Carbon dioxide controlled earthquake distribution pattern in the NW Bohemian swarm earthquake region,western Eger Rift,Czech Republic – gas migration in the crystalline basement

The ascent of magmatic carbon dioxide in the western Eger (Oh?e) Rift is interlinked with the fault systems of the Variscian basement. In the Cheb Basin, the minimum CO₂ flux is about 160 m³ h^?1, with a diminishing trend towards the north and ceasing in the main epicentral area of the Northwest Bohemian swarm earthquakes. The ascending CO₂ forms Ca‐Mg‐HCO₃ type waters by leaching of cations from the fault planes and creates clay minerals, such as kaolinite, as alteration products on affected fault planes. These mineral reactions result in fault weakness and in hydraulically interconnected fault network. This leads to a decrease in the friction coefficient of the Coulomb failure stress (CFS) and to fault creep as stress build‐up cannot occur in the weak segments. At the transition zone in the north of the Cheb Basin, between areas of weak, fluid conductive faults and areas of locked faults with frictional strength, fluid pressure can increase resulting in stress build‐up. This can trigger strike‐slip swarm earthquakes. Fault creep or movements in weak segments may support a stress build‐up in the transition area by transmitting fluid pressure pulses. Additionally to fluid‐driven triggering models, it is important to consider that fluids ascending along faults are CO₂‐supersaturated thus intensifying the effect of fluid flow. The enforced flow of CO₂‐supersaturated fluids in the transitional zone from high to low permeability segments through narrowings triggers gas exsolution and may generate pressure fluctuations. Phase separation starts according to the phase behaviour of CO₂‐H₂O systems in the seismically active depths of NW Bohemia and may explain the vertical distribution of the seismicity. Changes in the size of the fluid transport channels in the fault systems caused, or superimposed, by fault movements, can produce fluid pressure increases or pulses, which are the precondition for triggering fluid‐induced swarm earthquakes.     

Surface controls on the characteristics of natural CO2 seeps: implications for engineered CO2 stores

CO₂ 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 Italy natural CO₂ degassing to the surface via seeps is widespread, providing an insight into the various styles of subsurface ‘plumbing’ as well as surface expression of CO₂ fluids. Here we investigate surface controls on the distribution of CO₂ 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 CO₂ 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 CO₂ flow. CO₂ vents typically occur along faults in rocks that are located above the water table or are low permeability. Diffuse seeps develop where CO₂ (laterally supplied by these faults) emerges from the vadose zone and where CO₂ degassing from groundwater follows a different flow path due to flow differences for water and CO₂ gas. Bubbling water seeps (characterized by water bubbling with CO₂) arise where CO₂ supply enters the phreatic zone or an aquifer. CO₂‐rich springs often emerge where valleys erode into CO₂ aquifers, and these are typically high flux seeps. Seep type is known to influence human health risk at CO₂ 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 CO₂ storage operations is therefore crucial to targeted site monitoring strategy and risk assessment. The surface geology and topography above a CO₂ store must therefore be characterized in order to design the most effective monitoring strategy.     

Geodynamic processes in the NW Bohemian swarm earthquake region,Czech Republic,identified by continuous gas monitoring

