bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of pore-scale displacement mechanisms and small-scale heterogeneities on oil recovery by surfactant flooding
Authors: Schilling Birgitte E. R. , Stensen Jan-Åge , Øren Pål-Eric
Published: April 2008

This paper describes the basic mechanisms by which waterflood residual oil is mobilized and recovered during tertiary surfactant flooding in strongly water-wet glass micromodels. Of the two models used, one displayed a uniform and the other one a more heterogeneous distribution of water flood residual oil. Mobilization of residual oil proceeds by an imbibition-drainage mechanism where a surfactant imbibition displacement is always associated with a corresponding oil-water displacement (drainage) process. The drainage results in reconnection of residual oil and leads to the formation of an oil bank whilst the imbibition process may lead to breakup and trapping of mobilized oil. Although the magnitude of the residual oil saturation was similar for both models, microscopic sweep and recovery efficiency are found to be significantly higher when the residual oil is uniformly distributed. For more heterogeneous distributions of residual oil, surfactant flow through low resistance water paths results in substantial dispersion and early surfactant breakthrough.
A preliminary evaluation of an LTPF process for Statfjord Field, Norway
Authors: Kalpakci B. , Arf T. G. , Grist D. M. , Hyde S. B. , Vikane O. , Espedal S.
Published: April 2008

A preliminary laboratory study was initiated in 1991 to assess the potential of Low Tension Polymer Flood (LTPF) in the Statfjord Field Upper Brent Formation. On the basis of phase behaviour and compatibility experiments, four formulations were identified for further evaluation and a live oil core flood test was conducted to assess the oil recovery potential of one formulation under reservoir conditions. Encouraged by the initial laboratory results, a three-step LTPF process design strategy was followed. A target area of the reservoir was identified, the performance of the waterflood in this area was history-matched using the LTPF simulator and a predictive simulation of several LTPF application strategies was performed.

The chemical flood simulator, BPOPE, was first employed to match the laboratory core flow test by incorporating the LTPF characteristics measured. Following this conditioning, it was then used to assess the application of LTPF in the formation selected. A variety of perforation strategies, LTPF slug sizes and operating conditions were tested, all with reservoir cooling due to the injection of cold sea water.

The modelling shows LTPF to be sensitive to slug size in particular, to surfactant phase behaviour and to adsorption loss levels. It was concluded that the LTPF process could be applied to the Tarbert formation, to yield additional recovery. However, an optimization is required to make the LTPF process attractive for implementation. This paper presents the essential phase behaviour data, the results of the live oil core flow test, and discussion of the computer simulation approach and the results.
Constraints for using high-frequency sedimentary cycles in cyclostratigraphy
Authors: Cotillon P.
Published: April 2008

High-frequency cycles can be used for setting accurate correlations within basins, between sites of the world-ocean or between two different continents. Besides, since Gilbert (1895,
Journal of Geology
,
3
, 121–127), the cycles forming marl-limestone couplets have been regarded as chronostratigraphic tools for estimating durations of some stages, formations and biostratigraphical units. But this use is dangerous if applied without caution. Firstly, the period of decimetric couplets can vary along sedimentary successions because it is negatively correlated to the sedimentation rate. Indeed, in a dilated and quickly deposited series, many low-order cycles, unapparent in slowly deposited sediments because of bioturbation, occur as centrimetric to decimetric units and can be gathered in groups differing from the 100 ka bundles. Examples from the Lower and Middle Cretaceous show that 21 ka cycles are well-expressed with sedimentation rates of 5–30 m Ma
−1
. Secondly, the recording of orbital cycles depends on the facies and on the characteristics of sedimentary fluxes. Thirdly, corrections must be done for the variation of some orbital frequencies with time.

In summary, facies suitable for cyclostratigraphic work show moderate sedimentation rate, homogeneous lithologies with a balance of limestone and marl and are related to high sea-level periods which coincide generally with regular fluxes.
Geochemical criteria for reservoir characterization
Authors: Anissimov Leonid
Published: April 2008

This paper shows how the basinal analysis of formation waters and petroleums may be used to deduce the general geological setting of the source-reservoir system. There are two extreme cases, principally depending on the relative volumes of the source and reservoir regions. If the source rock region dominates, a ‘closed’ system results. Otherwise, where the reservoir volume is large the system is ‘open’. The ‘reservoir’ may be thought of as including the ‘carrier bed’ if appropriate.

