Showing papers in "Geofluids in 2010"

Fluid-induced processes: metasomatism and metamorphism

Abstract: Metamorphism and metasomatism both involve the reequilibration of mineral assemblages due to changes in pressure, temperature and ⁄or chemical environment. Both processes involve material transport but on different length scales, so every metamorphic reaction is metasomatic on a local scale. Fluids provide a transport mechanism which is orders of magnitude faster than solid state diffusion and induce reequilibration through dissolution of parent phases and reprecipitation of products. Chemical weathering (kaolinitization and serpentinization), and albitization are used as examples to describe the coupling between dissolution and precipitation. Albitization of feldspars in nature and in experiments is a pseudomorphic replacement which generates porosity in the albite. Porosity generation associated with interface-coupled dissolution-precipitation allows rapid material transport and together with fluid induced fracturing, is the mechanism of pervasive fluid flow through reacting crystals. Examples of metamorphic reactions in granulite-eclogite rocks illustrate the role of fluids in inducing chemical changes along fluid pathways. Microstructural criteria for a metamorphic event (i.e. change in P, T) are critically reviewed by describing the corona formed by reaction between kyanite and garnet, as well as partial replacement textures. We conclude that both corona structures and partial replacement textures are equally indicative of a metasomatic reaction (driven by a fluid-induced compositional change) as they may be of a metamorphic reaction driven by a change in P and ⁄or T. This raises the question of the extent to which fluids play not only a catalytic role but also a thermodynamic role in determining the course of a metamorphic reaction.

Permeability of the Continental Crust: Dynamic Variations Inferred from Seismicity and Metamorphism

Abstract: The variation of permeability with depth can be probed indirectly by various means, including hydrologic models that use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geothermal and metamorphic data combine to indicate that mean permeability (k) of tectonically active continental crust decreases with depth (z) according to log k ! )14‐3.2 log z, where k is in m 2 and z in km. Other independently derived, crustal-scale k‐z relations are generally similar to this power-law curve. Yet there is also substantial evidence for local-to-regional-scale, transient, permeability-generation events that entail permeabilities much higher than these mean k‐z relations would suggest. Compilation of such data yields a fit to these elevated, transient values of log k ! )11.5‐3.2 log z, suggesting a functional form similar to that of tectonically active crust, but shifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of active prograde metamorphism, permeability in the deeper crust will decay toward values below the mean k‐z curves. Several lines of evidence suggest geologically rapid (years to 10 3 years) decay of high-permeability transients toward background values. Crustal-scale k‐z curves may reflect a dynamic competition between permeability creation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such as compaction, hydrothermal alteration, and retrograde metamorphism.

The application of failure mode diagrams for exploring the roles of fluid pressure and stress states in controlling styles of fracture‐controlled permeability enhancement in faults and shear zones

Abstract: Geofluids (2010) 10, 217–233 Abstract Permeability enhancement associated with deformation processes in faults and shear zones plays a key role in facilitating fluid redistribution between fluid reservoirs in the crust. Especially in high fluid flux hydrothermal systems, fracture-controlled permeability can be relatively short-lived, unless it is repeatedly regenerated by ongoing deformation. Failure mode diagrams in pore fluid factor and differential stress space, here termed λ–σ failure mode diagrams, provide a powerful tool for analysing how fluid pressure and stress states drive failure, associated permeability enhancement and vein styles during deformation in faults and shear zones. During fault-valve behaviour in the seismogenic regime, relative rates of recovery of pore fluid factor, differential stress and fault cohesive strength between rupture events impact on styles of veining and associated, fracture-controlled permeability enhancement in faults and shear zones. Examples of vein-rich fault zones are used to illustrate how constraints can be placed, not just on fluid pressure and stress states at failure, but also on the fluid pressurization and loading paths associated with failure and transitory permeability enhancement in faults and shear zones. This provides insights about when, during the fault-valve cycle, various types of veins can form. The use of failure mode diagrams also provides insights about the relative roles of optimally oriented faults and misoriented faults as hydraulically conductive structures. The analysis highlights the dynamics of competition between fluid pressures and loading rates in driving failure and repeated permeability regeneration in fracture-controlled, hydrothermal systems.

