bluuebunny/crossref_metadata_2025_split huggingface dataset:
Planktonic foraminifera of the coastal upwelling area off Peru during the Pleistocene
Authors: Ibaraki Masako 
Published: April 2008

Abstract

Planktonic foraminifera of the coastal upwelling area off Peru during the Pleistocene have been quantitatively examined in ODP Leg 112 cores obtained from five sites beneath the upwelling area. Laminated diatomaceous facies in some cores suggest they had been in upwelling centres during the Pleistocene. The Pleistocene fauna dominantly consists of cold water and eurythermal elements, but also contains diverse tropical-subtropical species. The fauna is similar to the living one in the modern upwelling area, although the former seems to have much more cold-water elements. The most important species in the coastal upwelling area off Peru during the Pleistocene were
Globigerina quinqueloba
, concentrated in upwelling centres,
Neogloboquadrina dutertrei
, dominantly found in surrounding areas, and many kinds of tropical-subtropical species. Typical temperate water dwellers are rare.
Palaeoproductivity oscillations during the last 130 ka along the Portuguese and NW African margins
Authors: Abrantes Fatima 
Published: April 2008

Abstract
Diatom abundance and taxonomic changes were quantitatively studied in the latest Quaternary (∼ 130 ka to present day) sequences of the upwelling zones associated with the continental margins of Portugal and northwest Africa. Marked oscillations in diatom abundances accompanied by changes in the assemblage composition are evident during the late Quaternary and reflect changes in primary productivity related to upwelling intensity. Upwelling increased during glacial episodes (Isotopic Stages 2 and 6) relative to interglacial intervals (Isotopic Stages 1, 3 and 5). During the late Holocene, primary productivity levels are similar to those for Stage 5, and, lower than today in the early Holocene.
The palaeoproductivity reconstruction based on the diatom record is supported by independent estimates of palaeoproductivity based on the organic carbon content of the sediments.
Outcrop gamma-ray logging to improve understanding of subsurface well log correlations
Authors: Slatt Roger M. , Jordan Douglas W. , D’Agostino Anthony E. , Gillespie Robert H. 
Published: April 2008

Abstract
Reliable, reproducible gamma-ray logs of outcrops have been generated by two techniques which aim to improve the visualization of interwell-scale lateral continuity (and discontinuity) of strata and to demonstrate the reliability and potential pitfalls of subsurface wireline log correlations. One innovative technique was developed which uses a standard gamma-ray sonde run from a logging truck in order to log vertical cliff or quarry faces. A second technique employs a hand-held gamma-ray scintillometer to log more easily accessible outcrops.
Examples are presented from the Jackfork Group (Pennsylvanian), Arkansas, USA, of outcrop gamma-ray logging of both laterally continuous and discontinuous turbidites in structurally simple and complex settings. Because the strata from which the logs were measured can be visualized and discussed at the outcrop, these examples can clearly illustrate the following aspects of wireline log correlation: (1) reliability can be greatly improved by understanding expected subsurface depositional geometries and lateral facies changes; (2) wireline log correlations in stratigraphically and structurally complex settings, such as many oil or gas fields, may not be reliable without sufficient coring and special logging in addition to well testing; (3) erroneous correlations can result from the common practice of inferring three-dimensional rock geometries from two-dimensional well log data. Additional examples demonstrate that correlations and interpretations of subsurface wireline logs in both oil/gas fields and in exploration areas can be improved by comparing the subsurface logs with outcrop gamma-ray logs of nearby analogous strata.
In situ stress orientations in the Witch Ground Graben, North Sea, revealed by borehole breakouts: preliminary results
Authors: Cowgill S. M. , Meredith P. G. , Murrell S. A. F. , Brereton N. R. 
Published: April 2008

Abstract

Directions of minimum horizontal stress (σ
h
) derived from borehole breakouts are presented for seven boreholes in the Witch Ground Graben, North Sea. We have found a clear tendency for a dominant N-S (more accurately a NNW-SSE) direction of σ
h
, approximately perpendicular to the graben axis. However, significant variations from this general trend were encountered even within this relatively small area, especially within the boreholes from the Tartan and Highlander fields. Contrasts in local structure and lithology within the area may explain these differences although such contrasts have yet to be quantified.

