bluuebunny/crossref_metadata_2025 huggingface dataset:
High-resolution biostratigraphy and sequence development of the Palaeocene succession, Grane Field, Norway
Authors: Mangerud G. , Dreyer T. , Søyseth L. , Martinsen O. , Ryseth A.
Published: April 2008

A detailed biostratigraphic zonation and sequence stratigraphic framework has been developed for the Grane Field (PL 169), situated in Block 25/11 in the North Sea. The study is focused on the Upper Palaeocene interval, where a high level of chronostratigraphic resolution has been achieved and where the depositional history can be understood in terms of a finite set of depositional cycles and subcycles.

This work documents that the Grane Field reservoir succession, which consists of alternating turbidite sandstones and marine basin plain mudstones, fall within two main biozones, corresponding to a general Late Palaeocene age. The reservoir succession is developed between two regional datums: last stratigraphical common occurrence of
A. gippingensis
and last stratigraphical occurrence of
Isabellidinium? viborgense
. This corresponds to our major cycles T40 and T50, in which the reservoir sands comprise lowstand units filling in local accommodation during stepwise encroachment of the submarine fan system into this area.

Within the reservoir interval the zonation is based on the appearance and extinction of species with a regional distribution that also have marked frequency variations. These include
A. gippingensis, P. bulliforme, C. striatum, P. pyrophorum
and
I? viborgense
. In addition, events representing quantitatively marked peaks in the frequency of long-ranging taxa are used for correlation purposes. These events need to be constrained by other events, but are good for local correlations. Examples from the Grane Field include influxes of
Deflandrea denticulata, Cometodinium comatum
and
Subtilisphaera
spp.

The high-resolution biostratigraphy serves as a template for the sequence stratigraphic framework in the study area. Sixteen correlatable zones have been established by linking each of the fossil events to a specific sequence stratigraphic surface. As shown by previous workers, surfaces associated with condensation (regional and subregional maximum flooding surfaces) prove to be the most useful ones for correlating the Tertiary of the North Sea. In addition, some of the biostratigraphic events seem to coincide with transgressive surfaces and sequence bounding unconformities. Application of the sequence stratigraphic framework indicates that several reservoir sand units, which appear correlatable on well logs, actually are of different age. They may therefore contain potential flow barriers, formed in periods of condensation between the lowstand events.
Reservoir biostratigraphy of the Pedernales Field, Venezuela
Authors: Jones R. W. , Jones N. E. , King A. D. , Shaw D.
Published: April 2008

The geographic and geological settings of the Pedernales Field, and its history from discovery to development, are briefly outlined. Biostratigraphy and palaeoenvironmental interpretation, and their impact on integrated reservoir description (IRD) and production geology, are discussed in detail.

Historical and modern vintage micropalaeontological data enable the recognition of a number of ‘zones’ over the reservoir interval. These ‘zones’ are diachronous, although they do have correlative value when integrated into the framework of isochronous tectonostratigraphic sequence boundaries and flooding surfaces identified on seismic sections and well logs. The ‘zones’ and their containing sediments can be tentatively calibrated against global standard biozones and hence sequence boundaries and flooding surfaces on the eustatic sea-level cycle chart.

Micropalaeontological and palynological data also enable the recognition of a number of biofacies interpreted on the basis of modern analogy as indicating a range of delta-top, delta-front and prodelta environments. The principal reservoir is interpreted as having been deposited in a range of delta-top to delta-front subenvironments, the seal in a range of distal prodelta subenvironments.

The ongoing high-resolution biostratigraphic work programme has revealed the existence of a large number of correlatable events over the reservoir interval. Integrated high-resolution biostratigraphic, well log and seismic correlation can be achieved both within and between the compartments of the field.

High-resolution biostratigraphic/biofacies data demonstrate the existence of a bathymetric gradient at the time the reservoir were being deposited. Gravity gliding along the observed gradient provides a possible mechanism for the locally observed section repetition and enhanced reservoir development, and also for the up-dip trapping mechanism.
High-resolution sequence biostratigraphy of a lowstand prograding deltaic wedge: Oso Field (late Miocene), Nigeria
Authors: Armentrout J. M. , Fearn L. B. , Rodgers K. , Root S. , Lyle W. D. , Herrick D. C. , Bloch R. B. , Snedden J. W. , Nwankwo B.
Published: April 2008

High-resolution biostratigraphic analyses provide calibration of both depositional systems and specific depositional environments. Such studies facilitate prediction of reservoir geometry and the lateral continuity of both reservoir sandstones and sealing mudstones. Data from the offshore Nigeria Oso Field are presented as a case study of one late Miocene deltaic depositional system. Within the Oso Field, marine mudstones are recognized as effective top seals across the entire field in contrast to marginal marine to non-marine mudstones which have more restricted distribution and act as intrafield baffles. Three sequence stratigraphic models have been considered: (1) the first is based on biostratigraphic data integrated with core sedimentology; (2) a second model constructed using core sedimentology; and (3) a third based on interpreted regional seismic reflection profiles and well-log data. Integration of all data shows that the lower Oso Field producing interval is interpreted as a lowstand prograding wedge, and the upper interval as a prograding distal transgressive or alternatively distal highstand system tract.
Templates from mainland Africa and the Red Sea for interpreting the early evolution of the South Atlantic
Authors: Crossley R. , Cripps D.
Published: April 2008

