bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-phase relative permeability of water-wet, intermediate-wet and oil-wet sandstone
Authors: Baker Lee E.
Published: April 2008

Knowledge of three-phase relative permeability is important to the successful design of many types of improved oil recovery projects. An extensive experimental study was made of three-phase relative permeability in water-wet, intermediate-wet and oil-wet rock. The data obtained in this study are the only complete relative permeability data set for rocks which are not water-wet.

The study confirms the commonly held assumptions about the relative permeability behaviour of wetting and non-wetting phases. Phase saturation and the saturation history (the path by which a particular saturation condition is reached) are major factors in determining relative permeability. In general, the more wetting a phase is, the more nearly the relative permeability of the phase depends only on the saturation of that phase (regardless of saturation history). The relative permeability of the non-wetting phase depends largely (but not entirely) on the saturation of the phase and on the saturation history. The relative permeability surface of an intermediate-wetting phase is affected by the saturations of all phases, particularly when one of the phases is strongly wetting.

Three-dimensional plots of the phase relative permeability surfaces show smooth variation of relative permeability with saturation and with saturation history.

The study provides much new data and guidance for developing and testing theoretical models of three-phase relative permeability. Examples of currently used correlations (Stone’s methods 1 and 2) and a linear interpolation relative permeability correlation are compared to the experimental results.
The role of wettability and spreading in gas injection processes under secondary conditions
Authors: Kalaydjian François , Vizika Olga , Moulu Jean-Claude , Munkerud Per Kristian
Published: April 2008

The objective of this paper is to demonstrate the impact of film continuity on oil recovery by gas injection under secondary conditions, e.g. when water is non-mobile. In this case two films have to be considered: a water film and an oil film. The continuity of the former is insured by the wettability conditions, whereas the continuity and stability of the latter depend on the ability of the oil phase to spread on the water substrate in the presence of a gas phase. This ability is expressed by the spreading coefficient,
S
, defined as a balance of the three interfacial tensions. To study the effect of wettability and spreading on the residual oil saturation obtained by gas flooding and on the capillary pressure and relative permeabilities, centrifuge, displacement and gravity drainage experiments have been performed for various wettability and spreading conditions.

It is shown that both parameters modify the flow characteristics, the residual oil saturation and the recovery kinetics. More specifically, in a water-wet porous medium, the residual oil saturation is found to be higher and the drainage capillary pressure and relative permeabilities lower when measured for a non-spreading oil than for a spreading one. For an oil-wet porous medium the impact of
S
is much less important. However, for the same positive spreading coefficient, although the capillary pressure threshold is almost the same in water-wet and oil-wet porous media, the recovery kinetics are essentially different, due to the different displacement mechanisms.
Gas-oil capillary pressure measurements at reservoir conditions: effect of interfacial tension and connate water saturation
Authors: Longeron Daniel , Kalaydjian François , Bardon Charles
Published: April 2008

This paper describes laboratory equipment and procedures aimed at determining drainage gas-oil capillary pressure functions under reservoir conditions. Two series of capillary drainage tests (porous plate method) were performed with sandstone core samples using binary mixtures of methane/
n
-heptane at different equilibrium pressures, leading to very wide variations in the gas-liquid interfacial tension (IFT) values. Measurements were performed first without connate water (two-phase conditions) and then in the presence of connate water saturation (three-phase conditions). This study has shown that the gas-oil capillary pressures measured at different levels of IFT and without connate water can be transformed into one capillary pressure function,
P
c
/IFT, down to IFT = 3.7 mN/m. Below this value, the capillary pressures measured were significantly lower than those estimated by the Young-Laplace equation. In the presence of connate water, the gas-oil capillary pressures were always higher than the ones measured without water saturation. The difference increased when gas saturation increased and gas-oil IFT decreased. Finally, this paper showed that it is not valid to deduce three-phase capillary pressure from two-phase measurements.
Foam barriers for thin oil rims: gas blockage with hydrocarbon foams
Authors: Dalland Mariann , Hanssen Jan Erik
Published: April 2008

Creation of
in situ
foam barriers to gas flow is a potential way to improve production from wells which suffer gas breakthrough and give high gas-oil ratios. In this paper, new results on gas blockage by foamed liquid hydrocarbons are reported. These foams are applicable in the foam-block process because they can be injected as low-viscous, low-density surfactant solutions which may allow deep penetration near the gas/oil contact. Foam barriers of large radial extent may be created as oil production is resumed by reservoir gas coning through the treated zone.

It is shown by experiments in a sector model that a hydrocarbon foam of lower density than the reservoir oil, when injected at a low rate to allow gravity segregation, will place itself near the gas-oil contact and, upon production start-up, create an effective barrier to reduce the effect of gas coning. Gas coning in the absence and the presence of foams is investigated and compared.