The time series of two continuously operating gas monitoring stations at Oldřišská and Nový Kostel located along seismoactive faults in the epicentral area of the NW Bohemian swarm earthquakes (Czech Republic) are compared with water level fluctuations in two boreholes positioned along these faults and with gas flux variations of a mofette at the Soos mofette field at 9 km distance. The seasonal trend of the monitored CO₂ concentration with a maximum in November and a minimum in March/April is governed by groundwater temperatures, superimposed in spring by soil temperatures. CO₂ concentration variations identified at Oldřišská are also reflected in gas flux variations in the Soos mofette and/or water level fluctuations of two boreholes. Variations in the gas monitoring recordings of station at Nový Kostel are also linked with variations at Oldřišská. In all data sets, diurnal variations generated by earth tides occur, reflecting a daily stress – fault permeability cycle. Additional stress interferes with this cycle. Significant, abrupt changes are attributed to geodynamic processes linked with seismic events, as revealed by local seismicity or by the transient of waves of a strong remote earthquake. Simultaneous variations of the gas concentrations in the Nový Kostel area and in the gas flux in the Soos point to an interconnected hydraulic conductive fault systems present in the northern part of the Cheb Basin. Sharp falls in gas concentration, during or subsequent to, earthquake swarms may reflect fault compression associated with impeded gas migration. However, gas variations also occur in periods without seismic activity, indicating changes in fault permeability were caused by local aseismic fault movements, as revealed by events with opposite trends in the gas recordings at Oldřišská, Nový Kostel and the Soos. Therefore, a mathematical approach to establish a correlation between seismicity and gas geochemical variations is not possible.     

Stress‐induced temperature variations in groundwater of the Monferrato area (north‐western Italy)

Although characterized by low seismicity, the Monferrato area of north‐western Italy was affected by earthquakes, of magnitude M5.1 and M4.8, in 2000 and 2001. At the same time, marked changes were recorded in water temperature and chemistry in several wells within the epicentral area. In May 2004, an automatic network for the continuous monitoring of groundwater was installed in selected wells to study the phenomenon. Here, we report on data collected during a 3‐year period of groundwater monitoring. During the first year, episodes of water heating (by up to 20°C) were observed in one monitored well. The temporal analysis of the seismic activity recorded in the area revealed as almost all seismic events occurred during the period of elevated water temperatures. The similar timing of earthquakes and groundwater‐temperature anomalies suggests that both may be triggered by the same processes acting in the crust.     

Analysis of strain‐induced ground‐water fluctuations at Devils Hole,Nevada

The pool in Devils Hole is a sensitive indicator of crustal strain and fluctuates in response to changes in atmospheric pressure, earth tides, earthquakes, large‐scale tectonic activity and ground‐water development. Short‐term and cyclic water‐level fluctuations caused by atmospheric pressure and earth tides were found to be on the order of millimeters to centimeters. The 1992 Landers/Little Skull Mountain earthquake sequence and the 1999 Hector Mine earthquake induced water‐level offsets of greater than ?12 and ?3.6 cm, respectively. The results of a dislocation model used to compute volumetric strain for each earthquake indicates that the coseismic water‐level offsets are consistent in magnitude and sense with poroelastic responses to earthquake‐induced strain. Theoretical postseismic fluid‐flow modeling indicates that the diffusivity of the system is on the order of 0.03 m² sec^?1, and identified areas of anomalous water‐level fluctuations. Interpretation of model results suggests that while the persistent post‐Landers rise in water‐level can be attributed to deformation‐induced channeling of fluid to the Devils Hole fault zone, the cause of the pre‐Hector Mine water‐level rise may be related to postseismic excess fluid pressures or preseismic strain accumulation.     

