bluuebunny/crossref_metadata_2025_split huggingface dataset:
Palaeomagnetic reorientation of cores and the magnetic fabric of hydrocarbon reservoir sands
Authors: Hailwood E. A. , Ding F. 
Published: April 2008

Abstract
The palaeomagnetic method is established as an important complement to more traditional methods for reorienting cores drilled in hydrocarbon exploration and production programmes. The method is based on the isolation of one or more components of remanent magnetization within the core. These provide records of the direction of magnetic north, either for recent times (carried by magnetic overprints in the recent geomagnetic field at the drill site) or at the time of formation of the sediment (carried by the primary magnetization). Provided that any spurious components of magnetization associated with drilling are removed by appropriate analysis in the palaeomagnetism laboratory, then the magnetic components of interest can be separately isolated. The mean direction of stable remanent magnetization, averaged over several tens of centimetres of core, defines the direction of geographical north in that core section. For a recent component, the direction of north is specified directly, whereas for an ancient component this is given by reference to the direction predicted from published apparent polar wander curves for that plate or geographical region.
An important attribute of the palaeomagnetic core reorientation method is that the measurements are made on samples from the cores themselves, rather than by downhole instruments such as multishot orientation tools, dipmeters or imaging tools. This makes the technique particularly cost-effective and frees it from reliance on the proper performance of downhole tools or the correlation of planar features observed in the cores with those in the well. Consequently, there is no requirement for the well to be deviated or for there to be a significant natural dip to the formation (although this does help in fitting adjacent core pieces together to allow extrapolation of the reorientation data to longer intervals of core). Furthermore, as the determinations are made directly on the cores, the precision of palaeomagnetic reorientation is inherently greater than for methods based on measurements made in the well. It is possible to apply the method to cores for which the need for orientation was not apparent at the time of drilling, but for which this need arises as a result of later developments. The technique can be applied equally well to recently drilled cores or cores which have been stored for many years.
An account is given of the palaeomagnetic reorientation method and its applications to (1) defining fracture orientations and (2) establishing sediment transport directions in a channel sand body. The transport directions are inferred from magnetic fabric measurements and the principles of the anisotropy of magnetic susceptibility technique are briefly reviewed.
Magnetic susceptibility of a 180 m sediment core: reliability of incremental sampling and evidence for a relationship between susceptibility and gamma activity
Authors: Løvlie Reidar , Van Veen Paul 
Published: April 2008

Abstract
The magnetic susceptibility of a 180 m long core from the North Sea, covering the Middle Brent Group to Lower Cretaceous, has been measured at 2 cm intervals with a whole core sensor loop (MS2C) and on standard paleomagnetic samples collected at approximately 1 m intervals. The stratigraphic variations between the data sets reveal comparable features, suggesting that the sampling interval used yields a reliable record along this core. A relationship is demonstrated between gamma activity and susceptibility, the latter exhibiting more pronounced variations than the gamma logs, suggesting that susceptibility may be a useful lithomagnetic parameter for quantitative lithostratigraphic correlation. Non-linearity between gamma activity and magnetic susceptibility suggest that the latter may be treated as an independent lithological parameter which may be used for lithostratigraphic correlation. Comparison between corresponding plug and whole core susceptibility values indicates some degree of lateral inhomogeneity in the mineralogy of the core.
Magnetic anisotropy of borehole core samples
Authors: Tarling D. H. , Shi H. 
Published: April 2008

Abstract
All rocks have petrofabrics that can be determined rapidly and precisely by magnetic anisotropy as this depends on the net alignment of the crystallographic properties and the shape orientations of all grains within the rock, i.e. the grains themselves act as fabric markers. When ferromagnetic minerals are present in quantities >0.0001%, their properties dominate the observed fabric. As most rocks contain at least this level of ferromagnetic content, most techniques reflect the petrofabric of the ferromagnetic minerals, although rocks where these are absent still have petrofabrics that can be measured quickly and precisely by magnetic methods. Such magnetic fabrics can be used to define the direction of flow and velocities of the currents when the sediment was being deposited. The original fabric may be modified during shallow burial, enabling estimation of the degree of bioturbation. Subsequent fluid migration (gases, water, hydrocarbons, etc.) through the sediments superimposes magnetic fabrics than can be used to define the three-dimensional nature of such secondary channels. The very high sensitivity of the technique also enables it to detect the onset of anchimetamorphic changes at lower levels than detectable by standard techniques.
Recovery as a function of community structure
Authors: Krassilov Valentin A. 
Published: April 2008