Effective gas blockage is demonstrated for the first time with hydrocarbon foams at low pressure and at reservoir pressure and temperature. Foam performance is studied in water-wet model porous media at varying temperature and surfactant concentration. Experiments with and without wetting water present show the same performance. A given foamer formulation produced poorer gas-blocking performance in reservoir-condition tests than in low pressure tests. This was found to be related to the loss of solution viscosity as a result of gas dissolving at high pressure. By blending the kerosene solvent with a viscous hydrocarbon liquid such that the viscosity of the high pressure system was matched to that of the low pressure system at the respective foaming conditions, the gas-blocking ability of the high pressure foam was restored to match that of the low pressure system.
Soluble gas injection for waterflood profile modification
Authors: Hazlett Randy Doyle
Published: April 2008

A method involving the addition of water-soluble gas to exploit relative permeability relationships is evaluated for its potential to modify a waterflood profile. In this process, a soluble gas is added to injected water in an amount such that a single-phase aqueous solution enters the formation; however, the bubble point pressure is crossed in the steeply varying pressure profile around an injector. Solution gas alters the relative permeability to water in those zones taking water. Thus, the current water injection profile transports profile modification agent to the zones which need it most. Analytical model results for the radial flow of dissolved gas and subsequent gas evolution indicate that gas saturation is partitioned according to the permeability-porosity ratio. For most cases, this results in beneficial profile modification by three-phase relative permeability effects. Relative permeability reduction by a factor of three is anticipated through the nucleation of a trapped gas saturation. Gas saturations accumulate with time, making the process more beneficial with continued implementation. Though initially this process yields a near-wellbore treatment, prolonged application produces a trapped gas zone deep into the reservoir. Gas evolution away from the injector aids in injectivity maintenance. Increase in microscopic displacement efficiency, in addition to volumetric sweep, is also expected. Nitrogen, methane, and carbon dioxide are screened as potential water-soluble additives.
Performance evaluation and optimization of the Kuparuk hydrocarbon miscible water-alternating-gas flood
Authors: Hallam Richard J. , Ma Tuan D. , Reinbold Eric W.
Published: April 2008

In 1988 a hydrocarbon miscible water-alternating-gas (MWAG) pilot was initiated in the Kuparuk River field, on the North Slope of Alaska. Project decline curve analysis showed a significant response to the MWAG process. The field data alone, however, could not be used to accurately project the ultimate performance of the process. A systematic, integrated approach to reservoir modelling was used to evaluate the MWAG process performance. Finely gridded one dimensional (1D) models were used to evaluate the process mechanisms. Then fine and coarsely layered 2D and 3D compositional models were used to evaluate the process efficiency. The areal effects, including faulting, were then incorporated by using a 3D, coarsely gridded, multi-well, modified black oil (MBO) model. The 3D, site-specific, model was history-matched for the waterflooding period. The miscible flood performance was then predicted using a tuned MBO parameter and compared to the field performance. The 3D MBO model predicted that the ultimate incremental recovery would be 11 0.000000or a 30% HCPV slug. The model indicated that the miscible process can accelerate production of the waterflood reserves. The model was also used to evaluate the effects of total slug size, maturity of the preceding waterflood, and WAG ratio on the process efficiency.
Waterflooding under fracturing conditions: from theoretical modelling to field process
Authors: Niko H. , Ovens J.
Published: April 2008

The paper describes how some of the fundamental rock mechanical, heat and fluid flow aspects, of importance in the process of fracturing of water injection wells, have been used in Shell to build simulation and well testing tools. Examples of application indicate that thermal fractures in highly permeable formations are usually short with negligible influence on sweep efficiency. In tight, thick reservoirs the 3D aspect of fracture modelling becomes very important, particularly that of fracture containment. A field example of a thermal fracture in the North Sea is given, together with a brief account of results of a pilot test on injection under fracturing conditions in the Lekhwair field in Oman, using state-of-the-art fracture monitoring techniques.
Inorganic gels for enhanced oil recovery at high temperature
Authors: Altunina L. K. , Bokserman A. A. , Kuvshinov V. A. , Polkovnikov V. V.
Published: April 2008

A novel physico-chemical EOR method using inorganic gel-forming aluminium salt-carbamide-water (GALCA) systems has been developed. The method is based on the generation of an inorganic gel and CO
2
in reservoirs with high formation temperature. The time needed for gel formation depends on temperature and composition of the mixture used. The gel may be characterized as a thixotropic pseudoplastic solid-like body with a coagulated structure. The gels enhance reservoir homogeneity, modify water injection profile and consequently increase oil production and reduce watercut. In numerous field tests carried out in West Siberia, this method has proven to be both technically and economically effective, as well as ecologically acceptable.
Orbital forcing timescales: an introduction
Authors: House Michael R.
Published: April 2008

A brief review is given of orbital patterns affecting the Earth which may be of use in establishing, for long or short periods, orbital forcing timescales (OFT). The metronomic variations of the Earth-Moon system and of the Earth-Sun orbital patterns produce gravitational and temperature effects which alter the physical environment on the Earth’s surface. These give an interpenetrating effect of forcing cycles ranging from twice daily tides, day-night alternations, various tidal patterns and the annual solar pattern. All of these have been used palaeontologically to give precision to short-term age determination in the past.

