bluuebunny/crossref_metadata_2025 huggingface dataset:
Acquisition of anhysteretic remanence and tensor subtraction from AMS isolates true palaeocurrent grain alignments
Authors: Borradaile Graham J. , Fralick Philip W. , Lagroix France
Published: April 2008

Anisotropy of magnetic susceptibility (AMS) is commonly used to detect subtle palaeocurrent directions. A Proterozoic subarkose shows extreme anomalous inverse AMS fabrics that cannot be attributed to the well-known effect of single-domain magnetite. This was confirmed by anisotropy of anhysteretic remanent magnetization (AARM). Further, by subtracting increasing proportions of the AARM tensor from the AMS tensor the paramagnetic/diamagnetic response of the silicate fabric was isolated. This reveals an inverse quartz fabric but analysis of the results shows that two grain alignments are present. A prominent inverse magnetic fabric is due to quartz long axes aligning with current flow revealed by the difference [AMS]-[AARM]. This is due to the avalanche flow of concentrated sand dispersions down the slip-face of cross-stratified units. Dispersive pressure caused by grain interactions and shear within the fluid produces a subhorizontal resultant force that aligns grains by frictional freezing during deposition. In some samples AARM isolates a secondary feeble alignment of magnetite nearly perpendicular to bedding. This is due to suspension rainout and sweeping by the reverse circulation separation eddy that supplies additional material to the top of the sand laminae. It particularly affects fine grains such as magnetite that infiltrate the framework’s pores to produce a secondary population of vertically oriented grains.
Sequence stratigraphic subdivision of the Humber Group in the Outer Moray Firth area (UKCS, North Sea)
Authors: Duxbury S. , Kadolsky D. , Johansen S.
Published: April 2008

The Humber Group (Late Oxfordian-Ryazanian) in the Outer Moray Firth has been subdivided into 19 depositional sequences, which are firmly related to a newly defined, consistent biostratigraphic framework. Each sequence has been defined by core sedimentology (where possible), biostratigraphy and electric log patterns. In non-marine to shallow-marine environments (e.g. Piper Formation
sensu lato
), well-defined transgressive and highstand (or regressive) systems tracts are recognized, which are extensively cored in the area due to their importance as reservoirs of major fields. In younger sections, sequence recognition is based on the presence of submarine fans and maximum flooding surfaces; sequences so defined are to some extent supported by the cyclicity of equivalent shallowmarine sand sequences near the basin margins. The sequence stratigraphy approach has clarified the lateral stratigraphic relationships of reservoir sand units within the Tartan Field, and the relationships between the reservoir sand units in the Tartan, Highlander and Petronella fields. The main causes of the observed cyclic sedimentation are interpreted as the complex interplay between sediment supply, subsidence rates and eustasy. Although the relative importance of the last two remains unclear, the sequence stratigraphic approach followed here has allowed rigorous description and consistent interpretation of vertical and lateral facies associations.
Well-site biostratigraphy of Danish horizontal wells
Authors: Shipp D. J.
Published: April 2008

The number of horizontal and subhorizontal wells drilled has significantly increased over the last decade, especially during the last five years, and the use of horizontal wells in oil-field production and development is now routine. In 1987 Maersk Olie and Gas AS drilled the first horizontal well on the Dan Field. Since that time biostratigraphers at Robertson Research International Limited have been involved in Maersk’s pioneering work on the development of steering techniques for horizontal wells in the Danish sector of the North Sea. Well-site biostratigraphy has been successfully used to aid in the steering of these horizontal wells.

The background studies leading to the erection of highly detailed local biozonal schemes capable of detecting changes over vertical distances as little as 1 foot are described. The logistics of how well-site biostratigraphy is carried out at the well site is explained. In addition to Dan, well-site biostratigraphy has been successfully applied to development wells from several other Danish North Sea carbonate fields operated by Maersk Olie og Gas AS on behalf of the Danish Underground Consortium (a joint venture between Shell, Texaco and AP Møller). These fields include Harald, Kraka, Gorm, Roar, Skjold, Svend, Tyra and Valdemar. Over 100 wells from Danish fields have to date been effectively steered using high-resolution biostratigraphy. Vertical targets of 10–20 feet or less have been successfully followed over distances of several thousand feet.
Forties Field (North Sea) revisited: a demonstration of the value of historical micropalaeontological data
Authors: Jones R. W.
Published: April 2008

The history of the discovery, appraisal and development of the Forties Field and the evolution of the reservoir model are briefly reviewed.

The process of integrated reservoir description (IRD) is still ongoing, iterating with the acquisition of new analytical or (re)interpreted biostratigraphic or sedimentological or other static (seismic, well) or dynamic data. IRD has already made a significant impact on the ongoing infill production drilling programme.

Even in the absence of newly acquired high-resolution palynostratigraphic and three-dimensional (3D) seismic data, historical high-resolution micropalaeontological data can be used to identify eight main sands in the reservoir system, separated vertically and/or laterally by shales (production barriers).