Hydrologic Responses to Earthquakes and a General Metric

Abstract: Hydrologic responses to earthquakes, including liquefaction, changes in stream and spring discharge, changes in the properties of groundwater such as geochemistry, temperature and turbidity, changes in the water level in wells, and the eruption of mud volcanoes, have been documented for thousands of years. Except for some water-level changes in the near field which can be explained by poroelastic responses to static stress changes, most hydrologic responses, both within and beyond the near field, can only be explained by the dynamic responses associated with seismic waves. For these responses, the seismic energy density e may be used as a general metric to relate and compare the various hydrologic responses. We show that liquefaction, eruption of mud volcanoes and increases in streamflow are bounded by e � 10 )1 Jm )3 ; temperature changes in hot springs are bounded by e � 10 )2 Jm )3 ; most sustained groundwater changes are bounded by e � 10 )3 Jm )3 ; geysers and triggered seismicity may respond to seismic energy density as small as 10 )3 and 10 )4 Jm )3 , respectively. Comparing the threshold energy densities with published laboratory measurements, we show that undrained consolidation induced by dynamic stresses can explain liquefaction only in the near field, but not beyond the near field. We propose that in the intermediate field and far field, most responses are triggered by changes in permeability that in turn are a response to the cyclic deformation and oscillatory fluid flow. Published laboratory measurements confirm that changes in flow and time-varying stresses can change permeability, inducing both increases and decreases. Field measurements in wells also indicate that permeability can be changed by earthquakes in the intermediate field and far field. Further work, in particular field monitoring and measurements, are needed to assess the generality of permeability changes in explaining far-field hydrologic responses to earthquakes.

Faults and fault properties in hydrocarbon flow models

Abstract: Geofluids (2010) 10, 94–113 Abstract The petroleum industry uses subsurface flow models for two principal purposes: to model the flow of hydrocarbons into traps over geological time, and to simulate the production of hydrocarbon from reservoirs over periods of decades or less. Faults, which are three-dimensional volumes, are approximated in both modelling applications as planar membranes onto which predictions of the most important fault-related flow properties are mapped. Faults in porous clastic reservoirs are generally baffles or barriers to flow and the relevant flow properties are therefore very different to those which are important in conductive fracture flow systems. A critical review and discussion is offered on the work-flows used to predict and model capillary threshold pressure for exploration fault seal analysis and fault transmissibility multipliers for production simulation, and of the data from which the predictions derive. New flow simulation models confirm that failure of intra-reservoir sealing faults can occur during a reservoir depressurization via a water-drive mechanism, but contrary to anecdotal reports, published examples of production-induced seal failure are elusive. Ignoring the three-dimensional structure of fault zones can sometimes have a significant influence on production-related flow, and a series of models illustrating flow associated with relay zones are discussed.

Thermodynamic Model for Mineral Solubility in Aqueous Fluids: Theory, Calibration and Application to Model Fluid‐Flow Systems

Abstract: We present a thermodynamic model for mineral dissolution in aqueous fluids at elevated temperatures and pressures, based on intrinsic thermal properties and variations of volumetric properties of the aqueous solvent. The standard thermodynamic properties of mineral dissolution into aqueous fluid consist of two contributions: one from the energy of transformation from the solid to the hydrated-species state and the other from the compression of solvent molecules during the formation of a hydration shell. The latter contribution has the dimension of the generalized Krichevskii parameter. This approach describes the energetics of solvation more accurately than does the Born electrostatic theory and can be extended beyond the limits of experimental measurements of the dielectric constant of H2O. The new model has been calibrated by experimental solubilities of quartz, corundum, rutile, calcite, apatite, fluorite and portlandite in pure H2O at temperatures up to 1100! C and pressures up to 20 kbar. All minerals show a steady increase in solubility along constant geothermal gradients or water isochores. By contrast, isobaric solubilities initially increase with rising temperature but then decline above 200‐400! C. This retrograde behavior is caused by variations in the isobaric expansivity of the aqueous solvent, which approaches infinity at its critical point. Oxide minerals predominantly dissolve to neutral species; so, their dissolution energetics involve a relatively small contribution from the solvent volumetric properties and their retrograde solubilities are restricted to a relatively narrow window of temperature and pressure near the critical point of water. By contrast, Ca-bearing minerals dissolve to a variety of charged species; so, the energetics of their dissolution reactions involve a comparatively large contribution from volume changes of the aqueous solvent and their isobaric retrograde solubility spans nearly all metamorphic and magmatic conditions. These features correlate with and can be predicted from the standard partial molar volumes of aqueous species. The thermodynamic model can be used over much wider range of settings for terrestrial fluid‐rock interaction than has previously been possible. To illustrate, it is integrated with transport theory to show quantitatively that integrated fluid fluxes characteristic of crustal shear zones are capable of precipitating quartz or calcite veins from low- and medium-grade metamorphic conditions, at a geothermal gradient of 20! C km )1 . For subduction zones, modeled by a geotherm of 7! C km )1 , the required fluid fluxes are one to two orders of magnitude lower and predict enhanced efficiency of mass transfer and metasomatic precipitation in comparison with orogenic settings. The new model thus can be applied to shallow hydrothermal, metamorphic, magmatic and subduction fluids, and for retrieval of dependent thermodynamic properties for mass transfer or geodynamic modeling.

