bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of post-depositional remanent magnetization and their relevance to the palaeomagnetic record
Authors: Pisarevsky S. A.
Published: April 2008

The results of experiments on the watering of different types of continental Quaternary sediments in the anomalous magnetic field show that it can cause strong changes in the direction of remanence. The direction of acquired post-depositional remanent magnetization is stable to thermal and alternating field (AF) demagnetization. These changes are not related to the chemical remagnetization and are probably caused by mechanical rotations of magnetic particles in the pore spaces. Post-depositional remagnetization, both in nature and after collection of sediment samples, can distort the palaeomagnetic record of geomagnetic excursions, events and polarity transitions, as well as altering the finestructure features, such as the palaeomagnetic record of secular variations.
Magnetic properties of sediments deposited in suboxic-anoxic environments: relationships with biological and geochemical proxies
Authors: Vigliotti L. , Capotondi L. , Torii M.
Published: April 2008

Several studies of the rock magnetic properties of sediments deposited in suboxicanoxic environments show that bacterial degradation of organic matter leads to selective dissolution of magnetite with a decrease in magnetic mineral concentration and an increase in magnetic grain size. This study of upper Quaternary sediments from both marine and lacustrine environments shows many similarities, although diagnetic processes appear to be enhanced in the marine sediments. When the remanence is controlled by fine-grained materials, the magnetic susceptibility can be considered a leading indicator of such changes, and the coercivity parameters, such as
S
ratio and
B
0 cr
, are also useful discriminants suggesting when antiferromagnetic minerals become more important in intervals characterized by reductive diagenesis. Generally, magnetic properties show close correlations with other environmental indicators: diatoms and foraminiferal concentrations and compositions and geochemical properties. This suggests that the magnetic analyses are very sensitive to environmental variations and provide powerful tools for monitoring past depositional and diagenetic environments.
Diagenesis and remanence acquisition in the Cretaceous carbonate sediments of Monte Raggeto, southern Italy
Authors: D’Argenio B. , Ferreri V. , Iorio M. , Raspini A. , Tarling D. H.
Published: April 2008

Lower Cretaceous limestones and dolomites drilled at Monte Raggeto, north of Naples, show a hierarchical organization in cycles, bundles and superbundles, similar to that found in other Cretaceous shallow-water carbonate rocks exposed in the Southern Apennines. These cycles have been interpreted as being caused by the influences of planetary orbital perturbations of the Earth (precession, obliquity), with the bundles and superbundles being considered to represent the short- and long-term eccentricity orbital cycles, respectively. The palaeomagnetic properties of an 88 m core (corresponding to a total interval of
c.
3.2 Ma with a rock accumulation rate averaging 1 cm per 360 ± 16 years) were determined at 2 cm intervals. These show the same spectral periodicities as for the sedimentary properties determined at 1 cm intervals, but there are no correlations between these sedimentological and palaeomagnetic sets of data, when analysed on the scale of the bore-core. An example is given of a 15.50 m length of the bore-core, comprising a single superbundle corresponding to one of the long eccentricity terms, and including eight elementary cycles (each topped by a brief emersion episode) grouped in three bundles. These shallow marine carbonate sediments underwent very early diagenesis accompanied by marine cementation in the pore spaces at or immediately below the water-sediment interface in the same way as in modern-day tropical-subtropical environments. During brief emersions at the top of each cycle, meteoric early diagenesis also occurred. All non-carbonate grains that were present in the original sediment, including ferromagnetic ones, are likely to have been cemented in during these very early phases of diagenesis. It is concluded that the studied interval provides a model of the diagenetic evolution of the whole core, where the early diagenesis controlled the locking-in of a nearly continuous record of the geomagnetic field behaviour for the the entire core. Thus depositional, diagenetic and palaeomagnetic properties can be used in conjunction to provide detailed information on the environmental variations and geomagnetic field behaviour during the evolution of this stratigraphic sequence at an ultra-fine resolution, as 1 cm of rock is equivalent, on average, to 360 ± 16 years.
Microfossil diachronism in southern Norwegian North Sea chalks: Valhall and Hod fields
Authors: Bergen J. A. , Sikora P. J.
Published: April 2008

An integrated late Cretaceous calcareous microfossil framework for the southern Norwegian chalks is presented based on detailed study of 13 wells (many with core) and circum-North Sea outcrops, in addition to many cosmopolitan events recognized in reference sections in Tunisia, North America and Europe. Previous biozonations for the North Sea chalks: (1) lack the biostratigraphic resolution needed in production geology; (2) are largely founded on benthic foraminifera and acme events; (3) fail to recognize numerous stratigraphic discontinuities within the North Sea chalks; and (4) are poorly calibrated. Application of this new integrated framework cautions towards the use of certain acme and benthic foraminifera events as chronostratigraphic horizons both within the basin and from the basin into circum-North Sea outcrops. An additional complicating factor is palaeogeographic segregation between low and high latitudes, which intensified during the middle Campanian and became problematic within the North Sea basin itself during the late Maastrichtian. A final consideration is redeposition, which can lead to erroneous correlations between wells, but may also affect reservoir quality. Established North Sea biozonations have not recognized this primary palaeoenvironmental signal and massive redeposition across major lithostratigraphic units, which has resulted in the correlation of diachronous facies rather than isochronous horizons. This could lead to complications in operational applications, such as biosteering.
Towards a stable and agreed nomenclature for North Sea Tertiary diatom floras — the ‘
Coscinodiscus
’ problem
Authors: Bidgood M. D. , Mitlehner A. G. , Jones G. D. , Jutson D. J.
Published: April 2008