Findings drawn from the Tertiary pre-rift, rift and post-rift sequences of Africa and the Red Sea are applied to the South Atlantic margins with a view of aiding hydrocarbon exploration. In relation to gravity and magnetic prediction of top basement, template rifts tend to follow pre-existing lines of weakness and the upper crust underlying these rifts may have geophysical characteristics which are different from those of the rift flanks. Template rifts may or may not be associated with volcanicity. In sparsely volcanic rifts, syn-rift volcanism is located on rift axis structural highs. Extensive volcanism is accompanied by regional doming and erosional loss of the pre-rift section is likely in rift sectors preceded by volcanism. Consequently, it is doubtful whether pre-rift sediments are preserved beneath the major volcanics offshore from the Parana-Etendeka areas in the South Atlantic. The Kalahari Group of southern Africa is identified as a good analogue for pre-rift reservoir objectives in the South Atlantic, and facies and thickness criteria are discussed that help discriminate pre-rift sequences from rift sequences. Where sedimentation has kept pace with rift subsidence, so that fault scarp facies have not developed, then the Okavango Delta provides a useful analogue. Parallels are drawn between major sand dykes interpreted from seismic in the deepwater deposits of Lake Tanganyika and similar units within similar facies in the Recôncavo Basin of Brazil. Direct analogue links between the Red Sea and the South Atlantic are less strong, mainly because marine incursions, and salt deposition, occurred at a later structural stage in the South Atlantic. The onset of salt deposition in the Gulf of Suez-Red Sea occurred whilst rift block faulting was still in progress, and the post-rift sag phase, though partly salt filled, rapidly turned to fully-marine deposition. The earliest part of the post-rift sag phase in the South Atlantic is represented by laterally extensive lacustrine source rocks. Salt deposition in the South Atlantic occurred only after full development of the sag phase.
Upper Cretaceous palaeoenvironments and benthonic foraminiferal assemblages of potential source rocks from the western African margin, Central Atlantic
Authors: Holbourn Ann , Kuhnt Wolfgang , El Albani Abderrazzak , Pletsch Thomas , Luderer Florian , Wagner Thomas
Published: April 2008

Upper Cretaceous benthonic foraminiferal assemblages, clay minerals, kerogen types and carbonate microfacies are examined in organic-rich sediments from western African coastal basins along a latitudinal transect extending from southern Morocco to Nigeria. Samples from onshore sections of the Tarfaya Basin, southern Morocco, from DSDP Site 369, from offshore commercial wells on the Casamance Shelf, Senegal, from DSDP Site 367, from ODP Site 959 and from onshore sections of the Benue Trough in Nigeria, are analysed. Organic-rich sediments from these sites are characterized by smectiterich clay mineral assemblages, high amounts of marine organic matter and a typical benthonic foraminiferal biofacies dominated by buliminids and bolivinids. Benthonic foraminifera provide robust proxy indicators of palaeoproductivity and organic matter flux to the seafloor. The palaeobiogeographic distribution of benthonic foraminiferal biofacies can be used to identify areas and stratigraphic intervals of hydrocarbon accumulation. Lower Turonian benthonic foraminiferal assemblages exhibit extremely low diversity and are strongly dominated by a
Gabonita
biofacies, in contrast to Coniacian to Maastrichtian assemblages, which display higher diversity and contain numerous species of
Buliminella, Praebulimina, Afrobolivina, Bolivina
and
Orthokarstenia
. The palaeobathymetric distribution of these high productivity benthonic foraminiferal biofacies reflects the establishment of a productivity driven oxygen minimum zone along the western African shelf, associated with an upwelling system active off coastal Africa during most of the Late Cretaceous. The extent of the high productivity equatorial belt appears to have shifted latitudinally during the Late Cretaceous as sea level and circulation patterns changed. The zone of highest productivity was broadest in the Early Turonian, when it coincided with the maximum sea-level rise and highest atmospheric temperature in the Late Cretaceous.
Tectono-sedimentary controls on Cretaceous black shale deposition along the opening Equatorial Atlantic Gateway (ODP Leg 159)
Authors: Wagner Thomas , Pletsch Thomas
Published: April 2008

The Cretaceous Equatorial Atlantic Gateway between the Central and South Atlantic basins is of interest not only for palaeoceanographic and palaeoclimatic studies, but also because it provided particularly favourable conditions for the accumulation and preservation of organic-rich sediments. Deposition of carbonaceous sediments along the Côte d’Ivoire-Ghana Transform Margin (Ocean Drilling Program Leg 159) was intimately linked to the plate tectonic and palaeoceanographic evolution of this gateway. Notably, the formation of a marginal basement ridge on the southeastern border of the transform margin provided an efficient shelter of the landward Deep Ivorian Basin against erosive and potentially oxidizing currents. Different subsidence histories across the transform margin were responsible for the development of distinct depositional settings on the crest and on both sides of the basement ridge. Whereas the southern, oceanward flank of the basement ridge was characterized by rapid, continuous deepening since the latest Albian-early Cenomanian, marine sedimentation on the northern, landward flank was interrupted by a period of uplift and erosion in the late Albian, and rapid subsidence started after the early Coniacian.