It is cycles of the Milankovitch band which are showing promise of enabling new practical timescales to be established for parts of geological time. These depend on changes in the Earth-Sun distance (perihelion and precession cycles of 19 and 23 ka at the present time), changes in the tilt of the Earth’s axis with respect to the Earth’s orbit round the Sun (the obliquity cycles of 41 and 54 ka), and changes in the geometry of the Earth’s orbit around the Sun (eccentricity cycles of 106 and 414 ka). Since the number of days in the year have changed through time; so have the periods of the perihelion and precession cycles. There is increasing evidence that small-scale sedimentary rhythmic couplets, often grouped into bundles, may represent the effect of some of these; often the precessional couplets are grouped into bundles of five or so within the lower eccentricity period. The disentangling of the interpenetrating cycles to produce an OFT is an exciting problem and challenge for palaeobiology and sedimentology. These should enable numerical dates to be given to biostratigraphic and chronostratigraphic timescales and eventually enable many earth processes to be analysed in real time.

26 Ma oscillations related to the Cosmic Year (
c.
260 Ma) have been invoked to explain periodic mass extinctions in the fossil record. But evidence is presented to suggest such extinctions are not, in fact, periodic.
Calibration, analysis and interpretation of depositional cycles in the Early Toarcian of Yorkshire, UK
Authors: Moses Gwyn P. G.
Published: April 2008

The Toarcian of Yorskhire, UK, has been analysed and interpreted in order to determine whether shale-nodule couplets or cycles represent a regular cyclicity, and, if so, what the cyclicity could represent. This was done by calculating rates of deposition by analysing shale laminae and calculating durations of hiatus from rates of growth of nodules which are believed to be formed during hiatuses. This has enabled a new ammonite geochronology to be calculated, giving average zonal durations of 700 ka and subzonal durations close to 300 ka. The cyclicity is not regular enough to represent Milankovitch cycles and a tectonic origin related to pulsed North Sea thermal doming is suggested as an alternative mechanism.
High-resolution palynofacies investigation of Kimmeridgian sedimentary cycles
Authors: Waterhouse Helen K.
Published: April 2008

Palynofacies analysis is used as a tool to investigate in detail the palaeoenvironmental variations through several sedimentary cycles in the Kimmeridge Clay of Kimmeridge Bay, Dorset, UK. Evidence is given of palaeoenvironmental variations within cycles corresponding to those expected for obliquity orbital forcing. In addition, a second cyclical palaeoenvironmental variation, probably precessional forced, is seen in the palynofacies data. Further small-scale variations in palynofacies characteristics, which are not evident in the sedimentology, are also identified and allow cycles to be divided into a number of distinct palaeoenvironmental units. It is proposed that the obliquity cycle had its greatest effects on the marine environment, while the precessional cycle mainly affected the terrestrial environment. The abundance of useful palaeoenvironmental and palaeoclimatic information obtainable through high-resolution sampling in conjunction with a tool such as palynofacies analysis, provides evidence for and information about orbital forcing additional to that of most orbital forcing studies as it allows variations within cycles to be investigated.
Periodicities of carbonate cycles in the Valanginian of the Vocontian Trough: a strong obliquity control
Authors: Giraud F. , Beaufort L. , Cotillon P.
Published: April 2008

A high-resolution study of variations in carbonate productivity, quantified by CaCO
3
content and by colour intensity (grey level) for a Valanginian rhythmic succession is presented in the Vocontian Trough. Spectral analysis techniques reveal the presence of strong CaCO
3
cycles linked to cyclic variations in the Earth’s orbit. The results of the analysis suggest that obliquity cycle is the most clearly defined signal whereas the presence of CaCO
3
cycles related to the precession signal is less distinct.

The transition from limestone-dominant alternations in the lower Valanginian to marl-dominant alternations in the upper Valanginian is characterized both by an increase of sedimentation rate and a change from the precession as the dominant forcing in the lower, calcareous part of the Valanginian, to obliquity as the dominant forcing for the upper marly part of the Valanginian.

An orbitally calibrated chronology is presented for the Valanginian based on the identification of the carbonate cycles. The application of band-pass filterings to the original carbonate record allows the extraction of 91 carbonate cycles related to precession and 137 cycles related to obliquity. The proposed duration for the Valanginian stage is 7.04 Ma.
Detailed relationships between platform and pelagic carbonates (Barremian, SE France)
Authors: Quesne Didier , Ferry Serge
Published: April 2008

Field observations suggest that prograding bioclastic wedges of depositional sequences in outer-platform settings correlate with packages of basinal limestone beds thought to represent third-order lowstand systems tracts. The rule is apparently the same for higher-frequency cycles within these sequences, since basinal beds seem to correlate with bioclastic clinoforms while basinal marly interbeds root between clinoforms. If confirmed in other sequences, it would suggest that limestone beds, or bundles of beds, in this basin have the same ‘lowstand’ significance ‘highstand’ vs intervening marls from second- to sixth-order cycles, whatever the source of the carbonate (periplatform oozes or planktonic rain). A major remaining challenge is to understand the mechanisms by which high-frequency sea-level changes, as documented in outer-platform bioclastic wedges, correlate so well with the changes in nannoflora (nannoconus-rich beds vs coccolith-rich marly interbeds) thought to be climatic in the pelagic Vocontian Trough series. Is it glacio-eustasy and how does it work to make the deep-water Cretaceous limestone-marl alternation?
Reservoir geochemistry: methods, applications and opportunities
Authors: Larter S. R. , Aplin A. C.
Published: April 2008

Petroleum reservoirs often contain compositionally heterogeneous petroleum, water and sometimes gas columns which can be interpreted in geologically useful ways. Kilometre-scale lateral compositional heterogeneities in fluid columns suggest petroleum emplacement directions, directions of water flow in biodegraded oilfields and the presence of major barriers to fluid flow within reservoirs. Vertical fluid column heterogeneities on a 10 m scale suggest that reservoirs are vertically compartmentalized. These data can be used as a tool in reservoir description and are thus relevant to many aspects of reservoir appraisal and development. The data supplement those generated by reservoir geologists and engineers, and can be obtained immediately after drilling, at reasonable cost.

