bluuebunny/crossref_metadata_2025 huggingface dataset:
The filling and emptying of the Ula Oilfield: fluid inclusion constraints
Authors: Oxtoby Norman H. , Mitchell Alan W. , Gluyas Jon G.
Published: April 2008

The Norwegian sector Ula Field contains 430 million barrels of oil reserves trapped in a dip-closed Upper Jurassic sandstone reservoir and has multiple oil-water contacts (OWCs). Historically, the depth to the contacts has been the subject of some debate and development drilling has now proven the existence of at least six. The precise nature of some of these contacts remains uncertain because all were discovered after production start-up and determination of their origin is critical to future field development. The relationship of oil inclusion abundances to OWCs and rock properties (porosity, permeability) has been evaluated (a) to gain an insight into the filling history and mechanisms, and (b) to attempt to define pre-production OWCs. Oil inclusion abundances decrease with depth and towards present OWCs in Ula, but oil inclusions also occur below them. They indicate that Ula filled from the crest down, and that the filling was synchronous with cementation to at least 3500 metres subsea. Below this depth, cementation appears to have reduced permeability to such an extent prior to charge that the resulting filling geometry was irregular. Local abundance peaks within the oil column correlate with the tops of coarsening-upward cycles. Rare oil inclusions below the oil legs may represent either migration pathways updip from the source or invasion downdip from the main oil leg along higher permeability zones. Oil inclusions persist to greater depths in the east and south-east of the field which indicates that oil charged the field from these directions. A number of models explaining the variability of the OWCs have been ruled out by this work, thereby reducing the uncertainty in field development planning.
Migration of hydrocarbons in the Tampen Spur area, Norwegian North Sea: a reservoir geochemical evaluation
Authors: Horstad I. , Larter S. R. , Mills N.
Published: April 2008

This paper describes the application of reservoir geochemical protocols to a study of oil-field and regional petroleum compositional variations in the major fields in the Tampen Spur area. The Tampen Spur high is located in the middle of several oil-mature to gas-mature source-rock kitchens and most of the structures might receive petroleum from several different directions. Based on the definition of several different petroleum populations within the Tampen Spur area, we define five major petroleum charging systems. Correlating the petroleum population variations with geological information allows us to derive likely migration/fill scenarios for many of the oil-fields and requires us to invoke a new Upper Jurassic carrier system in parts of the area connecting Brent Group reservoirs across major faults. Based on this hypothesis, two exploration wells were drilled on structures to test this carrier and petroleum was found in one of the structures. An understanding of the migration pathways and petroleum drainage efficiency of the basins is of great economic interest, because most of the smaller substructures are marginally economic. Thus, it is important to provide an effective risk analysis and ranking of the satellite prospects prior to drilling. This study has demonstrated that filling and migration in a geological province are controlled by a series of complex processes and that the filling history of the structures within the Tampen Spur is more complicated than suggested earlier.
Modelling density-driven mixing rates in petroleum reservoirs on geological time-scales, with application to the detection of barriers in the Forties Field (UKCS)
Authors: England W. A. , Muggeridge A. H. , Clifford P. J. , Tang Z.
Published: April 2008

This paper discusses how to deduce the strength of intra-field barriers in reservoirs which have significant present-day differences in reservoir fluid density. The method combines a knowledge of the geological time available since filling, with estimates of the rate of density-driven mixing. If the observed density differences are inconsistent with a barrier-free reservoir model, we show how limits can be set on the strength of the barrier needed to account for any observed lack of mixing. A case history from the Forties Field is presented, illustrating these principles. It is shown that a transmissibility barrier of at least 1% must be included to account for the present-day lack in density equilibration between Forties and SE Forties. The results are validated against pressure decline data since production start-up. Two methods of estimating density-driven mixing rates are given. One is based on an analytical formula applicable to a rectangular geometry, and the other uses an unmodified black oil simulator. One significant result is the weak dependence of mixing rates on vertical permeability over a wide range of values, for many typical reservoir configurations. This implies that in many geological circumstances, subsurface density differences (coexisting at the same depth) can be used to infer the presence of intra-reservoir barriers.
Geological setting and ecology of the Broken Spur hydrothermal vent field: 29°10′N on the Mid-Atlantic Ridge
Authors: Murton Bramley J. , Van Dover Cindy , Southward Eve
Published: April 2008