In the ‘closed’ system, for example the carbonate reefs of the Western Canada Basin, water-soluble products of petroleum generation (e.g. benzene and H
2
S) are preserved in the reservoir. There is insufficient water to remove these components by aqueous solution. The partial pressure of methane is expected to be the same in formation waters and petroleums, since they can equilibrate in a ‘closed’ system. In an ‘open’ system the volume of water in the ‘reservoir’ is large compared to the source. Water and petroleum expelled from the mature source rock kitchen cannot reach equilibrium with the vast volume of formation fluids. Regions which connect with local source kitchens are expected to experience alterations in salinity and dissolved organics. Concentrations of water-soluble components will therefore be reduced in petroleums.

Data from ‘open’ and ‘closed’ systems are presented from the Devonian of the Volga-Ural province. A reduction in water salnity below 3 km, near the source kitchen is attributed to expulsion of a low salinity water during oil generation. Related changes in aqueous benzene concentrations are also seen.
Diagenesis of the Rotliegend Sandstones in the V-Fields, southern North sea: a fluid inclusion study
Authors: McNeil B. , Shaw H. F. , Rankin A. H.
Published: April 2008

Illite, quartz, Fe-dolomite (with some dolomite), anhydrite and barite are the principal pore-filling cements in the analysed samples of the Rotliegend sandstones from the Vulcan, South Valiant and Vanguard Fields of the southern North Sea. Apart from illite, these cements also infill fractures cutting the Rotliegend sandstones, but in the fractures there is a predominance of anhydrite and barite whereas in the pore infills, quartz and Fe-dolomite predominate. The quartz, Fe-dolomite and sulphate cements have been studied using a variety of analytical techniques but with the emphasis on fluid inclusion microthermometry. The results indicate that fluids at temperatures of 70 to 125°C, with salinities of 17 to 24 eq. wt% NaCl and an overall composition of Na-Ca-Mg-K-Cl-SO
4
, were involved in authigenic mineral formation in pore spaces and fractures. Hydrocarbons were detected in the fluid inclusions hosted in the cements and indicate that cementation occurred in the presence predominantly of methane and only rarely liquid hydrocarbons. Relating the microthermometry homogenization/formation temperatures to the burial history, quartz cementation occurred mainly in the late Permian to late Cretaceous, Fe-dolomite during the Jurassic to mid-Cretaceous, and anhydrite and barite mainly in the mid-to late Cretaceous.
Sour gas and water chemistry of the Bridport Sands reservoir, Wytch Farm, UK
Authors: Aplin Andrew C. , Coleman Max L.
Published: April 2008

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

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

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

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

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

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

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

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

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

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

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

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

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.
A wide-angle reflection profile across the Iceland-Faeroe Ridge
Authors: Makris Jannis , Lange Knuth , Savostin Leo , Sedov Vladimir
Published: April 2008

A wide-angle reflection profiling (WARP) survey was carried out in July 1993 across the Iceland-Faeroe Ridge in the North Atlantic. Sleeve guns were employed as energy source and ocean bottom seismometers (OBS) used to observe the seismic energy. Results of the first evaluation show that the crust below the Iceland-Faeroe Ridge is stretched continental, with an average thickness of 23 km at its central part. Towards the NE end of the profile, the transition to oceanic crust of the Norwegian Basin has been identified. The upper crust has
P
-wave velocity (
V
p
) values which range between 5.8 and 6.3 km/s, while the lower crust is more homogeneous with
V
p
values between 6.7 and 7.0 km/s. The upper crust, excluding sediments and volcanics, is about 6 km thick at the crest of the ridge and thins with increasing water depths to the northeast and southwest. The lower crust can be separated from the upper part by a first order discontinuity, a
P
i
P
reflector, that has been identified in several travel-time sections.
P
-wave velocities below the Moho have values of approximately 7.9 km/s.
Beyond the turbidite paradigm: physical models for deposition of turbidites and their implications for reservoir prediction
Authors: Kneller Ben
Published: April 2008