Historical micropalaeontological and sedimentological data can also be used to refine the interpretation of the depositional environment of the reservoir section. It indicates the development of both high-density channel-axis turbidite and low-density channel and overbank turbidite facies in pay zones, and interturbidite facies (with low potential as production barriers) and hemipelagite facies (with high potential as production barriers) in non-pay zones. The progressive elimination of infaunal and epifaunal benthonic foraminifera during the time the reservoir section was deposited indicates a progressive deterioration of the physics or chemistry of the benthonic environment (ultimately resulting in the development of a non-bioturbated laminite facies).
Constraints on the application of palynology to the correlation of Euramerican Late Carboniferous clastic hydrocarbon reservoirs
Authors: McLean D. , Davies S. J.
Published: April 2008

Palynology represents the standard tool for subsurface biostratigraphical correlation of Euramerican Late Carboniferous sequences. While palynomorphs are usually absent from sandstone lithologies, they form diverse and abundant assemblages in fine-grained sediments and coals. The occurrence of inter- and intrasandstone horizons that contain distinctive palynological assemblages forms the basis for the correlation and discrimination of sandstone reservoirs, and the application of palynology to development drilling programmes. The successful application of development palynology is contingent upon many factors related to geology (sandstone-body type, occurrence and lateral consistency of distinctive horizons, nature of intrareservoir units, degree of reservoir incision, etc.) and to practical constraints (such as the nature of databases, sampling strategies with ditch cuttings, standardization of preparation and data acquisition techniques, and time and cost limitations). The development palynologist needs to be critically aware of the existence of such constraints, and should limit the confidence attached to interpretations and predictions made accordingly.
The Mesozoic denudation history of the Atlantic margins of southern Africa and southeast Brazil and the relationship to offshore sedimentation
Authors: Gallagher Kerry , Brown Roderick
Published: April 2008

A new interpretation of regional apatite fission track data suites from western southern Africa (South Africa, Namibia) and southeast Brazil, in terms of the post-break-up denudation chronologies, is presented. The data reveal that although significant denudation occurs parallel to the present-day margin (associated with the inland escarpment) there are regional variations along-strike and with time. Moreover, the data reveal a significant amount of post-break-up denudation from within the continental interior of southern Africa. The data indicate relatively little post-rift denudation but the rates increase dramatically in both regions in the late Cretaceous-Early Tertiary, more than 50 Ma after initial rifting. The inferred denudation chronologies are broadly consistent with the sedimentary record in the offshore basins, which can show increases in sedimentation rate, contemporaneous unconformities and major changes in depositional character.
Western Africa: an unfinished story of oil and gas exploration
Authors: Dolan P.
Published: April 2008

Understanding the future oil and gas exploration potential of western Africa’s South Atlantic margin depends not only on access to modern, high-quality geological and geophysical data, but also on correctly interpreting the motivations for earlier exploration campaigns and the basic geological characteristics which can be used to sub-divide the margin of western Africa.
The distribution of petroleum reserves in basins of the South Atlantic margins
Authors: Coward M. P. , Purdy E. G. , Ries A. C. , Smith D. G.
Published: April 2008

The opening of the South Atlantic created a series of passive margin basins on both sides of the new ocean. This paper reviews the distribution of petroleum reserves in these basins in terms of their tectono-stratigraphic position within the framework of the rift-drift succession. Seven megasequences are recognized. Three are in the drift succession, three are in the rift succession and one comprises the pre-rift succession. Ninety-three per cent of the presently discovered recoverable hydrocarbons are reservoired in the drift succession, 6% are located in the rift succession and 1% is associated with the pre-rift units. The basins group geographically into seven sectors within which both margins share some common features. Sixty-five per cent of the reserves are contained in Sector V which is dominated by the Niger Delta, and 28% are positioned in Sector III, which includes the Campos and Lower Congo Basins. The new deepwater giant discoveries of Angola, which are located in Sector III, are the Africa counterparts of the earlier deepwater Campos Basin discoveries. In general sectors III–V, located between the Walvis Ridge and the Equatorial Atlantic transforms, are most favourable as they are characterized by the most prolific source rocks at all horizons.
Tectonics and hydrocarbon distribution along the Brazilian South Atlantic margin
Authors: Davison Ian
Published: April 2008

Break-up of the Brazilian margin produced a wide variety of structural styles depending on opening kinematics and thermal configuration of the lithosphere. Wide margins developed in Santos and Campos, where stretched continental crust extends for up to 600 km from the shoreline; whereas narrow margins may be only 40 km wide. The wide margins host the largest oil fields, such as those discovered in the Campos Basin. Wide margins have good hydrocarbon potential in the post-rift sequence, but narrow margins probably have the better unknown syn-rift plays which have still to be drilled in deepwater. Little is known of the narrow margins, such as the Jacuípe and Pernambuco-Cabo Basin area, as most of the prospective hydrocarbon structures lie in water depths > 1500 m.