Organic-rich sediments occur throughout almost the entire Cretaceous section, but hydrogen-rich marine black shales were exclusively recovered from core sections above an uplift-related unconformity. These black shales formed when separation of Africa and South America was sufficient to allow permanent oceanic midwater exchange after the late Albian. Four periods of black shale accumulation are recorded, some of them are correlated with the global oceanic anoxic events: in the latest Albian-earliest Cenomanian, at the Cenomanian-Turonian boundary, during the middle Coniacian-early Campanian, and in the mid-Maastrichtian. These periods were characterized by increased carbon fluxes to the seafloor, induced by enhanced palaeoproductivity and intensified supply of terrestrial organic matter. Black shale deposition appears to be intimately linked to periods of rising or maximum eustatic sea level and to the expansion of the oxygen minimum zone, as indicated by foraminiferal biofacies. Intervals between black shale units, in contrast, indicate a shrinking oxygen minimum zone and enhanced detrital flux rates, probably related to lowering sea level.

Upper Cretaceous detrital limestones with high porosities may provide excellent hydrocarbon reservoirs, although their areal extent appears to be limited. Palaeogene porcellanites, capped by Neogene pelagic marls and clays, extend over a wider area and may provide another target for hydrocarbon exploration.
Miocene benthonic foraminiferal morphogroups in an oxygen minimum zone, offshore Cabinda
Authors: Preece R. C. , Kaminski M. A. , Dignes T. W.
Published: April 2008

Two hundred and seventy-eight benthonic foraminiferal species were recorded (126 agglutinating and 152 calcareous) from CABGOC 128-3, offshore Angola. This assemblage is comprised of a cosmopolitan fauna named the Agua Salada Fauna, known from the Caribbean, in the Vienna Basin, West Africa, Libya and Borneo. The fluctuating relative abundance of benthonic foraminiferal morphogroups in combination with ‘total organic carbon’ (TOC) data is suggested to reflect variation in the intensity of the oxygen minimum zone as it steadily increases upwards. An infaunal agglutinated ‘opportunistic’ (
r
-selected) group moves from antiphase into phase with TOC upwards in CABGOC 128-3. A morphologically comparative calcareous group displays the opposite trend, moving from phase into antiphase with TOC data. Planispiral agglutinated foraminifera emulate the steady increase in TOC upwards in CABGOC 128-3. This group includes
Cyclammina cyclops
, previously only recorded in the Arctic. Elongate, flattened calcareous morphologies undergo a sudden expansion towards the top of the well and are apparently unaffected by TOC. This overall trend is supported by concurrent poor preservation amongst distinct calcareous groups susceptible to dissolution, as expected in an area of dysaerobic conditions. The disappearance of all agglutinated faunas in the late Miocene is considered to be a reflection of oceanographic change.
Onshore equivalents of the main Kudu gas reservoir in Namibia
Authors: Stanistreet I. G. , Stollhofen H.
Published: April 2008

Radiometric dates of Jurassic and Cretaceous flood basalts of the southern South Atlantic maritimes have thrown the stratigraphic contexts of Karoo and post-Karoo sequences into considerable question. Central to the problem is the status of volcano-sedimentary units, characterized by aeolian-related sedimentary interlayers, containing evaporites and other lacustrine elements. The main Kudu gas reservoir (‘Lower Gas Sand’) offshore Namibia, interleaved with undated flood basalts, is a prime example. A previously proposed onshore correlate is the fluvio-aeolian Etjo Sandstone, with a central Namibian type area, and comparisons have been made, particularly with units traditionally classified as Etjo Sandstone, near the Etendeka Plateau base in northwest Namibia, now recognized to be of younger age. We time-correlate the main Kudu Sandstone reservoir, however, with evaporitic fluvio-lacustrine and aeolian interlayers beneath and interleaved with Early Jurassic Kalkrand flood basalts of southern Namibia, situated onshore almost directly opposite the Kudu Field. The main Kudu Sandstone reservoir is transitional in palaeoenvironmental setting between Early Jurassic onshore analogues. Main Kudu reservoir geometries and associated lithologies can be better constrained with knowledge derived from Kalkrand sedimentary interlayers and their tectonic and volcanic controls. Kalkrand interlayer thicknesses are controlled primarily by down-to-the-east extensional fault system half-graben geometries, antithetic to an inferred down-to-the-west master detachment. Within resulting sub-basinal accommodation space, lava topography controlled thickness variation. When Kalkrand insights are taken into account with radiometric dating and biostratigraphy, a Namibian Mesozoic framework fixes Kudu reservoirs and analogues into a southern Gondwanan context. At least in its type area in central Namibia, the Etjo Formation is Early Jurassic, discounting a previously proposed widespread <100 Ma hiatus along a sub-Etjo disconformity. Instead, sequence boundary hiatuses of Permian-Triassic, Early Jurassic and Middle Jurassic-Early Cretaceous ages represent time-stratigraphic gaps, relating to major faulting episodes, recording extensional events associated with continental margin evolution. Understanding onshore analogues and refining the Namibian Mesozoic framework should help constrain seismic analysis of the Namibian margin and hydrocarbon reservoir geometries. Consequently, exploration focus should widen to include Karoo sequences beneath the basal Early Cretaceous transgression.
Are gold deposits in the crust fractals? A study of gold mines in the Zimbabwe craton
Authors: Blenkinsop Tom G. , Sanderson David J.
Published: April 2008