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

Abstract: 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 Gros Schonebeck (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

Role of saline fluids in deep‐crustal and upper‐mantle metasomatism: insights from experimental studies

Abstract: Chloride-rich brines are increasingly recognized as playing an important role in high pressure and temperature metamorphic and magmatic systems. The origins of these saline multicomponent fluids are debated, but experimental evidence suggests that regardless of their origin they must be important agents of rock alteration and mass transfer wherever they occur. Studies of the solubility of quartz in H2O, CO2‐H2O and salt‐H2O solutions provide a framework for understanding the role of brines in the deep crust and upper mantle. While quartz solubility in the system SiO2‐H2O‐NaCl‐CO2 is maximal at a given high pressure and temperature if the solvent is pure H2O, the decline in quartz solubility with NaCl content (salting-out) is less severe than in CO2‐H2O fluids at comparable H2O activities. Moreover, at lower pressures, quartz solubility initially salts in at low salt contents, before reaching a maximum and then declining. The behavior of quartz solubility in salt‐H2O solutions has not yet been fully explained and is the subject of active debate. Experimental investigations of the solubility of some other rock-forming oxides and silicates show enhancements due to NaCl addition. As illustrated by the well-studied CaO‐Al2O3‐SiO2‐NaCl‐H2O system, enhancements initially increase to maxima, and then decline. This behavior can be explained by formation of a range of hydrated aqueous complexes and clusters with specific NaCl:H2O stoichiometries. In contrast, solubilities of calcium salts, including calcite, fluorite, fluorapatite and anhydrite, rise monotonically with increasing NaCl, implying complexing to form anhydrous ionic solutes and⁄or ion pairs. The experimental studies offer new insights into fluid-rock interaction in a range of settings, including carbonatite‐fenite complexes, granulite-facies metamorphism, porphyry ore deposits and aluminum-silicate vein complexes in high-grade metamorphic terranes.

Geophysical signatures associated with fluid flow and gas hydrate occurrence in a tectonically quiescent sequence, Qiongdongnan Basin, South China Sea

Abstract: Interpretation of high-resolution two-dimensional (2D) and three-dimensional (3D) seismic data collected in the Qiongdongnan Basin, South China Sea reveals the presence of polygonal faults, pockmarks, gas chimneys and slope failure in strata of Pliocene and younger age. The gas chimneys are characterized by low-amplitude reflections, acoustic turbidity and low P-wave velocity indicating fluid expulsion pathways. Coherence time slices show that the polygonal faults are restricted to sediments with moderate-amplitude, continuous reflections. Gas hydrates are identified in seismic data by the presence of bottom simulating reflectors (BSRs), which have high amplitude, reverse polarity and are subparallel to seafloor. Mud diapirism and mounded structures have variable geometry and a great diversity regarding the origin of the fluid and the parent beds. The gas chimneys, mud diapirism, polygonal faults and a seismic facies-change facilitate the upward migration of thermogenic fluids from underlying sediments. Fluids can be temporarily trapped below the gas hydrate stability zone, but fluid advection may cause gas hydrate dissociation and affect the thickness of gas hydrate zone. The fluid accumulation leads to the generation of excess pore fluids that release along faults, forming pockmarks and mud volcanoes on the seafloor. These features are indicators of fluid flow in a tectonically-quiescent sequence, Qiongdongnan Basin.