Broad regional trends of σ
h
are apparent throughout Northwest Europe, although previously published data show a certain amount of variability. It has been shown that the minimum compressive stress direction changes significantly across many of the graben structures within the North Sea basin, and this also appears to be the case within the Witch Ground Graben.
Fracture permeability and alteration in gabbro from the Atlantis II Fracture Zone
Authors: Goldberg D. , Broglia C. , Becker K. 
Published: April 2008

Abstract
Geophysical logs recorded over 500 m in Ocean Drilling Program Hole 735B have been analysed to identify the in situ properties and the fracture permeability of basement rocks in the Atlantis II Fracture Zone. Hydrous alteration minerals were most abundant in the upper section of the hole. Packer tests also indicate the highest permeabilities in this zone. This interval is characterized by the presence of several open fractures imaged by the acoustic televiewer and by temperature log indications of fluid flow through transmissive fractures. Negative temperature gradient and large semblance anomalies were found to correlate well with the largest of the observed fracture zones. Most of the 70 identified fractures strike subparallel to the north-south fracture zone and dip steeply WSW. These fractures probably formed as a result of normal faulting on the transform side of the uplifted ridge in the Atlantis II Fracture Zone, where 735B was sited, rather than as a direct result of horizontal plate tectonic motion.
Thermal conductivity prediction from petrophysical data: a case study
Authors: Griffiths C. M. , Brereton N. R. , Beausillon R. , Castillo D. 
Published: April 2008

Abstract
Thermal conductivity measurements were carried out on over 400 samples from Tertiary and Cretaceous sediments and Jurassic basalts during Ocean Drilling Program Leg 123 in the Argo Abyssal Plain, North West Australia. Ship-board physical property measurements were obtained from cores over the same intervals, and petrophysical wireline logs were run. New algorithms are demonstrated relating thermal conductivity to matrix grain type and porosity, derived only from wireline log data. Comparison with heat flows calculated using high-resolution temperature measurements at Site 765 suggest that thermal conductivities predicted from wireline logs can be used to estimate heat flow. This implies that existing exploration and production wells in which high resolution temperature measurements and thermal conductivities have not been measured could still be used to prepare regional maps of heat flow variation.
Determination of Young’s modulus of the rock mass from geophysical well logs
Authors: McCann D. M. , Entwisle D. C. 
Published: April 2008

Abstract
Geophysical logging of site investigation boreholes can provide a wide range of useful geotechnical information ranging from the formation density (gamma/gamma log) to the dynamic elastic moduli (full wave train sonic log). In this paper, the use of Young’s modulus as a measure of the deformation characteristics of the rock mass is examined. A comparison is made between the dynamic value obtained from geophysical borehole logging and the static value obtained directly from the laboratory testing of samples collected from the borehole. The data are obtained from a number of BGS boreholes. It is shown that although the prediction of the static value of Young’s modulus from the dynamic value is dependent on a knowledge of the borehole lithology, the use of the dynamic elastic moduli can yield considerable useful engineering information in the assessment of the deformation properties of the rock mass.
The Temperature Decay Log: a different approach to presenting a temperature survey
Authors: Alm Per-Gunnar 
Published: April 2008

