedral overgrowths into the free pore space, indicating a former transient porosity of up to 20%. We conclude that the epidosites formed by reaction between newly intruded basaltic dykes and actively circulating black smoker fluid leading to extensive dissolution of primary dyke minerals. This reaction generated the porosity in the stripes and transiently led to a much increased permeability, allowing the rapid penetration of the black smoker fluid into the dykes and flow along them in fingers. As the system evolved, the same flow regime allowed mineral precipitation and partial infilling of the porosity. This mechanism allows rapid recrystallization of the rock with release of metals and other components into the fluid. This explains the depletion of these components in epidosites and their enrichment in black smoker vent fluids and the relatively constant composition of vent fluids as fresh rock is continually mined. Stripes of high permeability in T roodos sheeted dykes arose from dissolution by black smoker fluid shortly after intrusion, within a few hundred years. SEM image 0.3 mm wide shows one pore.