Simulation of the impact of faults on CO2 injection into sandstone reservoirs

Numerical simulations of multiphase CO₂ behavior within faulted sandstone reservoirs examine the impact of fractures and faults on CO₂ migration in potential subsurface injection systems. In southeastern Utah, some natural CO₂ reservoirs are breached and CO₂‐charged water flows to the surface along permeable damage zones adjacent to faults; in other sites, faulted sandstones form barriers to flow and large CO₂‐filled reservoirs result. These end‐members serve as the guides for our modeling, both at sites where nature offers ‘successful’ storage and at sites where leakage has occurred. We consider two end‐member fault types: low‐permeability faults dominated by deformation‐band networks and high‐permeability faults dominated by fracture networks in damage zones adjacent to clay‐rich gouge. Equivalent permeability (k) values for the fault zones can range from <10^?14 m² for deformation‐band‐dominated faults to >10^?12 m² for fracture‐dominated faults regardless of the permeability of unfaulted sandstone. Water–CO₂ fluid‐flow simulations model the injection of CO₂ into high‐k sandstone (5 × 10^?13 m²) with low‐k (5 × 10^?17 m²) or high‐k (5 × 10^?12 m²) fault zones that correspond to deformation‐band‐ or fracture‐dominated faults, respectively. After 500 days, CO₂ rises to produce an inverted cone of free and dissolved CO₂ that spreads laterally away from the injection well. Free CO₂ fills no more than 41% of the pore space behind the advancing CO₂ front, where dissolved CO₂ is at or near geochemical saturation. The low‐k fault zone exerts the greatest impact on the shape of the advancing CO₂ front and restricts the bulk of the dissolved and free CO₂ to the region upstream of the fault barrier. In the high‐k aquifer, the high‐k fault zone exerts a small influence on the shape of the advancing CO₂ front. We also model stacked reservoir seal pairs, and the fracture‐dominated fault acts as a vertical bypass, allowing upward movement of CO₂ into overlying strata. High‐permeability fault zones are important pathways for CO₂ to bypass unfaulted sandstone, which leads to reduce sequestration efficiency. Aquifer compartmentalization by low‐permeability fault barriers leads to improved storativity because the barriers restrict lateral CO₂ migration and maximize the volume and pressure of CO₂ that might be emplaced in each fault‐bound compartment. As much as a 3.5‐MPa pressure increase may develop in the injected reservoir in this model domain, which under certain conditions may lead to pressures close to the fracture pressure of the top seal.     

Injection‐induced seismicity in Carbon and Emery Counties,central Utah

Utah is one of the top producers of oil and natural gas in the United States. Over the past 18 years, more than 4.2 billion gallons of wastewater from the petroleum industry has been injected into the Navajo Sandstone, Kayenta Formation, and Wingate Sandstone in Carbon and Emery Counties, central Utah, where seismicity has increased during the same period. Previous studies have attributed this seismicity to coal mining. Here, we present evidence for wastewater injection being a major cause of the increased seismicity. We show that, in the coal mining area, seismicity rate increased significantly 1–5 years following the wastewater injection, and the earthquakes, mostly with magnitudes <3.0, are concentrated in areas seismically active prior to the injection. Using simple analytical and numerical models, we show that the injection in central Utah can sufficiently raise pore pressure to trigger seismicity within 10–20 km of the injection wells, and the time needed for the diffusion of pore pressure may explain the observed lag of seismicity increase behind the commencement of injection. The b‐value of these earthquakes increased following the wastewater injection, which is consistent with these events being injection‐induced. We conclude that the marked increase in seismicity rate in central Utah is induced by both mining activity and wastewater injection, which raised pore pressure along preexisting faults.     

Gas breakthrough experiments on pelitic rocks: comparative study with N2, CO2 and CH4

The capillary‐sealing efficiency of intermediate‐ to low‐permeable sedimentary rocks has been investigated by N₂, CO₂ and CH₄ breakthrough experiments on initially fully water‐saturated rocks of different lithological compositions. Differential gas pressures up to 20 MPa were imposed across samples of 10–20 mm thickness, and the decline of the differential pressures was monitored over time. Absolute (single‐phase) permeability coefficients (k_abs), determined by steady‐state fluid flow tests, ranged between 10^?22 and 10^?15 m². Maximum effective permeabilities to the gas phase k_eff(max), measured after gas breakthrough at maximum gas saturation, extended from 10^?26 to 10^?18 m². Because of re‐imbibition of water into the interconnected gas‐conducting pore system, the effective permeability to the gas phase decreases with decreasing differential (capillary) pressure. At the end of the breakthrough experiments, a residual pressure difference persists, indicating the shut‐off of the gas‐conducting pore system. These pressures, referred to as the ‘minimum capillary displacement pressures’ (P_d), ranged from 0.1 up to 6.7 MPa. Correlations were established between (i) absolute and effective permeability coefficients and (ii) effective or absolute permeability and capillary displacement pressure. Results indicate systematic differences in gas breakthrough behaviour of N₂, CO₂ and CH₄, reflecting differences in wettability and interfacial tension. Additionally, a simple dynamic model for gas leakage through a capillary seal is presented, taking into account the variation of effective permeability as a function of buoyancy pressure exerted by a gas column underneath the seal.     

