bluuebunny/crossref_metadata_2025 huggingface dataset:
Lithological and fracture response of common logs in crystalline rocks
Authors: Bremer Martin H. , Kulenkampff Johannes , Schopper Jürgen R.
Published: April 2008

Log responses in crystalline rocks are a function of the lithology (rock matrix), macro- and microfractures. Basically, the porespace-geometry dependent petrophysical parameters (porosity, internal surface area, permeability, formation factor) are similarly interrelated as in sediments, but mineralogy and fractures exert greater control than in sediments.

A qualitative log interpretation method was developed and tested. Fractured and otherwise deteriorated zones with anomalous properties are identified, using a suitable combination of five standard logs. Before a quantitative well-log analysis of petrophysical parameters is possible, the logs are corrected for lithology effects, using a quick-look lithology identification program. For lithology identification seven logs are considered sufficient. Their mean lithology-dependent response in the German KTB borehole is reported.
Application of dipmeter data in structural interpretation, Niger Delta
Authors: Adams J. T. , Ayodele J. K. , Bedford J. , Kaars-Sijpesteijn C. H. , Watts N. L.
Published: April 2008

Modern 3D seismic techniques provide both high resolution and dense coverage for interpretation of subsurface structures, and are a great improvement on older 2D methods. Features at the lower limit of seismic resolution, however, are often not resolved and interpretation can be ambiguous. Structures at this scale, in particular small-scale faults, are critical for optimum placement of production wells and enhanced field development. Integration of structural analysis techniques from borehole dipmeter data with 3D seismic data allows re-examination of an interpretation and enhancement of such small-scale features.

Dipmeter analysis of a faulted structure in the Niger Delta has enhanced the structural model derived from 3D seismic. The faulted intervals were first identified using vertical dip trend analysis, and the faults were orientated by applying stereographic techniques to bedding dips in zones of fault-related deformation. Near-wellbore structure was then modelled by constructing dipmeter-based cross-sections using an interactive workstation, and minimum fault throw was estimated from these models. Accurate positioning of the faults on the seismic data was achieved by time-indexing the dipmeter log, and the structure was integrated with the present-day stress regime using borehole breakout analysis.

Application of these methods has allowed faults that were below seismic resolution to be identified and oriented, and for these to be explained in terms of the regional structural regime.
Enhanced resolution resistivity logging for fracture studies
Authors: Jackson P. D. , Shedlock S. , Willis-Richards J. , Green A. S. P.
Published: April 2008

Electrical resistivity techniques have been developed which have a higher resolution than standard logging methods. The paper describes an enhanced focusing technique which is shown to increase the resolution to an extent where individual fractures can be identified within what appears to be a single fracture from conventional focused logs. The measurements were made in the Carnmenellis Granite and the problems of making resistivity measurements in this resistive environment are discussed in terms of electric current flow in the borehole which is controlled by the proximity of connected fractures and the resistivity contrast between the fluid and the formation.
Laboratory measurements of the seismic properties of sedimentary rocks
Authors: McCann C. , Sothcott J.
Published: April 2008

A laboratory technique for the measurement of compressional and shear wave velocities and attenuations (quality factors) of sedimentary rocks at pressures up to 70 MPa and at frequencies up to 1 MHz was published by Winkler and Plona in 1982. In the method the rock sample is enclosed between buffer rods. The arrival times and amplitudes of the signals reflected from its top and base are compared. A similar technique has been used for many years in the non-destructive testing of metals and plastics. This paper describes the use of a single-frequency pulsed sine wave for the velocity and attenuation measurements, in addition to the transient pulse used in the Schlumberger method. Measurements were made on samples with accurately known compressional and shear wave properties and on replicated rock samples. The results demonstrate that the determinations of velocities are accurate to ± 0.3%, and that the determinations of attenuation are accurate to ±0.1 dB/cm at 0.85 MHz.
A new approach to the interpretation of nuclear borehole logs
Authors: Ølgaard P. L.
Published: April 2008

When interpreting nuclear borehole logs it is usually assumed for simplicity that the tool signal and a formation property, e.g. its density, are uniquely related. However, while the tool signal will usually depend primarily on one formation property, it will also be affected by other formation properties. To take this multi-parameter dependence of all the nuclear tools into account, a new approach to nuclear log interpretation has been developed. In this approach four nuclear tools are considered: the gamma density tool, the neutron porosity tool, the pulsed neutron tool and the natural gamma tool. The responses of these four tools depend on formation density (ρ), porosity (φ), neutron absorption (σ
a
), and geological type (including shale content). For each of the four tools a physical model has been developed. The signals measured with the four tools and the four models represent together four equations with four unknowns: ρ, φ, σ
a
and geological type. These four equations may be solved by iteration and a consistent set of formation properties obtained. The interpretation approach has been implemented on a personal computer. Examples of results of such interpretations are presented.
Sandstone diagenesis: framework of a forward modelling approach by integrating wireline and other geological data
Authors: Cheshire M. J. , Sellwood B. W.
Published: April 2008