Much recent work has focused on polar nitrogen (N) and oxygen (O) containing compounds such as quinolines, carbazoles and phenols. These compounds interact relatively strongly with water, rock minerals and each other, and may significantly influence the viscosity and pressure-volume-temperature (PVT) properties of petroleum, and also reservoir wettability. There are obvious links here between reservoir geochemistry and the more traditional disciplines of petroleum and reservoir engineering which we expect to be exploited in the coming years.

In the absence of phase changes during migration, the composition of reservoired oil reflects the expelled oil plus changes induced by adsorption, especially of polar compounds, onto solid phases or by partitioning into porewaters along secondary migration pathways. The extent of change, as measured by the relative abundances of selected compounds in reservoired oil and associated water, is potentially a direct measure of migration volume and thus of the relative amounts of water, oil and rock on a migration pathway. We present a simplified, equilibrium model which may allow one to assess migration volumes, and use
n
-hexane/benzene ratios of some North Sea oils to suggest that these oils have contacted fewer than one hundred volumes of water on their migration pathways. Future work should aim to quantify the interactions of migrating and reservoired petroleum with rock and water, and should yield a more quantitative assessment of both migration volumes and the controls of reservoir wettability.
The reservoir geochemistry of the Eldfisk Field, Norwegian North Sea
Authors: Stoddart D. P. , Hall P. B. , Larter S. R. , Brasher J. , Li Maowen , Bjorøy M.
Published: April 2008

A detailed geochemical study has been performed on the North Sea Eldfisk Field chalk reservoir, which forms part of a larger regional evaluation of the Greater Ekofisk area. Utilizing rapid screening methods (Rock-Eval, Iatroscan), conventional biomarker maturity parameters, stable carbon isotope data (bulk and individual n-alkanes) and pyrrolic nitrogen distributions, it is shown that the Eldfisk Field petroleum is heterogeneous, both laterally on a kilometre scale and vertically on a metre scale. The chemical heterogeneities throughout the field led to a better understanding of the charging of the reservoir, the nature of communication between the Upper Jurassic source rocks and trap, and the identification of geological barriers to petroleum mixing within the field itself. The biomarker maturity and pyrrolic nitrogen compound data suggest that charging of the Eldfisk Field ocurred from the south, petroleum moving northwards up and along the Skrubbe fault zone charging the Eldfisk Alpha and Bravo structures. Although filling has largely taken place since c. 10 Ma
BP
, initial charging of the Eldfisk structure started c. 20 to 25 Ma
BP
. The data suggest that petroleum emplacement was via faults and fractures, with communication between the source rocks and trap being discontinuous. The distribution of the different petroleum populations within the field was controlled significantly by the internal field plumbing which is sedimentologically influenced. Sharp, step-like biomarker-parameter maturity variations downhole in several wells have been identified and verified statistically. These geochemial discontinuities can be correlated with specific geological features (hard-grounds and tight zones) which have prevented mixing of the petroleum column. In one case, the presence of a barrier in the reservoir has been confirmed by repeat formation test (RFT) pressure data. The identification of lithofacies variations laterally across the field and barriers vertically in the petroleum column may become important when assessing production scenarios.
The reservoir geochemistry and petroleum charging histories of Palaeogene-reservoired fields in the Outer Witch Ground Graben
Authors: Mason P. C. , Burwood R. , Mycke B.
Published: April 2008

Post-Chalk, particularly Tertiary, reservoired oils of the North Sea show a wide range of gross composition and frequent indications of biodegradative alteration. The latter has been associated with shallower reservoir depths and complex emplacement and accumulation histories. Comprising a detailed case study, lateral and vertical variations in petroleum composition within the Eocene-reservoired Chestnut and Alba Fields in the Outer Witch Ground Graben have been evaluated using bulk properties gas chromatography (GC), GC-mass spectrometry and isotopic techniques. The oils have experienced varying degrees of in-reservoir biodegradation. They were generated at an early main phase of source-rock maturity. The migration pathway to the Eocene reservoir is thought to be controlled by selective reactivation of Jurassic faults at times of compressional movement during the Tertiary. Using this model in conjunction with subsidence-history profiles, the timing of generation has been investigated and the temperature of the Eocene reservoir at petroleum emplacement has been estimated. The dominant control on biodegradative alteration is believed to be associateded with the distance from meteoric water influx and the available oxygen budget. According to distance from the hydrostatic head, an apparent decline in biodegradation, albeit showing contradictory levels of alteration, was observed along the trend. Various in-reservoir mechanisms to account for this apparent decline, including topping-up, intermediate transformation products and differential oil-column attack, have been evaluated. The study continues with the objectives of investigating the compositional variations of petroleums reservoired within Lower Cretaceous sandstones and the charging relationships between these and the Eocene reservoirs.
Regional setting of hydrothermal activity
Authors: German C. R. , Baker E. T. , Klinkhammer G.
Published: April 2008