Numerical model of pore‐pressure diffusion associated with the initiation of the 2010–2011 Guy–Greenbrier,Arkansas earthquakes

We model pore‐pressure diffusion caused by pressurized waste‐fluid injection at two nearby wells and then compare the buildup of pressure with the observed initiation and migration of earthquakes during the early part of the 2010–2011 Guy–Greenbrier earthquake swarm. Pore‐pressure diffusion is calculated using MODFLOW 2005 that allows the actual injection histories (volume/day) at the two wells to diffuse through a fractured and faulted 3D aquifer system representing the eastern Arkoma basin. The aquifer system is calibrated using the observed water‐level recovery following well shut‐in at three wells. We estimate that the hydraulic conductivities of the Boone Formation and Arbuckle Group are 2.2 × 10^?2 and 2.03 × 10^?3 m day^?1, respectively, with a hydraulic conductivity of 1.92 × 10^?2 m day^?1 in the Hunton Group when considering 1.72 × 10^?3 m day^?1 in the Chattanooga Shale. Based on the simulated pressure field, injection near the relatively conductive Enders and Guy–Greenbrier faults (that hydraulically connect the Arbuckle Group with the underlying basement) permits pressure diffusion into the crystalline basement, but the effective radius of influence is limited in depth by the vertical anisotropy of the hydraulic diffusivity. Comparing spatial/temporal changes in the simulated pore‐pressure field to the observed seismicity suggests that minimum pore‐pressure changes of approximately 0.009 and 0.035 MPa are sufficient to initiate seismic activity within the basement and sedimentary sections of the Guy–Greenbrier fault, respectively. Further, the migration of a second front of seismicity appears to follow the approximately 0.012 MPa and 0.055 MPa pore‐pressure fronts within the basement and sedimentary sections, respectively.     

The upper continental crust, an aquifer and its fluid: hydaulic and chemical data from 4 km depth in fractured crystalline basement rocks at the KTB test site

Detailed information on the hydrogeologic and hydraulic properties of the deeper parts of the upper continental crust is scarce. The pilot hole of the deep research drillhole (KTB) in crystalline basement of central Germany provided access to the crust for an exceptional pumping experiment of 1‐year duration. The hydraulic properties of fractured crystalline rocks at 4 km depth were derived from the well test and a total of 23100 m³ of saline fluid was pumped from the crustal reservoir. The experiment shows that the water‐saturated fracture pore space of the brittle upper crust is highly connected, hence, the continental upper crust is an aquifer. The pressure–time data from the well tests showed three distinct flow periods: the first period relates to wellbore storage and skin effects, the second flow period shows the typical characteristics of the homogeneous isotropic basement rock aquifer and the third flow period relates to the influence of a distant hydraulic border, probably an effect of the Franconian lineament, a steep dipping major thrust fault known from surface geology. The data analysis provided a transmissivity of the pumped aquifer T = 6.1 × 10^?6 m² sec^?1, the corresponding hydraulic conductivity (permeability) is K = 4.07 × 10^?8 m sec^?1 and the computed storage coefficient (storativity) of the aquifer of about S = 5 × 10^?6. This unexpected high permeability of the continental upper crust is well within the conditions of possible advective flow. The average flow porosity of the fractured basement aquifer is 0.6–0.7% and this range can be taken as a representative and characteristic values for the continental upper crust in general. The chemical composition of the pumped fluid was nearly constant during the 1‐year test. The total of dissolved solids amounts to 62 g l^?1 and comprise mainly a mixture of CaCl₂ and NaCl; all other dissolved components amount to about 2 g l^?1. The cation proportions of the fluid (X_Ca approximately 0.6) reflects the mineralogical composition of the reservoir rock and the high salinity results from desiccation (H₂O‐loss) due to the formation of abundant hydrate minerals during water–rock interaction. The constant fluid composition suggests that the fluid has been pumped from a rather homogeneous reservoir lithology dominated by metagabbros and amphibolites containing abundant Ca‐rich plagioclase.     