The hypothesis that gold deposits in the earth are fractal dusts is tested using data on gold mine distribution in the Zimbabwe craton. Mine distribution reflects deposit distribution only indirectly because of undiscovered deposits and economic factors. Nevertheless, box counting shows an approximately linear relationship between the logarithm of the number of squares necessary to cover all mines in the craton and the logarithm of the square size between 10 and 60 km, indicating a fractal distribution. More detailed data from two areas within the craton have a similar relationship between square sizes of 2.5–20 km. Departure from the ideal fractal relationship is termed ‘roll-off’, and is very satisfactorily modelled by random sampling of a fractal dust. The results are consistent with the hypothesis that gold deposits are fractal dusts with fractal dimensions of
c.
1.0 and the distribution of mines can be regarded as a random sample of the distribution of deposits. Making this assumption, regions that have been incompletely but randomly explored can be identified and distinguished from regions containing large subareas that have been completely unexplored, and appropriate exploration strategies can be devised in either case.
Nickel ore troughs in Archaean volcanic rocks, Kambalda, Western Australia: indicators of early extension
Authors: Brown M. A. N. , Jolly R. J. H. , Stone W. , Coward M. P.
Published: April 2008

The Kambalda nickel sulphide deposits are located in the 2700 Ma Eastern Goldfields Greenstone Belt, Yilgarn Craton, Western Australia. The main lithologies are basalts and komatiites. Nickel sulphide ore occurs within the komatiite at the contact with the underlying basalt. This contact is structurally complex, the result of several phases of compressive deformation that post-date ore formation. Ore is confined to trough structures at the basalt-komatiite contact, these may be tens to hundreds of metres wide, several kilometres in length and tens of metres deep. Troughs are subparallel to each other and to the regional structural trend, ore bodies have a characteristic ribbon-like geometry. Various models have been proposed to explain trough formation but none account for the variety of features seen in the area. A new model is presented explaining the Kambalda structures by analogy with present-day graben structures associated with Icelandic-style fissure swarms. It is proposed that the initial geometries of the Kambalda trough structures, prior to compressive deformation, were similar to the present-day structures seen in Iceland. Komatiitic lavas subsequently exploit this graben topography, resulting in distinctive orebody geometries. Thus, it is suggested that the Kambalda troughs reflect an early phase of extension in the region.
The Mediterranean Basins: Tertiary Extension within the Alpine Orogen — an introduction
Authors: Jolivet L. , Frizon de Lamotte D. , Mascle A. , Séranne M.
Published: April 2008

The recent evolution of ideas on the Mediterranean region has been triggered by very active data acquisition over the last 15 years. Seismic tomography provides an unique view of mantle heterogeneities, space geodesy leads to precise determinations of the present strain and velocity fields, the combination of structural geology, radiometric dating and metamorphic petrology allows the description ofP-T-t-Dpaths of exhumed metamorphic rocks, and exploration geophysics, onshore and offshore, gives a detailed view of the crustal geometry. Extension started in the Gulf of Lion and propagated eastwards and southwestwards to form the Liguro-Provençal basin, Tyrrhenian Sea and the Alboran Sea. It also started, at much the same time, in the Panonnian basin as well as in the Aegean back-arc region. Thus a seminal event occurred some 30 Ma ago that produced a sharp change from overall compression to back-arc extension. Although gravitational forces due to the collapse of a thick crust have affected most basins, it is now almost certain that this event ultimately originated in the mantle, either by slab detachment, slab rollback or both processes acting in sequence.
The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview
Authors: Séranne Michel
Published: April 2008

