Undersaturated Fluid Research Articles

Porosity generation via spatially uncoupled dissolution precipitation during plagioclase replacement in quartz undersaturated fluids

The replacement of feldspars is commonly characterized by pseudomorphism and reaction-induced pore generation. However, the effects of compositions of feldspars and fluids on porosity generation during alteration are still poorly understood. In this study, we conducted a series of hydrothermal experiments on plagioclase replacement by 2 M KCl or NaCl aqueous solutions at 600 °C and 150 MPa for 1–8 days, using plagioclase with different compositions (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99) with or without quartz. Albite replacement by K-feldspar was not affected by the presence of quartz, whereas anorthite was unaltered in the quartz-absent fluid. The replacement of labradorite by KCl(aq) showed different results: in the presence of quartz, labradorite was altered by K-feldspar, whereas in the absence of quartz, alteration proceeded significantly with the generation of large pores hosted by secondary anorthite coupled with euhedral K-feldspar overgrowth. Such textural relationship and oxygen isotope-labeled experiments reveal that silica-deficient fluid enhances the uncoupled dissolution reprecipitation process. The Si and Al ions in the reacted aqueous solution diffused outside the labradorite grains and encountered K+-rich solutions to grow K-feldspar. The experiments with polycrystalline rocks composed of amphibole + labradorite using 2 M KCl aqueous solution indicated the replacement of labradorite grains by anorthite and K-feldspar overgrowth, as found in single-crystal experiments. Our results indicate that the silica concentration in the fluids has different influences on the saturation indices of albite, anorthite, and K-feldspars in saline fluids, which significantly affect the replacement textures and porosity generation in crustal rocks.

Whiteschist genesis through metasomatism and metamorphism in the Monte Rosa nappe (Western Alps)

Whiteschists from the Monte Rosa Nappe were examined in the field as well as with petrographic, geochemical, and isotopic methods to constrain the controversial origin of these rocks in their Alpine metamorphic context. Whiteschists occur as ellipsoidal-shaped, decametric-sized bodies, within a Permian metagranite, and consist mainly of chloritoid, talc, phengite, and quartz. The transition from whiteschist to metagranite is marked by multiple sharp mineralogical boundaries defining concentric zones unrelated to Alpine deformation. The development of reaction zones, as well as the geometry of the whiteschist suggest a pervasive fluid infiltration, facilitated and canalized by reaction fingering. Whole-rock compositions of whiteschists and metagranites indicate an enrichment in MgO and H2O and depletion of Na2O, CaO, Ba, Sr, Pb, and Zn in the whiteschist relative to the metagranite. Trace- and rare-earth elements, together with all other major elements, notably K2O and SiO2, were within uncertainty not mobile. Such a K and Si saturated, Na undersaturated fluid is not compatible with previous interpretations of fluids derived from ultramafic rocks, evaporites, or Mg-enriched seawater due to mantle interactions. Together with the large variations in δD and δ18O values, this indicates large fluid fluxes during metasomatism. Calculated δD and δ18O values of fluids in equilibrium with the whiteschist support a magmatic–hydrothermal fluid source, as does the chemical alteration pattern. Bulk rock 87Sr/86Sr ratios in whiteschists confirm a pre-Alpine age of fluid infiltration. The 87Sr/86Sr ratios in whiteschists were subsequently partially homogenized in a closed system during Alpine metamorphism. In conclusion, the granite was locally affected by late magmatic–hydrothermal alteration, which resulted in sericite–chlorite alteration zones in the granite. The entire nappe underwent high-pressure metamorphism during the Alpine orogeny and the mineralogy of the whiteschist was produced during dehydration of the metasomatic assemblage under otherwise closed-system metamorphism. While each whiteschist locality needs to be studied in detail, this in-depth study suggests that many whiteschists found in granitic bodies in the Alps might be of similar origin.

