bluuebunny/crossref_metadata_2025_split huggingface dataset:
Sour gas and water chemistry of the Bridport Sands reservoir, Wytch Farm, UK
Authors: Aplin Andrew C. , Coleman Max L. 
Published: April 2008

Abstract

Co-produced waters and gaseous H
2
S were sampled at four wells at Wytch Farm oil field, UK, over a one year period. The chemical and stable isotopic (H, O) compositions of the waters were determined, plus the sulphur isotopic composition of dissolved sulphate and gaseous sulphide. Formation waters contain about 70 000 mg 1
−1
total dissolved solids (TDS) and are evolved meteoric waters; there is a 10% fieldwide variation in TDS. H
2
S was formed by (bacterial?) reduction of formation-water sulphate. A high but variable fraction of the sulphide has been subsequently lost, perhaps by reaction with abundant Fe-bearing minerals. At least some, and possibly a significant fraction, of the sulphide in the reservoir is dissolved in formation water. Simple mass-balance models show that when reservoir sulphide is pre-dominantly within formation water, the concentration of H
2
S in produced gas will increase dramatically with increasing water cuts. In this case, increasing concentrations of H
2
S in produced gas do not necessarily imply that souring has been caused by production practices. Seawater breakthrough is recognized and quantified in one well using chemical and stable isotopic data. K, Mg and SO
4
are all lost from injected seawater during its passage through the reservoir. Geochemical modelling and isotopic data fail to identify the fate of the missing sulphate, although the isotopic data tentatively suggest that some may have been converted to sulphide. However, the levels of sulphide produced from the breakthrough well are no higher than in other wells, suggesting that reduction of seawater sulphate contributes a small percentage of fieldwide H
2
S production.
Characteristics of hydrothermal discharge following a magmatic intrusion
Authors: Baker Edward T. 
Published: April 2008

Abstract

Seafloor hydrothermal systems are profoundly altered by magmatic fluctuations, which are inherently episodic and generally unpredictable. At present, three examples of hydrothermal systems perturbed by magmatic intrusion have been identified and sampled: 1986 on the Cleft segment of the Juan de Fuca Ridge; 1991 at 10°N on the East Pacific Rise; and 1993 on the CoAxial segment of the Juan de Fuca Ridge. From the fragmentary observations at each site three trends can be identified that may be common to magmatically perturbed hydrothermal systems. The flux of heat and mass can increase by orders of magnitude virtually simultaneously with a magmatic intrusion by the sudden and short-lived release of event plumes, while chronic discharge remains elevated for months or years afterwards. Vent fluid composition is altered by at least two processes. Phase separation is initiated or enhanced, producing fluids highly enriched in the vapour phase. The conjugate brine-enriched fluid may be stored in the crust to be flushed months or years later by convecting seawater. Magmatic degassing increases the flux of volatiles, temporarily elevating
3
He/temperature ratios. Time series observations of magmatically altered vent fields are vital, because chemical budget extrapolations and hypotheses derived only from observations of stable hydrothermal discharge may be incomplete or unreliable.
Particle formation, fallout and cycling within the buoyant and non-buoyant plume above the TAG vent field
Authors: Rudnicki Mark D. 
Published: April 2008

Abstract

The relationship between nephelometer and transmissometer measurements in the buoyant and non-buoyant plume above the TAG vent field have been investigated by calculating a phase angle between these two instruments, denoted α
N-Tr
. This method indicates three distinct particle populations, corresponding to buoyant plume water (α
N-Tr
<−20°), ambient seawater (−20°<α
N-Tr
<−10°) and non-buoyant plume water (−6°<α
N-Tr
<−5°). There is a dramatic change in the nature of hydrothermally produced particles in the buoyant plume, which may be due to the formation and fallout of large sulphide particles. There is no apparent evolution of the non-buoyant plume particle population at distances of several kilometres from the vent site. Although particle processes in the buoyant plume are characterized by formation and fallout, particles settling out at the top of the buoyant plume may be recaptured and recycled. Plume-driven recirculation leads to an increase in the residence time of particles within the plume system. The residence time of particles recycled at the top of the buoyant plume has been estimated from thorium isotope distributions as
c.
50–60 days. This model assumes that there is diffusion of Th into the plume system and the thickness of the diffusive interface has been calculated from the balance between the scavenging flux and the diffusive flux as
c.
7m.
Selective fault reactivation during basin inversion: potential for fluid redistribution through fault-valve action
Authors: Sibson Richard H. 
Published: April 2008