Hydrocarbons are hosted by reservoirs ranging from fractured Neocomian volcanics to Miocene turbidites, with over 80% of the reserves trapped in the Campos Basin turbidite sandstones. These sandstones occur as constrained channel-fills on the upper platform and slope, and as unconstrained lobes up to 30 km in long dimension on the mid to lower continental slope which are situated in 500+ m water depths at the present day. The wide margins will continue to be the most productive in Brazil, but deepwater production technology has advanced enough to make the deeper parts of the narrower margins attractive exploration areas.
Petroleum systems in the South Atlantic margins
Authors: Schiefelbein C. F. , Zumberge J. E. , Cameron N. R. , Brown S. W.
Published: April 2008

Results obtained from the detailed geochemical analysis of more than 150 crude oils were used to : (1) identify a number of different petroleum systems present in the South Atlantic margins; and (2) establish geochemical criteria useful in distinguishing lacustrine-derived oils from marine-derived oils. The multiplicity and type of effective source rocks were determined by using the geochemical characteristics of the crude oils to establish the number of compositionally distinct oil families, and by inferring palaeoenvironmental conditions of source rock deposition and possible age. Using multivariate statistical techniques and bulk geochemical parameters, isotopic compositions and biomarker distributions, a number of general petroleum systems were identified, differing from each other in terms of source palaeoenvironment and age. The oils are separated into two broad groups: lacustrine and marine. Further detailed examination of each broad group, sometimes using more specific geochemical criteria, typically resulted in the establishment of sub-groups of oils according to specific source environment, such as lacustrine fresh v. saline conditions. Areas where oils of mixed provenance occur are also identified. The geochemical criteria used in discriminating between lacustrine and marine oil types were determined based on geochemical differences that exist between known marine-sourced (mainly post-salt) oils from the Kwanza and Gabon (Port Gentil area) Basins and known lacustrine-sourced (pre-salt) oils from the Congo Basin.
Angola: source rock control for Lower Congo Coastal and Kwanza Basin petroleum systems
Authors: Burwood R.
Published: April 2008

The purpose of this paper is to provide an overview of petroleum occurrence and provenance for the 1000 km West African Atlantic Margin from Cabinda to mid-Angola. Over this margin the Lower Congo Coastal and Kwanza provinces cumulatively account for reserves of
c.
6 gigabarrels oil recoverable (GBOR). These are dominantly reservoired in Pinda carbonate traps of the former basin. However, with production from a range of aggradational wedge, carbonate platform and pre-salt reservoirs, a diversity in oil character presupposes complex hydrocarbon habitats charged by multiple sourcing.

Each of these two major Atlantic margin salt basins constitutes a different, source rock driven, hydrocarbon habitat. As classic passive margin pull-apart basins, Early Cretaceous initiated rift events (Pre-rift, Syn-rift I, II, etc.) evolved into the drift phase opening of the southern Atlantic. A striking feature of this progression was widespread evaporite deposition of the Aptian Loeme salt. This separates two distinct sedimentary and tectonic domains of the Pre- and Post-Salt.

The core Lower Congo Coastal habitat is dominated by the Pre-Salt Bucomazi Formation sourced ‘poly’ petroleum system. These lacustrine, often super-rich, sediments reveal considerable organofacies variations between their basin fill (Syn-rift I) and sheet drape (Syn-rift II) development, accounting for the compositional diversity in their progenic petroleums. Of crucial impact is a cognate diversity in their kerogen kinetic behaviour. This controls the conditions and timing of generation and realization of charge potential.

With the Lower Congo Coastal habitat extending southwards towards the Ambriz Spur, the Bucomazi facies proper appears restricted to the northern and deeper proto-lake trend. Over the more weakly subsident margins such troughs host inferior sheet drape potential. Elsewhere, Upper Cretaceous-Palaeogene marine clastic Iabe Formation sourced petroleum systems are hydrocarbon productive, yielding petroleums of unique, and/or mixed Pre-Salt hybrid, source provenance.

A less attractive Pre-Salt source system is reflected in the Kwanza habitat. Consequential effects of salt movement have proved important for halokinetic induced structuration, Cretaceous block rafting and pseudograben (Tertiary trough) source deposition. Marginal reserves of Infra-Cuvo and Cuvo derived petroleums are known. With limitations in charge potential at this level, the modest Kwanza reserves (0.1 GBOR) have a Post-Salt provenance deriving primarily form the Eocene Margas Negras and subordinate Aptian Middle Binga sourced petroleum systems. Cases of multi-sourced hybrid petroleums abound. In addition to the organic-rich Albian micrites, Upper Cretaceous black shales provide unproven possible offshore potential.
Palaeoenvironmental evolution of the Late Cretaceous sequence in the Tarfaya Basin (southwest of Morocco)
Authors: El Albani A. , Kuhnt W. , Luderer F. , Herbin J. P. , Caron M.
Published: April 2008