The integration of wireline log information with other forms of geological data allows the recognition of certain diagenetic effects in sedimentary rocks. Diagenetic modification of a sandstone causes changes in the rock properties that will produce subtle shifts in the wireline log characteristics. Such shifts are often difficult to interpret. However, by reference to a now extensive diagenetic information base it is possible to construct models for processes controlling some types of diagenetic change. So it is possible both to model and predict the spatial distribution of diagenetic effects. To interpret diagenetic effects on wireline logs it is necessary to integrate existing geological databases with information derived from wireline logs in a form that can be handled by computer. Interactive modelling involves the iterative refinement of modelled features. This is done by comparing synthetic logs with actual logs over intervals from which core-derived information is available (Rotliegend and Sherwood sandstones provide ideal test-beds for this approach). Predictive models may then be generated by an interactive process of confirming or denying modelled features. This approach allows refinement of the interpretations made. Eventually we expect that a means of directly interpreting certain important diagenetic effects will be possible from wireline log data.
Structural influences on the occurrence of groundwater in SE Zimbabwe
Authors: Greenbaum D.
Published: April 2008

The Zimbabwe Craton is a heterogeneous assemblage of crystalline basement rocks in which the groundwater reservoirs are structurally controlled or confined to the weathered overburden. Lineaments, interpreted from Landsat imagery and aerial photographs, represent faults, joints and dykes of several ages. Such fissured rocks are more susceptible to deep weathering and are a main target for borehole siting. An analysis of borehole yields indicates that, although failure rates can be high, fractures of all trends are capable of providing an adequate hand-pump yield (0.25 l/s). Thus, despite their origin as compressional shears, many fractures now appear to provide open conduits for groundwater. It is concluded that within the near-surface zone of most importance to basement rock aquifers (40 to 80 m) fractures of all origins are in a state of tension probably as a result of recent uplift and erosional unloading.
Geomorphological controls on borehole yields: a statistical study in an area of basement rocks in central Malawi
Authors: McFarlane M. J. , Chilton P. J. , Lewis M. A.
Published: April 2008

Data from about 1500 boreholes in an area of African erosion surface in central Malawi were statistically analysed to attempt to correlate terrain characteristics, profiles and yields. The objectives were to seek terrain criteria which (a) allow regional evaluation of hydrogeological potential and (b) facilitate borehole site selection. From the 26 1:50 000 topographic maps analysed, a close map-by-map correlation was shown between extensive areas of dambo (seasonally waterlogged bottomland), low stream frequency and low relief as expressed by mean, median and modal values of the relative relief of individual km squares. Using this map-by-map approach no strong correlations were established between yield, profile and terrain characteristics.

There were only slight variations in mean yield of the two borehole types: those which finished in the regolith and those penetrating the underlying hard rock (55 1/min and 68 1/min respectively). Division of the total borehole data into relative relief subsets showed clear associations of low relative relief, thicker total and thicker saturated regolith and higher yields, where relative relief is <250 ft/km
2
. Subsetting the data with respect to the eight minor erosion surfaces which here comprise the African surface also showed significant variations in mean yield associated with these surfaces. Low relative relief emerged as the prime target for borehole siting, with sites at altitudes between the surfaces more likely to be higher yielding than comparably low relative relief sites within the altitudinal range of the minor surfaces. Sites close to the base of inselbergs yielded less well than those in the lowest relief category. The emergence of strong relationships between terrain and borehole performance, regardless of lithological variations, places the effects of rock type on borehole performance of secondary importance to those of geomorphology and leaching history in the study area.

This study allows regional assessment of hydrogeological potential in terms of terrain type. It also provides borehole site selection options within 1 km of a given site, using tabulated mean yields in the relative relief classes in various positions in relation to the minor erosion surfaces. Only the information available on the 1:50 000 topographic maps, i.e. altitude and relative relief as represented by the 50 ft contours, is necessary to apply the siting options.
Guillemot D: a successful appraisal through alternative interpretation
Authors: Banner J. A. , Chatellier J-Y. , Feurer J. R. , Neuhaus D.
Published: April 2008

Guillemot D is a small Tertiary oil accumulation situated at the edge of the Western Platform in the UK sector of the Central North Sea. This salt-induced dome was drilled as early as 1969 by the crestal well 21/30-1 which found thin oil-bearing sand stringers in the Palaeocene Forties and Eocene Rogaland Formations. The structure was re-evaluated in 1987 when additional potential was recognized at Forties level downflank to the north of the discovery wells. This concept was tested late 1988 by well 21/30-13 which found a 119 ft oil column in thick Forties sands of excellent reservoir quality (619 ft gross, net-to-gross 0.89) only 1 km away from well 21/30-1.

The 1989 mapping by Production Geology benefited from the availability of a new good quality 1987 2D seismic grid which allowed a better understanding of the subtle Lower Tertiary stratigraphy. Regional geochemical data significantly contributed to an alternative geological model which postulated additional potential in the Eocene Tay sands. Two appraisal wells, i.e. 21/30-14 and 15, were successful drilled, doubling the reserves.