High-temperature hydrothermal venting, in which heat is transferred from the lithosphere to the oceans, is intimately associated with all forms of active plate boundaries — fast- and slow-spreading centres, fracture zones and even with subduction zones, in the form of back-arc spreading centres. A range of steady-state hydrothermal vent sites are described in various tectonic settings. A selection of the more unusual hydrothermal features and their geological environments are also considered, including hydrothermal ‘events’, gas-rich vents, and sites of phase separation. Modelling of hydrothermal plumes is introduced and the importance of focused high-temperature venting is reviewed with respect to global hydrothermal fluxes.
Geological setting of high-temperature hydrothermal activity and massive sulphide formation on fast- and slow-spreading ridges
Authors: Krasnov Sergey G. , Poroshina Irina M. , Cherkashev Georgiy A.
Published: April 2008

Geological features which control massive sulphide formation on the fast-spreading East Pacific Rise (EPR) and slow-spreading Mid-Atlantic Ridge (MAR) can be specified based on data from the Sevmorgeologija Association (St Petersburg) research cruises and results of other studies. Wide crestal surfaces of undisturbed axial volcanoes and the presence of axial grabens that indicate voluminous subsurface magma chambers represent the sites most favourable for sulphide formation on the EPR. The elevation of rift segments and distance from major ridge-axis discontinuities are less important for sulphide formation. Sites of localized magma delivery from subcrustal zones, as indicated by Mg anomalies in basalts, may be favourable. However, at least one site near 21°30′S on the EPR shows evidence of along-axial magma penetration from the central part of the rift segment to its tip, resulting in a lateral shift of hydrothermal activity with time. Higher crustal permeability for magma is required for the formation of subsurface chambers which initiate hydrothermal convection on the magmatically less active MAR. Rift valley marginal faults, and especially their intersections with minor transverse dislocations, locally control hydrothermal activity where magma laterally penetrates from the extrusive zones of the adjacent rift segments.
Time series studies of vent fluids from the TAG and MARK sites (1986, 1990) Mid-Atlantic Ridge: a new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulphide orebodies
Authors: Edmond J. M. , Campbell A. C. , Palmer M. R. , Klinkhammer G. P. , German C. R. , Edmonds H. N. , Elderfield H. , Thompson G. , Rona P.
Published: April 2008

The hot springs at TAG and MARK on the Mid-Atlantic Ridge have been resampled after an interval of four years (1986–1990). The fluid compositions show the same temporal stability as observed elsewhere, e.g. 21°N, East Pacific Rise (EPR) and the Guaymas Basin. Although the MARK fluids have no chemical characteristics that would distinguish them from those on the faster spreading ridges of the Pacific, TAG has pronounced differences. The depletion in B and the small shift in δ
11
B indicate a substantial degree of reaction at intermediate temperatures along the recharge path that is unique to TAG. In addition, the TAG mound contains a large cluster of sphalerite-rich ‘onion domes’ that have been formed from an approximately 5:1 mixture of the primary hydrothermal end-member fluid and seawater that is formed within the deposit. This has resulted in the extensive precipitation of FeS within the mound and a resulting decrease in pH to values below 3. The low pH causes the large-scale remobilization of Zn from the interior of the deposit and its reprecipitation on the surface as the domes. Such compositional zoning is a common feature of ophiolite-type massive sulphide ore bodies and probably results by the same mechanism. The end-member data from both hydrothermal areas fall on the mixing planes defined by the EPR and Juan de Fuca data in a new three-component mixing model, indicating the presence of a phase-separated brine pool at depth under these Mid-Atlantic Ridge systems which is very similar in composition to those on the Pacific ridges.
Dissolved methane and hydrogen in the Steinahóll hydrothermal plume, 63°N, Reykjanes Ridge
Authors: Palmer M. R. , Ludford E. M. , German C. R. , Lilley M. D.
Published: April 2008

Dissolved concentrations of CH
4
, H
2
and Mn have been measured in a hydrothermal plume above the shallow water (
c.
250m) Steinahóll vent site at 63°N on the Reykjanes Ridge. Samples were taken in October 1990 and June 1993. In 1990 dissolved CH
4
levels of up to 108 nmol/l and dissolved H
2
levels of up to 42 nmol/l were recorded. By 1993 these had declined to a maximum of 18nmol/l for CH
4
and 30nmol/l for H
2
. Re-occupation of the site in 1993 included a higher density of sampling, suggesting that the fall in measured levels of these dissolved gases was not a sampling artefact. The high dissolved CH
4
and H
2
concentrations reported for 1990 were measured immediately after an episode of earthquake activity which may have resulted in the injection of magmatic gases into the local hydrothermal system. By 1993 the dissolved CH
4
levels had returned to values more typical of submarine hydrothermal systems elsewhere on the Mid-Atlantic Ridge. Dissolved H
2
levels remained high at Steinahóll, relative to other mid-ocean ridge sites, and it is believed that this is due to a combination of inorganic and biogenic processes. The gas chemistry of the Steinahóll hydrothermal site bears some similarities to that of the geothermal waters on the nearby Reykjanes Peninsula, Iceland, whereas the high levels of dissolved Mn in the plume are more typical of a submarine hydrothermal system. It appears, therefore, that the Steinahóll hydrothermal site may represent a transition between submarine and subaerial geothermal systems in its gas chemistry.
Hydrothermal deposits and metalliferous sediments from TAG, 26°N Mid-Atlantic Ridge
Authors: Mills Rachel A.
Published: April 2008