The Gulf of Lion margin is one of the Tertiary extensional basins of the western Mediterranean that opened during convergence of Africa and Europe. This Oligocene-Aquitanian rifted margin and associated Burdigalian oceanic basin have been used as case study for stretching models of ‘Atlantic-type’ margins. However, when the Integrated Basin Study (IBS) project was initiated, several outstanding questions remained about the present structure and the geodynamic setting of the margin within the Western Mediterranean. IBS-Gulf of Lion research was based on the existing onshore and offshore, industrial and academic data, which were heterogeneous and unevenly distributed. Compilation of the stratigraphic correlations on a regional scale allowed precise calculation of the timing of rifting, and clarification of the relationships with Alpine and Mediterranean geodynamics. Reprocessing of the existing ECORS deep seismic reflection profiles shed new light on the extensional structure and mechanisms of extension of the continental margin. Structural and sedimentological studies onshore led to the definition of new tectonostratigraphic models for extensional basins. Results of structural analyses showed a partitioning of the extensional deformation processes across the continental margin. 3D gravity modelling of the margin and basin area led to the production of a new map of the Moho depth by inversion, and testing several hypotheses for the origin of the present day subsidence. Although the Gulf of Lion margin displays structural and stratigraphic features similar to ‘Atlantic-type’ margins, its structure and evolution corresponds to that of a rifted margin of a large continent formed during the opening of a marginal basin. Integration of the new results of IBS-Gulf of Lion within the geodynamic evolution of the western Mediterranean suggests that the Oligocene rifting of the Gulf of Lion represents the initial stage of a succession of rifting events and back-arc basin formation, due to continuously retreating subduction during convergence of Africa and Europe.
Constraints on the Neogene Mediterranean kinematic evolution along a 1000 km transect from Iberia to Africa
Authors: Vergés J. , Sàbat F.
Published: April 2008

We present in this paper a simple kinematic model to illustrate the evolution of the western Mediterranean since upper Oligocene times. The five-step mass balanced reconstructions of the region are constructed along a 1000 km long crustal-scale transect from Iberia to Africa across the València and Algerian basins. The model uses constant divergence and convergence rates and fits the geological and geophysical data well along the transect. The amount of convergence and divergence along the studied transect can be resolved by the combination of two tectonic mechanisms: the northern motion of the African plate and the southwards retreat of a subducting Tethyan oceanic slab located between the pre-Neogene Iberian and African margins.

The evolution of the western Mediterranean region during the last 30 Ma can be summarized as follows: (a) pre-Neogene (late Oligocene) Iberian and African margins separated by a north-dipping fragment of Tethyan oceanic crust; (b) widespread extension affecting the Iberian margin developing the València trough and the Algerian basin from upper Oligocene to the top of Langhian; (c) end of the formation of the València trough and the collision between the Kabylies and Africa domains at
c.
13 Ma; (d) end of the Algerian basin opening and northern Kabylies and Tellian thrust systems coupled tectonic domains in the upper Tortonian (8 Ma); (e) shortening within the whole domain by the northwards motion of Africa after late Tortonian times. The propagation of stress within the western Mediterranean region seems to affect the whole region as far north as the northern side of the Pyrenean range located a distance of 1200 km from the south Atlas front, the southern limit of the system.
Interaction between faulting, drainage and sedimentation in extensional hanging-wall syncline basins: example of the Oligocene Matelles basin (Gulf of Lion rifted margin, SE France)
Authors: Benedicto A. , Séguret M. , Labaume P.
Published: April 2008

A tectono-sedimentary model for hanging-wall syncline basins emphasizing the role of erosion in the basin architecture is proposed, based on the well-exposed Matelles basin. Detailed geological mapping is the base for the analysis of the extensional double ramp-flat Matelles fault system, hanging-wall deformation and relationships between faulting/drainage/sedimentation. Displacement of the hanging-wall above the fault resulted in the formation of (a) upper and lower rollovers associated with the upper and lower rampflat couples, (b) a hanging-wall ramp syncline above the lower ramp and (c) a half graben associated with the emergent ramp and upper flat couple. Only the hanging-wall ramp syncline and the lower rollover are preserved. Ramp syncline basin-fill architecture is characterized by progressive unconformity and faultward migration of the growth synclines ‘climbing up’ the monoclinal prerift strata corresponding to the ramp-side limb of the ramp syncline. Clast composition analysis of the synrift deposits indicates that sediments were mainly supplied from the hanging-wall flat, and allows to propose palinspastic reconstructions of the hanging wall in order to establish the palaeo-drainage system and its evolution. ‘Upper hanging-wall flat catchments’ and ‘lower rollover catchments’ are proposed as main features of the drainage system in extensional hanging-wall syncline basins.
New insights from
40
Ar/
39
Ar laserprobe dating of white mica fabrics from the Pelion Massif, Pelagonian Zone, Internal Hellenides, Greece: implications for the timing of metamorphic episodes and tectonic events in the Aegean region
Authors: Lips A. L. W. , Wijbrans J. R. , White S. H.
Published: April 2008