This paper discusses the key structural, seismo-stratigraphical and geochemical considerations in this appraisal campaign.
Sequence architecture, facies development and carbonate-cemented horizons in the Troll Field reservoir, offshore Norway
Authors: Gibbons K. , Hellem T. , Kjemperud A. , Nio S. D. , Vebenstad K.
Published: April 2008

The Sognefjord Formation in the Troll Field area was deposited during a general rise in sea-level during the Late Callovian to Early Volgian. A detailed study including palynology, sedimentology, response modelling and petrography of carbonate cement has been performed on 15 wells primarily from the western part of the field. All wells have been extensively cored through the reservoir enabling detailed palynological and sedimentological study of the depositional sequences in the wells. The Sognefjord Formation is interpreted, in this study, as a shoreline-attached tidally-influenced shelf complex.

The study is based on the general consistency found between the eustatic sea-level curve published by Haq
et al.
in 1987 and the biostratigraphical zonation for the Troll Field described here. The response to sea-level fluctuations in a shallow marine environment is discussed and a response model is established. A theoretical assessment of the criteria for recognizing important events is presented. The maximum flooding surfaces (mfs) and sequence boundaries (sb) are easiest to recognize. Theoretically there is a higher probability of forming carbonate cements in connection with maximum flooding surfaces.

The stratigraphically important dinocysts on Troll are classified according to their relation to sea-level fluctuations. Most of the palynozone boundaries are related to maximum flooding surfaces, except for a few which coincide with sequence boundaries. A stratigraphical subdivision of the Troll Field reservoir is presented based on a comprehensive palynological study.

Lithofacies analysis in conjunction with palynofacies interpretation have been applied to delineate the depositional environment. As an aid to the interpretation a theoretical model was established. The predicted succession and variability of sequences and facies types outlined by the theoretical model could be recognized in all wells.

Petrographical analysis shows that early diagenetic, near-surface cementation has occurred in connection with maximum flooding surfaces and sequence boundaries on Troll.

With the help of the stratigraphical framework the presence or absence of sequence boundaries, transgressive systems tracts, maximum flooding surfaces, and highstand systems tracts in each cycle has been defined in each well. This has given the detailed geometry of the reservoir sands of the field, and also allowed correlation of carbonate cemented horizons which are associated with bounding surfaces of the systems tracts. Knowledge of the distribution and extent of these cemented horizons has considerable significance for oil production strategy, particularly the siting of horizontal wells.
A discussion on the use of analogues for reservoir geology
Authors: Alexander Jan
Published: April 2008

Whilst it may be easy to describe and quantify a sedimentary succession from outcrop or core, using these data to predict sedimentary facies, diagentic features and fracture distribution in unsampled areas still requires considerable intuition. Frequently, at the current state of geological knowledge, reservoir datasets must be supplemented by consideration of analogues. The aim of this paper is to sound caution on the indiscriminate use of analogues in reservoir geology.

Analogues used in reservoir geology fall into three main categories: comparisons with modern depositional processes, experimental simulations, and rock record analogues. All analogues used for reservoir geology suffer from limitations due to the physical and temporal scales at which the comparisons are made. Difficulty in selecting appropriate analogues results from the amount of interpretation of the nature of the reservoir required before an analogue may be selected, the large number of degrees of freedom in the processes of formation of reservoir rocks, and the limited numbers of well-documented analogues which are easily available.
Analogue dipmeter logs through a prograding deltaic sandbody
Authors: Cameron Gavin I. F. , Collinson John D. , Rider Malcolm H. , Xu Li
Published: April 2008

Detailed analogue dipmeter measurements have been made along a 430 m roadcut in the Upper Namurian Rough Rock at Elland in Yorkshire. A series of 21 vertical sections were measured with dip and azimuth (dip-log) readings taken every 0.2 m, which were calibrated to previously described sedimentological sections and recordings of surface natural gamma ray profiles.

The inter-relationship between the scale of sedimentary structures and dipmeter processing parameters is shown by dip-log measurements in medium scale, crossbedded sedimentary structures. The analysis of grouped dip-log data is seen to give palaeocurrents very similar to those measured by traditional methods at outcrop. The relationship between sedimentological facies and ‘dip-log facies’ is applied to reservoir characterization, and statistical analysis of dip-log data is demonstrated to be an aid to inter-well comparisons.

Analysis techniques, mainly computer based and applicable in the subsurface, are applied to the outcrop data and the implications for subsurface dipmeter studies are stressed.
Eocene-Miocene high latitude biostratigraphy
Authors: Radford Sally S. , Qianyu Li
Published: April 2008

Studies of high-latitude planktonic foraminiferal faunas indicate that certain biostratigraphic problems may be resolved by the application of bioevents among the three microperforate families, Guembelitriidae, Chiloguembelinidae and Tenuitellidae. With the aid of the scanning electron microscope, these small tests appear to be useful stratigraphic markers in the condensed sequences of deep-sea cores, where their evolution can be correlated with magnetostratigraphy.