Diatoms are one of the most useful microfossil groups to be found in Tertiary (particularly Palaeogene) sequences of the North Sea subsurface for their biostratigraphic utility, especially where other mineralized-walled microfossil groups (e.g. foraminifera, calcareous nannoplankton, etc.) are absent. An example from one offshore borehole (Shell UK; 29/25-1) is given. However, their further use in biostratigraphic and sequence correlation is hampered by the lack of any stable nomenclature applied to their taxonomy. The numbers of so-called ‘in-house’ taxa are legion with duplication of forms almost inevitable. Few of these many and varied taxa are directly comparable between these schemes, and this leads to almost inevitable confusion for exploration, production and development geoscientists trying to correlate between the various schemes.

One stratigraphically important and familiar diatom taxon (
Coscinodiscus
sp. 1) is formally described in this paper as
Fenestrella antiqua
(Grunow) Swatman. In the example shown here it is demonstrated that four forms, previously identified as four independent taxa in open nomenclature (with distinctive stratigraphic ranges), are in fact separate manifestations of the life habitat of this one single species. It is hoped that through an awareness of the biological complexity of these forms, and their relationships to morphology, a stable taxonomy will eventually arise. It is furthermore hoped that this will lead to a stimulus in the study of the biostratigraphic and palaeoenvironmental applications of this important microfossil group to exploration, production and development geoscience.
Biosignals from the EA Field, Nigeria
Authors: Van Der Zwan C. J. , Brugman W. A.
Published: April 2008

The EA Field study was undertaken to determine reservoir connectivity across growth faults. In this high subsidence area, conventional biostratigraphy does not have sufficient resolution to provide a production-scale time frame. Consequently, a new tool had to be developed which would provide such accuracy. This new high-resolution correlation tool, Biosignals, is based on the relationship ‘pollen-parent plant-ecology’. Pollen, thus related, have been grouped into ‘vegetation’ zones, which have been logged per well and summarized on saw blade diagrams. Sequences of vegetation zones could be recognized, showing an upward trend from spore dominated, via swamp, rain forest, savanna to montane.

A number of potential regional time lines could be recognized: (1) a climate curve; (2) a change from the freshwater alga
Pediastrum
to the brackish water alga
Botryococcus
; and (3) the top of frequent fungi.

Based on this study the following conclusions may be drawn: (i) vegetational stacking patterns reflect fourth-order sequence stratigraphy; (ii) Biosignals suggest the tidal channels in well EA-13 to be lowstand deposits, although many of them belong to truncated sequences (only lowstand systems tract (LST) and transgressive systems tract (TST) preserved); (iii) on a third-order scale, the sandy middle part of the sequence (EA-1, 6000–6500 feet) correlates with dry climatic conditions and to low eustatic sea level. The two regionally extensive maximum flooding surfaces correlate with humid climatic intervals and eustatic high sea level; and (iv) the new high-resolution correlation tool provides a detailed biostratigraphical subdivision, within the existing conventional biozonation, and thus enables correlation of reservoir bodies across growth faults.
Deformational controls on the dynamics of fluid flow in mesothermal gold systems
Authors: Cox Stephen F.
Published: April 2008

Coupling between deformation processes and rock permeability is a major factor influencing fluid migration and fluid-rock interaction during the formation of mesothermal lode gold systems in mid- to upper crustal regimes. The evolution of permeability during deformation is controlled by a dynamic competition between deformation-induced porosity creation processes and porosity destruction processes. Localization of deformation in faults and shear zones leads to flow localization, with large scale flow systems forming when active faults and shear zones link to create percolation networks. Broad regions of fluid focusing develop around the upstream segments of active shear networks and fluid discharge regions develop in the downstream parts of these systems.

The architecture of flow within shear networks is influenced by the relative proportions of backbone, dangling and isolated structures within the network. Connectivity in the network may increase with growth of the shear network, but is expected to continually change as the locus and intensity of deformation changes. The typical distribution of mesothermal lode deposits within crustal scale shear networks indicates that mesothermal systems might develop most efficiently in networks that are close to the percolation threshold, i.e. with most flow occurring along a flow backbone forming only a small part of the total shear network. Dangling elements adjacent to the backbone, particularly in the downstream (discharge) parts of the system, provide some of the best potential for gold deposition by fluid-rock interaction processes.