Deep-towed sidescan sonar and manned submersible studies have shown that hydrothermal activity within the Broken Spur vent field, located at 29°10′N on the Mid-Atlantic Ridge, is controlled by a combination of recent volcanic and tectonic activity. Three sulphide mounds, with high-temperature fluid vents, and two weathered sulphide mounds, with low-temperature fluid seeps, are aligned across an axial summit graben that lies along the crest of a neovolcanic ridge within the axial valley floor. The largest high-temperature venting sulphide mound, which is up to 40 m high, lies in the centre of the graben. Two further, but smaller, high-temperature sulphide mounds are located to the east and west of the larger mound. All three high-temperature venting mounds lie on an axis that strikes 115°, orthogonal to the trend of the axial summit graben and neovolcanic ridge. These directions are similar to the strike of two sets of faults that locally intersect at the vent site and which probably control the location of hydrothermal emission. The fauna colonizing the vents are distinct, at least at a species level, from those found at other hydrothermal sites on the Mid-Atlantic Ridge. New species of bresiliid shrimp, and a new genus of brittle star have been found along with other fauna in an ecosystem that is otherwise similar to those found at high-temperature hydrothermal sites elsewhere on the Mid-Atlantic Ridge. However, the populations of the bresiliid shrimp at Broken Spur are significantly lower in abundance than those of the same genus found elsewhere. The size, shape and state of alteration of the sulphide mounds and the extent of their oxyhydroxide sediments and weathered sulphide talus aprons, suggests that hydrothermal activity at the Broken Spur vent field has been long-lived, probably for several thousand years. This is supported by the unique speciation of fauna present, which also suggests that the Broken Spur vent field is isolated (in terms of faunal accessibility) from other vent sites on the Mid-Atlantic Ridge. The low population of the shrimp, in an otherwise active and long-established hydrothermal habitat, suggests that the hydrothermal activity at Broken Spur is in a state of change and may have been recently rejuvenated.
Venting from the Mid-Atlantic Ridge at 37°17′N: the Lucky Strike hydrothermal site
Authors: Klinkhammer G. P. , Chin C. S. , Wilson C. , German C. R.
Published: April 2008

The Lucky Strike hydrothermal field on the Mid-Atlantic Ridge (MAR) at 37°17′N was discovered in 1992 during dredging operations carried out as part of the FARA Program (French-American Ridge Atlantic). The vent field at Lucky Strike lies in a depression between three cones that form the summit of an axial seamount. The summit rises 400 m above the rift valley floor to a water depth of 1570 m. Hydrothermal fluids stream from several vent complexes with exit temperatures up to 333°C. This paper presents the first observations of hydrothermal plumes in the water column over Lucky Strike, including anomalies in light scattering with a nephelometer and dissolved Mn with the zero-angle photon fibre optic spectrometer. Plumes at Lucky Strike are small compared with those at other MAR hydrothermal sites such as TAG (26°N) and MARK (23°N). These small plumes are consistent with the relatively low Fe and Mn concentrations found in associated vent waters. The Sr/Ca ratios of these fluids are low, suggesting that the basaltic substrate below Lucky Strike is highly altered, but rare earth element concentrations are also low, indicating reduced mobilization. These observations together with the low fluid temperatures are suggestive of an old and waning hydrothermal system with extensive zones of subfloor alteration.
Hydrothermal plumes at Broken Spur, 29°N Mid-Atlantic Ridge: chemical and physical characteristics
Authors: James R. H. , Elderfield H. , Rudnicki M. D. , German C. R. , Palmer M. R. , Chin C. , Greaves M. J. , Gurvich E. , Klinkhammer G. P. , Ludford E. , Mills R. A. , Thomson J. , Williams A. C.
Published: April 2008

Two distinct hydrothermal plumes have been identified and sampled in the vicinity of active hydrothermal vents recently discovered at a site named Broken Spur, 29°10.15′N 43°10.28′W, on the Mid-Atlantic Ridge. The northern plume (29°10.13′N 43°10.61′W) most closely associated with Broken Spur consists of a weak plume (heat flux
c.
5 MW), whereas plumes detected to the south (29°09.11′N 43°10.54′W) are much stronger (heat flux
c.
57 MW). The plumes are characterized by dissolved
222
Rn, Fe and Mn anomalies of up to 20 dpm/100 kg, 32 and 14 nmol/l, respectively, and enhanced concentrations of Fe, Mn, Cu, Zn, Cd, Co and Pb in the particulate phase. Comparison of Fe:Mn ratios in the plume (3.7) to Fe:Mn in vent fluids collected at Broken Spur (7.9) suggests that
c.
530f hydrothermal Fe is removed during plume evolution.
Mineralogy and sulphur isotope characteristics of a massive sulphide boulder, Galapagos Rift, 85°55′W
Authors: Knott Richard , Fallick Anthony E. , Rickard David , Bäcker Harald
Published: April 2008

The submarine hydrothermal sulphide deposits of the Galapagos Rift are characterized by being exceptionally Cu-rich. Fe- and Cu-Fe sulphides are extremely heterogeneous, display granular textures and evidence of open space growth. Core was examined from a 1.5 m diameter massive sulphide boulder collected by TV grab on the GARIMAS II cruise. The main growth stage is dominated by pyrite and chalcopyrite, forming granular aggregates from <1 mm to >5 cm. These sulphides have δ
34
S from +2.7 to + 5.5‰. The Fe-Cu sulphides are modified by minor seawater oxidation and a late stage marcasite-sphalerite-galena-barite assemblage. δ
34
S varies along the core in a systematic manner and there appears to be an isotopically heavy deviation from the modal +3.9‰. This is related to particular zones and suggests a second, isotopically distinct S source, which is interpreted to be a hydrothermal diagenetic solution derived from a shallow seawater-hydrothermal circulation system.
Distribution and transformation of Fe and Mn in hydrothermal plumes and sediments and the potential function of microbiocoenoses
Authors: Sudarikov S. M. , Davydov M. P. , Bazelyan V. L. , Tarasov V. G.
Published: April 2008