Abstract
According to the climax cut-off model, ecological succession can be truncated at an early stage by environmental factors thus causing elimination of climax species and their replacement by new dominants derived from successional species. Such profound restructurings are caused by prolonged stresses rather than by episodic impacts. Recovery of a dominant group is due to its representatives in the early stages of ‘mixed’ successions. In the ‘graded’ successions with the serial and climax species belonging to different evolutionary grades (e.g. dinosaurs and mammals in the terminal Cretaceous communities) such recoveries are less probable.
Ostracode speciation following Middle Ordovician extinction events, north central United States
Authors: Swain F. M. 
Published: April 2008

Abstract
Studies by several workers show that major extinctions of trilobites, echinoderms, brachiopods and gastropods (but not conodonts) occurred at the horizon of a middle Ordovician K-bentonite (Deicke metabentonite) in the north central United States. Extinctions of Ostracoda are also represented at the Deicke horizon. The metabentonite, a Rocklandian stage ash fall the age of which is approximately 454 Ma, middle Caradocian, originated in the eastern United States.
About 160 species of Ostracoda have been recorded in the middle and upper Ordovician of the north central USA. Ostracode speciation was low prior to the Deicke extinction event, consisting of a few eoleperditiids, leperditellids, drepanellids, aparchitids and bairdiids of the Glenwood and Platteville formations (Blackriveran-early Rocklandian). Following the extinction event, during the next approximately 1.5 Ma, 85 ostracode species appeared in the uppermost Platteville Limestone and in the overlying Decorah Shale (late Rocklandian-early Shermanian). The known Ostracoda species durations averaged about 2 Ma, although some are only half that length of time.
Additional episodes of ash falls, following the Deicke and associated episodes, occurred in the next approximately 7.5 Ma in the region. The preceding Decorah ostracode fauna disappeared from the area and about 50 recorded new or immigrant taxa appeared following the last volcanic event in the late Edenian and Maysvillian stages (Dubuque and Maquoketa formations). Species duration during this time interval apparently averaged less than in the Decorah, about 1 Ma.
The volcanic episodes appear to have provided nutrient-rich habitats that resulted in evolutionary spurts in the Ostracoda, following at least two of the episodes. Duration of the species was perhaps related to the total nutrient budget provided by the ash falls as well as to salinity regimes, palaeoclimates and other factors.
Diachronous recovery patterns in Early Silurian corals, graptolites and acritarchs
Authors: Kaljo Dimitri 
Published: April 2008

Abstract
The extinctions that occurred in the latest Ordovician and earliest and latest Wenlock, were the most impressive in the biotic history of this period. They were probably caused by glaciations. Recovery processes, more or less, follow the classical scenario: extinction-low-diversity survival interval-recovery through radiation events. Rather often there occurs diachrony of the phases of this scenario in the different groups discussed. Graptolite recovery was the most rapid with a diversity maximum in the mid-Llandovery. Their main extinction and low-diversity interval were in the latest Ordovician. The evolution of corals and acritarchs was slower — after a few lower-scale origination events a diversity burst was reached in the late Llandovery. An analogous but lower-scale pattern was noted at the very beginning and in the late Wenlock. The difference was caused by evolutionary and ecological reasons. A good correlation between diversity changes and terrestrial environmental events (glaciations, sea-level movements, stable isotope records) is noted.
The mid-Carboniferous rugose coral recovery
Authors: Kossovaya O. L. 
Published: April 2008