The Trans-Atlantic Geotraverse (TAG) hydrothermal site at 26°N on the Mid-Atlantic Ridge has been the focus of many studies since the early 1970s. The hydrothermal field includes a wide variety of deposits ranging from high-temperature sulphide chimneys, low-temperature Mn deposits, diffuse flow to relict, inactive mounds. These deposits reflect the different styles of venting and the evolution of the hydrothermal system over time. Metalliferous sediments in this area are derived from both mass wasting of mound features and fall out of plume particulates. The active TAG mound was drilled by the Ocean Drilling Programme in October/November 1994, which provided the first samples from beneath the surface at this site. The current state of knowledge of processes occurring within TAG metalliferous deposits are reviewed here.
Noble gas isotopes in 25 000 years of hydrothermal fluids from 13°N on the East Pacific Rise
Authors: Stuart F. M. , Harrop P. J. , Knott R. , Fallick A. E. , Turner G. , Fouquet Y. , Rickard D.
Published: April 2008

Noble gas isotopes have been measured in fluid inclusions in sulphides spanning 25 000 years of hydrothermal activity at 13°N on the East Pacific Rise. The
3
He/
4
He ratios are typical of mid-ocean ridge hydrothermal fluids, albeit slightly higher than contemporary vent waters, and reveal no temporal variation or correlation with the δ
34
S of the host sulphide. The absence of radiogenic He in fluids from the 25 000 year old mineralization on the SE Seamount suggests that the hydrothermal circulation occurred within an active magmatic system and not within the underlying 130 ka oceanic crust. This implies that seamount volcanism and hydrothermal activity occurred simultaneously off-ridge, and that magmatic activity shifted approximately 5 km off-ridge at this time. Helium concentrations in fluid inclusions from three samples are significantly greater than the end-member hydrothermal fluids at mid-ocean ridges. Small excesses of
40
Ar in the included fluids demonstrate that mantle-derived
40
Ar has been degassed along with primordial helium. Both are consistent with the direct addition of magmatic volatiles into the hydrothermal system at times during the history of hydrothermal activity at the site.
Preliminary modelling of hydrothermal circulation within mid-ocean ridge sulphide structures
Authors: Dickson Penny , Schultz Adam , Woods Andrew
Published: April 2008

The spatial distribution of venting sites at mid-ocean ridge crest hydrothermal fields, in combination with direct observations of the temperature and velocity of (primarily diffuse) effluent, provides constraints on the circulation of fluids both within exposed hydrothermal edifices and in the seafloor below. Before any constraints can be placed on either the interior structure of these bodies or the associated heat and mass transfer between them, their role in controlling the flow needs to be investigated. A simplified finite difference forward model of an idealized sulphide edifice allows the calculation of the interior velocity and temperature distributions and, more importantly, observation of the effect that the flow-governing components have on the circulation. Changes in permeability structure and in basement heat and mass fluxes have an important effect on the flow.
Modelling diffuse hydrothermal flow in black smoker vent fields
Authors: Pascoe A. Rachel , Cann Johnson R.
Published: April 2008

An important end-member of hydrothermal circulation at mid-ocean ridges is the low-temperature diffuse flow found at all known black smoker vent fields. Diffuse flow is the result of the subsurface mixing of cold seawater with hot black smoker fluid. The conditions that allow this mixing to take place are poorly understood. To gain further understanding of diffuse flow a simple pipe model has been used to explore the relationship between the sub-seafloor permeability structure and the temperature of the exiting fluid. Calculations with this simple model show a wide and continuously varying exit temperature from black smoker temperatures to diffuse flow temperatures as the permeability structure is changed. The results show that for low temperatures to be observed at the surface, the upper part of the circulation for that part of the discharge has to be relatively permeable, and that the deeper sections of the upflow must be much more impermeable. The model suggests that the early stages of evolution of a hydrothermal system are characterized by widespread diffuse flow and that black smokers develop as subsurface precipitation reduces the permeability of the upper section of the crust.
Mineralogy and sulphur isotope geochemistry of the Broken Spur sulphides, 29°N, Mid-Atlantic Ridge
Authors: Duckworth Rowena C. , Knott Richard , Fallick Anthony E. , Rickard David , Murton Bramley J. , Van Dover Cindy
Published: April 2008