Abstract

Inversion structures, associated with the compressional reactivation of moderate to steeply dipping normal faults inherited from earlier crustal extension, form important structural traps for hydrocarbons. Migration into these traps must occur syn- or post-inversion. Seismic reflection profiles show that inversion is frequently highly selective, with only some of an existing normal fault set being reactivated. Neglecting marked stress-field heterogeneity, frictional mechanics suggests three possible explanations for this selective reactivation: (1) preferential reactivation of shallowest-dipping normal faults in a region that previously underwent the greatest extensional ‘dominoing’ of fault blocks; (2) the presence of anomalously low friction material along particular faults; and (3) a heterogeneous distribution of fluid overpressures (P
f
> hydrostatic) with preferential reactivation occurring in the area of most intense overpressuring. The last possibility is favoured by the likelihood that fluid overpressures develop during inversion as a consequence of the dramatic increase in mean stress that accompanies the transition from an extensional stress regime, with high capacity to store fluids, to a compressional regime with comparatively low storage capacity.

Compressional reactivation of moderately to steeply dipping faults likely requires significant overpressures; in the case of faults with dips in excess of the frictional ‘lock-up’ angle (typically 50–60°), supralithostatic fluid pressures (P
f
> σ
3
= σ
v
) are a necessary prefailure condition in rupture nucleation sites. Extreme fault-valve action then becomes possible, with postseismic flushing of fluids upwards along faults from overpressured compartments. Evidence for such activity comes from mesothermal Au-quartz veins hosted in steep reverse faults, where repeated attainment of supralithostatic pressures alternated with discharge episodes along the faults. Episodes of vertical hydrocarbon migration along reverse faults are therefore a likely accompaniment to basin inversion; there are, for instance, historical records of postseismic discharge of aqueous and hydrocarbon fluids from the overpressured basins of the Western Transverse Ranges, California, where steep reverse faults remain active today. Careful assessment of the relative timing of fault reactivation during inversion and hydrocarbon migration is needed to evaluate the hypothesis for ancient inverted basins.
3D sandbox models of positive inversion
Authors: Keller J. V. A. , McClay K. R. 
Published: April 2008

Abstract

3D scaled physical models of positive inversion involving idealized, simple cylindrical listric and ramp-flat listric fault systems were carried out. In both types of models extension and inversion produced strongly segmented faults with fault displacement varying along-strike. Displacement transfer was achieved through relay ramp structures between overlapping faults. No transfer faults developed during extension or inversion. Inversion produced four distinct responses, basal décollement reactivation, folding, faulting and back-rotation of faults associated with localized layer-parallel shortening. Upon inversion reverse faults nucleated at the tips of pre-existing extensional faults. Reverse faults formed longer segments and accommodated less displacement than extensional faults of the same length. Maximum fault displacement/length ratios (γ) for extensional and reverse faults in the models varied between 2.5 × 10
−1
and 7.0 × 10
−3
(best fit values range from 1.1 × 10
−2
to 6.4 × 10
−2
) and are within the range measured for natural faults. γ values for reverse faults are consistently lower than for extensional faults in both listric or ramp-flat experiments. The results of these analogue models are comparable to natural examples of inverted fault systems.
Basin inversion in a strike-slip regime: the Tornquist Zone, Southern Baltic Sea
Authors: Deeks Nigel R. , Thomas Stefan A. 
Published: April 2008