The simple physical models which are popularly used to describe deposition from turbidity currents are based on the notion of unidirectional waning flow, resulting in the familiar Bouma sequence (Ta-e) or its high-density counterpart, the Lowe sequence (S1-3). Most geologists working on turbidite successions know only too well how wide is the range of facies which do not fit into the standard facies models. The application of simple equations of motion shows that deposition can occur beneath flows that are steady or even waxing. The various combinations of different spatial and temporal accelerations produce markedly different vertical and lateral variations in the resulting turbidite. On these grounds alone, we should expect not one but at least five basic types of sequence in turbidite beds, for which likely candidates can be found in many deep water clastic systems. This has important implications for deposit geometry, gradients in reservoir properties, and geological interpretation and correlation of well cores. Since many basins are confined,
a priori
reasoning tells us to expect interactions between turbidity currents and topography, producing anomalous intrabed vertical sequences, multiple current directions and locally enhanced deposition. Flume experiments with radially spreading scaled sediment gravity flows demonstrate how the geometry of sandstone deposits may be controlled by topography. Interactions between the flow and topographic features are dependent upon the shape and orientation of the obstacle, and upon its size relative to the height of the flow. Some of the results are counter-intuitive, and may have significant impact on location of reservoir sands around basin-floor and marginal topography.
A classification scheme for shale clasts in deep water sandstones
Authors: Johansson Melissa , Stow Dorrik A. V.
Published: April 2008

Shale clasts are widespread in turbidites and associated facies of all ages. They may yield important information regarding the depositional process of the host sandstone and its specific environmental setting. Previous work has tended to generalize the variety of shale clasts that exist and no comprehensive classification has yet been developed. The scheme proposed here is a descriptive classification based on the nature and arrangement of shale clasts within mainly structureless sandstones. Twelve types of clasts are recognized on the basis of the clast’s position in the bed and/or characteristic features, and these have been grouped according to whether they have been derived from either basal erosion or disturbance (Group A), flow modification (Group B) or post-depositional disturbance (Group C). These clasts have also been organized into characteristic assemblages which are thought to be indicative of particular depositional environment. The classification scheme when used in association with facies analysis can provide an additional tool for core investigations, contributing to the understanding of permeability barriers and relevant architecture.
The onset of interaction between the hydrosphere and oceanic crust, and the origin of the first continental lithosphere
Authors: De Wit Maarten J. , Hynes Andrew
Published: April 2008

New continental crust forms above subduction zones through the recycling of hydrated oceanic lithosphere. The most efficient process known for oceanic lithosphere hydration takes place at the submerged mid-ocean ridges where the lithosphere is young and warm, and cools through hydrothermal convection. Such mid-ocean ridge hydrothermal interactions were operative at least as far back as 3.5–3.8 Ga. The apparent absence of preserved continental crust older than 4.0 Ga may reflect the absence of hydrothermal interaction before that time. This model requires that prior to about 4.0 Ga mid-ocean ridges stood above sea level. Our calculations show, however, that on a plate-tectonic early Earth with substantially less continent, realistic higher heat flow, and a volume of sea water similar to that of today’s ocean, Archaean mid-ocean ridges would have remained below sea level. Only with a substantial reduction of surface water would Earth have been able to recycle dry oceanic lithosphere, and thus prevent the present day style of continental crust formation.

A 30% reduction of surface water is required to elevate early Earth’s ridges above sea level. This excess water may have been stored in nominally anhydrous minerals of the mantle. Early Earth’s mantle may have released a significant proportion of its initial water only gradually through convective overturn of the oceanic floor. Given realistic ocean-floor creation rates, it would have taken roughly 500 Ma to process the early Earth’s mantle through a MORB generation event if only the upper mantle was involved and considerably longer if whole mantle convection was involved. The inefficiency of water extraction during this process is illustrated by the amount of water apparently present in the source regions for present-day MORB. In this scenario, the Hadean-Archaean transition may mark the time when Earth changed its style of cooling from one dominated by heat exchange directly to the atmosphere to one dominated by heat exchange with the hydrosphere, which still buffers Earth’s heat loss today.
Lode-gold deposits of the Yilgarn block: products of Late Archaean crustal-scale overpressured hydrothermal systems
Authors: Groves D. I. , Ridley J. R. , Bloem E. M. J. , Gebre-Mariam M. , Hagemann S. G. , Hronsky J. M. A. , Knight J. T. , McNaughton N. J. , Ojala J. , Vielreicher R. M. , McCuaig T. C. , Holyland P. W.
Published: April 2008