Massive sulphide-sulphate-oxides from the newly discovered Broken Spur hydrothermal field at 29°10′N on the slow-spreading Mid-Atlantic Ridge have been examined to determine their mineralogical, chemical and sulphur isotope characteristics. The chimney and spire samples are dominated by chalcopyrite-isocubanite-pyrrhotiteanhydrite, with the uppermost active areas composed of anhydrite-rich ‘beehive’ structures which act as diffusers for the hydrothermal fluids and scaffolding for the chimney construction. Delicate ‘organ-pipe’ chimney tips grow on the beehive structures. Samples from mounds and the base of active edifices are more enriched in Fe disulphides and sphalerite, with significant amounts of Fe oxyhydroxides and amorphous silica. Sulphur isotope values of sulphides range from −0.8 to 2.4‰, similar to those reported from the Snakepit hydrothermal vent field. This range is significantly lower than the range reported from the fast-spreading East Pacific Rise, and this may be a function of fluid flow paths in differing hydrothermal settings.
Hydrothermal processes and contrasting styles of mineralization in the western Woodlark and eastern Manus basins of the western Pacific
Authors: Scott Steven D. , Binns Raymond A.
Published: April 2008

The western Woodlark Basin (initial rifting of continental crust) and eastern Manus Basin (rifted arc crust) offshore eastern Papua New Guinea display contrasting styles of hydrothermal activity and mineralization. In the eastern Manus basin, en echelon felsic and mafic volcanic ridges have formed in a pull-apart basin of the rifted New Britain arc terrane. Here, the PACMANUS Cu-Zn-Pb-Ag-Au sulphide deposit is forming within an area of about 800 × 350 m on the flank of a dacite lava dome atop a prominent 20 km long and 250–350 m high volcanic ridge. The ridge is andesitic in its lower reaches, dacitic to rhyolitic on top and is adjacent to an extensive field of basalt. At Woodlark, submarine rhyolite domes are devoid of hydrothermal products, but extensive Fe-Si-Mn oxyhydroxide deposits are forming from low temperature fluids on Franklin Seamount, an axial basaltic andesite volcano near the tip of the oceanic propagator. Protruding through and perhaps underlying these oxyhydroxides are inactive, higher temperature, precious metal-rich (Ag to 545 ppm, Au to 21 ppm), barite-silica spires. The Franklin Seamount deposits are thought to cap a disseminated sulphide deposit within the volcano and represent a failed massive sulphide system. Both Franklin Seamount and PACMANUS provide models for ancient ores on land.
Hydrothermal input into sediments of the Mid-Atlantic Ridge
Authors: Cherkashev G. A.
Published: April 2008

The analysis of geochemical profiles in metalliferous sediment cores obtained from the Mid-Atlantic Ridge rift valley at the TAG, MARK and 15°N hydrothermal vent fields and near 27°10′N allowed a study of the history of hydrothermal processes. Of the three hydrothermal zones of the TAG field, the North hydrothermal zone reached its maximum activity between 6000 and 25 000 years ago, the MIR Mound 10 000 years ago and the Active mound 15 000 years ago. Evidence of intensive hydrothermal sedimentation is not seen near the MARK field. The MARK sulphide mounds were probably formed in the course of a single hydrothermal episode that began 4000 years ago. Most of the sulphides of the newly found hydrothermal field near 15°N were formed in the course of an event traced by a Fe-enriched layer 70–90 cm below the seafloor. At present, this field is only slightly hydrothermally active. Three Fe-enriched layers correspond to the possible stages of ore formation at a site of presumed hydrothermal activity near 27°10′N. Their relative intensity is decreasing with time.
Hydrothermal sedimentation at ODP Sites 834 and 835 in relation to crustal evolution of the Lau Backarc Basin
Authors: Hodkinson R. A. , Cronan D. S.
Published: April 2008

Hydrothermally metal-enriched sediments from ODP Leg 135 Sites 834 and 835 in the Lau Backarc Basin have been studied with respect to basin evolution. Multivariate statistics (factor analysis) indicate element associations in the non-detrital, carbonate-free sediment fraction characteristic of hydrothermal oxide plume fallout. Selective chemical leach, X-ray diffraction and lithological studies confirm that the hydrothermal enrichment in the sediments at both sites results from distal oxide plume fallout. No evidence of a sulphide/weathered sulphide component is seen, suggesting that the sites have never been proximal to a hydrothermal discharge source. Factor scores for the hydrothermal oxide phase of the sediments reflect the hydrothermal activity associated with the propagation of the Eastern Lau Spreading Centre (ELSC) into the basin. The subsequent propagation of the Central Lau Spreading Centre (CLSC) into the basin is not clearly recorded. Non-detrital element accumulation rates for Mn and Fe confirm this. However, the results suggest a ‘time lag’ between the closest passage of the ELSC past the latitude of Sites 834 and 835 and maximum hydrothermal plume fallout flux to the sediment section. This may be due to a delay between ridge formation and either the initiation of hydrothermal systems on the ridge and/or their full development. In addition, however, plume dispersion patterns may have been affected by palaeo-current directions and basin topography. Element accumulation rate values show that the hydrothermal flux from the ELSC was of much greater intensity than that from the CLSC. The latter was comparable with values in presently accumulating sediments proximal to the CLSC and within the range typical of East Pacific Rise (EPR) values. The ELSC accumulation rates are comparable with, or higher than, the highest reported values from the EPR. Compared with EPR sediments, the Lau Basin hydrothermal plume fallout is rich in Mn relative to Fe. In particular, plume fallout associated with the ELSC exhibits a two- to four-fold enrichment of Mn relative to Fe compared with the EPR.
Ecology of Mid-Atlantic Ridge hydrothermal vents
Authors: Dover Cindy Lee van
Published: April 2008