Gas breakthrough pressure for hydrocarbon reservoir seal rocks: implications for the security of long-term CO2 storage in the Weyburn field

This paper reports a laboratory study of the gas breakthrough pressure for different gas/liquid systems in the Mississippian‐age Midale Evaporite. This low‐permeability rock formation is the seal rock for the Weyburn Field in southeastern Saskatchewan, Canada, where CO₂ is being injected into an oil reservoir for enhanced recovery and CO₂ storage. A technique for experimentally determining CO₂ breakthrough pressure at reservoir conditions is presented. Breakthrough pressures for N₂, CO₂ and CH₄ were measured with the selected seal‐rock samples. The maximum breakthrough pressure is over 30 MPa for N₂ and approximately 21 MPa for CO₂. The experimental results demonstrate that the Weyburn Midale Evaporite seal rock is of high sealing quality. Therefore, the Weyburn reservoir and Midale Beds can be used as a CO₂ storage site after abandonment. The measured results also show that the breakthrough pressure of a seal rock for a gas is nearly proportional to the interfacial tension of the gas/brine system. The breakthrough pressure of a CO₂/brine system is significantly reduced compared with that of a CH₄/brine system because of the much lower interfacial tension of the former. This implies that a seal rock that seals the original gas in a gas reservoir or an oil reservoir with a gas cap may not be tight enough to seal the injected CO₂ if the pressure during or after CO₂ injection is the same or higher than the original reservoir pressure. Therefore, reevaluation of the breakthrough pressure of seal rocks for a given reservoir is necessary and of highest priority once it is chosen as a CO₂ storage site.     

Tectonic control over large-scale diffuse degassing in eastern Sicily (Italy)

Eastern Sicily (southern Italy) is characterised by the presence of many natural gas emissions (mofettes, mud volcanoes). These gases are mostly carbon dioxide and methane, with minor amounts of helium, hydrogen, carbon monoxide and hydrocarbons. In this study, the extent and orientation of soil gas anomalies (He and CO₂) were investigated on a wide area (approximately 110 km²) located just SW of Mt. Etna. From a structural point of view, this area lays on a typical foredeep–foreland system that marks the boundary between the southern part of the Eurasian plate and the northern part of the African plate in the central Mediterranean. No tectonic structure was revealed in this area by surface geological surveys. Very high soil emissions were found, and their spatial pattern reveals the existence of some active faults all directed about N50°E. This direction coincides with that of two major fault systems that cut eastern Sicily and are evident, respectively, NE and SW of the study area. Soil gas data suggest that these fault systems are the expression of a single continuous structural line which is probably responsible for the past and present magma uprise in eastern Sicily. Isotopic values of carbon of CO₂ suggest a minor contribution of organic carbon. Moreover, in the highest degassing sites the isotopic values of He found in association with CO₂ (He abundance = 11–70 p.p.m.; R/R_a between 6.0 and 6.2) suggest that both gases are mantle derived. The extent of the areas affected by high gas emissions and the amounts of deep CO₂ emitted in the investigated area (several hundred tonnes per day) may provide additional supporting evidence of a mantle upwelling taking place beneath this region.     