The role of fluid chemistry on permeability evolution in granite: Applications to natural and anthropogenic systems

Efforts to maintain and enhance reservoir permeability in geothermal systems can contribute to sourcing more sustainable energy, and hence to lowering CO2 emissions. The evolution of permeability in geothermal reservoirs is strongly affected by interactions between the host rock and the fluids flowing through the rock's permeable pathways. Precipitation of secondary mineral phases, the products of fluid-rock interactions, within the fracture network can significantly reduce the permeability of the overall system, whereas mineral dissolution can enhance reservoir permeability. The coupling between these two competing processes dictates the long-term productivity and lifetime of geothermal reservoirs. In this study, we simulate the conditions within a geothermal system from induced fracturing to the final precipitation stage. We performed batch and flow-through experiments on cores of the Carnmenellis granite, a target unit for geothermal energy recovery in Cornwall (UK), to understand the role of mineral dissolution and precipitation in controlling the permeability evolution of the system. The physico-chemical properties of the cores were monitored after each reaction-phase using ICP-OES, SEM, hydrostatic permeability measurements, and X-ray Computed Tomography. Results show that permeability evolution is strongly dependent on fluid chemistry. Undersaturated alkaline fluids dissolve the most abundant mineral phases in granite (quartz and feldspars), creating cavities along the main fractures and generating pressure-independent permeability in the core. Conversely, supersaturated alkaline fluids, resulting from extended periods of fluid-rock interactions, promote the precipitation of clay minerals, and decrease the permeability of the system. These results suggest that chemical dissolution during geothermal operations could generate permeable pathways that are less sensitive to effective stress and will remain open at higher pressures. Similarly, maintaining the circulation of undersaturated fluids through these granitic reservoirs can prevent the precipitation of pore-clogging mineral phases and preserve reservoir permeability in granite-hosted geothermal systems.

A review of EOR techniques for carbonate reservoirs in challenging geological settings

A large portion of the remaining oil world-wide is located in tight formations, often of carbonate origin, with substantial heterogeneity. In this review article, we aim to study EOR opportunities from a reservoir characterisation point of view. The main categories chosen include heavily-fractured reservoirs, communicating reservoir layers with large permeability contrast, low-permeability reservoirs and finally reservoirs with large oil-water transition zones.The literature sources, which are critically reviewed, have highlighted suitable methods in each of the four categories. In many cases, field implementation suggests that EOR is indeed economically feasible and a number of key learnings can be extracted from these successful projects.In general, the earlier EOR can be implemented, the better. This reduces the incremental cost required for EOR and provides an earlier pay-back in terms of higher production. The increment due to EOR may become more difficult to quantify since the baseline case would have to be entirely based on numerical simulation. A low baseline recovery without EOR shifts the economics in favour of EOR. Use of the existing well stock is not always sufficient or suitable; in most cases EOR must be complemented by drilling of new wells with appropriate well type, well placement, sufficient monitoring and surveillance.Large fields offer more EOR opportunities because injectant breakthrough can be better managed by redirecting injectant to poorly swept areas. EOR can also be taken offshore; industry experience shows that if the size of the prize is big enough, EOR projects are being sanctioned.Gas injection has the broadest range of applicability for a number of reasons. Occasional lack of market to export produced gas from remotely-located fields provides a strong incentive for gas re-injection. Furthermore, gas offers better injectivity compared to water or chemical EOR methods. Gas injection does not require development of miscibility to be successful, although miscibility helps. Swelling of highly under-saturated fluids leads to oil viscosity reduction and oil saturation increase and delays premature gas breakthrough. However, gas based EOR may suffer from poor sweep efficiency due to geological heterogeneity, gravity override and viscous fingering leading to unfavourable mobility ratio and premature gas breakthrough. Hence, gas-based EOR will require mobility control to improve sweep efficiency.Appetite for risk taking varies among operators. Unique problems require unique solutions. Operators who demand firm proof before field testing may have to adjust their mind-set and pilot a new concept in a statistically significant number of wells to fully evaluate the potential.

Effects of injection fluid type on pressure-dependent permeability evolution of fractured rocks in geothermal reservoirs: An experimental chemo-mechanical study

In geothermal reservoirs, coupled thermal-hydrological-mechanical-chemical processes lead to gradual closure of fractures and consequent production decline. The objective of this study was to investigate the effects of injected fluid type on the evolution of fracture aperture/permeability at various stress levels through a series of flow-through experiments on a fractured phyllite specimen, retrieved from a production well in the Blue Mountain geothermal field. The injected fluids included deionized water, silica super- and under-saturated fluids, and the geothermal fluid extracted from the Blue Mountain geothermal field. It was found that injection of geothermal and silica super-saturated fluids led to lowest and highest permeability reduction, respectively. In addition, the degree of permeability recovery was lower in the experiment using geothermal fluid compared to the experiment using silica super-saturated fluid. On the other hand, chemical analysis of the effluent samples revealed that injection of deionized water resulted in dissolution of feldspar and quartz, while precipitation of silica occurred in the experiment using geothermal fluid. Post-test observation by scanning electron microscopy of fracture surface area in the test using silica super-saturated fluid indicated some degree of mineral precipitation. A comparison between pre- and post-test computer tomography scan images for the experiment with injected silica super-saturated showed that mostly decrease in fracture aperture occurred, with increase in fracture aperture in a few regions along the fracture surface. Finally, the Three-Element rheological model successfully predicted the fracture permeability decay. The results of this study suggested that precipitation of silica, a major problem in permeability loss of geothermal systems, can be potentially minimized by using an injection fluid that is closer to chemical equilibrium state with the host rock.