The spatial separation of suspended Fe and Mn in plumes, recorded at Mid-Atlantic Ridge (MAR) sites, some regions on the East Pacific Rise (EPR) and in the western Pacific, is reflected in the Fe and Mn distribution in metalliferous sediments of hydrothermal origin. Investigations at 21.5–23.5°S EPR show that at a downstream distance of 10 km from the vents the total Fe and Mn contents are lower than those on the flanks. Primary suspended Mn precipitates more than 20 km from the high-temperature vents, whereas the main suspended Fe precipitates within 10 km of the active vent area. In the vicinity of the vents a zone of ‘geochemical minimum’ was observed. Bacterial masses were observed in plumes and sediments in hydrothermal sites of the Pacific. Microbial coenoses were studied to show the increased activity of heterotrophic and Fe-Mn bacteria at several horizons of MAR hydrothermal plumes. Near-surface plumes studied in Matupi Harbour (New Britain) were characterized by both the separation of suspended Fe and Mn horizontally and a corresponding increased presence of micro-organisms. The activity of micro-organisms accompanied differentiation of the suspended and dissolved Mn at the periphery of the plume. Evidence suggests a significant role for bacteria in removing Mn from plumes in different regions of the ocean.
Composition and morphogenesis of the tubes of vestimentiferan worms
Authors: Shillito Bruce , Lechaire Jean-Pierre , Goffinet Gérard , Gaill Francoise
Published: April 2008

The structure and chitin-protein content of the tubes of different vestimentiferan species are compared.
Riftia pachyptila
and
Tevnia jerichonana
were collected at a hydrothermal vent site at 13°N on the East Pacific Rise;
Ridgeia piscesae
was from a hydrothermal vent site on the Juan de Fuca Ridge and
Lamellibrachia
sp. came from a cold seep on the Louisiana continental slope in the Gulf of Mexico. The tube ultrastructure shows chitin microfibrils organized in parallel bundles with various orientations. The chitin content varies considerably between species and is unrelated to seep or vent origin, whereas the protein content is much higher in the cold seep species than the hydrothermal vent species. All the species have specialized chitin secreting glands, as described for
R. pachyptila
. Based on this investigation, a tentative model of tube morphogenesis under cell control is proposed.
Geochemistry of the Snake Pit vent field and its implications for vent and non-vent fauna
Authors: Sudarikov S. M. , Galkin S. V.
Published: April 2008

Observations made during a cruise of the R/V
Professor Logachev
at the Snake Pit hydrothermal vent field provide a large database with which to examine the relationships between the geochemical characteristics of the hydrothermal sediments and the fauna that inhabit the area. Correlations between the nature and geochemistry of sediments, the distribution of hydrothermal and peripheral fauna and the hydrothermal activity are established. Three geochemical zones in the Snake Pit area are distinguished. (i) The Central Zone, located within 10–100 m of black smoker chimneys, consisting mainly of hydroxide-sulphide sediments with Fe sulphides predominant. Barite is found only in this zone and vent-associated organisms distinctly predominate over non-vent fauna. (ii) The Intermediate Zone encompasses high-temperature springs at a distance from 50 to 150 m. Sediments are chiefly sulphide-hydroxide composition and are dominated by Fe hydroxide and sulphide minerals in an oxide envelope. The non-vent specific fauna show maximum concentrations. (iii) The Outer Zone is located within a radius of 150–500 m from the smokers. The sediment composition is characterized by a predominance of sulphide-hydroxide transitional differences with notably varying compositions of oxide and sulphide minerals. Minor sestonophages prevail in the fauna, which is typical of communities with a pronounced oligotrophic structure. From the Central Zone towards the Outer Zone, the frequency of the bacterial mats seen on photographs decreases and the hydrothermal sediments show a similar trend. Analysis of the hydrothermal sediment obtained from the cores shows: (i) Mn is absent; (ii) barite is found only in the Central Zone; and (iii) chalcopyrite and pyrrhotite concentrations decrease and opal and quartz concentrations increase with increasing distance from the nearest smoker. Comparisons between the Mid-Atlantic Ridge hydrothermal fields of TAG, Snake Pit and 15°N indicate a decrease in the hydrothermal and biological activities with latitude (TAG, Snake Pit, 15°N), which might be due to their evolutionary history.
The molecular identification of early life-history stages of hydrothermal vent organisms
Authors: Dixon D. R. , Jollivet D. A. S. B. , Dixon L. R. J. , Nott J. A. , Holland P. W. H.
Published: April 2008

Amplification of diagnostic genomic DNA sequences using the highly sensitive polymerase chain reaction (PCR) technique provides a fast, sensitive and relatively inexpensive approach to species identification where there is a lack of diagnostic morphological characters. This applies particularly to the early life-history stages of marine invertebrates. Using PCR primers designed to amplify diagnostic length variants within phylogenetically widespread genes (in this case an expansion segment within the 28S rRNA gene), a first attempt is presented to produce a DNA database for use in the identification of hydrothermal vent larvae. In addition, a scanning electron microscopy study of particulates recovered from the neutrally buoyant plumes of hydrothermal vents on the Mid-Atlantic Ridge revealed evidence of biological material derived both from the vent environment and from the sea surface as marine ‘snow’. This investigation represents the first stage in the development of a bottom-mounted recorder to study the spatial and temporal aspects of larval dispersal in the hydrothermal vent environment. Larval dispersal processes are fundamental to the biogeography, genetics and evolution of the hydrothermal vent fauna.
Hydrothermal plumes: a review of flow and fluxes
Authors: Speer Kevin G. , Helfrich Karl R.
Published: April 2008