Large‐scale chemical stratification of fluids in the crust: hydraulic and chemical data from the geothermal research site Urach,Germany

The Urach 3 research borehole in SW Germany has been drilled through a sedimentary cover sequence and reached gneisses of the Variscan crystalline basement at 1604 m below surface. An additional 2840 m has been drilled through fractured basement rocks. The borehole has been used for hydraulic tests in the context of a ‘hot dry rock’ (HDR) project. The sedimentary cover ranges from the Carboniferous to the Middle Jurassic (Dogger) in age and comprises mostly clastic sediments in the Paleozoic and limestone and shale in the Mesozoic. Water composition data from 10 different depths include samples from all major lithological units. The total dissolved solids (TDS) increases from the surface to about 650 m where it reaches 4.1 g l^?1 in Triassic limestone. In lower Triassic sandstones, TDS increases very sharply to 28.5 g l^?1 and the water is saturated with pure CO₂ gas. With increasing depth, TDS does not change much in the clastic sediments of the Permian and Carboniferous. The crystalline basement is marked by a very sharp increase in TDS to 55.5 g l^?1 at about 1770 m depth. TDS increases within the basement to more than 78.5 g l^?1 at about 3500 m depth. The data suggest that there is limited vertical chemical communication over long periods of time. The CO₂ gas cap in the lower Triassic sandstones requires a gastight cover. The chemical stratification of the fluids relates to the permeability structure of the crust at the Urach site and fits well with hydraulic and thermal data from the site.     

The effect of deformation on permeability development in anhydrite and implications for caprock integrity during geological storage of CO2

Geological storage of CO₂ in depleted oil and gas reservoirs is one of the most promising options to reduce atmospheric CO₂ concentrations. Of great importance to CO₂ mitigation strategies is maintaining caprock integrity. Worldwide many current injection sites and potential storage sites are overlain by anhydrite‐bearing seal formations. However, little is known about the magnitude of the permeability change accompanying dilatation and failure of anhydrite under reservoir conditions. To this extent, we have performed triaxial compression experiments together with argon gas permeability measurements on Zechstein anhydrite, which caps many potential CO₂ storage sites in the Netherlands. Our experiments were performed at room temperature at confining pressures of 3.5–25 MPa. We observed a transition from brittle to semi‐brittle behaviour over the experimental range, and peak strength could be described by a Mogi‐type failure envelope. Dynamic permeability measurements showed a change from ‘impermeable’ (<10^?21 m²) to permeable (10^?16 to 10^?19 m²) as a result of mechanical damage. The onset of measurable permeability was associated with an increase in the rate of dilatation at low pressures (3.5–5 MPa), and with the turning point from compaction to dilatation in the volumetric versus axial strain curve at higher pressures (10–25 MPa). Sample permeability was largely controlled by the permeability of the shear faults developed. Static, postfailure permeability decreased with increasing effective mean stress. Our results demonstrated that caprock integrity will not be compromised by mechanical damage and permeability development. Geofluids (2010) 10 , 369–387     

Hydrothermal monitoring in a quiescent volcanic arc: Cascade Range,northwestern United States

Ongoing (1996–present) volcanic unrest near South Sister, Oregon, is accompanied by a striking set of hydrothermal anomalies, including elevated temperatures, elevated major ion concentrations, and ³He/⁴He ratios as large as 8.6 R_A in slightly thermal springs. These observations prompted the US Geological Survey to begin a systematic hydrothermal‐monitoring effort encompassing 25 sites and 10 of the highest‐risk volcanoes in the Cascade volcanic arc, from Mount Baker near the Canadian border to Lassen Peak in northern California. A concerted effort was made to develop hourly, multiyear records of temperature and/or hydrothermal solute flux, suitable for retrospective comparison with other continuous geophysical monitoring data. Targets included summit fumarole groups and springs/streams that show clear evidence of magmatic influence in the form of high ³He/⁴He ratios and/or anomalous fluxes of magmatic CO₂ or heat. As of 2009–2012, summit fumarole temperatures in the Cascade Range were generally near or below the local pure water boiling point; the maximum observed superheat was <2.5°C at Mount Baker. Variability in ground temperature records from the summit fumarole sites is temperature‐dependent, with the hottest sites tending to show less variability. Seasonal variability in the hydrothermal solute flux from magmatically influenced springs varied from essentially undetectable to a factor of 5–10. This range of observed behavior owes mainly to the local climate regime, with strongly snowmelt‐influenced springs and streams exhibiting more variability. As of the end of the 2012 field season, there had been 87 occurrences of local seismic energy densities approximately ≥ 0.001 J/m³ during periods of hourly record. Hydrothermal responses to these small seismic stimuli were generally undetectable or ambiguous. Evaluation of multiyear to multidecadal trends indicates that whereas the hydrothermal system at Mount St. Helens is still fast‐evolving in response to the 1980–present eruptive cycle, there is no clear evidence of ongoing long‐term trends in hydrothermal activity at other Cascade Range volcanoes that have been active or restless during the past century (Baker, South Sister, and Lassen). Experience gained during the Cascade Range hydrothermal‐monitoring experiment informs ongoing efforts to capture entire unrest cycles at more active but generally less accessible volcanoes such as those in the Aleutian arc.     