Coupled dissolution–precipitation reactions of tennantite-tetrahedrite minerals in the Darasun gold deposit (Eastern Transbaikalia, Russia)

Heterogeneous rhythmic-zonal aggregates of tennantite-IV replaced partly or completely early homogeneous Zn-tetrahedrite-I and euhedral (Fe-Zn)-tennantite-I crystal were found in ores of the Darasun gold deposit. Different replacement stages of fahlore were observed. It initiates at grain boundaries and is terminated by a complete transformation into pseudomorphic newly formed (Zn-Fe)-tennantite-IV aggregates rimed with Zn-tetrahedrite-IV. These aggregates associated intimately with bournonite and galena and their deposition initiated the pseudomorph formation. EMPA revealed that (Fe-Zn)-tetrahedrite richer in As relative to Zn-tetrahedrite-I was deposited at initial stage. Tennantite with wide variation in Sb/(Sb + As) and Fe/(Fe + Zn) ratios predominantes in heterogenous zonal aggregates of (Fe-Zn)-tetrahedrite-tennantite-IV. A negative correlation between the Sb/(Sb + As) and Fe/(Fe + Zn) was found in these minerals. In each site at the contact between Zn-tetrahedrite-I and newly formed (Fe-Zn)-tetrahedrite-tennantite-IV a miscibility gap between As and Sb a sharp drop in the Sb/(Sb + As) ratio and an increase in Fe/(Fe + Zn) ratio occur. Sharp saw-shape boundaries between Zn-tetrahedrite-I and tennantite-IV and voids in newly formed aggregates are considered to be evidence for couple dissolution-precipitation reactions. The dissolution was initiated due disequilibrium between Zn-tetrahedrite-I and an undersaturated fluid resulted from deposition of galena and bournonite. Precipitation of tetrahedrite-tennantite-IV occurred under oscillation in Sb/(Sb + As) and Fe/(Fe + Zn) ratios due to the metal and semimetal contents in the fluid. Crystallization temperature of zonal-heterogenous tennantite-IV aggregates was calculated by sphalerite-fahlore geothermometer which shows (134161) 20 С. Instability of early Zn-tetrahedrite-I resulted from fluid cooling, decreasing in fluid salinity, changing in tetrahedrite and tennantite solubility due to an evolution of migration conditions of semimetals.

Coupled Dissolution–Precipitation Reactions of Tennantite–Tetrahedrite Minerals in the Darasun Gold Deposit (Eastern Transbaikalia, Russia)

Heterogeneous rhythmic-zonal aggregates of tennantite-IV replaced partly or completely early homogeneous Zn-tetrahedrite-I and euhedral (Fe-Zn)-tennantite-I crystal were found in ores of the Darasun gold deposit. Different replacement stages of fahlore were observed. It initiates at grain boundaries and is terminated by a complete transformation into pseudomorphic newly formed (Zn-Fe)-tennantite-IV aggregates rimed with Zn-tetrahedrite-IV. These aggregates associated intimately with bournonite and galena and their deposition initiated the pseudomorph formation. EMPA revealed that (Fe-Zn)-tetrahedrite richer in As relative to Zn-tetrahedrite-I was deposited at initial stage. Tennantite with wide variation in Sb/(Sb + As) and Fe/(Fe + Zn) ratios predominantes in heterogenous zonal aggregates of (Fe-Zn)-tetrahedrite-tennantite-IV. A negative correlation between the Sb/(Sb + As) and Fe/(Fe + Zn) was found in these minerals. In each site at the contact between Zn-tetrahedrite-I and newly formed (Fe-Zn)-tetrahedrite-tennantite-IV a miscibility gap between As and Sb a sharp drop in the Sb/(Sb + As) ratio and an increase in Fe/(Fe + Zn) ratio occur. Sharp saw-shape boundaries between Zn-tetrahedrite-I and tennantite-IV and voids in newly formed aggregates are considered to be evidence for couple dissolution-precipitation reactions. The dissolution was initiated due disequilibrium between Zn-tetrahedrite-I and an undersaturated fluid resulted from deposition of galena and bournonite. Precipitation of tetrahedrite-tennantite-IV occurred under oscillation in Sb/(Sb + As) and Fe/(Fe + Zn) ratios due to the metal and semimetal contents in the fluid. Crystallization temperature of zonal-heterogenous tennantite-IV aggregates was calculated by sphalerite-fahlore geothermometer which shows (134–161) ± 20 °С. Instability of early Zn-tetrahedrite-I resulted from fluid cooling, decreasing in fluid salinity, changing in tetrahedrite and tennantite solubility due to an evolution of migration conditions of semimetals.

