Evolution of the transport properties of fractures subject to thermally and mechanically activated mineral alteration and redistribution

Strong feedbacks link temperature (T), hydrologic flow (H), mechanical deformation (M), and chemical alteration (C) in fractured rock. These processes are interconnected as one process affects the initiation and progress of another. Dissolution and precipitation of minerals are affected by temperature and stress, and can result in significant changes in permeability and solute transport characteristics. Understanding these couplings is important for oil, gas, and geothermal reservoir engineering, for CO2 sequestration, and for waste disposal in underground repositories and reservoirs. To experimentally investigate the interactions between THMC processes in a naturally stressed fracture, we report on heated (25°C up to 150°C) flow‐through experiments on fractured core samples of Westerly granite. These experiments examine the influence of thermally and mechanically activated dissolution of minerals on the mechanical (stress/strain) and transport (permeability) responses of fractures. The evolutions of the permeability and relative hydraulic aperture of the fracture are recorded as thermal and stress conditions' change during the experiments. Furthermore, the efflux of dissolved mineral mass is measured periodically and provides a record of the net mass removal, which is correlated with observed changes in relative hydraulic fracture aperture. During the experiments, a significant variation of the effluent fluid chemistry is observed and the fracture shows large changes in permeability to the changing conditions both in stress and in temperature. We argue that at low temperature and high stresses, mechanical crushing of the asperities and the production of gouge explain the permeability decrease although most of the permeability is recoverable as the stress is released. While at high temperature, the permeability changes are governed by mechanical deformation as well as chemical processes, in particular, we infer dissolution of minerals adjacent to the fracture and precipitation of kaolinite.     

Stable–unstable flow of geothermal fluids in fractured rock

Density-driven geothermal flow in 3-D fractured rock is investigated and compared with density-driven haline flow. For typical matrix and fracture hydraulic conductivities, haline flow tends to be unstable (convecting) while geothermal flow is stable (non-convecting). Thermal diffusivity is generally three orders of magnitude larger than haline diffusivity and, as a result, large heat conduction diminishes growth of geothermal instabilities while low mass diffusion enables formation of unstable haline 'fingering' within fractures. A series of thermal flow simulations is presented to identify stable and unstable conditions for a wide range of hydraulic conductivities for matrix and fractures. The classic Rayleigh stability criterion can be applied to classify these simulations when fracture aperture is very small. However, the Rayleigh criterion is not applicable when the porous matrix hydraulic conductivity is very small, because stabilizing fracture–matrix heat conduction is independent of matrix hydraulic conductivity. In that case, the numerically estimated critical fracture conductivity is nine orders of magnitude larger than the theoretically calculated critical fracture conductivity based on Rayleigh theory. The numerical stability analysis presented here may be used as a guideline to predict if a geothermal system in 3-D fractured rock is stable or unstable.     

3D numerical modeling of hydrothermal processes during the lifetime of a deep geothermal reservoir

Understanding hydrothermal processes during production is critical to optimal geothermal reservoir management and sustainable utilization. This study addresses the hydrothermal (HT) processes in a geothermal research doublet consisting of the injection well E GrSk3/90 and production well Gt GrSk4/05 at the deep geothermal reservoir of Groß Schönebeck (north of Berlin, Germany) during geothermal power production. The reservoir is located between ?4050 to ?4250 m depth in the Lower Permian of the Northeast German Basin. Operational activities such as hydraulic stimulation, production (T = 150°C; Q = ?75 m³ h^?1; C = 265 g l^?1) and injection (T = 70°C; Q = 75 m³ h^?1; C = 265 g l^?1) change the HT conditions of the geothermal reservoir. The most significant changes affect temperature, mass concentration and pore pressure. These changes influence fluid density and viscosity as well as rock properties such as porosity, permeability, thermal conductivity and heat capacity. In addition, the geometry and hydraulic properties of hydraulically induced fractures vary during the lifetime of the reservoir. A three‐dimensional reservoir model was developed based on a structural geological model to simulate and understand the complex interaction of such processes. This model includes a full HT coupling of various petrophysical parameters. Specifically, temperature‐dependent thermal conductivity and heat capacity as well as the pressure‐, temperature‐ and mass concentration‐dependent fluid density and viscosity are considered. These parameters were determined by laboratory and field experiments. The effective pressure dependence of matrix permeability is less than 2.3% at our reservoir conditions and therefore can be neglected. The results of a three‐dimensional thermohaline finite‐element simulation of the life cycle performance of this geothermal well doublet indicate the beginning of thermal breakthrough after 3.6 years of utilization. This result is crucial for optimizing reservoir management. Geofluids (2010) 10 , 406–421     