Abstract
In connection with ‘shut-in’ temperature surveys numerous temperature logs are run over a period of time. When it comes to presenting the results, several of the recorded temperature logs are often shown in the same figure. In order to increase the readability of the figure, some of the recorded logs might have to be excluded. This can cause vital information concerning the formation surrounding the borehole to be excluded. The paper gives an example of a different way to present the recorded temperature logs. Normally the presentation shows temperature versus depth. A different approach is to study the change of temperature versus time, after shut-in. In this case, the temperature behaviour at every depth is studied throughout the survey. Thereafter all the temperature logs are included in one final temperature decay log. This means that all the temperature logs run during the survey are used in the final presentation. The method gives as a result one single curve showing the temperature recovery after shut-in. The curve shows the time of recovery versus depth. In the figure, the different zones of the borehole are easily discerned. By use of several of the above mentioned temperature decay logs it is possible to distinguish parts of the borehole with uniform temperature behaviour. The paper gives an example from a fractured formation where the two methods of presenting temperature logs are compared. (The conventional one and the one suggested in the paper.)
Determining total organic carbon contents from well logs: an intercomparison of GST data and a new density log method
Authors: Myers K. J. , Jenkyns K. F. 
Published: April 2008

Abstract
A detailed case study shows that more accurate in situ organic carbon (TOC) estimates can be made by the interpretation of conventional density logs than by the carbon/oxygen log method, and at a fraction of the cost. The performance of the two methods has been compared in a cored section of the Schiste Carton source-rock in the Paris Basin. The density log method described in this paper method is relatively quick and simple to use, whereas the C/O method involves the interpretation of a specially run log.
Results show that with the density log method 80% of the TOC estimates fall within ±20% of the core measured values in a TOC range 2–6.5 wt%. The C/O log method, which involves 30 ten minute stationary measurements or three passes at 600 ft/hour, achieves comparable accuracies at higher TOCs (> 5wt%), but is considerably less accurate at lower TOCs with only 50% of values within ±20% of the core measured values.
The hydrology of basement complex regions of Africa with particular reference to southern Africa
Authors: Farquharson F. A. K. , Bullock A. 
Published: April 2008

Abstract
Basement complex rocks occur extensively throughout Africa and experience a wide range of climatic conditions. Over much of the continent, potential evaporation exceeds rainfall and runoff is consequently limited. Much of the continent is short of reliable water supplies, and hence understanding the hydrological behaviour of river basins throughout this important region of Africa is of vital importance to the development of the area.
There is a considerable body of rainfall and river flow data throughout the continent of Africa, but whilst climatic data has been collated and analysed on the regional or continental scale, river flow data has been analysed only at the national or sub-regional scale.
An attempt is made here to summarize the hydrological characteristics of those regions of Africa having outcrops of basement complex rocks, illustrating general observations with the results of detailed analyses of available data from Malawi and Zimbabwe. It is suggested that the results of studies in these two countries may be of wider benefit to workers in southern and eastern Africa in particular.
An exploration strategy for higher-yield boreholes in the West African crystalline basement
Authors: Boeckh Eduard 
Published: April 2008

Abstract
In Burkina Faso investigations have been carried out to test satellite imagery and electromagnetic (EM) methods as a means of improving the siting of water supply boreholes in fissured rocks of the crystalline basement. The results show that these methods may be used to locate boreholes with above average groundwater yields, and to reduce the failure rates.
Geometrical analysis of Landsat-MSS lineaments reveals the relevant regional fracture pattern, the present-day principal stress field and the resulting system of oriented hydraulic conductivities. Structural analysis of the lineament pattern and identification of dry-season-active vegetation on Landsat-TM imagery permit promising locations to be pinpointed for groundwater abstraction. EM-measurements on a two-dimensional grid using the ‘very low frequency’ (VLF) and ‘horizontal loop EM’ (HLEM) methods are used to locate potential fractures on a local scale as a basis for rational borehole siting.
The methods, applied separately or in combination, offer an economical approach for use in rural and community water supply, small-scale irrigation and water resources planning. The methods appear to be generally transferable to fissured hard rock aquifers in other regions. The best suited settings are believed to be on cratons and platforms in a semi-arid climate.
Thermal history reconstruction using apatite fission track analysis and vitrinite reflectance: a case study from the UK East Midlands and Southern North Sea
Authors: Bray R. J. , Green P. F. , Duddy I. R. 
Published: April 2008