Potential of palaeofluid analysis for understanding oil charge history

Abstract: Geofluids (2010) 10, 73–82 Abstract Fluid inclusion data, particularly the distribution of hydrocarbon fluid inclusions and their chemistry, can provide insights into oil charge in a petroleum-prospective region. Examples from the UK Atlantic margin show how we can understand thermal regime, timing and chemistry of oil charge. Data from the UK Atlantic margin based on fluid inclusion temperature profiles shows anomalously high temperatures which are highest at the top of the Triassic–Eocene sequence. This is interpreted as a product of hot fluid flow, probably reflecting hydrothermal activity related to intrusion of sills at depth. The preservation of high temperatures also implies rapid migration from depth through fracture systems. Ar–Ar analysis of oil-bearing K-feldspar cements, and petrographic studies of oil inclusion distribution help delimit timing and migration pathways for the hot fluid charge and later fluid migration events. Coupled with compositional data for oils measured destructively (organic geochemistry) or non-destructively (fluorescence), these approaches allow the development of oil charge histories based on real data rather than theoretical modelling.

Magmatic fluids immiscible with silicate melts: examples from inclusions in phenocrysts and glasses, and implications for magma evolution and metal transport

Abstract: The first occurrence of immiscibility in magmas appears to be most important in the magmatic-hydrothermal transition, and thus studies of magmatic immiscibility should be primarily directed towards recognition of coexisting silicate melt and essentially non-silicate liquids and fluids (aqueous, carbonic and sulphide). However, immiscible phase separation during decompression, cooling and crystallization of magmas is an inherently fugitive phenomenon. The only remaining evidence of this process and the closest approximation of natural immiscible magmatic liquids and vapours can be provided by melt and fluid inclusions trapped in silicate glasses and magmatic phenocrysts. Such inclusions are often used as a natural experimental laboratory to model the process of exsolution and the compositions of volatile-rich phases from a wide range of terrestrial magmas. In this paper several examples from recent research on melt and fluid inclusions are used to demonstrate the significance of naturally occurring immiscibility in understanding some large-scale magma chamber processes, such as degassing and partitioning of metals.

Geochemistry of H2 and CH4 enriched hydrothermal fluids of Socorro Island, Revillagigedo Archipelago, Mexico. Evidence for serpentinization and abiogenic methane

Abstract: Socorro Island is the exposed part of an approx. 4000-m-high volcanic edifice rising from the oceanic floor to approx. 1000 m asl at the northern part of the Mathematician Ridge, Eastern Pacific. The volcano is active, with the most recent basaltic eruption in 1993. Moderate fumarolic activity and diffuse degassing with a total CO2 flux of approx. 20 total day )1 are concentrated in the summit region of the volcano composed of a group of rhyolite domes. Low-temperature, boiling point, fumaroles discharge gas with high H2 (up to 20 mol% in dry gas) and CH4 (up to 4 mol%). Both carbon and He isotopic ratios and abundances correspond to those in MORB fluids (d 13 CCO2 � )5&; 3 He ⁄ 4 He = 7.6 Ra ,C O2 ⁄ 3 He = (2‐3) · 10 9 , where Ra is the atmospheric ratio 3 He ⁄ 4 He of 1.4 · 10 )6 ). Light hydrocarbons (CH4 ,C 2H6 ,C 3H8, and C4H10) are characterized by a high C1 ⁄C2+ ratio of approx. 1000. Methane is enriched in 13 C( d 13 CCH4 from )15 to )20&) and 2 H( d 2 H from )80 to )120&), and hydrocarbons show an inverse isotopic trend in both d 13 C and d 2 H (ethane is isotopically lighter than methane). These isotopic and concentration features of light hydrocarbons are similar to those recently discovered in fluids from ultramafic-hosted spreading ridge vents and may be related to the serpentinization processes: H2 generation and reduction of CO2 to CH4 within high-temperature zone of volcano-seawater hydrothermal system hosted in basaltic and ultramafic rocks beneath a volcano edifice. The thermodynamic analysis of this unusual composition of the Socorro fluids and the assessment of endmember compositions are complicated by the near-surface cooling, condensation and mixing with meteoric water.