Long‐term stability of fracture systems and their behaviour as flow paths in uplifting granitic rocks from the Japanese orogenic field

In granitic rocks, fracture networks typically provide pathways for groundwater flow and solute transport that need to be understood to assess the long‐term performance of deep underground storage or disposal facilities such as radioactive waste repositories. However, relatively little is known about the long‐term processes of fracturing and/or the longevity of flow paths (FP) in granitic rocks distributed within orogenic belts. To clarify these issues, Japanese plutons of different ages and in situ fractures in granite at the Mizunami Underground Research Laboratory (MIU) located in central Japan were studied. Detailed structural characterization and geochemical analysis of in situ fracture fillings sampled from a depth of 300 m were carried out to clarify the relationship between fracturing and mineral infilling processes. Different plutons show identical episodes of fracturing and fracture filling, consisting of: brittle tensile fracturing, due to decreasing temperature through the ductile–brittle transition after plutonic intrusion (Stage I); relatively rapid uplifting (ca. a few mm/year) accompanied by hydrothermal water circulation, which produced uncrushed layered mineral fillings (Stage II); and a period of low‐temperature meteoric water circulation following exposure after uplift (Stage III). The parageneses of carbonate mineral fracture fillings and their carbon isotopic compositions (¹⁴C, δ¹³C) show that there were distinct episodes of carbonate mineral precipitation during the rapid uplifting of a pluton. The carbonate minerals that formed during each episode incorporated carbon from a distinct source. The evolution of fillings identified here enables development of a specific model of fracturing and persistence of fluid‐conducting systems in the plutons of the orogenic field.     

Modeling fluid flow in a heterogeneous, fault-controlled hydrothermal system

Previous studies have shown that most hydrothermal systems discharging at the land surface are associated with faulting, and that the location, temperature and rate of discharge of these systems are controlled by the geometry and style of the controlling fault(s). Unfortunately, the transport of heat and fluid in fault-controlled hydrothermal systems is difficult to model realistically; although heterogeneity and anisotropy are assumed to place important controls on flow in faults, few data or observations are available to constrain the distribution of hydraulic properties within active faults. Here, analytical and numerical models are combined with geostatistical models of spatially varying hydraulic properties to model the flow of heat and fluid in the Borax Lake fault of south-east Oregon, USA. A geometric mean permeability within the fault of 7 × 10⁻¹⁴ m² with 2× vertical/horizontal anisotropy in correlation length scale is shown to give the closest match to field observations. Furthermore, the simulations demonstrate that continuity of flow paths is an important factor in reproducing the observed behavior. In addition to providing some insight into possible spatial distributions of hydraulic properties at the Borax Lake site, the study highlights one potential avenue for integrating field observations with simulation results in order to gain greater understanding of fluid flow in faults and fault-controlled hydrothermal and petroleum reservoirs.     

Mechanisms of fluid flow and fluid–rock interaction in fossil metamorphic hydrothermal systems inferred from vein–wallrock patterns, geometry and microstructure

Comparison of mass transfer patterns, geometry and microstructures developed within and around veins allows the interpretation of processes of fluid flow during deformation, metamorphism and mineralization. A classification of vein types based on the degree of interaction with wallrock (using petrological, geochemical or isotopic indicators) can be used to identify a range of processes, from closed system behaviour in which the vein mass is derived from local wallrock, through to open system behaviour in which the vein mass is derived externally. Microstructural characteristics, such as wallrock selvages, multiple growth events recorded by vein seams and vein crystal morphology, also help to constrain mass transfer patterns during vein formation. We present a range of processes for vein formation, including: (i) the formation of closed system fibrous veins by dissolution–precipitation creep, including varieties in which tensile failure is not required; (ii) pressure‐ or kinetically dependent closed system segregation veins in which transfer of soluble components from wallrock to vein leaves behind a residual selvage; (iii) similar vein–selvage patterning, but with mass imbalances between vein and wallrock requiring fluid advection through both interconnected fracture networks and in the surrounding permeable rock; and (iv) the proposed formation of veins by fluid ascent in mobile hydrofractures, in which isotopic or chemical disequilibrium within and around the vein suggests that the crack and fluid within it moved essentially as one. The postulate of rapid fluid and mass transfer via such mobile hydrofractures has implications for the release of volatiles from metamorphic terrains. Also, consideration of a broad range of possible vein‐forming mechanisms is highly desirable when dealing with mineral deposits found in deformed, metamorphosed rocks, because closed system veining may produce patterns that, if erroneously recognized as being open systems, could lead to false interpretations of the role of tectonic fracturing in ore genesis.     