Abstract
Apatite fission track analysis (AFTA) is a method of obtaining thermal history information in sediments. In sections that have been hotter in the past, AFTA provides estimates of maximum palaeotemperatures, and the time at which a sedimentary section began cooling from maximum palaeotemperatures. It is useful for studying the thermal history of sequences containing source rocks, and can provide information critical to the understanding of the timing of oil generation. Integration of AFTA and vitrinite reflectance (VR) data allows a coherent thermal history framework to be established and a formal method of analysing palaeogeothermal gradients has been developed by combining AFTA and VR data. This approach allows definition of the major facets of thermal history based on directly measurable parameters, rather than on assumed values of palaeo-heatflow, and provides rigorous estimates of the amount of uplift and erosion. The case study demonstrates the application of this approach to the East Midlands Shelf region. Here AFTA data have revealed an episode of maximum palaeotemperatures prior to cooling commencing at around 60 Ma. Combined AFTA and VR data show that this cooling was due to kilometre-scale early Tertiary uplift and erosion under near normal geothermal gradients. Our estimates of uplift and erosion are significantly greater than those of pervious workers and show that uplift and erosion was not restricted to the recognized axes of inversion, but was regional in extent. This has implications for the evolution of source rock maturity in this region and in other basins in around the UK where AFTA has indicated early Tertiary heating and where the extent and amount of Tertiary uplift and erosion may have also been underestimated by previous workers.
Estimation of Cenozoic water depths in the Western Central Graben, UK North Sea, by subsidence modelling
Authors: Joy Aidan M. 
Published: April 2008

Abstract
Variations in water depth with time at a point within a marine sedimentary basin can in theory be evaluated by fitting a tectonic subsidence curve for the point to a subsidence model. By combining results from numerous points, palaeobathymetric maps can be constructed for specified time horizons. This has been attempted for the Cenozoic interval in part of the Western Central Graben in the United Kingdom sector of the North Sea Basin. A modified version of the instantaneous extension model has been used as the basis for the method. The initial results suggest that water depths over most of the study area increased dramatically from the end of the Danian to the end of the Eocene. Thereafter sedimentation outpaced subsidence, reducing water depths at first slowly (Oligocene-Pliocene) and then rapidly (Pleistocene). These results are reviewed with reference to observational data: palaeontological and sedimentological data, stratigraphical data from seismic reflection profiles and data on the uplift and denudation histories of the adjacent land masses. The review procedure both illustrates the strengths of the method and highlights areas in which refinements are necessary. The iterative nature of this study, in which the observational data are used progressively to constrain the tectonic subsidence model, is emphasized.
The Angus Field, a subtle trap
Authors: Hall Stephen A. 
Published: April 2008

Abstract
The Angus Field is situated near the centre of the North Sea in UKCS Blocks 31/21 and 31/26a. High-quality shallow marine sands of late Kimmeridgian to early Ryazanian age (the Angus Sands) form the reservoir. Although the field is a simple four-way-dip closure at Base Chalk level, the distribution of the reservoir sands beneath this surface is complex and reflects the impact of basement faulting on both sediment deposition and preservation. Five facies have been recognized within the Angus Sands and a depositional model has been developed that relates their distribution to the structural history of the area. Four of these facies are of good to excellent reservoir quality, the fifth is only poorly developed but may act as a local permeability barrier on the southern flank of the field. Over much of the field area the reservoir sands are too thin to be seismically definable. In such areas a sand isopach has been created using well data points, the basement fault pattern and the results of reservoir modelling. Based on this isopach the Angus Field is calculated to contain 18.5 MMB STOIIP. By adopting an innovative approach to development that makes use of the high sustainable productivity of the reservoir Amerada Hess believes that it can develop the field economically on a standalone basis.
Facies analysis of the Kopervik sand interval, Kilda Field, Block 16/26, UK North Sea
Authors: Guy Martin 
Published: April 2008