In the early-mid-Eocene, the
Jenkinsina-Cassigerinelloita
lineage assists the recognition of zones based on the larger, muricate
Acarinina
. Four zones are proposed in a revised lineage zonation of the tenuitellids, presented as a complementary biostratigraphic scheme for differentiating the Eocene-Oligocene boundary interval in high latitudes: (1) the Praetenuitella insolita Zone in the late Eocene; (2) the Tenuitella gemma Zone in the early Oligocene; (3) the Tenuitella munda Zone in the mid-Oligocene; (4) the Tenuitellinata angustiumbilicata Zone in the late Oligocene.
Ostracoda as biostratigraphical indices in Cenozoic deep-sea sequences
Authors: Whatley R. C.
Published: April 2008

Some of the disadvantages inherent in the use of benthonic deep-sea Ostracoda such as their lack of pelagic larval stages and the extent to which relative paucity in deeper abyssal environments is prejudicial to diversity, are balanced against the advantages of being pandemic in distribution and independent of latitudinal and surface current temperature controls. The application of Ostracoda to the biostratigraphy of the deep water Cenozoic of the North Atlantic is outlined in some detail and it is shown that podocopid Ostracoda can be used to determine the various nannofossil zones.
Strontium isotope stratigraphy for the Late Cretaceous: a new curve, based on the English Chalk
Authors: McArthur J. M. , Thirlwall M. F. , Gale A. S. , Kennedy W. J. , Burnett J. A. , Mattey D. , Lord A. R.
Published: April 2008

Marine
87
Sr/
86
Sr decreases from 0.70775 in the Cenomanian to 0.70730 in the middle Turonian before increasing in a near-linear manner to >0.70775 in the early Maastrichtian. This variation has been defined using samples from the English Chalk that are closely integrated with the macrofossil and microfossil biostratigraphy of northwestern Europe. With this new isotope curve a stratigraphic resolution is attainable in correlation that is typically ±0.8 Ma for the Santonian and Campanian stages. Isotopic and biostratigraphic correlations between Dorset and Norfolk, in the UK, agree within the limit of analytical error in
87
Sr/
86
Sr.
Transatlantic correlations in the Campanian-Maastrichtian stages by eustatic changes of sea-level
Authors: Hancock Jake M.
Published: April 2008

There is good evidence that high sea-levels peaked simultaneously during the Late Albian to Turonian in the USA and northwest Europe. Therefore, it is to be expected that the principal peaks during the Campanian-Maastrichtian, now well dated in northwest Europe, should be detectable in the USA. Of the four main peaks in Europe, no. 3, low in the Zone ofBelemnitella langei, is the most prominent, and can be recognized with confidence in the Western Interior, New Jersey, Alabama-Mississippi and probably in Texas. The other peaks can be identified with various degrees of confidence, but the sea did not persist long enough in the Western Interior for the last peak, early in the Late Maastrichtian, to be developed there. The results change the generally assumed position of the Campanian-Maastrichtian boundary on the foraminiferal scale: the base of the Maastrichtian on the belemnite scale is a considerable distance above the top of the Zone ofGlobotruncanita calcarata, possibly as high as the base of the Zone ofGansserina gansseriin north temperate regions. On the ammonite scale in the western interior of the USA the boundary is probably at the base of the Zone ofBaculites baculusat 70.7 Ma, but could be at the base of the Zone ofB. eliasi.
High resolution biostratigraphy
Authors: Cope John C. W.
Published: April 2008

The degree of stratigraphical resolution obtainable from biostratigraphical data provides, for most parts of the Phanerozoic, a finer discrimination than that obtained by other means. The full potential of biostratigraphy has not yet been realised in many parts of the geological column and much time and effort, in many cases including primary taxonomic work, is needed, in a field which is currently considered unfashionable. Examples are selected from the Phanerozoic to illustrate the degree of resolution currently achievable.
Devonian goniatite biostratigraphy and timing of facies movements in the Frasnian of the Canning Basin, Western Australia
Authors: Becker R. Thomas , House Michael R. , Kirchgasser William T.
Published: April 2008

As part of an international study of Devonian facies movements against cratonic areas an investigation has been undertaken of the timing of facies movements associated with the Frasnian and Famennian reef complexes of the Canning Basin, Western Australia. This led to the discovery of rich goniatite faunas which enable significant improvements to be made to the high resolution ammonoid time scale. A new goniatite classification for the Frasnian is proposed for international use based on the entry of particular genera; this gives 12 divisions. A much more detailed regional zonation is proposed for the Canning Basin. As part of this, a description is given of the following new genera of the Gephuroceratidae:
Gogoceras, Playfordites, Serramanticoceras
and
Mixomanticoceras
. The genus
Probeloceras
is used in a restricted sense and recognised as the first member of the Beloceratidae;
Acanthoclymenia
is used for forms previously assigned loosely to
Probeloceras
.