Origin of the megabreccias in the Katanga Copperbelt (D.R.Congo)

The megabreccias in the Katanga part of the Neoproterozoic Central African Copperbelt contain up to several km-long blocks and fragments of the Mines Subgroup which host most of the stratiform Cu-Co deposits. New observations, particularly on cores from boreholes drilled at Luiswishi indicate three types of fracturing: 1) brittle post-folding in the Mines Subgroup; 2) hydraulic; and 3) ductile in soft incompetent siltstones of the R.A.T. and Dipeta subgroups. These fracturing phases dislocated the Roan succession into blocks and fragments and, in particular, clearly showed that there is an evolution from an in situ hydraulic fracturing, to a heterometric brecciation implying some movement and abrasion of the fragments. The process points to significant compression, and was accompanied by fluid expulsion and precipitation of dolomite after decompression. Fluid inclusion microthermometry in dolomite grains shows that the fluids were of high salinity and high temperature, suggesting dissolution of evaporites most likely contained in the Roan sedimentary pile. These saline fluids allowed the fluidization of the breccias, facilitating the displacement of the nappes, pinching out (extrusion-like) megabreccias along thrust-faults, and resulting in intrusion of breccias between the blocks or into large fractures. Breccias between the blocks are clearly identified as friction breccias. They contain a fine material, as part of the matrix, resulting from abrasion of the fragments during transportation. Abrasion and attrition explain the rounding of the fragments. A late cementation phase from less saline and lower temperature fluids suggests the addition of meteoric water in the system, and the mixing with the ambient fluids. The minimum burial depth of the meteoric water incursion is estimated at 2.8 km. Such under-saturated fluids may have contributed to the dissolution of residual evaporites and of the evaporitic material from the Kiubo rocks at the base of the nappes, and led to further brecciation, possibly explaining the multi-phase features of the breccia. The megabreccias occur at the base of the thrusts sheets and are marked by thrust-fault zones. Results of the study support a process of formation of the megabreccias related to a fold-and-trust event, and invalidate a syn-orogenic sedimentary origin as an olistostrome formed by subaqueous conglomeratic debris flows and clastic syn-orogenic sediments. They also contradict a pure salt tectonic hypothesis that propose the extrusions and enlargements of allochthonous evaporites-gigabreccia before the Lufilian deformation. However, the model is compatible with a “fluid behaviour” of pressured saline fluids trapped in folds and/or thrust sheets, and resulting from evaporites dissolution at variable depth.

Cyanobacterial mineralisation of posnjakite (Cu4(SO4)(OH)6·H2O) in Cu-rich acid mine drainage at Yanqul, northern Oman

This is the first detailed account of the copper sulfate posnjakite (Cu4(SO4)(OH)6·H2O) coating cm-long filaments of a microbial consortium of four cyanobacteria and Herminiimonas arsenicoxydans. It was first observed on immersed plant leaves and stalks in a quarry sump of the abandoned Yanqul gold mine in the northern region of Oman; rock surfaces in the immediate vicinity show no immediate evidence of posnjakite. However, a thin unstructured layer without filaments but also containing the brightly coloured turquoise posnjakite covers ferruginous muds in the sump. Although copper is a potent bactericide, the microbes seem to survive even at the extreme heavy metal concentrations that commonly develop in the sump during the dry season (Cu2+≈2300ppm; Zn2+=750ppm; Fe2+≈120ppm; Ni2+=37ppm; Crtotal=2.5ppm; Cl−=8250ppm; and SO42−=12,250ppm; pH ∼2.6), thus leading to the precipitation of posnjakite over a large range of physicochemical conditions. Upon exposure to the prevailing arid climate, dehydration and carbonation quickly replace posnjakite with brochantite (Cu4(SO4)(OH)6) and malachite (Cu2(CO3)(OH)2). To characterise and understand the geochemical conditions in which posnjakite precipitates from undersaturated fluids (according to our thermodynamic modelling of the dominant elements), waters from rainy and dry periods were analysed together with various precipitates and compared with the observed field occurrences. The findings imply that posnjakite should not form in the examined environment through purely inorganic mechanisms and its origin must, therefore, be linked to the encountered microbial activities.