Contrasting styles of fluid flow are expected between the seismogenic and aseismic regimes of the shear- or fault-hosted hydrothermal systems associated with mesothermal lode gold formation. Below the seismic-aseismic transition, creep processes can lead to near-steady state permeabilities and produce continuous fluid flow regimes. In the seismogenic regime, large cyclic changes in fault permeability, fluid pressures and fluid flux occur during the seismic cycle, and lead to fault-valve behaviour and episodic fluid flow. The seismogenic regime allows for a richer variety of gold deposition processes than is generally available in the aseismic regions of hydrothermal systems associated with lode gold formation.
Thrust-fracture network and hydrothermal gold mineralization: Witwatersrand Basin, South Africa
Authors: Jolley S. J. , Henderson I. H. C. , Barnicoat A. C. , Fox N. P. C.
Published: April 2008

This paper examines the geometry and genesis of a mineralized thrust-related fracture system, developed at a major rheological interface between Witwatersrand sediments and Ventersdorp volcanics in the Witwatersrand Basin of South Africa. At the Elandsrand Gold Mine, the conglomeratic Ventersdorp Contact Reef, which separates these successions, is imbricated by kilometre-wide zones of minor thrusts containing ultracataclastic fault rocks associated with hydrothermal fluid flow. Several sets of fractures are intimately associated with the imbricates, which link to underlying kilometre-scale thrusts. The fractures provided a linked fluid flow pathway and structural sites for precipitation of gold within the Ventersdorp Contact Reef, carrying >900f observed gold, with the remaining 100ccupying post-diagenetic matrix corrosion spaces within a few 100 µm of fractures. The relatively rigid volcanics deformed into low-amplitude kilometre-wavelength flexures, centred on the imbricate zones and underlying kilometre-scale thrusts. These major structures controlled the thrust-fracture system formation along the Ventersdorp Contact Reef-lava contact. Three progressive stages of structural evolution were identified: (1) layer-parallel shear across the contact, with stratigraphy-focused shallow fracture formation; (2) propagation of a kilometre-scale thrust towards the contact, leading to kilometre-scale flexure of the volcanics, with layer-parallel shearing in the limbs producing steep fractures and minor thrusts; (3) propagation of the major thrust along the contact, imbricating the sediments with shallow fracture formation in ramps.
Adularia-sericite gold deposits of Marmato (Caldas, Colombia): field and petrographical data
Authors: Rossetti Piergiorgio , Colombo Fabrizio
Published: April 2008

At Marmato (Caldas, Colombia), epithermal gold mineralization is associated with volcanic to subvolcanic (dacite to andesite) bodies intruding a late Miocene sedimentary sequence and older basement rocks. Mineralization occurs in dilational veins related to a post-magmatic brittle deformation event, possibly connected with reactivation of the Cauca-Romeral fault system. Within the vein system, a vertical zonation is observed. In the lower part the veins are mainly composed of adularia + pyrite ± quartz ± calcite. In the upper part the dominant assemblage is calcite + quartz + pyrite + sphalerite + arsenopyrite + galena + chalcopyrite, and minor argentite, pyrrhotite, polybasite and freibergite. Data from drill cores show that gold is principally related to the hydrothermal veins and dissemination in the wall rock is of minor importance. Wall-rock alteration around the veins is mainly represented by a strong, often complete, sericitization, with minor silicification. The sericitization post-dates an earlier propylitic alteration event, unrelated to gold deposition. The hydrothermal circulation was characterized by ascent of low salinity fluids with a low CO
2
content focused along a structurally controlled fracture system. Boiling was a major controlling factor in deposition in the lower part of the system, while in the upper part mixing also occurred.
Fluidized hydrothermal breccia in dilatant faults during thrusting: the Colombian emerald deposits
Authors: Branquet Yannick , Cheilletz Alain , Giuliani Gaston , Laumonier Bernard , Blanco Oscar
Published: April 2008

The emerald deposits of Muzo and Coscuez hosted within the western flank of the Colombian Eastern Cordillera formed during thrust tectonics affecting series of lower Cretaceous black shales. An unusual polygenetic breccia fills opened fractures and dilatant sites within thrusts and tear faults. Detailed field and petrographic studies show that the breccia formed during a fluidization process in which a hydrothermal pulp was injected into dilatant sites. Fluidization is synchronous with fault propagation. Multistage brecciation is observed and appears to correspond to successive faulting-fluid flow pulses. The opening of dilatant sites during compressive tectonics and the combination of fluidization and hydraulic fracturing suggests that these pulses may be related to successive build-up and drop of the fluid pressure.
Age of mineralization of carbonate-hosted, base metal deposits in the Rathdowney Trend, Ireland
Authors: Sevastopulo G. D. , Redmond P.
Published: April 2008

The mineralization which resulted in the base metal deposits hosted in carbonates of Lower Carboniferous (Dinantian) age in Ireland is in most deposits younger than their mainly Tournaisian-aged host rocks. The stratigraphical age of mineralization at Navan has been shown to be mostly pre-Arundian in age. In the Lisheen deposit in the Rathdowney Trend, development of massive sulphide mineralization post-dated regional dolomitization of late Tournaisian Waulsortian Limestone. Estimates from fluid inclusion and oxygen isotope data of the temperatures at which dolomitization took place suggests that the depth of burial at the time of dolomitization must have been greater than that achieved by early Arundian time, indicating that mineralization at Lisheen was later, and perhaps substantially later, than at Navan. Mineralization coincided with, or post-dated, maximum downthrow on extensional faults atLisheen; maximum downthrow, therefore, was not attained until after the beginning of Arundian time. If minealization of the Irish deposits, rather than being restricted to a narrow interval of time (Lower Viséan-early Arundian), as has been suggested in recent reviews, continued into the younger parts of the Viséan, later Viséan rocks in Ireland should be considered to be potential targets for exploration.
Constraints on Moho depth and crustal thickness in the Liguro-Provençal basin from a 3D gravity inversion: geodynamic implications
Authors: Chamot-Rooke N. , Gaulier J.-M. , Jestin F.
Published: April 2008