40
Ar/
39
Ar laserprobe dating of mylonites fabrics from the Pelion Massif in the Pelagonian Zone of mainland Greece has characterized its Mid-Late Alpine deformation history. Following high pressure (HP) metamorphism, ductile deformation occurred under greenschist-facies conditions from
c.
54 Ma, and continued to affect the Pelion Massif until
c.
15 Ma. The prolonged episode of ductile deformation in the Pelion Massif has resulted in the formation of an Oligocene-Early Miocene ductile domal structure. The new geochronological data obtained for the Pelion contribute to a detailed record of the Alpine kinematic history in the Pelagonian Zone and allow a discussion of
P-T-t
data from Aegean HP rocks to characterize the regional thermotectonic history. Comparison with the
P-T-t
data from the Cycladic region reinforces the point that Mid-Eocene phengite ages, commonly taken as the age of peak HP metamorphism in the Cyclades, do not always reflect the metamorphic culmination, but rather the retrograde paths of the HP rocks. It is shown that, on a regional scale, termination of HP metamorphism is a diachronous process in the Aegean region, being
c.
54 Ma in the north (Pelagonian Zone) and shifting to younger ages, chiefly
c.
40 Ma in the Cyclades, and
c.
20 Ma on Crete, as the present-day subduction zone is approached. In contrast to the diachronous exhumation of Aegean HP assemblages, the well documented Miocene phase of ductile regional extension appears to be synchronous across the whole Aegean region and affected basement rocks until
c.
15 Ma.
Petroleum systems of Alpine-Mediterranean foldbelts and basins
Authors: Ziegler P. A. , Roure F.
Published: April 2008

The Alpine-Mediterranean area hosts a number of large and smaller hydrocarbon provinces in platforms, rifts, foreland basins and the frontal parts of thrust belts. These hydrocarbon provinces are related to a wide range of source rocks occurring in Palaeozoic, Mesozoic and Cenozoic series. Source rocks are alternatively localized in prerift, syn-rift or passive-margin sequences, the age of which is highly variable. Moreover, syn-orogenic foreland basin sequences can contain important source-rocks and are frequently the locus of biogenic gas generation.

Most Mesozoic passive margins of the Western Tethys were destroyed during the Alpine orogenic cycle and were overthrusted by or incorporated into orogenic wedges, the external parts of which host a number of highly diversified petroleum systems. Late Variscan and Alpine compressional intraplate deformation had severe repercussions on the hydrocarbon habitat of some of the Peri-Tethyan platforms. Late Cretaceous and Palaeogene intracratonic and back-arc rifts host some important petroleum provinces; the Euphrates, Abu Gharadig and Sirte basins rely for hydrocarbon charge on syn-rift source rocks whereas post-rift series contain the source rocks of the Black Sea back-arc basin. Oligocene and younger rift systems host the petroleum provinces of the Gulf of Suez, Pelagian Shelf, Valencia Trough and Rhine Graben; these rely variably on pre- and syn-rift source rocks. The Neogene Pannonian system of transtensional back-arc basins relies for hydrocarbon charge mainly on syn-rift source rocks whereas the Vienna Basin is exclusively charged by Mesozoic post-rift source rocks.

The complex evolution of the Western Tethys belt, contrasted thermal regimes and successive episodes of sedimentary and tectonic burial account for the great diversity of its petroleum systems. Each hydrocarbon province is characterized by a very distinct scenario for the accumulation of source rocks, timing of their maturation, hydrocarbon expulsion and migration from effective kitchens to potential trapping domains, and the preservation of hydrocarbon accumulations.

Future exploration in the external parts of the Alpine orogenic belts, their forelands and beneath the Neogene fill of the Pannonian Basin is likely to yield further oil and gas discoveries. In contrast to the Black Sea Basin, the hydrocarbon potential of the Algero-Provençal, Tyrrhenian and Aegean back-arc basins is questionable. The thick sedimentary sequences of the deep East Mediterranean Basin may contain effective source rocks.
Chemical containment of waste in the geosphere
Authors: Metcalfe Richard , Rochelle Christopher A.
Published: April 2008

The aim of this introductory paper is to highlight those underlying chemical principles that are common to all forms of waste management by geological means, and that rely to some extent upon chemical containment. Until recently, chemical processes were usually considered mainly because they can affect the physical performance of engineered containment systems. However, in recent years, many researchers have recognized that chemical processes themselves can offer containment to wastes. Thus, it is no longer possible to view physical and chemical containment processes separately. The containment system can be optimized only if both the engineered and natural barriers are considered together, and if the engineered barrier system is designed taking the features of the geosphere into account. However, there has been relatively little reliance upon the geosphere itself as a chemical barrier. It is concluded that the potential for chemical containment should be considered in all forms of geological waste management. Even if the chemical barrier function of the geosphere is not relied upon to meet safety targets, the confidence of regulators and public alike will be enhanced if it can be demonstrated that the geosphere at the site functions as a chemical barrier.
Regulatory objectives and safety requirements for the disposal of radioactive wastes in Switzerland
Authors: Frank E.
Published: April 2008

According to Swiss Law the producers of radioactive waste are responsible for management and safe disposal of all types of radioactive waste. Therefore, in 1972, the electricity utilities involved in nuclear power production formed the National Cooperative for the Disposal of Radioactive Waste (NAGRA) which is responsible for final disposal and related work. The general disposal strategy includes two repository siting programmes, a deep repository for high-level waste in Northern Switzerland (crystalline basement or Opalinus claystone) and a repository with horizontal access for low- and intermediate-level waste in the Alps (marls and shales of the Wellenberg-site).