Abstract
The southern section of the deep seismic reflection profile BABEL crosses the Tornquist Zone between the mainland of Southern Sweden (Scania) and the island of Bornholm. This marks the boundary between the Sorgenfrei Tornquist Zone to the northwest and the Teisseyre Tornquist Zone to the southeast. The region represents a large-scale releasing bend in the dextral strike-slip system of Tornquist and has led to the development of pull-apart basins such as the Rønne and Arnager grabens, from the Carboniferous to the Late Jurassic.
The area around BABEL is covered by proprietary surveys (courtesy of JEBCO Seismic Ltd, BGR Germany and the Swedish Geological survey) which allow the three-dimensional interpretation of sediment/structure interactions within these basins. Within the area various classical (positive) inversion structures are seen on seismic sections, including harpoon structures and inversion monoclines. These features are found in association with both basin-bounding and intra-basinal faults. There is also evidence for inversion of basement blocks within the upper crust. Inversion took place between the late Cretaceous and early Tertiary, probably as a response to Carpathian/Alpine movements to the south. Strike-slip structures indicate that movements were dominantly dextral with rigid blocks such as Bornholm channelling deformation around them. At the same time, progradational sequences of sediment are seen to build off these blocks.
The BABEL section images a number of features believed to have resulted from the post-Carboniferous strike-slip movements of the Tornquist fault system. Structures imaged within the upper part of the section are consistent with those interpreted in three dimensions from the Jebco survey, as a brittle response within the upper crust. Below these, between 7 and 12 seconds TWT, a zone of high reflectivity is observed. This is similar in character to regions of lower crust imaged on either side to the northeast and southwest, but it is shallower below the Tornquist Zone. This is interpreted as a region of lower crust in which deformation is transferred in a more ductile fashion. Below this again there are coherent reflectors that may represent a more brittle transfer of the deformation into the upper mantle. The extent of upper crustal deformation imaged on this profile is around 40 km in width whilst in the lower crust it is at least 90 km.
High-resolution chemostratigraphy of Quaternary distal turbidites: a case study of new methods for the analysis and correlation of barren sequences
Authors: Pearce Timothy J. , Jarvis Ian 
Published: April 2008

Abstract

The late Quaternary distal turbidites of the NE Atlantic Madeira Abyssal Plain (MAP), represent one of the most intensely studied sequences in the modern deep ocean. More than 160 sediment cores have been recovered from the 68 × 10
3
km
2
of the plain, and existing litho- and biostratigraphies, when integrated with oxygen stable-isotope data for other areas of the NE Atlantic, provide a tight spatial and temporal framework. We have used the MAP as a test-bed to assess the applications of inorganic geochemistry as a sedimentological and stratigraphic tool. Data for 22 major and trace elements were obtained in over 500 samples taken from representative cores. Chemostratigraphic sequences are highly uniform across the plain, and several turbidites have unique geochemical signatures, providing a means of establishing bed-by-bed correlations over distances of > 500 km. Cluster and principal component analysis confirm the statistical validity of empirically derived correlations and groupings. Vertical and lateral geochemical trends within beds document the sedimentological evolution of flows, and may be used to establish palaeotransport pathways. Four compositional groups are interpreted as representing derivations from different source areas: (1) the margins of the NW Africa off Morocco; (2) off Western Sahara; (3) the Canary Islands; (4) the Cruiser-Great Meteor East seamount chains. The study amply demonstrates the viability of chemostratigraphy for the correlation and analysis of sedimentary sequences, providing a stratigraphic resolution and reliability which matches or exceeds that obtainable by other techniques.
SHRIMP zircon age control of Gondwanan sequences in Late Carboniferous and Early Permian Australia
Authors: Roberts John , Claoue-Long Jonathan , Jones Peter J. , Foster Clinton B. 
Published: April 2008