3D gravity modelling is combined with seismic refraction and reflection data to constrain a new Moho depth map in the Liguro-Provençal basin (Western Mediterranean Sea). At seismically controlled points, the misfit between the gravimetric solution and the seismic data is about 2 km for a range of Moho depths between 12 km (deep basin) and 30 km (mainland). The oceanic crust thickness in the deep basin (5 km) is smaller than the average oceanic crust thickness reported in open oceans (7 km), pointing to a potential mantle temperature 30–50°C below normal and/or very slow oceanic spreading rate. Oceanic crust thickness is decreasing towards the Ligurian Sea and towards the continent-ocean boundary to values as small as 2 km. Poor magma supply is a result of low potential mantle temperature at depth, lateral thermal conduction towards an unextended continental margin, and decrease of the oceanic spreading rate close to the pole of opening in the Ligurian Sea. Re-examination of magnetic data (palaeomagnetic data and magnetic lineations) indicates that opening of the Liguro-Provençal Basin may have ceased as late as Late Burdigalian time (16.5 Ma) or even later. The absence of a significant time gap between cessation of opening in the Liguro-Provençal Basin and rifting of the Tyrrhenian domain favours a continuous extension mechanism since late Oligocene time driven by the African trench retreat.
Early Tertiary structural evolution of the border zone between the Pannonian and Transylvanian Basins
Authors: Györfi István , Csontos László , Nagymarosy András
Published: April 2008

The Mid-Hungarian Lineament is the most important tectonic feature of the Intra-Carpathian area. Its evolution is closely related to the Early Tertiary (Palaeogene-Early Miocene) episodes of basin formation. This paper attempts to explain the structural relationships between the Transylvanian Palaeogene Basin and the Mid-Hungarian Lineament. Microtectonic data collected on the field, combined with the available geological and geophysical data point to the following conclusions.

(1)

The first compressional event is probably Early Oligocene in age, and is characterized by σ1 oriented ENE-WSW. We assume that the Oligocene basin is flexural in origin.

(2)

The second phase of compression occurred during Early Miocene times with σ1 oriented NNW-SSE. The last thrust emplacement accounts for the Ottnangian-Karpatian overthrusting of the Alcapa block on top of the Tisza-Dacia block. This deformational phase could have induced large-scale block rotations in both units.

(3)

Late Miocene left lateral faulting occurred along the Dragos-Voda fault system, the effects of which can be traced as far as Hungary. This event most probably corresponds to an eastward escape of the Tisza-Dacia block with respect to surrounding terranes.
Protecting the redox stability of a deep repository: concepts, results and experience from the Äspö hard rock laboratory
Authors: Banwart Steven A. , Wikberg Peter , Puigdomenech Ignasi
Published: April 2008

Redox potential and net reducing capacity are rigorously defined geochemical parameters that provide a framework for assessing the redox-related performance of waste containment in the sub-surface. These concepts are illustrated by performance assessment issues in the geological disposal of spent nuclear fuel, and also by consideration of chromium and tetrachloromethane as illustrative compounds representing other types of hazardous waste. Recent and ongoing research at the Äspö Hard Rock Laboratory, a full-scale prototype repository, provides a case study of the redox-related performance of the geological barrier and its function to protect a deep repository from intrusion of dissolved oxygen. Enhanced recharge during construction of the underground laboratory at Äspö strongly influenced redox conditions in the shallow bedrock. The rapid recharge, caused by tunnel construction through a vertical fracture zone, mobilized organic carbon into the groundwater. Microbial processes were critical to the performance of the shallow bedrock, as a barrier to oxygen intrusion during enhanced recharge, by efficiently scavenging oxidants through intense respiration of the added carbon. Because the study site was mainly exposed bedrock with very little soil cover, this effect may be even more pronounced at potential repository sites with thicker overburden.
The role of sorption onto rocks of the Borrowdale Volcanic Group in providing chemical containment for a potential repository at Sellafield
Authors: Berry J. A. , Baker A. J. , Bond K. A. , Cowper M. M. , Jefferies N. L. , Linklater C. M.
Published: April 2008