The Late Cretaceous sedimentary evolution of the Tarfaya Basin is reviewed based on late Albian-Campanian outcrops and two cored exploration wells (S13 and S75) with a stratigraphic range from the early Cenomanian to the late Turonian. Standard sedimentological and micropalaeontological methods were used, together with clay mineralogy and organic geochemistry. Two distinct depositional environments were found. The first is late Albian-early/middle Cenomanian in age and dominated by terrigenous silici- and bioclastic sediments containing primarily terrestrial organic matter with a low organic content. Abundant illite and chlorite, eroded from crystalline rocks outcropping in the Anti-Atlas mountain and Mauritanides is present, as is abundant kaolinite reworked from soils surrounding the Tarfaya Basin. The second environment comprises late Cenomanian-Campanian sediments characterized by pelagic marls and limestones with a high organic content and a abundant pelagic macro- and microfauna. These sediments, which were probably deposited in an upwelling system, are characterized by higher contents of smectite.
The interpretation of passive margin depositional processes using seismic stratigraphy: examples from offshore Namibia
Authors: Bagguley Joanne , Prosser Sarah
Published: April 2008

Seismic stratigraphic analysis of the post-rift megasequence of the Namibian passive continental margin has enabled depositional features to be interpreted in terms of the likely controls on their formation. The scale of the information derived from this manner of depositional system analysis is fundamental in understanding processes of sedimentation on passive margins; processes which may not be so readily apparent at field or well-log scale studies. Inner shelf channel forms are interpreted as incised valleys, created during a fall in relative sea level. The horizontal nature of the seismic facies infilling these valleys suggests that they filled during progressive periods of rising relative sea level, implying late lowstand or transgressive sediments. A series of low mounded features can also be identified on the shelf, which may be interpreted as palaeodune fields, carbonate build-ups or longshore clastic mounds. The outer shelf and upper slope positions reveal interpreted canyon and channel forms. The infill to these features contrasts with that of the incised valleys by having a more draped from (sometimes with a chaotic unit below); a geometry consistent with submarine origins. The submarine canyons may be termed attached or detached depending on whether they can be shown to link back into fluvial systems on the shelf; attached canyons will be more likely to be able to support the development of deepwater, sand-rich submarine fans. In the slope/base of slope area both regionally extensive and more localized mounds can be interpreted as mass transport deposits, the trigger for which may be attributed to a relative sea-level fall (due to their connection to the valley and canyon systems), though a transgressive origin may also be consistent in some cases. The mounds seen in positions deepest in the basin have a markedly different seismic facies to those at the base of slope and a different depositional system may be invoked, perhaps that dominated by distal turbidity currents or waning bottom currents.
Facies architecture of the Etjo Sandstone Formation and its interaction with the Basal Etendeka Flood Basalts of northwest Namibia: implications for offshore prospectivity
Authors: Jerram Dougal A. , Mountney Nigel , Stollhofen Harald
Published: April 2008

The Basal Etendeka Flood Basalt stratigraphy in the Huab Basin of northwest Namibia comprises a series of lava flows interleaved with aeolian sandstone bodies of the Etjo Sandstone Formation. The sandstone units are characterized by three main types: (1) the major erg — a mixed aeolian and fluvial facies up to 150 m thick; (2) minor ergs — aeolian facies which occur directly above the first volcanic units and are up to 60 m thick; (3) isolated bodies — multidune, single dune and lava topography infills. A variety of bypass surfaces identified by sand-filled cracks and sediment-lava breccias occur on lava top surfaces. Preserved ripples and pahoehoe lava imprints indicate that the aeolian sand dunes were actively migrating during basalt emplacement.

Observations recorded in the Basal Etendeka Flood Basalts which may be of relevance to offshore hydrocarbon exploration include: a major-minor erg relationship resulting in large sandbodies up to 60 m thick which occur directly after the first volcanic units; the occurrence of sand-filled fissures up to 36 m in depth which would greatly influence connectivity in an offshore setting; the identification of bypass surfaces as marker horizons which may laterally correlate with isolated sandbodies.
Basin evolution and sedimentary fill in the Palaeozoic sequences of the Falkland Islands
Authors: Meadows Neil S.
Published: April 2008

The sedimentary sequences exposed on the Falkland Islands can be referred to two major episodes of basin evolution. On the basis of fieldwork and recent palynological determinations carried out as part of the study from which this publication is taken, the older of these is largely Middle Devonian in age and comprises a marine transgressive-regressive system culminating in extensive fluvial braidplain deposits which are referred to the Gran Malvinas Group. The younger is largely Permian in age, commences with a range of glacial deposits and includes lacustrine, shallow marine and deeper marine deposits before also culminating in fluvial channel deposits and is referred to the Lafonian Supergroup. The facies identified at outcrop and their relationships are described and presented herein as depositional models for the two major systems, while inferences derived from these models are used both to suggest the structural and tectonic history of the islands, and to erect a revised lithostratigraphy which relates to these episodes of basin evolution. It is recognized that this is an interim measure pending much more intensive work on the geology of the Falkland Islands which is currently taking place.
Fractal analysis and percolation properties of veins
Authors: Roberts S. , Sanderson D. J. , Gumiel P.
Published: April 2008