Abstract

The taxonomic diversity of the rugose corals following the mid-Carboniferous ‘lesser mass extinction event’ (Alekseev 1989) may be subdivided into two intervals. The survival interval includes long-ranging survivors, short-ranging survivors (
Dibunophyllum, Palaeosmilia, Diphyphyllum
), survivors with post-crisis ontogenetic changes (
Caninia, Bothrophyllum
), newly-appeared taxa with hypothetical ancestors (
Profischerina, Protodurhamina
) and those of cryptogenic origin (
Lytvophyllum
). The lower limit of the survival interval coincides with the
Eumorphoceras/Homoceras
Zone and can be identified in the Donetz Basin, Gornaya Bashkiria, Novaya Zemlja, North Timan, Cantabrian Mountains (Rodriguez
et al.
1986) and the Western Interior of the USA (Sando 1989).

The recovery interval is established by the appearance of the immigrant genus
Petalaxis
and by increasing specific diversity in pre-existing lineages. Its lower limit corresponds to the
Pseudostaffella praegorsky—Profusulinella staffelliformis
boundary, the lower boundary of the Westphalian A in the Cantabrian Mountains and the lower boundary of the Atokan Series in the Western Interior of the USA.

The radiation interval was marked by the appearance of astreoid colonies in the Petalaxis—Ivanovia lineage at the
Fusulinella calaniae—F. vozhgalensis—F. kamensis
lower boundary.
Recoveries and Radiations: Gastropods After the Permo-Triassic Mass Extinction
Authors: Erwin Douglas H. , Hua-Zhang P. 
Published: April 2008

Abstract
The biotic recovery following the end-Permian mass extinction begins with a long lag phase characterized by a low-provinciality, depauperate biota. Normal marine communities do not reappear until the latest Early Triassic (Spathian) and do not become common until the early Middle Triassic (Anisian). Three explanations have been offered for this pattern: first, that the end-Permian extinction so disrupted marine ecosystems that community assembly rules needed to be rewritten prior to the re-establishment of normal community structures; second, that the recovery was delayed by continuing harsh environmental conditions; and third, that the preservational and sampling bias is significant and the pattern more apparent than real.
Data on the stratigraphic, biogeographical and environmental distribution of Wordian-Anisian marine gastropod genera and species were used to evaluate these hypotheses. The results demonstrate (1) an initial removal of endemic (narrowly distributed) genera during the Wordian and Capitanian; (2) the presence of a low provinciality gastropod assemblage during the Djulfian and Changxingian; and (3) the persistence into the earliest Triassic of geographically widespread, environmentally tolerant genera, as well as the apparent (but unobserved), occurrence of numerous ‘Lazarus’ taxa. The data indicate that while preservational bias was significant, environmental dampening was also important.
Planktonic foraminifera recovery from the Cenomanian-Turonian mass extinction event, northeastern Caucasus
Authors: Tur Nataliya A. 
Published: April 2008

Abstract
The changing structure of the planktonic foraminiferal assemblages is examined from the Upper Cenomanian-Lower Coniacian succession of ‘Carbonate Dagestan’.
The analysis of the morphological and taxonomical diversity shows that the deep-water forms are most severely affected by the mass extinction event. The repopulation of their post-crisis assemblages is rapid. The composition of the shallow-water population remains more or less stable.

The foraminiferal dynamics suggests the main developmental phases. The Extinction Phase occurs during the
R. cushmani
Zone, when the bathypelagic rotaliporids became extinct as a result of an expansion of the oxygen-minimum conditions. During the
D. hagni
Zone the survival community is characterized by the primitive forms and forms with an incipient keeled structure. The Early Recovery Phase, occurring during the
H. helvetica, M. coronata
and early
M. tarfayaensis
Zones, is characterized by the appearance of the first marginotruncanids. The Later Recovery Phase, beginning in the later
M. tarfayaensis
Zone, involves a maximum of morphological diversity of marginotruncanids in the
M. concavata
Zone.
Validation of seismic data processing and interpretation with integration of gravity and magnetic data
Authors: Horscroft T. R. , Bain J. E. 
Published: April 2008