The current Nirex design concept for the disposal of intermediate-level and certain low-level radioactive wastes in the UK involves emplacement in a cementitious repository deep underground. A site near Sellafield has been investigated to see if it is suitable. The Nirex Safety Assessment Research Programme includes a study of the sorption of radionuclides on the potential host rock formation, as sorption processes are important mechanisms for retarding radionuclides emanating from a repository. The potential host rock is the Borrowdale Volcanic Group (BVG) which is comprised of tuffs characterized by low porosity and permeability. This paper gives an overview of studies of the sorption of radionuclides, in particular actinides, on BVG rock and includes a description of experimental techniques in use at AEA Technology, the difficulties inherent in working with intact-rock samples of such low porosity and permeability and the use of data from both crushed and intact-rock experiments in performance assessment studies. The main sorbing minerals in the BVG are described, and the study of perturbing influences such as the presence of degradation products from organic materials in the wastes and highly alkaline porewater from the cementitious backfill are also discussed. Illustrative results are presented.
Geochemical interactions between landfill leachate and sodium bentonite
Authors: Spooner A. J. , Giusti L.
Published: April 2008

The disposal of municipal, commercial and industrial waste by landfill has been, and is, widely used around the world. Within the landfill, leachate is generated with the input of atmospheric water, and by mixing with the moisture already present from the decomposition of waste materials. The potentially hazardous components of leachate have the ability to pollute surface and groundwaters unless stringent criteria are met for the location of landfills, suitable materials are used to build basal barriers and capping systems, and modern leachate collection and disposal techniques are employed. Different types of barriers have been used, including naturally occurring soil/clay liners, compacted clay liners, geosynthetic materials, and composite sealing systems composed of a mineral layer and a synthetic membrane. One of the substances most commonly used in the sealing of landfill sites, either on its own or in combination with synthetic materials, is sodium bentonite. Despite its widespread use, there is some evidence that this material may not be capable of attenuating some of the substances contained in landfill leachate, and that the clay mineral structure itself may be damaged as a result of long-term exposure with the various organic/inorganic components of municipal landfill leachate. In particular, bentonites tend to collapse and shrink once exposed to certain toxic effluents. The stability, chemical resistance and permeability of a bentonite clay liner appear to be significantly affected by landfill leachates and other inorganic/organic solutions used in experimental work. The main aims of this paper are: (i) to describe the properties of bentonite and municipal solid waste leachates, and (ii) to summarize the available information on the interactions between bentonite and leachate.
Metal retention from landfill leachate by glacial clays: laboratory experiments
Authors: Hurst E. L. , Holmes S. P.
Published: April 2008

Two sets of laboratory experiments were carried out to investigate the role of clay liners on the retention of metals present in landfill leachate. In the first experiment leachate was passed through clay filled columns and comparisons made of the metal content in treatment columns (leachate) with that in the control columns (de-ionized water). The metals Na, K, Mg, Ca, Al and Fe show elevated concentrations when compared with the control columns. In addition, a black sludge was observed on the upper surface of the treatment columns. On analysis this sludge was found to be Fe-rich when compared with the surrounding leachate/clay matrix. In the second experiment a simple tank system was used to investigate sludge formation at the leachate: clay liner interface. The observed sludge was found to have elevated metal concentrations, with a corresponding decrease in metal levels in leachate samples, compared with analysis before the experiment. From these observations it was concluded that the sludge may be acting as a sink for key metal contaminants from leachate.
Evaluation of the containment properties of geological and engineered barriers by pore-water extraction and characterization
Authors: Reeder Shaun , Cave Mark R.
Published: April 2008

In the construction of underground repositories for the disposal of potentially toxic wastes, it is critical to the development of a safety case that the chemical composition of waters flowing through the proposed containment barrier is fully understood. Because the barrier material is usually chosen to have low permeability, to minimize the risk of long-term transport of toxic material into the biosphere, it is often not possible to obtain sufficient flowing groundwater samples to allow complete hydrogeochemical characterization. In order to study the chemical composition of the water in these formations it is necessary, therefore, to use specialized pore-water extraction techniques.

In this study, the practicalities of three methods for extracting and characterizing porewaters from potential containment materials are discussed. These are: (i) mechanical squeezing using a hydraulic press; (ii) centrifugation using a heavy liquid displacent; and (iii) aqueous leaching of residual salts from crushed core material.

Three case studies are presented which demonstrate how each of the different pore-water characterization methods may be applied practically, and how the data obtained can prove invaluable in understanding the hydrogeochemistry of sites.
Microresistivity and ultrasonic imagers: tool operations and processing principles with reference to commonly encountered image artefacts
Authors: Cheung Philip S.
Published: April 2008

Borehole images obtained from microresistivity and ultrasonic imager tools form an important source of geological information. The interpretation of these images is often hampered by the presence of artefacts, arising from peculiarities of the logging tools, and/or unexpected borehole conditions. In this paper, the operations of microresistivity and ultrasonic imager tools are reviewed, with a special emphasis on the tool features and borehole conditions that may give rise to artefacts in the resultant borehole images. The origin and characteristics of some commonly encountered artefacts are discussed, pointing out how they can be identified. Processing methods that may help to remove or reduce the effects of the artefacts are described. This information should help an interpreter to initially better understand the origin and significance of commonly encountered artefacts in terms of tool response, and secondly ensure that undesirable effects of such artefacts are minimised by using available data processing methods.
Application of borehole image logs in constructing 3D static models of productive fracture networks in the Apulian Platform, Southern Apennines
Authors: Trice Robert
Published: April 2008