In the past year, the number of explored deep-water vent sites on the Mid-Atlantic Ridge has doubled. The fauna of Atlantic vents consists for the most part of a subset of invertebrate types found elsewhere in chemosynthetic ecosystems, with taxonomic differentiation usually at the species or genus level. Despite this similarity in taxonomic composition, the ecology of Atlantic vents differs from the ecology of Pacific vents in ways that highlight aspects of biogeography, trophic ecology and sensory adaptations.
Hydrothermal fluxes of metals to the oceans: a comparison with anthropogenic discharge
Authors: German Christopher R. , Angel Martin V.
Published: April 2008

Published data for Cu, Zn, Pb, Mn, Co and Cd concentrations in hydrothermal vent fluids have been combined with best estimate vent fluid volume fluxes to generate annual gross fluxes of these dissolved metals to the oceans. These are compared with the gross annual fluxes for anthropogenic metal release to the environment, which must ultimately enter the oceans. Comparison of gross fluxes suggests that hydrothermal discharge is significant for Mn and, to a lesser extent, for Co, Zn, Cd and Cu; anthropogenic discharge is dominant for Pb. Dissolved metal fluxes from high temperature vents are modified by the precipitation of various metal sulphide and oxide phases in hydrothermal plumes. Similarly, river-borne fluxes of anthropogenic metals to the oceans are modified by the processes of dissolved metal flocculation and suspended particle coagulation and deposition in estuaries. Quantification of these processes using available published data suggests that for all metals except Mn, the net flux of metals to the oceans from hydrothermal discharge must be negligible when compared with the net anthropogenic flux.
Mechanics of basin inversion from worldwide examples
Authors: Lowell James D.
Published: April 2008

Inversion is dependent on pre-existing basin configuration in the initial subsidence, usually extensional phase, and the resolution of compressional forces in the later shortening phase. External horizontal, rather than isostatic vertical forces are required for inversion because many deep sediment-filled basins around the world have never been inverted, but should have been, if isostatic rebound were the driving mechanism.

Inversion can range in scale from basins to sub-basins to selected structures within basins. Typically, rift basins can be later inverted. Mainly by reactivation of older normal faults, inversion selects rift basins where, in pure shear, weakening because of necking or thinning of lithosphere has occurred; and where, in simple shear, mechanical detachment surfaces are available for subsequent movement. Some pre-existing lows or sags can apparently be inverted in the absence of reactivated normal faults, as in the southern Altiplano of Bolivia and offshore Sabah, Borneo.

Basins can be inverted by dominantly strike-slip with some convergent component, e.g. offshore northeast Brazil, and by almost direct compression, Atlas Mountains, Morocco. A prime difference is that convergent strike-slip can reactivate relatively high-angle normal faults as reverse faults, whereas these faults tend to lock when subjected to more direct compression which then creates lower angle contractional faults. Usually, inversion is caused by a combination of compression and strike-slip (transpression) because the azimuth of maximum principal compressional stress to the direction of original basin trend vectors into an oblique-slip component. This is illustrated in northern Argentina where compression derived from Andean deformation is resolved obliquely (about 45°) against an older rift basin in the foreland nearby. Interestingly, for this area at least, the 45° angle which affords an equal contribution of strike slip and compression results only in reactivation of older normal faults and does not create younger contractional features.

Apparently compressive forces can be transmitted backward from the lead edge of an underthrusted foreland plate to invert rather remote regions and basins that are carried on that plate; this may be a significant part of the mechanism for basin inversion in northwest Europe and the southern North Sea as that foreland region underthrusted along the Alpine system.
Fold belt in the San Jorge Basin, Argentina: an example of tectonic inversion
Authors: Homovc J. F. , Conforto G. A. , Lafourcade P. A. , Chelotti L. A.
Published: April 2008

The San Jorge basin, located in southern Argentina, extends over the central part of the Patagonian region. The sedimentary fill of the basin is related to different rift and sag tectonic phases, from Triassic to Cretaceous. During Late Cretaceous-Early Tertiary, a marine transgression from the Atlantic developed; Tertiary sediments completed the basin fill. Late Tertiary compression produced a narrow structural belt that crops out in the San Bernardo Range and is present in the subsurface of the Santa Cruz and Chubut Provinces. The fold belt is 150 km long and 50 km wide. In its northern zone, it is formed by a series of anticlinal structures related to high angle reverse faults; in the southern part, these structures have similar characteristics, being associated with reverse faults which are the result of the reactivation of pre-existing normal faults. This report describes the structural features of a wide subsurface area of the fold belt, with high hydrocarbon potential. The development of the Neocomian sequence is used to establish the relationship between tectonism and sedimentation.