Estimation of methane flux offshore SW Taiwan and the influence of tectonics on gas hydrate accumulation

Abstract: Widely distributed bottom simulating reflectors (BSRs) imply the potential existence of gas hydrates offshore southwestern Taiwan. To compare the distribution of methane concentrations along passive and active margins in the region, bottom waters and cored sediments were collected during four cruises from 2005 to 2006. The results reveal that sites with high methane concentrations are predominantly distributed in the active margin and site GS5 is the only site that contains very high methane concentrations in the passive margin of studied area. Anomalously high methane fluxes still can be obtained from the calculation of diffusive methane flux, although there might be some gas leakage during or after sampling procedures. The profiles of methane and sulfate concentration reveal very shallow depths of the sulfate–methane interface (SMI) at some sites. There is evidence that sulfate reduction is mainly driven by the process of anaerobic methane oxidation. Thus, sulfate fluxes can be used as a proxy for methane fluxes through the use of diffusion equations; and the results show that the fluxes are very high in offshore southwestern Taiwan. The depths of the SMI are different at sites GH6 and C; however, both methane profiles reveal parallel methane gradients below the SMI. This might be because of methane migration to surface sediments from the same reservoir with the same diffusion rates. Although BSRs are widely distributed both in the active margin and in the passive margin, most sites with high methane concentrations have been found in the active margin. Therefore, the specific tectonic settings in offshore SW Taiwan might strongly control the stability of gas hydrates, and thus affect the methane concentrations and fluxes of the sediments and sea waters. Furthermore, the carbon isotopic composition of methane shows that a biogenic gas source is dominant at shallower depth; however, some thermogenic gases might be introduced through the fracture/fault zones from deeper source in the active region of studied area.

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

Abstract: The Seferihisar–Balcova 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

Metal complexation and ion association in hydrothermal fluids: insights from quantum chemistry and molecular dynamics

Abstract: Geofluids (2010) 10, 41–57 Abstract Complexation by ligands in hydrothermal brines is a fundamental step in the transport of metals in the Earth's crust and the formation of ore deposits. Thermodynamic models of mineral solubility require an understanding of metal complexation as a function of pressure, temperature and composition. Over the past 40 years, mineral solubilities and complexation equilibria under hydrothermal conditions have been predicted by extrapolating thermodynamic quantities using equations of state based on the Born model of solvation. However, advances in theoretical algorithms and computational facilities mean that we can now explore hydrothermal fluids at the molecular level. Molecular or atomistic models of hydrothermal fluids avoid the approximations of the Born model and are necessary for any reliable prediction of metal complexation. First principles (quantum mechanical) calculations based on density functional theory can be easily used to predict the structures and relative energies of metal complexes in the ideal gas phase. However, calculations of metal complexation in condensed fluids as a function of temperature and pressure require sampling the configuration degrees of freedom using molecular dynamics (MD). Simulations of dilute solutions require very large systems (thousands of atoms) and very long simulation times; such calculations are only practical by treating the interatomic interactions using classical two- or three-body interatomic potentials. Although such calculations provide some fundamental insights into the nature of crustal fluids, simple two- or three-body classical potentials appear to be inadequate for reliably predicting metal complexation, especially in covalent systems such as Sn2+, Au3+ and Cu+. Ab initio MD (i.e. where the bonding is treated quantum mechanically, but the molecular motions are treated classically) avoids the use of interatomic potentials. These calculations are practical for systems with hundreds of atoms over short times (<10 psec) but enable us to predict complexation as a function of pressure, temperature and composition. In this paper, I provide an introductory outline of the computational methods and illustrations of their application to NaCl brines and the complexation of Cu, Au, Sn and Zn.

Fluids associated with hydrothermal dolomitization in St. George Group, western Newfoundland, Canada

Abstract: Dolomite reservoirs are increasingly recognized as an important petroleum exploration target, although the application of a hydrothermal dolomite exploration model to these reservoirs remains controversial. The St. George Group of western Newfoundland consists of a sequence of dolomitised carbonates, with significant porosity development (up to 30%) and petroleum accumulations. Fluid inclusion microthermometry and bulk fluid leach analyses indicated that fluids responsible for matrix dolomitization (associated with intercrystalline porosity) and later saddle dolomitization are CaCl2 ± MgCl2 rich, high salinity (up to 26 eq. wt% NaCl) brines. Integration of fluid inclusion data with thermal maturation histories from the St. George Group show that these dolomites formed at temperatures higher than the ambient rock temperature, and are therefore hydrothermal in origin. Bulk leach analyses show that dolomitization is associated with influxes of postevaporitic brines (±Cl enriched magmatic fluids) late in the diagenetic history of these carbonates. This dolomitization is possibly Devonian in age, during a period of significant magmatic activity, extensional tectonics and development of hypersaline basins. Petrographic and geochemical similarities between Paleozoic hosted hydrothermal dolomitization in western Newfoundland, eastern Canada and the northeastern United States are consistent with a regional-scale hydrothermal dolomitization event late in the diagenetic history of these carbonates. Geofluids (2010) 10, 422–437