Fractured reservoirs of the Apulian Platform carbonates present significant challenges from the perspective of formation evaluation. Evaluation problems manifest themselves at all stages of reservoir development from exploration and production drilling through to reserves calculation and reservoir simulation. A significant aspect of these problems arises from the difficulty in locating and testing productive fractures during drilling operations. Once productive fractures have been identified the problem then becomes how to model them away from the borehole and to assign field scale fracture parameters. A key data set in such evaluations is provided by borehole image logs. This paper discusses how borehole image logs have been used to describe the Apulian Platform carbonates with respect to: (a) identify producing fractures, (b) to understand and model the relationship of fractures to the stress field, (c) to provide input to 3D models of producing fracture systems, and (d) how the 3D models were used to quantify the fracture system. The combination of these elements has led to a practical methodology by which a significantly improved understanding of the Apulian Platform fracture system has been achieved.
A sedimentological application of ultrasonic borehole images in complex lithologies: the Lower Kimmeridge Clay Formation, Magnus Field, UKCS
Authors: Goodall Ian , Lofts Jeremy , Mulcahy Matthew , Ashton Michael , Johnson Sam
Published: April 2008

Ultrasonic borehole image tools are generally considered to be of limited value as an interpretative sedimentological tool because thick packages of a single lithology have similar acoustic properties resulting in resolution of very little diagnostic detail. In addition, build-up of mudcake in porous sandstones typically masks geological information by impeding the acoustic signal. In contrast, in heterolithic sediments, differences in acoustic impedance between intercalated lithologies enable clear distinction of sandstones and mudrocks from ultrasonic amplitude images. Whilst resolution of internal detail is still limited, valuable diagnostic data may be obtained for the sandstones by careful examination of the geometries and orientations of their lower and upper contacts with enclosing mudrocks. This is the case in the Lower Kimmeridge Clay Formation reservoir in the Magnus Field, UKCS, using the Schlumberger Ultrasonic Borehole Imager tool (UBI). Here, the bulk of the reserves are held in relatively thin, depositionally flat-lying, high-density turbidite sandbodies which are donors to variably inclined and complexly bifurcating sandstone injections. Recognition of sandstone injections and an understanding of their orientations is crucial to future development of this reservoir because they may be the principal means of communication between sandbodies and also have pay potential. Whilst this is not possible using conventional openhole logs it is easily achieved using UBI amplitude images.
FMS Images from carbonates of the Bahama Bank Slope, ODP Leg 166: Lithological identification and cyclo-stratigraphy
Authors: Williams Trevor , Pirmez Carlos
Published: April 2008

Ocean Drilling Program (ODP) Leg 166 cored a transect of holes through the prograding carbonate sequences that form the western slope of the Great Bahama Bank, with the aim of detailing the relationship between the sequences and changes in sea-level over the last 25 Ma. A total of 1200m of FMS resistivity images from Site 1003 (lower slope) and Site 1005 (mid-slope) were divided into three image facies types, with the aid of calibration against the recovered core. Type 1 was conductive (poorly cemented) sediment dominated by pelagic components, Type 2 was resistive (well cemented) sediment dominated by platform (neritic) components, and Type 3 was highly resistive (very well cemented) sediment, usually calciturbidites but occasionally hardgrounds. Much of the section is composed of metre-scale alternations between Type 1 and Type 2 sediment. We have used the cycle thicknesses in the Middle Miocene to obtain a sedimentation rate curve and to refine the biostratigraphy. The cyclicity is modulated by the precessional astronomical cycle. The FMS images were used to evaluate the lithostratigraphic position and significance of prominent isolated uranium peaks. The peaks tend to occur just below the tops of calci-turbidite-rich units, sometimes coincident with sequence boundaries and maximum flooding surfaces.
Contributions of acoustic imaging to the development of the Bruce Field, Northern North Sea
Authors: Parkinson D. N. , Dixon R. J. , Jolley E. J.
Published: April 2008

An interpretation of four acoustic images from the Bruce Field is presented. It provides an example of what can be achieved using moderate-quality data from a North Sea siliciclastic reservoir. Faults have been detected and oriented using evidence from large-scale block rotations, small-scale drag, direct imaging of fault planes, identification of fault damage zones and breakout rotation. Fracture frequency is highly variable and is dominantly controlled by fault-related damage, though rock properties may also be important. Damage zones are some 25–50 m thick and appear to be concentrated on the hanging walls of their controlling faults. These damage zones are important barriers to flow. Fractures are orientated NNE-SSW. A stress-field map has been produced combining image and dip-log data. The dominant orientation of the minimum horizontal principal stress is parallel to the fractures, though it is influenced locally by structure. The combination of stress field and fracture studies predict the dominance of east-west fluid flow in the reservoir, ‘baffled’ by north-south fracture barriers. Palaeocurrent information has been used to help construct a suite of time-slice palaeogeographies which describe the sedimentary heterogeneity.
Previous ideas and models of the stratigraphy, structure and mineral deposits of the Manx Group, Isle of Man
Authors: Ford T. D. , Wilson Eva , Burnett D. J.
Published: April 2008