The new zonation provides a framework for analysis of facies movements. Occasions of anoxic or hypoxic events are indicated by rich, hematized goniatite faunas. Reef backstepping associated with deepening is dateable by goniatites in associated marginal slope deposits. Several of the international eustatic sea-level changes are recognised, noticeably those correlative with the Genundewa, Middlesex and Rhinestreet deepenings of New York. No evidence has been found for the internationally widespread ‘Kellwasserkalk’ facies in the late Frasnian, but a succession of terminal Frasnian sea-level changes is recognized. The ‘Irtidium Anomaly’ level lies well above the Frasnian/Famennian boundary and hence provides no confirmation of a bolide to explain Frasnian/Famennian boundary extinctions. In the Famennian, evidence for the European Condroz regression and Annulata deepening is presented.
Towards a carbon isotope stratigraphy of the Cambrian System: potential of the Great Basin succession
Authors: Brasier M. D.
Published: April 2008

The Cambrian spans a time of major biosphere change, during which the major invertebrate groups appeared and radiated. Recent discoveries of major carbon isotope excursions in carbonate rocks around the world have stimulated a working group on ‘Late Precambrian-Cambrian Event Stratigraphy’ to explore their potential for correlation (IGCP Project 303). Bioevents, sequence boundaries and trace element markers may also be used for calibration. A major impediment to this objective has been the endemicity and diachroneity of many polymeroid trilobites and early skeletal microfossil taxa associated with shallow water carbonates. The use of deeper-water carbonates, bearing more widely-distributed agnostoids and polymeroids should, however, provide an opportunity to test independently the correlative value of carbon isotope excursions on a global scale. Studies on the Cambrian of the Great Basin, western USA, show that such facies can bear similar δ
13
C signatures to contemporaneous shallow-water carbonates with more endemic trilobites. Hence ‘agnostoid facies’ do have the potential to provide a global and integrated carbon isotope and fossil stratigraphy. A test case is provided by a major positive carbon excursion in the late Dresbachian (or Steptoean) Stage, coincident with shoaling and emergence at the top of the Sauk II sequence across the Laurentian craton. If global, this distinctive marker should be traceable into other Cambrian carbonate successions since it lies between two of the most widely correlatable biohorizons: above the last occurrence of
Glyptagnostus reticulatus
and below the first appearance of
Irvingella
spp.
Analogue sandbox modelling of Miocene extensional faulting in the Outer Moray Firth
Authors: Higgs W. G. , McClay K. R.
Published: April 2008

The Palaeogene sequence of the Outer Moray Firth in the UK sector of the North Sea consists of a series of stacked submarine-fans and associated shelf deltaic deposits. The sequence was derived from the elevated Shetland Platform to the northwest. The clastic material was transported southeastwards along the axis of the Witch Ground Graben towards the basin low in the Central Graben.

Towards the top of the Palaeogene submarine-fan slope deposits, there is a disturbed sequence, Oligocene in age, which is interpreted to have been deformed by Miocene age extensional faulting. A semi-regional study of Quadrants 15, 16, 21 and 22 has revealed that the faults, which cut the Eocene to Lower Miocene section, terminate at the Middle Miocene unconformity. Fault activity coincided with a Middle Miocene tilting event indicated by the onlap of Upper Miocene sequences higher on the shelf slope. This tilting event acted as a trigger for extensional fault movement.

Detailed study of the extensional fault geometries illustrates the complex nature of the deformation. The dominant fault trend is northeast-southwest, commonly with the downthrown side towards the northwest. The faults do not exhibit a marked listric geometry nor do they have a common detachment horizon. A common feature is the decrease in observed brittle deformation with depth.

Analogue sandbox modelling illustrates the development of extensional faults associated with the gravitational collapse of a tilted sequence. Deformation within the sandbox models is dominated by non-rigid block rotation.

The Miocene-aged faulting in the Outer Moray Firth is interpreted to be a result of the gravitational collapse of the Palaeogene slope sequence. Fault dip towards the shelf was probably controlled by shear stresses within the deforming sequence and a downslope resistance to deformation. A non-rigid block rotation model is proposed as a possible mechanism for the deformation. A basal detachment is not a requirement of this model.
Seismic sequence stratigraphy and tectonics offshore Namibia
Authors: Light M. P. R. , Maslanyj M. P. , Greenwood R. J. , Banks N. L.
Published: April 2008

High quality seismic data recently acquired offshore Namibia have revealed the complex interplay between sedimentation and rifting that accompanied the breakup of Gondwanaland. Seismic sequence analysis has demonstrated a complex series of superimposed phases of rifting.

Jurassic early-rift sediments were deposited in a Basin & Range setting with marine embayments. Mid- to Late Jurassic extension probably culminated in acidic to intermediate volcanicity. Subsequently, late-rift fill sedimentary accumulations were terminated locally by a major period of plateau-forming effusive activity. This was coincident with the start of Early Cretaceous spreading in the South Atlantic. Shelf, slope and basinal facies are clearly defined in the post-rift succession.

Although virtually undrilled, the area is considered to have very considerable petroleum potential. Seismic stratigraphy will continue to play a vital role in defining the critical juxtapositions of source rock, reservoir and seal.
Sand transport by wind: a review of the current conceptual model
Authors: McEwan Ian K. , Willetts Brian B.
Published: April 2008

Rapid progress has been made in the last decade towards a more comprehensive model of wind blown sand transport extending and sharpening Bagnold’s classic model in significant aspects. This paper reviews the current physical model and attempts to indicate future directions of research.