Dinaride evaporite mélange: Diagenesis of the Kosovo polje evaporites

Permian to Triassic age and floored by the various Adriatic carbonate platform units. The tectonic history of the Dinaridic evaporite melange is related to long term burial and Palaeogene to Neogene exhumation processes. Evaporiterocks of the Kosovo polje deposits are associated with carbonates, clastic and minor albitized, subvolcanic, neutralto basic rocks. They are capped by a clayey Quaternary cover of variable thickness, and a several metres thick cavernouscarbonate breccia – rauhwacke. Evaporite rocks show two distinct facies: laminated evaporite-carbonate, composed of evaporite and dolomicrite (± organic matter, pyrite, halite) intercalations, and evaporite-carbonate breccia, composed of fragments of laminated evaporites, carbonate and siltite, cemented by massive gypsum ± sulfur, occurringat the shallower levels and related to the emplacement. The Quaternary cover is composed of 47-88% clayey material, comprising illite, kaolinite and most likely vermiculite, 9-49 % carbonate and 3-4% evaporite minerals. The thickness of the evaporite rehydration zone correlates negatively with the thickness of the overlying Quater nary cover and with the amount of clayey material within it. Rauhwacke are composed of partly dedolomitized dolostone and leachedgypsum fragments, cemented with late stage calcite. Diagenetic processes are related to early diagenesis and the burialphase began with the formation of diagenetic halite from an oversaturated subsurface or surface (lagoon) brine, and was followed by the biochemical reduction of evaporite sulphate to sulfide and formation of pyrite within organic-richcarbonate laminae. These processes triggered early dolomitization (in a shallow burial realm), and the formation of idiomorphic planar-e type of dolomite crystals at temperatures below 50-60°C. Planar dolomite suppressed diagenetichalite. With an increase in burial depth, gypsum dehydrates to anhydrite and when temperatures exceed ~50 °C, precipitation of fine grained non-planar –a type of dolomite began. Halite molds are partly replaced with non-planar-atype of dolomite. During regional uplift and exhumation, anhydrite rehydrated to gypsum under the influence of low-temperature undersaturated fluid (meteoric water), whereas hydrogen-sulfide oxi dized to elementary sulfur, observed as cement in the evaporite-carbonate breccia. Rauhwacke at the uppermost part of the deposit are formed by severe tectonic movements associated with the gypsum-driven dedolomitization process.

Simulation of Hydrate Dynamics in Reservoirs

Summary Gas hydrates in reservoirs are generally not in thermodynamic equilibrium, and there may be several competing phase transitions involving hydrate. Formation of carbon dioxide (CO2) hydrates during aquifer storage of CO2 involves roughly 10 vol% increase compared with groundwater. Dissociation of hydrate toward undersaturated fluid phases involves the same level of contraction. Hydrate phase transitions are generally fast (scales of seconds) compared with mineral dissolution and precipitation, and it is unlikely that a time-shifted explicit coupling to geomechanical analysis will be able to capture the appropriate dynamic couplings between flow and changes in stress. The need for geomechanical integrity of the storage site therefore requires a reservoir simulator with an implicit solution of mass flow, heat flow, and geomechanics. And because CO2 involved in hydrate is also involved in different geochemical reactions, we propose a scheme where all possible hydrate formation (on water/ CO2 interface, from water solution, and from CO2 adsorbed on mineral surfaces) and all different possible dissociations are treated as pseudoreactions, but with kinetics derived from advanced theoretical modeling. The main tools for generating these models have been phase-field-theory (PFT) simulations, with thermodynamic properties derived from molecular modeling. The detailed results from these types of simulations provide information on the relative impact of mass transport, heat transport, and thermodynamics of the phase transition, which enable qualified simplifications for implementation into RetrasoCodeBright (RCB) (Saaltink et al. 2004). The primary step was to study the effect of hydrate growth or dissociation with a certain kinetic rate on the mechanical properties of the reservoir. Details of the simulator and numerical algorithms are discussed, and relevant examples are shown.