Since Bishop Wilson’s [Wilson, Bishop T. (1772)
The Works of Bishop T. Wilson
, Cruttwell, Bath] account of the geology of the Isle of Man there has been a succession of increasingly sophisticated accounts of the Manx slates. Outlines were provided by Woods [Woods, G. (1811)
An account of the Past and Present State of the Isle of Man, Including … a Sketch of its Geology
, Baldwin, London], Berger [Berger, J. F. (1814)
Transactions of the Geological Society of London
,
2
; (1824)
Annals of Philosophys, Series 2
,
viii
] and MacCulloch [MacCulloch, J. (1819)
A Description of the Western Isles of Scotland, Including the Isle of Man
, Constable, London] in the early nineteenth century but the first attempt at a stratigraphic subdivision was by Cumming in 1846 [Cumming, J. G. (1846)
Quarterly Journal of the Geological Society
,
31
]. Lamplugh’s map and memoir at the turn of the century provided the first full account [Lamplugh, G. W. (1903)
The Geology of the Isle of Man
, HMSO]. Later, Gillott [e.g. Gillott, J. E. (1955)
Geological Magazine
,
92
] examined the Manx slate in more detail but it was not until the 1960s that Simpson [Simpson, A. (1963)
Quarterly Journal of the Geological Society
,
119
] defined an 11 unit stratigraphic succession and delineated a refolded synclinal structure. Subsequently, discoveries of graptolites and acritarchs have necessitated revisions to both Simpson’s succession and structure. Correlation with some parts of the Skiddaw Group is now well established.
Magnetic survey of the Poortown Dolerite, Isle of Man
Authors: Piper J. D. A. , Biggin A. J. , Crowley S. F.
Published: April 2008

A ground magnetic survey of a 1.5 × 1 km
2
area surrounding the Poortown Dolerite emplaced into Early Ordovician Manx Group metasediments has identified two regions of high-amplitude and short-wavelength anomalies; the first region extends for 300 m north of the quarry outcrop and the second lies between 100 and 500 m to the east. Linear anomalies have trends ranging from east-west to east-northwest-west-southwest. Palaeomagnetic and rock magnetic studies of the exposed intrusion show that the magnetization resides in Ti-poor titanomagnetite, which is predominantly multidomained. The natural remanent magnetization is dominated by a viscous remanence in the present geomagnetic field and the magnetic anomalies can therefore be modelled using the ambient field direction. A smaller high-blocking temperature component has a westerly direction of negative inclination and is compatible with a normal magnetization acquired during Late Ordovician times. 2D Geometrical models are developed to fit the observed magnetic anomalies. They show that the igneous complex comprises one or two north dipping sheets at its eastern and western peripheries, which expand into a set of multiple sheets separated by screens of Manx Group country rock in the area immediately north of the present quarry outcrop. The sheets may be components of a smaller number of sill-like intrusions repeated by faulting. This general model is supported by results from six boreholes.
Geological structure and tectonic evolution of the Lower Palaeozoic rocks of the Isle of Man
Authors: Fitches W.R. , Barnes R. P. , Morris J. H.
Published: April 2008

The Lower Palaeozoic sedimentary rocks of the Isle of Man, deposited near the margin of the Avalonian plate, were folded and cleaved during continental closure and collision. Although folds on a scale of several kilometres can be inferred, the large-scale structure of the Isle of Man remains uncertain in the absence of detailed and widespread biostratigraphical controls on the stratigraphy and other difficulties. A working model suggests that the island is composed of several northeast trending, strike-parallel ‘tracts’ separated by vertical or steeply northwest dipping faults. Stratigraphic sequences may be apparent within tracts but cannot easily be correlated between tracts. The within-tract structure is the result of two main stages of deformation (D1 and D2) and one or more localized later events (D3). F1 folds have axial surfaces which dip steeply northwest or southeast, near-horizontal hinges, and wavelengths ranging up to several kilometres. The F2 folds, seen mostly on a small scale, are reclined to recumbent and coaxial with F1 folds. The D1 and D2 structures were imposed on the Wenlock rocks of the Niarbyl Formation but are absent from the Peel Sandstone (late Silurian-early Devonian), constraining the main deformation to the Caledonian (Acadian) orogeny.

The D1 structures are the products of collisional tectonics but the origin of the flat-lying D2 structures is unclear, although vertical flattening is more likely than thrusting. Most of the inferred tract-bounding structures cannot be characterized in the absence of exposure or precise stratigraphical controls. Two of them, however, are inferred to be northwest dipping faults. One of these faults cuts D1 structures whilst the other appears to be pinned by a syn-D1 intrusion, suggesting broadly syn-D1 age. Localized, steep northeast striking zones of late to post-D1 high strain, which are exposed on the west coast of the island, in part reflect partitioning of coaxial or transpressive deformation into the boundaries between rocks of contrasted ductility. The Niarbyl Fault Zone (‘shear zone’) is shown to be a belt of ductile and brittle deformation whose tectonic history and regional significance have yet to be fully resolved. Brittle thrust faults are exposed in several parts of the Isle of Man and include the Niarbyl Thrust, which, in one interpretation, is taken to separate the Niarbyl high-strain phyllonite belt from the overlying Niarbyl Formation. Most thrusts are north to northwest dipping and displacements are typically on a metre scale. Many of them may be attributed to the latest stages of the Caledonian collision processes on the basis that similar structures also deform the late Silurian-early Devonian Peel Sandstone of the Isle of Man and rocks of similar age elsewhere in the region.
The Auckland volcanic field, New Zealand: geophysical evidence for its eruption history
Authors: Cassidy John , Locke Corinne A. , Miller Craig A. , Rout David J.
Published: April 2008