Systematic sampling of vein systems often reveals a power-law distribution of the number of veins (
N
) to the vein thickness (
t
) of the form
N
=

Ct
-D

. A comparison of vein thickness distributions from different geological settings establishes systematic variations between vein populations, with lower
D
-values characterizing well-connected vein systems. Low
D
-value vein systems are reported from mineral deposits and from veins developed within and around fault zones, often at constant vein frequency. Simple models are described to explain the field observations which suggest that the observed variations in vein thickness distributions are directly related to the connectivity of the sampled vein network.
Geometry and population systematics of a quartz vein set, Holy Island, Anglesey, North Wales
Authors: Stowell Julia F. W. , Watson Adrian P. , Hudson Neil F. C.
Published: April 2008

A steeply dipping set of quartz-chlorite-muscovite-biotite veins was emplaced, at depths of at least 14 km and temperatures >400°C, into Môna Complex metasedimentary rocks of Holy Island, after the D4 event. Vein trends range from northeast-southwest to north-northwest-south-southeast. Vein offshoots, consistently oriented
c.
10–35° anticlockwise relative to the main vein, are common. Quartz fibres within different veins show a range of orientations, from northwest-southeast to east-west, and are interpreted as tracking vein opening directions. Vein and fibre orientations are integrated into a four-stage model for vein emplacement. At each stage, new main veins either open extensionally, with fibres subnormal to their boundaries, or by hybrid extension-shear. Hybrid main veins commonly have extensional offsoots. Extensional main veins are orientated progressively further anticlockwise with time from a dominantly northeast-southwest trend to a dominantly north-south trend, but hybrid main veins have out-of-sequence orientations and probably utilized pre-existing fractures. The dominance of extensional and hybrid extension-shear vein opening indicates that the differential stress was small and pore fluid pressure was high. A study of length-thickness relationships has revealed that this vein set is self-affine with vein widening progressing more quickly than elongation during growth. Linear transect analyses show that the vein spacings have fractal characteristics.
Influence of layering on vein systematics in line samples
Authors: Gillespie P. A. , Johnston J. D. , Loriga M. A. , McCaffrey K. J. W. , Walsh J. J. , Watterson J.
Published: April 2008

The thickness and spacing systematics of 30 line samples of vein arrays at ten localities have been analysed. The analytical methods were tested on four synthetic data sets. The synthetic and natural data sets were each analysed with respect to: cumulative thickness v. distance; thickness population; spacing population; the coefficient of variation of spacing, which measures clustering; the mass function, which measures the scaling of strain heterogeneity.

A fundamental distinction is apparent between stratabound arrays, vein arrays in layered rocks where the veins are confined to individual mechanical units, and non-stratabound arrays which occur in rock volumes lacking persistent crack-stopping discontinuities. The stratabound arrays show regular spacing controlled by layer thickness and non-power-law thickness distributions. The non-stratabound arrays are clustered and have power-law thickness distributions resulting from the lack of a controlling length scale. Exponents of the thickness distributions have a modal value of 0.8 and do not vary significantly with lithology. Ore mineralization is favoured in non-stratabound arrays as they are more likely than stratabound arrays to form large clusters connected to a remote fluid source.
Critical stress localization of flow associated with deformation of well-fractured rock masses, with implications for mineral deposits
Authors: Sanderson David J. , Zhang Xing
Published: April 2008

Fluid flow and deformation of fractured rock masses is investigated using numerical models based on distinct element methods (UDEC) which couple mechanical and hydraulic behaviour. A series of numerical experiments is presented in which diffuse flow through fracture networks changes to highly localized flow at a critical stress state. Slip on parts of the fracture network causes the opening up of some fractures, often at fracture intersections. Thus, a change in the stress state results in a sudden localization of deformation and fluid flow. A series of plots of hydraulic conductivity v. differential stress are presented for some natural fracture networks to demonstrate the influence of fracture network geometry and
in situ
stress on the critical phase boundary between diffuse and localized flow.

The onset of localized deformation and fluid flow is a critical point phenomenon, resulting in fractal properties. Fractal and multifractal techniques are used to characterize heterogeneity of the flow in order to generalize the results of the modelling of discrete systems to a continuum-based description of the flow. The significance of the models and their implications for crustal behaviour are discussed in relation to hydrothermal mineralization and natural vein systems.
Alpine plate kinematics in the western Mediterranean: a westward-directed subduction regime followed by slab roll-back and slab detachment
Authors: Zeck H. P.
Published: April 2008