Stress‐dependent transport properties of fractured arkosic sandstone

We measure the fluid transport properties of microfractures and macrofractures in low‐porosity polyphase sandstone and investigate the controls of in situ stress state on fluid flow conduits in fractured rock. For this study, the permeability and porosity of the Punchbowl Formation sandstone, a hydrothermally altered arkosic sandstone, were measured and mapped in stress space under intact, microfractured, and macrofractured deformation states. In contrast to crystalline and other sedimentary rocks, the distributed intragranular and grain‐boundary microfracturing that precedes macroscopic fracture formation has little effect on the fluid transport properties. The permeability and porosity of microfractured and intact sandstone depend strongly on mean stress and are relatively insensitive to differential stress and proximity to the frictional sliding envelope. Porosity variations occur by elastic pore closure with intergranular sliding and pore collapse caused by microfracturing along weakly cemented grain contacts. The macroscopic fractured samples are best described as a two‐component system consisting (i) a tabular fracture with a 0.5‐mm‐thick gouge zone bounded by 1 mm thick zones of concentrated transgranular and intragranular microfractures and (ii) damaged sandstone. Using bulk porosity and permeability measurements and finite element methods models, we show that the tabular fracture is at least two orders of magnitude more permeable than the host rock at mean stresses up to 90 MPa. Further, we show that the tabular fracture zone dilates as the stress state approaches the friction envelope resulting in up to a three order of magnitude increase in fracture permeability. These results indicate that the enhanced and stress‐sensitive permeability in fault damage zones and sedimentary basins composed of arkosic sandstones will be controlled by the distribution of macroscopic fractures rather than microfractures.     

Investigation of coupled hydraulic–geomechanical processes at the KTB site: pressure‐dependent characteristics of a long‐term pump test and elastic interpretation using a geomechanical facies model

The German Continental Deep Drilling Program comprising a pilot borehole down to 4000 m and a main borehole down to 9101 m in southeast Germany (KTB) is continuing to provide a unique opportunity for the identification of important factors and processes in deep‐seated fluid and energy transfer. In situ stress conditions significantly impact flow, transport and exchange characteristics of fracture networks that dominate the permeability of crystalline reservoir rocks. In this paper, several scales of information are combined to present a fully three‐dimensional hydraulic finite element model of the principal KTB fault zones, and linked to a geomechanical model describing the alteration of the hydraulic parameters with stress changes caused by fluid extraction. The concept of geomechanical facies is introduced to define and characterize architectural elements in the subsurface system. Evaluation of a long‐term pump test in the KTB pilot hole, June 2002–July 2003, coupled with a geomechanical model gives an insight into some of the elastic and nonelastic processes controlling hydraulic transport in the basement rocks. Trends in the decline of the permeability and the degree of storage in the system could only partially be explained by elastic processes, clearly indicating the importance of nonelastic processes. A number of inelastic processes are suggested as areas for further research.     

Salt tectonics and shallow subseafloor fluid convection: models of coupled fluid‐heat‐salt transport