Simulation of adsorption of gold nanoparticles carried by gas ascending from the Earth’s interior in alluvial cover of the middle–lower reaches of the Yangtze River

Abstract: The adsorption onto other minerals of charged gold nanoparticles, carried by gas ascending from the Earth’s interior, is an important component of their transport and deposition in surficial cover such as alluvial, aeolian, and glacial sediments. To simulate the adsorption of these particles, an experiment was conducted in which a flow of air that contained gold nanoparticles was passed upward through a sample of alluvium from the middle–lower reaches of the Yangtze River. These experiments showed that gold nanoparticles are adsorbed on kaolinite, halloysite, goethite, and hematite in the alluvial cover. Both the gold nanoparticles and minerals (i.e., kaolinite, halloysite, goethite, and hematite) carry surface charges that provide them with excellent adsorption properties. This study showed that the specific mineral composition of surficial alluvial cover affects the concentration of gold nanoparticles in the ascending gas. This phenomenon may plausibly be used in exploration for concealed gold, copper–gold, and silver–gold deposits in areas of thick alluvial cover. Geofluids (2010) 10, 438–446

Downward hydrocarbon migration predicted from numerical modeling of fluid overpressure in the Paleozoic Anticosti Basin, eastern Canada

Abstract: The Anticosti Basin is a large Paleozoic basin in eastern Canada where potential source and reservoir rocks have been identified but no economic hydrocarbon reservoirs have been found. Potential source rocks of the Upper Ordovician Macasty Formation overlie carbonates of the Middle Ordovician Mingan Formation, which are underlain by dolostones of the Lower Ordovician Romaine Formation. These carbonates have been subjected to dissolution and dolomitization and are potential hydrocarbon reservoirs. Numerical simulations of fluid-overpressure development related to sediment compaction and hydrocarbon generation were carried out to investigate whether hydrocarbons generated in the Macasty Formation could migrate downward into the underlying Mingan and Romaine formations. The modeling results indicate that, in the central part of the basin, maximum fluid overpressures developed above the Macasty Formation due to rapid sedimentation. This overpressured core dissipated gradually with time, but the overpressure pattern (i.e. maximum overpressure above source rock) was maintained during the generation of oil and gas. The downward impelling force associated with fluid-overpressure gradients in the central part of the basin was stronger than the buoyancy force for oil, whereas the buoyancy force for gas and for oil generated in the later stage of the basin is stronger than the overpressure-related force. Based on these results, it is proposed that oil generated from the Macasty Formation in the central part of the basin first moved downward into the Mingan and Romaine formations, and then migrated laterally up-dip toward the basin margin, whereas gas throughout the basin and oil generated in the northern part of the basin generally moved upward. Consequently, gas reservoirs are predicted to occur in the upper part of the basin, whereas oil reservoirs are more likely to be found in the strata below the source rocks. Geofluids (2010) 10, 334–350

Using seafloor heat flow as a tracer to map subseafloor fluid flow in the ocean crust

Abstract: We describe how seafloor heat flow is determined, review current understanding of advective heat loss from oceanic lithosphere, and present results from three field areas to illustrate how heat flow measurements are used (along with complementary data) to resolve fluid flow rates and patterns. Conductive heat flow through much of the seafloor is lower than predicted by lithospheric cooling models as a result of hydrothermal circulation; this discrepancy is the basis for global estimates of the magnitude of advective cooling of oceanic lithosphere. Hydrothermal circulation also redistributes heat within the ocean crustal aquifer, leading to local variability. Heat flow studies in Middle Valley, a sedimented spreading center in the northeastern Pacific Ocean, indicate multiple scales of fluid circulation, delineate conditions at the top of a hydrothermal reservoir, and show the influence of primary and secondary convection. Heat flow studies on the eastern flank of the Juan de Fuca Ridge document the thermal influence of isolated basement outcrops surrounded by thick, low-permeability sediments. Warm hydrothermal fluids seep from the crust through a small volcanic edifice, having flowed into the crust through a larger outcrop � 50 km to the south. These fluids generate a local geothermal anomaly, but have little influence on regional heat loss from the plate. In contrast, heat flow surveys on part of the Cocos Plate, eastern Equatorial Pacific Ocean, indicate that regional conductive heat loss is just 10‐40% of predictions from lithospheric cooling models. Basement outcrops in this area focus massive discharge of cool, hydrothermal fluid and associated heat (4‐80 · 10 3 ls -1 of fluid, 0.8‐1.4 GW of heat). Seafloor heat flow studies will be increasingly important in coming years for understanding marine hydrogeologic regimes and the role of fluids in a variety of Earth processes