Integration of a wide range of multidisciplinary data, including seismic tomography features, regional tectonic transport directions and ages of subduction/collision, lithospheric extension and slab detachment, suggests a new model for the development of the Alpine belt in the western Mediterranean. The model implies that the backbone of the Alpine Orogeny was formed by a composite SW-NE-striking subduction system, active until some time before 22 Ma. The system which consumed Mesozoic Tethyan lithosphere was dipping westward under the leading edge of Iberia which was drifting eastward with respect to North America under the influence of the opening of the North Atlantic. Allowing for a series of late-stage extensional regimes, with local formation of Neogene oceanic lithosphere, in the Valencia Gulf, Provençal-Algerian basin and southern Tyrrhenian basin, and inherent slab roll-back, the original collision belt may be reconstructed comprising all present Alpine metamorphic core complexes in the western Mediterranean: Betic-Rif, Kabylies and the Sicily-Apennines(-Corsica) belt. It follows that in the western Mediterranean the northward drift of Africa against Iberia/Europe, although influential, e.g., in creating the E-W grain of the Betic Cordilleras, has not been the controlling factor. In contrast, in the eastern Mediterranean the influence of N-S convergence has been much more pronounced due to the hinged sinistral movement of the African plate. A major part of the Europe-Africa N-S convergence was accommodated in a coherent N-dipping subduction zone running from North Africa through Sicily-Calabria to Crete, becoming increasingly significant to the east.
The petroleum systems of the Southeast Basin and Gulf of Lion (France)
Authors: Mascle Alain , Vially Roland
Published: April 2008

The Southeast Basin is the thickest onshore French sedimentary basin where up to 10 km of Mesozoic-Cenozoic sediments can locally be found. This basin is surrounded to the east and to the south by two segments of the Alpine Thrust Belt, the Western Alps and the Pyrenees-Provence respectively, and to the west by recently uplifted elements of the Palaeozoic Basement (Massif Central). The development of the basin was related to several stages of subsidence between late Carboniferous and late Cretaceous times. Partial tectonic inversion took place during two Alpine compressive events in early Tertiary and late Tertiary times. They were separated by an intervening stretching event of Oligocene age which further south led to the opening of the western Mediterranean oceanic basin in Burdigalian times and, as a result, to the formation of the Gulf of Lion passive continental margin. In Neogene times the Palaeozoic basement of the Massif Central was uplifted to approximately 2000 m as the result of an ascending athenospheric plume.

A large oil seep near the city of Gabian has been exploited since the beginning of the seventeenth century. Most of the exploration undertaken from 1945 (onshore) and 1965 (offshore) to the present time has, however, been disappointing as no significant oil or gas accumulations have been discovered, despite drilling of about 150 wells. A recent re-assessment of the potential remaining prospectivity of the Southeast Basin and Gulf of Lion basin has been undertaken by IFP. This study has benefited from scientific researches developed within the Integrated Basin Studies programme. This paper focuses on the source rocks and the petroleum system aspects. The review of all potential source rocks indicates that, from a qualitative and quantitative point of view, the best source rocks are located within three specific stratigraphic intervals (Stephanian-Autunian, late Lias and late Eocene-Oligocene). This analytical work allowed the reconstruction of the history of the different petroleum systems from the Jurassic to the present day. Because of the severe tectonic disturbances that these areas have experienced in Tertiary times, we can conclude that the best potential for economical discoveries are within the Gulf of Lion and some sub-basins of the Southeast Basin where subsidence has been active in Neogene times, or, in other words, where the processes of hydrocarbon generation, expulsion and migration can still be active today.
Lithospheric structure of the Pannonian basin derived from seismic, gravity and geothermal data
Authors: Tari Gábor , Dövényi Péter , Dunkl István , Horváth Frank , Lenkey László , Stefanescu Mihai , Szafián Péter , Tóth Tamás
Published: April 2008

The structure of the Pannonian basin is the result of distinct modes of Mid-Late Miocene extension exerting a profound effect on the lithospheric configuration, which continues even today. As the first manifestation of extensional collapse, large magnitude, metamorphic core complex style extension took place at the beginning of the Mid-Miocene in certain parts of the basin. Extrapolation of the present-day high heat flow in the basin, corrected for the blanketing effect of the basin fill, indicates a hot and thin lithosphere at the onset of extension. This initial condition, combined with the relatively thick crust inherited from earlier Alpine compressional episodes, appears to be responsible for the core complex type extension at the beginning of the syn-rift period. This type of extension is well documented in the northwestern Pannonian basin. Newly obtained deep reflection seismic and fission-track data integrated with well data from the southeastern part of the basin suggests that it developed in a similar fashion.

Shortly after the initial period, the style of syn-rift extension changed to a wide-rift style, covering an area of much larger geographic extent. The associated normal faults revealed by industry reflection seismic data tend to dominate within the upper crust, obscuring pre-existing structures. However, several deep seismic profiles, constrained by gravity and geothermal modeling, image the entire lithosphere beneath the basin. It is the Mid-Miocene synrift extension which is still reflected in the structure of the Pannonian lithosphere, on the scale of the whole basin system.