The safety of waste disposal is assessed on the basis of specific protection objectives formulated by the Swiss regulatory authority. The protection objectives limit the annual dose equivalent or the radiological risk to an individual and they require that post-closure safety must not depend on long-term monitoring or any other interventions to ensure safety. After a final repository has been sealed, it must be possible to dispense with safety and surveillance measures.
The chemical basis of near-field containment in the Swiss high-level radioactive waste disposal concept
Authors: Alexander W. R. , McKinley I. G.
Published: April 2008

Concepts for the disposal of high-level radioactive waste (HLW) vary from country to country but, in Switzerland, a key contribution to safety is the chemistry of the near-field. In this paper, the development of the Swiss repository design is discussed as a basis for explaining the chemical confinement of radionuclides within the engineered barrier system (EBS). The expected performance of the EBS is described and the chemistry of the system explained. Potential perturbations to the EBS are examined and the methodology of assessing the long-term behaviour of the EBS is briefly described. The relevance of HLW near-field design to chemo-toxic wastes is discussed and an extensive reference list on the chemical aspects of the EBS is provided.
Experimental simulation of the alkaline disturbed zone around a cementitious radioactive waste repository: numerical modelling and column experiments
Authors: Bateman K. , Coombs P. , Noy D. J. , Pearce J. M. , Wetton P. , Haworth A. , Linklater C.
Published: April 2008

One approach to describe the migration of an alkaline plume from the cementitious engineered barriers of a geological disposal facility for radioactive wastes is to employ coupled chemistry and flow computer models. Although evidence from natural systems is useful to constrain reaction mechanisms and minerals to be incorporated into such models, time-dependent information is generally lacking. A series of laboratory column experiments has been conducted in order to test the capabilities of two of the currently available, coupled models to predict product solids and output fluid compositions with time. The coupled models PRECIP and CHEQMATE were used to provide predictive calculations based upon known experimental parameters and data available from the literature. The predictions did not replicate all the variations in mineralogy observed in the experiments, primarily due to restrictions in the availability of kinetic and thermdynamic data for the range of secondary phases of interest. However, the model predictions did reproduce the general variation of secondary phases with time and distance along the columns.
What is the porosity of a mudrock?
Authors: Pearson F. J.
Published: April 2008

Several types of porosity (total or physical, advective flow, advective and diffusive transport, and geochemical) can be distinguished in a mudrock. Each may have a different value. Physical porosity is the volume not occupied by mineral grains. Interconnected physical porosity can be found using injection techniques and water-content measurements. Advective and diffusive flow and transport porosities are used when describing the movement of fluids and solutes. Advective porosity relates the average linear velocity of a fluid to its Darcy flux (velocity), and in mudrock is relevant principally to fractures. Diffusive porosity relates diffusion in rock to diffusion in pure water and depends on the diffusing substance and the medium. In mudrock, water diffusion porosities equal water content porosities but solute diffusion porosities are smaller because solutes do not have access to the entire water content of a rock. Inaccessible water may be in pores too small to admit solutes, or present as interlayer or surface-layer water. Geochemical porosity is required to model pore-water composition and reactive transport. It represents the fluid volume in which reactions occur, and for mudrocks, seems equivalent to solute-diffusion porosity. Review of studies of several mudrocks shows halide geochemical porosities from 0.3 to 0.7 of other porosity values. Uncertainties of this size are unimportant for transport calculations, but lead to significant uncertainties in modelled pore-water compositions. To make acceptably accurate calculations of water-rock reactions and transport in mudrock, it is important to define the geochemical porosity.
Permeability and fluid flow in natural mudstones
Authors: Dewhurst David N. , Yang Yunlai , Aplin Andrew C.
Published: April 2008

Mudstone permeabilities vary by ten orders of magnitude and by three orders of magnitude at a single porosity. Much of the range at a given porosity can be explained by differences in grain size; at a given effective stress, coarser-grained mudstones are more permeable than finer-grained mudstones, although the difference diminishes with increased burial. Pore size distributions illustrate why more silt-rich mudstones are more permeable than finer mudstones and also show that the loss of porosity and permeability with increasing effective stress is driven primarily by the preferential collapse of large pores. Pore size distributions can also be used to estimate permeability rapidly. None of the existing models are ideal and need to be adjusted and validated through the acquisition of a much larger permeability database of well-characterized mudstones. We also examine the role of faults and fractures as fluid conduits in mudstones. The occurrence of microscopic hydrofractures is inferred from the observation that fluid pressures in sedimentary basins rarely exceed minimum leak-off pressures. The extent to which microfractures enhance mudstone permeability, both instantaneously and over longer periods of geological time, is poorly constrained. Although fault zones in mudstones have generally low permeability, there is abundant evidence for episodic flow along faults in tectonically active regions. The role of faults as fluid conduits during periods of tectonic quiescence is less certain, and the timing and extent of any enhanced permeability and enhanced flow are not well known. In general, conditions conducive to fluid flow along muddy faults include an increase in the activity of the fault, high fluid pressures within the fault zone and the extent of overconsolidation and lithification of the mudstones.
Permeability anisotropy of consolidated clays
Authors: Clennell M. B. , Dewhurst D. N. , Brown K. M. , Westbrook G. K.
Published: April 2008