Spatial variations in the salinity of pore waters in northern deep water Gulf of Mexico sediments: implications for pathways and mechanisms of solute transport

Abstract: Geofluids (2010) 10, 83–93 Abstract Spatial variations in the salinity of pore waters in sedimentary basins can provide important insight into basin-scale hydrogeologic processes. Although there have been numerous studies of brine seeps in the deep water Gulf of Mexico, much less is known about porewater salinities in the vast areas between seeps. A study has been made of spatial variation in pore water salinities in sediments in an approximately 500 km by 200 km area of the northern deep water (water depth >500 m) Gulf of Mexico sedimentary basin (GOM) to provide insight into pathways and mechanisms of solute transport in this portion of the basin. A second objective was to document salinities in the upper 500 m of the sedimentary section, the approximate depth to which methane hydrates, a potential future energy resource, may be stable. Elevated salinities would reduce the P–T stability range of hydrates. Salinities were calculated from borehole logs using a dual electrical conductivity model. Even though much of the northern GOM is underlain by allochthonous salt most of the undisturbed shallow sedimentary section has not been permeated by hypersaline waters. These waters are limited to areas near brine seeps. Hypersaline waters having salinities in excess of 100 g l−1 become more common at subseafloor depths of 2 km and greater. A field study at Green Canyon 65 and published numerical simulations of fluid flow above tabular salt bodies suggest that brines produced by salt dissolution migrate laterally and pond above salt and/or within minibasins and that the dominant mechanism of vertical solute transport is a combination of compaction-driven advection and diffusion, not large-scale thermohaline overturn. Superimposed on this diffuse upward flux of dissolved salt is the more focused and localized expulsion of saline fluids up faults.

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

Abstract: 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

The interplay of permeability and fluid properties as a first order control of heat transport, venting temperatures and venting salinities at mid-ocean ridge hydrothermal systems

Abstract: Geofluids (2010) 10, 132–141 Abstract While the fundamental influence of fluid properties on venting temperatures in mid-ocean ridge (MOR) hydrothermal systems is now well established, the potential interplay of fluid properties with permeability in controlling heat transfer, venting temperatures, and venting salinities has so far received little attention. A series of numerical simulations of fully transient fluid flow in a generic, across-axis model of a MOR with a heat input equivalent to magmatic supply at a spreading rate of 10 cm year−1 shows a strong dependence of venting temperature and salinity on the system’s permeability. At high permeability, venting temperatures are low because fluid fluxes are so high that the basal conductive heating cannot warm the large fluid masses rapidly enough. The highest venting temperature around 400°C as well as sub-seafloor fluid phase separation occur when the permeability is just high enough that the fluid flux can still accommodate all heat input for advection, or for lower permeabilities where advection is no longer capable to transfer all incoming magmatic heat. In the latter case, additional mechanisms such as eruptions of basaltic magma may become relevant in balancing heat flow in MOR settings. The results can quantitatively be explained by the ‘fluxibility’ hypothesis of Jupp & Schultz (Nature, 403, 2000, 880), which is used to derive diagrams for the relations between basal heat input, permeability and venting temperatures. Its predictive capabilities are tested against additional simulations. Potential implications of this work are that permeability in high-temperature MOR hydrothermal systems may be lower than previously thought and that low-temperature systems at high permeability may be an efficient way of removing heat at MOR.

Methane emission from natural gas seeps and mud volcanoes in Transylvania (Romania).