Thermohaline convection associated with salt domes has the potential to drive significant fluid flow and mass and heat transport in continental margins, but previous studies of fluid flow associated with salt structures have focused on continental settings or deep flow systems of importance to petroleum exploration. Motivated by recent geophysical and geochemical observations that suggest a convective pattern to near‐seafloor pore fluid flow in the northern Gulf of Mexico (GoMex), we devise numerical models that fully couple thermal and chemical processes to quantify the effects of salt geometry and seafloor relief on fluid flow beneath the seafloor. Steady‐state models that ignore halite dissolution demonstrate that seafloor relief plays an important role in the evolution of shallow geothermal convection cells and that salt at depth can contribute a thermal component to this convection. The inclusion of faults causes significant, but highly localized, increases in flow rates at seafloor discharge zones. Transient models that include halite dissolution show the evolution of flow during brine formation from early salt‐driven convection to later geothermal convection, characteristics of which are controlled by the interplay of seafloor relief and salt geometry. Predicted flow rates are on the order of a few millimeters per year or less for homogeneous sediments with a permeability of 10^?15 m², comparable to compaction‐driven flow rates. Sediment permeabilities likely fall below 10^?15 m² at depth in the GoMex basin, but such thermohaline convection can drive pervasive mass transport across the seafloor, affecting sediment diagenesis in shallow sediments. In more permeable settings, such flow could affect methane hydrate stability, seafloor chemosynthetic communities, and the longevity of fluid seeps.     

Geometry‐coupled reactive fluid transport at the fracture scale: application to CO2 geologic storage

Water acidification follows CO₂ injection and leads to reactive fluid transport through pores and rock fractures, with potential implications to reservoirs and wells in CO₂ geologic storage and enhanced oil recovery. Kinetic rate laws for dissolution reactions in calcite and anorthite are combined with the Navier‐Stokes law and advection–diffusion transport to perform geometry‐coupled numerical simulations in order to study the evolution of chemical reactions, species concentration, and fracture morphology. Results are summarized as a function of two dimensionless parameters: the Damköhler number Da which is the ratio between advection and reaction times, and the transverse Peclet number Pe defined as the ratio between the time for diffusion across the fracture and the time for advection along the fracture. Reactant species are readily consumed near the inlet in a carbonate reservoir when the flow velocity is low (low transverse Peclet number and Da > 10^?1). At high flow velocities, diffusion fails to homogenize the concentration field across the fracture (high transverse Peclet number Pe > 10^?1). When the reaction rate is low as in anorthite reservoirs (Da < 10^?1), reactant species are more readily transported toward the outlet. At a given Peclet number, a lower Damköhler number causes the flow channel to experience a more uniform aperture enlargement along the length of the fracture. When the length‐to‐aperture ratio is sufficiently large, say l/d > 30, the system response resembles the solution for 1D reactive fluid transport. A decreased length‐to‐aperture ratio slows the diffusive transport of reactant species to the mineral fracture surface, and analyses of fracture networks must take into consideration both the length and slenderness of individual fractures in addition to Pe and Da numbers.     

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.     

Deep geothermal groundwater flow in the Seferihisar–Balçova area,Turkey: results from transient numerical simulations of coupled fluid flow and heat transport processes

The Seferihisar–Balçova Geothermal system (SBG) is characterized by complex temperature and hydrochemical anomalies. Previous geophysical and hydrochemical investigations suggest that hydrothermal convection in the faulted areas of the SBG and recharge flow from the Horst may be responsible for the observed patterns. A numerical model of coupled fluid flow and heat transport processes has been built in order to study the possible fluid dynamics of deep geothermal groundwater flow in the SBG. The results support the hypothesis derived from interpreted data. The simulated scenarios provide a better understanding of the geophysical conditions under which the different fluid dynamics develop. When recharge processes are weak, the convective patterns in the faults can expand to surrounding reservoir units or below the seafloor. These fault‐induced drag forces can cause natural seawater intrusion. In the Melange of the Seferihisar Horst, the regional flow is modified by buoyant‐driven flow focused in the series of vertical faults. As a result, the main groundwater divide can shift. Sealing caprocks prevent fault‐induced cells from being overwhelmed by vigorous regional flow. In this case, over‐pressured, blind geothermal reservoirs form below the caprocks. Transient results showed that the front of rising hot waters in faults is unstable: the tip of the hydrothermal plumes can split and lead to periodical temperature oscillations. This phenomenon known as Taylor–Saffman fingering has been described in mid‐ocean ridge hydrothermal systems. Our findings suggest that this type of thermal pulsing can also develop in active, faulted geothermal systems. To some extent, the role of an impervious fault core on the flow patterns has been investigated. Although it is not possible to reproduce basin‐scale transport processes, this first attempt to model deep groundwater geothermal flow in the SBG qualitatively supported the interpreted data and described the different fluid dynamics of the basin. Geofluids (2010) 10 , 388–405     