Insights into the Liquid Bismuth Collector Model Through Analysis of the Bi-Au Stormont Skarn Prospect, Northwest Tasmania

The liquid bismuth collector model has previously been proposed to explain the common association between gold and bismuth seen in many ore deposits, and involves the scavenging of gold from hydrothermal fluid by liquid bismuth. Here, textural relationships and temperature and chemical conditions of mineralization at the Stormont Bi-Au skarn prospect in northwestern Tasmania are analyzed to investigate whether the liquid bismuth collector model can explain gold accumulation at this prospect. The calc-silicate skarn assemblage containing grandite, hedenbergite, actinolite, magnetite, epidote, quartz, and calcite suggests that initial metasomatism involved temperatures >400° to 500°C. Visible gold is associated with native bismuth, bismuthinite, maldonite, galena, and an unknown Bi-Te-S mineral. Native bismuth is directly associated with grandite, which formed during initial stages of metasomatism. This implies that bismuth would have precipitated from a hydrothermal fluid as a liquid. Heating experiments showed that bismuth alloy inclusions in garnet melt between temperatures of 250° and 288°C. The mineralization setting at Stormont and textural relationships suggest that gold was scavenged from hydrothermal fluids by liquid bismuth. Rhythmically zoned grandite crystals provide evidence for fluctuating hydrothermal fluid conditions, which may have contributed to dissolution and reprecipitation of gold, thus facilitating an ore zone refining process within the prospect. This refining process is made possible by the tendency of liquid bismuth to scavenge gold from significantly under-saturated fluids. Infiltrating gold-undersaturated fluids can dissolve gold not in contact with bismuth, which can then be preferentially redeposited upon encountering a bismuth droplet, enhancing the correlation between the two elements. A long-lived and focused fluid flow system would maximize the efficiency of this ore zone refining process, which is likely to be relevant to gold deposition at a range of deposit types.

Salinity-Based Pump-and-Dump Strategy for Drilling Salt With Supersaturated Fluids

Summary Riserless drilling with weighted-drilling-fluid systems, commonly referred to as a "pump-and-dump" drilling strategy, is an established drilling technique used on deepwater wells with shallow hazards. Large holes and high flow rates result in very large volumes of fluid being required to drill to total depth (TD), circulate the well clean, and cement the conductor-casing string. Fluids management becomes a major issue in the riserless hole section. In the Gulf of Mexico, mud is often densified in excess of well requirements and then blended with seawater in a "cut-back" operation to reach the desired density to pump downhole. When performing riserless drilling into salt, a pump-and-dump strategy is often used. Dilution with seawater, however, results in an undersaturated fluid. This fluid leaches the salt, resulting in substantial hole enlargement. The hole enlargement can result in poor cementing jobs that require remediation or even an additional string of casing. A unique operation has been employed in the Santos basin of Brazil where a supersaturated-brine fluid was used to conduct a pump-and-dump operation with the goal of drilling with a saturated-brine fluid and minimizing hole enlargement. This paper details the planning of the operation, fluid design, and pilot testing. Fluids management, equipment rig up, and results are discussed in detail. The operation has been executed successfully twice, with both operations achieving the set objectives for the wells. Unforeseen complications that were encountered are discussed along with lessons learned that have been applied to subsequent operations.

Methane under-saturated fluids in deep-sea sediments: Implications for gas hydrate stability and rates of dissolution

Deep-sea sediments contain Earth's largest reservoir of methane (CH 4, 3000 GTons C) trapped within ice-like crystals known as gas hydrates. Understanding the controls on gas hydrate stability is critical because methane released from hydrate destabilization is hypothesized to be a powerful agent of past and potentially future climate change. Hydrates are stable under high pressure, low temperature, moderate salinity, and saturated gas conditions. Yet, the degree of gas saturation is rarely known in nature because in situ dissolved pore-water CH 4 concentrations are rarely measured. Here, we report measurements of these concentrations in sediments immediately surrounding deep-sea gas hydrate deposits and show that pore-fluids are greatly under-saturated with respect to expected values for equilibrium with methane gas hydrate. This indicates that the hydrates are dissolving, even though they are found within the appropriate pressure and temperature stability field. However, dissolution rates calculated from the in situ CH 4 data are significantly less than dissolution rates predicted for methane-under-saturated pore-water in direct contact with pure methane gas hydrate if equilibrium CH 4 concentrations exist immediately adjacent to the hydrate surface. Diffusion-retarding factors found naturally in ocean sediments, such as oil coatings or biofilms, appear to enhance stability in outcropping hydrate deposits. The in situ seafloor evidence provided herein leads us to hypothesize that the stability of the worldwide hydrate deposits may be much greater than predicted from diffusion kinetics because biological (microbial excretion of slime or surfactants) and/or physical processes (oil coatings) effectively armor and stabilize exposed hydrate surfaces, substantially retarding their dissolution.