Abstract: Gas seepage from petroleum basins is the second largest natural source of methane to the atmosphere, after wetlands. The uncertainty in global emission estimates should be reduced by extending the flux database which is fundamental for defining the emission factors and the actual area of seepage adopted for up-scaling. As a contribution to this goal, we report a new seepage data-set for the Transylvanian Basin, one of the largest natural gas producing regions of Europe, that is characterized by the widespread occurrence of natural leakages of gas at the surface, including at least 73 mud volcanoes and gas seeps. In this study, methane flux was measured using closed-chambers, from 12 seepage sites, in correspondence with focused gas vents (mud volcano craters, bubbling pools, and flammable gas leaks), in the soil surrounding the vents, and at 15 sites located far from macro-seep zones but close to gas fields. Fluxes from individual vents (macro-seeps) were found to reach orders of kg CH4 m−2 day−1 (up to 12 kg m−2 day−1) and diffuse fluxes from soils (miniseepage) were found to be up to a few g CH4 m−2 day−1. Far from seep zones, positive CH4 fluxes (microseepage) may occur locally, typically on the order of tens to hundreds of mg m−2 day−1. The values, as well as the occurrence of seepage even far from vent zones and in mud volcanoes that are apparently extinct, are coherent with results obtained in other countries. Gas fluxes from macro-seeps and soils may change seasonally, but the interannual variation of the average emission factor was found to be minimal. The total CH4 output for Transylvania macro-seeps is estimated conservatively to be around 680 t year−1; the total geo-CH4 seepage emission from the Transylvania petroleum system could be approximately 40 × 103 t year−1, and at least 100 × 103 t year−1 for all Romanian petroleum systems, that is roughly 10% of the total anthropogenic CH4 emission in the country.

The experimental investigation of soil gas radon migration mechanisms and its implication in earthquake forecast

Abstract: Laboratory experiments were performed to help understand the fluctuations and the spike-like anomalies of Rn in a time series that was recorded continuously at monitoring stations One of the experiments indicated that Rn is adsorbed on the surface of sand particles and can be liberated with minor changes in the physical conditions of the containing medium Another experiment indicated that the liberation of ultra-trace Rn, adsorbed on the surface of sand, was not very sensitive to small temperature and pressure changes but was responsive to the flow of carrier gases Among the carrier gases tested, CO2 was preferred because it has a boiling point similar to that of Rn However, all other gases that are inert to Rn can also be carrier gases Temperature variation in the supra soil layer can be measured fairly accurately inside double-insulated PVC pipes that also house the Rn detecting system Temperature variation appears to be related to localized strain heating that is a part of the earthquake energy variation cycle Up-flow of soil gas, caused by the strain heating, induced the sudden release of Rn, which thus appears as a spike-like anomaly The migration of soil gases is expected to follow the thermal cycle corresponding to each earthquake cycle Therefore, the spike-like anomalies can be used, in conjunction with the temperature variation cycle, as time markers to forecast the time, place, and magnitude of a coming earthquake

Radon and radium content of some cold and thermal aquifers from Bihor County (northwestern Romania)

Abstract: In the present study, two of the major naturally occurring radionuclides (226Ra and 222Rn) were analyzed in water samples from shallow, medium-depth, and deep geothermal aquifers, all of which are located in Bihor County, northwestern Romania. Here, the results of radon and radium measurements, performed from 2008 to 2009 in 50 locations, are reported. Radon proved to have a wide range of activity, with values from 4.5 to 110.8 Bq l−1 for shallow aquifers, from 9.3 to 106.0 Bq l−1 for medium-depth aquifers, and from 10.1 to 34.8 Bq l−1 for deep geothermal aquifers. The shallow aquifers have lower radium concentrations (0.06 to 0.48 Bq l−1), compared to medium-depth aquifers (0.1 to 0.52 Bq l−1) and deep geothermal aquifers (0.27 to 1.8 Bq l−1). The principal aim was a thorough investigation into the possible correlations between the occurrence of radon and radium in different aquifers and the hydrogeological, hydrogeochemical, and geothermal features of the area. Besides the direct link between the occurrence of uranium and thorium and the 226Ra and 222Rn contents in groundwater, the measurements we performed have allowed us to identify a secondary control on radionuclide distributions caused by the adsorption of dissolved radium onto clay minerals in exchange for sodium (for the sandy clay aquifer of Sacuieni), the high competition for adsorption sites in aquifers with high concentrations of dissolved calcium and magnesium (for the limestone dolomite aquifer of Oradea), and the role of thermal processes.