Consolidation of clays tends to result in changes in particle orientation and pore size distribution as well as progressive reduction of porosity and permeability with increasing effective stress. Clay particles are expected to rotate normal to an axial load, thus decreasing flow path tortuosity parallel to the particle alignment direction and increasing tortuosity normal to the particle alignment. This results in the development of anisotropic permeability, such that the horizontal permeability of a consolidated sediment is greater than the vertical permeability at any given porosity. Within any uniform layer, levels of permeability anisotropy are modest. Typically, permeability anisotropy produced by consolidation of natural clays is in the range 1.1–3 and does not reach the high levels predicted by simple models of clay particle reorientation. The discrepancy arises from particle clustering and irregularities in particle packing. Although somewhat higher levels of anisotropy may exist as a consequence of lamination within individual beds, values > 10 that are known to exist on the formation scale are produced by strong contrasts between the permeabilities of interlayered beds. As argillaceous sediments have permeability ranges of many orders of magnitude, apparently subtle lithological layering in a shale unit may lead to a highly anisotropic flow behaviour.
Top-seal leakage through faults and fractures: the role of mudrock properties
Authors: Ingram Gary M. , Urai Janos L.
Published: April 2008

Mudrocks are effective top seals for hydrocarbon accumulations because they possess very low permeabilities, high capillary entry pressures, and are often laterally continuous basin-wide. For leakage through the seal to take place, an additional mechanism must provide enhanced permeability in mudrocks. Tectonically induced, dilatant faulting and fracturing in brittle rocks is such a mechanism. The effectiveness of mudrocks as seals may be compromised by a number of other factors, such as tectonic fault displacements in excess of the seal thickness; tensile fracturing under extreme fluid pressure conditions; and leakage via a network of juxtaposed thin leaky beds across sub-seismic faults within the seal. Before hydrocarbon trap integrity analysis, seismic interpretations should honour the fundamental geometry of the trap as closely as possible because top seals can be reliably appraised only when basic geometries are accurately determined. Fluid pressure is a proven risk in many exploration provinces, in terms of mudrock top-seal leakage via opening mode fractures. This natural hydrofracturing can take place if buoyancy pressures, combined with fluid overpressures, exceed the minimum
in situ
horizontal stress, plus the tensile strenght of the seal, an occurrence that leads to mechanical failure of the seal rock. The rheology of a seal is another important factor as it determines the failure mode, i.e. whether the rocks are ductile and remain sealing after deformation or whether they deform in a brittle manner to create permeable leak paths. Several techniques have been developed to predict rock rheology; these rely on uplift, sonic velocity and clay content data. Burial curves can be used to determine overconsolidation of mudrocks. Sonic velocity data can be used to estimate unconfined compressive strength in mudrocks. The most direct method utilizes relationships between mudrock friction angle, swelling clay content and mudrock surface area to determine ductility. The effect on trap integrity of sub-seismic faults is also quantifiable because advances in structural geological technology now permit predictions of the numbers of sub-seismic faults in a trap. This information, combined with detailed top-seal stratigraphic data, provides the power to screen traps for the risk of top-seal leakage via sub-seismic fault juxtapositions within the seal. A simple strategy incorporating these factors and linking them to other structural and stratigraphic information can contribute to reducing uncertainty about top-seal leakage caused by tectonic deformation.
Porosity characteristics of Cambrian mudrocks (Oak Ridge, East Tennessee, USA) and their implications for contaminant transport
Authors: Dorsch J. , Katsube T. J.
Published: April 2008

Thirty-eight mudrock samples from the Cambrian Conasauga Group were analysed by immersion, helium and mercury porosimetry. The data generated from these techniques include effective porosities, pore throat size distributions and sample densities. The purpose of these analyses was to obtain information required to evaluate the capacity of the Conasauga Group mudrock to retard transport of and to store contaminants on the Oak Ridge Reservation (East Tennessee, USA). Results indicate that the three porosities were 9.90±2.61%, 3.8±0.7% and 8.1±4.3% for immersion (φ
I
), mercury (φ
Hg
) and helium porosity (φ
he
), respectively. These values are considerably higher than those previously reported for these rocks (0.1–3.4%). The φ
Hg
values are commonly the smallest of the three. Although the three porosities generally show little change with depth, an expected trend as a result of the maximum burial depth of the Conasauga Group of >4 km, a decrease with depth is seen in some of the coreholes for only φ
I
. Furthermore, inconsistencies between the different porosity types provide an insight into the mudrock pore structure and related geochemical characteristics. For example, they indicate the existence of considerable pore space accessed only through <3 nm pore throats and the possibility of meteoric cement having replaced original diagenetic cement for some mudrock samples, which would lead to a possible instability to immersion with contaminated water. These characteristics have significant implications for the evaluation of the mudrock retardation and storage capacity for contaminants, and the design of remediation measures.