Abstract
integration of gravity and magnetic data with seismic interpretations provides a definitive means of section validation. Traditionally this has been achieved as a two stage and separate process — seismic interpretation in the time domain and the potential fields model in the depth domain.
Recent advances in software technology now enables interactive simultaneous real-time modelling of seismic gravity and magnetic data. In addition to conventional interpretations, modern uses of modelling now includes: quality control of seismic depth conversion, control of the potential field model using offset raytracing; derivation of a high quality depth model as a starting point for processes like pre-stack depth migration or turning wave analyses. Integration too can be used to differentiate between geometrically similar but lithologically different seismic structures.
This paper describes the advantages of dynamically linking velocity, density and susceptibility in the modelling process illustrated by three different North Sea geological examples.

(a)
Depth-velocity modelling of salt structures in the Central Graben.

(b)
Structural interpretations of the Rattray volcanic rocks in the Outer Moray Firth.

(c)
Shallow sand channels and velocity models from high resolution aeromagnetic data.
Discussion of potential errors in fault heave methods for extension estimates in rifts, with particular reference to fractal fault populations and inherited fabrics
Authors: Morley C. K. 
Published: April 2008

Abstract
Fault heave methods for estimating extension (including balanced cross-sections and the flexural cantilever model) are important because they are the only methods that are universally applicable to estimating extension from the syn-rift stratigraphic section. One of the main criticisms of the methods when based on reflection seismic data is the failure to account for crustal extension along sesimically invisible faults (displacements up to a few tens of metres). The fractal distribution of several fault populations has been cited as evidence for the existance of such minor faults. It has been suggested by several workers that the discrepancies between extension estimates for the North Sea rifts calculated from the syn-rift section and by backstripping (using the McKenzie model) can be explained by a substantial amount of crustal extension having taken place on seismically invisible faults. This paper examines whether it is reasonable to extrapolate such conclusions to other rifts.
Estimates for the contribution of minor faults to crustal extension based on fractal distributions of faults need to be treated with caution for several reasons: (1) in some rifts large boundary faults (several kilometres displacement) may account for 80–95% of the total extension amount. In such rifts minor faults may not contribute significantly to crustal extension (less than 5%). (2) The different distribution of fault size and spacing between different zones within rifts suggests that a single fractal distribution along a regional rift traverse is commonly unrealistic. (3) Fractal fault populations measured in syn-rift sedimentary sequences may be measuring faults involved in gravity tectonics and compaction, not just those faults that contribute to crustal extension.
Regions with very complex pre-rift structural histories (e.g. North Sea) appear to have significant amounts of extension on minor faults. This contrasts with rifts developed on crusts with a relatively stable history (e.g. East African rift), where much of the extension is accommodated on a few large faults. The differences between these two rifts (despite both rifts having undergone similar amounts of extension) illustrate that pre-existing fabrics are extremely significant for understanding fault populations, fault orientations, partitioning of extension between different sizes of fault, the types of linkage between faults (transfer zones), syn-rift basin subsidence and fault lengths. An appreciation of this variability should lead to more accurate prediction of the contribution of seismically invisible faults to extension on balanced cross-sections.
Finite element modelling of the competition between shear bands in the early stages of thrusting: Strain localization analysis and constitutive law influence
Authors: Barnichon J. D. , Charlier R. 
Published: April 2008

Abstract
Finite element simulation of a sandbox model of thrusting is performed using large strain analysis and two different non-associated elastoplastic constitutive laws (namely Drücker-Prager and Van Eekelen criteria). The analysis of strain localization using the Rice bifurcation criterion coupled with a kinematic indicator shows that, in the early stages of imbricated thrusting, the development of major shear bands can be influenced by some competition with second order bands. The influence of the vertical/horizontal stress ratio is checked, as well as the influence of Lode angle in the constitutive law. Significant differences are found between a classical plasticity criterion (Drücker-Prager) and a more realistic one (Van Eekelen) regarding the resistance of the model and the stress paths. These differences may result in erroneous fault type prediction.
Mathematical modelling of growth strata associated with fault-related fold structures
Authors: Hardy Stuart , Poblet Josep , McClay Ken , Waltham Dave 
Published: April 2008

