bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

Abstract

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

Abstract
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

Abstract
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

Abstract
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

Abstract

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

Abstract

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

Abstract
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

Abstract

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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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.