The currently accepted physical model of aeolian sand transport reduces the sand transport system to four distinct sub-processes: aerodynamic entrainment; the trajectory of the wind driven sand grains; the grain/bed collision; and the modification of the wind by the driven sand. The isolation and separate treatment of these sub-processes has been an important factor in the recent rapid development of aeolian sand transport mechanics. It is, however, their interaction that produces the rich behaviour of the system. Anderson & Haff (1991) and McEwan & Willetts (1991) have synthesized the four sub-processes and constructed full saltation models which follow the system from incipience to steady-state saltation. These computer simulations provide a stern test for the physical model as the results calculated can be compared to experimental observations from field and wind tunnel. The models compare well with available data; thus we can have some confidence that the physical model is realistic. Moreover, these computer simulations have become a powerful investigative tool and have highlighted areas where our understanding is deficient.
Thermoluminescence dating of periods of sand movement and linear dune formation in the northern Negev, Israel
Authors: Rendell H. M. , Yair A. , Tsoar H.
Published: April 2008

The dating of periods of sand movement and dune formation in the Negev has, until the present study, been based on the presence of stone artefacts on, within or beneath sand dunes, or upon14C dating of associated deposits. We present the results of a preliminary thermoluminescence (TL) dating study of sand and interdune-playa deposits from the Sede Hallamish area of the northern Negev. The results appear to indicate that: (a) sediments in the interdune area were deposited over a period from at least 43 ka to 9 ka and include several depositional cycles; and (b) the lower parts of the linear dunes were formed 6–10 ka and the dune flanks have been stabilized since then. These results are placed in the context of existing studies of the palaeoclimate of the Negev during the Late Pleistocene.
A chronostratigraphic re-evaluation of the Tusayan Dunes, Moenkopi Plateau and southern Ward Terrace, Northeastern Arizona
Authors: Stokes Stephen , Breed Carol S.
Published: April 2008

Optical dating of the Tusayan Dunes of northeastern Arizona indicates that at least three distinct late Holocene phases of aeolian reactivation took place in the area, partly upon pre-existing, Pleistocene linear dunes. The reactivation phases occurred approximately 400, 2000–3000, and 4700 years ago. We hypothesize that the large discrepancy in age between the basal and upper parts of the linear dunes relates to the reactivation of only the linear ridge crests, leaving an undisturbed plinth. If so, linear dunes hold a potential for preserving long-term records of successive aeolian depositions. Evidence of regional alluvial-fill episodes and palaeoclimatic data for the area indicate that sediment supply, in addition to aridity and effective sand-transporting winds, is a key factor in determining the timing of aeolian reactivation.
Modes of linear dune activity and their palaeoenvironmental significance: an evaluation with reference to southern African examples
Authors: Livingstone Ian , Thomas David S. G.
Published: April 2008

Supposedly relict continental desert sand dunes have frequently been used as palaeoenvironmental indicators. Often the argument has been, first, that vegetated dunes are fixed relicts, and second, that fixed dunes indicate past aridity. Recent work suggests that neither of these maxims is necessarily true. Using the example of the linear dunes of southern Africa, this paper examines the extent to which we can continue to support a simple assumption that vegetated dunes are inactive relicts. In particular, it addresses the problem of what constitutes activity on linear dunes, and suggests some hypotheses for investigating the ‘grey’ area between active, unvegetated dunes and densely wooded, inactive dunes. The concept of episodic activity is introduced in the context of partially vegetated linear dune forms.
Fluvial-aeolian interactions in a Proterozoic alluvial plain: example from the Mancheral Quartzite, Sullavai Group, Pranhita-Godavari Valley, India
Authors: Chakraborty Tapan , Chaudhuri A. K.
Published: April 2008

The coarse-grained alluvial succession of the Proterozoic Mancheral Quartzite encloses within it a few 2–6 m-thick, typically salmon red, fine-grained, very well-sorted sandstone units. Some 25–400f the fine-grained sandstone units is aeolian — mostly adhesion laminae with subordinate adhesion cross-laminae and wind-ripple strata. The remainder of the sequence is aqueous; either massive or with faintly-developed trough cross-bedding. Based on their grain size, sorting and roundness, the bulk of the aqueous deposits appear to be reworked aeolian deposits. Thick adhesion laminated units within them show a number of superimposed drying-upwards sequences, represented by an upward decrease in laminae spacing within each of the sequences. Individual drying-upwards sequences, 5–15 cm thick, are in places bounded by disconformity surfaces marked by iron crusts.

Stratigraphic relationships with associated fluvial facies indicate that these aeolian sandstones formed in the distal part of the floodplain. An arid climate, vegetation-free landscape, quickly avulsing channel behaviour and rapid basin subsidence favoured development and preservation of these rather thick alluvial plain aeolian deposits. Fluvial dynamics, in contrast to the erg dynamics as interpreted from other interlayered aeolian-fluvial deposits, was the primary control on the depositional features and their internal organization within these aeolian sandstone units.
A complex dune system in Baix Empordà (Catalonia, Spain)
Authors: Cros Lluisa , Serra Jordi
Published: April 2008

The dune system of Baix Empordà, Catalonia, Spain is composed of sediments supplied by the Ter and Daró rivers, transported by the effective north-northwesterly local wind called the
Tramuntana
.