The physics of high-temperature hydrothermal venting and in particular the effects of the buoyancy flux on the background oceanic circulation are reviewed. The central theme is that the venting is dynamically active and forces oceanic flows on a variety of spatial and temporal scales. Vent fluid is not simply advected by the prevailing currents, but can actively set the scales of the flow. The cascade of scales (from fast to slow and small to large) discussed is: the buoyant plume, lateral spreading and mesoscale geostrophic vortices and the basin-scale plume. The focus is on models and observations relevant to the last two phases for which the Earth’s rotation plays a significant part. Some discussion of the observational requirements for the resolution of these flows is given. The fact that small-scale buoyant convection can drive flows on scales many orders of magnitude larger can have important effects on the dispersal of vent fluid, estimates of hydrothermal heat flux and the establishment and maintenance of vent field biological communities.
The link between sedimentary basin inversion and igneous underplating
Authors: Brodie James , White Nicky
Published: April 2008

There is now general agreement that many sedimentary basins on the northwest continental shelf of Europe underwent permanent exhumation during the Tertiary. The most dramatic indicator of this process is the present-day absence of up to 3 km of anticipated post-rift subsidence in the midlands of Britain and in the East Irish Sea. Any explanation must take into account the fact that the entire shelf has very small, long wavelength, free-air gravity anomalies. This constraint is of fundamental importance and implies either that the crust has been thickened, that phase changes have occurred within the lithosphere, or that low density material has been added to the lithosphere. Tertiary epeirogenic uplift and exhumation is often attributed to horizontal shortening which is assumed to be related in a general sense to Alpine mountain building. However, the removal of 3 km of sediment from a basin, which was originally 100 km wide, requires 20–30 km of shortening. Whilst minor Tertiary shortening is observed all over the continental shelf, nowhere is it sufficient to account for the inferred amount of denudation. More significantly, exhumation is thought to have commenced in the Early Tertiary and dramatically increases from south to north. Shortening is generally younger (mid-Tertiary) and decreases in intensity from south to north. Here we argue that Tertiary uplift and denudation are a consequence of regional igneous underplating. At the beginning of the Tertiary, rifting associated with the initiation of the Iceland plume generated substantial volumes of melt. Petrological arguments and the results from inversion of rare earth element concentrations of MgO-rich igneous rocks suggest that a minimum of 2–5 km of melt were produced beneath a substantial part of the continental shelf. We infer that much of this melt was trapped within the lithosphere, presumably close to the Moho, which would have acted as a density filter. Such underplating will have caused rapid surface uplift whilst maintaining isostatic equilibrium. Simple calculations based on deep seismic reflection data and the high P-wave velocities observed beneath Scotland are consistent with the petrological arguments.
The geometries and kinematics of inverted fault systems: a review of analogue model studies
Authors: McClay K. R.
Published: April 2008

The geometries and kinematics of 2D inverted extensional fault systems are reviewed using the results of scaled physical models together with case histories from inverted basin fault systems. 2D analogue models of detached terranes, listric, planar, ramp/flat listric and domino arrays of planar faults were constructed from homogeneous sandpacks and from anisotropic sand/mica layers. The models were first extended and then subjected to horizontal compression in order to reactivate the extensional fault systems. Upon extension simple listric faults produce a characteristic roll-over anticline and crestal collapse graben. Inversion of this system produces reactivation of the main detachment with the development of a fault-bounded wedge of syn-extensional strata elevated above regional together with tightening of the crestal collapse graben. New thrust faults initiate from the tips of the crestal collapse extensional faults. Characteristic ‘harpoon’ structures develop associated with the reactivation of the main detachment. Similar inversion architectures were also produced for the planar fault systems. Extension of ramp-flat listric fault systems produces an upper roll-over and crestal collapse graben together with a ramp syncline and lower roll-over and crestal collapse graben. Inversion of this system only reactivates a part of the main detachment producing a hangingwall shortcut fault that bypasses the upper roll-over system. Inversion of domino fault arrays produced characteristic harpoon geometries of the syn-rift wedge together with shortcut faults in the footwalls of the main domino faults.

Analysis of the progressive deformation during both extension and inversion has been carried out using marker points embedded in the models. In the analogue models of detached terranes hangingwall collapse during extension occurs along planar to curved shear surfaces, whereas upon inversion, different, lower angle shear trajectories are utilized. In the domino fault arrays more complicated shear paths are observed as a result of the footwall shortcut faults developed during inversion. These results have important implications for the analysis of extensional and inverted terranes and for fault reconstruction and section balancing techniques.