AbstractPrevious geometric models of fault-related folding have been limited in their ability to model sedimentation contemporaneous with contractional deformation. Waltham’s 1992 general forward modelling equation allows more realistic models of sedimentation to be combined with mathematical models of fault-bend folding, fault-propagation folding and limb-rotation associated with detachment folding. Using average rates of fault slip and sedimentation derived from the literature, the mathematical models are used to predict stratal geometries associated with each of the major styles of fault-related folding in both sub-marine and sub-aerial settings. Both background sedimentation and local erosion, transport and deposition as a result of fold growth are modelled. Comparison is made with natural examples and the utility of growth strata architectures in the assessment of fault-fold kinematics assessed.
An insight on salt tectonics in the Angola Basin, South Atlantic
Authors: Spathopoulos Fivos 
Published: April 2008

Abstract
Salt movements have dominated the post-Aptian tectonic process of the West Africa Salt Basin, which extends from northern Namibia to southern Cameroon. The thick salt layer was deposited during the Aptian within the shallow rift basins of the proto-South Atlantic and was overlain by an extensive carbonate platform. As a result of the basinward tilt of the margin, the carbonate platform and the overlying Upper Cretaceous sequence slid on the salt layer and broke up into smaller blocks. This process caused a mobilization of the salt and the formation of salt rollers, salt diapirs and a whole suite of raft tectonics structures such as turtlebacks, carbonate rafts and severe folding at the toe of the salt basin. Although this process is seen throughout the Angola Salt Basin, the areal distribution of the salt movement-induced structures shows a significant differentiation. Burial history curves also show important differences along the margin in the post-Aptian subsidence. Analysis of recent seismic data suggests that the thin-skinned extension needed for salt and raft tectonics is accommodated basinward by folding of the overburden up to a point where all the extension has been taken up. From there to the toe of the salt layer, the overburden is largely undisturbed. The salt and raft tectonics in the Angola Basin was the main factor that influenced the distribution of oil reserves in the region and its understanding is very important for hydrocarbon exploration there.
Controls on Eocene sedimentation in the central North Sea Basin: results of a basinwide correlation study
Authors: Joy Aidan M. 
Published: April 2008

Abstract
The Eocene of the central North Sea Basin consists of mudstones with locally developed deltaic and gravity-flow sandstones, some of which are important hydrocarbon reservoirs. The pronounced lateral variability in sandstone thickness, combined with the difficulties of seismic interpretation in the Eocene, obliges explorationists to develop a regional understanding of Eocene depositional systems in order to predict sandstone distribution. However the development of such a regional understanding is hampered by the need to correlate between widely spaced wells. The correlation of basinal sections with their marginal equivalents has proved to be a particularly intractable problem.
This study utilizes a simple subdivision of the Eocene into two sequences based upon the recognition of a small number of areally extensive wireline-log and seismic markers. Though the sediments belonging to these two sequences are very similar, the nature and distribution of the large-scale depositional systems developed during the two intervals are quite different. During sequence I time (early to mid Eocene) sand supply was dominated by two deltas, one in UK Quadrant 9 and one in UK Quadrant 21. The late Paleocene Moray Delta formed a positive bathymetric feature in UK Quadrant 15 at this time, suppressing delta development in this region. By sequence II time (mid to late Eocene) a rise in relative sea level had created space in Quadrant 15, allowing a single major deltaic depocentre to develop here; this delta dominated sand supply during sequence II time. Both the location and the morphology of this delta suggest that it was fluvially dominated.
This interpretation suggests that the availability of accommodation space for delta development was a fundamental controlling factor in the supply of sand to the basin. There is no evidence for a eustatic control on sand supply.