Dune formation is mostly related to the vegetation on the plains area and to topographic obstacles in the Begur Massif zone. Morphologically, these dunes are mainly parabolics and blowouts. Climbing, falling and ‘passadis’ dunes are present in the mountainous zone, the latter being a newly proposed name for a type of lee dune.

The sands are differentiated into distinct groups according to their sedimentological features and marine faunal content. Their evolution and age have been determined by mapping, and by geological and historical information, respectively.
Late Holocene dune formation on the Sefton coast, northwest England
Authors: Pye Kenneth , Neal Adrian
Published: April 2008

Coastal dunes fringe the coast between Southport and Liverpool in northwest England, forming a natural barrier which prevents marine flooding of low-lying agricultural land in West Lancashire and north Merseyside. The dunes are best developed at Formby Point, where they reach 25 m above sea-level and blown sand extends some 4 km inland. This paper presents the results of an initial programme of field drilling, sediment analysis and radiocarbon dating which suggest that the dune complex developed from a large offshore sandbank, which was in existence by 6800 years ago. Behind the bank lay a large area of intertidal mudflats and sandflats with fringing saltmarsh. Dunes may have first become established on the emergent sand bank around 6000 years ago, but the oldest dunes for which there is direct dating evidence formed between 5700 and 5800 years ago. A period of dune stability, possibly associated with seaward progradation of beach and foredune ridges, appears to have occurred between 2500 years ago and the early Middle Ages. During the earlier part of this period oak and alder woodland covered the dunes and the sands were deeply podsolized. Historical and documentary evidence indicates that extensive sandblowing occurred all along the coast during the early Middle Ages, which was a period of exceptional storminess and rapid coastal erosion. Several settlements and farms were lost to the sea or were overwhelmed by blown sand as the coast eroded. Active sand tongues extended across the full width of the barrier and buried the backbarrier peats and silts along the western margin of Downholland Moss. Dune instability has continued intermittently until the present, although on a smaller scale, and consequently most of the present dune forms are relatively recent. Sand stabilization and foreshore reclamation measures undertaken in the second half of the nineteenth century were highly effective and contributed to a net seaward progradation of the entire Formby coastline between 1850 and 1900. Abandonment of these measures around the time of the First World War contributed to, but did not ultimately cause, a return to shoreline erosion and a new phase of frontal dune instability which is still in progress.
Progressive vadose diagenesis in late Quaternary aeolianite deposits?
Authors: Gardner Rita A. M. , McLaren Susan J.
Published: April 2008

Since the mid-1960s a number of models (as exemplified by Gavish & Friedman (1969) and Land
et al.
(1967)) have been developed which suggest that a progressive sequence of morphological, mineralogical and chemical changes occurs during carbonate diagenesis in the vadose zone. The models raise the possibility that the degree of cementation and diagenetic alteration can be used as a guide to interpreting the relative, and even the absolute, age of deposits. A study of the early vadose diagenesis of a wide range of late Quaternary age aeolianites from Oman, South India, the Bahamas, Tunisia and Mallorca has been undertaken to assess the wider applicability of the models. The results presented in this paper indicate that the rigid application of such models to areas outside those for which they have been developed is inappropriate. In the aeolianites analysed, spatial and temporal heterogeneity is the key characteristic. All tend towards an end point of low-Mg calcite allochems and meniscus, rim and pore-filling cement types but at different rates in different places, both locally and regionally, and via several different diagenetic routes.
Contrasting origin and character of Pleistocene and Holocene dust falls on the Canary Islands and southern Morocco: genetic and climatic significance
Authors: Coudé-Gaussen Geneviève , Rognon Pierre
Published: April 2008

Various surficial calcitizations and laminar calcretes overlie non-carbonate basement rocks in southern Morocoo (Ifni and Anti-Atlas areas) and basalt outcrops or biodetrital sands in the eastern Canary Islands (mainly Fuerteventura). Geomorphological, sedimentological and micromorphological data provide evidence for their origin by deposition of Pleistocene calcitic dust and syn- to post-depositional colluvial reworking. These dusts were derived from the comminution of marine and local biodetrital sands mobilized by the westerlies on the emergent offshore shelves during times of Pleistocene low sea-level.