Book Reviews

Book Reviews

Bulk composition and phase behaviour of petroleum sourced by the Bakken Formation of the Williston Basin

Abstract The Bakken Formation is currently regarded primarily as a self-contained, unconventional petroleum system. While previously viewed as a source for oil occurring in overlying formations, it is now predicted that resources of more than 3.5 billion barrels of oil are trapped intraformationally. New insights into the formation's open v. closed nature are presented here using the physical properties of natural petroleum, source rock characteristics and the numerical modelling of phase behaviour. In the mature western part of the basin petroleum accumulations have been postulated to be continuous in nature, characterized by very short migration distances of indigenous hydrocarbons. This necessitates that the composition and therefore physical properties of the generated hydrocarbons must be controlled by the maturity of the source rock in the immediate vicinity. This assumption is not supported by the clustering of higher gas–oil ratios and lighter oil gravities along the locations of the anticlines in the basin. We have used open and closed system pyrolysis techniques to predict the bulk composition of the petroleum generated at different transformation stages, both cumulatively and instantaneously. Based on these predictions the Bakken would contain dominantly undersaturated fluids throughout the basin. Differences in predicted GORs of cumulative and instantaneous models support the conclusion that the reported hydrocarbon compositions cannot completely be explained by a tight self-contained petroleum system. The observed variability of in-place hydrocarbon compositions is readily explained by lateral migration of petroleum in the main middle Bakken carrier, and vertical leakage of emplaced hydrocarbons from the fractured reservoir at anticline locations. This has resulted in the loss of the early generated petroleum, and led to a present-day dominance of late generation products. These results reveal that the Bakken Formation is a partly open petroleum system, at least along the major anticlines of the Williston Basin.

Drilling Salt Formations Offshore With Seawater Can Significantly Reduce Well Costs

Summary The rate of penetration (ROP) increases significantly if salt formations are drilled with undersaturated fluids. This is especially true when drilling riserless. Of primary concern is the amount of hole enlargement that will occur. However, if managed, then a step change in drilling performance and costs can be achieved. This paper presents in detail the results of a comprehensive and large-scale laboratory testing program performed on outcrop salt samples that replicate drilling salt formations with undersaturated drilling fluids with flow rates of up to 1,000 gal/min (gpm). The laboratory testing program includes tests performed in a computerized tomographic (CT) scanner to map hole enlargement in real time, as well as variously sized borehole leaching tests with borehole diameters of up to 6 in. Flow rates are scaled to produce comparable levels of turbulence occurring in the field. An analytical hole-enlargement prediction model is presented that incorporates the effects of ROP, pump rate, drill-fluid saturation, dissolving salt drill cuttings, and salt leaching from the borehole wall. This model accurately predicts to within 10% the measured hole enlargements produced in the scaled laboratory tests for a wide range of flow rates and fluid saturations. Predictions of field performance are made, and the implications of the predicted hole geometry are discussed. Provided that high rates of penetration are realized, acceptable hole geometries will result, even when pumping seawater at flow rates of up to 1,600 gpm. A field application is described whereby the historical practice of drilling the last 200 ft of the 20-in. hole into salt using a saturated brine drilling fluid was discontinued in preference to the continued use of seawater. The competing influences of increasing rate of penetration and avoiding the cost of a sacrificial mud system, offset by the increased cost of cement, resulted in cost savings of U.S. $250,000 per well. Implemented in a multiwell subsalt development, cost savings of more than U.S. $1.5 million will be realized in the drilling program. Further applications of this technology are now being sought.