Although the lode-gold deposits of the Yilgarn block are hosted by a variety of rocks, and their structural style, associated alteration and ore mineralogy are also variable, common parameters suggest that they represent a coherent group of epigenetic deposits, most of which formed during a widespread (500 000 km
2
) and broadly synchronous (2635 ± 10 Ma) hydrothermal event in the closing stages of the Late Archaean tectonothermal evolution of the host granitoid-greenstone terrains. Progressive variations in deposit parameters can be correlated with the metamorphic grade of the enclosing greenstone successions. These systematic variations, combined with evidence for their timing and the
P-T
conditions of their formation, indicate that the deposits form a continuum in which gold deposition took place from < 5 km depth (1 kbar, 180°C) to > 15 km depth (> 5 kbar, 700°C), marking hydrothermal fluid flow and fluid evolution through the middle and upper crust.

The primary ore fluid appears to have been an overpressured, low salinity H
2
O-CO
2
-CH
4
fluid originating from a deep source. Upward fluid advection was strongly channelized along vertically extensive conduits. Although there is a gross regional association between clusters of gold deposits and craton- or greenstone-scale deformation zones, these do not appear to have been the primary fluid conduits, at least during their major phase of structural and magmatic activity. Further, all kinematic types of lower-order structure are mineralized, and the fossil fluid conduits exposed at the mine scale are extremely variable, some with no obvious fault or shear control. A potentially unifying hypothesis that can explain this extreme variability in the nature of the conduits is that fluid flow was focussed into zones of low mean stress in the granitoid-greenstone terrains. This can explain the selective occurrence of gold deposits adjacent to irregular or fault-bounded granitoid contacts in some goldfields, the selective mineralization of competent units (e.g. dolerites) in elongate greenstone belts that contain such units and are oriented sub-perpendicular to the far-field compressive stress, and the lack of mineralization in major, largely planar, shear zones undergoing simple shear. The orientation of the greenstone belts with respect to the far-field compressive stress appears to be a crucial factor in defining the potential for strike-extensive zones of low mean stress. This potentially can explain why granitoid-greenstone belts with a high density of sub-parallel craton- and greenstone-scale deformation zones are most highly mineralized and commonly contain the giant gold deposits which, themselves, are located in geometrically anomalous zones within these greenstone belts.
Global sea-level and the (pen-)insularity of late Cenozoic Britain
Authors: Funnell Brian M.
Published: April 2008

During mid-Pliocene times, Britain was surrounded by warm temperate seas. In the south, these received relatively little clastic input, accumulated mainly biogenic sediments, and appear to have allowed free circulation of marine waters between the North Sea and the Atlantic Ocean, either around or across southern Britain.

In late Pliocene times, commencing at about 2.5 Ma
bp
, the first major global sea-level falls associated with northern hemisphere glaciation occurred. At about the same time, climatic changes appear to have accelerated the rate of headwater extension and bedload transport of the principal rivers flowing towards the southern North Sea. The combination of sea-level fall and progressive deltaic progradation into the southern North Sea converted Britain into a European peninsula.

By the mid-Quaternary Cromerian stage, the Great European (Ur-Frisia) Delta top was sufficiently moribund, and subject to subsidence, to allow interglacial sea-levels to encroach southwards across it. During the following Anglian Glaciation, impounded glacio-marine or glacio-lacustrine waters appear to have extended southwards, across both the former delta top and the bounding Weald-Artois chalk ridge, into the English Channel.