Abstract

Australia during the Late Carboniferous formed part of the Gondwana supercontinent and was close to the South Pole. Resulting continental glacigene deposits and cold water marine sequences in the Southern New England Orogen cannot be correlated biostratigraphically with Late Carboniferous successions in the northern hemisphere because they contain a low diversity biota endemic to Gondwana. Magnetostratigraphic correlation via the Permian-Carboniferous reversed magnetic superchron is presently uncertain. Sensitive high-resolution ion microprobe (SHRIMP) zircon dating of volcanics horizons in the sedimentary sequence of the Southern New England Orogen is now establishing relationships between the Gondwanan faunas and warm-climate European equivalents. In Continental sediments, correlation of the major Gondwanan glaciation of Late Carboniferous Australia is revised from Stephanian to early Namurian-Westphalian in age, a timing that matches that of the Hoyada Verde glaciation in Argentina. A major hiatus of > 15 Ma between Carboniferous rocks of the Southern New England Orogen and Permian rocks of the Sydney Basin of eastern Australia probably reflects the first deformational movements within the accretionary prism of the orogen.
Nothorhacopteris
, traditionally a Late Carboniferous indicator in Australia, now ranges from late Viséan to Westphalian, and an ‘enriched’ assemblage, previously correlated with the top of the
Nothorhacopteris
flora, is confined to the late Viséan to early Namurian. Palynofloras assigned to the
Spelaeotriletes ybertii/Diatomozonotriletes birkheadensis
Zones (Namurian-early Westphalian), are now confined to the Namurian, and a mid to late Westphalian age is indicated for a more diverse palynoflora identified as uppermost
D. birkheadensis
Zone (? =
Asperispora reticulatispinosus
Zone). In the cold climate marine sequence, the
Levipustula levis
brachiopod Zone appears to be confined to the early Namurian, and so partly corresponds in age to the continental glaciation.
The application of samarium-neodymium (Sm-Nd) Provenance Ages to correlation of biostratigraphically barren strata: a case study of the Statfjord Formation in the Gullfaks Oilfield, Norwegian North Sea
Authors: Dalland A. , Mearns E. W. , McBride J. J. 
Published: April 2008

Abstract
Samarium-neodymium (Sm-Nd) isotope analyses indicate that the Statjord Formation in the Gullfaks Oilfield was derived from two compositionally distinct clastic sediment source terrains. One provenance area produced sediment with ‘low’ Provenance Ages (< 1800 Ma), which are similar to those of numerous other North Sea formations. The sedimentary rocks of this group were deposited by palaeocurrents which flowed towards the NNE and it is likely that they were derived from Triassic, and possibly Devonian, strata to the southwest of the Gullfaks area (e.g. East Shetland Platform). The other provenance area produced sediment with anomalous ‘high’ Provenance Ages (> 1800 Ma). These sediments were transported predominantly towards the south and are compositionally less mature and have poorer reservoir properties than those with ‘low’ Provenance Ages. The source terrain for this provenance group is considered to lie to the north of the Gullfaks Oil Field area, where exposed Lewisian gneisses probably formed a major clastic sediment source terrain. Switching between these contrasting provenance areas gave rise to the interdigitating, low-to-high Provenance Age profile which characterizes the Statfjord Formation. The Provenance Age profiles are used in combination with conventional well logs and sedimentological core descriptions to construct a detailed reservoir zonation. The method has allowed confident correlation of channel sandstones with overbank mudstone deposits. The resulting correlation scheme has revealed the presence of intraformational erosional surfaces and normal faults that were previously undetected.
Pliocene-Pleistocene radiolarian biostratigraphy and palaeoceanography of the North Atlantic
Authors: Haslett Simon K. 
Published: April 2008

Abstract

The Plio-Pleistocene radiolarian record of the North Atlantic differs considerably from that of the Pacific and Indian Oceans. The standard cosmopolitan low-latitude radiolarian zonal scheme can be applied to the tropical Atlantic, but a separate zonation has been developed for the endemic radiolarian faunas of the high latitude North Atlantic. A new (preliminary) radiolarian zonation for the Plio-Pleistocene is offered here for middle latitudes, consisting of five zones defined on Last Appearance Datum levels only:
Cycladophora davisiana
zone (0–0.5 Ma),
Stylatractus universus
zone (0.5–1.75 Ma),
Antarctissa whitei
zone (1.75–3 Ma),
Sphaeropyle langii
zone (3–4.75 Ma) and the
Stichocorys peregrina
zone (4.75 Ma to Late Miocene). This new zonation, being geographically transitional, contains elements of both the previous high- and low-latitude zonations.