Thermal convection in faulted extensional sedimentary basins: theoretical results from finite-element modeling

Thermal convection has the potential to be a significant and widespread mechanism of fluid flow, mass transport, and heat transport in rift and other extensional basins. Based on numerical simulation results, large‐scale convection can occur on the scale of the basin thickness, depending on the Rayleigh number for the basin. Our analysis indicates that for syn‐rift and early post‐rift settings with a basin thickness of 5 km, thermal convection can occur for basal heat flows ranging from 80 to 150 mW m^?2, when the vertical hydraulic conductivity is on the order of 1.5 m year^?1 and lower. The convection cells have characteristic wavelengths and flow patterns depending on the thermal and hydraulic boundary conditions. Steeply dipping extensional faults can provide pathways for vertical fluid flow across large thicknesses of basin sediments and can modify the dynamics of thermal convection. The presence of faults perturbs the thermal convective flow pattern and can constrain the size and locations of convection cells. Depending on the spacing of the faults and the hydraulic properties of the faults and basin sediments, the convection cells can be spatially organized to align with adjacent faults. A fault‐bounded cell occurs when one convection cell is constrained to occupy a fault block so that the up‐flow zone converges into one fault zone and the down‐flow zone is centred on the adjacent fault. A fault‐bounded cell pair occurs when two convection cells occupy a fault block with the up‐flow zone located between the faults and the down‐flow zones centred on the adjacent faults or with the reverse pattern of flow. Fault‐bounded cells and cell pairs can be referred to collectively as fault‐bounded convective flow. The flow paths in fault‐bounded convective flow can be lengthened significantly with respect to those of convection cells unperturbed by the presence of faults. The cell pattern and sense of circulation depend on the fault spacing, sediment and fault permeabilities, lithologic heterogeneity, and the basal heat flow. The presence of fault zones also extends the range of conditions for which thermal convection can occur to basin settings with Rayleigh numbers below the critical value for large‐scale convection to occur in a basin without faults. The widespread potential for the occurrence of thermal convection suggests that it may play a role in controlling geological processes in rift basins including the acquisition and deposition of metals by basin fluids, the distribution of diagenetic processes, the temperature field and heat flow, petroleum generation and migration, and the geochemical evolution of basin fluids. Fault‐bounded cells and cell pairs can focus mass and heat transport from longer flow paths into fault zones, and their discharge zones are a particularly favourable setting for the formation of sediment‐hosted ore deposits near the sea floor.     

Multiphase flow simulation through porous media with explicitly resolved fractures

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+1DG‐P_NDG 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.     

Fluid effect on hydraulic fracture propagation behavior: a comparison between water and supercritical CO2‐like fluid

The initiation of hydraulic fractures during fluid injection in deep formations can be either engineered or induced unintentionally. Upon injection of CO₂, the pore fluids in deep formations can be changed from oil/saline water to CO₂ or CO₂ dominated. The type of fluid is important not only because the fluid must fracture the rock, but also because rocks saturated with different pore fluids behave differently. We investigated the influence of fluid properties on fracture propagation behavior by using the cohesive zone model in conjunction with a poroelasticity model. Simulation results indicate that the pore pressure fields are very different for different pore fluids even when the initial field conditions and injection schemes (rate and time) are kept the same. Low viscosity fluids with properties of supercritical CO₂ will create relatively thin and much shorter fractures in comparison with fluids exhibiting properties of water under similar injection schemes. Two significant times are recognized during fracture propagation: the time at which a crack ceases opening and the later time point at which a crack ceases propagating. These times are very different for different fluids. Both fluid compressibility and viscosity influence fracture propagation, with viscosity being the more important property. Viscosity can greatly affect hydraulic conductivity and the leak‐off coefficient. This analysis assumes the in‐situ pore fluid and injected fluid are the same and the pore space is 100% saturated by that fluid at the beginning of the simulation.     