Syntectonic hydrothermal calcite in a faulted carbonate platform margin (Albian of Jorrios, northern Spain)

ABSTRACT A calcite mass more than 1·5 km long and 20 m wide crops outs along the faulted margin of the Albian carbonate platform of Jorrios in northern Spain. The mass contains abundant dissolution cavities up to 7 m long and 1 m high, filled with cross‐stratified quartz sandstone and alternating sandstone–calcite laminae. Similar cavities are also present in a 50‐m‐wide zone of platform limestones adjacent to the calcite mass that are filled with limestone breccias and sandstone. The calcite mass has mean δ18O values of 19·6‰ (SMOW), whereas platform limestones have mean δ18O values of 24·4‰ (SMOW). Synsedimentary faulting of the carbonate margin and circulation of heated fault‐related waters resulted in replacement of a band of limestone by calcite. Soon after this replacement, dissolution by undersaturated fluids affected both the calcite mass and the adjacent limestones. Percolating marine quartz sand filled all dissolution cavities, sometimes alternating with precipitating calcite. The resulting cavities and fills, which recall products of meteoric diagenesis, are attributed to a hydrothermal origin based on their geometry, occurrence along the profile and synsedimentary tectonic relationships. The early faulting and diagenesis are related to local extensional tectonism in a large‐scale strike‐slip setting. Movements occurred during the early dispar/appenninica zone of the Late Albian.

Analysis of individual fluid inclusions using synchrotron X-ray fluorescence microprobe: progress toward calibration for trace elements

A critical problem for conducting quantitative analysis of individual fluid inclusions using Synchrotron X-Ray Fluorescence (SXRF) technique relates to the standardization and the calibration of the X-ray spectra. In this study, different approaches have been tested for calibration purposes: (a) the use of chlorine when Cl content can be estimated either from melting point depressions of undersaturated fluid inclusions or from saturation limits for halite-bearing fluid inclusions, (b) the use of calcium from synthetic fluid inclusions of known CaCl2 content as an external standard. SXRF analysis was performed on individual fluid inclusions from the Chivor and Guali emerald deposits, Columbia. These well-known samples contain a single fluid inclusion population for which detailed crush-leach analyses are available, thus providing a relevant compositional reference frame. Concentration estimates were also compared to Particle Induced X-ray Emission (PIXE) analysis carried out independently on the same fluid inclusions.Results of the calibration tests indicate that major (Cl, K, Ca, Fe, Mn) and trace element (Cu, Zn, As, Br, Rb, Sr, Ba, Pb) concentration estimates can be performed without precise knowledge of the analytical volume and the inclusion’s 3D geometry. Although the standard deviation of the SXRF results can be relatively high depending on the calibration mode used, mean concentration estimates for most elements are in good agreement with PIXE and crush-leach analysis. Elemental distributions within single fluid inclusions were also established. Associated correlation diagrams argue for the homogeneous distribution of most elements in the fluid inclusion. In contrast, Br shows a bimodal distribution interpreted to reflect a significant enrichment of the vapor portion of the inclusion fluid.

Long-term flow/chemistry feedback in a porous medium with heterogenous permeability; kinetic control of dissolution and precipitation

The kinetics of dissolution and precipitation is of central importance to understanding the long-term evolution of fluid flows in crustal environments, with implications for problems as diverse as nuclear waste disposal and crustal evolution. The authors examine the dynamics of such evolution for several geologically relevant permeability distributions (models for en-echelon cracks, an isolated sloping fractured zone, and two sloping high-permeability zones that are close enough together to interact). Although the focus is on a simple quartz matrix system, generic features emerge from this study that can aid in the broader goal of understanding the long-term feedback between flow and chemistry, where dissolution and precipitation is under kinetic control. Examples of thermal convection in a porous medium with spatially variable permeability reveal features of central importance to water-rock interaction. After a transient phase, an accelerated rate of change of porosity may be used with care to decrease computational time, as an alternative to the quasi-stationary state approximation (Lichtner, 1988). Kinetic effects produce features not expected by traditional assumptions made on the basis of equilibrium, for example, that cooling fluids are oversaturated and heating fluids are undersaturated with respect to silicic acid equilibrium. Indeed, the authors observe regions of downwelling oversaturatedmore » fluid experiencing heating and regions of upwelling, yet cooling, undersaturated fluid. When oscillatory convection is present, the amplitudes of oscillation generally increase with time in near-surface environments, whereas amplitudes tend to decrease over long times near the heated lower boundary. The authors examine the scaling behavior of characteristic length scales, of terms in the solute equation, and of the typical deviation from equilibrium, each as a function of the kinetic rate parameters.« less