The architecture of inverted fault systems developed in the analogue models show striking similarities to the inverted basin geometries found in natural fault systems such as those in the North Sea and South East Asia. Examples from these terranes are compared and contrasted with the results of the analogue models. Conceptual models for inversion kinematics and fault system architectures are presented.
Estimating inversion: results from clay models
Authors: Eisenstadt Gloria , Withjack Martha Oliver
Published: April 2008

Physical models using wet clay show that, although both fold and fault geometries change with increasing amounts of inversion, it is difficult to estimate the magnitude of inversion. During extension, a listric main normal fault and a rollover fold cut by secondary normal faults developed above two diverging, basal plates. With 50 0nversion (4 cm extension, 2 cm shortening), the main normal fault underwent reverse displacement, and an asymmetric syncline formed in the pre-growth layers in the hangingwall. Secondary antithetic and synthetic normal faults passively rotated. Faults on the synclinal limb near the main normal fault rotated away from the fault, whereas those on the opposing limb rotated towards the fault. A broad anticline formed at shallow levels above the original half graben. Compressional reactivation of the main normal fault ceased between 50% and 100 0nversion (4 cm extension, 4 cm shortening). Instead, low-angle thrust faults with small displacements accommodated most shortening. The shallow anticline above the original half graben expanded laterally and vertically. With 200 0nversion (4 cm extension, 8 cm shortening), low-angle thrust faults with large displacements deformed the hangingwall and footwall of the main normal fault. Thrust faults cut the main normal fault and many of the secondary normal faults. The geometry of the hangingwall fold varied with depth. A broad anticline deformed the upper growth layers, changing to a syncline in the lower growth layers and an anticline in the pre-growth layers.

Estimates of inversion from null point analysis, amount of uplift above regional datum, or line length balancing, grossly underestimate the amount of shortening in the models. In addition, both the 50% and 100 0nversion models lack the large-scale deformation features typically associated with compression. These results suggest that both quantitative and qualitative techniques used to calculate inversion magnitude can significantly underestimate the amount of shortening. Furthermore, if the inversion uplift is eroded or if many small-scale deformation features are not imaged seismically, then inversion may go unrecognized.
A comparison of inverted basins of the Southern North Sea and inverted structures of the external Alps
Authors: Huyghe Pascale , Mugnier Jean-Louis
Published: April 2008

For several years, the relationships between extensional and contractional features have been studied in detail by numerous authors, and structural rules suggested regardless of the size and the geodynamic context of the inverted structures. Examples from the external part of the Western Alps and its foreland follow these rules (oblique-slip reactivation, buttress effect of normal faults, short-cuts, preserved half-grabens beneath decollement, forced folds, etc.), but are not controlled by the same geodynamic conditions as intra-plate inverted basins. The Jura platform, which is not controlled by extensional tectonics, shows fault reactivation, buttress effects and preserved structures beneath the decollement. Inverted structures are nicely illustrated in the Dauphinois domain of the stretched western Tethyan margin, but the effect of lithospheric weakening inherited from the stretching was reduced at the time of the Neogene shortening, and the maximum burial is induced by nappe loading. Therefore the relative timing of inversion tectonics, maximum burial and maximum temperature are very different in the Bourg-d’Oisans half-graben of the Dauphinois domain than for intra-plate inverted basins of the Southern North Sea, e.g. the Broad Fourteens Basin. Nonetheless, fold structures overlapped by major unconformities developed during early deformation events in restricted areas of the Western Alps. The example of structures in the Devoluy is linked to the inversion of the Vocontian basin, and is followed by a renewal of subsidence prior to the later Alpine shortening. Therefore, the Alps provide geometrical field examples of the interaction between stretching and shortening events, and may give some insight into the geometry of structures on the seismic scale. Intra-plate inverted basins provide examples of folded and faulted structures overlapped by sediments and provide a means of explaining the early shortening events that occurred in the Western Alps which were not related to the alpine evolution.
Structural analysis of the inverted Bristol Channel Basin: implications for the geometry and timing of fracture porosity
Authors: Nemčok Michal , Gayer Rod , Miliorizos Marios
Published: April 2008