Radiolaria also possess great potential for palaeoceanographical analysis in the North Atlantic. The recognition of subarctic, boreal, subtropical and tropical radiolarian assemblages may prove useful in tracking water masses at times of climatic and oceanographic change. Radiolarian species characteristic of Indo-Pacific upwelling areas, e.g.
Pterocanium auritum
Nigrini & Caulet,
Lamprocyrtis nigriniae
(Caulet),
Acrosphaera murrayana
(Haeckel),
Pterocorys minythorax
(Nigrini) and
Lithostrobus hexagonalis
Haeckel, have been found in the eastern tropical North Atlantic (ODP Hole 658C), and it may be possible, through future research, to develop an Upwelling Radiolarian Index for the Atlantic. This would be capable of documenting upwelling histories and help to interpret ocean-atmosphere interaction in the eastern tropical Atlantic during Plio-Pleistocene time.
Geometry and tectonic evolution of megasequences in the Plymouth Bay Basin, English Channel
Authors: Ruffell A. , Coward M. P. , Harvey M. 
Published: April 2008

Abstract
A complete Carboniferous-Permian-Triassic succession is found within the anomalously thick (10 km to acoustic basement) Plymouth Bay Basin (PBB). The evolution of this succession provides us with the most complete tectonic/stratigraphic history of the late Palaeozoic to Triassic interval of the North Atlantic area. Deep and commercial seismic data from the PBB reveal the existence of four prominent seismic megasequences, bounded by strong and laterally persistent reflections. Isochore mapping and seismic facies analysis of each megasequence demonstrate that depocentres changed upward through the succession in a consistent manner, from E-W to elongate, NW-SE-orientated depocentres. This change may have occurred in the late Carboniferous or early Permian. The two-fold tectonic history of the PBB explains the thickness of sedimentary fill: an early basin formed above a deep crustal ramp, developed during Variscan compression along the Lizard-Start thrusts (and others); a later NW-SE early extensional or strike-slip basin depocentre is coincident with the earlier basin centre, creating further accommodation space.
Permo-Triassic development of the Celtic Sea region, offshore Ireland
Authors: Shannon P. M. 
Published: April 2008

Abstract
The Celtic Sea region, lying to the south of Ireland, contains a set of linked post-Palaeozoic sedimentary basins. Petroleum exploration in the region has concentrated upon Cretaceous and Jurassic targets with relatively little known about the Permo-Triassic evolution. Seismic reflection profiles and available well data are used to analyse the Permo-Triassic development of the basins lying in the Irish sector of the Celtic Sea. Occasional, small, fault-controlled Permian intermontane basins were developed in the Celtic Sea region. Rifting in Early Triassic times produced a number of fault-bounded basins, with clastic facies deposited in a fluvial-dominated arid setting. Late Triassic basins, largely controlled by post-rift thermal subsidence and consisting of coastal sabkha or supratidal flat deposits, extended beyond the limits of the Early Triassic basins. The Pembrokeshire Ridge-Labadie Bank, partially cored by Variscan granites, acted as a partial boundary between the North and South Celtic Sea Basins during Triassic times. Lower Triassic sandstone deposits are best developed in the North Celtic Sea and Fastnet Basins. Upper Triassic salt is widespread in the South Celtic Sea Basin and occurs locally along the southern margin of the North Celtic Sea Basin. Upper Triassic marls and mudstones are regionally developed throughout the basins.
Evolution of the Cockburn Basin: implications for the structural development of the Celtic Sea basins
Authors: Smith C. 
Published: April 2008

Abstract
The Cockburn Basin is one of a series of sub-parallel basins in the Celtic Sea-Western Approaches area to the south and southwest of Ireland. These basins share broadly similar structural and depositional histories, and are regarded as the remnants of a regionally extensive Triassic-Middle Jurassic basin which was dissected by Late Jurassic-Early Cretaceous tectonics. During the late Paleogene the basin was offset dextrally by 4–5 km across the Cockburn Fault, part of a major Tertiary wrench zone. Displacement on the Cockburn Fault is entirely Tertiary in age; however, the adjacent Fastnet segment of the Goban-Cockburn Fault Zone appears to have exerted a degree of control on the pre-Aptian development of the Fastnet Basin. At least one major basement structure, the Scilly Wrench Zone, appears to have entirely avoided reactivation during the post-Variscan development of the Western Approaches Trough. It would appear that reactivation of basement structures in the Celtic Sea-Western Approaches area has occurred in a patchy and unpredictable manner and, although pre-existing faults locally exert considerable influence, there is no fundamental basement control of the evolution of sedimentary basins in the region.