Fracture-fill calcite as a record of microbial methanogenesis and fluid migration: a case study from the Devonian Antrim Shale, Michigan Basin

The Devonian Antrim Shale is an organic‐rich, naturally fractured black shale in the Michigan Basin that serves as both a source and reservoir for natural gas. A well‐developed network of major, through‐going vertical fractures controls reservoir‐scale permeability in the Antrim Shale. Many fractures are open, but some are partially sealed by calcite cements that retain isotopic evidence of widespread microbial methanogenesis. Fracture filling calcite displays an unusually broad spectrum of δ¹³C values (+34 to ?41‰ PDB), suggesting that both aerobic and anaerobic bacterial processes were active in the reservoir. Calcites with high δ¹³C values (>+15‰) record cementation of fractures from dissolved inorganic carbon (DIC) generated during bacterial methanogenesis. Calcites with low δ¹³C values (13C values between ?10 and ?30‰ can be attributed to variable organic matter oxidation pathways, methane oxidation, and carbonate rock buffering. Identification of ¹³C‐rich calcite provides unambiguous evidence of biogenic methane generation and may be used to identify gas deposits in other sedimentary basins. It is likely that repeated glacial advances and retreats exposed the Antrim Shale at the basin margin, enhanced meteoric recharge into the shallow part of the fractured reservoir, and initiated multiple episodes of bacterial methanogenesis and methanotrophic activity that were recorded in fracture‐fill cements. The δ¹⁸O values in both formation waters and calcite cements increase with depth in the basin (?12 to ?4‰ SMOW, and +21 to +27‰ PDB, respectively). Most fracture‐fill cements from outcrop samples have δ¹³C values between ?41 and ?15‰ PDB. In contrast, most cement in cores have δ¹³C values between +15 and +34‰ PDB. Radiocarbon and ²³⁰Th dating of fracture‐fill calcite indicates that the calcite formed between 33 and 390 ka, well within the Pleistocene Epoch.     

Physical properties of carbonate fault rocks,fucino basin (Central Italy): implications for fault seal in platform carbonates

We documented the porosity, permeability, pore geometry, pore type, textural anisotropy, and capillary pressure of carbonate rock samples collected along basin‐bounding normal faults in central Italy. The study samples consist of one Mesozoic platform carbonate host rock with low porosity and permeability, four fractured host rocks of the damage zones, and four fault rocks of the fault cores. The four fractured samples have high secondary porosity, due to elongated, connected, soft pores that provide fluid pathways in the damage zone. We modeled this zone as an elastic cracked medium, and used the Budiansky–O'Connell correlation to compute its permeability from the measured elastic moduli. This correlation can be applied only to fractured rocks with large secondary porosity and high‐aspect ratio pores. The four fault rock samples are made up of survivor clasts embedded in fine carbonate matrices and cements with sub‐spherical, stiff pores. The low porosity and permeability of these rocks, and their high values of capillary pressure, are consistent with the fault core sealing as much as 77 and 140 m of gas and oil columns, respectively. We modeled the fault core as a granular medium, and used the Kozeny–Carmen correlation, assigning the value of 5 to the Kozeny constant, to compute its permeability from the measured porosities and pore radii. The permeability structure of the normal faults is composed of two main units with unique hydraulic characteristics: a granular fault core that acts as a seal to cross‐fault fluid flow, and an elastic cracked damage zone that surrounds the core and forms a conduit for fluid flow. Transient pathways for along‐fault fluid flow may form in the fault core during seismic faulting due to the formation of opening‐mode fractures within the cemented fault rocks.     

Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well,Lower Cretaceous,Valencia Trough,Spain)