The Bristol Channel basin was developed as an early Mesozoic half-graben, with the Bristol Channel fault zone (BCFZ) representing the principal down-to-the-south basin controlling structure. The basin was inverted during the Tertiary. This paper illustrates the application of structural analysis to the northern and southern margins of the Bristol Channel basin, where on shore medium- to small-scale structures provide analogues for the BCFZ. Throughout the outcrop four distinct structural mechanisms and associated fracture systems can be recognised. i) The oldest are normal faults, in many cases resulting from the reactivation of underlying WNW-ESE to NE-SW Late Palaeozoic Variscan thrusts. These are linked by N-S transfer faults and associated with WNW-ESE to ENE-WSW extensional veins. Evidence for synsedimentary development of both normal faults and extensional veins has been found in the Triassic and Lower Liassic sequences. Palaeostress analysis suggests an approximately NE-SW oriented σ
3
. ii) During inversion the normal faults acted as buttresses, forming WNW-ESE and younger NW-SE trending folds. Associated thrusts and oblique- to strike-slip faults reactivated the earlier transfer faults. The orientation of σ
1
was NE-SW during this deformation. iii) A regionally consistent system of NE-SW striking extensional veins formed at the close of the strike-slip inversion event. iv) Following primary inversion, lateral escape structures formed against the buttresses, with the development of oblique- and strike-slip faults in a variable and locally controlled stress field. Fracture porosity determined at sites throughout the outcrop show highest levels, up to 20%, formed in association with normal faults during the rifting event, but also significant amounts developed during the inversion event. Veins associated with strike-slip faulting of the latter give average porosities of 6.5%, whilst the later inversion-related extensional veins give average porosities of 0.8%. All these porosities show a high degree of directional permeability. It is argued that the oil generation window was reached during burial in the later stages of the rifting event, and that rapid polyphase fluid discharge from over-pressured fracture-bounded compartments allowed hydrocarbon migration into normal fault-related fracture porosity to form traps. The development of fracture porosity during inversion produced a long-lasting directed permeability and allowed many of the traps to drain. Only those traps associated with normal faults not directly affected by the inversion will have survived. A strategy to discover such a play requires a well targeted and detailed structural study.
3D analysis of inverted extensional fault systems, southern Bristol Channel basin, UK
Authors: Dart Chris J. , McClay Ken , Hollings Peter N.
Published: April 2008

East-West trending inverted extensional fault systems offset Triassic to Lower Jurassic strata close to the southern margin of the Bristol Channel basin along the north Somerset coast. Field mapping, using exceptionally detailed aerial photographs, has revealed a three phase tectonic evolution. (i) North South orientated stretching, resulting in a well developed extensional fault system. The faults are segmented, linked by relay ramps and horsetail toward their tips. (ii) North-South oriented compression, resulting in partial inversion of the extensional fault system, with the development of hangingwall buttress anticlines and zones of intense folding. (iii) North-South orientated compression, resulting in NW-SE trending dextral and NE-SW trending sinistral strike-slip faults. Comparison of hangingwall buttress anticlines exposed in North Somerset with similar larger scale structures observed on seismic profiles from the Bristol Channel shows that both are directly analogous. Correlation with regional data on the tectonic evolution southwest England, and the Bristol Channel, indicates that extension occurred during the Jurassic-Lower Cretaceous, and contractional inversion and strike-slip deformation during the Tertiary.
Inversion of an extensional-ramp basin by a newly formed thrust: the Cameros basin (N. Spain)
Authors: Guimerà Joan , Alonso Ángela , Mas José Ramón
Published: April 2008

The Cameros basin (Iberian Chain) was developed under a very subsident extensional regime during the latest Jurassic-Early Cretaceous. Its sedimentary fill constitutes a megasequence of more than 5000 m of vertical thickness which contains six depositional sequences. Most of the sediments are continental (alluvial and lacustrine) with only minor marine intercalations. A lateral accretionary geometry at the basin scale shows a SSW-NNE migration of the depocentre and an onlap to the north of the sedimentary sequences on the Mesozoic substratum of the northern boundary of the basin.

The overall Mesozoic structure of the basin, as seen from field studies and seismic profile interpretation, is a gentle WNW-ESE synclinorium 30–70 km wide and 150 km long. Extension in the pre-basin Mesozoic rocks is very small. The basin boundaries are defined by an unconformity of the basin filling rocks on the former Mesozoic substratum; it is bounded only locally by major faults at surface. The basin is interpreted as an extensional-ramp basin produced over an S-dipping ramp in a blind, flat-lying extensional fault some kilometres deep in the basement. The extensional displacement on the fault produced a synclinal basin which progressively widened with time. The depocentres of the successive depositional sequences were always located above the ramp and migrated to the north, inside the basin, as a result of the hangingwall displacement to the south. From a computer-modelled cross-section, we estimate the total displacement on the extensional fault to be about 33 km. This extension, in the hangingwall, would have taken place in the Pyrenean basin, north of the Cameros basin.

Tertiary compression (Palaeogene to Lower-Middle Miocene) inverted the basin by means of a newly formed E-W north-directed thrust. Overthrusting in the Tertiary Ebro basin fill, this new fault formed in the Keuper beds, in a weakness zone with uniform dip within the extensional hangingwall (about 30 km in cross-section). During the deformation it expanded to the north and south until it branched to the Iberian sole thrust, which might coincide with the Mesozoic extensional sole fault. Although this thrust produced a slight (2–3 km) inversion of the basement-Mesozoic cover unconformity, its maximum displacement was of about 28 km. Pre-basin Mesozoic rocks overlie the thrust surface and are overlain by the basin-filling rocks. A south-directed imbricate-fan thrust system developed in the southern margin. The total shortening was about 38 km, leading to the complete inversion of the basin.
Inversion-controlled uplift of an outer-arc ridge: Nias Island, offshore Sumatra
Authors: Samuel M. A. , Harbury N. A. , Jones M. E. , Matthews S. J.
Published: April 2008