Upper crustal fluid flow in the outboard region of the Southern Alps,New Zealand

The currently active fluid regime within the outboard region of the Southern Alps, New Zealand was investigated using a combination of field observations, carbon‐ and oxygen‐stable isotopes from fault‐hosted calcites and interpretation of magnetotelluric (MT) data. Active faulting in the region is dominated by NE striking and N striking, oppositely dipping thrust fault pairs. Stable isotopic analyses of calcites hosted within these fault zones range from 10 to 25‰δ¹⁸O and from ?33 to 0‰δ¹³C. These values reflect mixing of three parent fluids: meteoric water, carbon‐exchanged groundwater and minor deeper rock‐exchanged fluids, at temperatures of 10–90°C in the upper 3–4 km of the crust. A broad, ‘U‐shaped’ zone of high electrical conductivity (maximum depth c. 28 km) underlies the central Southern Alps. In the ductile region of the crust, the high‐conductivity zone is subhorizontal. Near‐vertical zones of high conductivity extend upward to the surface on both sides of the conductive zone. On the outboard side of the orogen, the conductive zone reaches the surface coincident with the trace of the active Forest Creek Faults. Near‐surface flow is shown to dominate the outboard region. Topographically driven meteoric water interacts, on a kilometre scale, with either carbon‐exchanged groundwater or directly with organic material within Pliocene gravels, resulting in a distinctive low ¹³C signal within fault‐hosted calcites of the outboard region. The high‐strain zone in the lower crust focuses the migration of deeply sourced fluids upward to the base of the brittle–ductile transition. Interconnected fluid is imaged as a narrow vertical zone of high conductivity in the upper crust, implying continuous permeability and possibly buoyancy‐driven flow of deeply sourced fluids to higher levels of the crust where they are detected by the isotopic analysis of the fault‐hosted calcites.     

Exploring the potential linkages between oil‐field brine reinjection,crystalline basement permeability,and triggered seismicity for the Dagger Draw Oil field,southeastern New Mexico,USA, using hydrologic modeling

We used hydrologic models to explore the potential linkages between oil‐field brine reinjection and increases in earthquake frequency (up to M_d 3.26) in southeastern New Mexico and to assess different injection management scenarios aimed at reducing the risk of triggered seismicity. Our analysis focuses on saline water reinjection into the basal Ellenburger Group beneath the Dagger Draw Oil field, Permian Basin. Increased seismic frequency (>M_d 2) began in 2001, 5 years after peak injection, at an average depth of 11 km within the basement 15 km to the west of the reinjection wells. We considered several scenarios including assigning an effective or bulk permeability value to the crystalline basement, including a conductive fault zone surrounded by tighter crystalline basement rocks, and allowing permeability to decay with depth. We initially adopted a 7 m (0.07 MPa) head increase as the threshold for triggered seismicity. Only two scenarios produced excess heads of 7m five years after peak injection. In the first, a hydraulic diffusivity of 0.1 m² s^?1 was assigned to the crystalline basement. In the second, a hydraulic diffusivity of 0.3 m² s^?1 was assigned to a conductive fault zone. If we had considered a wider range of threshold excess heads to be between 1 and 60 m, then the range of acceptable hydraulic diffusivities would have increased (between 0.1–0.01 m² s^?1 and 1–0.1 m² s^?1 for the bulk and fault zone scenarios, respectively). A permeability–depth decay model would have also satisfied the 5‐year time lag criterion. We also tested several injection management scenarios including redistributing injection volumes between various wells and lowering the total volume of injected fluids. Scenarios that reduced computed excess heads by over 50% within the crystalline basement resulted from reducing the total volume of reinjected fluids by a factor of 2 or more.     