The gradually diminishing extension during the Late Miocene/Pliocene could not advance to the localization of extension into narrow rift zones in the Pannonian region, except some deep subbasins such as the Makó/Békés and Danube basins. These basins are underlain coincidently by anomalously thin crust (22–25 km) and lithosphere (45–60 km). Significant departures (up to 130 mW m
−2
) from the average present-day surface heat flow (
c.
90 mW m
−2
) and intensive Pliocene alkaline magmatism are also regarded as evidence for the initiation of two newly defined narrow rift zones (Tisza and Duna) in the Pannonian basin system. However, both of these narrow rifts failed since the final docking of the Eastern Carpathians onto the European foreland excluded any further extension of the back-arc region.
Geochemical factors in the selection and assessment of sites for the deep disposal of radioactive wastes
Authors: Savage David , Arthur Randolph C. , Saito Shigeyuki
Published: April 2008

Geochemical parameters which are important to waste isolation include (with optimum conditions in parenthesis): groundwater pH (10 > pH > 8), redox (
Eh
< −200 mV),
P

co
2

(<10
−4
bars), and ionic strength (< 0.5 M); the colloid (< 0.01 ppm), organic (< 5 ppm) and inorganic ligand (Cl
−
> HCO
−
3
, low abundances of phosphate) content of groundwaters; and the mineralogical composition and structure of groundwater flowpaths (zeolites, clays, iron oxyhydroxides with appreciable diffusive porosity). pH, redox, and in some environments,
P

co
2

, will be controlled by water-rock reaction; other parameters are influenced more by geological history or environment. pH and redox buffer intensities of likely mineral assemblages and groundwater compositions under possible physical conditions of deep disposal have been calculated. For pH, reactions between aluminosilicates, carbonates and water should dominate over those in the fluid phase alone. The reaction of small amounts of ferrous iron-bearing minerals (≥ 5 vol %) is sufficient to maintain chemically reducing conditions under likely hydraulic conditions at depth. Consideration of laboratory data suggest that pH and redox reactions should not be hindered by kinetics under likely physical conditions for deep disposal, but further field and laboratory experimental data are required to verify these conclusions. The most suitable geochemical environments for waste isolation are basic or intermediate igneous rocks in tectonically stable, low-lying terrain away from sedimentary basins. Geochemical criteria should not be used in isolation of other factors for the selection of sites, but suitable geochemical conditions may help relax the requirements for optimum performance dictated by other geological and hydrogeological factors.
Characterizing the chemical containment properties of the deep geosphere: water-rock interactions in relation to fracture systems within deep crystalline rock in the Tono area, Japan
Authors: Iwatsuki T. , Yoshida H.
Published: April 2008

A water/rock interaction study has been integrated with interpretations of geological structures, in order to understand the geochemical conditions in deep granitic rock at the Tono study site, central Japan. Geological investigations show that fracture systems in the granitic rock can be classified into: intact zones; moderately fractured zones; and intensely fractured zones. This classification is based upon the frequency and width of fractures and fractured zones. Isotopic analyses of the groundwaters indicate that the groundwater at SL 80m (SL: sea level) has recharged within the last 30 years. Petrological and mineralogical observations were undertaken, and coupled with theoretical calculations using the PHREEQE geochemical computer code. The results of this approach suggest that the chemical evolution of the groundwater is generally controlled by water-rock interactions involving plagioclase, clay minerals and ferric hydroxide in the intact zones and moderately fractured zones. The geochemical condition of the groundwater is correlated closely with the nature of the fracture systems, mineral compositions and water-rock interaction processes. Different fractures contain chemically distinct groundwaters, which have differing potential capacities to mobilize radionuclides. The investigation method, based on a comparison between geological structures and groundwater chemistry, can be applied to develop a realistic hydrogeochemical model for deep, fractured granitic rocks. Such an approach is necessary if the chemical containment potential of such lithologies is to be evaluated adequately. The work demonstrates that structural controls on chemical heterogeneities in groundwaters should be understood, before the disposal of redox-sensitive wastes can be undertaken safely in fractured crystalline rocks.
The underground sequestration of carbon dioxide: containment by chemical reactions in the deep geosphere
Authors: Rochelle C. A. , Pearce J. M. , Holloway S.
Published: April 2008

Anthropogenic emissions of carbon dioxide (CO
2
) have been linked to increasing levels in the atmosphere and to potential global climate change. The capture of CO
2
from large point sources, followed by its sequestration as a supercritical fluid into the deep geosphere, is one potential method for reducing such emissions without a drastic change in our energy-producing technologies. Once emplaced underground, geochemical and hydrogeological processes will act to ‘trap’ the CO
2
as dissolved species and in carbonate minerals. Although dry supercritical CO
2
appears to cause little reaction with the host rocks, once dissolved in water mineral dissolution and precipitation reactions can result. From a geochemical standpoint, sandstones appear to be preferable to carbonates for sequestration operations because fluid-mineral reactions within them have a better capacity for pH buffering. However, individual host lithologies will vary in structure, mineralogy and hydrogeology, and individual sequestration operations will have to take account of local geological, fluid chemical and hydrogeological conditions. This paper summarizes some of the recent laboratory experimental, natural analogue and computer modelling approaches directed at understanding reactions involved in the chemical containment of CO
2
.
The geochemical engineering of landfill liners for active containment
Authors: Thornton S. F. , Bright M. I. , Lerner D. N. , Tellam J. H.
Published: April 2008