This study presents data from Nias Island which lies on an outerarc ridge 150 km to the west of mainland Sumatra. A new stratigraphy, geological map and cross-section of Nias have been produced from field data collected over all the island supplemented with aerial photography, LANDSAT and SAR (synthetic aperture radar) studies and these data form the basis for a new interpretation of the island. Where exposed the basement consists of ophiolitic rocks, originating from a variety of tectonic settings, and with a thin sedimentary cover. Three main sub-basins are identified on Nias. Late Paleogene-Neogene sedimentation in these basins was controlled primarily by major extensional faults though deposition within sub-basins was influenced by the presence of sub-basin transecting faults with significant vertical throws. Two phases of inversion are recognized: the first occurred during the Early Miocene, was limited to western regions and is identified by the presence of a localized unconformity and vitrinite and apatite fission track data. The second inversion initiated in Pliocene times and affected all the sub-basins on Nias. Latest Pliocene-Pleistocene rocks unconformably overlie the Miocene strata. Deformational styles within separate fault-bounded segments vary markedly; structural mapping demonstrates uplift and deformation has been largely controlled by reactivation of sub-basin extensional faults and oblique-slip movements on transecting faults. Diapiric melanges developed during inversion suggesting an intimate relationship between uplift and remobilization of overpressured mudrocks. This study concludes that the uplift of the sub-basins on Nias resulted from inversion of basin successions by reactivation of original major extensional faults rather than on thrust-slices developed in an accretionary prism. Nias does not form part of an accretionary complex; the accretionary prism has developed to the southwest of Nias.
Fold growth during basin inversion — example from the East China Sea Basin
Authors: Wang Guang Ming , Coward Mike P. , Yuan Wenguang , Liu Shenshu , Wang Wenqiang
Published: April 2008

Understanding the progressive growth of fold structures during basin inversion is an important part of basin analysis. To achieve this, a complex inversion-related anticline in the East China Sea Basin has been studied to examine variations in the growth rate and growth kinematics during the Miocene inversion of the basin. Detailed studies of onlaps and truncations of seismic reflections, particularly within the syn-growth package, indicate that different parts of the anticlinal structure grew at slightly different times, i.e. the start and cessation of growth vary along and across the structure. Growth was related to the reactivation of the pre-existing, dominantly W-dipping early normal faults. The reactivation of these early normal faults and the development of new reverse faults progressed laterally and transversely across the structure. Sections have been depth-converted, back-stripped and reconstructed to restore the progressive development of the fold. The concept of this study may be applied to many other basins with a similar evolutionary history.
Correlation of sandstones using heavy minerals: an example from the Statfjord Formation of the Snorre Field, northern North Sea
Authors: Morton Andrew , Hurst Andrew
Published: April 2008

Correlation of sandstones using heavy minerals is dependent on the recognition and quantification of provenance-sensitive parameters. Suitable criteria include ratios of minerals that have similar hydraulic and diagenetic behaviour, and properties of populations of individual mineral species. Sandstones of the Statfjord Formation in the Snorre Field display marked variations in their heavy mineral assemblages. Although hydraulic and diagenetic processes have played some part in generating these variations, major changes in the nature of the source material can be detected on the basis of heavy mineral ratio and varietal data. These changes in sand provenance can be used to produce a detailed stratigraphic breakdown of the sequence and to correlate from well to well. Eleven heavy mineral zones have been recognized on the basis of variations in monazite/zircon, garnet/zircon and rutile/zircon ratios, on the incoming of chloritoid, and on the composition of garnet populations. The marked changes in provenance result from the proximity of the depositional location to the hinterland and it is considered unlikely that the detailed correlation made in the Snorre Field area applies to Statfjord sandstones in other parts of the northern North Sea. However, there is evidence for some degree of allogenic control on the heavy mineral variations and therefore some events may have regional significance.
Direct Pb/Pb dating of Silurian macrofossils from Gotland, Sweden
Authors: Russell Jonathan
Published: April 2008

Pb/Pb and U-Pb methods of direct isotopic dating, characteristically associated with igneous and metamorphic rocks, have been applied successfully to well-preserved stromatoporoidal carbonates from Gotland, Sweden. A Pb/Pb date of 432 ± 15 Ma for Wenlock-Ludlow boundary carbonates is in good agreement with independent chronometric estimates and is interpreted as the age of deposition/early diagenesis. A Pb/Pb date of 326 ± 11 Ma for older Middle Wenlock samples is
c.
100 Ma too young for its stratigraphic position and is interpreted as a late diagenetic age associated with fluid movement along sub-surface faults. U-Pb systematics in both samples have been disturbed by U remobilization in response to recent weathering associated with post-glacial uplift in the Baltic.

For many years, the direct age determination of sediments has been an irritating problem to earth scientists. Direct Pb/Pb dating of carbonates should be considered a credible source of chronometric information. Application of the method to poorly constrained Precambrian stromatolitic carbonates is potentially powerful since host sediment ages may be only poorly constrained through indirect reference to basement and intrusive rocks. A broad geochemical review is presented to explain the suitability of certain carbonates for this method of isotopic investigation.
Wireline log-cyclicity analysis as a tool for dating and correlating barren strata: an example from the Upper Rotliegend of The Netherlands
Authors: Yang Chang-Shu , Kouwe Wim F. P.
Published: April 2008

The stratigraphic subdivision and correlation of the Upper Rotliegend Group in The Netherlands offshore is difficult due to lack of biostratigraphic data. A regional study of 150 released wells and core data has revealed the preservation of five supersequences, 12 sequences and short-term cyclicities in the Upper Rotliegend deposits. They are interpreted to reflect long- and short-term climate changes, which controlled the deposition of Rotliegend sequences.