Samples from the Amposta Marino C2 well (Amposta oil field) have been investigated in order to understand the origin of fractures and porosity and to reconstruct the fluid flow history of the basin prior, during and after oil migration. Three main types of fracture systems and four types of calcite cements have been identified. Fracture types A and B are totally filled by calcite cement 1 (CC1) and 2 (CC2), respectively; fracture type A corresponds to pre‐Alpine structures, while type B is attributed to fractures developed during the Alpine compression (late Eocene‐early Oligocene). The oxygen, carbon and strontium isotope compositions of CC2 are close to those of the host‐rock, suggesting a high degree of fluid‐rock interaction, and therefore a relatively closed palaeohydrogeological system. Fracture type C, developed during the Neogene extension and enlarged by subaerial exposure, tend to be filled with reddish (CS3r) and greenish (CS3g) microspar calcite sediment and blocky calcite cement type 4 (CC4), and postdated by kaolinite, pyrite, barite and oil. The CS3 generation records lower oxygen and carbon isotopic compositions and higher ⁸⁷Sr/⁸⁶Sr ratios than the host‐limestones. These CS3 karstic infillings recrystallized early within evolved‐meteoric waters having very little interaction with the host‐rock. Blocky calcite cement type 4 (CC4 generation) has the lowest oxygen isotope ratio and the most radiogenic ⁸⁷Sr/⁸⁶Sr values, indicating low fluid‐rock interaction. The increasingly open palaeohydrogeological system was dominated by migration of hot brines with elevated oxygen isotope ratios into the buried karstic system. The main oil emplacement in the Amposta reservoir occurred after the CC4 event, closely related to the Neogene extensional fractures. Corrosion of CC4 (blocky calcite cement type 4) occurred prior to (or during) petroleum charge, possibly related to kaolinite precipitation from relatively acidic fluids. Barite and pyrite precipitation occurred after this corrosion. The sulphur source associated with the late precipitation of pyrite was likely related to isotopically light sulphur expelled, e.g. as sulphide, from the petroleum source rock (Ascla Fm). Geofluids (2010) 10 , 314–333     

On the geochemistry of gases and noble gas isotopes (including 222Rn) in deep crustal fluids: the 4000 m KTB-pilot hole fluid production test 2002–03

The concentrations of H₂, O₂, CO₂, and concentrations and isotopic composition of the noble gases (including ²²²Rn), N₂, CH₄, and higher hydrocarbons dissolved in 4000 m deep‐seated fluids from a 12‐month fluid production test in the KTB pilot hole were analyzed. This determination of the gas geochemistry during the test in combination with the knowledge of the hydraulic data provides relevant information about the fluid hydraulics of the deep system. All gas concentrations and isotopic signatures, except for ²²²Rn, showed constancy during the course of the test. This, in combination with large fluid flow rates at a moderate water table drawdown, imply an almost infinite fluid reservoir in 4000 m depth. From the change in ²²²Rn‐activity as a function of pump rate, the contribution of smaller and wider pores to the overall fluid flow in an aquifer can be deduced. This ²²²Rn‐activity monitoring proved therefore to be a valuable instrument for the qualitative observation of the scavenging of pore and fracture surfaces, a hydraulic feature invisible to standard hydraulic testing tools. The observance of this scavenging effect is due to (i) the continuous on‐line geochemical monitoring, (ii) the durability of the test, (iii) a change in pump rate during the course of the test, and (iv) due to the short half‐life of ²²²Rn. The fluids have a 5.9% mantle He component, and a δ²¹Ne excess of 14%, and a noble gas model age of about (5.5–6.2) ± 2.0 Myr. The mean N₂/Ar‐ratio of 516 and δ¹⁵N‐data of about +1.5‰ indicates sedimentary or metamorphic origin of N₂. The hydrocarbons, amounting to 33 vol.% in the gas phase, are derived from thermal decomposition of marine organic matter of low maturity. But a key question, the identification of the potential source region of the fluids and the migration pathway, is still unidentified.     

Fluid sinks within the earth's crust

The Urach 3 research borehole in south‐west (SW) Germany has been drilled through the sedimentary cover, and the gneisses of the Variscian crystalline basement at 1600 m below the surface (Black Forest basement) has been reached. An additional 2800 m has been drilled through the fractured crystalline rocks, and the borehole has been used for a number of hydraulic tests in the context of a ‘hot‐dry rock’ (HDR) project exploring for geothermal energy. The fracture system of the basement is saturated with a NaCl brine with about 70 g L^?1 dissolved solids. Water table measurements in the borehole cover a period of 13 years of observation, during which the water table continuously dropped and did not reach a steady‐state level. This unique set of data shows that the hydraulic potential decreases with depth, causing a continuous flow of fluid to the deeper parts of the upper continental crust. The potential decrease and the associated downward migration of fluid is an evidence for the progress of water (H₂O)‐consuming reactions in the crystalline rocks. Computed stability relations among relevant phases at the pressure temperature (PT) conditions in the fracture system and documented fossil fracture coatings in granites and gneisses suggest that the prime candidate for the H₂O‐consuming reaction is the zeolitization of feldspar. The potential of the gneisses to chemically bind H₂O matches the estimated amount of migrating H₂O.