Density and isothermal compressibility of supercritical H2O–NaCl fluid: molecular dynamics study from 673 to 2000 K, 0.2 to 2 GPa,and 0 to 22 wt% NaCl concentrations

We report on molecular dynamics (MD) simulations for predicting the density and isothermal compressibility of an H₂O–NaCl fluid as a function of temperature (673–2000 K), pressure (0.2–2.0 GPa), and salt concentration (0.0–21.9 wt%). The atomistic behavior was analyzed via the hydration number of ions and number of ion pairs. Hydration numbers of Na⁺ and Cl^? increased with increasing pressure and decreasing temperature. Conversely, the fraction of Na–Cl ion pairs increased with decreasing pressure and increasing temperature. This hydration and association behavior is consistent with the low dielectric constant of H₂O under these conditions. The presence of polynuclear clusters of Na–Cl was confirmed at high temperatures, low pressures, and high salt concentrations. We propose a purely empirical equation of state (EoS) for H₂O–NaCl fluids under high temperatures and pressures that should be useful for estimating the fluid distribution in Earth's crust and upper mantle in relation to effects on earthquakes and volcanic eruptions.     

Is the ion composition of the outer ionosphere related to seismic activity?

In analyzing the ion composition data from the low-altitude Intercosmos 24 satellite, significant longitudinal deviations in the latitudinal density variation of light ions, H⁺ and He⁺, were observed in the inner plasmasphere at altitudes of 2000–2500 km. These deviations display surprising correlation with the seismic activity in the Earth's crust. H⁺ and He⁺ concentrations significantly enhanced with respect to values observed above seismically inactive regions were found above the epicenters of future earthquakes for up to two days before shocks. These findings indicate that, at least under some conditions, processes of the coupling between the solid Earth and the ionosphere may be indicated by in situ measurements of plasma parameters in the Earth environment.     

Bleached mudstone,iron concretions,and calcite veins: a natural analogue for the effects of reducing CO2‐bearing fluids on migration and mineralization of iron,sealing properties,and composition of mudstone cap rocks

This study investigates the Wangfu Depression of the Songliao Basin, China, as a natural analogue site for Fe migration (bleaching) and mineralization (formation of iron concretions) caused by reducing CO₂‐bearing fluids that leak along fractures after carbon capture, utilization, and storage. We also examined the origin of fracture‐filling calcite veins, the properties of self‐sealing fluids, the influence of fluids on the compositions of mudstone and established a bleaching model for the study area. Our results show that iron concretions are the oxidative products of precursor minerals (pyrite and siderite) during uplift and are linked to H₂S and CO₂ present in early stage fluids. The precipitation of calcite veins is the result of CO₂ degassing and is related to CO₂, CH₄, and minor heavy hydrocarbons in the main bleaching fluids. In our model, fluids preferentially enter high‐permeability fracture systems and result in the bleaching of surrounding rocks and precipitation of calcite veins. The infilling of calcite veins significantly decreases the permeability of fractures and forces the fluids to slowly enter and bleach the mudstone rocks. The Fe²⁺ released during bleaching migrates to elsewhere with the solutions or is reprecipitated in the calcite veins and iron concretions. The formation of calcite veins reduces the fracture space and effectively prevents fluid flow. The fluids have an insignificant effect on minerals within the mudstone. In terms of the chemistry of the mudstone, only the contents of Fe₂O₃, U, and Mo change significantly, with the content of U increasing in the mudstone and the contents of Fe₂O₃ and Mo decreasing during bleaching.