The Late Quaternary monogenetic basalt volcanoes of the Auckland volcanic field exhibit styles of eruption ranging from phreatomagmatic to magmatic. New detailed aeromagnetic and other geophysical data from the southern half of the field provide constraints on the style and relative timing of eruptions. Concealed basalt bodies are shown to be common beneath maars indicating deep excavation by phreatomagmatic events and subsequent filling by magma. Depth of excavation is unrelated to the presence of surficial and potentially saturated Plio-Pleistocene sediments but commonly involves magma-water interaction in widespread aquifers within the underlying Miocene sediments. Coincident anomalous bulk magnetization directions show that at least three volcanoes were active contemporaneously and suggest a very short duration of activity which is probably typical of other centres in the field. These results emphasize the spasmodic nature of activity in the field over the last 140 ka culminating in the most recent centre, Rangitoto, which erupted after a long quiescent period and represents a large spasm of activity, confined to a single centre. The locations of contemporaneous centres are tentatively correlated with the regional NNW-SSE structural trend.
Volcanic hazards in Auckland, New Zealand: a preliminary assessment of the threat posed by central North Island silicic volcanism based on the Quaternary tephrostratigraphical record
Authors: Newnham Rewi M. , Lowe David J. , Alloway Brent V.
Published: April 2008

The City of Auckland (population
c.
1 million), built on a basaltic volcanic field active as recently as
c.
ad
1400, faces an additional volcanic threat: that from several large and productive rhyolitic and andesitic eruptive centres of the central North Island, 140–280 km to the south and southeast. Non-basaltic tephra fallout layers originating from these distal eruptive centres are numerous and widespread in the Auckland region and have primary thicknesses ranging from
c.
1 mm to ⩾ = 0.6 m; ignimbrites up to 9 m thick are also documented but are uncommon. The assessment of volcanic hazards in Auckland is made problematical by the different types of volcanic threat posed by these two spatially distinct source areas, and by the lack of recognition hitherto given to the threat from the distal sources. This paper reviews the Quaternary records of distal volcanism affecting Auckland and outlines current investigations into the assessment of environmental impacts of past eruptions. Our preliminary results indicate that the potential threat to Auckland from the distal volcanic sources has been underestimated and that further research into the impacts of Quaternary volcanism on Auckland’s environment and infrastructure is essential. The importance of this threat was underscored in mid-1996 when a small magnitude eruption of Mt Ruapehu necessitated closure of Auckland International Airport for three nights.
Modelling the impact of Icelandic volcanic eruptions upon the prehistoric societies and environment of northern and western Britain
Authors: Grattan John , Gilbertson David , Charman Daniel
Published: April 2008

Many studies now address the impact of Icelandic volcanic eruptions upon the societies and environment of Britain and Ireland. It has become apparent that the assumptions of the magnitude of volcanic impact inherent in these studies are open to question. The scale of climatic change following many eruptions has been observed to be ephemeral or non-existent, whilst the impact of toxic volatile gases and aerosols is dependent, to a large degree, on the vulnerability of the receptor rather than the scale of deposition. The scale and extent of volcanic climatic-forcing mechanisms are examined and the popular perception of the effectiveness of these is challenged. In particular, the concepts of harsh volcanic winters and cool volcanic summers is examined via case studies of the impacts of the three largest eruptions of recent and historical times, i.e. those of the Laki fissure, Tambora and Mount Pinatubo. Alternative volcanic forcing mechanisms for environmental change are considered, principally those concerned with the transport and deposition of toxic volatile material. The need for caution in the adoption of these models is also urged. It is suggested that workers in this field need to adopt a clearer theoretical framework when contemplating the association of palaeoenvironmental data and volcanic events. Such a framework is proposed.
Crustal anisotropy and differential uplift: their role in long-term landform development
Authors: Battiau-Queney Y.
Published: April 2008

The British Isles belong to a passive continental margin where recently acquired geophysical data have confirmed the great heterogeneity of the crust. In such an area, models based on time-related landform evolution leading to planation fail to explain the long-term development, at least since the North Atlantic began to open. The concept of a morphotectonic system taking into account a continuous input of energy (due to plate tectonics) and a state of stress controlled by pre-existing crustal discontinuities is well adapted to the British area and other similar areas. The spatial units of a morphotectonic system are defined by the crustal framework and do not generally coincide with drainage basins. Regional-scale landforms are not time-related but depend on crustal properties, and denudation rates might strongly differ on either side of main crustal hinges, whatever the proximity of the sea.