The performance of engineered containment barriers presently used to line UK landfills is uncertain over the design lives of most sites. Failure of key components of the barrier technology can be expected within a timescale under which leachate will still present a threat to groundwater resources. Additional environmental control measures are therefore necessary to mitigate the effects of these future impacts. A methodology for the geochemical engineering design of high-attenuation landfill liners using low-cost natural and industrial waste materials which are available in the UK is presented. Anaerobic laboratory column experiments are used to test the performance of various materials as liners in chemically attenuating contaminants in a methanogenic leachate spiked with a suite of heavy metals and organic micropollutants. The basic liner design recipe which maximizes the attenuation of key contaminants in leachate has been confirmed. Variability in material composition does not significantly affect liner performance provided this recipe is used. Important design issues (liner exhaustion and attenuation reversibility effects) are examined in the study. These liners may be routinely constructed from measurements of a limited suite of material properties using standard laboratory procedures. The application of these liners within the current framework of landfill design is outlined.
Radioactive waste containment in indurated shales: comparison between the chemical containment properties of matrix and fractures
Authors: De Windt L. , Cabrera J. , Boisson J. Y.
Published: April 2008

The chemical containment properties of poorly consolidated, plastic clays have received considerable attention, particularly during investigations related to radioactive waste disposal. However, the chemical containment properties of indurated shales have received rather less consideration. This paper summarizes those properties of consolidated shales which make them suitable for the chemical containment of radionuclides, using the example of investigations at the Tournemire research site in the south of France. The problems associated with investigating such low-permeability media are highlighted, and solutions to these problems are presented. At this research site, tectonic processes have induced centimetre-scale fractures and decimetre-scale faults in the shales, whereas excavation of a tunnel has generated metre-scale fracturing. Some low water flows through the fracture networks have been detected. However, the Tournemire shales are characterized by very low matrix pore-water contents, and any limited fluid mobility through the matrix is caused mainly by diffusion. Slowly flowing waters in fractures are chemically analogous to these matrix pore-waters. The hydrochemistry is of Na-Cl-(

HCO
3
−

) type, and the waters are slightly alkaline, strongly reducing and of low ionic strength. The resemblance between the chemistries of pore- and fracture-waters arises from the buffering of the pore-water chemistry by matrix mineral/water interactions, and possible subsequent diffusion of the pore-water into the fractures. The chemical buffering mechanisms controlling the water chemistries (especially pH, alkalinity and
Eh
), are discussed and modelled. The chemical conditions will minimize the mobility of many radionuclides in both shale matrix and in fractures. Chemical evidence from the fracture-filling calcite is consistent with this hypothesis and demonstrates the past immobility of U and Cs. Batch experiments carried out in gloveboxes under a N
2
atmosphere and with synthetic fracture-water show weaker retention of Cs by the calcite fillings than by the shale matrix. Due to its very low porosity, the shale matrix should behave like a membrane filter for colloids, further reducing the mobility of radionuclides. In fractures, interface structures between matrix walls and calcite veins should also hinder colloid migration (except perhaps in occasional centimetre-scale apertures).
Failure envelopes of mudrocks at high confining pressures
Authors: Petley D. N.
Published: April 2008

Whereas models for the mechanical behaviour of many types of sedimentary rocks at high mean effective stresses (2–50 MPa) are now reasonably well developed, few attempts have been made to generate models for mudrocks in the same way. This paper combines a new set of undrained shear experiments on London Clay with a summary of other mudrock datasets as described in the literature, including Kimmeridge Clay, London Clay, Boom Clay and clay from mud volcanoes. It has been demonstrated previously that at low mean effective stresses, most undisturbed mudrocks behave in a brittle manner, showing a distinct peak strength before undergoing failure and strain weakening to a residual strength. At very high mean effective stresses they tend to behave in a ductile manner, with the maintenance of peak strength to large strains. This paper shows for the first time that a ‘transitional’ regime can be defined for most mudrocks. In this transitional regime, undrained shear deformation leads to the maintenance of peak strength to a given axial strain in a manner that is similar to ductile deformation, before the initiation of strain weakening to a residual strength. Detailed analysis has demonstrated that during the maintenance of peak strength the sample is undergoing pervasive micro-fracturing and is thus behaving in a ductile manner on the macro-scale and in a brittle manner on the micro-scale. As a result of this study it has been possible to construct a model for the form of the failure envelopes of most mudrocks, with two fundamentally important envelopes being defined: (1) a linear residual strength envelope: at low and medium mean effective stresses the stress path will tend towards this envelope after brittle failure; (2) a peak strength envelope: at low mean effective stresses this envelope has a linear form, but once the transitional regime is attained it reduces in gradient, eventually intercepting the residual strength envelope at the boundary of the ductile regime. This new framework for the behaviour of mudrocks during undrained shear deformation is generally consistent with those developed for other sedimentary rocks.