Abstract

The Kilda Field is a large gas condensate accumulation located in UK Block 16/26. The reservoir sands, informally termed Kopervik, are Lower Cretaceous in age and are developed on the southern dip slope of the Fladen Ground Spur. They represent a localized sandy development within the lower Sola and upper Valhall formations, associated with the Austrian orogeny. Sands were carried out into the basin by a variety of sediment gravity flow processes, and were deposited as an apron near the foot of the slope. Their distribution is controlled largely by the nature of the flow and by pre-existing topography. Six main sand facies have been recognized, three of which are high-density turbidite (HDT) types and contain virtually all reservoir quality rock. HDT S
3
sands resulted from suspension collapse. They exhibit porosities around 15%, and permeabilities around 40 md. HDT Laminated sands probably formed from traction sedimentation. They have similar average porosities but reduced permeabilities between 10 and 15 md. Sands of the Marginal facies are of an inferior reservoir quality. The remaining three facies consist of non-reservoir quality sands deposited by liquefaction, debris or low-density turbidite flow. Post-depositional disturbance suggests that deposition took place on an unstable palaeoslope. Subsequent porosity loss within clean sand intervals has been dominantly through compaction.
The Nelson Field: a successful application of a development geoseismic model in North Sea exploration
Authors: Whyatt M. , Bowen J. M. , Rhodes D. N. 
Published: April 2008

Abstract
UK North Sea Block 22/11 has had a long history of exploration involving six companies drilling four wells over two decades. The latent exploration potential of the block was recognized by Enterprise in 1985 following geoseismic studies carried out in the neighbouring Montrose and Arbroath Fields. The modest reserves attributed to the oil and gas shows encountered in the poor quality Palaeocene reservoir of the first well, 22/11-1, drilled in 1967 were considered uneconomic at the time. The true reservoir potential of the Palaeocene appeared to have been overlooked. Enterprise pursued the new play concept, firstly by farming in and thereafter by acquiring the sole exploration rights of the block in a series of complex commercial asset and acreage deals.
Applying modern and ancient submarine fan analogues with a detailed appreciation of local geology, Enterprise recognized sand-prone ‘channels’ on reflection seismic data and established that the 22/11-1 well had penetrated an ‘interchannel’ facies. Subsequent drilling confirmed this model and led to the discovery of the Nelson Field, in the centre of what is considered to be a mature hydrocarbon province.
The geoseismic model, exploration concepts and reservoir geology of this late Palaeocene turbidite clastic play are presented to illustrate how a detailed production geological/geophysical model can significantly benefit exploration.
Reservoir heterogeneity in the Vanguard Field, UKCS
Authors: Lahann Richard W. , Ferrier Jon A. , Corrigan Susan 
Published: April 2008

Abstract
The Rotliegendes Group in the Vanguard Field, UKCS, consists of a series of aeolian, fluvial, and reworked aeolian sediments, organized into four broadly correlatable lithostratigraphical units. Within these units, reservoir properties are strongly linked to lithofacies, with aeolian dune sandstones possessing the best porosity and permeability.
A comparison of production log spinner profiles (PLT) with detailed sedimenttological logs from three development wells shows that gas production is intimately associated with specific lithofacies and their spatial organization. Although dune sandstones in the upper section of the Rotliegendes have good petrophysical characteristics, they do not contribute significantly to production because they are contained within laterally extensive and relatively impermeable interdune sandstones. Most production comes from 15–30 feet thick and highly permeable (100–1000 mD) dune foreset intervals deposited near the bases of large aeolian dunes.
Significant interwell variation in production rates and in porosity exists for all lithofacies due to the variable extent of diagensis. Petrographical studies, combined with whole-rock geochemical analysis, have outlined a diagenetic sequence for the field. Two end-member mineralogical assemblages are identified which are closely linked to reservoir quality. High porosity wells contain a chlorite-K-feldspar-quartz-dolomite assemblage while low porosity wells contain an illite-quartz-ankerite assemblage. Empirical evidence shows that porosity loss is associated with conversion of K-feldspar, chlorite and dolomite to illite and ankerite, and is due to compaction rather than cementation. Post-depositional faults acted as conduits for fluid which altered mineralogy, removed soluble components and, in one case, introduced anhydrite cement.