An opposite aeolian system related to Holocene and present-day Saharan dusts explains the formation of silts resting on the slopes of Fuerteventura and the fine fraction of the colluvial soils of southern Morocco. These deposits often lie unconformably on the calcitic bodies and their sedimentological and micromorphological characters are similar to those of the modern Saharan dusts. This suggests opposite major climatic conditions for the two aeolian dust systems: the calcitic dust system was controlled by a southerly shift of the midlatitude circulation in the Pleistocene; the later Saharan dust contribution seems to be related to the re-establishment of the trade wind circulation and Saharan air outbreaks, particularly in the early Holocene when the continuance of blocking situations in the midlatitude Atlantic may have been notable.
The magnetic mineralogy of a loess section near Lanzhou, China
Authors: Rolph Timothy C. , Shaw John , Derbyshire Edward , Jingtai Wang
Published: April 2008

A collection of almost 500 hand samples from a 250 m loess section near Lanzhou, Gansu province, have been investigated to determine both the nature of the magnetic mineralogical differences between the loess and inter-bedded soils and the time variance of the magnetic mineralogy. Results indicate that the parent magnetic material, which is aeolian in origin, is cation-deficient magnetite/maghaemite and the strong magnetic susceptibility contrast, particularly in the younger material, between loess and soil, is mostly due to pedogenic enhancement of the magnetic content of soils by in situ formation of ultrafine magnetite, although we should not ignore possible changes in the nature of the available aeolian material during the warmer soil-forming periods. Compared to loess sections further to the east, the degree of enhancement in the soils is small, on average having a ferrimagnetic content which is only 17 0reater than that of the loess. Trends in the magnetic mineralogy of the section indicate a possible link with the long-term climate change of Northern China, the average ferrimagnetic grain size increasing with decreasing age in response to increased wind velocity and a colder, drier climate. At a point approximately 110 m above the base there is a dramatic change in the nature and concentration of the magnetic material. This change indicates either a rapid deterioration in the average climate or a sudden change in the provenance of the source material, although at this stage we cannot discount the possibility of some time gap at this position. The loess/palaeosol susceptibility contrast is noticeably reduced below this level, perhaps indicating less extremes of climate.
Fluvial scour and incision: models for their influence on the development of realistic reservoir geometries
Authors: Salter T.
Published: April 2008

The three-dimensional geometry of fluvial sandbodies may be very complicated. Factors influencing this complexity include the processes of fluvial scour and incision, both of which can promote local or widespread increases in sandbody thickness. Incision, as caused by downstream relative base-level fall, produces entrenched valleys that control the location of subsequent alluvial deposits. A simple model is presented that attempts to quantify the scale of this extrinsic incision and, in turn, highlights several sedimentary implications of base-level fall. In contrast, scour is an intrinsic process common to all fluvial systems, and is promoted by constriction, curvature or confluences within the major channels. Existing empirical relationships that predict fluvial sandbody width from thickness are oversimplistic, failing to consider the modification of sandbody dimensions by such intrinsic processes. At low aggradation rates sandbody thickness may exceed channel bankfull depth by 100%, due purely to the effects of scour.

It is proposed that future fluvial width: thickness databases should stem from the classification of deposits according to the aggradational regime during their deposition. This will encourage the specific recognition of scour and incision, particularly in coastal plain alluvial deposits, and lead to an improved understanding of the controls upon sandbody deposition.
Dimensions and quality of reservoirs originating in low and high sinuosity channel systems, Lower Cretaceous Travis Peak Formation, East Texas, USA
Authors: Davies David K. , Williams Brian P. J. , Vessell Richard K.
Published: April 2008

Quantification of the dimensions and quality of fluvial reservoirs requires knowledge of channel style, depositional environment and diagenesis. Two styles of fluvial channel co-existed during the rapid aggradation of >600 m of Lower Cretaceous, Travis Peak sediments in East Texas: (1) high sinuosity (meandering) channels, and (2) low sinuosity (straight) channels. Each was restricted to a specific geographic area, the result of long-term, geomorphic stability. In both channel systems, reservoir sandstones originated principally in channel and crevasse splay environments.

Reservoir models developed through sedimentological analysis are similar to those developed independently through reservoir simulation studies. Reservoir sandstones originating as point bars in high sinuosity channel systems are relatively small (1.2 km
2
or less), thin (3.6 m), heterogeneous, and isolated within non-pay mudrocks. Reservoir sandstones originating as medial and transverse/oblique bars in the low sinuosity system are areally extensive (>20 km
2
) thick (3.6 to 13.7 m), homogeneous, and display complex pressure relationships due to avulsion of long stream segments, and the lateral and vertical stacking of successive channels in well-defined channel belts. The greatest volume of channel sandstone occurs in the low sinuosity channel system. The high sinuosity system is dominated by overbank deposits.

Travis Peak sandstones have been extensively modified by compaction and cementation. Despite extensive diagenesis, permeability values reflect original depositonal environment and bedding style, even in rocks which have lost more than 80% of their original, depositional porosity. Channel sandstones have higher permeability than associated splay sandstones. Within a channel sandstone, the highest values of permeability occur in destratified, and flat- to low-angle cross-bedded sandstones: planar cross-bedded and ripple-bedded sandstones have the lowest values of permeability. Original depositional environment and bedding style exercise important control on permeability (particularly potential gas flow) even in rocks in which the pore systems have been modified significantly by diagenesis. A knowledge of depositional environment and bedding style is therefore important in predicting potential producibility in tight, gas sandstones such as the Travis Peak formation.