The supersequences (second order) serve best for regional correlation and prediction of sand distribution. The sequences (third order) are best suited for detailed block- or field-scale correlation and distribution of (un)favourable lithofacies. The short-term (higher order) cycles represent orbital-forcing (Milankovitch) cycles and are used as quantitative parameters in wireline log cyclicity analysis. This includes the calibration of sequence boundaries and maximum flooding surfaces, the determination of the absolute age, the estimation of the duration time and the net accumulation rate of the preserved rock succession, and the pattern recognition of the trend of net accumulation rate variations within the third-order sequence. The method has revealed a new possibility of dating and correlating strata such as the Upper Rotliegend.
Detachment tectonics during Atlantic rifting: analysis and interpretation of the S reflection, the west Galicia margin
Authors: Reston T. J. , Krawczyk C. M. , Hoffmann H.-J.
Published: April 2008

Beneath the tilted fault blocks of the western Galicia rifted margin an unusually bright reflection, the S reflection, is observed. The waveform, polarity and amplitude of S indicate that it is a reflection from a seismic interface across which the acoustic impedance increases sharply. This result is consistent with its interpretation as a detachment fault juxtaposing a low velocity and density upper plate and a high velocity and density lower plate. The lower plate may represent partially serpentinized mantle material, brought into contact with pre-rift sediments and upper crustal basement by tectonic denudation.

Pre-stack depth migration is applied to determine the true geometry of S, and its relationships with the overlying faults. It is found that S passes continuously beneath the upper crustal faults, which detach onto S. However, S does appear to be truncated westwards by east-dipping reflections associated with the peridotites exposed at the seafloor. We interpret these reflections as the continuation of the top-to-the-east extensional shear zone sampled within the peridotite, and suggest that S is either antithetic to a master mantle detachment, or that S is cut by a later mantle shear zone.
Evolution of a major oceanographic pathway: the equatorial atlantic
Authors: Jones E. J. W. , Cande S. C. , Spathopoulos F.
Published: April 2008

The history of continental separation in the equatorial Atlantic is important to our understanding of the events which have led to the establishment of the present patterns of water circulation. Orientations of oceanic basement lineaments determined from bathymetric, seismic, magnetic and satellite altimetry data, and the distribution of seismic reflectors in deep-water sediments indicate that during its early opening stages the Atlantic was bounded to the south by the Guinea Fracture Zone. Using stage poles obtained from South Atlantic spreading patterns, basement ages and the palaeobathymetry of the equatorial region have been derived. The proximity of magnetic anomaly M0 to the present continental slopes suggests that the deep-water basins began to form in the Aptian. During the early stages of basin development water circulation was greatly restricted by fracture zone ridges, leading to the formation of thick sequences of carbonaceous shales. Outflow of dense, saline water from the equatorial basins may have been an important factor in controlling deposition along the Atlantic margins, contributing to the development of unconformities within the Cretaceous sedimentary record. By Santonian time the equatorial rift had reached a width of
c.
1200 km, water depths close to the continental margins exceeded 5000 m and the transfer of surface water between the North and South Atlantic was well established. The conjugate Sierra Leone and Ceara Rises were built up during the late Cretaceous and existed as separate features by Early Oligocene time. Both the Romanche and Vema Fracture Zones have acted as important conduits for the transfer of bottom-water from the western to the eastern equatorial basins, with seismic profiles providing evidence for vigorous bottom water flow during the Eocene and later Tertiary. In the Sierra Leone Basin the circulation of bottom water may have reversed during the late Tertiary as a result of the movement of the eastern portion of the Romanche Fracture Zone north of the equator.
The tephrochronology and radiocarbon dating of North Atlantic, Late-Quaternary sediments: an example from the St. Kilda Basin
Authors: Hunt John B. , Fannin Nigel G. T. , Hill Peter G. , Peacock J. Douglas
Published: April 2008

A sequence of disseminated basaltic tephras of Icelandic provenance has been investigated in sediments of Late Quaternary age recovered from the St Kilda Basin, on the Scottish continental shelf. The tephras were deposited from gradually melting rafted pack ice, transported on an anti-clockwise surface current originating to the north of Iceland. The presence of these ice-rafted tephras extends the zone of this current activity well beyond its previously documented western limit, demonstrating current impingement on the UK continental shelf. The evidence of ice-rafting, together with the biostratigraphy and a series of AMS
14
C dates, confirm that this deposition occurred during the Younger Dryas chronozone. Electron probe microanalysis (EPMA) of glass-shard geochemistry is used to ralate the St Kilda tephras to tephras found in marine and terrestrial deposits throughout the North Atlantic area, and to possible volcanic centres in Iceland. The joint role of tephrochronology and radiocarbon dating is discussed in relation to the comparative reliability of marine and terrestrial timescales. Problems with the chronology of the terrestrial equivalents of these tephras in Northern Iceland are highlighted.