Subsequently a marine connection between the North Sea and the Atlantic Ocean, via the English Channel, was alternately established and broken, as global sea-level rose and fell (synchronously with interglacial and glacial periods), cyclically converting Britain from island to peninsula and back again.
Insularity and Quaternary vertebrate faunas in Britain and Ireland
Authors: Stuart A. J.
Published: April 2008

Terrestrial vertebrate faunas (mammals, reptiles and amphibians) from the Middle and Upper Pleistocene of Britain, Ireland and adjacent Continental Europe are compared. Problems arise from uncertainties of status and correlation of stages. Evidence of insularity is provided by reduced diversity in (a) Britain
versus
the Continent, and (b) Ireland
versus
Britain. British temperate/interglacial faunas older than the Last Interglacial are very similar to Continental faunas, notably the Cromerian faunas of West Runton, England, and Voigtstedt, Germany. The earliest indications of insularity are in Last Interglacial faunas, in which, for example, pine voles
Pitymys
spp., extinct rhinoceros
Stephanorhinus kirchbergensis
, horse
Equus ferus
and humans, present in equivalent Continental faunas, are absent from Britain. Local extinction of horses and humans is implied. The British Last Cold Stage (Devensian) faunas are very similar to those of the adjacent Continent, indicating unimpeded migration. However, Irish Last Cold Stage faunas are impoverished (absences include woolly rhinoceros
Coelodonta antiquitatis
and humans), indicating no connection from Ireland to Britain. Early Holocene separation of Britain is reflected in the absences of several small vertebrates found on the adjacent Continent. Irish Holocene faunas are much more impoverished, lacking, for example, voles, frogs and snakes, indicating continued isolation from Britain.
Sea-levels and the evolution of island endemics: the dwarf red deer of Jersey
Authors: Lister Adrian M.
Published: April 2008

The dwarfing of red deer on Jersey in the Last Interglacial allows a detailed investigation of the ‘island dwarfing’ phenomenon. Uniquely, the island furnishes normalsized deer in stratified deposits both preceding and succeeding the dwarfing episode. Timing the dwarfing process depends on a number of assumptions about land-sea history, but suggests a maximum duration of 6000 years. The changes in size and body form of the deer are discussed in terms of adaptation, development and evolutionary rate, and are compared with modern analogues.
Deglaciation, Earth crustal behaviour and sea-level changes in the determination of insularity: a perspective from Ireland
Authors: Devoy Robert J. N.
Published: April 2008

Insularity is defined here in terms of the presence or absence of land-bridge connections between adjacent landmasses. The development of a land-bridge depends upon the existence of a special combination of environmental conditions. These are created through the interaction of climate, sea-level and other physical environmental controls, together with changes in palaeogeography. Such factors operate together to produce conditions suitable for a land-bridge over only relatively short time-scales (10 to 100 years). New palaeoenvironmental data about such factors have emerged in recent years from the Irish Sea and from neighbouring regions. These data, derived from a variety of disciplines, have contributed to earlier discussions about the existence here of land-bridge links in the Late Quaternary. Some of this information, particularly that concerning patterns of Earth crustal behaviour, former ice distributions, deglacial history and sea-level changes, is reviewed. An assessment is given of their significance for possible land-bridge operation at this time.
The what, why, when, how, where and who of geological data management
Authors: Giles Jeremy R. A.
Published: April 2008

The variety, form and volume of data available to geologists have increased significantly over the past few decades. It has become essential to use databases (either analogue or digital) to turn this avalanche of data into usable information. A significant percentage of databases that are created do not repay the cost and effort of their creation and can therefore be considered to be failures. The probability of creating a successful database can be increased by careful consideration of a few simple questions. What is the objective that the database will meet? Why is a digital database being considered? When, if ever, will the cost and effort of creating the database be repaid? How will the database be designed and created? Where are the users? And finally, who will be responsible for maintaining the integrity of the database?
Data modelling a general-purpose petroleum geological database
Authors: Chew K. J.
Published: April 2008

Experience gained while modelling a petroleum geological database can be summarized as follows. General-purpose database models should reflect the real world, complete with imperfections, and avoid being application-oriented in design. In geological database modelling it is necessary to separate the recording of descriptive features (empirical data) from that of geological processes (concepts). Detailed analysis reveals that many geological relationships assumed to be inherently hierarchical are in fact many-to-many. Ambiguity and inconsistency in definitions should be avoided, if necessary by adopting a definition at variance with industry norms. Use of generic entities simplifies the modelling exercise and can benefit the end-user.